A method for preparing large-area gallium nitride thin films

Abstract

This invention relates to a method for preparing large-area gallium nitride (GaN) thin films, the method comprising the following steps: (1) pretreating a GaN wafer with electrodes on its surface using oxygen plasma; (2) placing the GaN wafer obtained in step (1) in an electrolyte using an anode electrode fixture without a support layer, applying a constant voltage, and performing electrochemical etching. After complete electrochemical etching, the GaN thin film is retrieved using a substrate; (3) cleaning and drying the GaN thin film on the substrate from step (2) to obtain a GaN thin film. This invention uses oxygen plasma pretreatment to enhance the hydrophilicity of the GaN surface, making the etching electrolyte easier to wet and allowing bubbles to detach quickly; simultaneously, it reduces local stress on the film surface, decreasing the probability of film breakage. The resulting GaN thin film, which does not require support layer removal, has the advantages of large area, low roughness, and easy integration.

Description

Technical Field

[0001] This invention relates to the field of heterogeneous integration technology of III-V compound semiconductor materials, and particularly to a method for preparing large-area gallium nitride thin films. Background Technology

[0002] GaN thin films are widely used in RF / power devices, Micro-LEDs, lasers, flexible sensors, and heterogeneous integrated devices due to their high breakdown field strength, high thermal conductivity, and high electron saturation drift velocity. High-quality, large-area, low-roughness self-supporting GaN thin films are key materials for the development of these devices towards low cost, large scale, and flexibility.

[0003] Currently, the acquisition of large-area GaN thin films mainly relies on the following processes: a) Laser Lift-off (LLO): This uses a KrF laser to generate localized high-temperature decomposition at the GaN / sapphire interface, but the equipment is expensive, has significant thermal shock, and low yield. b) Mechanical grinding + chemical mechanical polishing: This process is lengthy and results in poor thickness uniformity. c) Electrochemical wet etching: Using GaN as the anode and Pt as the cathode, an anodic voltage is applied in an HF / H2C2O4 / HNO3 electrolyte, causing GaN to be oxidized to Ga2O3 and dissolved, thereby separating the film. This method has simple equipment and low cost, but it has the following drawbacks: 1. Nitrogen gas accumulates at the interface, generating localized stress that leads to film rupture; 2. Dislocation defects preferentially break down, forming pinning points, reducing integrity and increasing roughness; 3. A support layer such as photoresist or SiO2 is required, and the photoresist / silicon oxide removal process is fragile and affects ion exchange, reducing etching efficiency.

[0004] In existing technologies, gallium nitride thin films are generally prepared using a support layer, such as a photoresist or a temporary protective layer like SiO2, to maintain the integrity of the film. This process has several drawbacks, including small area (traditional electrochemical etching can only produce fragments <1 cm²), low integrity (dislocation pinning leads to crack propagation), high roughness (uniform electric field at surface defects, resulting in large differences in etching rates), the introduction of additional processes and contamination by the support layer, and long processing time (the support layer hinders ion diffusion).

[0005] Therefore, how to provide an electrochemical stripping method for gallium nitride thin films with large area, low roughness, and high yield has become an urgent problem to be solved. Summary of the Invention

[0006] To address the aforementioned technical problems, the present invention aims to provide a method for preparing large-area gallium nitride (GaN) thin films. The electrochemical lift-off method for GaN thin films based on oxygen plasma pretreatment described in this invention eliminates the need for a support layer. This novel oxygen plasma pretreatment process achieves a single lift-off area ≥ 2 inches (50 mm) in diameter, a breakage rate < 5%, and an N-polarity RMS roughness ≤ 1 nm (5 × 5 μm).2 It achieves excellent results by reducing the peeling time by ≥40%, and the process used is compatible with existing electrochemical equipment, eliminating the need for expensive lasers and reducing costs.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing a large-area gallium nitride thin film, the method comprising the following steps:

[0009] (1) Perform oxygen plasma pretreatment on gallium nitride wafers with electrodes on their surface;

[0010] (2) Without the support layer, the gallium nitride wafer obtained in step (1) is placed in the electrolyte through the anode electrode clamp, and a constant voltage is applied to perform electrochemical etching;

[0011] (3) The gallium nitride wafer obtained in step (2) is vertically immersed in deionized water with the electrochemical etched surface and peeled off to obtain a gallium nitride thin film.

[0012] This invention employs oxygen plasma pretreatment (OPT) technology to enhance the hydrophilicity of gallium nitride (GaN) surfaces, reducing the contact angle from 45° to below 10°. This makes the etching electrolyte easier to wet and allows bubbles to detach quickly. Furthermore, OPT can passivate GaN surface defects, reducing the probability of preferential corrosion at defect sites. Oxygen free radicals form Ga-O bonds on the GaN surface, making the surface bandgap more gradual, the electric field more uniform, and reducing the roughness of the thin film.

[0013] On the one hand, oxygen plasma pretreatment can introduce oxygen-containing functional groups (such as hydroxyl -OH). Under the action of oxygen plasma, the original chemical bond Ga-N on the material surface is broken, forming free radicals, which then react with active oxygen species in the plasma (such as O, O2). + O - The surface readily combines with polar hydroxyl (-OH) groups, which are highly hydrophilic and can significantly reduce the water contact angle, thus enhancing surface wettability. Furthermore, the inventors' experimental characterization showed that the contact angle decreased from 42.0° to below 3.8° after oxygen plasma treatment, indicating the formation of numerous hydrophilic groups on the surface.

