Gallium nitride material epitaxial structure for infrared spectrum test under in-situ electric field and test method thereof

Abstract

The application discloses a gallium nitride material epitaxial structure for in-situ infrared spectrum test under electric field and a test method thereof, and belongs to the technical field of semiconductors. The gallium nitride material epitaxial structure comprises a substrate, a stress control layer, an n-type gallium nitride layer and a p-type gallium nitride layer, wherein the p-type gallium nitride layer is an alternatingly-doped superlattice p-type gallium nitride or a gradually-doped p-type gallium nitride; a test electrode with an inner circle-outer ring metal pattern is arranged on the surface of the gallium nitride material epitaxial structure, and the inner circle electrode and the outer ring electrode form ohmic contacts with the p-type gallium nitride layer and the n-type gallium nitride respectively. The gallium nitride material epitaxial structure breaks through the limitation of the p-type gallium nitride layer bearing voltage in the traditional epitaxial structure, overcomes the blocking effect of the traditional metal electrode on infrared light, and is designed by modifying the sample seat of an infrared reflectance spectrometer, so that the infrared reflectance spectrometer can truly have the ability of testing the infrared spectrum of the p-type gallium nitride under in-situ electric field while applying pressure to the sample, and technical support is provided for the dynamic evolution research of material defects.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor technology, specifically relating to a gallium nitride epitaxial structure for in-situ electric field infrared spectroscopy testing and its testing method. Background Technology

[0002] Gallium nitride (GaN) materials possess advantages such as a large bandgap, high breakdown field strength, high electron saturation drift velocity, and high temperature and radiation resistance, making them widely used in high-power, high-frequency electronic devices. GaN-based high electron mobility transistors (HEMTs) play a prominent role in power electronics and radio frequency electronics due to the unique ultra-high mobility of their two-dimensional electron gas at heterojunction interfaces. Among these, p-type GaN gate HEMTs are the most widely adopted normally-off HEMT devices, and the evolution of the magnesium (Mg) acceptor dopant in the p-type GaN gate under an electric field is particularly important.

[0003] In p-type gallium nitride, Mg impurities mainly exist as acceptor Mg. Ga The Mg atoms exist in the form of Ga sites, carrying a negative charge. Under the influence of an electric field, especially at a certain temperature, they can diffuse around donor defects (which are positively charged), forming new defect complexes through Coulomb interactions. This "defect reaction" not only significantly reduces the concentration of effective acceptors, thereby reducing the hole concentration in p-type gallium nitride, but also introduces new defect energy levels, leading to threshold voltage drift and dynamic resistance degradation in HEMT devices. In severe cases, it can even cause direct failure, making it impossible to achieve normally-off operation.

[0004] Fourier transform infrared (FTIR) spectroscopy is an effective technique for characterizing the lattice occupancy and atomic structure of point defects in materials. However, most current infrared spectroscopic characterizations of impurity defects remain at a static level, meaning that in-situ infrared spectroscopy under an electric field cannot be achieved. This is limited by the difficulty in designing in-situ electric field devices, and also because light is difficult to penetrate metals, with most infrared light being absorbed by the metal electrodes on the sample surface. Furthermore, for Mg impurities in p-type gallium nitride, due to their conductivity, only a small portion of the voltage applies to this layer, with the majority falling into the underlying transition layer. This means that only a small portion of the applied vertical electric field can be applied to the structure of interest, significantly increasing the difficulty of characterization and making it impossible to overcome the impurity migration barrier. Summary of the Invention

[0005] In order to solve and overcome the above-mentioned problems in the existing technology, the purpose of this invention is to provide a gallium nitride epitaxial structure and testing method that can be used for infrared reflection spectrum testing under an in-situ electric field.

[0006] To achieve infrared reflectance spectroscopy testing of p-type gallium nitride under an in-situ electric field, this invention first provides a gallium nitride epitaxial structure with high Mg impurity concentration and capable of withstanding a higher proportion of applied voltage. (See [link to relevant documentation]). Figure 1 It includes a substrate, and a stress control layer, a Si-doped n-type gallium nitride layer, and a Mg-doped p-type gallium nitride layer located on the substrate, wherein the p-type gallium nitride layer is an alternately doped superlattice p-type gallium nitride or a graded-doped p-type gallium nitride; a test electrode is disposed on the surface of the epitaxial wafer, the test electrode is an inner circle-outer ring type metal pattern electrode, consisting of an inner circle electrode located at the center and an outer ring electrode concentric with it, wherein the inner circle electrode is located on the p-type gallium nitride layer and forms an ohmic contact with it, and the outer ring electrode is located on the n-type gallium nitride layer and forms an ohmic contact with it.

