A spectroscopic method for measuring the built-in electric field strength of a gallium nitride crystal surface

Abstract

The application discloses a kind of spectroscopy methods for measuring the built-in electric field intensity of gallium nitride crystal surface.Utilizing femtosecond transient reflection spectroscopy technology, the carrier dynamics process in the built-in electric field of gallium nitride crystal surface is studied, the measurement of its surface built-in electric field intensity is realized, including the following steps: build pump-probe transient reflection light path, test the transient reflection spectrum of gallium nitride epitaxial surface, test the evolution process of surface built-in electric field intensity change with time under different initial carrier distribution by changing different excitation wavelength, obtain the depletion layer thickness of gallium nitride surface built-in electric field by fitting carrier diffusion-drift coupling model, and calculate the intensity of built-in electric field.The application is suitable for studying the carrier dynamics process of heavily doped gallium nitride surface as a kind of ultrafast time-resolved pump-probe spectroscopy technology, and provides theoretical guidance for optimizing the performance of GaN-based optoelectronic devices.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of semiconductor technology, and particularly to a spectroscopic method for measuring the built-in electric field intensity of a gallium nitride crystal surface. BACKGROUND

[0002] In semiconductor materials and devices, the built-in electric field is an important factor affecting their electronic and optical properties. The built-in electric field, which is caused by uneven charge distribution or polarization effect, usually exists in pn junctions, heterojunctions, quantum wells, and material surfaces and interfaces. Accurate measurement and understanding of the built-in electric field intensity is crucial for regulating the electrical, optical, and thermal characteristics of semiconductor devices, especially in microelectronic and optoelectronic devices such as field effect transistors, diodes, light emitting diodes, and solar cells. Gallium nitride (GaN) and its related materials such as InGaN and AlGaN are important wide-bandgap semiconductors, widely used in the production of high-power, high-frequency electronic devices and high-efficiency light-emitting devices such as LEDs and lasers. GaN materials have strong spontaneous polarization and piezoelectric polarization effects, which result in strong built-in electric fields in the material interior and surface. The built-in electric field has a significant impact on the carrier transport, energy band structure, and optical performance of GaN-based devices. Therefore, developing an effective experimental method to measure and study the built-in electric field of GaN surface is of great significance for understanding and optimizing device performance.

[0003] Ultrafast transient reflectance spectroscopy is an emerging optical measurement technique that uses ultra-short pulse laser (usually in femtosecond) to irradiate the sample and measures the change of reflectance spectrum in real time, which can non-contact dynamically measure the built-in electric field of semiconductor materials and provides a powerful means to study the evolution of semiconductor surface electric field. Therefore, the purpose of the present application is to develop an ultrafast transient reflectance spectroscopy method for measuring the built-in electric field intensity of the surface of a gallium nitride crystal, which is used to study the intensity of the built-in electric field of the surface of a gallium nitride crystal. SUMMARY

[0004] The technical problem to be solved by the present application is to provide an ultrafast transient reflectance spectroscopy method for measuring the built-in electric field intensity of the surface of a gallium nitride crystal. The pump-probe technique is used to control the initial carrier concentration distribution by changing the pump light wavelength, and the evolution of reflectivity is monitored in real time using probe light. The built-in electric field intensity is obtained by fitting the carrier diffusion-drift coupling model.

[0005] The technical solution adopted by the present application to achieve the above-mentioned purpose is: a spectroscopic method for measuring the built-in electric field intensity of the surface of a gallium nitride crystal, comprising the following steps:

[0006] The laser is used to excite the electron to the conduction band by single photon excitation or two-photon excitation, and the valence band leaves a hole; white light is used as probe light to detect the change of excited carrier in real time;

[0007] The incident angle of the probe light on the gallium nitride film sample is adjusted, and the pump light emitted by the optical parametric amplifier is incident on the front surface of the gallium nitride film sample;

[0008] The probe light reflected on the surface of the gallium nitride film sample is collected to test the transient absorption spectrum;

[0009] According to the spectral information, the ΔR / R data is obtained; wherein, R represents the reflectivity of the probe light with pump light excitation, and ΔR represents the difference between the reflectivity of the probe light with pump light excitation and the reflectivity of the probe light without pump light excitation;

[0010] According to the ΔR / R data, the depletion layer width is obtained by fitting; and the built-in electric field strength is obtained according to the depletion layer width.

