Surface characterization of materials using cathodoluminescence

Abstract

The method and system include generating an electron beam within a vacuum chamber from an electron beam generator. A mounting platform within the vacuum chamber is configured to support a material. The electron beam is directed at a surface region of the material at a grazing angle. A detector assembly, which may have an optical penetration path positioned above the surface region, receives cathodoluminescence emissions resulting from the electron beam transferring energy to the surface region. The detector assembly determines spectral characteristics of the cathodoluminescence emissions to characterize the surface region.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application No. 63 / 378,684, filed October 7, 2022, entitled "Surface Characterization of Materials Using Cathodoluminescence," the contents of which are incorporated herein by reference. [Background technology]

[0002] Many techniques are known in the art for characterizing material properties, including elemental composition, bonding state, crystal structure and parameters, and other physical properties. For materials that include multiple layers or thin films (e.g., semiconductor structures), specific techniques can be used to characterize the surface of the material. Generally, energy (e.g., x-rays, electrons, or light) is used to stimulate the surface, and then particles ejected from the surface by the stimulation are analyzed.

[0003] In one example, X-ray photoelectron spectroscopy (XPS) utilizes the photoelectric effect by irradiating a material with X-rays and analyzing the resulting emitted electrons. XPS can be used to analyze the surface region of a material (depths of approximately 10 nm or less) and identify elements in the material along with their electronic structure and chemical state. Grazing incidence X-ray diffraction (GIXD) is another material analysis technique that involves directing an X-ray beam at a shallow angle toward the surface to limit the beam's penetration into the material. GIXD is useful for analyzing thin films (e.g., regions up to approximately 1 μm deep) to determine their crystal structure and lattice constant, among other properties. Auger electron spectroscopy (AES) is a technique in which Auger electrons are emitted after being excited by an electron beam. AES can provide extremely high surface sensitivity with spatial resolution on the order of a few nanometers.

[0004] Cathodoluminescence (CL) works using the luminescence effect, where an electron beam excites a material, resulting in the emission of photons. In semiconductor luminescence, an impinging primary electron excites a secondary electron, which excites a valence electron, which can then recombine with a hole in the valence band to generate a photon. CL systems are typically incorporated into a transmission electron microscope (TEM) or scanning electron microscope (SEM) and utilize the TEM's or SEM's electron beam generator to deliver the electron beam. The luminescent light can be reflected by a parabolic mirror above the material's surface to a detector. The detector characterizes the light emitted from the sample using, for example, a monochromator and a photomultiplier tube. Summary of the Invention [Means for solving the problem]

[0005] A method for characterizing a surface region of a material includes generating an electron beam from an electron beam generator within a vacuum chamber and directing the electron beam at a grazing angle toward the surface region of the material. A detector assembly receives cathodoluminescence emission resulting from the electron beam transferring energy to the surface region, the detector assembly being positioned above the surface region. The detector assembly may operate within a vacuum environment. The method also includes determining spectral characteristics of the cathodoluminescence emission to characterize the surface region.

[0006] A method for characterizing a surface region of a material includes generating an electron beam within a vacuum chamber from an electron beam generator coupled to a sidewall of the vacuum chamber. The electron beam is directed at the surface region of the material at a grazing angle. A detector assembly receives cathodoluminescence emission resulting from the electron beam transferring energy to the surface region, with the detector assembly's optical penetration path positioned above the surface region. The detector assembly determines spectral characteristics of the cathodoluminescence emission to characterize the surface region.

[0007] A system for characterizing a surface region of a material includes a vacuum chamber, a mounting platform within the vacuum chamber, an electron beam generator coupled to the vacuum chamber, and a detector assembly. The mounting platform is configured to support the material. The electron beam generator is configured to direct an electron beam at a grazing angle toward the surface region of the material. The detector assembly is positioned above the surface region to receive cathodoluminescence emissions resulting from the electron beam transferring energy to the surface region. Optical components within the detector assembly can be configured to be housed within the vacuum environment.

[0008] A system for characterizing a surface region of a material includes a vacuum chamber, a mounting platform within the vacuum chamber, an electron beam generator coupled to a sidewall of the vacuum chamber, and a detector assembly. The mounting platform is configured to support the material. The electron beam generator is configured to direct an electron beam at a grazing angle toward the surface region of the material. The detector assembly has an optical penetration path positioned above the surface region to receive cathodoluminescence emissions resulting from the electron beam transferring energy to the surface region.

[0009] Embodiments of the present disclosure will be described with reference to the accompanying drawings. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a flowchart for a method for characterizing a surface region of a material, according to some embodiments. [Figure 2] FIG. 1 is a schematic diagram of a system for characterizing a surface region of a material, according to some embodiments. [Figure 3] FIG. 1 is a schematic diagram of a system for characterizing a surface region of a material, according to some embodiments. [Figure 4] FIG. 1 is a perspective view of a system for characterizing a surface region of a material, according to some embodiments. [Figure 5] FIG. 5 is a top view of the system shown in FIG. 4, according to some embodiments. [Figure 6] 5 is a perspective end view of a mounting arrangement for the system shown in FIG. 4, according to some embodiments. [Figure 7] 1 is a table of example materials that may be characterized, according to some embodiments. [Figure 8A] FIG. 1 is a perspective cross-sectional view of a detector assembly for detecting cathodoluminescence emission from a material, according to some embodiments. [Figure 8B] FIG. 1 illustrates a cross-sectional side view of a system for characterizing a surface region of a material, according to some embodiments. [Figure 8C] FIG. 1 illustrates a cross-sectional side view of a system for characterizing a surface region of a material, according to some embodiments. [Figure 9A] 5 is a diagrammatic side view of the vacuum sample chamber shown in FIG. 4 illustrating the arrangement of multi-layer materials, according to some embodiments. [Figure 9B] FIG. 9B is a detailed view of the detector assembly and materials of FIG. 9A, according to some embodiments. [Figure 10A] FIG. 1 is a schematic diagram of a light focusing arrangement, according to some embodiments. [Figure 10B] FIG. 1 illustrates a refractive optical configuration, according to some embodiments. [Figure 10C] FIG. 1 illustrates properties of ultraviolet-transmissive optical materials, according to some embodiments. [Figure 11-1] 1A and 1B are schematic diagrams of a reflected light collection arrangement, according to some embodiments. [Figure 11-2] FIG. 1C is a plot of effective focal length as a function of mirror radius according to some embodiments. [Figure 11-3] FIG. 1D is a plot of reflectivity as a function of wavelength for reflector and coating materials according to some embodiments. [Figure 12] 1A-1C are diagrams illustrating side views of cathodoluminescence emission from a surface region as a result of impingement from an electron beam incident at a grazing angle, according to some embodiments. [Figure 13]FIG. 1 illustrates a semiconductor material having a complex refractive index, according to some embodiments. [Figure 14] FIG. 1 is a schematic diagram of light processing components of a cathodoluminescence system, according to some embodiments. [Figure 15] FIG. 1 is a schematic diagram of a grating-based spectrometer, according to some embodiments. [Figure 16] FIG. 1 is a schematic diagram of another grating-based spectrometer, according to some embodiments. [Figure 17] FIG. 1 is a schematic illustration of an electrodynamic process initiated by high-energy electron impact excited cathodoluminescence, according to some embodiments. [Figure 18] FIG. 18 illustrates a physical representation of the process described in FIG. 17. [Figure 19A] FIG. 1 illustrates an exemplary structure utilized for selective area cathodoluminescence characterization, according to some embodiments. [Figure 19B] FIG. 1 illustrates an exemplary structure utilized for selective area cathodoluminescence characterization, according to some embodiments. [Figure 20] FIG. 1 illustrates a Monte Carlo particle simulation for dynamic scattering trajectories of incident electrons, according to some embodiments. [Figure 21A] 1 is a graph of the distribution of energy transferred to a crystal by scattered electrons as a function of depth into a structure, according to some embodiments. [Figure 21B] 1 is a graph of the distribution of energy transferred to a crystal by scattered electrons as a function of depth into a structure, according to some embodiments. [Figure 22] 1 is a plot of backscattered electron fraction as a function of incident electron beam angle, according to some embodiments. [Figure 23A] 1 is a graph of the calculated spatial dependence of cathodoluminescence due to electron impact excitation as a function of depth into a structure, according to some embodiments. [Figure 23B]1 is a graph of the calculated spatial dependence of cathodoluminescence due to electron impact excitation as a function of depth into a structure, according to some embodiments. [Figure 23C] 1 is a graph of the calculated spatial dependence of cathodoluminescence due to electron impact excitation as a function of depth into a structure, according to some embodiments. [Figure 23D] 1 is a graph of the calculated spatial dependence of cathodoluminescence due to electron impact excitation as a function of depth into a structure, according to some embodiments. [Figure 23E] 1 is a graph of the calculated spatial dependence of cathodoluminescence due to electron impact excitation as a function of depth into a structure, according to some embodiments. [Figure 23F] 23A-23E. [Figure 24] 1A and 1B are energy-momentum band structure diagrams according to some embodiments. [Figure 25] FIG. 1 is a simplified schematic diagram of the band structure and subgap defect states responsible for CL emission, according to some embodiments. [Figure 26] 1 is a plot of CL emission from bulk cubic MgO crystals excited by electron impact excitation, according to some embodiments. [Figure 27A] 1 is a plot of CL emission according to some embodiments. [Figure 27B] 1 is a plot of CL emission according to some embodiments. [Figure 27C] 1 is a plot of CL emission according to some embodiments. [Figure 28] FIG. 1 is a schematic diagram of a reflection high-energy electron diffraction (RHEED) setup, according to some embodiments. [Figure 29] 1 is a schematic diagram of an embodiment of a CL system, according to some embodiments. [Figure 30] 1 is a simplified diagram of a high-energy electron accelerator forming part of an electron beam source, according to some embodiments. [Figure 31]1 is a plot of de Broglie wavelength versus kinetic energy according to some embodiments. [Figure 32] FIG. 1 is a functional block diagram representing a CL system, according to some embodiments. [Figure 33] FIG. 33 is a schematic diagram of FIG. 32 with additional features, according to some embodiments. [Figure 34] FIG. 1 is a block diagram of a system for pulsing an electron beam of a cathodoluminescence measurement system, according to some embodiments. [Figure 35] 1 is a plot of CL emission as a function of wavelength according to some embodiments. [Figure 36] 1 is a plot of CL emission as a function of energy for various grazing angles, according to some embodiments. [Figure 37] 37 is a plot of the relative intensity of the CL emission peak shown in Figure 36 as a function of grazing angle, and also shows the change in simulated peak electron penetration depth as a function of grazing angle, according to some embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0011] In the following description, like reference numerals designate like or corresponding parts throughout the figures.

[0012] This disclosure describes systems and methods for characterizing the surface region of a material using cathodoluminescence (CL) in a manner that enables characterization of thin films at controlled depths with greater sensitivity than conventional systems. In some aspects, the technique can be used to characterize a specific layer of a multilayer structure (e.g., an epitaxial layer (“epilayer”) formed on a substrate). Characterization can include elemental composition, bonding state, crystal structure and parameters (e.g., crystalline properties), and other physical properties. Embodiments include detection of cathodoluminescence light in the deep ultraviolet (DUV), vacuum ultraviolet (VUV), or extreme ultraviolet (EUV) range (e.g., cathodoluminescence light with wavelengths between 110 nm and 400 nm or between 110 nm and 280 nm), thus enabling measurement of cathodoluminescence emission from ultrawide bandgap (UWBG) semiconductors (e.g., semiconductors with bandgaps between about 4 eV and about 10 eV).

[0013] Standard electron microscopes probe large volumes and depths of materials because the electron beam is delivered at normal incidence. That is, the electron beam is perpendicular to the surface of the sample being analyzed (i.e., directed perpendicular to the surface). As a result, the excitation volume and corresponding solid angle of emission are large, making them unsuitable for characterizing only the surface region of a material. In multilayer materials (e.g., epitaxial structures), penetration must be limited to depths on the order of 10 nm to 10 μm (e.g., 100 nm to 1 μm) to characterize the semiconducting behavior of the surface epilayer(s) without clouding the data with information about the underlying layers and / or substrate. The desired penetration depth depends on the thickness of the layers present in the structure.

[0014] In this disclosure, CL systems and methods are implemented that use a grazing incidence approach to achieve electron impact excitation of a thin volume (referred to as the surface region) of a material just below the surface. The surface region can have a thickness (depth from the surface) of, for example, 1 nm to 500 nm, 10 nm to 500 nm, 1 nm to 1 μm, 10 nm to 1 μm, 100 nm to 1 μm, 1 nm to 10 μm, 10 nm to 10 μm, or 100 nm to 10 μm. The grazing angle can be adjusted to control the depth of electron beam penetration, thus enabling selective excitation of crystalline epilayer material at a controllable depth. In embodiments, directing an electron beam toward the surface region of the material includes setting the grazing angle and beam energy of the electron beam to adjust the penetration depth of the electron beam into the surface region. In embodiments, CL is produced by an electron beam at a low angle relative to the surface and with high energy. Controlled surface area excitation results in minimizing CL from the substrate or other interlayers (between the substrate and the surface area) by confining the electron beam excitation / scattering to within the desired surface area (e.g., epilayer).

