N-plane GaN-based epitaxial structures and their preparation methods based on inverted transfer technology

Abstract

This invention discloses an N-plane GaN-based epitaxial structure based on inverted transfer technology and its fabrication method. The method includes: growing an AlN nucleation layer, an undoped GaN buffer layer, an AlN insertion layer, an undoped GaN layer, an AlGaN layer, an undoped GaN channel layer, an AlN spacer layer, an AlGaN barrier layer, and a Si-doped graded AlN layer on a Si substrate. x Ga 1‑ x An N-plane barrier layer, a Si-doped GaN spacer layer, and an Fe-doped semi-insulating GaN layer are constructed. A SiC substrate is bonded to the Fe-doped semi-insulating GaN layer using an inverted transfer technique. The Si substrate is removed down to an undoped GaN buffer layer, and self-terminating etching is achieved in the AlN insertion layer using a high etch selectivity ratio between the undoped GaN buffer layer and the AlN insertion layer. The AlN insertion layer is then removed to obtain an N-plane GaN-based epitaxial structure. This invention avoids the difficulties of directly growing N-plane GaN, improves heat dissipation, and allows for precise control of the remaining material thickness at the etch stop layer.

Description

Technical Field

[0001] This invention belongs to the field of semiconductors, specifically relating to an N-plane GaN-based epitaxial structure based on inverted transfer technology and its preparation method. Background Technology

[0002] With the development of technology, the increasing demand for high data transmission rates has driven the frequency utilization of the millimeter-wave band (30–300 GHz). Compared to materials such as Si, GaAs, and InP, GaN and SiC have larger band gaps (Eg) and higher critical breakdown field strengths, making them more suitable for high-voltage, high-power applications. Compared to 4H-SiC, GaN not only exhibits a better electron saturation velocity, but GaN-based heterostructures can also achieve higher electron mobility and carrier concentration. Furthermore, GaN-based epitaxial structures can be grown on high thermal conductivity substrates such as SiC and diamond, providing excellent heat dissipation characteristics. Due to these advantages, GaN-based materials exhibit excellent JFOM (Johnson Figure of Merit), BFoM (Baliga Figure of Merit), and BHFFoM (Baliga High-Frequency Figure of Merit), thus finding wide applications in RF microwaves, power switches, 5G communications, satellite communications, and other civilian and military radar applications.

[0003] GaN, a group III-V compound semiconductor, has a wurtzite structure as its most commonly used and stable structure. Because the positive and negative charges in GaN crystals are not centrosymmetric along the c-axis, a large spontaneous polarization field exists in the c-axis direction. Depending on the polarization direction, GaN grown along the

[0001] direction is Ga-polarized, while GaN grown along the [000-1] direction is N-polarized. Early research on GaN HEMTs largely focused on devices grown along the Ga-polarized direction. However, with the increase in application frequency bands, the principle of proportional scaling places higher demands on the design and fabrication of small-size device structures. Traditional Ga-polarized GaN materials have almost reached their application bottleneck and are difficult to meet the future needs of millimeter-wave and higher frequency RF power applications. Compared with conventional Ga-polar GaN materials, N-polar GaN materials have unparalleled superior properties: (1) The N-polar GaN-based heterostructure has a natural back barrier, which can confine the wave function of 2DEG to the GaN / AlGaN interface, improve carrier confinement, and give the device better off-state characteristics; (2) In N-polar GaN-based HEMT devices, the formation of ohmic tunneling only needs to pass through the GaN layer with a smaller bandgap, which is conducive to achieving lower ohmic contact resistance and reducing knee voltage; (3) For ultra-high frequency applications, when realizing small-size GaN-based HEMT devices, N-polar GaN-based HEMTs are based on flip-chip structures, which effectively shortens the distance between the gate and the channel, making it more conducive to realizing high-performance high-frequency power devices. Therefore, N-polar GaN-based epitaxial structures have greater potential for applications in millimeter-wave and higher frequency bands.

[0004] N-faceted GaN materials (i.e., N-polar GaN materials) are generally grown using direct epitaxial growth techniques such as MOCVD (metal-organic chemical vapor deposition) and MBE (molecular beam epitaxy). However, direct growth of N-faceted GaN materials is difficult and costly. Currently, only the University of California, Santa Barbara (UCSB) can obtain high-quality N-faceted GaN-based epitaxial materials suitable for radio frequency (RF) applications through direct epitaxy. Moreover, while growing GaN materials on Si substrates is less expensive, its heat dissipation is poor, limiting its application in RF power applications.

[0005] The inverted transfer technique avoids the difficulties of directly growing N-polar GaN. This technique utilizes the characteristic that GaN's two polarity directions differ by 180°, obtaining N-polar GaN through processes such as bonding and removing the original substrate of the Ga-polar GaN-based epitaxial material. However, during the etching process to remove the original material, it is difficult to precisely control the thickness of the remaining GaN, leading to uncertainty in the thickness of the remaining N-polar GaN material and causing significant etching damage, resulting in increased surface roughness.

[0006] Therefore, how to solve the above problems and obtain high-quality N-plane GaN-based epitaxial structures is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] To address the aforementioned problems in the prior art, this invention provides an N-plane GaN-based epitaxial structure based on inverted transfer technology and its fabrication method. The technical problem to be solved by this invention is achieved through the following technical solution:

[0008] In a first aspect, embodiments of the present invention provide a method for preparing an N-plane GaN-based epitaxial structure based on inverted transfer technology, the method comprising:

[0009] After obtaining and pre-treating the Si substrate, an AlN nucleation layer and an undoped GaN buffer layer are grown sequentially on it.

