Enhancing the Doping Efficiency of Ultra-Wide Bandgap Semiconductors by Metal-Semiconductor-Assisted Epitaxy

Abstract

Disclosed is an epitaxial growth process of ultra-wide bandgap aluminum gallium nitride (AlGaN), called metal-semiconductor junction assisted epitaxy. The epitaxy of AlGaN is carried out using plasma-assisted molecular beam epitaxy under conditions rich in metal (e.g., rich in Ga). The excess Ga layer leads to the formation of a metal-semiconductor junction during the epitaxy of Mg-doped AlGaN, which decouples the Fermi level from the valence band at the growth front. The Fermi level position is decoupled from the Mg-doped element. That is, although a p-type dopant is incorporated, the surface band bending still allows the formation of a near n-type growth front. By controlling the regulation of the Fermi level through an in-situ metal-semiconductor junction during epitaxy, effective p-type conduction can be achieved for wide-bandgap AlGaN.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims priority to U.S. Provisional Patent Application No. 62 / 838,842, filed on April 25, 2019, titled "Enhanced Doping Efficiency of Ultrawide Bandgap Semiconductors by Metal-Semiconductor Assisted Epitaxy" by Liu et al., and also claims priority to U.S. Provisional Patent Application No. 62 / 676,828, filed on May 25, 2018, titled "Doped AlGaN Grown by Molecular Beam Epitaxy" by Liu et al. BACKGROUND OF THE INVENTION

[0003] Wide bandgap semiconductors, such as gallium nitride (GaN), aluminum nitride (AlN), and their alloys, have become the materials of choice for high-power and high-frequency electronic devices and various photonic devices, including ultraviolet (UV) light-emitting diodes (LEDs), lasers, and solar-blind photodetectors. Ultrawide bandgap semiconductors are important for a wide variety of electronic and photonic devices, but their practical applications are limited by poor current conduction.

[0004] Precise control of the doping levels in the different layers of a device structure is crucial for the operation and performance of these devices. However, to date, achieving efficient p-type conduction in AlN and AlGaN with relatively high Al content remains extremely challenging, which has been identified as one of the major obstacles to realizing high-performance optoelectronic devices operating in the mid-UV and deep-UV spectral ranges.

[0005] Magnesium (Mg) has been identified as the only viable p-type dopant in group-III nitride semiconductors. However, it exhibits a very large activation energy (up to 500 - 600 meV) in Al-rich AlGaN, severely limiting the doping efficiency and the achievement of a large hole concentration at room temperature. The solubility of the Mg acceptor dopant in AlGaN also decreases significantly with increasing Al concentration. In addition, as the incorporation of Mg dopants increases, the formation energy of donor-like defects, including nitrogen vacancies (V N and V N 3+ ) decreases significantly, which leads to a strong self-compensation effect.

[0006] For AlGaN with an Al content of approximately 50 - 60%, due to the large lattice mismatch between AlN and GaN and the substrates usually available, the formation of a large density of defects and dislocations further affects the realization of effective p-type conduction. Previously, the epitaxy of Mg-doped AlGaN has been extensively studied by using metalorganic chemical vapor deposition (MOCVD). The reported free hole concentration in the Al-rich AlGaN epitaxial layer is 10 15 to 10 17 cm -3 at room temperature. By using δ-doping, short-period superlattices, polarization-induced doping, metal-modulated epitaxy, and indium surfactants, improved hole concentrations have been reported, but for Al-rich AlGaN at room temperature, the mobility values are low, approximately 1 - 3 cm 2 / (V·s). As a result, for AlGaN epitaxial layers with an Al composition in the range of 60 - 70%, the resistivity values typically measured are in the range of several or dozens of Ω·cm, compared to less than 1 Ω·cm for p-type GaN.

[0007] Various studies have been conducted to achieve p-type AlGaN with low resistivity, including using a high V / III ratio to suppress the formation of compensating nitrogen vacancies, superlattices composed of alternating AlGaN layers, metal-modulated epitaxy (MME), Mg δ-doping, indium as a surfactant, and polarization-induced doping, but with limited effectiveness. For example, the lowest resistivity reported so far for p-type Al 0.85 Ga 0.15 N epitaxial layers far exceeds 10 3 Ω·cm, which is three orders of magnitude larger than the resistivity of Mg-doped GaN. SUMMARY OF THE INVENTION

[0008] To obtain p-type aluminum gallium nitride (AlGaN) with a large hole concentration and low resistivity, a high density of magnesium (Mg) doping atoms must be incorporated. At very large concentrations (∼10 19 -10 20 cm -3), it is expected to form an Mg impurity band, thereby realizing hole transition conduction. In addition, the acceptor energy level significantly broadened at a large Mg doping concentration, as well as the band tailing effect, also reduce the ionization energy of a part of the Mg dopants. However, in practice, since Mg substituting for Al in the AlGaN lattice has a greater formation enthalpy (lower solubility) compared with Ga, it becomes more difficult to incorporate Mg into AlGaN as the Al concentration increases. Theoretical calculations confirm the difficulty of incorporating substitutional Mg, especially in the aluminum lattice. The formation energies of various compensating point defects also strictly depend on the position of the Fermi level. During the conventional epitaxial process, when incorporating Mg doping, the Fermi level shifts towards the valence band edge, which significantly reduces the formation energies of carbon, oxygen, and nitrogen vacancies. These defects have a strong compensating effect and further degrade the structural, electronic, and optical properties of Mg-doped AlGaN. In addition, the formation energies of N-substituted and interstitial Mg incorporation decrease sharply with the incorporation of Mg, and when the Fermi level is near the valence band edge, it is comparable to the formation energy of AI(Ga)-substituted Mg incorporation, further limiting the doping efficiency and the achievement of a large hole concentration.

