A method for fabricating HEMTs based on ultrathin gallium nitride self-supporting substrates

Abstract

This invention discloses a method for fabricating HEMTs based on an ultrathin gallium nitride self-supporting substrate, comprising: thinning an undoped self-supporting substrate; heating the substrate to 700-900 degrees Celsius in an atmosphere containing impurity element gas and holding it at that temperature for at least 8 hours to allow impurities to diffuse into the substrate; growing N-type GaN on the substrate surface; epitaxially growing AlGaN on the N-type GaN surface to form a two-dimensional electron gas layer at the interface between the N-type GaN and AlGaN; epitaxially growing P-type GaN on the AlGaN surface; fabricating electrodes and drain electrodes at both ends of the P-type GaN; etching away the areas in the P-type GaN where the electrodes and drain electrodes are fabricated; etching and thinning both ends of the AlGaN; and depositing a metal film on the surface of the P-type GaN to fabricate a gate electrode. This invention, by heating in an atmosphere containing impurities, uses thermal diffusion to dope carbon impurities into the GaN substrate to control the resistivity of the substrate. When sufficient carbon impurities are incorporated into the substrate, a high resistivity is formed, which is beneficial for its use as a high electron mobility transistor substrate.

Description

Technical Field

[0001] This invention relates to the semiconductor field, and in particular to a method for fabricating HEMTs based on an ultrathin gallium nitride self-supporting substrate. Background Technology

[0002] Gallium nitride (GaN) is an important third-generation semiconductor material with superior properties such as a large bandgap, high breakdown electric field, high thermal conductivity, high electron saturation drift velocity, and strong radiation resistance. More importantly, the AlGaN / GaN heterojunction interface forms a two-dimensional electron gas (2DEG) with high electron concentration and high electron mobility. Therefore, gallium nitride has very important application prospects in high-temperature, high-frequency, high-power, radiation-resistant microwave devices or high-power electronic devices and circuits. It also shows excellent application prospects and market potential in solid-state light sources, power electronics, and microwave RF devices.

[0003] To achieve the pinch-off and other performance characteristics of gallium nitride-based high electron mobility transistors (HEMTs), the conductive channels in the HEMT device material structure must be grown on a semi-insulating substrate. The crystal quality and high-resistivity characteristics of this substrate directly affect the device's pinch-off characteristics, breakdown voltage, leakage current, lifetime, and reliability. Therefore, the choice of substrate is particularly important. Most gallium nitride-based device epitaxial materials are heteroepitaxially grown on sapphire, silicon carbide, or silicon substrates using metal-organic chemical vapor deposition (MOCVD) equipment.

[0004] In existing technologies, gallium nitride-based thin films (i.e., epitaxial films) grown on silicon substrates (or silicon carbide substrates) are heteroepitaxial, which results in significant thermal mismatch between the epitaxial film and the substrate, limiting the performance of the fabricated devices. Furthermore, the existing high-resistivity substrates are thicker, increasing the device size and hindering miniaturization. The methods for fabricating high-resistivity substrates also have high processing costs, making large-area fabrication difficult. Summary of the Invention

[0005] This invention provides a method for fabricating HEMTs based on an ultrathin gallium nitride self-supporting substrate, comprising the following steps:

[0006] The undoped self-supporting substrate is thinned by heating it to 700-900 degrees Celsius in an atmosphere containing impurity element gas and holding it at that temperature for at least 8 hours to allow the impurities to diffuse into the substrate.

[0007] n-type GaN is grown on the substrate surface, and AlGaN is epitaxially grown on the n-type GaN surface to form a two-dimensional electron gas layer at the interface between n-type GaN and AlGaN. p-type GaN is epitaxially grown on the AlGaN surface.

[0008] Electrodes and drain electrodes are fabricated at both ends of a p-type GaN, the regions where electrodes and drain electrodes are fabricated in the p-type GaN are etched away, the two ends of the AlGaN are thinned by etching, and a metal film is deposited on the surface of the p-type GaN to fabricate the gate electrode.

[0009] Furthermore, it also includes:

[0010] Thinning is achieved by removing the poor-quality crystal portion of the undoped self-supporting substrate through laser ablation.

[0011] Furthermore, the thickness of the thinned undoped self-supporting substrate is 150-200 μm.

[0012] Furthermore, it also includes:

[0013] Inside the tube furnace, nitrogen gas carrying methane is introduced to heat the substrate to 800-850 degrees Celsius and hold it at that temperature for at least 8-10 hours.

[0014] Furthermore, n-type GaN with a thickness of 8-10 μm was grown on the GaN surface by organic chemical vapor deposition, wherein the carrier concentration was approximately 7-9 x 10⁻⁶. 15 cm -3 .