[0010] An AlN insertion layer is grown on the undoped GaN buffer layer as an etch stop layer;

[0011] An undoped GaN layer, an AlGaN layer, an undoped GaN channel layer, an AlN spacer layer, an AlGaN barrier layer, and a Si-doped graded Al layer are sequentially grown on the AlN insertion layer. x Ga 1-x N barrier layer, Si-doped GaN spacer layer and Fe-doped semi-insulating GaN layer;

[0012] After processing the Fe-doped semi-insulating GaN layer and the obtained SiC substrate, the SiC substrate is bonded to the Fe-doped semi-insulating GaN layer using an inverted transfer technique to obtain the bonded structure.

[0013] The Si substrate, AlN nucleation layer, and undoped GaN buffer layer in the bonded structure are removed sequentially, and the etching self-termination of the AlN insertion layer is achieved by utilizing the high etching selectivity ratio between the undoped GaN buffer layer and the AlN insertion layer.

[0014] For the obtained material structure, the AlN insertion layer is removed by ALE etching technology, and the undoped GaN layer with N polarity exposed after the removal of the AlN insertion layer is surface treated to obtain an N-plane GaN-based epitaxial structure.

[0015] In one embodiment of the present invention, the preprocessing includes:

[0016] The Si substrate was thermally cleaned in hydrogen gas.

[0017] In one embodiment of the present invention, the process of sequentially growing an AlN nucleation layer and an undoped GaN buffer layer includes:

[0018] Using trimethylaluminum (TMAl) and ammonia (NH3) as precursors for Al and N respectively, AlN nucleation layers were grown in multiple stages under set temperature and pressure conditions by adjusting the flow rates of TMAl and NH3.

[0019] At 1150°C, trimethylgallium (TMGa) and NH3 were used as precursors for Ga and N, respectively, and hydrogen was used as the carrier gas. The flow rate of TMGa gas was set to 300 sccm to form an undoped GaN buffer layer on the AlN nucleation layer.

[0020] In one embodiment of the present invention, an AlN insertion layer is grown on the undoped GaN buffer layer as an etch stop layer, comprising:

[0021] At 1100°C, TMAl and NH3 were used as precursors for Al and N, respectively, and hydrogen was used as the carrier gas. The flow rate of TMAl was set to 280 sccm to form an AlN insertion layer on the undoped GaN buffer layer.

[0022] In one embodiment of the present invention, the thickness of the AlN insertion layer is 1. ～ 5nm.

[0023] In one embodiment of the present invention, an undoped GaN layer, an AlGaN layer, an undoped GaN channel layer, an AlN spacer layer, an AlGaN barrier layer, and a Si-doped graded AlN layer are sequentially grown on the AlN insertion layer. x Ga 1-x The structure comprises an N-type barrier layer, a Si-doped GaN spacer layer, and an Fe-doped semi-insulating GaN layer, including:

[0024] At 1100℃, TMGa and NH3 were used as Ga and N precursors, respectively, and hydrogen was used as carrier gas. The flow rate of the TMGa gas source was set to 100 sccm to form an undoped GaN layer on the AlN insertion layer.

[0025] At 1100℃, TMAl, TMGa and NH3 are used as precursors for Al, Ga and N respectively, and hydrogen is used as a carrier gas to react the three to form an AlGaN layer on the undoped GaN layer, wherein the aluminum composition of the AlGaN layer is 0.22 to 0.27.

[0026] At 1100℃, TMGa and NH3 were used as precursors for Ga and N, respectively, and hydrogen was used as the carrier gas. The flow rate of the TMGa gas source was set to 100 sccm to form an undoped GaN channel layer on the AlGaN layer.

[0027] At 1100℃, TMAl and NH3 were used as Al and N precursors, respectively, and hydrogen was used as carrier gas. The flow rate of the TMAl gas source was set to 260 sccm to form an AlN spacer layer on the undoped GaN channel layer.

[0028] At 1100℃, TMAl, TMGa and NH3 are used as precursors for Al, Ga and N respectively, and hydrogen is used as a carrier gas to carry out the reaction of the three to form an AlGaN barrier layer on the AlN spacer layer; wherein the aluminum composition in the AlGaN barrier layer is 0.33 to 0.39.

[0029] At 1100℃, using TMAl, TMGa, and NH3 as precursors for Al, Ga, and N respectively, and hydrogen as a carrier gas, the three react to form Si-doped graded Al on the AlGaN barrier layer. x Ga 1-x N-barrier layer; wherein the Si-doped graded Al x Ga 1-x In the N-type barrier layer, the aluminum composition x gradually changes from 0.05 to 0.05, and the Si doping concentration is 3 × 10⁻⁶. 18 ~6×10 18 cm -3 ;

[0030] At 1100°C, using TMGa and NH3 as Ga and N precursors respectively, and hydrogen as the carrier gas, the Si-doped graded Al... x Ga 1-x A Si-doped GaN spacer layer is formed on the N-type barrier layer; wherein the Si doping concentration in the Si-doped GaN spacer layer is 3 × 10⁻⁶. 18 ~6×10 18 cm -3 ;

[0031] At 1100℃, TMGa and NH3 are used as precursors for Ga and N, respectively, and hydrogen is used as a carrier gas to allow TMGa, NH3 and ferrocene to react simultaneously, so as to form an Fe-doped semi-insulating GaN layer on the Si-doped GaN spacer layer.

[0032] In one embodiment of the present invention, after processing the Fe-doped semi-insulating GaN layer and the obtained SiC substrate, the SiC substrate is bonded to the Fe-doped semi-insulating GaN layer using an inverted transfer technique to obtain a bonded structure, including:

[0033] The surfaces of the Fe-doped semi-insulating GaN layer and the SiC substrate to be bonded are chemically and mechanically polished, followed by surface cleaning.