[0011] The wavelength of the probe light covers 340nm-800nm.

[0012] The incident angle of the probe light on the gallium nitride film sample is 45°.

[0013] The pump light emitted by the optical parametric amplifier is incident on the front surface of the gallium nitride film sample, and the incident angle is 0-10°.

[0014] The pump light excitation wavelength is changed to 260nm and 400nm to test the transient reflection spectrum.

[0015] The ΔR / R data is obtained, and the depletion layer width of the built-in electric field is obtained by fitting the experimental data by using the carrier drift-diffusion model.

[0016] The built-in electric field strength is obtained according to the depletion layer width, and is obtained by the following formula:

[0017]

[0018] Wherein, F is the built-in electric field strength, W0 is the depletion layer width, q is the electronic charge, N D is the doping concentration, and ε is the relative dielectric constant, and ε0 is the vacuum dielectric constant.

[0019] The present application has the following advantages and benefits:

[0020] 1. The transient reflection spectrum is an optical method, so it can be measured without contacting the sample, avoiding the interference introduced by electrode contact in the traditional electrical method. This non-contact measurement is particularly suitable for surface-sensitive or easily-damaged semiconductor materials.

[0021] 2. Traditional electrical measurements sometimes require the preparation of electrodes on the sample or the introduction of an external electric field, which may cause certain physical or chemical effects on the material surface. Transient reflectance spectroscopy, on the other hand, is completely non-invasive and does not damage the sample, making it particularly suitable for measuring fragile or difficult-to-handle materials.

[0022] 3. Transient reflectance spectroscopy can achieve micrometer-level or even nanometer-level spatial resolution through optical focusing, thereby measuring the built-in electric field in a localized area of ​​the semiconductor surface. In contrast, traditional electrical methods generally have difficulty achieving such high spatial resolution. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The optical path diagram of the ultrafast transient reflection spectrum.

[0024] Figure 2 This is a transient reflection time-sharing spectrum diagram of GaN epitaxy in the embodiment.

[0025] Figure 3 Graph showing carrier dynamics of GaN epitaxy under conditions of different excitation wavelengths in the embodiment.

[0026] Figure 4 Flow chart of the method of the present invention. DETAILED DESCRIPTION

[0027] The present invention will be described in further detail below with reference to examples and drawings, but the embodiments of the present invention are not limited thereto.

[0028] The present invention discloses a spectroscopic method for measuring the built-in electric field strength on the surface of gallium nitride crystals. Using femtosecond transient reflection spectroscopy, the carrier dynamics in the built-in electric field on the surface of gallium nitride crystals were studied, and the measurement of the built-in electric field strength on the surface was achieved. The method includes the following steps: establishing a pump-probe transient reflection optical path, testing the transient reflection spectrum of the gallium nitride epitaxial surface, testing the time evolution of the built-in electric field strength under different initial carrier distributions by changing different excitation wavelengths, obtaining the depletion layer thickness of the built-in electric field on the gallium nitride surface by fitting a carrier diffusion-drift coupling model, and calculating the built-in electric field strength. As an ultrafast time-resolved pump-probe spectroscopy technique, the present invention is suitable for studying the surface carrier dynamics of heavily doped gallium nitride, and provides theoretical guidance for optimizing the performance of GaN-based optoelectronic devices.

[0029] like Figure 4 Said present invention comprises the following steps:

[0030] (1) Construction of the optical path for femtosecond transient reflectance spectroscopy: This system mainly consists of three components: a femtosecond Ti:sapphire laser system (800nm, 35fs, 1kHz), an optical parametric amplifier (OPA), and a transient spectrometer. TOPAS OPA is mainly used to generate wavelength-tunable pump light (240nm to 2600nm); while the transient spectrometer includes a variable delay control device, a calcium fluoride crystal window, a chopper, and a detector. The system can achieve a time resolution of 35fs, a maximum delay time of 8ns, and a probe light (white light) wavelength covering 340nm to 800nm. The incident angle of the probe light on the thin film sample is adjusted to about 45°, and the probe light reflected by the sample surface enters the fiber spectrometer; the pump light is incident on the front of the thin film sample with an incident angle of 0-10°.