[0007] like Figure 2 As shown, the method for preparing the gallium nitride epitaxial structure includes the following steps:

[0008] 1) Select commonly used substrates such as silicon, sapphire, or silicon carbide;

[0009] 2) Epitaxially grow a stress control layer on the selected substrate;

[0010] 3) Epitaxially layer a silicon-doped n-type gallium nitride layer;

[0011] 4) Grow alternating doped superlattice p-type gallium nitride layers or graded doped p-type gallium nitride layers on silicon-doped n-type gallium nitride layers;

[0012] 5) Prepare a test electrode with an inner circle-outer ring metal pattern on the epitaxial wafer obtained in step 4).

[0013] Furthermore, in step 2) above, a nucleation layer and a stress control layer are typically grown sequentially. The nucleation layer and the stress control layer can be AlN, GaN, InN or their ternary compounds, or their quaternary compounds, or a combination of the above materials.

[0014] Furthermore, the method for growing the alternating doped superlattice p-type gallium nitride structure in step 4) above can be as follows: first, epitaxially grow a layer of weakly p-type gallium nitride (thickness range 200~500 nm, doped with a low concentration of Mg, concentration range 1×10⁻⁶). 17 / cm 3 ~5×10 17 / cm 3 Then, an epitaxial layer of strong p-type gallium nitride (thickness range 20~50 nm, doped with a high concentration of Mg, concentration range 7×10⁻⁶) is added. 18 / cm 3 ~ 3×10 19 / cm 3A set of units is formed, and this set of units is repeated 5-10 times to finally obtain a superlattice p-type gallium nitride material epitaxial structure that has both a high Mg acceptor concentration (multiple sets of thin strong p-type gallium nitride layers) and the ability to withstand a large proportion of applied voltage (multiple sets of thick weak p-type gallium nitride layers).

[0015] The method for growing the graded-doped p-type gallium nitride structure in step 4) above can be: first, epitaxially grow a p-type gallium nitride structure. -- Gallium nitride (thickness range 300~700 nm, doped with low concentration of Mg, concentration range 1×10⁻⁶) 16 / cm 3 ~ 5×10 16 / cm 3 ), and then extend another layer of p - Gallium nitride (thickness range 200~500 nm, doped with low concentration of Mg, concentration range 1×10⁻⁶) 17 / cm 3 ~ 5×10 17 / cm 3 Then, an epitaxial layer of p-type gallium nitride (thickness range 100~300 nm, doped with Mg of general concentration, concentration range 1×10⁻⁶) is added. 18 / cm 3 ~ 5×10 18 / cm 3 Finally, an outer layer p + Gallium nitride (thickness range 30~70 nm, doped with a high concentration of Mg, concentration range 8×10⁻⁶) 18 / cm 3 ~ 3×10 19 / cm 3 Ultimately, a graded-doped p-type gallium nitride epitaxial structure was obtained, which has both a high Mg acceptor concentration and can withstand a large proportion of applied voltage.

[0016] After the gallium nitride epitaxial wafer is fabricated, an electrode structure for in-situ electrified infrared reflectance spectroscopy testing is prepared. This test electrode is an inner circle-outer ring type metal pattern electrode, including a central "inner circle" portion and a concentric "outer ring" portion. The radius of the "inner circle" portion can be 3~5 mm, the inner circle radius r of the "outer ring" portion can be 7~8 mm, the outer circle radius R can be 10~12 mm, and the ring width d = Rr. The fabrication process of this test electrode includes the following steps ( Figure 3 ):

[0017] a) Perform photolithography on the cleaned epitaxial wafer. The photolithographic pattern is an "inner circle". Through the process of coating, developing and exposing, photoresist is covered in the inner circle area for etching protection, while other areas are exposed.

[0018] b) Perform ICP etching on the above epitaxial wafer. Since the "inner circle" area is protected by photoresist, the actual etching area is other areas, ensuring that the etching depth exceeds the p-type gallium nitride layer and reaches the n-type gallium nitride layer.

