Epitaxial gallium nitride photoconductive device based on intrinsic light vertical back incidence and preparation method thereof

Abstract

According to the epitaxial gallium nitride photoconductive device based on intrinsic light vertical back incidence and the preparation method, diamond or sapphire is used as a substrate, and epitaxial high-quality gallium nitride is used as a conductive channel, so that intrinsic light vertical back incidence can be realized, and the photoelectric conversion efficiency and the power capacity are further improved; higher quantum efficiency can be obtained by adopting intrinsic light triggering; and the material has extremely low carrier lifetime, and is a core factor for realizing high-frequency light guide microwave output. In addition, the anti-reflection film improves the photocurrent amplitude at the angle of improving the quantum efficiency, and the surface passivation layer weakens the edge electric field of the electrode, thereby improving the breakdown voltage of the device. The problem that it is difficult for a gallium nitride photoconductive switch to achieve vertical back incidence of intrinsic light is solved, and the photoelectric conversion efficiency is improved as much as possible on the premise that high-frequency response is guaranteed.

Description

Technical Field

[0001] The present invention belongs to the field of photoconductive semiconductor devices, and in particular relates to an epitaxial gallium nitride photoconductive device based on vertical back incidence of intrinsic light and a preparation method thereof. Background Art

[0002] Optical microwaves have the advantages of good photoelectric isolation, wide range of microwave output parameters, flexible adjustment, all-solid-state, modularization, compact structure, etc. At present, optical microwaves are developing towards high frequency, high power and long life. Planar optical devices have better frequency response at high frequencies due to their lower stray capacitance between electrodes.

[0003] Existing photoconductive switching devices are mainly composed of a pair of metal electrodes and the substrate of the photoconductive device. When a bias voltage is applied, the substrate of the photoconductive device will generate a large number of photogenerated carriers under laser irradiation, resulting in a dynamic change in the device resistance and stimulating a photocurrent between the cathode and the anode. According to the layout of the electrodes, photoconductive semiconductor devices can be divided into two structures: coplanar and heteroplanar. In the heteroplanar structure, the electrodes are distributed on two different planes of the wafer, which makes the current distribution more uniform and improves the withstand voltage performance. However, this structure does not support intrinsic light triggering and is easily affected by stray electrical parameters, limiting the improvement of response speed. The electrodes of the coplanar structure are all located on the same plane of the wafer, allowing the use of intrinsic light triggering, showing the potential for achieving high photoelectric efficiency and fast response. However, this design also brings challenges. Due to the coplanar layout of the electrodes, the uneven distribution of the photocurrent may lead to excessive surface electric field strength, current concentration and local overheating, which in turn causes surface breakdown, limiting the improvement of the power capacity of the device.

[0004] In the past, planar photoconductive semiconductor devices were mainly concentrated on silicon (Si) and gallium arsenide (GaAs) materials. However, the small breakdown field strength and short linear region severely limited the development of these two materials as photoconductive devices. In recent years, the third-generation semiconductors represented by silicon carbide and gallium nitride have excellent properties such as high breakdown field strength, high temperature resistance, and strong radiation resistance, and have gradually begun to be widely studied. Currently, the most studied photoconductive devices are silicon carbide photoconductive devices. However, the doping concentration of vanadium, a deep energy level impurity used to control the carrier lifetime of silicon carbide, in silicon carbide has an upper limit (about 3×10 17 cm -3 ), making it difficult for vanadium-doped silicon carbide to further reduce the carrier lifetime. Therefore, at higher frequency bands, silicon carbide devices will have a long tail and the modulation index will continue to decrease. Gallium nitride (GaN) stands out with its wide band gap, high critical breakdown electric field, high carrier mobility, high electron saturation velocity and excellent high-frequency characteristics, and has become the mainstream material in the field of RF microwaves; GaN materials can achieve lower carrier lifetimes through doping, which means that higher frequency modulated microwaves can be achieved.