[0034] On the surface of the obtained Fe-doped semi-insulating GaN layer, a silicon nitride layer is deposited using PECVD. Subsequently, Ni / Cu / Au stacked metal is obtained by electron beam evaporation on the deposited silicon nitride surface, thus obtaining the first material structure.

[0035] On the surface of the obtained SiC substrate, a layer of silicon nitride is deposited using PECVD. Subsequently, Ni / Cu / Au multilayer metal is obtained by electron beam evaporation on the deposited silicon nitride surface, thus obtaining the second material structure. The thicknesses of each metal layer in the Ni / Cu / Au multilayer metal are 50 nm, 500 nm, and 20 nm, respectively.

[0036] The Fe-doped semi-insulating GaN layer in the first material structure is bonded to the SiC substrate in the second material structure using Au-Au thermal compression bonding, so that the second material structure is stacked in reverse on top of the first material structure; then the entire structure is flipped 180° so that the SiC substrate is at the bottom, resulting in the bonded structure.

[0037] In one embodiment of the present invention, the Si substrate, AlN nucleation layer, and undoped GaN buffer layer in the bonded structure are removed sequentially, and the high etch selectivity ratio between the undoped GaN buffer layer and the AlN insertion layer is utilized to achieve self-termination of the etch in the AlN insertion layer, including:

[0038] The surface of the Si substrate located at the top of the bonded structure is subjected to chemical mechanical polishing.

[0039] The Si substrate in the bonded structure is removed by etching using an inductively coupled plasma etching process, taking advantage of the high etching selectivity of SF6 plasma for Si / AlN.

[0040] After etching away the Si substrate, the exposed N-polar AlN layer surface is first removed by BCL3 / CL2 plasma etching to remove the AlN nucleation layer and part of the undoped GaN buffer layer. Then, the remaining undoped GaN buffer layer is removed by BCL3 / Ar plasma etching. Taking advantage of the high etching selectivity of GaN / AlN, the etching self-termination is achieved in the AlN insertion layer. The flow rate of BCL3 is 100 sccm, the flow rate of Ar is 80 sccm, the upper electrode power of the etching equipment is 100 W, the lower electrode power is 20 W, and the pressure is 5 mTorr.

[0041] In one embodiment of the present invention, for the obtained material structure, the AlN insertion layer is removed by ALE etching technology, and the undoped N-polar GaN layer exposed after the removal of the AlN insertion layer is surface treated to obtain an N-plane GaN-based epitaxial structure, including:

[0042] The AlN insertion layer was removed by etching at a preset rate using CL2 / Ar plasma and atomic layer etching technology. The flow rate of CL2 was 100 sccm, the flow rate of Ar was 80 sccm, the upper electrode power of the etching equipment was 100 W, the lower electrode power was 10 W, and the pressure was 5 mTorr.

[0043] The surface of the exposed N-polar undoped GaN layer was chemically and mechanically polished, followed by surface cleaning, to obtain an N-plane GaN-based epitaxial structure.

[0044] Secondly, embodiments of the present invention provide an N-plane GaN-based epitaxial structure based on inverted transfer technology, which is prepared according to the preparation method of the N-plane GaN-based epitaxial structure based on inverted transfer technology described in the first aspect.

[0045] This invention utilizes GaN-based materials and employs an inverted transfer technique to fabricate a high-quality N-plane GaN-based epitaxial structure. It offers the following advantages:

[0046] 1. Avoiding the difficulty of directly growing N-face GaN-based epitaxial materials. Specifically, this invention employs an inverted transfer technique, inverting high-quality Ga-face GaN-based epitaxial material by 180°, and obtaining N-face GaN-based epitaxial materials through bonding, etching, and other processes, effectively avoiding the difficulty of directly epitaxially growing N-face GaN-based epitaxial materials.

[0047] 2. Improving the problem of poor heat dissipation on Si substrates. Although growing GaN materials on existing Si substrates is low-cost, its poor heat dissipation limits its application in radio frequency power. This invention removes the original Si substrate and uses SiC as the substrate for N-plane GaN-based epitaxial materials through bonding, greatly improving the thermal conductivity of the material.

[0048] 3. Based on inverted transfer technology, the thickness of the remaining N-polar GaN is precisely controlled, reducing surface roughness and process complexity. After bonding, the original material needs to be removed by etching. Precise control of the remaining GaN thickness during this process is difficult, leading to significant etching damage. This invention inserts an AlN layer as an etching stop layer between the undoped GaN buffer layer and the undoped GaN layer. Utilizing the high etching selectivity ratio (above 60 for N-polar GaN to AlN) between the undoped GaN buffer layer and the AlN insertion layer, self-termination of etching is achieved on the AlN insertion layer, enabling precise control of the remaining epitaxial material thickness. Attached Figure Description

[0049] Figure 1 This is a schematic flowchart illustrating a method for fabricating an N-plane GaN-based epitaxial structure based on inverted transfer technology, as provided in an embodiment of the present invention.

[0050] Figures 2a to 2m This is a schematic diagram illustrating the fabrication process of an N-plane GaN-based epitaxial structure based on inverted transfer technology, as provided in an embodiment of the present invention. Detailed Implementation

[0051] The present invention will be further described in detail below with reference to specific embodiments, but the implementation of the present invention is not limited thereto.

[0052] In a first aspect, embodiments of the present invention provide a method for preparing an N-plane GaN-based epitaxial structure based on inverted transfer technology, such as... Figure 1 As shown, the method may include the following steps:

[0053] S1, after obtaining the Si substrate and performing pretreatment, an AlN nucleation layer and an undoped GaN buffer layer are grown sequentially on it.