[0019] c) After removing the photoresist with chemical reagents and cleaning, perform the second photolithography step. The photolithographic pattern is a square slightly smaller than the epitaxial wafer. Cover the square area with photoresist for etching protection.

[0020] d) ICP etching of the above epitaxial wafers, so that the epitaxial wafers in other areas outside the square pattern are etched into the stress control layer, in order to achieve current path isolation after power-on. After etching, the photoresist is chemically cleaned.

[0021] e) Use a mask to cover the area outside the "inner circle" with photoresist to block the metal;

[0022] f) Grow p-type electrodes using electron beam evaporation or magnetron sputtering and then perform appropriate annealing processes to make them form good ohmic contact with the p-type gallium nitride layer;

[0023] g) Strip and chemically clean to remove excess photoresist and metal;

[0024] h) Perform the third step of photolithography on the epitaxial wafer. The photolithographic pattern is an "outer ring". Cover the area outside the "outer ring" with photoresist to block the metal.

[0025] i) An n-type electrode is grown in the “outer ring” region by electron beam evaporation or magnetron sputtering and then subjected to a corresponding annealing process to form a good ohmic contact with the n-type gallium nitride layer;

[0026] j) Strip and chemically clean to remove excess photoresist and metal.

[0027] Based on the above-mentioned gallium nitride epitaxial structure, when performing infrared spectroscopy testing on p-type gallium nitride under an in-situ electric field, the gallium nitride epitaxial structure sample is placed on the electrical sample holder of the infrared spectrometer sample rod, so that the inner circular electrode and the outer ring electrode are electrically connected to the external voltage source. The built-in light source of the infrared spectrometer illuminates the sample, while the external voltage source controls the vertical electric field applied to the sample, thereby realizing in-situ electric infrared reflectance spectrum testing.

[0028] During testing, when a vertical voltage is applied, one electrode of the external voltage source is connected to the inner circular electrode, and the other electrode is connected to the outer ring electrode. The voltage levels of the two electrodes are determined as needed. For example, if it is desired to generate an electric field from the p-type layer towards the substrate, the inner circular electrode is connected to a high voltage, and the outer ring electrode to a low voltage. Specifically, the epitaxial wafer is placed on the electrical sample holder, and the outer ring electrode and inner circular electrode of the epitaxial wafer are electrically connected to the first and second electrical plates on the electrical sample holder, respectively. Then, the two electrical plugs on the infrared spectrometer sample rod are inserted into the sockets of the first and second electrical plates, respectively, to establish an electrical connection between the infrared spectrometer sample rod and the sample under test through the electrical plates. The positive and negative terminals of the external voltage source are connected through the electrical sockets on the infrared spectrometer sample rod, thereby establishing an electrical connection between the epitaxial wafer and the external voltage source, completing the sample preparation step for in-situ electrically applied infrared reflectance spectroscopy testing.

[0029] The testing principle is as follows: According to Ohm's law, when a voltage falls across a series resistor, the higher the resistance, the higher the voltage carried. The p-type gallium nitride epitaxial structure provided by this invention effectively inserts a sufficiently thick, high-resistance (low-conductivity) weak p-type layer, maintaining a certain Mg acceptor concentration while ensuring a certain voltage drop. Simultaneously, since infrared light cannot penetrate the metal electrodes used for applying current to the material surface, the inner circle-outer ring metal pattern scheme provided by this invention effectively leaves a large area for infrared light contact. Furthermore, due to the principle of conductor equipotentiality, the strong p-type gallium nitride layer on the surface achieves a uniform electric field distribution, ensuring that the applied electric field also exists within the electrode-free region between the inner circle and outer ring.

[0030] The beneficial effects of this invention are:

[0031] This invention provides a testing method for in-situ infrared reflectance spectroscopy testing under an electric field, specifically including an epitaxial structure, electrode design, and testing apparatus. The substrate can be any of the currently available conventional epitaxial gallium nitride substrates; different substrate choices can simplify the upper stress-controlled transition layer structure to varying degrees.