[0015] Furthermore, in the two-dimensional electron gas layer formed at the interface between AlGaN and N-type GaN, the Al composition is greater than 0 and less than 0.5, and the thickness is 80-120 nm.

[0016] Furthermore, the p-type GaN has a thickness of 400-600 nm and a carrier concentration of approximately 2 x 10⁻⁶. 20 cm -3 .

[0017] Furthermore, the gas containing impurity elements is methane.

[0018] Furthermore, the substrate is at least one of sapphire substrate, SiC substrate, Si substrate or undoped self-supporting GaN substrate.

[0019] Furthermore, the impurities are carbon, boron, or helium.

[0020] This invention proposes a novel ultrathin gallium nitride self-supporting substrate. By heating the substrate in an atmosphere containing impurities, carbon impurities are doped into the GaN substrate through thermal diffusion, thereby controlling the substrate's resistivity. When sufficient carbon impurities are incorporated, a high-resistivity substrate is formed, which is beneficial for its use as a high-electron-mobility transistor substrate. Furthermore, the process flow is relatively simple and easily implemented in practical applications. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 A schematic diagram illustrating the process of forming a semi-insulating substrate by doping with carbon impurities, as provided in an embodiment of the present invention;

[0023] Figure 2 This is a schematic diagram of the structure of a HEMT device fabricated using a self-supporting substrate, provided in an embodiment of the present invention.

[0024] Figure 3 The output characteristic test diagram of a HEMT device fabricated using a self-supporting substrate is provided in an embodiment of the present invention. Detailed Implementation

[0025] To enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0026] This invention provides a method for fabricating HEMTs based on an ultrathin gallium nitride self-supporting substrate, comprising the following steps:

[0027] Step 1: Thin the undoped self-supporting substrate by heating it to 700-900 degrees Celsius in an atmosphere containing impurity element gas and holding it at that temperature for at least 8 hours to allow the impurities to diffuse into the substrate.

[0028] In this embodiment of the invention, a double-sided polished undoped GaN self-supporting substrate with a thickness of approximately 300-500 μm is prepared, such as... Figure 1 The bottom substrate shown is thinned by removing the portion with poor crystal quality from the GaN self-supporting substrate using laser ablation. The thickness of the removed portion is approximately 100-150 μm, resulting in an ultrathin substrate thickness of approximately 150-200 μm. The substrate is at least one of sapphire, SiC, Si, or undoped self-supporting GaN substrates.

[0029] Further, within a tube furnace, nitrogen gas carrying methane is introduced to heat the substrate to 800-850 degrees Celsius and hold it at that temperature for at least 8-10 hours. The nitrogen atmosphere carrying methane provides carbon impurities, followed by holding at 700-900 degrees Celsius for 8-10 hours. During this process, the carbon impurities in the atmosphere diffuse thermally, allowing them to dope onto the substrate surface and further diffuse downwards into the interior of the GaN substrate. Figure 2The diagram shown illustrates this. The gas containing impurity elements is methane. Possible impurities include carbon, boron, or helium. After diffusion, a high-resistivity, ultrathin gallium nitride self-supporting substrate is fabricated. The substrate's resistance can be controlled by adjusting the content of the impurity gas.

[0030] Step 2: Epitaxially grow n-type GaN on the substrate surface, epitaxially grow AlGaN on the n-type GaN surface to form a two-dimensional electron gas layer at the interface between n-type GaN and AlGaN, and epitaxially grow p-type GaN on the AlGaN surface.

[0031] Specifically, n-type GaN with a thickness of 8-10 μm is grown on the GaN surface by organic chemical vapor deposition, wherein the carrier concentration is approximately 7-9 x 10⁻⁶. 15 cm -3 The thickness of p-type GaN is 400-600 nm, and the carrier concentration is approximately 2 x 10⁻⁶. 20 cm -3 In the two-dimensional electron gas layer formed at the interface between AlGaN and n-type GaN, the Al content is greater than 0 and less than 0.5, and the thickness is 80-120 nm.

[0032] In this embodiment of the invention, the substrate that has undergone thermal diffusion doping is cleaned, and a 10 μm n-type GaN layer is grown on the GaN buffer layer by organic chemical vapor deposition (MOCVD), with a carrier concentration of approximately 8 x 10⁻⁶. 15 cm -3 Hydrogen-based silanes can be used as Si raw materials for n-type dopants (donors). For example... Figure 1 As shown, AlGaN material is grown on the surface of epitaxial n-type GaN to form a heterojunction with the underlying n-type GaN, forming a two-dimensional electron gas layer (2DEG) at the interface. The Al composition is greater than 0 and less than 0.5, and the thickness is 100 nm. If the composition is too high, cracks are likely to occur, and if the thickness is too low, it is not enough to form a 2DEG at the GaN material interface.