[0014] On the other hand, oxygen plasma pretreatment can achieve surface cleaning and micro / nano structure changes. Oxygen plasma pretreatment (OPT) also has the effect of suppressing gallium nitride surface states, removing surface organic contaminants, and helping water molecules spread more easily, thereby further enhancing hydrophilicity.

[0015] During oxygen plasma treatment, oxygen atoms diffuse into the surface or near-surface region of the material and combine with dangling bonds at dislocations to form Ga-O bonds. These Ga-O bonds can saturate the non-bonding electronic states at dislocations, thereby reducing the electrical activity of dislocations and achieving a "passivation" effect. Furthermore, oxygen plasma treatment may also form a thin oxide layer (such as Ga2O3) on the surface. This oxide layer can shield the influence of dislocations on carrier transport and, through stress field adjustment, reduce dislocation density or inhibit their expansion, indirectly playing a passivation role.

[0016] Compared with existing technologies, the method provided by this invention simplifies the process and avoids debonding and breakage without the need for a support layer. The electrochemical etching time is shorter (etching time is reduced from 60 min to 10 min for the same area), while the yield is improved (the breakage rate is reduced from 40% to <5%). The prepared film has a large area (2-inch wafer can be completely peeled off in one go, and it can be extended to 4-6 inches) and low roughness. Furthermore, the method of this invention is equipment compatible and only requires the addition of a plasma pretreatment process before the traditional electrochemical reaction, resulting in low cost.

[0017] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The technical objectives and beneficial effects of the present invention can be better achieved and realized through the following preferred technical solutions.

[0018] The thickness of the GaN thin film on the surface of the gallium nitride wafer in step (1) is 50nm-1μm, for example, it can be 50nm, 100nm, 150nm, 200nm, 220nm, 250nm, 280nm, 300nm, 320nm, 350nm, 380nm, 400nm, 450nm, 500nm, 550nm, 600nm, 650nm, 700nm, 750nm, 800nm, 850nm, 900nm, 950nm or 1μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 200nm-500nm.

[0019] This invention further controls the thickness of the gallium nitride thin film on the surface of the gallium nitride wafer to 50nm-1μm. Oxygen plasma pretreatment has failure problems. As the thickness of the gallium nitride thin film on the surface of the gallium nitride wafer increases, the failure mechanism induced by oxygen plasma changes from "dominated by penetrating defects in thin layers" to "dominated by stress-cracks and interface state accumulation in thick layers". The self-limiting thickening of the oxide layer reduces deep lattice damage, but the mismatch stress of Ga2O3 / GaN on the surface increases, which can easily lead to microcracks and local peeling. At the same time, the interface traps formed by oxygen diffusion can still lead to high field concentration and premature breakdown. The overall failure threshold increases with the increase of thickness.

[0020] In step (1) of this invention, the electrode preparation on the surface of gallium nitride wafer can be directly carried out using existing technologies, such as using simple methods like silver plating or indium spotting with a soldering iron as electrodes; or by metal deposition (such as Ti / Al) followed by rapid annealing at 800℃-850℃ in a nitrogen atmosphere to obtain a gallium nitride wafer with ohmic electrodes on its surface.

[0021] The gallium nitride wafers used in this invention are prepared by epitaxial growth methods, such as MOCVD / MBE, with MOCVD being preferred for growing gallium nitride epitaxial structures.

[0022] The structure of the gallium nitride wafer used in step (1) of this invention is as follows: Figure 5 As shown, 1: Substrate (silicon, sapphire, self-supporting gallium nitride, etc.); 2: Buffer layer: unintentionally doped gallium nitride (including nucleation layer to improve the growth quality of subsequent layers), thickness 1-3 μm; 3: Current diffusion layer: lightly silicon-doped gallium nitride (3-7 × 10⁻⁶ m² / 440 nm). 18 cm -3 The thickness is 200-800 nm. When voltage is applied, a lightly doped current diffusion layer ensures uniform voltage across the entire etched sample. (Note: The doping concentration must be lower than that of the sacrificial layer. When applying the optimal etching voltage, the sacrificial layer must be completely etched (non-porous etching), while the current diffusion layer must be kept uncorroded or only slightly etched, as corrosion of the current diffusion layer leads to uneven voltage transmission, thus affecting the uniformity of film etching); 4: Current blocking layer: Unintentionally doped gallium nitride (200-800 nm) as an intermediate layer to avoid interference between the current diffusion layer and the sacrificial layer; 5: Sacrificial layer: Heavy silicon doped gallium nitride (0.8-10×10⁻⁶ nm). 19 cm -3 The thickness is 200nm-1μm. Highly doped gallium nitride will be oxidized to an intermediate product under a suitable voltage, and then dissolved and reacted with the etching solution; 6: Target layer: Undoped gallium nitride thin film (50nm-1μm). Unintentionally doped gallium nitride, after being peeled off using oxygen plasma treatment to prepare a high-quality gallium nitride thin film, is transferred to the target substrate (Si, SOI, PDMS, etc.) for subsequent device fabrication.