[0015] The embodiments also include detection optics positioned directly above and near the surface region to be characterized, thereby enabling highly efficient collection of cathodoluminescence light. In conventional CL systems, the detection optics are located to the side of the system, away from the sample. That is, CL measurements are traditionally performed with far-off-axis devices added to standard SEM tools, where the incident electron beam is substantially perpendicular to the sample surface. Therefore, the light collection optics must be configured to be not only off-axis but also far from the excitation region of the sample, resulting in insufficient light collection. In embodiments of the present disclosure, the optics are positioned near (e.g., directly above) the region of interest and can be configured in a vacuum environment, enabling detection of vacuum ultraviolet (VUV) to UV wavelengths. Notably, conventional SEM-modified CL applications are limited to the near-UV / visible region (≥ 300 nm) due to the selection of optical materials and components that are not VUV-grade (e.g., absorbing and / or highly dispersive). Embodiments of the present disclosure provide the ability to perform optical band-edge measurements of CL excitation of thin films with ultra-wide bandgaps, which are not typically measurable using photoluminescence. In contrast to CL, which uses electrons as the excitation source and photons as the output, photoluminescence uses a light beam as the excitation source and light / photons as the output. For example, CL can provide excitation in the deep UV region, which is not possible with photoluminescence (PL). In CL, the energy of the electrons that excite the material is high enough to generate CL, but there is a lack of practical lasers for photoluminescence systems with wavelengths low enough (and therefore high enough energy) to optically excite ultra-wide bandgap materials.

[0016] In one example, the band gap of wide band gap materials (e.g., metal oxides) can be characterized. Wide band gap materials, especially ultra-wide band gap materials, require excitation at wavelengths and energies that are difficult to achieve with conventional systems. In one example, embodiments enable screening of the quantum efficiency of direct band gap metal oxide phosphors (MOxPs) for implementation in optoelectronic devices. The CL measurements described herein also enable understanding of direct electron impact excitation of MOxPs.

[0017] Some embodiments also include components that pulse the electron beam and analyze the resulting CL emission to achieve a frequency-specific response with a high signal-to-noise ratio (SNR).

[0018] The term "grazing angle" in this disclosure refers to the angle α between the plane of the top surface of the material being characterized and a beam directed toward the surface. The terms glancing angle, grazing incidence, and small grazing incidence angle are used interchangeably in this disclosure. Grazing angles are defined as shallow angles, for example, from 0.1° to 45°, or from 0.1° to 25°, or from 0.1° to 15°, or from 1° to 10°, or from 0.1° to 5°, or on the order of 1 degree. In various examples, the grazing angle can be 45° or less, or 40° or less, or 35° or less, or 30° or less, or 25° or less, or 20° or less.

[0019] FIG. 1 shows a flowchart of a method 100 for characterizing a surface region of a material, according to some embodiments. In block 110, an electron beam is generated in a vacuum chamber. That is, block 110 includes providing a vacuum environment for a detector assembly while receiving CL light by the surface region in block 130. The electron beam generator is positioned to the side of (e.g., laterally adjacent to) the material, rather than directly above it as in conventional CL systems. Block 120 includes directing the electron beam to impinge on the surface region of the material at a grazing angle. By impinging on the sample at a shallow angle (grazing angle), the electron beam penetrates the surface region to a limited depth. The depth can be controlled, at least in part, by the angle of incidence. Some embodiments include setting the grazing angle in block 125 to achieve a predetermined penetration depth by the electron beam. For example, the grazing angle can be set so that the electron beam penetrates the surface region without significantly penetrating the material below the surface region. In another example, the material includes an epitaxial layer on a substrate, and the grazing angle is set so that the electron beam penetrates the epitaxial layer without penetrating the substrate. In some embodiments, block 125 includes setting the grazing angle to control the maximum penetration depth of the electron beam in the surface region. The maximum penetration depth is, for example, in the range of 100 nm to 1 μm or 10 nm to 10 μm.

[0020] Electrons from the electron beam transfer most of their kinetic energy to material within an energy transfer region, characterized by the relative energy transfer or penetration depth of the electron beam. In this embodiment, by impinging the electron beam at a shallow angle, the electron beam transfers a substantially majority of its kinetic energy to the surface region. That is, the electron beam energy transfer region overlaps with and is substantially bounded by the surface region, with minimal energy being transferred to material below the surface region. In some examples, the amount of energy transferred from the electron beam to the characterized surface region is greater than 60%, greater than 70%, greater than 80%, or greater than 90% of the electron beam energy. Accordingly, in some examples, the amount of energy transferred from the electron beam to layers below the surface region (e.g., the substrate and / or intermediate layers between the substrate and the surface region) is less than 40%, less than 30%, less than 20%, or less than 10% of the electron beam energy.

[0021] Block 130 includes receiving cathodoluminescence emission light resulting from the electron beam delivering energy to the surface region using a detector assembly. The detector assembly is positioned above the surface region (or above the region of the mounting platform on which the surface region is located) rather than to the side of the sample as in conventional CL systems. Embodiments of block 130 may include collimating the cathodoluminescence emission light with a collimator within the detector assembly. The collimator may include non-refractive optics (e.g., a reflector and / or an objective mirror) to enable processing of deep UV wavelengths. Block 140 includes determining spectral characteristics of the cathodoluminescence emission light to characterize the surface region.

[0022] FIG. 2 shows a schematic side view of a system 200 for characterizing a surface region 213 of a material 210. The system 200 is operable to perform the method 100. The material 210 can be, for example, a bulk material, a material made up of multiple layers, an epitaxial structure of one or more epitaxial layers (epilayers) stacked on a substrate, or other types of material. In the example shown in FIG. 2, the material 210 includes a substrate 211 and an epilayer 212 (which may represent one or more epilayers) on the substrate 211, and the surface region 213 is the portion of the epilayer 212 to be characterized.

[0023] The system 200 includes a vacuum chamber 250, a mounting arrangement 260 within the vacuum chamber 250 for supporting the material 210, an electron beam generator 220 coupled to a first sidewall 251 of the vacuum chamber 250, and a detector assembly 230 coupled to a top wall 253 of the vacuum chamber 250. In one example, the operating pressure of the vacuum chamber 250 is less than 10 Torr, which is generated by a vacuum pump 255 coupled to the vacuum chamber 250. The electron beam generator 220 forms an electron beam 225 and transfers energy to the material 210 to a desired depth in the surface region 213 by directing the electron beam 225 along a grazing angle α relative to a top surface of the epilayer 212. The electron beam generator 220 is positioned to the side of the material 210 (e.g., laterally to the side of the material 210, rather than above the top surface of the material 210) to achieve the grazing angle. Detector assembly 230 receives cathodoluminescence emission 240 from surface region 213 at a detection angle β defined relative to the plane of surface region 213. In one example, the arrow indicating cathodoluminescence emission 240 represents the light entry path for detector assembly 230, and the detector assembly is positioned such that the light entry path is approximately perpendicular to the surface region (i.e., detection angle β=90°, or approximately 85°-95°). In another embodiment, the detection angle can be between 80°-100°. In yet another embodiment, the detection angle can be between 70°-110°.

[0024] In one example, detector assembly 230 is coupled to vacuum chamber 250 by viewport 235 and oriented to receive cathodoluminescent emission 240 from surface region 213. Cathodoluminescent emission 240 is emitted at a detection angle β relative to the plane of surface region 213. In one example, as shown in FIG. 2 , detection angle β is substantially perpendicular to surface region 213, and detector assembly 230 is positioned above (e.g., directly above) surface region 213. In some examples, as described later in this disclosure, the distance between detector assembly 230 and surface region 213 may be adjustable, for example, with a portion of detector assembly 230 extending into vacuum chamber 250.

[0025] In some examples, the detector assembly 230 and the electron beam generator 220 may have a fixed orientation relative to each other (e.g., 90 degrees relative to each other), and the sample may be tilted to change the grazing angle and detection angle.

[0026] In one example, the electron beam generator 220 generates an electron beam 225 with a beam energy E b The electron gun is configured to emit electrons in the range of 0.5 to 30 keV with beam spot sizes ranging from 50 μm to 1 mm. In one example, the total beam current at the sample varies from 1 nanoampere (nA) to 10 milliamperes (mA), depending on the electron beam spot size. The energy and grazing angle of the electron beam 225 both contribute to determining the penetration depth and the excess energy of the free electrons, which ultimately cause the material to emit light via recombination and radiative channels. In this disclosure, the penetration depth of the electron beam into the surface region describes the depth to which the majority of the electron beam energy is transmitted (which can result in cathodoluminescence emission). In embodiments, the penetration depth is controlled by various factors, such as the grazing angle, the electron beam energy provided by the electron beam generator, the material being analyzed (i.e., the surface region material), and the electron beam spot size.

[0027] In embodiments, the electron beam generator 220 and / or the mounting arrangement 260 are configured to generate the electron beam at an incident angle α relative to the surface region. In some examples, α is selected to be in the range of 0.1° to 15°, or 0.1° to 25°, or 0.1° to 30°, or 0.1° to 45° to probe the surface region 213 to a predetermined depth. In other examples, α is selected to be in the range of 0.1° to 5° to probe the surface region 213 to a shallower depth than when α is in the range of up to 15°. The angle α can be adjusted, for example, by rotating or tilting the mounting platform of the mounting arrangement 260 (relative to the electron beam generator 220) within the vacuum chamber 250.

[0028] In one example, material 210 includes a substrate 211, and surface region 213 is part of a surface epitaxial semiconductor layer (epilayer) 212 deposited on the substrate or an intermediate layer. In another example, the surface epitaxial semiconductor layer is an epitaxial oxide layer. In one example, the surface epitaxial semiconductor layer has a bandgap energy E g and the electron beam energy E b is the energy

number

[0029] In one example, mounting arrangement 260 is a five-degree-of-freedom mount, as described in more detail in connection with Figure 6. Linear translational motion is available in the X, Y, and Z axes (Figure 2), and rotation is available about the Z axis (i.e., azimuth angle φ) and the x axis (i.e., tilt angle θ).

[0030] 3 is a side schematic view of a system 300 for characterizing a surface region 213 of a material 210, similar to system 200, but which also includes a reflection high-energy electron diffraction (RHEED) instrument 310 for further characterizing the crystalline properties of surface region 213. In the depiction of FIG. 3, RHEED instrument 310 is coupled to a second sidewall 252 of vacuum chamber 250. Second sidewall 252 faces first sidewall 251 to which electron beam generator 220 is coupled.

[0031] In this example, the RHEED device 310 includes a photoluminescence detector 320 (e.g., a fluorescent screen configured to measure spatial properties of electrons 325 after diffraction of the electron beam 225 by atoms in the surface region 213). The initial electron beam 225 is oriented to be incident at a shallow grazing angle (e.g., α having a value described herein). The RHEED device 310 is positioned to receive electrons 325 from atoms in the surface region 213 that are diffracted from the electron beam 225.

[0032] Systems 200 and 300 may include additional aspects as described in more detail in subsequent figures. In one example, mounting arrangement 260 is further configured to cool material 210 (the sample to be characterized) to cryogenic temperatures (e.g., using liquid nitrogen or liquid helium) to manage the heat load (reducing or altering the degree of quenching of cathodoluminescence emission due to heating). In another example, mounting arrangement 260 may be configured to heat material 210 above room temperature.

[0033] In one example, the mounting arrangement 260 is configured to apply a bias voltage to the material being characterized. For example, the mounting platform may be coupled to a power source, and electrical contacts are provided on the material sample. Electrical components (e.g., probes, wires, or clips) on the mounting platform can be coupled to the electrical contacts on the sample to apply a bias voltage to the material using the power source. The applied bias voltage can be in the range of, for example, 1 V to 10 kV. In one example, the electrical contacts are positioned laterally across the sample. In another example, contacts can be positioned on the top and backside of the material.

[0034] In one example, modification of the CL emission from a material can be achieved by applying a bias voltage to the material being characterized, thereby creating an internal electric field to further manipulate the region where cathodoluminescence emission occurs. This can provide insight into the excess electron energy required for electron impact excited cathodoluminescence emission, which can be used to guide the electro-optical device configuration and minimum electron energy threshold required for a light emitting device.

[0035] In another example, a bias voltage may be applied to the material being characterized either when an electron beam is applied to cause electroluminescence or in the absence of an incident electron beam. Current-voltage measurements of the material or structure can then be used to confirm the excess energy threshold found by observing cathodoluminescence.