[0031] (2) GaN epitaxial transient reflection spectrum test: The sample is a Si-doped (0001) GaN epitaxial wafer with a thickness of 4.5 μm. Electrons are excited to the conduction band using single-photon excitation and two-photon excitation, respectively. The pump light wavelengths are 260 nm and 400 nm, respectively, and the probe light wavelength range is 340 nm-800 nm. The probe light is collected by a UV detector to test the transient absorption spectrum.

[0032] (4) Calculation of the built-in electric field strength of GaN: On the surface of the semiconductor, carriers can drift, diffuse and recombine. Drift and diffusion will lead to the enhancement of the modulated electric field ( Figure 3 The signal observed in the photoelectron scattering increases from 0 to -1), while carrier recombination causes a decrease in the photogenerated electric field (the signal observed decays from -1 to 0). It is assumed here that carriers within the depletion region primarily drift, while those outside the depletion region (neutral region) primarily diffuse. Carriers residing in the neutral region can diffuse into the depletion region or into the bulk phase driven by a concentration gradient.

[0033] (5) Collect ΔR / R data and fit the experimental data of ΔR / R varying with the delay time using the carrier drift-diffusion coupling model to obtain the depletion layer width. Here, R represents the reflectivity of the probe light on the sample surface without pump light excitation, and ΔR represents the difference between the reflectivity of the probe light with pump light excitation and the reflectivity of the probe light without pump light excitation.

[0034] The change of carrier concentration distribution in the neutral region of GaN film over time can be expressed as:

[0035]

[0036] Here, N represents the carrier concentration, D is the minority carrier (hole) diffusion coefficient, τ is the bulk recombination lifetime. t is the delay time after the sample is excited, and x is the depth from the sample surface.

[0037] For n-type doped GaN, there is an upward band bending on the surface, the photo-generated holes diffuse into the depletion layer, and then the holes are swept to the surface under the action of the built-in electric field; the photo-generated electrons are swept to the surface of the neutral region under the action of the built-in electric field, and then diffuse into the bulk. The final transient electric field strength depends on the number of photo-generated holes from the bulk to the depletion layer. A common method is to assume a virtual boundary surface to separate the depletion region and the neutral region, and the carrier flux at the boundary is characterized by the charge carrier velocity, and the boundary condition that the carrier flux needs to satisfy is as follows:

[0038]

[0039] Here, W0 is the depletion layer width, S V is the hole thermal velocity at the virtual boundary of the depletion layer, and J represents the carrier flux. The final transient reflection signal is proportional to the carrier concentration, that is

[0040]

[0041] Different excitation wavelengths have different absorption coefficients, so the initial carrier concentration distribution is also different, and the initial carrier concentration distribution is represented as

[0042] N(x, 0) =N0·exp(-αx) (5)

[0043] Where α is the absorption coefficient, and N0 is the carrier concentration at time t=0. Using the carrier drift-diffusion coupling model composed of formulas (1)-(5), the depletion layer width and the minority carrier (hole) diffusion coefficient D can be obtained by fitting the TR kinetics.

[0044] (6) Calculation of built-in electric field strength: The formula of the built-in electric field strength is as follows:

[0045]

[0046] Where q is the electronic charge, N D is the doping concentration, ε is the relative dielectric constant, and ε0 is the vacuum dielectric constant.

[0047] Embodiment:

[0048] This embodiment describes a method for measuring the built-in electric field strength of the epitaxial surface of gallium nitride, which uses Si-doped gallium nitride epitaxial on the (0001) crystal surface of the sapphire substrate as a demonstration object, which specifically includes the following steps:

[0049] 1. Build the ultrafast time-resolved transient reflectance spectroscopy optical path based on pump-probe technology, the specific details are shown in Figure 1 .