[0032] This invention overcomes the limitations of voltage carrying capacity in traditional p-type gallium nitride (GaN) layers in epitaxial structures and also overcomes the blocking effect of traditional metal electrodes on infrared light. It truly possesses the capability for in-situ electric field infrared spectral characterization of p-type GaN, providing a solid foundation for research on the evolution of Mg impurities under external electric fields, their interaction mechanisms with other defects, and diffusion mechanisms. It also offers important guidance for future improvements in the threshold voltage stability of normally-off GaN-based HEMTs and reduction of dynamic on-resistance. The in-situ electric infrared reflectance spectroscopy testing method in this invention is compatible with vacuum and liquid helium cryogenic conditions, surpassing the testing capabilities of traditional infrared spectroscopy and providing advanced technical support for the study of the dynamic evolution of defects in solid materials. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of a gallium nitride epitaxial structure for in-situ infrared reflection spectrum testing under an electric field according to the present invention; wherein, (a) is a top view and (b) is a front view; 1 is a substrate, 2 is a stress control layer, 3 is an n-type gallium nitride layer, 4 is a p-type gallium nitride layer, 5 is an outer ring electrode, and 6 is an inner circular electrode.

[0034] Figure 2 This is a flowchart illustrating the overall implementation scheme of a gallium nitride epitaxial structure for in-situ infrared reflectance spectroscopy testing under an electric field, as described in this invention.

[0035] Figure 3 The diagram shows the structural steps of preparing the inner circle-outer ring type metal pattern test electrode according to the present invention. In each step diagram, the top is the top view and the bottom is the front view.

[0036] Figure 4 This is a flowchart illustrating the preparation of a gallium nitride epitaxial structure for in-situ infrared reflectance spectroscopy testing under an electric field, as described in Embodiment 1 of the present invention.

[0037] Figure 5 This is a flowchart illustrating the preparation of a gallium nitride epitaxial structure for in-situ infrared reflectance spectroscopy testing under an in-situ electric field, as described in Embodiment 2 of the present invention.

[0038] Figure 6 This is a schematic diagram of the device for in-situ electrically applied infrared reflectance spectroscopy testing according to the present invention, wherein: 5 is the outer ring electrode, 6 is the inner circular electrode, 7 is the electrical sample holder, 8 is the first electrical plate, 9 is the metal area on the first electrical plate, 10 is the socket of the first electrical plate, 11 is the metal area on the second electrical plate, 12 is the socket of the second electrical plate, 13 is the infrared spectrometer sample rod, 14 is the electrical socket of the infrared spectrometer sample rod (for connecting to an external power supply), 15 is the vertically incident infrared light, 16 is the light-transmitting area between the inner and outer ring electrodes, and 17 is the epitaxial wafer; A and B are the electrical plugs of the infrared spectrometer sample rod (for connecting the sample rod and the electrical plate). Detailed Implementation

[0039] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments, but this does not limit the scope of the present invention in any way.

[0040] Example 1

[0041] Please see Figure 4 The process of achieving the target on a silicon substrate using an alternating doped superlattice p-type gallium nitride structure combined with patterned electrodes includes the following steps:

[0042] Step 1: Select a silicon substrate. Specifically, the selected silicon substrate is a commonly used p-type silicon substrate with a thickness of 3 mm and a square shape with a side length of 2 cm.

[0043] Step 2: Deposit an AlN / AlGaN stress control layer. Specifically, the stress control layer is deposited using MOCVD. This superlattice structure consists of 40 groups with a total thickness of 300 nanometers, and the Al composition in AlGaN is 70%.

[0044] Step 3: Epitaxial n-type GaN layer, specifically, the epitaxial method is MOCVD, the dopant is silicon, and the doping concentration is 5 × 10⁻⁶. 17 / cm 3 The thickness is 500 nanometers;

[0045] Step 4, extension p - The GaN layer is specifically epitaxially formed by MOCVD, with magnesium as the dopant and a doping concentration of 1×10⁻⁶. 17 / cm 3 The thickness is 200 nanometers;

[0046] Step 5: Epitaxially grow a p-type GaN layer. Specifically, the epitaxial method is MOCVD, the dopant is magnesium, and the doping concentration is 1×10⁻⁶. 19 / cm 3 The thickness is 30 nanometers;

[0047] Step 6: Repeat steps 4 and 5 alternately for a total of 5 sets;

[0048] Step 7: Remove the surface oxide layer of the heterostructure from the epitaxial wafer by inorganic cleaning, and then remove organic contaminants by organic cleaning; the inorganic cleaning can be soaking in hydrochloric acid for 5 min, and the organic cleaning can be ultrasonicating with acetone, ethanol and deionized water for 5 min each.