[0033] Step 3: Prepare electrodes and drain electrodes at both ends of the p-type GaN, etch away the areas in the p-type GaN where electrodes and drain electrodes are prepared, etch and thin the two ends of the AlGaN, and deposit a metal film on the surface of the p-type GaN to prepare the gate electrode.

[0034] like Figure 2 As shown, a 500 nm p-type GaN material was epitaxially grown with a carrier concentration of approximately 2 x 10⁻⁶. 20 cm -3 Magnesium thiocene can be used as a p-type dopant donor to deplete the two-dimensional electron gas at the GaN material interface.

[0035] like Figure 2As shown, the grown GaN epitaxial wafer undergoes photolithography and etching to remove some of the p-GaN, exposing the areas at both ends of the device for source and drain electrodes. Following this, a further photolithography and etching process removes some AlGaN, reducing the thickness of the AlGaN to decrease the mobility of the two-dimensional electron channel. After photolithography, Ti / Al / Ni / Au is deposited to form the source and drain electrodes. On top of the existing structure, Ni / Au is deposited on the p-GaN using an overlay method to fabricate the gate electrode. After removing residual resist, the device fabrication is complete.

[0036] This invention proposes a novel ultrathin gallium nitride self-supporting substrate. By heating the substrate in an atmosphere containing impurities, carbon impurities are doped into the GaN substrate through thermal diffusion, thereby controlling the substrate's resistivity. When sufficient carbon impurities are incorporated, a high-resistivity substrate is formed, which is beneficial for its use as a high-electron-mobility transistor substrate. Furthermore, the process flow is relatively simple and easily implemented in practical applications.

[0037] Depend on Figure 3 As can be seen, the reverse leakage current decreased by nearly three orders of magnitude during the transition from -10V to 0V, and the device's on / off ratio was also significantly improved, reflecting the effect of thermally diffused doping on device performance. This is because carbon impurities enter the gallium nitride energy level, resulting in the formation of a high-resistivity region that effectively blocks leakage current.

[0038] The above description is only a partial embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for fabricating HEMTs based on gallium nitride self-supporting substrates, characterized in that, Includes the following steps: The undoped self-supporting substrate is thinned by heating it to 700-900 degrees Celsius in an atmosphere containing impurity element gas and holding it at that temperature for at least 8 hours to allow the impurities to diffuse into the substrate. n-type GaN is grown on the substrate surface, and AlGaN is epitaxially grown on the n-type GaN surface to form a two-dimensional electron gas layer at the interface between n-type GaN and AlGaN, and p-type GaN is epitaxially grown on the AlGaN surface. Electrodes and drain electrodes are fabricated at both ends of a p-type GaN, the regions where electrodes and drain electrodes are fabricated in the p-type GaN are etched away, the two ends of the AlGaN are thinned by etching, and a metal film is deposited on the surface of the p-type GaN to fabricate a gate electrode. Among them, the impurity element gas is a gas containing carbon impurity elements.

2. The preparation method according to claim 1, characterized in that, Also includes: Thinning is achieved by removing the poor-quality crystal portion of the undoped self-supporting substrate through laser ablation.

3. The preparation method according to claim 1, characterized in that, The thickness of the thinned undoped self-supporting substrate is 150-200 μm.

4. The preparation method according to claim 1, characterized in that, Also includes: Inside the tube furnace, nitrogen gas carrying methane is introduced to heat the substrate to 800-850 degrees Celsius and hold it at that temperature for 8-10 hours.

5. The preparation method according to claim 1, characterized in that, n-type GaN with a thickness of 8-10 μm was grown on the GaN surface by organic chemical vapor deposition (MOCVD), with a carrier concentration of 7-9 x 10⁻⁶. 15 cm -3 .

6. The preparation method according to claim 1, characterized in that, In the two-dimensional electron gas layer formed at the interface between AlGaN and n-type GaN, the Al composition is greater than 0 and less than 0.5, and the thickness is 80-120 nm.

7. The preparation method according to claim 1, characterized in that, p-type GaN, with a thickness of 400-600 nm and a carrier concentration of approximately 2 x 10⁻⁶. 20 cm -3 .

8. The preparation method according to claim 1, characterized in that, The gas containing impurity elements is methane.

9. The preparation method according to claim 1, characterized in that, The substrate is at least one of sapphire substrate, SiC substrate, Si substrate or undoped self-supporting GaN substrate.

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