[0036] In one example, the detector assembly 230 includes a spectrometer for determining the spectral characteristics of the cathodoluminescence emission 240. In another example, the detector assembly 230 includes a monochromator for fine wavelength resolution. In another example, the detector assembly 230 includes a wavelength-selective photodetector. In another example, the detector assembly 230 has a light sensor (e.g., a photomultiplier tube (PMT) or a photodiode).

[0037] In one example, the spectral characteristics are determined over a wavelength range of about 110 nm to about 400 nm. In another example, the spectral characteristics are determined over a wavelength range of 110 nm to 280 nm. In these deep-UV wavelength ranges, the detector assembly 230 may require a vacuum configuration including a separate vacuum system, or may be purged with an inert gas to minimize absorption losses, since deep-UV wavelengths are absorbed by air. In another example, the detector assembly may be located within the same vacuum environment as the vacuum chamber 250, which may reduce coupling optics.

[0038] In one example, the detector assembly 230 is based on a transmissive optical system. In another example, the detector assembly 230 is based on a reflective optical system to minimize dispersion.

[0039] In one example, the electron beam generator 220 is pulsed to manage the heat transferred into the material 210 .

[0040] 4 and 5 show perspective and top views, respectively, of a system 400 for characterizing a surface region of a material, according to some embodiments. In one example, system 400 may be utilized to characterize a surface region that forms part of an epitaxial layer formed on a substrate.

[0041] In this example, system 400 includes a vacuum chamber 450 and an associated vacuum pump 405 coupled to vacuum chamber 450. System 400 also includes an electron beam generator 420 and an optional RHEED arrangement 480. In this example, electron beam generator 420 is a RHEED-type electron gun capable of providing an electron beam with an energy ranging from 1 keV to 15 keV and a spot size of 50 μm. In other embodiments, the electron beam energy can vary from 1 keV to 30 keV and a spot size ranging from 10 μm to 10 mm.

[0042] In this example, the vacuum chamber 450 has six sides or windows to which other components can be coupled. The electron beam generator 420 and the RHEED arrangement 480 are coupled to opposite sides of the vacuum chamber 450, e.g., along the y-axis. The vacuum pump 405 and the sample loading chamber 490 are coupled to the vacuum chamber 450 on two other opposite sides of the vacuum chamber 450, e.g., along the x-axis. Note that the sample loading chamber 490 is shown without certain components (e.g., the exterior door and windows in this view) so that the interior of the chamber can be viewed. The detector assembly 700 (described below in connection with FIG. 8A ) is coupled to the vacuum chamber 450 in the z-direction.

[0043] System 400 further includes mounting arrangement 600, which in this example is attached to sample loading chamber 490 by a mounting flange 605. In this example, sample loading chamber 490 is separated from vacuum chamber 450 by an isolation gate valve 495. Isolation gate valve 495 can be opened to place material 210 (FIGS. 2-3) into vacuum chamber 450 after loading and evacuation of sample loading chamber 490.

[0044] The top view of FIG. 5 shows further details of the mounting arrangement 600, including a mounting platform 601 at one end of the mounting arrangement 600. The mounting platform 601 is the area where the material to be characterized is placed. That is, the mounting platform 601 is configured to support the material. The mounting platform 601 is located within the vacuum chamber. As indicated by the double-headed arrow in the x-direction, the mounting arrangement 600 is configured to allow the mounting platform 601 to move between a retracted position within the sample loading chamber 490 (to allow samples to be loaded or unloaded) and an extended position within the vacuum chamber 450 (where the sample is analyzed). FIG. 5 also shows an inlet for a coolant (e.g., liquid nitrogen (N) or helium (He)). The coolant can be circulated within the mounting arrangement 600 as described below.

[0045] Referring now to FIG. 6, a perspective end view of a mounting arrangement 600 is shown. The mounting platform 601 includes a first mounting platform component 610, a second mounting platform component 620, and one or more linear stages 622. The first mounting platform component 610 is attached to a linearly extendable arm 650. The arm 650 is extendable from a retracted position (shown in FIG. 6, with the first mounting platform component 610 near the mounting flange 605) for loading the material 210 into the loading chamber 490 to an extended position (shown schematically in FIG. 5) in which the material 210 is placed into the vacuum chamber 450 for characterization. In the illustrated example of FIGS. 4-6, the distance between the retracted and extended positions is approximately 40 cm. However, in other examples, the travel distance of the arm 650 can be adjusted according to the size of the system 400.

[0046] FIG. 6 illustrates that the first mounting platform component 610 can have an adjustable tilt angle θ. That is, the first mounting platform component 610 can rotate about an axis defined by the extension of the extendable arm 650 (in this example, the longitudinal x-axis). The rotation changes the tilt angle θ of the first mounting platform component 610, and this tilt can be used to adjust the grazing angle of the electron beam on the material 210, as described in connection with FIG. 1 . Embodiments include adjusting the tilt angle of the mounting platform to change the grazing angle. In some examples, the detector assembly 230 and the electron beam generator 220 can have fixed orientations relative to one another. For example, referring again to FIGS. 2 and 3 , the electron beam generator 220 and the detector assembly 230 can be oriented at approximately 90 degrees relative to one another (e.g., the electron beam generator 220 generates an electron beam that is approximately horizontal, and the detector assembly 230 has an optical penetration path that is approximately vertical). In such an example, tilting the material 210 by 5° toward the electron beam generator 220 (i.e., pointing the surface region 213 more toward the electron beam generator 220) changes the grazing angle by 5° (e.g., increasing the grazing angle from 10° to 15°) and the detection angle by 5° (e.g., increasing the detection angle from 90° to 95°). In various examples, the detection angle β can be set (e.g., by tilting the material 210 in the vacuum chamber) to be approximately perpendicular to the surface region (e.g., between 85° and 95°), or between 80° and 100°, or between 70° and 110°.

[0047] A second mounting platform component 620 is attached to the first mounting platform component 610. The second mounting platform component 620 is controllable to move along the X, Y, and Z axes relative to the first mounting platform component 610 and to rotate about an axis extending perpendicularly from the second mounting platform component 620 (i.e., azimuth angle φ). In one example, the second mounting platform component 620 is a circular plate coupled to the first mounting platform component 610. The coupling is by one or more linear stages or actuators capable of translation in the X and Y directions and elevation in the Z direction. The second mounting platform component 620 may also be coupled to the first mounting platform component 610 by a motor capable of rotating the second mounting platform component 620 in the azimuth angle φ. The first and second mounting platform components 610 and 620 together form a five-axis mount (i.e., the mounting platform 601 has five degrees of freedom). In a specific example, the platform component 620 has approximately 10 mm of travel along any of the X, Y, and Z axes.

[0048] In FIG. 6 , mounting arrangement 600 further comprises an inlet 671 and an outlet 672 for cooling material 210 by cooling second mounting platform component 620. For example, inlet 671 and outlet 672 may be coupled to channels in second mounting platform component 620 through which a liquid coolant can flow. Cooling can manage heat loads to reduce or alter the degree of quenching of cathodoluminescence emission due to heating. Inlet 671 and outlet 672 may be coupled to a coolant source (e.g., liquid N or He, as shown in FIG. 5 ) via conduits routed through extendable arm 650 (conduits not shown for clarity) to circulate the coolant and cool the material on the mounting platform. In one embodiment, sample (material 210) may be cryogenically cooled. For example, second mounting platform component 620 may be cooled by a liquid nitrogen-based arrangement, which, in turn, cools the supported test material. In such an example, liquid N2 is circulated through second mounting platform component 620 via inlet 671 and outlet 672. In this manner, the temperature of platform component 620 can vary from approximately 77 K to 300 K, or approximately 10 K to 300 K when using liquid helium. In some embodiments, this ability to control the temperature of second mounting platform component 620 (and consequently material 210) can be used to characterize the bandgap energy of surface region 213 as a function of temperature.

[0049] In other embodiments, the mounting arrangement 600 may be configured to heat the material 210 above room temperature. For example, the second mounting platform component 620 may include a resistive heater or other type of heating element that heats the material 210 so that the bandgap energy of the surface region 213 can be characterized as a function of temperature.

[0050] In an embodiment, the method includes providing a mounting platform 601 in a vacuum chamber, the mounting platform configured to support a material, and using the mounting platform to cool or heat the material during cathodoluminescence emission resulting from an electron beam impinging on a surface region.

[0051] Referring again to FIG. 1 , the surface region of a material may be characterized in block 110 by generating an electron beam that impinges on the surface region to a determined depth. In one example, this may be achieved by varying the angle of incidence of the electron beam relative to the surface region (see, for example, FIG. 8A ). In one example, varying the angle of incidence is achieved by manipulating the tilt angle of the mounting arrangement 600 of FIG. 6 (thus tilting the material 210). The angle of incidence may be shallow relative to the surface region (e.g., a grazing angle). In another example, both the angle of incidence and the electron beam energy are varied to achieve a desired penetration depth of the electron beam in the surface region.

[0052] In block 120, cathodoluminescence emission from the surface region resulting from the electron beam impinging on the surface region is received by detector assembly 700. In block 130, the spectral characteristics of the cathodoluminescence emission are determined. This process may be repeated at multiple locations on the material to scan the surface region at different surface locations. In one example, the surface region is scanned over a two-dimensional grid that spans or partially spans the material by moving the material (e.g., using mounting arrangement 600) in the plane of the surface region by translation and / or rotation.

[0053] The present systems and methods advantageously provide the ability to characterize direct bandgap materials, particularly oxide-based materials.

[0054] In embodiments, the present systems and methods can be used to characterize semiconductor structures including substrates with one or more epitaxial layers. In particular, the epitaxial layers can be formed of metal oxides (e.g., direct bandgap metal oxides). Exemplary substrates that can be used in such structures can include Al2O3, Ga2O3, MgO, LiF, MgAl2O4, SiC, silica, silicon, AlN, GaN, and ScMgAlO4. Exemplary epilayer materials include oxides selected from Table 690 shown in FIG. 7, which is a quaternary oxide material formed by combining together materials from Table 690, as described in U.S. Pat. No. 11,342,484, entitled "Metal oxide semiconductor-based light emitting device," owned by the assignee of the present application.

[0055] 8A shows a cross-sectional perspective view of a detector assembly 700 for detecting cathodoluminescence emission 240 resulting from electron beam 225 impinging on surface region 213 of material 210, according to an exemplary embodiment. Detector assembly 700 may be coupled to vacuum chamber 250 or 450 by flange 705, such that the optical components within the detector assembly are contained within a vacuum environment. In this example, detector assembly 700 is configured as a monochromator including a light collection arrangement in the form of a collimator assembly 710 that functions to collect and collimate cathodoluminescence emission 240 resulting from a focal point corresponding to surface region 213. Detector assembly 700 also includes a grating and slit assembly 740 and a detector 780. Grating and slit assembly 740 separates cathodoluminescence emission 240 into its component wavelengths. Detector 780 determines the intensity for each component wavelength to generate an overall emission spectrum for cathodoluminescence emission 240.

[0056] The cathodoluminescence emission 240 enters the detector assembly 700 through an aperture 715. In one example, the detector assembly 700 includes an initial proximity coupling and high numerical aperture (NA) light collection arrangement to generate collimated light from the sample. In various examples of systems according to the present disclosure, the collimator 700 in the detector assembly includes non-refractive optics, may include no refractive optics, or may include refractive optics, e.g., only refractive optics, or a combination of refractive and non-refractive optics. In methods according to the present disclosure, the detector assembly 700 receives cathodoluminescence emission resulting from an electron beam delivering energy to a surface region, and receiving includes collimating the cathodoluminescence emission by a collimator in the detector assembly. In various examples, the collimator may include non-refractive optics, may include no refractive optics, or may include refractive optics, e.g., only refractive optics, or a combination of refractive and non-refractive optics.

[0057] In this example, the collimating assembly 710 is configured as a Newtonian reflector including a reflector 711 and a centrally located objective mirror 712. The collimating assembly 710 receives the cathodoluminescence emission 240 from the surface region 213 through an aperture 715 and forms a collimated beam 718. The collimated beam 718 then enters a grating and slit assembly 740. The collimating assembly 710 is configured to have a relatively short focal length (e.g., on the order of 15 cm). The use of non-refractive optics (reflector 711 and objective mirror 712) in the collimating assembly 710 advantageously enables detection of cathodoluminescence emission in the deep UV region (wavelength range 110 nm to 400 nm). In contrast, conventional configurations of refractive optics are unable to maintain the focal length due to dispersion in the deep UV region.

[0058] In one example, collimated light (collimated beam 718) is directed to be incident on a dispersive optical grating, and the diffracted beam from the grating is then focused onto an exit slit coupled to an optical detector. In the example of Figure 8A, grating and slit assembly 740 includes grating 741 mounted on a rotatable mount 742. Focusing mirror 745 focuses the diffracted components of collimated beam 718 from grating 741 onto slit 746 for measurement by detector 780. In this example, detector 780 is a photomultiplier tube selected for enhanced sensitivity over the wavelength range of interest.