[0050] Wherein, the incident angle of the probe light on the GaN film sample is about 45°; adjust the angle of the mirror so that the pump light is perpendicular to the surface of the GaN film sample, the incident angle is 0°.

[0051] 2. The band gap of gallium nitride is 3.4eV, select 260nm as the excitation light (photon energy is about 4.8ev), through single photon excitation technology to excite the valence band electrons to the conduction band, the valence band leaves a hole; supercontinuum white light as the probe light to detect the change of excited state carriers in real time. As shown in Figure 2 , it is the time-sharing spectrum of GaN transient reflectance spectroscopy, it can be seen that there is spectral oscillation above the band gap, which is caused by FKO oscillation of built-in electric field.

[0052] 3. Use 260nm and 400nm as pump light respectively, extract the dynamics at 366nm, as shown in Figure 3 . Use formulas (1), (2), (3), (4), (5) to write fitting program, and the experimental data can be fitted, it can be found that the experimental data and the fitting curve are completely consistent. Among them, the first order decay rate is fixed as 1E+9s -1 , the GaN epitaxial thickness is 2μm, Sv is 1E+7cm / s, and the surface recombination rate is 1000cm / s. The final fitting result: the diffusion coefficient D is 0.466cm 2 / s, and the depletion layer thickness W0 is 31.0nm.

[0053] 4. According to formula (6), calculate the built-in electric field intensity of GaN surface, wherein q is 1.6*10 -19 C, W0 is 31.0nm, N D is 1*10 18 cm -3 , ε is 9.5, and ε0 is 8.85*10 -12 F / m, put into formula (6), the built-in electric field intensity is 3.1*10 4 V / cm.

Claims

1. A spectroscopic method for measuring the built-in electric field strength on the surface of a gallium nitride crystal, characterized in that: The following steps are involved: The laser uses single-photon excitation or two-photon excitation to excite electrons to the conduction band, leaving holes in the valence band; White light is used as the probe light to detect the changes of excited state carriers in real time; The incident angle of the probe light on the GaN thin film sample is adjusted, and the pump light emitted by the optical parametric amplifier is incident on the front side of the GaN thin film sample; Collect the probe light reflected from the surface of the GaN film sample and test the transient absorption spectrum; Obtain ΔR / R data based on the spectral information; where R represents the reflectivity of the probe light with pump light excitation, and ΔR represents the difference between the reflectivity of the probe light with pump light excitation and the reflectivity of the probe light without pump light excitation; According to the ΔR / R data, the depletion layer width is obtained by fitting; and the built-in electric field strength is obtained according to the depletion layer width.

2. The spectroscopic method for measuring the built-in electric field strength on the surface of a gallium nitride crystal according to claim 1, characterized in that: The detection light wavelength covers 340nm to 800nm.

3. The spectroscopic method for measuring the built-in electric field strength on the surface of a gallium nitride crystal according to claim 1, characterized in that: The incident angle of the detection light on the gallium nitride film sample is adjusted to 45°.

4. The spectroscopic method for measuring the built-in electric field strength on the surface of a gallium nitride crystal according to claim 1, characterized in that: The pump light emitted by the optical parametric amplifier is incident on the front side of the gallium nitride thin film sample at an incident angle of 0-10°.

5. The spectroscopic method for measuring the built-in electric field strength on the surface of a gallium nitride crystal according to claim 1, characterized in that: The pump light excitation wavelength was changed to 260 nm and 400 nm to test the transient reflection spectrum.

6. The spectroscopic method for measuring the built-in electric field strength on the surface of a gallium nitride crystal according to claim 1, wherein the ΔR / R data is obtained and the depletion layer width of the built-in electric field is obtained by fitting the experimental data using a carrier drift-diffusion model.

7. The spectroscopic method for measuring the built-in electric field strength on the surface of a gallium nitride crystal according to claim 1, wherein the built-in electric field strength is obtained based on the depletion layer width, and is obtained by the following formula: in, F is the built-in electric field strength, W0 is the depletion layer width, q is the electron charge, N D is the doping concentration, ε is the relative dielectric constant, and ε0 is the vacuum dielectric constant.

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