[0009] If the Fermi level at the growth front can be shifted away from the valence band during the epitaxy of p-type (e.g., Mg-doped) AlGaN, the key problem of achieving efficient p-type conduction in AlGaN can be solved.

[0010] According to an embodiment of the present disclosure, the above challenges are overcome by a novel epitaxial growth process (referred to herein as metal-semiconductor junction-assisted epitaxy). Through metal-semiconductor junction-assisted epitaxy, molecular beam epitaxy (MBE) is used to form a liquid metal layer on the growth surface during epitaxy to enhance the incorporation of dopants and reduce the formation of compensating defects.

[0011] It has been demonstrated that metal-semiconductor junction-assisted epitaxy can be used for the growth of Mg-doped AlGaN epitaxial layers. In these embodiments, the epitaxy of AlGaN is performed using, for example, plasma-assisted MBE under metal-rich (e.g., Ga-rich) conditions. The excess Ga layer results in the formation of a metal-semiconductor junction during the epitaxy of Mg-doped AlGaN, which shifts the Fermi level away from the valence band at the growth front. During metal-semiconductor junction-assisted epitaxy, the presence of the excess Ga layer creates a separation between the Fermi level and the valence band. In this novel epitaxial process, the Fermi level position is decoupled from the Mg doping element; that is, although p-type dopants are incorporated, the surface band bending still allows the formation of a nearly n-type growth front, which is in direct contrast to the fixed Fermi level position near the valence band edge in the conventional Mg-doped AlGaN epitaxial process. In this way, even when a very high density of Mg doping atoms is incorporated, the formation energy of substitutional Mg is significantly reduced, while the formation energy for compensating defects is increased. For example, the spacing between the Fermi level and the valence band changes from that in conventional Mg-doped Al 0.5Ga 0.5 The increase during the epitaxial process of N is from about 0.12 eV to about 2.2 eV using metal-semiconductor junction assisted epitaxy.

[0012] Under metal-rich conditions, during the AIGaN epitaxy, the regulatory role of the Fermi level at the growth front and its influence on enhancing Mg substitutional bonding and suppressing the formation of compensating defects have not been confirmed.

[0013] By increasing the spacing between the Fermi level and the valence band, for Al 0.5 Ga 0.5 For the metal-semiconductor junction assisted epitaxy of N, the formation energy is reduced to only about 0.43 eV, which is about 1.6 eV lower than the growth energy during conventional epitaxial growth. The disclosed method utilizes excess Ga to spontaneously form a metal-semiconductor junction at the growth surface and can be integrated into the manufacturing systems in use. During the growth process, the Al composition of AIGaN can be controllably changed by adjusting the material flux of Al while keeping the nitrogen gas flow rate constant. It is preferred to form Al-N during epitaxy, and any excess Ga will accumulate on the surface to form a metal-semiconductor junction.

[0014] Compared with the conventional growth process, using the disclosed process, the incorporation of Mg in AIGaN is increased by nearly an order of magnitude. For example, for a moderate Mg flux of about 7×10 8 Torr, a Mg concentration of about 2×10 0.75 Ga 0.25 was measured in N. Through detailed secondary ion mass spectrometry (SIMS) measurements, a significant reduction in the incorporation of carbon impurities was also confirmed. Significantly, for Al 20 cm -3 Ga 0.9 N, the measured free hole concentration is about 4.5x10 0.1 cm 17 , and the resistivity value is less than 5 Ω·crn, which is reduced by nearly three orders of magnitude compared with the conventional method. -3

[0015] Compared with the samples grown using the conventional growth mode, the ultraviolet 280 nm light-emitting diodes fabricated according to the disclosed embodiments show significant improvements in the device characteristics and external quantum efficiency of the samples grown by metal-semiconductor junction assisted epitaxy.

[0016] ​As described above, wide-bandgap semiconductors are important for various electronic and photonic devices, but their practical applications are limited by poor current conduction. As disclosed herein, effective p-type conduction can be achieved for large-bandgap AlGaN by the controlled tuning of the Fermi level through in-situ metal-semiconductor junctions during epitaxy. This unique technique can be extended to the epitaxy / synthesis of various wide-bandgap semiconductors to achieve effective current conduction that was previously impossible.

[0017] After reading the following detailed description of the embodiments shown in the various figures, those of ordinary skill in the art will recognize these and other objects and advantages of the various embodiments of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The drawings incorporated in and forming a part of this specification illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. Throughout the drawings and the description, the same numbers represent the same elements. These numbers may not be drawn to scale.

[0019] Figure 1 is a flowchart of a method of manufacturing a device (metal-semiconductor junction assisted epitaxy) according to an embodiment of the invention.

[0020] Figure 2 and Figure 3 illustrate structures (parts of a device) formed during metal-semiconductor junction assisted epitaxy in accordance with an embodiment of the invention.

[0021] Figure 4A illustrates the energy band diagram of a Mg-doped AlGaN layer during conventional epitaxy.