[0059] In this example, collimating assembly 710 is configured to translate in a direction perpendicular to material 210 (z-direction) to focus surface region 213 as needed. Translation of collimating assembly 710 can be achieved, for example, using translational focus adjustment mechanism 760, by the movement indicated by arrows 762 in FIGS. 8A and 8B. FIG. 8B is a schematic side view similar to FIG. 2, showing more components of detector assembly 700. Reference numbers from FIGS. 2 and 8A apply to FIG. 8B. In one example, collimating assembly 710 can translate ±50 mm relative to a mean position.

[0060] In one example that may be used in other embodiments of the present disclosure, a spectrometer (eg, a 190-800 nm fiber coupled (FC) spectrometer) may be used in place of the monochromator in detector assembly 700 .

[0061] In another example, shown in the side schematic view of FIG. 8C, the light collecting arrangement or collimating assembly 710 can be replaced with a focusing lens 770 and / or other refractive optical elements, allowing a narrow wavelength range to be probed and the effects of chromatic dispersion to be managed. The focusing lens 770 is positioned at a focal distance f from the sample. In one example, the focusing lens 770 can be in the form of a plano-convex lens formed of MgF2 (as shown) or CaF2. Also shown in FIG. 8C are a concave viewport 235 with an EUV-FS (extreme ultraviolet fused silica) window 236 and a photoluminescence detector 320 of the RHEED device. While material 210 (including substrate 211 and epilayer 212) is shown approximately horizontal with respect to the vacuum chamber 250, its tilt angle θ can be adjusted to reorient it as shown by material 210*. The tilt angle adjustment can be used to achieve a desired grazing angle of the electron beam 225.

[0062] In some embodiments, a vacuum environment is provided to the detector assembly while receiving cathodoluminescence emissions. The vacuum environment may be provided by the same vacuum system as vacuum chamber 250 or 450, or may be generated by a separate vacuum system.

[0063] 9A is a side view of vacuum chamber 450 showing the orientation of material 210 loaded into vacuum chamber 450 relative to electron beam generator 420, detector assembly 700, and RHEED arrangement 480. FIG. 9A also shows material 210* in different orientations to change the angle of incidence and detection. The orientation can be adjusted using the tilt angle of mounting platform 601. Collimating assembly 710 is also shown, showing that detector assembly 700 (i.e., aperture 715 through which cathodoluminescence emission 240 enters collimating assembly 710) is close to surface region 213.

[0064] An enlarged simplified diagram of the positioning of collimating assembly 710 relative to surface region 213 is shown in FIG. 9B, which shows the distance 717 between surface region 213 and aperture 715 of collimating assembly 710. As a result, collimating assembly 710 is closely coupled to the solid angle of cathodoluminescence emission 240 resulting from the focused electron beam spot on surface region 213 (i.e., the collimating assembly has a short focal length and a large numerical aperture). In one example, the numerical aperture and focal length of collimating assembly 710 are configured to provide an expected etendue limit (i.e., a limit on the amount of light the optical system can receive from the light source) for cathodoluminescence emission 240. The numerical aperture (NA) is the ratio of entrance aperture D to focal length f: NA=D / f.

[0065] The proximity and location of the detector assembly 700 directly above the surface region 213 provides greater sensitivity in detection than conventional CL systems by allowing a large solid angle of the cathodoluminescence emission 240 to be collected by the detector assembly 700. That is, by having the detection optics so close to and directly above the surface region, in combination with the detection optics having a high numerical aperture, the detection system can capture a high percentage (e.g., almost all) of the emitted photons. In one example, the detector assembly has an aperture that receives the cathodoluminescence emission, and the aperture is positioned a distance from the surface region. This distance partially determines the solid angle over which the cathodoluminescence emission is received (i.e., in combination with other factors, such as aperture size and the direction of the cathodoluminescence emission).

[0066] In some embodiments, the angle of incidence is selected to be a relatively large angle rather than a grazing angle (e.g., by increasing the tilt angle as shown by oriented multilayer material 210* so that the electron beam penetrates further into surface region 213). Electron beam 225 may be configured to sample both the surface epilayer and the underlying substrate. These measurements can then be compared to those taken at a lower angle of incidence, which primarily samples the epilayer, and the underlying substrate can be characterized by deconvolution of the two spectra (the spectrum collected at the high angle of incidence and the spectrum collected at a grazing angle of incidence).

[0067] Further details of the detector assembly optics will now be described. Figure 10A shows two configuration embodiments for focusing and collecting vacuum-UV and UV (VUV-UV) photons emitted from a surface region. Light focusing arrangement 1002 provides a refractive objective lens, while arrangement 1001 provides a fully reflective lens. The effective focal lengths of the lens arrangements are either located at the sample surface (surface of material 1005) or slightly detuned below the surface.

[0068] The refractive arrangement 1002 is constructed using at least a plano-concave lens (PCCL) and, optionally, an additional biconvex lens (BCVL). Optically transparent materials are used for the lenses, and in embodiments, the refractive index of the lenses can be selected to minimize chromatic dispersion over a predetermined wavelength range. For UV operation, materials can be selected, for example, from (i) low-hydrogen fused silica (F-SiO2) and (ii) fluoride-based glasses, such as calcium fluoride (CaF2) and magnesium fluoride (MgF2). A low optical loss system for the arrangement 1002 can be implemented. However, even with low-loss materials in the UV region, limitations exist. The refractive index dispersion with wavelength is sufficiently large that the effective focal length (EFL) exhibits substantial chromatic dispersion over a wavelength range of, for example, Δλ ∼ 100 nm centered at λ = 250 nm. Combinations of achromatic, superachromatic, apochromatic, and athermal achromatic triplet objectives are also possible (e.g., those described in "Method to design apochromat and superachromat objectives," Opt. Eng. 56(10), 105106 (2017)), and can be formed using materials of different refractive indices and lens curvatures to minimize chromatic aberrations. However, below about 170 nm, it is difficult to obtain optically transparent materials (i.e., low absorption losses), which further limits refractive lens configurations (arrangement 1002) for VUV applications.

[0069] The light-focusing arrangement 1001 is constructed using UV low-loss curved reflective mirrors in a concentric Schwarzschild microscope arrangement ("The Design of Reflecting Microscope Objectives", W.H. Steel, 1950; and "Applied Optics and Optical Design", A.E. Conrady, 1929, Oxford University Press). For example, the reflective surfaces can be formed and coated with UV-enhanced aluminum metal. A fully reflective system (arrangement 1001) provides broadband wavelength operation in the vacuum-UV and UV regions without the previously mentioned adverse chromatic dispersion. For example, aluminum is one of the few metals that exhibits low absorption losses in the VUV-UV wavelength region, which can be further improved by additional optical coating using an anti-reflective MgF2 layer.

[0070] Embodiments of the present disclosure provide optical objectives that exhibit both: (i) a large numerical aperture (NA), where NA=D / EFL (D=diameter of the entrance pupil collection optic), e.g., 0.2≦NA≦1.5, or 0.5≦NA≦1.2, and (ii) a relatively short EFL, e.g., 1 mm≦EFL≦200 mm, or 10 mm≦EFL≦50 mm. In embodiments, a detector assembly has an aperture that receives cathodoluminescence emission, the aperture being positioned at a distance of 1 mm to 200 mm from the surface region. In embodiments, the detector assembly has a numerical aperture in the range of 0.2 to 1.5.

[0071] Light collected from the sample area is processed by a lens system and preferably collimated on output: the reflective lenses of arrangement 1001 form an annular collimated beam 1003, while the refractive lenses of arrangement 1002 form a solid collimated beam 1004.

[0072] 10B shows the simplest implementation of the refractive configuration of arrangement 1002, utilizing a plano-convex lens (PCVL). A spherical plano-convex lens 1006 is shown formed with a radius of curvature R, a center thickness CT, a diameter D, and an edge height h.

[0073] 10C shows the properties of various UV-transmitting optical materials used to form an equivalent PCVL. In this example, a design EFL of 45 mm is selected for the PCVL at a particular exemplary wavelength, λ = 220 nm, resulting in a particular radius of curvature, R, for each material, as shown. The variation of EFL with wavelength over the range 170 nm ≤ λ ≤ 350 nm shows a large chromatic dispersion of the EFL, indicative of a refractive geometry 1002. That is, when the PCVL is positioned at a distance to the reflective surface equal to the design EFL of 45 mm and λ = 220 nm, it produces a parallel output beam that is defocused at wavelengths, e.g., 190 nm and 350 nm.

[0074] 11A and 11B disclose embodiments of fully reflective mirror objective designs (i.e., the collimator includes non-refractive optics and no refractive optics) suitable for forming chromatic dispersion-free, and therefore wavelength-independent, EFL light collection optics. The configurations of FIGS. 11A and 11B are known as Schwarzschild and Cassegrain objectives, respectively. The mirrors can be spherical or aspherical, with radii ±R. 1=1,2 For example, the configuration of FIG. 11A may have a primary reflector R2 that may be spherical, while R1 may be selected from either an aspheric or spherical shape to achieve a high NA. In embodiments, the configuration of FIG. 11A is utilized herein for its ability to increase the working distance from the physical edge of the lens to the sample, resulting in an increased NA (i.e., improved light collection capability). For example, a possible design for the objective lens of FIG. 11A is shown in FIG. 11C, where the ratio of the spherical radii,

[0075] R1 / R2=√5-1 / √5+1=0.38197

[0076] is chosen to achieve minimum spherical aberration, so that

[0077] EFL=1 / 2(1 / R1-1 / R2)

[0078] FIG. 11D shows an embodiment of a reflector material (Al metal) and coating (MgF2 / Al metal) to achieve operation at VUV-UV wavelengths. Unoxidized Al metal has the highest reflectivity of all known metals in the VUV-UV band. In an embodiment, the Al metal can be stabilized against oxidation using a protective coating of MgF2, as shown.

[0079] FIG. 12 is a diagrammatic illustration of cathodoluminescence emission from a surface region 213 of an epilayer 212 on a substrate 211 after impingement from an electron beam incident at a grazing angle, where the solid angle of the cathodoluminescence emission is determined by the respective refractive indices of the surface region 213 and the vacuum environment (i.e., n epi Swimming vac and n epi >n vac = 1). Furthermore, the refractive index of the epitaxial layer 212 (n epi ) is formed on the substrate 211 (having a refractive index n sub If the band gap is larger than epi >n sub As can be seen, for a particular cathodoluminescence emission site 216 occurring at a given penetration depth, when the angle of incidence exceeds the critical angle, the cathodoluminescence emission is limited by total internal reflection, resulting in the emission being confined to approximately a cone around the normal direction. Alternatively, if the epitaxial layer 212 (refractive index n epi ) is formed on the substrate 211 (having a refractive index n sub The band gap may be smaller than that of epi >n sub is expected.

[0080] Figure 13 shows the complex refractive index n epi13 shows a semiconductor material having a planar light-emitting surface 1310 exposed to vacuum and a laterally extended optically stimulated light-emitting region 1311 emitting a transverse electric TEz plane wave 1312. A finite-difference time-domain (FDTD) model is used to show the near-field angular emission profile 1313 from the surface 1310, the escape cone of light from the interior of the slab, and the "rabbit ear" shape 1314 of the forward cone. Clearly, a light collection and detector located at position 1315 is preferable to an off-axis angular position 1316. For most materials, the practical refractive index is 1.2≦n epi ≦3.0. For VUV-UV optically active materials, 1.4≦n epi ≦2.2, and the critical angle for the escape cone is θ c =sin -1 (n vac / n epi ) and, for example, 27 ≤ θ measured from the surface normal c < 45 degrees. In an embodiment, the NA of the objective lens is selected to optimize the required EFL working distance and angular light collection of the radiation emitted from the sample.

[0081] FIG. 14 shows the functional components of a CL system, including a device under test (DUT) 1410, which includes at least one epilayer 1412 and a substrate 1411. An electron beam 1420 is coupled (directed) into the DUT, forming an optically active region 1413. Cathodoluminescence light 1440 outcoupled from the DUT surface is collected by an objective lens 1421, which may be selected from a singlet plano-convex lens, an achromat, a reflective lens, etc. The lens 1421 is positioned substantially EFL relative to the DUT surface and processes the collimated light in a spectral processor 1422, which then transmits the processed wavelengths into an optoelectronic detector 1425. The optical (spectral) processor 1422 may include spectrally selective bandpass filters 1424 and 1423 or may be a wavelength-dispersive processor (e.g., a diffraction grating-based device). The wavelengths processed by the spectral processor 1422 are then converted into an electronic signal by an optoelectronic detector 1425 (detection module). The electronic signal may have an active spectral response of the form shown schematically as a narrowband response 1426 or a broadband response 1427.