[0005] At present, conventional planar GaN photoconductive devices with front light triggering have problems such as poor device life, low laser damage threshold, and easy breakdown. Vertical back incidence of intrinsic light has been proven to be a better triggering method and device configuration. Studies have shown that planar devices with vertical back incidence of intrinsic light have higher power capacity and device photoelectric conversion efficiency. However, the absorption depth of GaN for intrinsic light is relatively shallow. The absorption depth of ultraviolet light of GaN material is only on the order of hundreds of nanometers. GaN single crystals need to be thinned to the micron level to have a high photoelectric conversion efficiency. However, micron-thick GaN single crystals will be too thin to make the device fragile and unusable. In addition, the technology for growing single crystal semi-insulating GaN substrates with low defect density and high body resistivity is not mature enough. The price of GaN single crystals is relatively expensive, and the presence of defects in the substrate will also affect the performance of the device.

[0006] Therefore, a new method with higher quality, mature technology and lower price is needed to replace the traditional single-crystal gallium nitride photoconductive switch and solve the problem of the device being too thin and fragile.

[0007] In addition, there is the problem of selecting substrate materials for GaN photoconductive switches. Using substrates such as silicon carbide and silicon will lead to the formation of a conductive channel inside the substrate under light triggering. At this time, although the longer carrier lifetime of the substrate will increase the photocurrent amplitude, it will bring about the problem of a slower falling edge, and the fast response characteristics of GaN cannot be brought into play.

[0008] Therefore, it is necessary to design a gallium nitride composite photoconductive device to solve the problem that gallium nitride photoconductive switches are difficult to achieve vertical back incidence of intrinsic light, and to improve the photoelectric conversion efficiency as much as possible while ensuring high-frequency response. Summary of the invention

[0009] In order to solve the above technical problems, the present invention provides an epitaxial gallium nitride photoconductive device based on vertical back incidence of intrinsic light, adopts gallium nitride material with shorter carrier lifetime, and solves the problem of substrate absorption of light slowing down response speed by adopting ultra-wide bandgap substrates such as sapphire, diamond, and aluminum nitride, thereby avoiding the formation of conductive channels in the substrate and improving the light transmittance of ultraviolet trigger light.

[0010] The present invention provides an epitaxial gallium nitride photoconductive device based on vertical back incidence of intrinsic light, comprising an insulating substrate, a semi-insulating gallium nitride epitaxial layer and a metal electrode;

[0011] The insulating substrate is a sapphire, diamond or aluminum nitride substrate with a thickness of 100-1000 μm;

[0012] The semi-insulating gallium nitride epitaxial layer is arranged on the upper part of the insulating substrate and has a thickness of 0.2 μm-5 μm;

[0013] The metal electrode comprises a first electrode and a second electrode, and the first electrode and the second electrode are both arranged on the upper surface of the semi-insulating gallium nitride epitaxial layer; the thickness ranges from 0.2 to 5 μm.

[0014] Furthermore, the semi-insulating gallium nitride epitaxial layer is heavily doped gallium nitride, the doping element is iron or carbon, and the doping concentration is 5×10 17 cm -3 to 1×10 19 cm -3 .

[0015] Furthermore, a passivation layer is provided on the upper surface of the semi-insulating gallium nitride epitaxial layer, and the passivation layer covers the upper surface of the semi-insulating gallium nitride epitaxial layer except the metal electrode; the thickness of the passivation layer ranges from 0.05 to 2 μm.

[0016] Preferably, the passivation layer is an aluminum oxide passivation layer.

[0017] Furthermore, a silicon dioxide anti-reflection film is provided at the bottom of the insulating substrate, and the thickness of the silicon dioxide anti-reflection film is in the range of 50nm-300nm.

[0018] The present invention provides a method for preparing an epitaxial gallium nitride photoconductive device based on vertical back incidence of intrinsic light, comprising the following steps:

[0019] (1) Select 100-1000 μm sapphire, diamond or aluminum nitride substrate as the insulating substrate and clean it thoroughly;

[0020] (2) heteroepitaxially heavily doping gallium nitride on the upper surface of the insulating substrate with a thickness of 0.2 μm to 5 μm to form a semi-insulating gallium nitride epitaxial layer, wherein the gallium nitride doping element is iron or carbon;

[0021] (3) coating, exposing, and developing the upper surface of the semi-insulating gallium nitride epitaxial layer, and forming two metal electrode patterns by photolithography;