[0022] Figure 4B illustrates the energy band diagram of a Mg-doped AlGaN layer with the formation of a metal-semiconductor junction during epitaxy with a fixed Fermi level far from the valence band at the growth front.

[0023] Figure 4C illustrates the calculation of the formation energy of Mg substituting for GaN, AIN, and Al 0.5 Ga 0.5 N as a function of the interval between the Fermi level and the valence band, with different growth Mg substitution formation energies.

[0024] Figure 5A and 5B illustrate examples of devices that can be manufactured using metal-semiconductor junction assisted epitaxy in accordance with an embodiment of the invention.

[0025] Figure 6A and 6BIt is a flowchart of a method for manufacturing a device using a metal-semiconductor junction-assisted epitaxy according to an embodiment of the present invention.

[0026] Figure 7A Shows the relationship between the Mg atom concentration and depth obtained by secondary ion mass spectrometry measurement on Mg-doped Al 0.75 Ga 0.25 N grown using metal-semiconductor junction-assisted epitaxy and using conventional epitaxy according to an embodiment of the present invention.

[0027] Figure 7B Shows the relationship between the Mg concentration and Mg flux of Mg-doped Al 0.75 Ga 0.25 N grown using metal-semiconductor junction-assisted epitaxy and using conventional epitaxy according to an embodiment of the present invention.

[0028] Figure 7C Shows the relationship between the incorporated Mg concentration and Mg flux of samples with different Al compositions grown using metal-semiconductor junction-assisted epitaxy according to an embodiment of the present invention.

[0029] Figure 7D Shows the photoluminescence spectrum of a Mg-doped Al 0.75 Ga 0.25 N sample grown using metal-semiconductor junction-assisted epitaxy with edge peaks and Mg acceptor peaks.

[0030] Figure 8A , 8B, 8C and 8D show the room-temperature Hall measurement data of Mg-doped AlGaN epitaxial layers with respect to the Al composition according to an embodiment of the present invention, respectively showing the free hole concentration, hole mobility, resistivity and resistance coefficient of the Mg-doped AlGaN layer.

[0031] Figure 9A , 9B and 9C show the temperature-dependent Hall measurements of Mg-doped AlGaN epitaxial layers grown by metal-semiconductor junction-assisted epitaxy according to an embodiment of the present invention, where the Al content is between 75% and 90%, and the reciprocal of the temperature is plotted for the hole concentration, hole mobility and resistivity, respectively.

[0032] Figure 10A Shows the electroluminescence spectrum of an ultraviolet light-emitting diode (UV LED) grown using metal-semiconductor junction-assisted epitaxy according to an embodiment of the present invention.

[0033] Figure 10B Shows the current-voltage characteristics of LEDs grown using metal-semiconductor junction-assisted epitaxy and using conventional epitaxy according to an embodiment of the present invention.

[0034] Figure 10C Shows the relationship between the external quantum efficiency and the current density of LEDs using metal-semiconductor junction assisted epitaxial growth and using conventional epitaxial growth in an embodiment according to the present invention. Detailed Description

[0035] Reference will now be made in detail to various embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Although described in connection with these embodiments, it is to be understood that they are not intended to limit the present disclosure to these embodiments. On the contrary, the present disclosure is intended to cover alternatives, modifications, and equivalents that may be included within the spirit and scope of the present disclosure. Additionally, in the following detailed description of the present disclosure, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. However, it will be understood that the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present disclosure.

[0036] Certain portions of the following detailed description are presented in terms of processes, logic blocks, processing, and other symbolic representations of operations for manufacturing devices, such as semiconductor devices. These descriptions and representations are the means by which those skilled in the art of device manufacturing most effectively convey the substance of their work to others skilled in the art. In the present application, a process, logic block, processing, etc. is considered to be a self-consistent sequence of steps or instructions that results in a desired outcome. These steps are those that require physical manipulation of physical quantities. However, it should be borne in mind that all such and similar terms are to be associated with appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise, it will be apparent from the following discussion that, throughout the present application, discussions using terms such as "epitaxy", "deposition", "formation", "variation", "removal", "incorporation", etc. refer to the actions and processes of device manufacturing.

[0037] Operations described as separate boxes may be combined and performed in the same processing step (i.e., within the same time interval, after a previous processing step and before a next processing step). Additionally, some operations may be performed in an order different from the order in which they are described below. Furthermore, manufacturing processes and steps may be performed in conjunction with the processes and steps discussed herein; however, the present invention is not limited thereto. That is, there may be many processing steps before, between, and / or after the steps shown and described herein. Importantly, embodiments according to the present invention may be implemented in conjunction with these other (perhaps conventional) processes and steps without significantly disturbing them. Generally, embodiments according to the present invention may replace or be integrated with a portion of a traditional process without significantly affecting the peripheral processes and steps.

[0038] It should be understood that the accompanying drawings are not necessarily to scale and show only a portion of the described devices and structures and the various layers forming those structures. For the sake of simplicity in discussion and illustration, although the process is described for one or two devices or structures, in fact one or more than two devices or structures may be formed.

[0039] Figure 1 is a flowchart of a method 100 for fabricating a device in an embodiment of the present invention. This method may be referred to as metal-semiconductor junction assisted epitaxy. In method 100, molecular beam epitaxy (MBE) is used to form a liquid metal layer on a growth surface during epitaxy to enhance the incorporation of dopants and reduce the formation of compensating defects.