[0082] For example, a narrow bandwidth CL response may only be required for quantification of a physical property of the DUT, and a narrow band filter (e.g., bandpass filter 1424) can be utilized in series with the optical path of the spectral processor 1422 and photoelectric detector 1425. This allows for rapid spatial mapping of the DUT.

[0083] Higher resolution spectral mapping generally requires a dispersion wavelength module such as that shown in FIG.

[0084] Conventional grating-based spectrometers utilize an entrance slit and an exit slit. Inside a grating-based spectrometer, an optical processor operates by imaging the entrance slit onto the exit slit plane. A first reflective focusing mirror collimates the polychromatic light from the entrance slit onto a diffraction grating. The grating then spatially disperses the collimated beam according to specific wavelengths. In the embodiment of FIG. 15, a DUT 1510 includes a surface region 1512 on a substrate 1511. A lens 1521 is a light-collecting optic that forms a collimated beam 1531 (e.g., a beam primarily from the surface region 1512) from the DUT 1510. The beam 1531 enters a diffraction grating 1533 in a dispersive spectrometer 1530. This arrangement significantly simplifies the optical system over prior art grating-based spectrometers by utilizing an optical focal point on the sample plane as the entrance pupil to the grating-based spectrometer. The spatial separation of the wavelengths contained within the incident beam is directed to a focusing mirror 1532, which focuses the spatially dispersed collimated beam 1534 onto an exit slit 1535. Angular control of the grating 1533 allows an optoelectronic detector 1525 coupled to the output of the exit slit 1535 to reconstruct an optical spectrum comprising the wavelengths contained within the incident beam emanating from the DUT 1510. The sample (DUT 1510) may be spatially positioned (e.g., horizontally and / or vertically, as indicated by arrows 1514 and 1515, respectively) with respect to the EFL of the imaging lens 1521, further enabling depth-dependent spectral imaging.

[0085] Embodiments utilize a modified grating-based spectrometer 1530 that is advantageously coupled to a vacuum system containing a sample under test. For VUV-UV wavelength operation, atmospheric absorption adversely affects optical signal propagation. Furthermore, high-energy electron impact excitation of the sample under test also requires a vacuum to propagate the electron beam over distances ranging from 10 cm to 1 m. Therefore, it is advantageous to simultaneously couple both the optical beam and the electron beam to the sample within a vacuum.

[0086] FIG. 16 shows further details of a grating-based spectrometer, including an entrance aperture 1645 (either fixed or variable) that defines a light path for the collimated beam 1631. Elements in FIG. 16 correspond to those with the same reference numbers in FIG. 15. The aperture 1645 further reduces stray light that is not related to the light substantially collected from the region at the EFL. The position of the objective lens 1621, with its EFL indicated by line 1642, can be adjusted vertically, as indicated by arrow 1646, to a new position 1641 with EFL 1643, etc. The sample (DUT 1510) may be positioned such that the lens is scanned to adjust the EFL relative to the DUT surface plane, providing the collimated beam 1631. One or both of the lens 1621 and the sample may be translated to achieve the goal of the collimated beam 1631. Lens 1621 in FIG. 16 and lens 1521 in FIG. 15 are shown as refractive optical components, but may also be reflective optical components as in FIG. 11A or 11B.

[0087] FIG. 17 shows a schematic diagram of the complex electrodynamic process of high-energy electron impact excitation cathodoluminescence, according to an embodiment of the present disclosure. Inspired by Feynman diagramming, the space-time coordinate system of an incident high-energy electron 1751 is shown propagating within an interaction vertex 1753. The interaction vertex 1753 can be associated with a scattering event that liberates a virtual particle 1754 that interacts with a crystalline ground state 1752, described, for example, by the energy-momentum structure of FIGS. 24A-24B. The virtual particle 1754 can include crystal lattice excitations (e.g., phonons), secondary particle excitations (e.g., non-equilibrium electron and hole states) created by impact ionization, and collective lattice excitations (e.g., polaritons). The excited crystalline state 1755 propagates forward in time as excited crystalline state 1757 along with the scattered electron state 1756. The high-energy scattered electron state 1756 can be scattered again at vertex 1759 to another energy-momentum state 1760. Virtual particle 1761 then interacts with excited crystalline state 1758, generating photon 1762 and annihilating the crystal lattice excited state (crystalline state 1758). For example, conservation of energy momentum may favor the recombination of an electron in the conduction band with a hole state in the valence band, generating photon 1762. The multiple such processes described above can also occur in various combinations, highlighting the complex processes underlying cathodoluminescence.

[0088] Figure 18 shows a physical representation of the process described in Figure 17. An incident high-energy electron 1871 enters a structure containing a first region 1878 and is scattered 1872 into a second region 1879. The electron generates a crystal lattice excitation 1874 and is then further scattered into state 1873. The lattice excitation 1874 may contain an electron-hole pair, which recombine to generate a photon 1875. The photon 1875 may be directed deeper into region 1879 or may be directed toward the surface region. If the refractive index of region 1879 is different from that of the first region 1878, a portion 1876 of the light (photon 1875) is Fresnel reflected. The remaining portion 1877 is transmitted into a strategically positioned detector 1880. If the surface region (first region 1878) and the second region 1879 are of the same material composition, Fresnel reflection also occurs at the vacuum-surface interface. That is, the cathodoluminescent photons 1875 may be partially absorbed, internally reflected, or transmitted to the exterior of the structure.

[0089] Figures 19A and 19B show exemplary structures that can be utilized for selective area cathodoluminescence characterization. Figure 19A shows an exemplary epitaxial layer formed on a substrate. Specifically, the epilayer comprises a single-crystalline zinc-aluminum-oxide cubic spinel structure, ZnAl2O4 (thickness LEPI), advantageously grown on a single-crystalline (001)-oriented magnesium oxide (MgO) substrate. The energy-momentum band structures for ZnAl2O4 and MgO both exhibit direct bandgaps, resulting in emission upon appropriate excitation. A more complex exemplary structure is shown in Figure 19B. The diagram includes multiple periodic stacks of alternating epilayers, ZnAl2O4 and MgAl2O4 (thicknesses LA and LB, respectively), with a period L. A periodic superlattice structure is formed with multiple periods to achieve a total thickness LT. An MgO (001)-oriented substrate can be used in conjunction with the growth process to form a single-crystalline structure. Exemplary structures and fabrication methods are disclosed in the above-referenced U.S. Patent No. 11,342,484. Characterization of wide-bandgap semiconductor and phosphor materials suitable for optical emitters at UV and VUV wavelengths is an important objective of this disclosure. Crystalline materials of composition ZnAl2O4, MgAl2O4, and MgO are used herein as example materials for VUV-UVCL photoemission, although other materials are possible (e.g., other oxides, such as LiGaO2). III-nitrides are also possible (e.g., AlGaN, BN), and fluorides (e.g., LiF, MgF2, etc.).

[0090] Figure 20 shows the energy

number

[0091] 21A and 21B are graphs corresponding to the structure of FIG. 19A, showing the distribution of energy transferred to the crystal by scattered electrons as a function of depth z into the structure. The scattered electron energy S e The (z) vs. z plot (penetration depth nm) has the intensity area scaled by the number of events relative to the total number of incident electrons. traj =2500

[0092] FIG. 21A shows the electron beam incident angle θ inc= 90°, showing that the energy fraction of the scattered electrons between 8 and 10 keV occurs deep within the structure, as indicated by the boxed region 2110. The model predicts that the majority of the electron-impact-induced cathodoluminescence comes from both the epilayer and the substrate. Furthermore, at normal beam incidence, the majority of the CL comes from the substrate rather than the epilayer.

[0093] In contrast, FIG. 21B shows the grazing electron beam incidence θ inc = 5°, showing that the energy portion of the scattered electrons between 8 and 10 keV occurs in a region confined to the top epilayer region, as indicated by the boxed region 2120. This model predicts that the majority of the electron-impact-induced cathodoluminescence originates primarily from the epilayer, rather than the substrate. Clearly, a shallow electron beam incidence is advantageous for selectively exciting the epilayer region for cathodoluminescence analysis. In an embodiment, directing the electron beam toward the surface region of the material includes setting a grazing angle such that the majority of the cathodoluminescence emission is emitted from the epilayer, rather than the substrate. For example, the grazing angle can be 15° or less.

[0094] Figure 22 shows the backscattered electron ratio for an example of a 200 nm ZnAl2O4 / MgO substrate structure.

number

[0095] The wave-particle duality of electrons is further exploited in this disclosure as a precise probe of the symmetry and crystallinity of a crystal surface: a grazing incidence high energy electron beam can be tuned to probe a small portion of the crystal surface due to the penetration depth of the electrons for a given kinetic energy.

[0096] In FIG. 22, the surface penetration depth 2220 (z surf ) are overlaid as a function of the angle of incidence. As a guide, the depth maximum distribution for 10 keV electrons at normal incidence is found from FIG. 21A. For the exemplary structure shown in FIG. 19A, θ inc The calculated peak spatial distribution for 9-10 keV electrons at z = 90 degrees is max =150nm occurs in.

[0097] Therefore, the shallow angle penetration depth can be estimated by the following geometric relationship:

[0098] z surf =z max sin -1 θ inc

[0099] This is because the shallow angle 1°≦θ inc ≦5°, the surface sampling depth is 2.6nm≦z surf ≦13 nm.

[0100] Electrons accelerated to a kinetic energy of 10 keV have a characteristic de Broglie wavelength λ dB Since θ = 12pm = 0.012nm = 0.12Å (see Figure 31), surface diffraction can occur for the regular arrangement of atoms that form the top atomic layer of a crystalline surface. This effect is exploited in a known method called Reflection High Energy Diffraction (RHEED). RHEED is used to quantitatively assess the surface crystallinity and symmetry of atomic arrangements of thin film materials and surfaces.

[0101] Shallow angle electron excited cathodoluminescence directly coupled with RHEED allows selective surface areas of the epilayer to be directly probed and compared with their crystalline structure and electron emission properties.

[0102] 23A-23F are charts of calculated / modeled cathodoluminescence according to example embodiments. Figure 23A shows the calculated spatial dependence of cathodoluminescence due to electron impact excitation as a function of depth into the structure. Three cases are plotted for comparison.

[0103] The structure S1 is inc 200 nm ZnAl2O4 bulk epitaxial layer on MgO substrate for θ = 5°, showing selective cathodoluminescence from the epitaxial layer. Compared with the bulk epitaxial layer structure S1, inc For the case of θ = 5°, we also show a superlattice structure S3 containing 10 periods of [10 nm ZnAl2O4 / 10 nm MgAl2O4], forming a total thickness of 200 nm, deposited on an MgO substrate. Both cases S1 and S3 show a higher total CL from the epilayer (up to a depth of 200 nm) relative to the integrated signal from the MgO substrate. The third case, S4, shows the same SL as S3, but at a normal incidence angle θ inc = 90°. S4 shows a dramatic reduction in epilayer CL and a large, broad, spatially integrated CL signal from the substrate.

[0104] FIG. 23B shows the shallow angle θ for the SL structure S3 at 10 keV. inc = 5° compared to a higher electron energy of 15 keV (designated S5). At higher electron impact energies, there is an overall increase in the CL signal with a slightly higher contribution from the MgO substrate.

[0105] Figure 23C shows the normal incidence of the SL structure [10 nm ZnAl2O4 / 10 nm MgAl2O4]. θ inc= 90° CL at electron energies of 10 keV (S4) and 15 keV (S6) is shown. As the electron impact energy increases, there is an overall increase in the CL signal, which originates from deeper within the MgO substrate.

[0106] Figure 23D shows the θ for the SL structure [10 nm ZnAl2O4 / 10 nm MgAl2O4]. inc The figure shows a comparison of CL signals at 5 and 90° (S3 and S4, respectively) at 10 keV electron energy. As the incident angle increases, there is an overall increase in the CL signal, which originates from deeper within the MgO substrate.

[0107] Figure 23E shows the θ for the SL structure [10 nm ZnAl2O4 / 10 nm MgAl2O4]. inc A comparison of the 5 and 90° CL (S5 and S6, respectively) using a higher electron energy of 15 keV is shown. The higher the electron impact energy, the greater the overall CL signal, originating from deeper within the MgO substrate.

[0108] Figure 23F is a summary of the integrated CL from the epilayer and substrate regions for the experimental configurations designated S1-S6 from Figures 23A-23E. The parameter β=(CL epi ) / (CL epi +CL sub ) represents the fraction CL generated by the epilayer compared to the total CL of the structure, where:

[0109]

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[0110] It is demonstrated that shallow angle electron injection is highly advantageous in improving the signal ratio of CL from the epilayer region compared to CL emanating from the substrate region.

[0111] Exemplary CL characteristics are now described with specific reference to the details of the materials investigated.