[0022] (4) vapor-depositing two metal electrodes on the upper surface of the semi-insulating gallium nitride epitaxial layer treated in step (3), with an electrode spacing of 0.5-5 mm, an electrode pattern of a rounded rectangle, 4 mm-8 mm long, 2 mm-4 mm wide, a fillet radius of 0.5 mm-1.5 mm, and an electrode thickness of 0.2 μm-2 μm. After the vapor deposition is completed, the adhesive is removed, cleaned, and peeled off;

[0023] (5) subjecting the device processed in step (4) to high temperature annealing at a temperature of 800° C. to 900° C., so that the metal electrode forms a good ohmic contact with the device;

[0024] (6) Cutting the wafer, cleaning it, welding it, and encapsulating it to prepare an epitaxial GaN photoconductive device.

[0025] Preferably, in step (1), the size of the insulating substrate is (8mm-12mm)×(8mm-12mm).

[0026] In step (2), the GaN doping concentration range is 5×10 17 cm -3 to 1×10 19 cm -3 .

[0027] Further, after step (5), the method further comprises:

[0028] (6) growing an aluminum oxide passivation layer on the upper surface of the semi-insulating gallium nitride epitaxial layer using ALD;

[0029] (7) coating, photolithography, and development on the surface of the aluminum oxide passivation layer to form an electrode window area;

[0030] (8) Dry etching the electrode window area to complete the metal electrode window;

[0031] (9) depositing a silicon dioxide antireflection film on the bottom of the insulating substrate (non-electrode side of the back of the device);

[0032] (10) Cutting the wafer, cleaning it, welding it, and encapsulating it to prepare an epitaxial gallium nitride photoconductive device.

[0033] Furthermore, the thickness of the aluminum oxide passivation layer is in the range of 0.05-2 μm.

[0034] Furthermore, the thickness of the silicon dioxide anti-reflection film ranges from 50nm to 300nm.

[0035] Working principle of the present invention:

[0036] The laser passes through the insulating substrate and is incident on the semi-insulating gallium nitride epitaxial layer. The triggering light passing through the insulating substrate is absorbed by the gallium nitride material, and a large number of photogenerated carriers are generated inside the semi-insulating gallium nitride epitaxial layer. Under the action of the bias voltage, a photocurrent is formed.

[0037] The insulating substrate is made of sapphire, diamond or aluminum nitride substrate, which improves the mechanical strength of the device while reducing light attenuation, improves the light transmittance of the ultraviolet trigger light, avoids the substrate from slowing down the switching speed, avoids the formation of a conductive channel in the substrate, and ensures the response speed of the device.

[0038] Intrinsic light triggering enables the device to have a higher photoelectric conversion efficiency, and vertical back-light input enables the device to have a higher photoelectric conversion efficiency while having a larger power capacity and light damage threshold; epitaxial gallium nitride is used instead of gallium nitride single crystal to obtain higher crystal quality, and epitaxial heavily doped gallium nitride also has a lower production cost, while meeting the device thickness required for back-light input, solving the problem of the device being too thin faced by back-side triggering.

[0039] In addition, the present invention proposes two solutions to improve the photoelectric conversion efficiency. One is to add an anti-reflection film to the device to improve the quantum efficiency and thus the photocurrent; the other is to optimize the carrier lifetime by regulating the concentration of iron or carbon and the ratio of shallow energy level impurities to the concentration of iron or carbon, so as to increase the output photocurrent as much as possible while ensuring high-frequency response.

[0040] Beneficial effects of the present invention:

[0041] The present invention is aimed at the application needs of photoconductive devices to expand to high frequency bands and high power, and proposes an epitaxial gallium nitride photoconductive semiconductor device based on vertical back incidence of intrinsic light and its preparation method. Diamond or sapphire is used as the substrate, and high-quality gallium nitride is epitaxially used as the conductive channel, which can realize vertical back incidence of intrinsic light, further improving the photoelectric conversion efficiency and power capacity; the use of intrinsic light triggering can obtain higher quantum efficiency; and the present invention has an extremely low carrier lifetime, which is the core factor in realizing high-frequency photoconductive microwave output. In addition, the anti-reflection film increases the photocurrent amplitude from the perspective of improving quantum efficiency, and the surface passivation layer weakens the edge electric field of the electrode, thereby increasing the device breakdown voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 is a schematic diagram of the cross-sectional structure of the epitaxial gallium nitride photoconductive device in Example 1;