[0023] Preferably, step (1) before preparing electrodes on the surface of the gallium nitride wafer includes cleaning the gallium nitride wafer.

[0024] Preferably, the cleaning includes sequential organic cleaning and inorganic cleaning.

[0025] Preferably, the organic cleaning includes ultrasonic cleaning with acetone, isopropanol and water for 5-10 minutes respectively, for example, 5 minutes, 8 minutes or 10 minutes, but not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0026] Preferably, the inorganic cleaning includes immersion in hydrochloric acid for 1-5 minutes, for example, 1 minute, 3 minutes or 8 minutes, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0027] Preferably, the equipment used for the oxygen plasma pretreatment (OPT) in step (1) is an oxygen plasma desmearing machine.

[0028] Preferably, during the oxygen plasma pretreatment process, the flow rate of O2 is 20 sccm-100 sccm, for example, it can be 20 sccm, 30 sccm, 40 sccm, 50 sccm, 60 sccm, 70 sccm, 80 sccm, 90 sccm or 100 sccm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 40 sccm-60 sccm.

[0029] This invention further controls the O2 flow rate to 20 sccm-100 sccm. The O2 flow rate affects the uniformity of charge density. Plasma pretreatment fundamentally changes the physicochemical state of the material surface through two mechanisms: physical bombardment and chemical reaction, thereby affecting its surface energy and ultimately manifesting as a change in surface wettability.

[0030] Surface wettability is usually measured by the water contact angle: a small contact angle indicates hydrophilicity; a large contact angle indicates hydrophobicity.

[0031] If the O2 flux is too low, the concentration of reactive oxygen species generated in the plasma will be relatively low. In this case, the physical bombardment effect may dominate, while the chemical effect will be weak. Although physical sputtering can effectively remove light organic contaminants and adsorbed hydrocarbons from the surface, the oxide layer may not be uniform or thick enough. The surface wettability changes from hydrophobic to weakly hydrophilic. This is because surface contaminants are removed and a small number of polar groups are introduced. However, the effect may be incomplete and unstable, and the treated surface may recover its hydrophobicity more quickly over time.

[0032] If the O2 flow rate is too high, the concentration of reactive oxygen free radicals and ions in the reaction chamber increases significantly, and the intensity and rate of the chemical reaction become dominant. High concentrations of reactive oxygen can react violently with the GaN surface, forming a thicker, potentially more porous Ga2O3 layer. This carries the risk of over-etching and may cause excessive oxidation of the GaN surface, even forming a rough or defect-prone oxide layer. If the oxide layer is too thick, its quality may be poor. The surface becomes highly hydrophilic due to the formation of a thick oxide layer rich in strong polar bonds such as Ga-O and Ga-OH, resulting in extremely high surface energy. However, the surface micro-roughness may also increase due to over-oxidation. While this further enhances hydrophilicity and prolongs its duration, it may introduce unwanted surface states.

[0033] Preferably, during the oxygen plasma pretreatment process, the radio frequency (RF) power is 30W-400W, for example, it can be 30W, 100W, 150W, 200W, 250W, 300W, 350W or 400W, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 70W-120W.

[0034] This invention further controls the radio frequency (RF) power to 30W-400W. The RF power affects the bombardment energy and the damage to the gallium nitride (GaN) surface. If the RF power is too high, the high-density plasma will rapidly and deeply oxidize the GaN through physicochemical synergy, and the high bombardment energy will easily damage the GaN surface, causing surface roughness / damage and surface lattice damage. If the RF power is too low, the plasma density will be low, resulting in mild oxidation, low processing efficiency, and insignificant effect.

[0035] RF power affects anisotropy and temperature; if the power is too high, the overall temperature of the sample will be too high, affecting the binding of hydroxyl groups, physicochemical synergy, anisotropy, high activity, extreme hydrophilicity, causing severe lattice damage and roughening; if the power is too low, the reaction becomes a purely chemical reaction, isotropic, with limited effect, incomplete treatment, and unstable effect.

[0036] Preferably, the oxygen plasma pretreatment time is 30s-150s, for example, it can be 30s, 50s, 80s, 100s or 150s, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 60s-90s.

[0037] This invention further controls the oxygen plasma pretreatment time to 30s-150s. The oxygen plasma pretreatment time affects the passivation depth and the roughness of the gallium nitride surface. If the oxygen plasma pretreatment time is too long, the efficiency is low, and damage will accumulate under high-energy conditions, easily causing an increase in the surface roughness of the sample. If the oxygen plasma pretreatment time is too short, the effect is insufficient, the hydrophilization effect is incomplete, uneven, and unstable, the pretreatment fails, and the effect is unreliable.

[0038] Preferably, during the oxygen plasma pretreatment process, the cavity pressure is 5mTorr-50mTorr, for example, it can be 5mTorr, 10mTorr, 15mTorr, 20mTorr, 25mTorr, 30mTorr, 35mTorr, 40mTorr, 45mTorr or 50mTorr, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 20mTorr-30mTorr.