[0112] Figures 24A and 24B show the electron energy momentum E band structures for single-crystal oxide semiconductors MgO and the newly developed ZnAl2O4. These materials are examples of VUV-UV emitting phosphors to be characterized according to the present disclosure. The band structures were calculated using density functional theory (DFT) methods and the Tran-Blaha modified Becke-Johnson (TBmBJ) exchange functional developed for this disclosure (see, e.g., International Patent Application No. PCT / IB2021 / 060466, "Epitaxial Oxide Material, Structures, and Devices," filed November 11, 2021).

[0113] Figure 24A is an Ek diagram for cubic MgO with a space group designated Fm3m. The crystal has a direct bandgap at the Brillouin zone center or Γ point (denoted by the conduction band minimum 2402 and the valence band maximum 2404).

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[0114] Figure 24B shows the Ek diagram for cubic ZnAlO with a space group designated Fd3m (spinel-like structure). The crystal has a direct bandgap at the Brillouin zone center or Γ point (denoted by the conduction band minimum 2412 and the valence band maximum 2414).

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[0115] Thus, the materials in this example have the following bandgap relationship:

[0116] E g (ZnAl2O4) <E g (MgO) <E g (MgAl2O4)

[0117] Clearly, therefore, large values ​​of the band gap require higher energy electronic excitation methods than conventional photoluminescence methods due to the lack of suitable optical excitation sources (i.e., lasers) for optical excitation above the band gap.

[0118] Figure 25 shows a simplified schematic of the band structure and subgap defect states involved in CL emission. The light emission process generally occurs in the lowest energy state of the crystal. Therefore, the complex band structures in Figures 24A and 24B are related to the band limit, E, which represents the relative energy of the conduction band minimum (valence band maximum). c (E v 24A and 24B, the conduction band minimum, Ec, is shown as 2402 and 2412, while the valence band maximum is shown as 2404 and 2414.

[0119] Incoming energetic electrons bound within a crystal can exchange sufficient kinetic energy to create "hot" electrons in the crystal through impact ionization events. "Hot" electrons are so named because they are created at energies significantly higher than the Ec in the crystalline material, far beyond the energy of equilibrium electronic states. These non-equilibrium hot electrons then relax their energy by combining with allowed crystalline states and quasiparticles, such as phonons, excitons, and polaritons. While an ideal crystal exhibits a perfect crystalline structure, actual "single crystal" structures contain various crystallographic imperfections (e.g., lattice defects) and may contain impurity atomic species within the structure.

[0120] Two main defects known as crystal structure defects and oxygen vacancies are found to occur in oxide materials. Structural defects are usually M =E GOxygen vacancies appear as deep sub-bandgap defect states (mid-gap states 2546) located approximately at / 2. Oxygen vacancies typically result in n-type conductivity in the material and are assigned to shallow donor states 2543 that reside below the conduction band edge. Donor-like states E D is located approximately 1 eV below Ec.

[0121] Therefore, the hot electrons 2541 can relax towards Ec by lattice thermalization by phonons. Bandgap-related photoemission E G can occur when a band-edge electron 2547 recombines with an available valence state 2548 (i.e., a hole). An alternative pathway is for the electron 2542 to relax non-radiatively to a donor state 2543, which then optically recombine with a hole (valence state 2544) to release energy E D <E G Yet another possible pathway is for the electron 2545 to radiatively recombine with the mid-gap state 2546, emitting a photon with energy E M <E D <E G Although other parasitic luminescence processes are possible, for the sake of clarity and the preceding discussion, the aforementioned process will be used to describe the CL from the experimental setup described.

[0122] In Figure 26, θ inc =5deg and

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[0123] 27A-27C are further calculated / modeled plots of CL emission. inc =5,90deg and

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[0124] Figure 27B shows the θ for a 200 nm thick single crystal epilayer of ZnAl2O4 deposited on a high quality MgO substrate. inc =5, 90deg and

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[0125] Figure 27C shows the CL emission for the SL structure [10 nm ZnAl2O4 / 10 nm MgAl2O4] deposited on an MgO substrate, with a change in θ inc =5, 90deg and

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[0126] The band edge emission from the n=1 SL state was measured at a peak of 2781(θ inc =5°) and 2782(θ inc = 90°), demonstrating the selectivity of the epilayer for shallow-angle excitation. The substrate band-edge emissions 2783 and 2784 are not significantly attenuated because the large MgAl2O4 band gap does not absorb them. Low-energy donor and mid-gap features 2785, 2786, and 2787 are also evident. Clearly, shallow-angle electron beam excitation is advantageous for selectively probing the electronic properties of the top epilayer.

[0127] A schematic diagram of a reflection high-energy electron diffraction (RHEED) setup is shown in Figure 28. A high-energy focused electron beam source 2891 is directed at grazing incidence 0<θ to the surface 2893 of the crystal under test. inc < 10°. In some embodiments, the beam 2892 is directed at an angle of incidence 0.5<θ inc <2° is used so that electron beam diffraction effects occur by binding only to a few monolayers of the top surface region of the crystal 2893 under test.

[0128] The regular surface arrangement of atoms 2894 constitutes part of a single crystal plane, forming a two-dimensional lattice. A regular 2D lattice has a well-defined in-plane symmetry. The incident electron beam is focused at a characteristic de Broglie wavelength λ dB is further adjusted to be less than or equal to the atomic lattice spacing in the perpendicular direction within and between the atomic planes. dB This occurs for a lattice spacing of 1 / 4. Atomically flat 2D surface arrangements of atoms form characteristic diffraction lines 2895 and streaks 2896 on the Ewald sphere where the phosphor screen 2898 intersects. Atomically non-flat surfaces cause modulation of the spots and / or streaks, which are a direct measure of the crystalline quality / symmetry of the crystalline material under test. As the angle of incidence of the electron beam increases, characteristic Kikuchi lines and spots are formed, providing further crystallographic information about the material under test. Clearly, simultaneous in situ coupling of RHEED diagnostics to CL emission from the region being probed is a valuable tool for characterizing materials.

[0129] FIG. 29 discloses a schematic diagram of an embodiment of a CL system having a high-energy electron beam source 2905 that generates an electron beam 2906. The electron beam 2906 is suitable for impact ionization excitation of a crystal 2915 to generate a characteristic cathodoluminescence beam 2907 generated within the excitation region, as well as for surface structure probing using RHEED. Reflected diffraction beams 2908 and 2909 of electrons from the top surface crystal layer are directed toward a phosphor screen 2912 that is activated by the incident beam 2908 / 2909. The image converter phosphor screen processes photons 2911 onto a 2D charge-coupled array device 2913 for further processing. Similarly, the CL beam 2907 is processed by a wavelength-selective detector or spectrometer 2910. The entire system is housed within a vacuum chamber 2925, allowing for a large electron mean free path and low-loss propagation of VUV-UV light. A pump 2922 draws a vacuum to create a range 10 -11 ≦φ≦10 -4 A vacuum environment 2920 is maintained having a vacuum pressure φ of Torr.

[0130] Figure 30 shows a simplified diagram of a high-energy electron accelerator that forms part of an electron beam source 3024. A filament 3021 coated with a material capable of releasing electrons via thermionic emission is resistively heated. The vacuum electrons are then attracted towards a first plate or grid and then accelerated by the action of a high potential difference 3022. The electrons 3023 reach sufficient kinetic energy determined by the potential 3022 (i.e., the potential difference). The emitted high-energy electrons 3025 have a characteristic particle-wave de Broglie wavelength λ that can be tuned according to the plot 3135 shown in Figure 31. dB For example, a 10 keV electron beam has λ dB = 12.2 pm. Since the lattice spacing of a typical crystal varies from 2 ≤ a ≤ 10 Å, λ dB can be used as a fine structure probe for RHEED.

[0131] Figure 32 shows a schematic functional block diagram representing the system disclosed herein. The electron source 3252 includes a source of electrons, a high-voltage accelerator, optional beam modulators, focusing lenses (i.e., magnetic, electromagnetic, or electrostatic), and a beam steering mechanism (e.g., quadrupole or electrostatic plates for XY deflection). Beam locking may also be part of the source to adjust grazing incidence on the sample surface. The device under test (DUT) is a semiconductor and crystalline structure coupled to a VUV-UV cathodoluminescence measurement apparatus and RHEED diagnostic (i.e., an image intensifying phosphor screen). The system is housed in a vacuum chamber 3255, which is actively pumped to achieve a predetermined vacuum pressure.

[0132] Figure 33 shows the schematic of Figure 32 with additional features. Additional features include the electron beam, regulated by parameter "A," being incident on the DUT with temperature control (e.g., cryogenic versus liquid helium or liquid nitrogen), as well as providing a heater ("B" represents the cooling liquid source and heater) to regulate the temperature to a desired constant temperature via a feedback loop as needed. The sample position relative to the electron beam and optics can be manipulated by a micropositioner "C." The RHEED image can be further processed by computer-aided image acquisition (RHEED light processor). The VUV-UV light signal can be further processed by computer-aided data acquisition (VUV-UV light processor).

[0133] In some embodiments, as illustrated by block diagram 3400 in FIG. 34 , the electron beam generator 3420 can be pulsed to manage heat transfer within the material 3410. The method includes pulsing the electron beam while directing the electron beam to impinge on a surface region of the material. In one example, the electron beam generator 3420 has an electrostatic deflection mechanism 3422. The electrostatic deflection mechanism 3422 can be electrically controlled by an external pulse generator or arbitrary waveform generator 3421 to control the pulse width and duty cycle of the emitted electron beam 3425 while maintaining peak electron energy. The electrostatic deflection mechanism 3422 can deflect the electron beam at desired intervals and for desired times to allow the electron beam to either impinge on the material 3410 (carrying beam 3426) or miss the material 3410 (deflected beam 3427). The pulse (or percentage of time) that the electron beam strikes the material can be short compared to the percentage of time that the electron beam misses the material (i.e., the duty cycle is low), reducing heating. Thus, pulsing allows the use of high electron energies without adversely heating the material.

[0134] A control system for synchronizing the fundamental frequency of the modulation source applied to the electron beam can be used to perform synchronous detection of the cathodoluminescence signal. As an example, the electro-optical detection system (detector assembly 3470) measuring the cathodoluminescence signal can be synchronized to the modulation frequency using a lock-in amplifier detection arrangement 3430. This results in a high SNR for the cathodoluminescence signal by measuring the signal primarily at the modulation frequency of the electron beam while rejecting noise sources at other frequencies.

[0135] The ability to tune the frequency and phase of the detection system also allows for the implementation of phase-sensitive detection to sense various physical processes, which may involve time delays and the deconvolution of competing physical processes. A large duty cycle (i.e., the interval between pulses) can reduce the effects of cumulative heat imparted to the material from the electron beam. If the time between pulses is greater than the thermal relaxation time of the material, any cathodoluminescence signal that may result from expected thermal effects (e.g., bandgap narrowing and thermal quenching of luminescence) can be mitigated by deconvolving direct recombination effects from thermal effects. This results in a dramatic increase in the signal-to-noise ratio, which is desirable for the low light levels typically associated with cathodoluminescence processes.

[0136] In one example, the pulse width of the electron beam 3425 can range from 1 nanosecond (ns) to 1 second. In another example, the pulse width can range from 10 ns to 100 microseconds (μs). In one example, the duty cycle of the pulsed electron beam 3425 can include 10 ns pulses with 100 ns off between pulses. In some examples, the modulation frequency can range from 1 Hz to 1 MHz or from 1 kHz to 10 kHz.

[0137] Example Figure 35 is a plot 3500 of CL emission as a function of wavelength (nm) showing the effect of substrate temperature on the measured CL emission intensity (arbitrary units). In this example, the sample includes an epitaxially deposited β-Ga2O3 layer approximately 1200 nm thick formed on a substrate including 4H polyhedral SiC material.

[0138] Similar to the system setup shown in FIG. 8C (where the sample (e.g., material 210) is tilted to achieve the desired grazing (and detection angle)), in this experimental measurement configuration of FIG. 35, the electron beam generator is oriented to the side (and horizontal) of the sample, the detector assembly is mounted above the sample, and the sample is tilted to affect the grazing angle relative to the electron beam generator. This tilt determines the detection angle of the detector assembly relative to the surface of the sample, because in this configuration the electron beam generator and detector assembly are fixedly oriented relative to each other. In this example, the grazing angle is set to 15 degrees, meaning the corresponding detection angle is 105 degrees.