[0054] The thickness of the Si substrate can be selected as needed. The pretreatment includes:

[0055] The Si substrate was thermally cleaned in hydrogen gas to remove residual oxides from the surface. For details on Si substrate pretreatment, please refer to [link to relevant documentation]. Figure 2a understand.

[0056] The process of sequentially growing an AlN nucleation layer and an undoped GaN buffer layer on the obtained Si substrate includes the following two steps, the results of which can be found in [link to documentation]. Figure 2b As shown:

[0057] 1) Using trimethylaluminum (TMAl) and ammonia (NH3) as precursors for Al and N respectively, AlN nucleation layers were grown in multiple stages under set temperature and pressure conditions by adjusting the flow rates of TMAl and NH3.

[0058] Specifically, TMAl and high-purity ammonia (NH3) were used as precursors for Al and N, respectively. TMAl gas was introduced at 1000℃ and 70 Torr pressure for a pre-seedling time of 20 s to prevent amorphous SiN from forming. x The formation of the layer involved a TMAl flow rate of 200 sccm. AlN nucleation layers of 15–30 nm thickness were deposited on a Si substrate at temperatures of 700, 800, 900, and 1100 °C, with TMAl and NH3 flow rates set to 300 sccm and 15 slm, respectively, and a growth pressure of 60 Torr. Subsequently, under constant source gas flow rate and growth pressure, an AlN nucleation layer of 150–180 nm thickness was grown at 1100 °C.

[0059] 2) At 1150°C, trimethylgallium (TMGa) and NH3 were used as precursors for Ga and N, respectively, and hydrogen was used as the carrier gas. The flow rate of TMGa gas was set to 300 sccm to form an undoped GaN buffer layer on the AlN nucleation layer.

[0060] The thickness of the undoped GaN buffer layer can be 1–1.3 μm.

[0061] S2, an AlN insertion layer is grown on the undoped GaN buffer layer as an etch stop layer;

[0062] Specifically, S2 may include the following steps:

[0063] At 1100°C, TMAl and NH3 were used as precursors for Al and N, respectively, and hydrogen was used as the carrier gas. The flow rate of TMAl was set to 280 sccm to form an AlN insertion layer on the undoped GaN buffer layer.

[0064] The AlN insertion layer will serve as the subsequent etch stop layer.

[0065] In one optional embodiment, the thickness of the AlN insertion layer is 1–5 nm.

[0066] The results of this step can be found in [link / reference]. Figure 2c As shown.

[0067] S3, on the AlN insertion layer, an undoped GaN layer, an AlGaN layer, an undoped GaN channel layer, an AlN spacer layer, an AlGaN barrier layer, and a Si-doped graded Al layer are sequentially grown. x Ga 1-x N barrier layer, Si-doped GaN spacer layer and Fe-doped semi-insulating GaN layer;

[0068] In one alternative implementation, please refer to Figure 2dS3 may include the following steps:

[0069] S31, at 1100℃, TMGa and NH3 are used as Ga and N precursors respectively, hydrogen is used as carrier gas, and the flow rate of the TMGa gas source is set to 100 sccm to form an undoped GaN layer on the AlN insertion layer.

[0070] The thickness of the undoped GaN layer can be 20–50 nm.

[0071] S32, at 1100℃, using TMAl, TMGa, and NH3 as precursors for Al, Ga, and N respectively, and hydrogen as a carrier gas, the three react to form an AlGaN layer on the undoped GaN layer.

[0072] The aluminum content of the AlGaN layer is 0.22–0.27; the thickness of the AlGaN layer can be 2–3 nm.

[0073] S33, at 1100℃, TMGa and NH3 are used as Ga and N precursors respectively, hydrogen is used as carrier gas, and the flow rate of the TMGa gas source is set to 100 sccm to form an undoped GaN channel layer on the AlGaN layer.

[0074] The thickness of the undoped GaN channel layer can be 8–15 nm.

[0075] S34, at 1100℃, TMAl and NH3 are used as Al and N precursors respectively, hydrogen is used as carrier gas, and the flow rate of the TMAl gas source is set to 260 sccm to form an AlN spacer layer on the undoped GaN channel layer.

[0076] The thickness of the AlN spacer layer can be 0.6–1 nm.

[0077] S35, at 1100℃, TMAl, TMGa and NH3 are used as precursors of Al, Ga and N respectively, and hydrogen is used as carrier gas to carry out the reaction of the three to form an AlGaN barrier layer on the AlN spacer layer.

[0078] The aluminum content in the AlGaN barrier layer is 0.33–0.39; the thickness of the AlGaN barrier layer can be 10–20 nm.

[0079] S36, at 1100℃, using TMAl, TMGa, and NH3 as precursors for Al, Ga, and N respectively, and hydrogen as a carrier gas, the three react to form Si-doped graded Al on the AlGaN barrier layer. x Ga 1-xN-barrier layer;

[0080] Among them, the Si-doped graded Al x Ga 1-x In the N-type barrier layer, the aluminum composition x gradually changes from 0.05 to 0.05, and the Si doping concentration is 3 × 10⁻⁶. 18 ~6×10 18 cm -3 Si-doped graded Al x Ga 1-x The thickness of the N-barrier layer can be 20 nm.

[0081] S37, at 1100℃, using TMGa and NH3 as Ga and N precursors respectively, and hydrogen as a carrier gas, in the Si-doped graded Al... x Ga 1-x A Si-doped GaN spacer layer is formed on the N-barrier layer;

[0082] The Si doping concentration in the Si-doped GaN spacer layer is 3 × 10⁻⁶. 18 ~6×10 18 cm -3 The thickness of the Si-doped GaN spacer layer can be 8–12 nm.