[0039] Preferably, during the oxygen plasma pretreatment, the temperature of the gallium nitride wafer is ≤60°C.

[0040] Preferably, the electrolyte in step (2) includes a first acid solution and / or a second acid solution.

[0041] Preferably, the first acid solution comprises nitric acid with a concentration of 0.1M-1M, such as 0.1M, 0.2M, 0.3M, 0.4M, 0.5M, 0.6M, 0.7M, 0.8M, 0.9M or 1M, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 0.1M-0.4M.

[0042] This invention further controls the concentration of nitric acid to 0.1M-1M. The concentration of acid affects the roughness of the final GaN film. Nitric acid is a strong acid that is completely ionized. When the acid concentration is low, the corrosion rate is slow, the original defects on the surface are not fully removed, and the roughness is high. At a medium concentration (0.05-0.5M), the corrosion is uniform and proceeds in an atomic-layer step manner, the roughness is reduced to the minimum, and the surface is the smoothest. However, at a high concentration (≥0.5M), defects are preferentially dissolved to form pits, and the roughness increases significantly. Therefore, the acid concentration needs to be controlled within a moderate range to obtain the best surface smoothness.

[0043] Preferably, the second acid solution comprises a mixture of oxalic acid and hydrofluoric acid.

[0044] Preferably, the concentration of oxalic acid is 0.1M-0.5M, for example, it can be 0.1M, 0.2M, 0.3M, 0.4M or 0.5M, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0045] Preferably, the concentration of the hydrofluoric acid is 0.1M-0.5M, for example, it can be 0.1M, 0.2M, 0.3M, 0.4M or 0.5M-4M, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0046] Preferably, the pH of the electrolyte is 0.3-1, for example, it can be 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0047] Preferably, during the electrochemical etching process, a gallium nitride wafer is used as the anode and a Pt sheet is used as the cathode, and the distance between the gallium nitride thin film electrochemically stripped from the anode based on oxygen plasma pretreatment and the cathode is 1cm-3cm.

[0048] Preferably, the power supply used in the electrochemical corrosion process is a Keithley 2450 power meter.

[0049] Preferably, the constant voltage applied during the electrochemical corrosion process is 10V-40V, for example, it can be 10V, 15V, 20V, 25V, 28V, 30V, 32V, 35V, 38V or 40V, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 20V-32V.

[0050] This invention further controls the application of a constant voltage of 10V-40V during the electrochemical corrosion process. The higher the voltage, the higher the corrosion rate, and the faster rate increases the roughness of the N-polar film.

[0051] Preferably, during the electrochemical corrosion process, when the current density decreases to 0 mA / cm² 2 Electrochemical corrosion is completed at that time.

[0052] Preferably, the substrate in step (3) includes polyethylene terephthalate (PET), silicon wafer, silicon oxide wafer or SOI wafer.

[0053] As a preferred embodiment of the method described in this invention, the method includes the following steps:

[0054] (I) For gallium nitride wafers with a GaN film thickness of 200nm-500nm to be stripped, use acetone, isopropanol and water to sonicate for 5min-10min in sequence, and then use hydrochloric acid (volume fraction of 40%-60%) to immerse for 1min-5min.

[0055] (II) The method for preparing electrodes on the surface of a cleaned gallium nitride wafer includes: using an electric soldering iron to plate indium particles on the wafer surface and sidewalls as electrodes to obtain a gallium nitride wafer with electrodes prepared on its surface;

[0056] (III) The gallium nitride wafer with electrodes on its surface is subjected to oxygen plasma pretreatment using an oxygen plasma resist remover. During the oxygen plasma pretreatment, the flow rate of O2 is 40 sccm-60 sccm, the cavity pressure is 5 mTorr-50 mTorr, the RF power is 70 W-120 W, and the oxygen plasma pretreatment time is 60 s-90 s.

[0057] (IV) Without the support layer, the gallium nitride wafer obtained in step (III) is placed in an electrolyte, with the gallium nitride wafer as the anode and the Pt sheet as the cathode, and a constant voltage of 20V-32V is applied for electrochemical etching. When the current density decreases to 0 mA / cm², the etching process continues. 2 During this process, electrochemical etching is completed, and GaN thin films are retrieved using a substrate.

[0058] The electrolyte includes nitric acid with a concentration of 0.1M-1M;

[0059] (V) Immerse the GaN film on the substrate from step (IV) in deionized water for cleaning, and then dry the substrate to obtain a gallium nitride film.

[0060] In a second aspect, the present invention provides a gallium nitride thin film, which is prepared according to the method described in the first aspect; the area of ​​the gallium nitride thin film is ≥2cm² and the roughness is <1nm.

[0061] The gallium nitride thin film provided by this invention has the advantages of large area, low roughness, and no support layer. The area of ​​a single peel can be scaled up to the wafer level, such as 2 inches (circle with a diameter of 50 mm) or 4 inches (circle with a diameter of 100 mm). The breakage rate is <5%, and the RMS roughness of the N-polar surface is ≤1 nm (5 μm). 2 ×5μm 2 ).

[0062] Thirdly, the present invention provides an application of gallium nitride thin film as described in the second aspect, wherein the gallium nitride thin film is used in fields such as integrated optoelectronics, flexible electronics, AR / VR displays, radio frequency and power.