[0043] Figure 2 is a schematic diagram of the top view of the epitaxial gallium nitride photoconductive device in Example 1;

[0044] Figure 3 is a schematic diagram of the cross-sectional structure of the SiC-GaN composite photoconductive device in Example 2;

[0045] Figure 4 Comparison of photocurrent of epitaxial GaN photoconductive device with back-light incident and GaN single crystal with front-light incident under 10kW laser;

[0046] Figure 5 Schematic diagram of the internal electric field distribution of the back-illuminated epitaxial GaN photoconductive device;

[0047] Figure 6 Schematic diagram of the internal electric field distribution of a GaN single crystal photoconductive device with incident light from the front;

[0048] Figure 7 Schematic diagram of the internal current distribution of the back-illuminated epitaxial GaN photoconductor device.

[0049] 1. Insulating substrate, 2. Semi-insulating gallium nitride epitaxial layer, 301. First electrode, 302. Second electrode, 4. Passivation layer, 5. Silicon dioxide anti-reflection film. DETAILED DESCRIPTION

[0050] Embodiment 1,

[0051] like Figure 1 , 2 As shown, this embodiment provides an epitaxial gallium nitride photoconductive device based on vertical back incidence of intrinsic light, comprising an insulating substrate 1, a semi-insulating gallium nitride epitaxial layer 2 and a metal electrode;

[0052] The insulating substrate 1 is a sapphire substrate with a size of 12 mm×12 mm and a thickness of 500 μm;

[0053] The semi-insulating gallium nitride epitaxial layer 2 is arranged on the upper part of the insulating substrate 1 and has a thickness of 1 μm. The semi-insulating gallium nitride epitaxial layer 2 is heavily doped gallium nitride, and the doping element is iron, and the doping concentration is 8×10 18 cm -3 .

[0056] The metal electrodes include a first electrode 301 and a second electrode 302 , which are both disposed on the upper surface of the semi-insulating gallium nitride epitaxial layer 2 ; the size is 3 mm×7 mm, the thickness is 0.8 μm, and the spacing is 1 mm.

[0057] Embodiment 2,

[0058] like Figure 3 As shown, this embodiment provides an epitaxial gallium nitride photoconductive device based on vertical back incidence of intrinsic light, which is a SiC-GaN composite photoconductive device. On the basis of Embodiment 1, the upper surface of the semi-insulating gallium nitride epitaxial layer 2 is further provided with a passivation layer 4, and the passivation layer 4 covers the upper surface of the semi-insulating gallium nitride epitaxial layer 2 except for the metal electrode; the thickness of the passivation layer 4 is 0.8μm. The passivation layer 4 is an aluminum oxide passivation layer. The bottom of the insulating substrate 1 is provided with a silicon dioxide anti-reflection film 5, and the thickness of the silicon dioxide anti-reflection film 5 is 50nm.

[0059] Working principle of the present invention:

[0060] The laser passes through the insulating substrate and is incident on the semi-insulating gallium nitride epitaxial layer. The triggering light passing through the insulating substrate is absorbed by the gallium nitride material, and a large number of photogenerated carriers are generated inside the semi-insulating gallium nitride epitaxial layer. Under the action of the bias voltage, a photocurrent is formed.

[0061] The schematic diagram of the comparison of the photocurrent of the epitaxial GaN photoconductive device with back-light incident and the GaN single crystal with front-light incident under 10kW laser is shown in the figure Figure 4 As shown;

[0062] Schematic diagram of the electric field distribution inside the back-illuminated epitaxial GaN photoconductor device Figure 5 As shown;

[0063] Schematic diagram of the internal electric field distribution of a GaN single crystal photoconductive device with incident light from the front Figure 6 As shown;

[0064] Schematic diagram of the internal current distribution of the back-illuminated epitaxial GaN photoconductor device Figure 7 shown.