[0040] Figure 2 and Figure 3 shows a structure 200 (a part of a device) formed during metal-semiconductor junction assisted epitaxy in an embodiment of the present invention, where a liquid gallium (Ga) layer exists on the surface during the epitaxy of a magnesium (Mg)-doped AlGaN layer.

[0041] In Figure 1 the block 102, also referring to Figure 2 and 3 , MBE (e.g., plasma-assisted MBE or ammonia MBE) is used to deposit Mg, aluminum (Al), Ga, and nitrogen (N) in a layer above the substrate 202 to form a Mg-doped AlGaN layer 204 by epitaxy. In an embodiment, the Mg-doped AlGaN layer 204 includes a part of a light-emitting diode (LED) heterostructure 506 ( Figure 5A ). In one embodiment, the substrate 202 is a sapphire substrate. However, other substrates may be used.

[0042] In Figure 1 the block 104, also referring to Figure 2 and Figure 3 , while depositing Mg, Al, Ga, and N, a liquid gallium layer 206 is formed on the Mg-doped AlGaN layer. The liquid gallium layer 206 causes the formation of a metal-semiconductor junction during the epitaxy of the Mg-doped AlGaN layer 204.

[0043] More specifically, during the epitaxy of the Mg-doped AlGaN layer 204, the composition of Al is controlled by adjusting the flux and growth rate of Al with the N flow rate. For example, the N flow rate can be kept constant, while the Al flux can be adjusted to regulate the alloy composition. Since AlN preferably forms over GaN in the Mg-doped AIGaN layer, this results in the accumulation of some Ga, thus forming a liquid Ga layer 206 in the Mg-doped AIGaN layer. In one embodiment, the flow rate of nitrogen is 0.4 standard cubic centimeters per minute; however, the present invention is not limited thereto, and different flow rates can be used. In one embodiment, the Mg-doped AlGaN layer 204 is grown at a temperature of approximately 700 °C; however, the present invention is not limited thereto, and the Mg-doped AlGaN layer can be grown over a wide temperature range.

[0044] Figure 4A The energy band diagram of the Mg-doped AlGaN layer during conventional epitaxy is shown. The Fermi level position is decoupled from the Mg-doped element; that is, although a p-type dopant is incorporated, the surface band bending still allows the formation of a nearly n-type growth front. Thus, even when a very high density of Mg-doped atoms is incorporated, the formation energy of the substitutional Mg is significantly reduced, while increasing the formation energy for compensating defects.

[0045] In an embodiment according to the present invention, the formation of the metal-semiconductor junction during epitaxy moves the Fermi level away from the valence band of the growth front, as Figure 4B shown.

[0046] Figure 4C Shows the calculated formation energies for Mg substituting GaN, AIN, and Al 0.5 Ga 0.5 N, which is a function of the spacing between the Fermi level and the valence band, where for different growth processes separately indicated by their respective arrows in the figure, there are different Mg substitution formation energies. By increasing the spacing between the Fermi level and the valence band, for the metal-semiconductor junction-assisted epitaxy of AIGaN, the formation energy is only reduced to about 0.43 eV.

[0047] Compared with the conventional growth process, the incorporation of Mg in AlGaN is improved by nearly an order of magnitude. For example, for a medium Mg flux of about 7×10 8 Torr, a Mg concentration of approximately 2×10 0.75 Ga 0.25 N is measured. Through detailed secondary ion mass spectrometry (SIMS) measurements, a significant reduction in the incorporation of carbon impurities is also confirmed. Significantly, for Al 20 Ga -3 N, the measured free hole concentration is about 4.5x10 0.9 Ga 0.1 N, the measured free hole concentration is about 4.5x10 17 cm-3 , the resistivity value is less than 5 Ω·cm, which is reduced by nearly three orders of magnitude compared with the conventional method.

[0048] Figure 5A and 5B show examples of devices that can be fabricated using metal-semiconductor junction assisted epitaxy in embodiments according to the present invention. Figure 5A shows the components of the optoelectronic device 500, while Figure 5B shows the components of the Schottky diode 510. Figure 6A and 6B are flowcharts 600 and 650 of methods for fabricating devices using metal-semiconductor junction assisted epitaxy in embodiments according to the present invention. Generally, in these methods, MBE is used to form a liquid metal layer on the growth surface during epitaxy to enhance the incorporation of dopants and reduce the formation of compensating defects.

[0049] In Figure 6A and Figure 6B in the block 602, also referring to Figure 5A and Figure 5B , in one embodiment, an (undoped) (Al)GaN buffer layer 503 is formed above the substrate 202. The substrate 202 can be a sapphire substrate, but the present invention is not limited thereto, and other substrates can be used.

[0050] In the block 604, in one embodiment, an undoped AlGaN layer 505 is formed above the AlN buffer layer 503.

[0051] In Figure 5A the embodiment of Figure 6A , referring to the block 606 of Figures 1 - 3 , a heterostructure 506 is formed above the undoped AlGaN layer 505. The heterostructure 506 includes a Si-doped AlGaN layer 507, a plurality of (Al)GaN (AlGaN and / or GaN) quantum wells 509, a Mg-doped AlGaN layer 204, and a p-doped contact layer 511 (e.g., a p-doped GaN or AlGaN contact layer). As described above (in combination with Figures 1 - 3 ), the Mg-doped AlGaN layer 204 is formed by the following steps: depositing Mg, Al, Ga, and N in the layer using MBE, where an excess of Ga is generated during this deposition. The excess Ga forms a liquid Ga layer on the Mg-doped AlGaN layer during the epitaxy of the Mg-doped AlGaN layer.