[0063] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0064] Compared with the prior art, the present invention has at least the following beneficial effects:

[0065] (1) The present invention uses oxygen plasma pretreatment technology to improve the hydrophilicity of gallium nitride surface and reduce the contact angle from 45° to below 10°, thereby making the corrosion electrolyte easier to wet and the bubbles to detach quickly; in addition, oxygen plasma pretreatment can also passivate dislocations on gallium nitride surface, reduce the probability of preferential corrosion at defects, and oxygen free radicals form Ga-O bonds on GaN surface, introduce fixed negative charges, make the surface band bending more gentle, and the electric field more uniform, reducing the roughness of the film, and improving the peeling quality and efficiency under the condition of no support layer.

[0066] (2) The gallium nitride thin film provided by the present invention has the advantages of large area, low roughness, and no support layer. The area of ​​a single peeling is ≥2 inch (50 mm) in diameter, the breakage rate is <5%, and the RMS roughness of the N polar surface is ≤1 nm (5 μm). 2 ×5μm 2 ). Attached Figure Description

[0067] Figure 1 This is a photograph of the contact angle of the GaN surface before OPT in Embodiment 1 of the present invention;

[0068] Figure 2 This is a photograph of the contact angle of the GaN surface after OPT in Embodiment 1 of the present invention;

[0069] Figure 3 The three-dimensional AFM morphology of the N-polar surface of the GaN thin film obtained in Example 1 of this invention (RMS=0.42nm);

[0070] Figure 4 This is an optical photograph of the GaN thin film obtained in Example 1 of the present invention;

[0071] Figure 5 This is the wafer structure of the GaN wafer in step (Ⅰ) of Embodiment 1 of the present invention, wherein 1-sapphire substrate, 2-buffer layer: undoped gallium nitride, 3-current diffusion layer: light silicon doped gallium nitride, 4-current blocking layer: undoped gallium nitride, 5-sacrificial layer: heavy silicon doped gallium nitride, 6-gallium nitride thin film;

[0072] Figure 6 This is the structure of the GaN wafer during electrochemical etching in step (Ⅳ) of Embodiment 1 of the present invention;

[0073] Figure 7 It is the GaN thin film obtained in step (V) of Embodiment 1 of the present invention. Detailed Implementation

[0074] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0075] In this invention, the terms "first aspect," "second aspect," "third aspect," "fourth aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first," "second," "third," "fourth," etc., serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on the quantity.

[0076] Unless otherwise specified, all reagents and consumables used in the following examples and comparative examples were purchased from conventional reagent manufacturers in the art; unless otherwise specified, the experimental methods and techniques used were conventional methods and techniques in the art.

[0077] Example 1

[0078] This embodiment provides a method for preparing a large-area gallium nitride thin film, the method comprising the following steps:

[0079] (I) The GaN wafer to be stripped with a GaN film thickness of 320 nm was ultrasonically treated with acetone, ethanol and water for 10 min in sequence, and then acid-immersed with hydrochloric acid for 3 min.

[0080] (II) The method for preparing electrodes on the surface of a cleaned GaN wafer includes: using an electric soldering iron to plate indium particles on the surface and sidewalls of the wafer as electrodes to obtain a GaN wafer with ohmic electrodes prepared on the surface.

[0081] (III) The GaN wafer with electrodes on its surface is subjected to oxygen plasma pretreatment (OPT) using an oxygen plasma stripper. During the oxygen plasma pretreatment, the flow rate of O2 is 50 sccm, the cavity pressure is 20 mTorr, the RF power is 80 W, the oxygen plasma pretreatment time is 30 s, and the temperature of the GaN wafer is ≤60℃.

[0082] (IV) Without the support layer, the GaN wafer obtained in step (III) is placed in an electrolyte, with the GaN wafer as the anode and the Pt sheet as the cathode, and a constant voltage of 30V is applied for electrochemical etching. When the current density decreases to 0mA / cm 2 Electrochemical etching was completed, and GaN thin films were retrieved using an SOI substrate;

[0083] The electrolyte includes 0.3M nitric acid and has a pH of 1.5.

[0084] (V) Immerse the GaN film on the SOI substrate from step (Ⅳ) in deionized water for cleaning, and then dry the substrate to obtain the GaN film.

[0085] Contact angle photos of GaN surfaces before and after OPT are compared to, for example... Figure 1 and Figure 2 As shown, from Figure 1 and Figure 2 As can be seen from the data, the contact angle before OPT was 48.0°, and the contact angle after OPT was 2.8°, indicating a significant increase in hydrophilicity.

[0086] The three-dimensional AFM morphology of the N-polar surface of the GaN thin film is as follows: Figure 3 As shown, from Figure 3 From this, we can see that RMS = 0.42nm;

[0087] Optical images of GaN thin films, such as Figure 4 As shown, the obtained GaN film is intact;

[0088] The initial wafer structure of the GaN wafer before electrochemical etching in step (I) is as follows: Figure 5 As shown, 1-sapphire substrate, 2-buffer layer: undoped gallium nitride, 3-current diffusion layer: light silicon-doped gallium nitride, 4-current blocking layer: undoped gallium nitride, 5-sacrificial layer: heavy silicon-doped gallium nitride, 6-gallium nitride thin film;

[0089] The structure of the GaN wafer during electrochemical etching in step (IV) is as follows: Figure 6 As shown, when a voltage is applied, the sacrificial layer 5 begins to etch laterally, its area continuously decreases, and the gallium nitride film 6 above it begins to release.