[0083] S38, at 1100℃, TMGa and NH3 are used as precursors for Ga and N respectively, and hydrogen is used as a carrier gas, so that TMGa, NH3 and ferrocene react simultaneously to form an Fe-doped semi-insulating GaN layer on the Si-doped GaN spacer layer.

[0084] The thickness of the Fe-doped semi-insulating GaN layer can be 0.5–1.5 μm.

[0085] In step S3, Ga polar GaN-based epitaxial material is grown on a Si substrate.

[0086] S4. After processing the Fe-doped semi-insulating GaN layer and the obtained SiC substrate, the SiC substrate is bonded on the Fe-doped semi-insulating GaN layer using inverted transfer technology to obtain the bonded structure.

[0087] Please see Figure 2e The two figures above and below illustrate that the Fe-doped semi-insulating GaN layer and the obtained SiC substrate need to be processed. Specifically, S4 may include the following steps:

[0088] S41, perform chemical mechanical polishing on the surface of the Fe-doped semi-insulating GaN layer and the surface of the SiC substrate to be bonded, followed by surface cleaning;

[0089] Chemical mechanical polishing (CMP) is a process that uses CMP (Chemical Mechanical Planarization / Polishing) to grind surfaces and reduce surface roughness.

[0090] Surface cleaning is performed using a sulfuric acid-hydrogen peroxide mixed solution.

[0091] S42, on the surface of the obtained Fe-doped semi-insulating GaN layer, a layer of silicon nitride is deposited using PECVD process, and then Ni / Cu / Au stacked metal is obtained by electron beam evaporation on the deposited silicon nitride surface, thereby obtaining the first material structure;

[0092] The thickness of the deposited silicon nitride can be 10–20 nm. For the first material structure, please refer to [link / reference needed]. Figure 2f Indication.

[0093] S43, a layer of silicon nitride is deposited on the surface of the obtained SiC substrate using PECVD process, and then Ni / Cu / Au stacked metal is obtained by electron beam evaporation on the deposited silicon nitride surface, thereby obtaining the second material structure;

[0094] The thickness of the deposited silicon nitride can be 10–20 nm.

[0095] In the Ni / Cu / Au multilayer metal, the thicknesses of each metal layer are 50nm, 500nm, and 20nm, respectively.

[0096] See the second material structure. Figure 2g Indication.

[0097] S44, the Fe-doped semi-insulating GaN layer in the first material structure is bonded to the SiC substrate in the second material structure by Au-Au thermal compression bonding, so that the second material structure is stacked on top of the first material structure in reverse; then the whole structure is flipped 180° so that the SiC substrate is at the bottom, and the bonded structure is obtained.

[0098] For the bonding process, please refer to [link / reference]. Figure 2h Understanding. For the bonded structure obtained after the overall flipping, please refer to [link / reference needed]. Figure 2i As can be seen, the SiC substrate is located at the bottom, and the Si substrate is located at the top.

[0099] S5, sequentially remove the Si substrate, AlN nucleation layer, and undoped GaN buffer layer from the bonded structure, and utilize the high etching selectivity ratio between the undoped GaN buffer layer and the AlN insertion layer to achieve self-termination of etching in the AlN insertion layer;

[0100] In one optional implementation, S5 may include the following steps:

[0101] S51, perform chemical mechanical polishing on the surface of the Si substrate located at the top of the bonded structure;

[0102] Chemical mechanical polishing (CMP) is a process of grinding surfaces to reduce surface roughness.

[0103] S52, utilizing the high etching selectivity of sulfur hexafluoride (SF6) plasma for Si / AlN, the Si substrate in the bonded structure is etched away by inductively coupled plasma etching process;

[0104] Please see Figure 2j Because of the high etching selectivity of sulfur hexafluoride (SF6) plasma for Si / AlN, it can etch Si but not AlN. Therefore, it is possible to etch the Si substrate while retaining the AlN layer.

[0105] S53, the surface of the N-polar AlN layer exposed after etching away the Si substrate is first removed by BCL3 / CL2 plasma etching to remove the AlN nucleation layer and part of the undoped GaN buffer layer; then the remaining undoped GaN buffer layer is removed by BCL3 / Ar plasma etching, and the etching self-termination is achieved in the AlN insertion layer by utilizing the high etching selectivity of GaN / AlN. The flow rate of BCL3 is 100 sccm, the flow rate of Ar is 80 sccm, the upper electrode power of the etching equipment is 100W, the lower electrode power is 20W, and the pressure is 5mTorr.

[0106] In step S53, the AlN nucleation layer and part of the undoped GaN buffer layer are first removed by rapid etching with BCL3 / CL2 plasma at a rate of 25 nm / min. Then, the remaining undoped GaN buffer layer is removed by slow etching with BCL3 / Ar plasma at a rate of 6 nm / min. During this etching process, due to the high etch selectivity of GaN / AlN, the undoped GaN buffer layer is etched away, but the AlN insertion layer is not etched.

[0107] S6. For the obtained material structure, the AlN insertion layer is removed by ALE (Atomic Layer Etching) etching technology, and the undoped GaN layer with N polarity exposed after the removal of the AlN insertion layer is surface treated to obtain an N-plane GaN-based epitaxial structure.

[0108] Please see Figure 2k In one optional implementation, S6 may include:

[0109] S61, using CL2 / Ar plasma, the AlN insertion layer is removed by atomic layer etching at a preset rate; wherein, the flow rate of CL2 is 100 sccm, the flow rate of Ar is 80 sccm, the upper electrode power of the etching equipment is 100W, the lower electrode power is 10W, and the pressure is 5mTorr.