[0052] In one embodiment, the heterostructure 506 consists of a 250-nm-thick Si-doped Al 0.7 Ga 0.3 N layer, a plurality of Al 0.45 Ga 0.55 N / Al0.7 Ga 0.7 N quantum well, a 60 nm thick Mg-doped AlGaN layer, and a 3 nm p-GaN contact layer. In one embodiment, the front (lower) half (e.g., the first 30 nm) of the Mg-doped AlGaN layer is graded from about 70% to 50% Al composition to maximize holes injected into the active region using polarization-induced doping, and the second (upper) half (e.g., the second 30 nm) of the layer is Mg-doped Al 0.5 Ga 0.5 N. In one embodiment, an AlGaN electron blocking layer (not shown) is also incorporated to reduce electron spillage.

[0053] In one embodiment, the device active region is calibrated to emit at approximately 280 nm.

[0054] Continuing reference Figure 5A and Figure 6A , in block 608, a metal (ohmic) contact 513 is formed on the p-doped (Al)GaN contact layer 511. In one embodiment, the metal contact 513 includes nickel (Ni), Al, and / or gold (Au); however, the present invention is not limited thereto.

[0055] In block 610, the portions of the heterostructure 506 not covered by the metal contact 513 (e.g., portions of the p-doped (Al)GaN contact layer 511, the Mg-doped AlGaN layer 204, and the (AI)GaN quantum well 509) are removed. For other types of devices, other layers of the heterostructure 506 may be removed.

[0056] In block 612, a metal layer is deposited at the location where the p-doped (AI)GaN contact layer 511, the Mg-doped AIGaN layer 204, and the quantum well 509 were removed to form a contact 515 on the Si-doped AIGaN layer 507. In one embodiment, the metal layer includes titanium (Ti); however, the present invention is not limited thereto.

[0057] In an embodiment, the manufacturing process of an ultraviolet (UV) light-emitting diode (LED) includes using lithography, dry etching, and contact metallization techniques. The area of the device mesa is 50 μm × 50 μm. A Ti(40 nm) / Al(120 nm) / Ni(40 nm) / Au(50 nm) metal stack is deposited on the n-AlGaN and annealed at 750 °C for 30 seconds in a nitrogen environment to form an n-metal contact. A Ni(20 nm) / Al(100 nm) / Au(20 nm) metal stack is deposited on the p-AlGaN and annealed at 500 °C for 5 minutes in air to form a p-metal contact.

[0058] Now referring Figure 5BIn the embodiment, reference is also made to Figure 6B In block 614, a Mg-doped AlGaN layer 204 is formed on the undoped AlGaN layer 505. As described above (in connection with Figures 1 - 3 ), the Mg-doped AlGaN layer 204 is formed by depositing Mg, Al, Ga, and N in the layer using MBE, and an excess of Ga is generated during the deposition. During the epitaxy of the Mg-doped AlGaN layer 204, the excess Ga forms a liquid Ga layer on the Mg-doped AlGaN layer.

[0059] In block 616, a p-doped contact layer 511 (e.g., a p-doped GaN or AlGaN contact layer) is formed on the Mg-doped AlGaN layer 204.

[0060] In block 618, a metal (ohmic) contact 513 is formed on the p-doped (Al)GaN contact layer 511.

[0061] In block 620, a contact 515 is formed on the Mg-doped AlGaN layer 204.

[0062] More specifically, in the embodiment, for the Schottky diode 510 fabricated on the Mg-doped AlGaN layer 204, a metal stack composed of Ni (20 nm) / Al (100 nm) / Au (20 nm) is first deposited for the ohmic contact 513 and annealed in air at 500 °C for five minutes. A low-power plasma etch with BCl3 / Cl2 chemistry is used to remove the p-doped (Al)GaN layer 511 outside the Ni / Al / Au metal contact 513. Then, a 200-nm-thick Ti layer is deposited for the Schottky contact 515. In one embodiment, the area of the diode is 500 μm × 500 μm.

[0063] Figure 7A Shows the relationship between the Mg atomic concentration and the depth obtained by SIMS measurement of Mg-doped Al 0.75 Ga 0.25 N grown using the metal-semiconductor junction-assisted epitaxy method and the conventional epitaxy method. Figure 7A Shows the Mg atomic concentration distributions of two samples, where different Mg-doped AlGaN layers are separated by undoped layers. The first sample is grown using metal-semiconductor junction-assisted epitaxy under Ga-rich conditions to ensure that the metal Ga completely covers the substrate surface during the growth process. For the corresponding layers, the second sample is grown using the conventional growth mode under near stoichiometry but the same conditions (e.g., the same growth rate and the same Mg flux). Subsequently, the Mg concentration is obtained using SIMS measurement. It can be seen that by using metal-semiconductor junction-assisted epitaxy, the Mg concentration is significantly higher than that grown using the conventional growth method.