[0090] After the GaN wafer obtained in step (V) is electrochemically etched, as shown below... Figure 7 As shown, after the sacrificial layer 5 is completely etched, the uppermost gallium nitride film 6 detaches and is released, yielding a GaN film, which is then transferred to the target substrate. After drying, the quality of the Ga-polar and N-polar films (such as roughness) can be characterized.

[0091] Example 2

[0092] This embodiment provides a method for preparing a large-area gallium nitride thin film, the method comprising the following steps:

[0093] (I) The GaN wafer to be stripped with a GaN film thickness of 200 nm was ultrasonically treated with acetone, ethanol and water for 5 min in sequence, and then acid-immersed with hydrochloric acid for 1 min.

[0094] (II) The method for preparing electrodes on the surface of a cleaned GaN wafer includes: using an electric soldering iron to plate indium particles on the surface and sidewalls of the wafer as electrodes to obtain a GaN wafer with ohmic electrodes prepared on the surface.

[0095] (III) The GaN wafer with electrodes on its surface is subjected to oxygen plasma pretreatment using an oxygen plasma stripper. During the oxygen plasma pretreatment, the flow rate of O2 is 20 sccm, the cavity pressure is 5 mTorr, the RF power is 35 W, and the oxygen plasma pretreatment time is 150 s.

[0096] (IV) Without the support layer, the GaN wafer obtained in step (III) is placed in an electrolyte, with the GaN wafer as the anode and the Pt sheet as the cathode, and a constant voltage of 20V is applied for electrochemical etching. When the current density decreases to 0mA / cm 2 Electrochemical corrosion is completed at that time;

[0097] The electrolyte comprises a mixture of 0.3 M oxalic acid and 0.1 M hydrofluoric acid at a volume ratio of 1:1.

[0098] (V) The GaN wafer obtained in step (IV) is vertically immersed in deionized water for peeling, retrieved using a polyethylene terephthalate substrate, and dried to obtain a GaN thin film.

[0099] Example 3

[0100] This embodiment provides a method for preparing a large-area gallium nitride thin film, the method comprising the following steps:

[0101] (I) The GaN wafer to be stripped with a GaN surface film thickness of 400 nm was ultrasonically treated with acetone, ethanol and water for 10 min in sequence, and then acid-immersed with hydrochloric acid for 5 min.

[0102] (II) The method for preparing electrodes on the surface of a cleaned GaN wafer includes: using an electric soldering iron to plate indium particles on the wafer surface and sidewalls as electrodes to obtain a GaN wafer with electrodes prepared on its surface;

[0103] (III) The GaN wafer with electrodes on its surface is subjected to oxygen plasma pretreatment using an oxygen plasma stripper. During the oxygen plasma pretreatment, the flow rate of O2 is 100 sccm, the cavity pressure is 50 mTorr, the RF power is 150 W, and the oxygen plasma pretreatment time is 100 s.

[0104] (IV) Without the support layer, the GaN wafer obtained in step (III) is placed in an electrolyte, with the GaN wafer as the anode and the Pt sheet as the cathode, and a constant voltage of 32V is applied for electrochemical etching. When the current density decreases to 0mA / cm 2 Electrochemical corrosion is completed at that time;

[0105] The electrolyte includes nitric acid with a concentration of 1M;

[0106] (V) The GaN wafer obtained in step (IV) is vertically immersed in deionized water for peeling, retrieved using a polyethylene terephthalate substrate, and dried to obtain a GaN thin film.

[0107] Example 4

[0108] This embodiment provides a method for preparing large-area gallium nitride thin films. The only difference from Embodiment 1 is that the flow rate of O2 in the oxygen plasma pretreatment in step (III) is 10 sccm, while the other preparation steps remain unchanged.

[0109] Example 5

[0110] This embodiment provides a method for preparing large-area gallium nitride thin films. The only difference from Embodiment 1 is that the flow rate of O2 in the oxygen plasma pretreatment in step (III) is 120 sccm, while the other preparation steps remain unchanged.

[0111] Example 6

[0112] This embodiment provides a method for preparing large-area gallium nitride thin films. The only difference from Embodiment 1 is that the RF power of the oxygen plasma pretreatment in step (III) is 20W, while the other preparation steps remain unchanged.

[0113] Example 7

[0114] This embodiment provides a method for preparing large-area gallium nitride thin films. The only difference from Embodiment 1 is that the RF power of the oxygen plasma pretreatment in step (III) is 420W, while the other preparation steps remain unchanged.

[0115] Example 8

[0116] This embodiment provides a method for preparing large-area gallium nitride thin films. The only difference from Embodiment 1 is that the oxygen plasma pretreatment time in step (III) is 15s, while the other preparation steps remain unchanged.