[0110] Please see Figure 2l In this step, the preset etching rate is an extremely low rate, specifically 0.2 nm / min, to slowly etch away the AlN insertion layer. This extremely low etching rate allows for precise control of the etching depth, ensuring that the AlN insertion layer is completely removed without significantly etching the underlying N-polarity undoped GaN layer.

[0111] After the AlN insertion layer is removed, an undoped GaN layer, which is N-polar, will be exposed.

[0112] S62, the surface of the exposed N-polar undoped GaN layer is chemically and mechanically polished, followed by surface cleaning, to obtain an N-plane GaN-based epitaxial structure.

[0113] Chemical mechanical polishing (CMP) is a process of grinding surfaces to reduce surface roughness.

[0114] Surface cleaning was performed using a sulfuric acid-hydrogen peroxide mixed solution. After surface polishing and cleaning, a smooth N-polar GaN surface was finally obtained.

[0115] For the final obtained N-plane GaN-based epitaxial structure (i.e., N-polar GaN-based epitaxial structure), please refer to [link to documentation]. Figure 2m As shown.

[0116] N-polar GaN materials are typically obtained using direct epitaxial growth techniques such as MOCVD and MBE. However, direct growth of N-polar GaN is difficult and costly. The inverted transfer technique can avoid these difficulties. This technique utilizes the characteristic that GaN's two polarity directions differ by 180°, obtaining N-polar GaN through processes such as bonding and removing the original substrate of the Ga-polar GaN-based epitaxial material. However, during the etching process to remove the original material, it is difficult to precisely control the thickness of the remaining GaN, leading to uncertainty in the thickness of the remaining N-polar GaN channel layer and causing significant etching damage and increased surface roughness.

[0117] Specifically, to ensure the growth quality of conventional Ga-polar GaN materials, a GaN buffer layer of more than 1 μm is usually grown on the substrate. However, in order to obtain N-polar GaN through the inversion transfer technique, this buffer layer needs to be etched away, and a portion of GaN is required as a channel layer, with a thickness of about 10-20 nm. Due to the large thickness of this GaN portion, it is necessary to balance the etching rate with the thickness of the remaining GaN material. Without an effective etching stop layer, it is difficult to achieve a balance between efficiency and accuracy. Furthermore, the resulting N-polar GaN-based epitaxial material has a simple structure and generally poor quality, making it unsuitable for use in RF power devices.

[0118] The AlN etch stop layer designed in this invention solves the above problems, is compatible with complex epitaxial structures, and precisely controls the remaining material thickness while ensuring process efficiency. This allows for the fabrication of high-quality and complex N-polar GaN-based epitaxial material structures, based on which high-performance RF power devices can be fabricated. Conventional AlN etch stop layers are Ga-polar and have never been used in the fabrication of N-polar GaN materials. This invention uses N-polar AlN as the etch stop layer, and the specific process parameters used for etching have been repeatedly modulated and verified.

[0119] In summary, this invention utilizes GaN-based materials and employs an inverted transfer technique to fabricate a high-quality N-plane GaN-based epitaxial structure. It offers the following advantages:

[0120] 1. Avoiding the difficulty of directly growing N-face GaN-based epitaxial materials. Specifically, this invention employs an inverted transfer technique, inverting high-quality Ga-face GaN-based epitaxial material by 180°, and obtaining N-face GaN-based epitaxial materials through bonding, etching, and other processes, effectively avoiding the difficulty of directly epitaxially growing N-face GaN-based epitaxial materials.

[0121] 2. Improving the problem of poor heat dissipation on Si substrates. Although growing GaN materials on existing Si substrates is low-cost, its poor heat dissipation limits its application in radio frequency power. This invention removes the original Si substrate and uses SiC as the substrate for N-plane GaN-based epitaxial materials through bonding, greatly improving the thermal conductivity of the material.

[0122] 3. Precisely control the thickness of the remaining N-polar GaN to reduce surface roughness and process complexity. After bonding, the original material needs to be removed by etching. Precisely controlling the thickness of the remaining GaN during this process is difficult, leading to significant etching damage. This invention inserts an AlN layer as an etching stop layer between the undoped GaN buffer layer and the undoped GaN layer. Utilizing the high etching selectivity ratio (above 60 for N-polar GaN to AlN) between the undoped GaN buffer layer and the AlN insertion layer, self-termination of etching is achieved on the AlN insertion layer, enabling precise control of the remaining epitaxial material thickness.

[0123] Secondly, corresponding to the above method embodiments, this invention also provides an N-plane GaN-based epitaxial structure based on inverted transfer technology, prepared using the method for preparing an N-plane GaN-based epitaxial structure based on inverted transfer technology described in the first aspect. The structure is as follows... Figure 2m As shown, from bottom to top, they include:

[0124] SiC substrate, silicon nitride layer, Ni / Cu / Au metal stack, Ni / Cu / Au metal stack, silicon nitride layer, Fe-doped semi-insulating GaN layer, Si-doped GaN spacer layer, Si-doped graded Al x Ga 1-x N-type barrier layer, AlGaN barrier layer, AlN spacer layer, undoped GaN channel layer, AlGaN layer, and undoped GaN layer.

[0125] The thickness of the SiC substrate can be set as needed.

[0126] The thickness of the silicon nitride can be 10–20 nm.

[0127] In the Ni / Cu / Au metal stack, the thickness of each metal layer can be 50nm, 500nm, or 20nm, respectively.