[0049] Step 8, refer to Figure 3 The fabrication of the metal patterned electrode is shown, including:

[0050] (a) Photolithography of the "inner circle" pattern: After uniformly spin-coating the positive adhesive Az6130, bake at 90°C for 90 seconds. Align the area to be photolithographically etched on the epitaxial wafer with the "inner circle" pattern photomask and perform photolithography. After exposure, remove the epitaxial wafer and place it in the developer. The development time can be 40 seconds. After development, bake at 120°C for 150 seconds.

[0051] (b) Etching the area outside the "inner circle": Use chlorine-based ICP to etch the area outside the "inner circle" pattern. The etching depth can be 1.2 μm to ensure that the n-type GaN layer is etched.

[0052] (c) After stripping the photoresist with an organic solvent, perform a second photolithography step with a square pattern slightly smaller than the epitaxial wafer. The side length of the square pattern can be 1 cm. The photolithography process is the same as in step (a).

[0053] (d) Etch the area outside the square pattern. The etching depth can be 2.5 μm to ensure that the stress control layer is etched.

[0054] (e) Use an "inner circle" photomask as a mask to cover the area outside the "inner circle" with photoresist to prevent metal deposition;

[0055] (f) Growth and annealing of p-type electrodes in the “inner circle”: depositing metal Ni / Au on the epitaxial wafer by electron beam evaporation, with a thickness of 10 nm / 50 nm, and annealing at 550 °C for 5 min in an oxygen atmosphere to achieve the formation of p-type ohmic contacts in the “inner circle” region.

[0056] (g) Remove excess photoresist and metal using organic solvents;

[0057] (h) Photolithography of the “outer ring” pattern: spin-coat the epitaxial wafer with negative photoresist, align the “outer ring” area of ​​the epitaxial wafer with the pattern on the photomask, perform photolithography, and after exposure, remove the epitaxial wafer and place it in the negative photoresist developer. The development time can be 15 s, and cover the area outside the “outer ring” with photoresist.

[0058] (i) Growing n-type electrodes in the “outer ring” and annealing: depositing metal Ti / Al / Ni / Au on the epitaxial wafer by electron beam evaporation, with a thickness of 30 nm / 175 nm / 35 nm / 60 nm, and annealing at 850 °C for 35 s in a nitrogen atmosphere to achieve the formation of n-type ohmic contacts in the “outer ring” region.

[0059] (j) Remove excess photoresist and metal using organic solvents.

[0060] Step 9: The epitaxial wafer 17 with inner circle-outer ring electrodes prepared in step 8 is attached to the electrical plate. Figure 6 The epitaxial wafer 17 is placed on the electrical sample holder 7 and placed between the first electrical board 8 and the second electrical board. The electrical sample holder 7 is then placed in a ball welder for wire bonding. One end of the aluminum wire is welded to the inner circular electrode 6 of the epitaxial wafer and the other end is welded to the metal area 11 on the second electrical board. Another aluminum wire is taken and one end is welded to the outer ring electrode 5 of the epitaxial wafer and the other end is welded to the metal area 9 on the first electrical board 8, so as to realize the electrical connection between the epitaxial wafer 17 and the electrical board.

[0061] Step 10, as follows Figure 6 As shown, the electrical plug A of the infrared spectrometer sample rod 13 is inserted into the socket 12 of the second electrical board, and the other electrical plug B is inserted into the socket 10 of the first electrical board, so as to realize the electrical connection between the infrared spectrometer sample rod 13 and the epitaxial wafer 17 through the electrical board.

[0062] Step 11: Connect the positive and negative terminals of the external voltage source through the electrical connector 14 on the sample rod 13 of the infrared spectrometer to realize the control of the vertical voltage of the gallium nitride epitaxial wafer between the inner circle and the outer ring. Specifically, the positive terminal of the voltage source is connected to the inner circle electrode 6 of the epitaxial wafer 17 through the electrical plug A of the sample rod, and the negative terminal of the voltage source is connected to the outer ring electrode 5 of the epitaxial wafer 17 through the electrical plug B of the sample rod.

[0063] Step 12: Place the prepared sample rod vertically in the infrared spectrometer, as shown. Figure 6 As shown, the built-in light source of the infrared spectrometer emits vertically incident infrared light 15 onto the epitaxial wafer, which excites the gallium nitride sample through the light-transmitting region 16 between the inner circle and outer ring electrodes. At the same time, an external voltage source controls the vertical electric field of the p-type gallium nitride epitaxial structure on the epitaxial wafer, realizing an in-situ electrically charged infrared reflection spectrum test experiment. This test method is compatible with vacuum and liquid helium cryogenic tests.