[0117] Example 9

[0118] This embodiment provides a method for preparing large-area gallium nitride thin films. The only difference from Embodiment 1 is that the oxygen plasma pretreatment time in step (III) is 350s, while the other preparation steps remain unchanged.

[0119] Example 10

[0120] This embodiment provides a method for preparing a large-area gallium nitride thin film. The only difference from Embodiment 1 is that the thickness of the GaN thin film on the surface of the gallium nitride wafer in step (I) is 40 nm, while the other preparation steps remain unchanged.

[0121] Example 11

[0122] This embodiment provides a method for preparing a large-area gallium nitride thin film. The only difference from Embodiment 1 is that the thickness of the GaN thin film on the surface of the gallium nitride wafer in step (I) is 1.3 μm, while the other preparation steps remain unchanged.

[0123] Example 12

[0124] This embodiment provides a method for preparing large-area gallium nitride thin films. The only difference from Embodiment 1 is that the concentration of nitric acid in step (III) is 0.05M, while the other preparation steps remain unchanged.

[0125] Example 13

[0126] This embodiment provides a method for preparing large-area gallium nitride thin films. The only difference from Embodiment 1 is that the concentration of nitric acid in step (III) is 1.3M, while the other preparation steps remain unchanged.

[0127] Comparative Example 1

[0128] This comparative example provides a method for electrochemical stripping of gallium nitride thin films. The only difference from Example 1 is that the oxygen plasma pretreatment in step (III) is replaced with nitrogen plasma pretreatment, while the other preparation conditions remain unchanged.

[0129] Comparative Example 2

[0130] This comparative example provides a method for electrochemical stripping of gallium nitride thin films. The only difference from Example 1 is that the oxygen plasma pretreatment in step (Ⅲ) is replaced with CF4 / O2 mixed plasma pretreatment, while the other preparation conditions remain unchanged.

[0131] Comparative Example 3

[0132] This comparative example provides a method for electrochemical stripping of gallium nitride thin films. The only difference from Example 1 is that the oxygen plasma pretreatment in step (Ⅲ) is replaced with ultraviolet ozone treatment, while the other preparation conditions remain unchanged.

[0133] test:

[0134] The GaN films obtained in the examples and comparative examples were tested, and the test results are shown in Table 1 below.

[0135] Table 1

[0136]

[0137] The test results show that:

[0138] (1) As can be seen from Examples 1-3, the electrochemical stripping method based on oxygen plasma pretreatment provided by the present invention uses oxygen plasma pretreatment (OPT) technology to improve the hydrophilicity of gallium nitride (GaN) surface, reducing the contact angle from 45° to below 10°, thereby making the etching electrolyte easier to wet and bubbles to detach quickly; in addition, oxygen plasma pretreatment can also passivate dislocations on the GaN surface, reduce the probability of preferential corrosion at defects, and oxygen free radicals form Ga-O bonds on the GaN surface, introducing fixed negative charges, making the surface band bending more gradual, the electric field more uniform, and reducing the roughness of the film. The present invention simplifies the process and avoids debonding and breakage without the need for a support layer, and the electrochemical etching time is shorter (etching time is reduced from 20 min to 10 min for the same area, and the stripping time is shortened by ≥40%), while the yield is improved (the breakage rate is reduced from 40% to <5%); the prepared film area is larger (2-inch wafers can be completely stripped in one go, and it can be extended to 4-6 inches) and the roughness is low (N-plane RMS≤1nm).

[0139] (2) By comparing Example 1 with Example 4-5, it can be seen that the present invention further controls the flow rate of O2 to 20sccm-100sccm. The flow rate of O2 affects the uniformity of charge density. When the flow rate of O2 is too high, it will cause severe oxidation to form an excessively thick / loose oxide layer, resulting in a decrease in interface quality and an increase in the cracking rate. If the flow rate of O2 is too low, there will be insufficient reactive oxygen, resulting in a thin oxide layer with poor quality and poor adhesion.

[0140] (3) By comparing Example 1 with Example 6-7, it can be seen that the present invention further controls the RF power to 30W-400W. The RF power affects the bombardment energy and the damage to the gallium nitride surface. If the RF power is too high, the high bombardment energy is likely to cause damage to the gallium nitride surface. If the RF power is too low, the plasma density is low, the treatment is incomplete, the adhesion is extremely poor, and the physical sputtering is weak.

[0141] (4) By comparing Example 1 with Example 8-9, it can be seen that the present invention further controls the oxygen plasma pretreatment time to 30s-150s. The oxygen plasma pretreatment time affects the passivation depth and the roughness of the gallium nitride surface. If the oxygen plasma pretreatment time is too long, it will easily cause the surface roughness of the sample to increase; if the oxygen plasma pretreatment time is too short, the effect will be insufficient, the oxide layer will be uneven, and the adhesion will be extremely poor.

[0142] (5) By comparing Example 1 with Examples 10-11, it can be seen that the present invention further controls the thickness of the gallium nitride wafer used to be 50nm-1μm. For very thin and very thick films, plasma treatment has failure problems. As the thickness of the gallium nitride film on the surface of the gallium nitride wafer increases, the failure mechanism induced by oxygen plasma changes from "through-type defects dominating when the layer is thin" to "stress-crack and interface state accumulation dominating when the layer is thick". The self-limiting thickening of the oxide layer reduces the damage to the deep lattice, but the mismatch stress of Ga2O3 / GaN on the surface increases, which can easily cause microcracks and local peeling. At the same time, the interface traps formed by oxygen diffusion can still lead to high field concentration and premature breakdown. The overall failure threshold increases with the increase of thickness.