[0128] The thickness of the Fe-doped semi-insulating GaN layer can be 0.5–1.5 μm;

[0129] The Si doping concentration in the Si-doped GaN spacer layer is 3 × 10⁻⁶. 18 ~6×10 18 cm -3 The thickness of the Si-doped GaN spacer layer can be 8–12 nm.

[0130] The Si-doped graded Al x Ga 1-x In the N-type barrier layer, the aluminum composition x gradually changes from 0.05 to 0.05, and the Si doping concentration is 3 × 10⁻⁶.18 ~6×10 18 cm -3 Si-doped graded Al x Ga 1-x The thickness of the N-barrier layer can be 20 nm;

[0131] The aluminum composition in the AlGaN barrier layer is 0.33–0.39; the thickness of the AlGaN barrier layer can be 10–20 nm.

[0132] The thickness of the AlN spacer layer can be 0.6–1 nm;

[0133] The thickness of the undoped GaN channel layer can be 8–15 nm;

[0134] The aluminum composition of the AlGaN layer is 0.22–0.27; the thickness of the AlGaN layer can be 2–3 nm.

[0135] The thickness of the undoped GaN layer can be 20–50 nm.

[0136] For specific details regarding the N-plane GaN-based epitaxial structure based on the inverted transfer technique, please refer to the relevant content in the first aspect, which will not be repeated here.

[0137] This invention provides a high-quality N-polar GaN-based epitaxial material obtained through an inverted transfer technique. Specifically, it involves bonding, etching, and chemical mechanical polishing to transform a high-quality Ga-polar GaN-based epitaxial material into a high-quality N-polar GaN-based epitaxial material, avoiding the difficulties of directly growing N-faceted GaN-based epitaxial materials. In this invention, the initial substrate for epitaxial GaN-based epitaxial material is a Si substrate, and the substrate to be bonded is a SiC substrate. Utilizing the high thermal conductivity of SiC, the thermal conductivity of the final epitaxial material is significantly improved, mitigating the poor heat dissipation of the Si substrate. This allows the device to operate at a higher bias point, ultimately achieving higher output power. Furthermore, the use of an AlN insertion layer as an etch stop layer (with an N-polar GaN to AlN etch selectivity ratio of over 60) allows for precise control of the remaining epitaxial material thickness, thus enabling precise control of the remaining N-polar GaN thickness and reducing surface roughness and process complexity.

[0138] It should be noted that, in the description of this invention, the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0139] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. In addition, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0140] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the scope of protection of the present invention.

Claims

1. A method for fabricating an N-plane GaN-based epitaxial structure based on inverted transfer technology, characterized in that, include: After obtaining and pre-treating the Si substrate, an AlN nucleation layer and an undoped GaN buffer layer are grown sequentially on it. An AlN insertion layer is grown on the undoped GaN buffer layer as an etch stop layer; An undoped GaN layer, an AlGaN layer, an undoped GaN channel layer, an AlN spacer layer, an AlGaN barrier layer, and a Si-doped graded Al layer are sequentially grown on the AlN insertion layer. x Ga 1-x N barrier layer, Si-doped GaN spacer layer and Fe-doped semi-insulating GaN layer; After processing the Fe-doped semi-insulating GaN layer and the obtained SiC substrate, the SiC substrate is bonded to the Fe-doped semi-insulating GaN layer using an inverted transfer technique to obtain the bonded structure. The Si substrate, AlN nucleation layer, and undoped GaN buffer layer in the bonded structure are removed sequentially, and the etching self-termination of the AlN insertion layer is achieved by utilizing the high etching selectivity ratio between the undoped GaN buffer layer and the AlN insertion layer. For the obtained material structure, the AlN insertion layer is removed by ALE etching technology, and the undoped GaN layer with N polarity exposed after the removal of the AlN insertion layer is surface treated to obtain an N-plane GaN-based epitaxial structure.

2. The method according to claim 1, characterized in that, The preprocessing includes: The Si substrate was thermally cleaned in hydrogen gas.

3. The method according to claim 1, characterized in that, The process of sequentially growing an AlN nucleation layer and an undoped GaN buffer layer includes: Using trimethylaluminum (TMAl) and ammonia (NH3) as precursors for Al and N respectively, AlN nucleation layers were grown in multiple stages under set temperature and pressure conditions by adjusting the flow rates of TMAl and NH3. At 1150°C, trimethylgallium (TMGa) and NH3 were used as precursors for Ga and N, respectively, and hydrogen was used as the carrier gas. The flow rate of TMGa gas was set to 300 sccm to form an undoped GaN buffer layer on the AlN nucleation layer.

4. The method according to claim 1, characterized in that, Growing an AlN insertion layer as an etch stop layer on the undoped GaN buffer layer includes: At 1100°C, TMAl and NH3 were used as precursors for Al and N, respectively, and hydrogen was used as the carrier gas. The flow rate of TMAl was set to 280 sccm to form an AlN insertion layer on the undoped GaN buffer layer.

5. The method according to claim 4, characterized in that, The thickness of the AlN insertion layer is 1–5 nm.

6. The method according to claim 1, characterized in that, An undoped GaN layer, an AlGaN layer, an undoped GaN channel layer, an AlN spacer layer, an AlGaN barrier layer, and a Si-doped graded Al layer are sequentially grown on the AlN insertion layer. x Ga 1-x The structure comprises an N-type barrier layer, a Si-doped GaN spacer layer, and an Fe-doped semi-insulating GaN layer, including: At 1100℃, TMGa and NH3 were used as Ga and N precursors, respectively, and hydrogen was used as carrier gas. The flow rate of the TMGa gas source was set to 100 sccm to form an undoped GaN layer on the AlN insertion layer. At 1100℃, TMAl, TMGa and NH3 are used as precursors for Al, Ga and N respectively, and hydrogen is used as a carrier gas to react the three to form an AlGaN layer on the undoped GaN layer, wherein the aluminum composition of the AlGaN layer is 0.22 to 0.