[0064] Example 2

[0065] Please see Figure 5 The process of achieving the target on a silicon substrate using a graded-doped p-type gallium nitride structure combined with patterned electrodes includes the following steps:

[0066] Step 1: Select a silicon substrate. Specifically, the selected silicon substrate is a commonly used p-type silicon substrate with a thickness of 3 mm and a square shape with a side length of 2 cm.

[0067] Step 2: Deposit an AlN / AlGaN stress control layer. Specifically, the stress control layer is deposited using MOCVD. This superlattice structure consists of 40 groups with a total thickness of 300 nanometers, and the Al composition in AlGaN is 70%.

[0068] Step 3: Epitaxial n-type GaN layer, specifically, the epitaxial method is MOCVD, the dopant is silicon, and the doping concentration is 5 × 10⁻⁶. 17 / cm 3 The thickness is 500 nanometers;

[0069] Step 4, extension p -- The GaN layer is specifically epitaxially formed by MOCVD, with magnesium as the dopant and a doping concentration of 1×10⁻⁶. 16 / cm 3 The thickness is 500 nanometers;

[0070] Step 5, extension p - The GaN layer is specifically epitaxially formed by MOCVD, with magnesium as the dopant and a doping concentration of 1×10⁻⁶. 17 / cm 3 The thickness is 300 nanometers;

[0071] Step 6: Epitaxially grow a p-type GaN layer. Specifically, the epitaxial method is MOCVD, the dopant is magnesium, and the doping concentration is 1×10⁻⁶. 1 / cm 3 The thickness is 100 nanometers;

[0072] Step 7, extension p + The GaN layer is specifically epitaxially formed by MOCVD, with magnesium as the dopant and a doping concentration of 1×10⁻⁶. 19 / cm 3 The thickness is 30 nanometers;

[0073] Step 8: Remove the surface oxide layer of the heterostructure from the epitaxial wafer by inorganic cleaning, and then remove organic contaminants by organic cleaning; the inorganic cleaning can be soaking in hydrochloric acid for 5 min, and the organic cleaning can be ultrasonicating with acetone, ethanol and deionized water for 5 min each.

[0074] Step 9, refer to Figure 3 The fabrication of the metal patterned electrode is shown, including:

[0075] (a) Photolithography of the "inner circle" pattern: After uniformly spin-coating the positive adhesive Az6130, bake at 90°C for 90 seconds. Align the area to be photolithographically etched on the epitaxial wafer with the "inner circle" pattern photomask and perform photolithography. After exposure, remove the epitaxial wafer and place it in the developer. The development time can be 40 seconds. After development, bake at 120°C for 150 seconds.

[0076] (b) Etching the area outside the "inner circle": Use chlorine-based ICP to etch the area outside the "inner circle" pattern. The etching depth can be 1.2 μm to ensure that the n-type GaN layer is etched.

[0077] (c) After stripping the photoresist with an organic solvent, perform a second photolithography step with a square pattern slightly smaller than the epitaxial wafer. The side length of the square pattern can be 1 cm. The photolithography process is the same as in step (a).

[0078] (d) Etch the area outside the square pattern. The etching depth can be 2.5 μm to ensure that the stress control layer is etched.

[0079] (e) Use an "inner circle" photomask as a mask to cover the area outside the "inner circle" with photoresist to prevent metal deposition;

[0080] (f) Growth and annealing of p-type electrodes in the “inner circle”: depositing metal Ni / Au on the epitaxial wafer by electron beam evaporation, with a thickness of 10 nm / 50 nm, and annealing at 550 °C for 5 min in an oxygen atmosphere to achieve the formation of p-type ohmic contacts in the “inner circle” region.

[0081] (g) Remove excess photoresist and metal using organic solvents;

[0082] (h) Photolithography of the “outer ring” pattern: spin-coat the epitaxial wafer with negative photoresist, align the “outer ring” area of ​​the epitaxial wafer with the pattern on the photomask, perform photolithography, and after exposure, remove the epitaxial wafer and place it in the negative photoresist developer. The development time can be 15 s, and cover the area outside the “outer ring” with photoresist.