[0064] Figure 7B shows the relationship between the Mg concentration and the Mg flux of Mg-doped Al grown using metal-semiconductor junction-assisted epitaxy and using conventional epitaxy. For both samples, the Mg concentration increases with the increase in the Mg beam equivalent pressure (BEP). However, compared with the sample grown using the conventional process ("Sample B"), the Mg atomic density of the sample grown using metal-semiconductor junction-assisted epitaxy ("Sample A") is one order of magnitude higher. The maximum Mg incorporation achieved in Sample A is approximately 2×10 0.75 Ga 0.25 Mg atoms cm 20 Mg atoms cm -3 −3, and no signs of saturation are shown. By fixing the Fermi level away from the valence band edge using a metal-semiconductor junction at the growth front, the formation energy of Al(Ga)-substituted Mg incorporation is reduced, thus greatly enhancing Mg incorporation.

[0065] In addition, compared with metalorganic chemical vapor deposition (MOCVD), using metal-semiconductor junction-assisted epitaxy significantly reduces the carbon (C) impurity concentration. For the sample grown using metal-semiconductor junction-assisted epitaxy, the carbon concentration is limited by the measurement background of SIMS (about 1×10 16 cm -3 −3). For comparison, carbon concentrations of about 5×10 16 cm -3 −3 to 2×10 18 cm -3 −3 are typically measured in Al-rich AlGaN grown by MOCVD.

[0066] Fixing the Fermi level at the growth front by metal-semiconductor junction-assisted epitaxy also results in a significant reduction in point defect formation, which explains the observation that the resistivity of the undoped (Al)GaN layer grown under Ga-rich conditions is approximately three orders of magnitude higher than that of the thin film grown under conventional conditions.

[0067] Figure 7C shows the relationship between the incorporated Mg concentration and the Mg flux for samples grown with three different Al compositions (60%, 75%, and 85%). These samples are grown using metal-semiconductor junction-assisted epitaxy to maximize Mg incorporation. For the same Mg flux used, as the Al content in the alloy increases, the Mg incorporation significantly decreases.

[0068] Figure 7D shows Mg-doped Al grown using metal-semiconductor junction-assisted epitaxy 0.75 Ga 0.25Photoluminescence spectra of N samples, where the band-edge peak and Mg-acceptor peak are as indicated by the arrows in the figure. A strong peak was measured near the band edge at about 255 nm (4.86 eV), and at the same time, there was a Mg-acceptor related transition at about 298 nm (4.16 eV). This emission originated from the donor-acceptor pair transitions in the Mg-doped AlGaN epitaxial layer. The broad linewidth of the Mg-acceptor related transition and its partial overlap with the band-edge luminescence emission indicate that the distribution of Mg-acceptor energy levels is very large and almost extends to the valence-band edge of AlGaN.

[0069] Therefore, significantly enhanced Mg doping can lead to the formation of impurity bands for hole transition conduction, but more importantly, it results in a significant reduction in the activation energy of a portion of the Mg dopants, thus making it possible to have a large hole carrier concentration in AlGaN at room temperature.

[0070] Figure 8A , 8B, 8C, and 8D show the room-temperature Hall measurement data of the Mg-doped AlGaN epitaxial layer plotted against the Al composition, showing the free hole concentration, hole mobility, resistivity, and resistance coefficient of the Mg-doped AlGaN layer in the examples according to the present invention (labeled "this work" in Figure 8D .

[0071] Metal-semiconductor junction-assisted epitaxy was used to grow a series of Mg-doped samples with Al compositions varying from about 75% to about 90%. Atomic force microscopy measurements showed that the smooth surface roughness of all samples was less than 1 nm. Hall measurements were performed on the samples using the van der Pauw method to determine the hole concentration, hole mobility, and resistivity of the AlGaN layer at temperatures ranging from room temperature to 500 °C. As Figure 8A shown, it was observed that the room-temperature hole concentration decreased monotonically with increasing Al content.

[0072] Figure 8B shows the variation of hole mobility with Al composition. The decrease in hole concentration and mobility with increasing Al composition can be explained by the reduced Mg incorporation in alloys with higher Al mole fractions due to the lower solubility of Mg and the increased formation enthalpy of Al substituting for Mg. Even for the incorporated Mg atoms, the increase in the activation energy of the Mg acceptor with increasing Al content further reduces the free hole concentration. The phenomenon of decreasing Mg concentration with increasing Al composition has been further confirmed by SIMS measurements on AlGaN samples with different alloy compositions but the same Mg BEP. These factors lead to an increase in the resistivity of the Mg-doped AlGaN layer with increasing Al composition, as Figure 8C shown. However, even for Al 0.9 Ga 0.1 N, the measured resistivity was still lower than 5 Ω·cm.

[0073] In Figure 8D plots of resistivity values of some previously reported Mg-doped AlGaN layers versus Al composition are presented, along with the resistivity of AlGaN layers obtained by metal-semiconductor junction assisted epitaxy. It can be seen that the resistivity of Mg-doped AlGaN grown by metal-semiconductor junction assisted epitaxy is nearly 1 - 3 orders of magnitude lower compared to the previously reported results.

[0074] Figure 9A , Figures 9B and 9C show the temperature-dependent Hall measurement results of Mg-doped AlGaN epitaxial layers grown by metal-semiconductor junction assisted epitaxy, where the Al content is between 75% and 90%, and the hole concentration, hole mobility, and resistivity are plotted respectively against the reciprocal of temperature.