[0143] (6) As can be seen from Example 1 and Comparative Example 1, when oxygen plasma pretreatment is replaced with nitrogen plasma treatment, N vacancies are introduced, which increases the defect corrosion rate.

[0144] (7) As can be seen from Example 1 and Comparative Example 2, when oxygen plasma pretreatment is replaced with CF4 / O2 mixed plasma treatment, F free radicals corrode GaN, resulting in surface damage.

[0145] (8) As can be seen from Example 1 and Comparative Example 3, when oxygen plasma pretreatment is replaced by ultraviolet ozone treatment, the uniformity is poor and the difference between the center and edge of the large area wafer is obvious.

[0146] In summary, this invention, through oxygen plasma pretreatment, solves the key problems of small area, low integrity, large roughness, and the need for a support layer in traditional electrochemically exfoliated GaN films without adding expensive equipment, providing a feasible and low-cost technical path for the large-scale preparation of high-quality self-supporting GaN films.

[0147] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing large-area gallium nitride thin films, characterized in that, The method includes the following steps: (1) Perform oxygen plasma pretreatment on gallium nitride wafers with electrodes on their surface; (2) Without the support layer, the gallium nitride wafer obtained in step (1) is placed in the electrolyte through the anode electrode fixture, and a constant voltage is applied to perform electrochemical etching. After the electrochemical etching is complete, the GaN film is retrieved using a substrate. (3) The GaN film on the substrate from step (2) is cleaned and dried to obtain a gallium nitride film; The thickness of the GaN thin film on the surface of the gallium nitride wafer in step (1) is 50 nm-1 μm; The equipment used for the oxygen plasma pretreatment in step (1) is an oxygen plasma desmearing machine; During the oxygen plasma pretreatment process, the O2 flow rate is 20 sccm-100 sccm; During the oxygen plasma pretreatment process, the RF power is 30W-400W; The oxygen plasma pretreatment time is 30s-150s; During the oxygen plasma pretreatment process, the cavity pressure is 5 mTorr-50 mTorr; The electrolyte in step (2) includes a first acid solution and / or a second acid solution; The first acid solution comprises nitric acid with a concentration of 0.1 mol / L to 1 mol / L; The second acid solution comprises a mixture of oxalic acid and hydrofluoric acid; The concentration of oxalic acid is 0.1 mol / L to 0.5 mol / L; The concentration of the hydrofluoric acid is 0.1 mol / L to 0.5 mol / L.

2. The method according to claim 1, characterized in that, Step (1) Before fabricating electrodes on the surface of the gallium nitride wafer, the gallium nitride wafer is also cleaned.

3. The method according to claim 1, characterized in that, The constant voltage applied during the electrochemical corrosion process is 10V-40V.

4. The method according to claim 1, characterized in that, During the electrochemical corrosion process, when the current density decreases to 0 mA / cm 2 Electrochemical corrosion is completed at that time.

5. The method according to claim 1, characterized in that, The method includes the following steps: (I) For gallium nitride wafers with a GaN film thickness of 200nm-500nm to be stripped, use acetone, isopropanol and water to sonicate for 5min-10min in sequence, and then use hydrochloric acid to immerse for 1min-5min. (II) The method for preparing electrodes on the surface of a cleaned gallium nitride wafer includes: using an electric soldering iron to plate indium particles on the wafer surface and sidewalls as electrodes to obtain a gallium nitride wafer with electrodes prepared on its surface; (III) The gallium nitride wafer with electrodes on its surface is subjected to oxygen plasma pretreatment using an oxygen plasma stripper. During the oxygen plasma pretreatment, the flow rate of O2 is 40 sccm-60 sccm, the cavity pressure is 20 mTorr-30 mTorr, the RF power is 70 W-90 W, and the oxygen plasma pretreatment time is 60 s-90 s. (IV) Without the support layer, the gallium nitride wafer obtained in step (III) is placed in an electrolyte, with the gallium nitride wafer as the anode and the Pt sheet as the cathode, and a constant voltage of 20V-32V is applied for electrochemical etching. When the current density decreases to 0mA / cm 2 During this process, electrochemical etching is completed, and GaN thin films are retrieved using a substrate. The electrolyte comprises nitric acid with a concentration of 0.1 mol / L to 1 mol / L; (V) Immerse the GaN film on the substrate from step (IV) in deionized water for cleaning, and then dry the substrate to obtain a gallium nitride film.

6. A gallium nitride thin film, characterized in that, The gallium nitride thin film is prepared by the method according to any one of claims 1-5; The gallium nitride thin film has an area of ​​≥2cm² and a roughness of ≤1nm.

7. An application of the gallium nitride thin film as described in claim 6, characterized in that, The gallium nitride thin film is used in integrated optoelectronics, flexible electronics, AR / VR displays, radio frequency and power applications.

Images