[0083] (i) Growing n-type electrodes in the “outer ring” and annealing: depositing metal Ti / Al / Ni / Au on the epitaxial wafer by electron beam evaporation, with a thickness of 30 nm / 175 nm / 35 nm / 60 nm, and annealing at 850 °C for 35 s in a nitrogen atmosphere to achieve the formation of n-type ohmic contacts in the “outer ring” region.

[0084] (j) Remove excess photoresist and metal using organic solvents.

[0085] Step 10: The epitaxial wafer 17 with inner circle-outer ring electrodes prepared in step 9 is attached to the electrical plate. Figure 6 The sample is placed on the electrical sample holder 7, and then the electrical sample holder 7 is placed in the ball welding machine to perform wire bonding operation. One end of the aluminum wire is welded to the inner circular electrode 6, and the other end is welded to the metal area 11 on the second electrical board. Another aluminum wire is taken and one end is welded to the outer ring electrode 5, and the other end is welded to the metal area 9 on the first electrical board 8 to realize the electrical connection between the epitaxial wafer 17 and the electrical board.

[0086] Step 11: Insert the electrical plug A of the infrared spectrometer sample rod 13 into the socket 12 of the second electrical board, and insert the other electrical plug B into the socket 10 of the first electrical board, so as to realize the electrical connection between the infrared spectrometer sample rod 13 and the epitaxial wafer 17 through the electrical board.

[0087] Step 12: Connect the positive and negative terminals of the external voltage source through the electrical connector 14 on the sample rod 13 of the infrared spectrometer to realize the control of the vertical voltage of the gallium nitride epitaxial wafer between the inner circle and the outer ring. Specifically, the positive terminal of the voltage source is connected to the inner circle electrode 6 of the epitaxial wafer 17 through the electrical plug A of the sample rod 13, and the negative terminal of the voltage source is connected to the outer ring electrode 5 of the epitaxial wafer 17 through the electrical plug B of the sample rod 13.

[0088] Step 13: Place the prepared sample rod 13 vertically in the infrared spectrometer, as shown. Figure 6 As shown, the infrared spectrometer has a built-in light source that emits vertically incident infrared light 15 onto the epitaxial wafer. The light is excited through the light-transmitting region 16 between the inner circle and outer ring electrodes, while an external voltage source controls the vertical electric field of the p-type gallium nitride epitaxial structure on the epitaxial wafer, thus realizing an in-situ electrically charged infrared reflection spectrum test experiment. This test method is compatible with vacuum and liquid helium cryogenic tests.

Claims

1. A gallium nitride epitaxial structure for in-situ electric field infrared spectroscopy testing, comprising a substrate, and a stress control layer, a Si-doped n-type gallium nitride layer, and a Mg-doped p-type gallium nitride layer located on the substrate, characterized in that, The p-type gallium nitride layer is either an alternately doped superlattice p-type gallium nitride or a graded-doped p-type gallium nitride. A test electrode with an inner circle-outer ring metal pattern is provided on the surface of the epitaxial wafer. The test electrode consists of an inner circle electrode located at the center and an outer ring electrode concentric with it. The inner circle electrode is located on the p-type gallium nitride layer and forms an ohmic contact with it, while the outer ring electrode is located on the n-type gallium nitride layer and forms an ohmic contact with it.

2. The gallium nitride epitaxial structure as described in claim 1, characterized in that, The p-type gallium nitride layer is an alternately doped superlattice p-type gallium nitride, which is composed of alternating stacks of weak p-type gallium nitride layers with a thickness of 200~500 nm and strong p-type gallium nitride layers with a thickness of 20~50 nm.

3. The gallium nitride epitaxial structure as described in claim 2, characterized in that, The Mg doping concentration of the weak p-type gallium nitride layer is 1×10⁻⁶. 17 / cm 3 ~ 5×10 17 / cm 3 The Mg doping concentration of the strongly p-type gallium nitride layer is 7 × 10⁻⁶. 18 / cm 3 ~ 3×10 19 / cm 3 .

4. The gallium nitride epitaxial structure as described in claim 1, characterized in that, The p-type gallium nitride layer is a graded-doped p-type gallium nitride layer, with a thickness of 300~700 nm from bottom to top. -- Gallium nitride layer with a thickness of 200~500 nm - Gallium nitride (GaN) layers of different thicknesses: p-type GaN with a thickness of 100–300 nm, and p-type GaN with a thickness of 30–70 nm. + Gallium nitride of different types is stacked sequentially.