[0075] Refer to Figure 9A , a low activation energy (about 10 - 20 meV) is found in the sample at temperatures below 600K, which is characteristic of hole hopping conduction in the impurity band and can also partly explain this, as a part of the Mg dopants have a significantly reduced activation energy, as demonstrated by the photoluminescence spectra shown by Figure 7D . At higher temperatures, more Mg dopants are activated, thus generating holes in the valence band. This results in a sharp increase in the hole concentration at high temperatures (>650K), which is characterized by a large activation energy (about 300 - 400 meV). However, this activation energy value is slightly lower than the theoretically expected value for AlGaN alloys with an Al composition of about 75 - 90%. This can be explained by the band-tail effect and the significantly broadened acceptor level distribution, which effectively reduces the activation energy of a part of the Mg acceptors.

[0076] As Figure 9B shown, the measured hole mobility has a monotonically decreasing trend with increasing temperature, as expected due to the increase in phonon scattering.

[0077] As Figure 9C shown, the resistivity is first observed to increase between temperatures of 300K and approximately 650K, due to the decrease in hole mobility and the relatively small change in hole concentration. At higher temperatures, when the Mg acceptors are thermally activated, the sharp increase in hole concentration leads to the observed decrease in resistivity.

[0078] Figure 10A shows the electroluminescence spectrum of a UV LED grown by metal-semiconductor junction assisted epitaxy. Figure 10B shows the current-voltage (I-V) characteristics of LEDs grown by metal-semiconductor junction assisted epitaxy and conventional epitaxy. Figure 10CShows the relationship between the external quantum efficiency (EQE) and the current density of these devices.

[0079] Metal-semiconductor junction-assisted epitaxy significantly reduces the resistivity of Mg-doped AlGaN, which is very important for improving the efficiency of optoelectronic devices operating at mid-ultraviolet and deep-ultraviolet wavelengths. The device characteristics of AIGaN UV LEDs grown using metal-semiconductor junction-assisted epitaxy emitting at approximately 280 nm were compared with those of the same LEDs grown using conventional epitaxy.

[0080] Figure 10A Shows a typical electroluminescence spectrum with a narrow linewidth of about 11 nm. The current-voltage characteristics are shown in Figure 10B which shows an onset voltage of approximately 7 volts for LEDs grown using metal-semiconductor junction-assisted epitaxy, while for conventional devices, an onset voltage of approximately 9 volts can be seen. Ineffective Mg doping may result in a higher onset voltage when using conventional epitaxy. The EQE can be measured directly on the wafer without any packaging, substrate removal, or cooling.

[0081] As Figure 10C shown, the maximum on-wafer EQE measured for devices grown using metal-semiconductor junction-assisted epitaxy at room temperature is approximately 3%, which is significantly better than that of devices grown using traditional epitaxy processes, with a maximum EQE of approximately 0.6%. The performance of this device is also better than that of other previously reported AIGaN UV LEDs grown by MBE near this wavelength. The improved device characteristics seen in samples grown using metal-semiconductor junction-assisted epitaxy highlight the importance of effective p-type conductivity for device performance. It is also worth noting that the measured EQE can be significantly increased by appropriate device packaging to improve light extraction efficiency and by adopting more comprehensive techniques to capture all emitted light (such as using an integrating sphere).

[0082] In summary, by using metal-semiconductor junction-assisted epitaxy to regulate the surface Fermi level, effective p-type conductivity of aluminum-rich AlGaN, which was previously unattainable, can be achieved. The presence of a metal-semiconductor surface at the growth front moves the Fermi level away from the valence band edge, which can significantly enhance the incorporation of AI(Ga) substituting for Mg doping and further reduce the formation of compensating point defects. The presence of surface states is strongly affected by the growth conditions, as previously described for polar and nonpolar surfaces, which can further move the Fermi level away from the valence band edge, although the surface state density structure during the temperature ramp required for crystal growth is still unknown. Therefore, a high concentration of Mg acceptors can be incorporated into aluminum-rich AIGaN, thus forming a Mg impurity band. Aluminum-rich AIGaN epitaxial layers have been measured with resistivity values below 1 Ω·cm for Al 0.75 Ga 0.25 N, for Al0.9 Ga 0.1 The N is about 4 Ω·cm, which is crucial for realizing efficient mid-UV and deep-UV optoelectronic devices. Compared with the devices using conventional epitaxial growth, the deep-UV LEDs using metal-semiconductor junction-assisted epitaxial growth show a great improvement in external quantum efficiency and lower turn-on voltage. The metal-semiconductor junction-assisted epitaxy can be further extended to the epitaxy / synthesis of various semiconductor nanostructures and heterostructures to achieve controlled doping incorporation and fundamentally improve their structural, electronic, and optical properties.

[0083] For purposes of illustration and description, the foregoing description of specific embodiments of the invention has been given. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications and variations are possible in light of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

Claims

1. A method of manufacturing a device, the method comprising: Depositing magnesium, aluminum, gallium, and nitrogen as a magnesium-doped aluminum gallium nitride epitaxial layer over a substrate using molecular beam epitaxy; And During the molecular beam epitaxy, forming a gallium liquid metal layer at a growth interface of the magnesium-doped aluminum gallium nitride epitaxial layer, wherein the gallium liquid metal layer is smooth and conformal over the magnesium-doped aluminum gallium nitride epitaxial layer throughout the forming process, and wherein while reducing the formation of compensation defects, the gallium liquid metal layer pins the Fermi level away from the valence band at the growth interface, thereby enhancing the incorporation of magnesium and aluminum dopants in the magnesium-doped aluminum gallium nitride epitaxial layer.