[0139] Figure 35 shows the effect of varying the substrate temperature (measured by a thermocouple) from room temperature 20°C (represented by CL emission spectrum 3510) down to -120°C (represented by CL emission spectrum 3580). CL emission spectra 3570, 3560, 3550, 3540, 3530, and 3520 are for increasing the substrate temperature from -120°C to 20°C in steps of approximately 20°C for incident beam energies selected to excite below-bandgap states. The spectra show the various transitions E from the conduction band to subgap states in the β-Ga2O3 layer. D and E M (See FIG. 25). The substrate temperature was controlled by a cooling system in the substrate mounting arrangement, as described in connection with FIG. 6. As can be seen by examining FIG. 35, the various spectra 3510-3580 are specific to the β-Ga2O3 layer and do not exhibit features indicative of substrate recombination. In particular, there is no sharp emission peak centered at 392 nm that would be expected from a SiC substrate. This indicates that the electron beam is interacting only with or probing the β-Ga2O3 layer, as desired.

[0140] As expected, the entire CL emission spectrum 3510-3580 increases in intensity as a function of decreasing temperature because decreasing temperature generally reduces non-radiative recombination pathways due to interactions with lattice phonons. Figure 35 shows that the substrate temperature can be varied and / or set while characterizing epitaxial materials on the substrate to enhance the clarity of the measurement results.

[0141] Figure 36 is a plot 3600 of CL emission spectra as a function of energy (wavelength in nanometers) showing the effect of varying the grazing angle for a sample including an epitaxially deposited β-GaO layer approximately 1200 nm thick formed on a 4H polyhedral SiC substrate. In this example, the top emission spectrum 3610 shown corresponds to a grazing angle of 0.7 degrees, the bottom emission spectrum 3620 corresponds to a grazing angle of 15.7 degrees, and the spectra in between are shown for 1-degree increments of grazing angle. The various curves are vertically offset in the graph for clarity; therefore, the emission counts on the Y-axis are in arbitrary units. Emission spectra generated using shallower grazing angles (toward emission spectrum 3610) exhibit lower intensity and more noise than those generated using higher grazing angles because shallower grazing angles sample a smaller volume of the material (because incident electrons penetrate less deeply into the material and more electrons reflect off the surface of the sample). FIG. 36 also annotates the emission spectral maxima at approximately 400 nm (i.e., P1) and approximately 500 nm (i.e., P2). As can be seen, the various emission spectra are again specific to the β-Ga2O3 layer and again do not exhibit features indicative of substrate recombination (i.e., the absence of a sharp SiC peak at 392 nm). The relative heights of the P1 and P2 maxima vary with grazing angle (as depicted in FIG. 37), indicating that in this example, the material properties closer to the surface differ from those further away. FIG. 36 demonstrates that depth-dependent material properties (of a layer, or various layers of a multilayer structure) can be characterized by varying the grazing angle (e.g., by decreasing the grazing angle to probe portions of the sample region closer to the surface). FIG. 36 demonstrates that the grazing angle can be varied and / or selected to characterize the material composition at different depths (e.g., thicknesses, or different layers) of epitaxial material on a substrate.

[0142] FIG. 37 is a plot 3700 including a curve 3710 of the relative intensity of the CL emission peak (i.e., the ratio P2 / P1 from FIG. 36) as a function of grazing angle. It also shows a curve 3750 of the peak electron penetration depth (nm) as a function of grazing angle, simulated using the CASINO Monte Carlo simulation tool (see, e.g., FIG. 20), which produced the CL emission spectrum shown in FIG. 36. It can be seen by inspection that the angular dependence of the ratio P2 / P1 (dependence on grazing angle, indicated by the directional ellipse) corresponds to the angular dependence of the electron penetration depth. That is, as the grazing angle increases, the ratio P2 / P1 decreases, while the CL peak penetration depth increases. This indicates that the defects giving rise to the CL emission peak P2 are more prevalent in the first few nanometers of the β-Ga2O3 layer (i.e., closer to the surface being probed and further away from the underlying SiC substrate) and, as expected, gradually decrease as the grazing angle increases.

[0143] As will be appreciated, Figure 37 is an example of how a very surface region (e.g., an epitaxial region) of a sample may be selectively probed and characterized, in accordance with the present disclosure, to characterize material properties at various depths within the surface region. The present disclosure describes using cathodoluminescence to selectively probe a surface region of a material at a desired depth within the surface region using a method that includes varying the grazing angle and / or penetration depth, and a system configured to enable the grazing angle and / or penetration depth to be varied. For example, the method and system may include setting the grazing angle and beam energy of the electron beam to adjust the penetration depth of the electron beam into the surface region, and / or adjusting the tilt angle of the mounting platform supporting the sample material to change the grazing angle and / or penetration depth.

[0144] Embodiment Aspect 1: In an aspect of the present disclosure, a method (e.g., method 100) for characterizing a surface region of a material includes generating an electron beam from an electron beam generator in a vacuum chamber; directing the electron beam at a grazing angle toward the surface region of the material; receiving, with a detector assembly, cathodoluminescence emission resulting from the electron beam transferring energy to the surface region, the detector assembly being positioned above the surface region and operating in a vacuum environment; and determining spectral characteristics of the cathodoluminescence emission with the detector assembly to characterize the surface region.

[0145] Aspect 2: In an aspect of the present disclosure, a method (e.g., method 100) for characterizing a surface region of a material includes generating an electron beam within a vacuum chamber from an electron beam generator coupled to a sidewall of the vacuum chamber; directing the electron beam at a grazing angle toward the surface region of the material; receiving, with a detector assembly, cathodoluminescence emission resulting from the electron beam transferring energy to the surface region, wherein an optical penetration path of the detector assembly is positioned above (e.g., directly above) the surface region; and determining spectral characteristics of the cathodoluminescence emission with the detector assembly to characterize the surface region.

[0146] Aspect 3: In the method according to aspect 1 or 2, directing the electron beam includes setting the grazing angle and the beam energy of the electron beam to adjust the penetration depth of the electron beam into the surface region.

[0147] Aspect 4: The method according to any one of aspects 1-3, further comprising:

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[0148] Aspect 5: The method according to any one of Aspects 1-4, wherein the material includes an epitaxial layer on a substrate, and directing the electron beam includes setting the grazing angle such that a majority of the cathodoluminescent light is emitted from the epitaxial layer rather than the substrate.

[0149] Embodiment 6: In the method according to any one of Embodiments 1-5, the grazing angle can be any value described herein, such as 45° or less, or 25° or less, or 15° or less.

[0150] Embodiment 7: The method according to any one of embodiments 1-6, wherein the detector assembly is positioned above (eg, directly above) the surface region.

[0151] Embodiment 8: In the method according to any one of embodiments 1 to 7, the detector assembly may have an optical intrusion path, and the detector assembly may be positioned such that the optical intrusion path is at a detection angle that is perpendicular (or nearly perpendicular, e.g., 85° to 95°) to the surface region, or at a detection angle of 70° to 110° relative to the surface region.

[0152] Aspect 9: The method according to any one of Aspects 1 to 8, wherein the detector assembly has an aperture for receiving the cathodoluminescence emission, the aperture being positioned at a distance of 1 mm to 200 mm from the surface region.

[0153] Aspect 10: In the method according to any one of aspects 1 to 9, the detector assembly has a numerical aperture in the range of 0.2 to 1.5 (e.g., in combination with the aperture being positioned at a distance of 1 mm to 200 mm from the surface region).

[0154] Embodiment 11: In the method according to any one of embodiments 1 to 10, receiving by the detector assembly includes collimating the cathodoluminescence emission by a collimator within the detector assembly, the collimator including a non-refractive optical component and no refractive optical component.

[0155] Embodiment 12: In the method according to any one of embodiments 1 to 10, receiving by the detector assembly includes collimating the cathodoluminescence emission by a collimator within the detector assembly, the collimator including a refractive optical component.

[0156] Embodiment 13: In the method according to any one of embodiments 1 to 10, receiving by the detector assembly includes collimating the cathodoluminescence emission by a collimator within the detector assembly, the collimator including a combination of refractive and non-refractive optical components.

[0157] Aspect 14: The method according to any one of Aspects 1 to 13, wherein the spectral characteristic is in the deep ultraviolet wavelength range of 110 nm to 400 nm or 110 nm to 280 nm.

[0158] Aspect 15: The method according to any one of aspects 1 to 14, wherein the detector assembly operates in a vacuum environment during said receiving (e.g., the spectral characteristics are in a deep ultraviolet wavelength range of 110 nm to 400 nm or 110 nm to 280 nm).

[0159] Embodiment 16: The method according to any one of embodiments 1 to 15 may further include providing a mounting platform within the vacuum chamber, the mounting platform configured to support the material, and using the mounting platform to cool or heat the material during the cathodoluminescence emission resulting from the electron beam impinging on the surface region.

[0160] Aspect 17: The method according to any one of Aspects 1 to 16 may further include providing a mounting platform within the vacuum chamber, the mounting platform configured to support the material, and adjusting an inclination angle of the mounting platform to change the grazing angle.

[0161] Embodiment 18: The method according to any one of Embodiments 1-17 can further include pulsing the electron beam during the directing to impinge on the surface region of the material.

[0162] Embodiment 19: The method according to any one of Embodiments 1 to 18 can further include measuring the crystalline properties of the surface region with a reflection high-energy electron diffraction (RHEED) instrument coupled to a second sidewall of the vacuum chamber.

[0163] Aspect 20: In an aspect of the present disclosure, a system for characterizing a surface region of a material includes a vacuum chamber, a mounting platform within the vacuum chamber, the mounting platform configured to support the material, an electron beam generator coupled to the vacuum chamber, and a detector assembly. The electron beam generator is configured to direct an electron beam at a grazing angle toward the surface region of the material. The detector assembly is positioned above the surface region to receive cathodoluminescence light resulting from the electron beam transferring energy to the surface region, and optics within the detector assembly are configured to be contained within a vacuum environment.

[0164] Aspect 21: In an aspect of the present disclosure, a system for characterizing a surface region of a material includes a vacuum chamber, a mounting platform within the vacuum chamber, the mounting platform configured to support the material, an electron beam generator coupled to a sidewall of the vacuum chamber, and a detector assembly. The electron beam generator is configured to direct an electron beam at the surface region of the material at a grazing angle. The detector assembly has an optical penetration path and is positioned above (e.g., directly above) the surface region to receive cathodoluminescence light emission resulting from the electron beam transferring energy to the surface region.

[0165] Embodiment 22: In the system according to embodiment 20 or 21, the material includes an epitaxial layer on a substrate, and the grazing angle is set such that a majority of the cathodoluminescent light is emitted from the epitaxial layer rather than the substrate.

[0166] Embodiment 23: In the system according to any one of embodiments 20-22, the grazing angle can be any value described herein, such as 45° or less, or 25° or less, or 15° or less.

[0167] Aspect 24: In the system according to any one of Aspects 20-23, the detector assembly is positioned directly above the surface region.

[0168] Embodiment 25: In a system according to any one of embodiments 20 to 24, the detector assembly has an optical intrusion path, and the detector assembly is positioned such that the optical intrusion path is at a detection angle that is perpendicular (or nearly perpendicular, e.g., 85° to 95°) to the surface area, or at a detection angle of 70° to 110° to the surface area.

[0169] Aspect 26: In the system according to any one of Aspects 20 to 25, the detector assembly has an aperture for receiving the cathodoluminescence emission, the aperture being positioned at a distance of 1 mm to 200 mm from the surface region.

[0170] Aspect 27: In a system according to any one of aspects 20 to 26, the detector assembly has a numerical aperture in the range of 0.2 to 1.5 (e.g., in combination with the aperture being positioned at a distance of 1 mm to 200 mm from the surface region).

[0171] Embodiment 28: In a system according to any one of embodiments 20 to 27, the detector assembly includes a collimator that collimates the cathode luminescence emission into a parallel beam, the collimator including non-refractive optical components and not including any refractive optical components, and a grating and slit assembly that receives the parallel beam.

[0172] Embodiment 29: In the system according to any one of embodiments 20-27, the detector assembly includes a collimator that collimates the cathodoluminescence emission into a parallel beam, the collimator including a refractive optical element.

[0173] Embodiment 30: In a system according to any one of embodiments 20 to 27, the detector assembly includes a collimator that collimates the cathodoluminescence emission into a parallel beam, the collimator including a combination of refractive and non-refractive optical components.

[0174] Embodiment 31: In the system according to any one of embodiments 20-30, the detector assembly is configured as a monochromator or spectrometer that determines the spectral characteristics of the cathodoluminescence emission to characterize the surface region.

[0175] Aspect 32: In the system according to any one of Aspects 20-31, the optical components in the detector assembly are housed in a vacuum environment.

[0176] Aspect 33: In a system according to any one of aspects 20 to 32, the mounting platform has five degrees of freedom, including linear translational movement in the X-axis, Y-axis, and Z-axis, rotational movement about the Z-axis (azimuth angle φ), and rotational movement about the x-axis (tilt angle θ).

[0177] Aspect 34: The system according to any one of Aspects 20-33, wherein the mounting platform has an adjustable tilt angle θ that changes the grazing angle.