[0065] Embodiment 3,

[0066] The present embodiment provides a method for preparing an epitaxial gallium nitride photoconductive device based on vertical back incidence of intrinsic light, comprising the following steps:

[0067] (1) Select 100-1000 μm sapphire, diamond or aluminum nitride substrate as the insulating substrate and clean it thoroughly;

[0068] (2) Heteroepitaxially heavily doped GaN with a thickness of 0.2 μm to 5 μm is formed on the upper surface of the insulating substrate to form a semi-insulating GaN epitaxial layer. The GaN doping element is iron or carbon with a doping concentration of 5×10 17 cm -3 to 1×10 19 cm -3 ;

[0069] (3) coating, exposing, and developing the upper surface of the semi-insulating gallium nitride epitaxial layer, and forming two metal electrode patterns by photolithography;

[0070] (4) evaporating two metal electrodes on the upper surface of the semi-insulating gallium nitride epitaxial layer treated in step (3), and removing the glue, cleaning, and peeling after the evaporation is completed;

[0071] (5) annealing the device after the treatment in step (4) at a high temperature so that the metal electrode forms a good ohmic contact with the device;

[0072] (6) Cutting the wafer into rectangular devices with a side length of 8 mm to 12 mm, cleaning, welding, and encapsulating to prepare epitaxial gallium nitride photoconductive devices.

[0073] Embodiment 4,

[0074] The present embodiment provides a method for preparing an epitaxial gallium nitride photoconductive device based on vertical back incidence of intrinsic light, comprising the following steps:

[0075] (1) Select 100-1000 μm sapphire, diamond or aluminum nitride substrate as the insulating substrate and clean it thoroughly;

[0076] (2) Heteroepitaxially heavily doped GaN with a thickness of 0.2 μm to 5 μm is formed on the upper surface of the insulating substrate to form a semi-insulating GaN epitaxial layer. The GaN doping element is iron or carbon, and the GaN iron or carbon doping concentration is 5×10 17 cm -3 to 1×10 19 cm -3 ;

[0077] (3) coating, exposing, and developing the upper surface of the semi-insulating gallium nitride epitaxial layer, and forming two metal electrode patterns by photolithography;

[0078] (4) evaporating two metal electrodes on the upper surface of the semi-insulating gallium nitride epitaxial layer treated in step (3), and removing the glue, cleaning, and peeling after the evaporation is completed;

[0079] (5) annealing the device after the treatment in step (4) at a high temperature so that the metal electrode forms a good ohmic contact with the device;

[0080] (6) growing an aluminum oxide passivation layer on the upper surface of the semi-insulating gallium nitride epitaxial layer by ALD; the aluminum oxide passivation layer has a thickness of 0.05 μm to 2 μm;

[0081] (7) coating, photolithography, and development on the surface of the aluminum oxide passivation layer to form an electrode window area;

[0082] (8) Dry etching the electrode window area to complete the metal electrode window;

[0083] (9) depositing a silicon dioxide antireflection film on the bottom of the insulating substrate (the non-electrode side of the back of the device), wherein the thickness of the silicon dioxide antireflection film is 200 nm to 300 nm;

[0084] (10) Cutting the wafer, cleaning it, welding it, and encapsulating it to prepare an epitaxial gallium nitride photoconductive device.

Claims

1. An epitaxial gallium nitride photoconductive device based on vertical back incidence of intrinsic light, characterized in that: It includes an insulating substrate, a semi-insulating gallium nitride epitaxial layer and a metal electrode; The insulating substrate is a sapphire, diamond or aluminum nitride substrate with a thickness of 100-1000 μm; The semi-insulating gallium nitride epitaxial layer is arranged on the upper part of the insulating substrate and has a thickness of 0.2 μm-5 μm; The metal electrode comprises a first electrode and a second electrode, and the first electrode and the second electrode are both arranged on the upper surface of the semi-insulating gallium nitride epitaxial layer; the thickness of the metal electrode ranges from 0.2 μm to 2 μm, and the electrode spacing is 0.5 to 5 mm.

2. The epitaxial GaN photoconductive device based on vertical back incidence of intrinsic light according to claim 1, characterized in that: The semi-insulating GaN epitaxial layer is heavily doped GaN, with the doping element being iron or carbon, and the doping concentration being 5×10 17 cm -3 to 1×10 19 cm -3 .