5. The gallium nitride epitaxial structure as described in claim 4, characterized in that, The p -- The Mg doping concentration of the gallium nitride layer is 1×10⁻⁶. 16 / cm 3 ~ 5×10 16 / cm 3 The p - The Mg doping concentration of the gallium nitride layer is 1×10⁻⁶. 17 / cm 3 ~ 5×10 17 / cm 3 The Mg doping concentration of the p-type gallium nitride layer is 1×10⁻⁶. 18 / cm 3 ~ 5×10 18 / cm 3 The p + The Mg doping concentration of the gallium nitride layer is 8 × 10⁻⁶. 18 / cm 3 ~ 3×10 19 / cm 3 .

6. The gallium nitride epitaxial structure as described in claim 1, characterized in that, The radius of the inner circular electrode is 3~5 mm; the inner circle radius r of the outer ring electrode is 7~8 mm, and the outer circle radius R is 10~12 mm.

7. A method for preparing a gallium nitride epitaxial structure according to any one of claims 1 to 6, comprising the following steps: 1) Select a substrate; 2) Epitaxial growth of a stress control layer on the substrate; 3) Epitaxial growth of an n-type gallium nitride layer doped with Si; 4) Epitaxial growth of alternating doped superlattice p-type gallium nitride layers or graded doped p-type gallium nitride layers; 5) Prepare a test electrode with an inner circle-outer ring metal pattern on the epitaxial wafer obtained in step 4).

8. The preparation method according to claim 7, characterized in that, Step 5) The process of preparing the test electrode includes: a) Clean the epitaxial wafer and perform a photolithography process. The photolithographic pattern is the inner circle of the inner circle-outer ring metal pattern. Photoresist is covered in the inner circle area for etching protection, while other areas are exposed. b) Perform ICP etching on the epitaxial wafer, with the etching depth exceeding the p-type gallium nitride layer and reaching the n-type gallium nitride layer; c) After removing the photoresist with chemical reagents and cleaning, perform the second photolithography step. The photolithographic pattern is a square slightly smaller than the epitaxial wafer. Cover this area with photoresist for etching protection. d) ICP etching etches the epitaxial wafer outside the square pattern into the stress control layer, followed by chemical cleaning of the photoresist. e) Use a mask to cover the area outside the inner circle with photoresist to block the metal; f) Grow p-type electrodes using electron beam evaporation or magnetron sputtering and then perform appropriate annealing processes to make them form good ohmic contact with the p-type gallium nitride layer; g) Strip and chemically clean to remove excess photoresist and metal; h) Perform a third photolithography step on the epitaxial wafer. The photolithographic pattern is the outer ring of the inner circle-outer ring metal pattern, and photoresist is covered in the area outside the outer ring to block the metal. i) An n-type electrode is grown in the outer ring region by electron beam evaporation or magnetron sputtering and then subjected to a corresponding annealing process to form a good ohmic contact with the n-type gallium nitride layer; j) Strip and chemically clean to remove excess photoresist and metal.

9. A method for infrared spectroscopy testing of p-type gallium nitride under an in-situ electric field, comprising first obtaining an epitaxial structure of gallium nitride material as described in any one of claims 1 to 6, then placing the gallium nitride epitaxial structure sample on the electrical sample holder of the sample rod of an infrared spectrometer, so that the inner circular electrode and the outer ring electrode are electrically connected to an external voltage source, the sample is irradiated by a built-in light source of the infrared spectrometer, and the vertical electric field applied to the sample is controlled by the external voltage source to achieve in-situ electric infrared reflectance spectrum testing.

10. The method as described in claim 9, characterized in that, The electrical sample holder is equipped with a first electrical plate and a second electrical plate. The gallium nitride epitaxial structure sample is placed on the electrical sample holder, and the outer ring electrode and the inner circular electrode are electrically connected to the first electrical plate and the second electrical plate, respectively. Then, the two electrical plugs of the infrared spectrometer sample rod are inserted into the sockets of the first and second electrical plates, respectively, to achieve electrical connection between the infrared spectrometer sample rod and the sample. The positive and negative terminals of the external voltage source are connected through the electrical sockets of the infrared spectrometer sample rod, thereby achieving electrical connection between the sample and the external voltage source, and thus enabling the external voltage source to control the vertical electric field of the sample.

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