[0178] Embodiment 35: In the system according to any one of embodiments 20-34, the mounting platform includes a conduit for circulating a refrigerant to cool the material on the mounting platform.

[0179] Aspect 36: In the system according to any one of Aspects 20-35, the mounting platform includes a heating element for heating the material on the mounting platform.

[0180] Aspect 37: In the system according to any one of Aspects 20-36, the electron beam generator is configured to pulse the electron beam while directing the electron beam toward the surface region of the material.

[0181] Embodiment 38: In the system according to any one of embodiments 20 to 37, the system further includes a reflection high-energy electron diffraction (RHEED) instrument coupled to a second sidewall of the vacuum chamber for measuring crystalline properties of the surface region.

[0182] Aspect 39: In the system according to any one of Aspects 20-38, the mounting platform is configured to apply a bias voltage to the material.

[0183] Features depicted in a figure may be utilized with other figures, even if not explicitly shown. For example, the RHEED device 310 of Figure 3 may be utilized in various configurations of the systems and methods of the present disclosure. In another example, pulsing the electron beam as described in connection with Figure 34 may be utilized in various configurations of the systems and methods of the present disclosure.

[0184] In some cases, a single embodiment may combine multiple features for brevity and / or to aid in understanding the scope of the present disclosure. In such cases, it should be understood that these multiple features may be provided separately (in separate embodiments) or in any other suitable combination. Alternatively, if separate features are described in separate embodiments, unless otherwise stated or implied, these separate features may be combined into a single embodiment. This also applies to claims that can be recombined in any combination. That is, a claim can be amended to include features defined in any other claim. Furthermore, a phrase referring to "at least one" of a list of items refers to any combination of those items, including single elements. As an example, "at least one of a, b, or c" is intended to cover a, b, c, ab, ac, bc, and abc.

[0185] Reference has been made in detail to embodiments of the disclosed invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided for the purpose of explaining the technology, not limiting it. Indeed, while the specification has described in detail certain embodiments of the invention, it will be understood that those skilled in the art, upon gaining an understanding of the foregoing, will readily conceive modifications, variations, and equivalents to these embodiments. For example, features illustrated or described as part of one embodiment can be used with another embodiment to yield yet a still further embodiment. It is therefore intended that the present subject matter encompass all such modifications and variations within the scope of the appended claims and their equivalents. These and other modifications and variations to the present invention can be practiced by those skilled in the art without departing from the scope of the invention, which is more particularly set forth in the appended claims. Moreover, those skilled in the art will recognize that the foregoing description is by way of example only and is not intended to limit the invention.

Claims

1. 1. A method for characterizing a surface region of a material, the method comprising: generating an electron beam in a vacuum chamber from an electron beam generator; directing the electron beam at a grazing angle toward the surface region of the material; receiving, with a detector assembly, cathodoluminescence light emission resulting from the electron beam transferring energy to the surface region, the detector assembly being positioned above the surface region and operating in a vacuum environment; and determining, by the detector assembly, spectral characteristics of the cathodoluminescence emission to characterize the surface region.

2. The method of claim 1 , wherein directing the electron beam comprises setting the grazing angle and a beam energy of the electron beam to adjust a penetration depth of the electron beam into the surface region.

3. High temperature charge carriers [Equation 1] beam energy E of the electron beam so as to be transmitted into the surface region at ｂ and further comprising: ｇ The method of claim 1 , wherein:

4. the material includes an epitaxial layer on a substrate; The method of claim 1 , wherein directing the electron beam comprises setting the grazing angle such that a majority of the cathodoluminescent light is emitted from the epitaxial layer rather than the substrate.

5. The method of claim 1 , wherein the grazing angle is less than or equal to 25°.

6. The method of claim 1 , wherein the detector assembly is positioned directly above the surface region.

7. 7. The method of claim 6, wherein the detector assembly has an optical penetration path, and the detector assembly is positioned at a detection angle such that the optical penetration path is approximately perpendicular to the surface area, the detection angle being between 85° and 95°.

8. 2. The method of claim 1, wherein the detector assembly has an aperture for receiving the cathodoluminescence emission, the aperture being positioned at a distance of between 1 mm and 200 mm from the surface region.

9. The method of claim 1 , wherein the detector assembly has a numerical aperture in the range of 0.2 to 1.

5.

10. 10. The method of claim 1, wherein receiving by the detector assembly comprises collimating the cathodoluminescence emission by a collimator within the detector assembly, the collimator including non-refractive optics and no refractive optics.

11. The method of claim 1 , wherein receiving by the detector assembly comprises collimating the cathodoluminescence emission by a collimator within the detector assembly, the collimator comprising a refractive optic.

12. 10. The method of claim 1, wherein receiving by the detector assembly comprises collimating the cathodoluminescence emission by a collimator within the detector assembly, the collimator comprising a combination of refractive and non-refractive optics.

13. 2. The method of claim 1, wherein the spectral characteristics are in the deep ultraviolet wavelength range of 110 nm to 400 nm or 110 nm to 280 nm.

14. providing a mounting platform within the vacuum chamber, the mounting platform configured to support the material; 10. The method of claim 1, further comprising: using the mounting platform to cool or heat the material during the cathodoluminescence emission resulting from the electron beam impinging on the surface region.

15. providing a mounting platform within the vacuum chamber, the mounting platform configured to support the material; The method of claim 1 , further comprising adjusting a tilt angle of the mounting platform to change the grazing angle.

16. The method of claim 1 , further comprising pulsing the electron beam to impinge on the surface region of the material while directing the electron beam.

17. The method of claim 1 further comprising applying a bias voltage to the material.

18. 1. A method for characterizing a surface region of a material, the method comprising: generating an electron beam within the vacuum chamber from an electron beam generator coupled to a sidewall of the vacuum chamber; directing the electron beam at a grazing angle toward the surface region of the material; receiving, with a detector assembly, cathodoluminescence light resulting from the electron beam transferring energy to the surface region, the detector assembly having an optical penetration path disposed above the surface region; and determining, by the detector assembly, spectral characteristics of the cathodoluminescence emission to characterize the surface region.

19. High temperature charge carriers [Equation 2] beam energy E of the electron beam so as to be transmitted into the surface region at ｂ and further comprising: ｇ 20. The method of claim 18, wherein: is the band gap energy of the material.

20. 20. The method of claim 18, wherein the light penetration path of the detector assembly is positioned at a detection angle of between 70° and 110° relative to the surface area.

21. 20. The method of claim 18, wherein the grazing angle is 25 degrees or less.

22. the detector assembly has an aperture for receiving the cathodoluminescence emission, the aperture being positioned at a distance of 1 mm to 200 mm from the surface region; The method of claim 18, wherein the detector assembly has a numerical aperture in the range of 0.2 to 1.

5.

23. 20. The method of claim 18, wherein receiving by the detector assembly comprises collimating the cathodoluminescence emission by a collimator within the detector assembly, the collimator comprising a non-refractive optical component and no refractive optical component.

24. 20. The method of claim 18, wherein receiving by the detector assembly comprises collimating the cathodoluminescence emission by a collimator within the detector assembly, the collimator comprising a refractive optic.

25. 20. The method of claim 18, wherein receiving by the detector assembly comprises collimating the cathodoluminescence emission by a collimator within the detector assembly, the collimator comprising a combination of refractive and non-refractive optics.

26. the detector assembly operates in a vacuum environment during the receiving; 19. The method of claim 18, wherein the spectral characteristics are in the deep ultraviolet wavelength range of 110 nm to 400 nm or 110 nm to 280 nm.

27. 20. The method of claim 18, further comprising pulsing the electron beam to impinge on the surface region of the material while directing the electron beam.

28. 20. The method of claim 18, further comprising measuring the crystalline properties of the surface region with a reflection high-energy electron diffraction (RHEED) instrument coupled to a second sidewall of the vacuum chamber.

29. 20. The method of claim 18, further comprising applying a bias voltage to the material.

30. 1. A system for characterizing a surface region of a material, comprising: A vacuum chamber; a mounting platform within the vacuum chamber, the mounting platform configured to support the material; and an electron beam generator coupled to the vacuum chamber, the electron beam generator configured to direct an electron beam at a grazing angle toward the surface region of the material; a detector assembly positioned above the surface region to receive cathodoluminescence light resulting from the electron beam transferring energy to the surface region, the detector assembly being configured to house optical components within a vacuum environment.

31. the material includes an epitaxial layer on a substrate; 31. The system of claim 30, wherein the grazing angle is set so that a majority of the cathodoluminescent light is emitted from the epitaxial layer rather than the substrate.

32. 31. The system of claim 30, wherein the grazing angle is less than or equal to 25 degrees.

33. 31. The system of claim 30, wherein the detector assembly is positioned directly above the surface region.

34. 34. The system of claim 33, wherein the detector assembly has an optical penetration path, and the detector assembly is positioned at a detection angle where the optical penetration path is normal to the surface area.

35. 31. The system of claim 30, wherein the detector assembly has an aperture for receiving the cathodoluminescence emission, the aperture being positioned at a distance of between 1 mm and 200 mm from the surface region.

36. 31. The system of claim 30, wherein the detector assembly has a numerical aperture in the range of 0.2 to 1.

5.

37. The detector assembly includes: a collimator that collimates the cathodoluminescence light into a parallel beam, the collimator including non-refractive optics and no refractive optics; and a grating and slit assembly for receiving the collimated beam.

38. 31. The system of claim 30, wherein the detector assembly includes a collimator that collimates the cathodoluminescence emission into a parallel beam, the collimator including a refractive optic.

39. 31. The system of claim 30, wherein the detector assembly includes a collimator that collimates the cathodoluminescence emission into a parallel beam, the collimator including a combination of refractive and non-refractive optics.

40. 31. The system of claim 30, wherein the detector assembly is configured as a monochromator or spectrometer that determines spectral characteristics of the cathodoluminescence emission to characterize the surface region.

41. 31. The system of claim 30, wherein the mounting platform has five degrees of freedom including linear translational movement in the x-, y-, and z-axes, rotational movement about the z-axis (azimuth angle φ), and rotational movement about the x-axis (tilt angle θ).

42. 31. The system of claim 30, wherein the mounting platform has an adjustable tilt angle θ to vary the grazing angle.

43. 31. The system of claim 30, wherein the mounting platform includes conduits for circulating a refrigerant to cool the material on the mounting platform.

44. 31. The system of claim 30, wherein the mounting platform includes a heating element for heating the material on the mounting platform.

45. 31. The system of claim 30, wherein the electron beam generator is configured to pulse the electron beam while directing the electron beam toward the surface region of the material.

46. The system of claim 30 , wherein the mounting platform is configured to apply a bias voltage to the material.

47. 1. A system for characterizing a surface region of a material, the system comprising: A vacuum chamber; a mounting platform within the vacuum chamber, the mounting platform configured to support the material; and an electron beam generator coupled to a sidewall of the vacuum chamber, the electron beam generator configured to direct an electron beam at a grazing angle toward the surface region of the material; a detector assembly having an optical penetration path positioned above the surface region to receive cathodoluminescence light resulting from the electron beam transferring energy to the surface region.

48. 48. The system of claim 47, wherein the grazing angle is 25 degrees or less.

49. 48. The system of claim 47, wherein the light penetration path of the detector assembly is positioned at a detection angle of between 70° and 110° relative to the surface area.

50. 48. The system of claim 47, wherein the light penetration path is perpendicular to the surface region.

51. the detector assembly has an aperture for receiving the cathodoluminescence emission, the aperture being positioned at a distance of 1 mm to 200 mm from the surface region; 48. The system of claim 47, wherein the detector assembly has a numerical aperture in the range of 0.2 to 1.

5.

52. The detector assembly includes: a collimator that collimates the cathodoluminescence light into a parallel beam, the collimator including non-refractive optics and no refractive optics; and a grating and slit assembly that receives the collimated beam.

53. 48. The system of claim 47, wherein the detector assembly includes a collimator that collimates the cathodoluminescence emission into a parallel beam, the collimator including a refractive optic.

54. 48. The system of claim 47, wherein the detector assembly includes a collimator that collimates the cathodoluminescence emission into a parallel beam, the collimator including a combination of refractive and non-refractive optics.

55. 48. The system of claim 47, wherein optical components in the detector assembly are contained in a vacuum environment.

56. 48. The system of claim 47, wherein the mounting platform includes conduits for circulating a refrigerant to cool the material on the mounting platform.

57. 48. The system of claim 47, wherein the mounting platform includes a heating element for heating the material on the mounting platform.

58. 48. The system of claim 47, wherein the electron beam generator is configured to pulse the electron beam while directing the electron beam toward the surface region of the material.

59. 48. The system of claim 47, further comprising a reflection high-energy electron diffraction (RHEED) device coupled to a second sidewall of the vacuum chamber for measuring crystalline properties of the surface region.

60. 48. The system of claim 47, wherein the mounting platform is configured to apply a bias voltage to the material.

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