3. The epitaxial GaN photoconductive device based on vertical back incidence of intrinsic light according to claim 1, characterized in that: A passivation layer is also provided on the upper surface of the semi-insulating gallium nitride epitaxial layer, and the passivation layer covers the upper surface of the semi-insulating gallium nitride epitaxial layer except the metal electrode; the thickness of the passivation layer ranges from 0.05 μm to 2 μm.

4. The epitaxial GaN photoconductive device based on vertical back incidence of intrinsic light according to claim 3, characterized in that: The passivation layer is an aluminum oxide passivation layer.

5. The epitaxial GaN photoconductive device based on vertical back incidence of intrinsic light according to claim 1, characterized in that: A silicon dioxide anti-reflection film is provided at the bottom of the insulating substrate, and the thickness of the silicon dioxide anti-reflection film ranges from 50nm to 300nm.

6. A method for preparing an epitaxial gallium nitride photoconductive device based on vertical back incidence of intrinsic light, characterized in that: The following steps are involved: (1) Select 100-1000 μm sapphire, diamond or aluminum nitride substrate as the insulating substrate and clean it thoroughly; (2) heteroepitaxially heavily doping gallium nitride on the upper surface of the insulating substrate with a thickness of 0.2 μm to 5 μm to form a semi-insulating gallium nitride epitaxial layer, wherein the gallium nitride doping element is iron or carbon; (3) coating, exposing, and developing the upper surface of the semi-insulating gallium nitride epitaxial layer, and forming two metal electrode patterns by photolithography; (4) evaporating two metal electrodes on the upper surface of the semi-insulating gallium nitride epitaxial layer treated in step (3), and removing the glue, cleaning, and peeling after the evaporation is completed; (5) subjecting the device processed in step (4) to high temperature annealing at a temperature of 800° C. to 900° C., so that the metal electrode forms a good ohmic contact with the device; (6) Cutting the wafer, cleaning it, welding it, and encapsulating it to prepare an epitaxial GaN photoconductive device.

7. The method for preparing an epitaxial gallium nitride photoconductive device based on vertical back incidence of intrinsic light according to claim 6, characterized in that: In step (2), the GaN doping concentration range is 5×10 17 cm -3 to 1×10 19 cm -3 .

8. A method for preparing an epitaxial gallium nitride photoconductive device based on vertical back incidence of intrinsic light, characterized in that: The following steps are involved: (1) Select 100-1000 μm sapphire, diamond or aluminum nitride substrate as the insulating substrate and clean it thoroughly; (2) heteroepitaxially heavily doping gallium nitride on the upper surface of the insulating substrate with a thickness of 0.2 μm to 5 μm to form a semi-insulating gallium nitride epitaxial layer, wherein the gallium nitride doping element is iron or carbon; (3) coating, exposing, and developing the upper surface of the semi-insulating gallium nitride epitaxial layer, and forming two metal electrode patterns by photolithography; (4) evaporating two metal electrodes on the upper surface of the semi-insulating gallium nitride epitaxial layer treated in step (3), and removing the glue, cleaning, and peeling after the evaporation is completed; (5) subjecting the device processed in step (4) to high temperature annealing at a temperature of 800° C. to 900° C., so that the metal electrode forms a good ohmic contact with the device; (6) growing an aluminum oxide passivation layer on the upper surface of the semi-insulating gallium nitride epitaxial layer using ALD; (7) coating, photolithography, and development on the surface of the aluminum oxide passivation layer to form an electrode window area; (8) Dry etching the electrode window area to complete the metal electrode window; (9) depositing a silicon dioxide antireflection film on the bottom of the insulating substrate; (10) Cutting the wafer, cleaning it, welding it, and encapsulating it to prepare an epitaxial gallium nitride photoconductive device.

9. The method for preparing an epitaxial gallium nitride photoconductive device based on vertical back incidence of intrinsic light according to claim 8, characterized in that: The thickness of the aluminum oxide passivation layer ranges from 0.05 μm to 2 μm.

10. The method for preparing an epitaxial gallium nitride photoconductive device based on vertical back incidence of intrinsic light according to claim 8, characterized in that: The thickness of the silicon dioxide antireflection film ranges from 50nm to 300nm.

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