27. At 1100℃, TMGa and NH3 were used as precursors for Ga and N, respectively, and hydrogen was used as the carrier gas. The flow rate of the TMGa gas source was set to 100 sccm to form an undoped GaN channel layer on the AlGaN layer. At 1100℃, TMAl and NH3 were used as Al and N precursors, respectively, and hydrogen was used as carrier gas. The flow rate of the TMAl gas source was set to 260 sccm to form an AlN spacer layer on the undoped GaN channel layer. At 1100℃, TMAl, TMGa and NH3 are used as precursors for Al, Ga and N respectively, and hydrogen is used as a carrier gas to carry out the reaction of the three to form an AlGaN barrier layer on the AlN spacer layer; wherein the aluminum composition in the AlGaN barrier layer is 0.33 to 0.

39. At 1100℃, using TMAl, TMGa, and NH3 as precursors for Al, Ga, and N respectively, and hydrogen as a carrier gas, the three react to form Si-doped graded Al on the AlGaN barrier layer. x Ga 1-x N-barrier layer; wherein the Si-doped graded Al x Ga 1-x In the N-type barrier layer, the aluminum composition x gradually changes from 0.05 to 0.05, and the Si doping concentration is 3 × 10⁻⁶. 18 ~6×10 18 cm -3 ; At 1100°C, using TMGa and NH3 as Ga and N precursors respectively, and hydrogen as the carrier gas, the Si-doped graded Al... x Ga 1-x A Si-doped GaN spacer layer is formed on the N-type barrier layer; wherein the Si doping concentration in the Si-doped GaN spacer layer is 3 × 10⁻⁶. 18 ~6×10 18 cm -3 ; At 1100℃, TMGa and NH3 are used as precursors for Ga and N, respectively, and hydrogen is used as a carrier gas to allow TMGa, NH3 and ferrocene to react simultaneously, so as to form an Fe-doped semi-insulating GaN layer on the Si-doped GaN spacer layer.

7. The method according to claim 1, characterized in that, After processing the Fe-doped semi-insulating GaN layer and the obtained SiC substrate, the SiC substrate is bonded to the Fe-doped semi-insulating GaN layer using an inverted transfer technique to obtain the bonded structure, including: The surfaces of the Fe-doped semi-insulating GaN layer and the SiC substrate to be bonded are chemically and mechanically polished, followed by surface cleaning. On the surface of the obtained Fe-doped semi-insulating GaN layer, a silicon nitride layer is deposited using PECVD. Subsequently, Ni / Cu / Au stacked metal is obtained by electron beam evaporation on the deposited silicon nitride surface, thus obtaining the first material structure. On the surface of the obtained SiC substrate, a layer of silicon nitride is deposited using PECVD. Subsequently, Ni / Cu / Au multilayer metal is obtained by electron beam evaporation on the deposited silicon nitride surface, thus obtaining the second material structure. The thicknesses of each metal layer in the Ni / Cu / Au multilayer metal are 50 nm, 500 nm, and 20 nm, respectively. The Fe-doped semi-insulating GaN layer in the first material structure is bonded to the SiC substrate in the second material structure using Au-Au thermal compression bonding, so that the second material structure is stacked in reverse on top of the first material structure; then the entire structure is flipped 180° so that the SiC substrate is at the bottom, resulting in the bonded structure.

8. The method according to claim 7, characterized in that, The Si substrate, AlN nucleation layer, and undoped GaN buffer layer in the bonded structure are removed sequentially. Then, utilizing the high etch selectivity ratio between the undoped GaN buffer layer and the AlN insertion layer, self-termination of the etch in the AlN insertion layer is achieved, including: The surface of the Si substrate located at the top of the bonded structure is subjected to chemical mechanical polishing. The Si substrate in the bonded structure is removed by etching using an inductively coupled plasma etching process, taking advantage of the high etching selectivity of SF6 plasma for Si / AlN. After etching away the Si substrate, the exposed N-polar AlN layer surface is first removed by BCL3 / CL2 plasma etching to remove the AlN nucleation layer and part of the undoped GaN buffer layer. Then, the remaining undoped GaN buffer layer is removed by BCL3 / Ar plasma etching. Taking advantage of the high etching selectivity of GaN / AlN, the etching self-termination is achieved in the AlN insertion layer. The flow rate of BCL3 is 100 sccm, the flow rate of Ar is 80 sccm, the upper electrode power of the etching equipment is 100 W, the lower electrode power is 20 W, and the pressure is 5 mTorr.

9. The method according to claim 8, characterized in that, For the obtained material structure, the AlN insertion layer is removed using ALE etching technology, and the exposed N-polarity undoped GaN layer after the AlN insertion layer removal is surface-treated to obtain an N-plane GaN-based epitaxial structure, including: The AlN insertion layer was removed by etching at a preset rate using CL2 / Ar plasma and atomic layer etching technology. The flow rate of CL2 was 100 sccm, the flow rate of Ar was 80 sccm, the upper electrode power of the etching equipment was 100 W, the lower electrode power was 10 W, and the pressure was 5 mTorr. The surface of the exposed N-polar undoped GaN layer was chemically and mechanically polished, followed by surface cleaning, to obtain an N-plane GaN-based epitaxial structure.

10. An N-plane GaN-based epitaxial structure based on inverted transfer technology, characterized in that, The N-plane GaN-based epitaxial structure based on the inverted transfer technique was prepared according to any one of claims 1-9.

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