2. The method according to claim 1 further comprises: During the epitaxy of the magnesium-doped aluminum gallium nitride layer, controlling the aluminum composition by adjusting the flux of aluminum and the growth rate by adjusting the nitrogen flow rate, and allowing an excess of gallium to form a liquid metal layer on the surface of the magnesium-doped aluminum gallium nitride layer.

3. The method according to claim 1, wherein, The magnesium-doped aluminum gallium nitride layer comprises a part of a light-emitting diode heterostructure, wherein the light-emitting diode heterostructure further comprises a silicon-doped aluminum gallium nitride layer, a plurality of aluminum gallium nitride or gallium nitride quantum wells, and a p-doped contact layer.

4. The method according to claim 1 further comprises: Before the epitaxy of the magnesium-doped aluminum gallium nitride layer: Forming one or more layers over the substrate, wherein the one or more layers are selected from the group consisting of: an undoped aluminum nitride buffer layer; an undoped gallium nitride layer; a doped gallium nitride layer; an undoped aluminum gallium nitride layer; and a doped aluminum gallium nitride layer.

5. The method according to claim 1, wherein, The molecular beam epitaxy is selected from the group consisting of: plasma-assisted molecular beam epitaxy; and ammonia molecular beam epitaxy.

6. A method of manufacturing a device, comprising: Epitaxially depositing a magnesium-doped aluminum gallium nitride epitaxial layer assisted by a metal-semiconductor junction over a substrate layer; And During the metal-semiconductor junction-assisted epitaxy, depositing a gallium liquid metal layer, wherein the gallium liquid metal layer is smooth and conformal over a first layer throughout the metal-semiconductor junction-assisted epitaxial deposition of the magnesium-doped aluminum gallium nitride epitaxial layer, and wherein while reducing the formation of defects, the gallium liquid metal layer pins the Fermi level away from the valence band at the growth interface between the gallium liquid metal layer and the magnesium-doped aluminum gallium nitride epitaxial layer, thereby enhancing the incorporation of magnesium and aluminum dopants in the magnesium-doped aluminum gallium nitride epitaxial layer.

7. The method of manufacturing a device according to claim 6, wherein, The magnesium-doped aluminum gallium nitride layer comprises a part of a heterostructure, wherein the heterostructure further comprises a silicon-doped aluminum gallium nitride layer, a plurality of aluminum gallium nitride or gallium nitride quantum wells, and a p-doped contact layer.

8. The method of manufacturing a device according to claim 7, further comprising one or more layers between the substrate layer and the heterostructure, wherein the one or more layers are selected from the group consisting of: an undoped aluminum nitride buffer layer; an undoped gallium nitride layer; and an undoped aluminum gallium nitride layer.

9. A method of manufacturing a device, the method comprising: Forming a buffer layer on a substrate; Forming an undoped aluminum gallium nitride layer over the buffer layer; And One or more layers are formed over the undoped aluminum gallium nitride layer, the one or more layers including a magnesium-doped aluminum gallium nitride layer and a p-doped contact layer, wherein the magnesium-doped aluminum gallium nitride layer is formed by: Depositing magnesium, aluminum, gallium, and nitrogen in a layer using molecular beam epitaxy, wherein during the molecular beam epitaxy, an excess of gallium is provided during the deposition and the excess gallium forms a liquid gallium layer on the magnesium-doped aluminum gallium nitride layer, wherein the liquid gallium layer is smooth and conformal on the magnesium-doped aluminum gallium nitride layer during the molecular beam epitaxy, and wherein while reducing the formation of defects, the liquid gallium layer pins the Fermi level away from the valence band at the growth interface between the liquid gallium layer and the magnesium-doped aluminum gallium nitride epitaxial layer, thereby enhancing the incorporation of magnesium and aluminum dopants in the magnesium-doped aluminum gallium nitride epitaxial layer.

10. The method according to claim 9 further comprises: During the epitaxy of the magnesium-doped aluminum gallium nitride layer, the aluminum composition is controlled by adjusting the aluminum flux and the growth rate is adjusted with the nitrogen flow rate, and an excess of gallium is allowed to form a liquid metal layer on the surface of the magnesium-doped aluminum gallium nitride layer.

11. The method according to claim 9, wherein, The molecular beam epitaxy is selected from the group consisting of: plasma-assisted molecular beam epitaxy; and ammonia molecular beam epitaxy.

12. The method according to claim 9, wherein The magnesium-doped aluminum gallium nitride layer includes a part of a light-emitting diode heterostructure, wherein the light-emitting diode heterostructure further includes a silicon-doped aluminum gallium nitride layer and a plurality of aluminum gallium nitride or gallium nitride quantum wells.

13. The method according to claim 12 further comprises: A metal contact is formed on the silicon-doped aluminum gallium nitride layer.

14. The method according to claim 9 further comprises: A metal contact is formed on the magnesium-doped aluminum gallium nitride layer.

15. The method according to claim 9 further comprises: A metal contact is formed on the p-doped contact layer.

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