N-surface GaN-based epitaxial structure based on inversion transfer technology and preparation method thereof

Abstract

The invention discloses an N-face GaN-based epitaxial structure based on an inversion transfer technology and a preparation method of the N-face GaN-based epitaxial structure. The method comprises the steps that an AlN nucleating 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, a Si-doped gradient Al < x > Ga < 1-x > N barrier layer, a Si-doped GaN spacer layer and a Fe-doped semi-insulating GaN layer are grown on a Si substrate; bonding a SiC substrate on the Fe-doped semi-insulating GaN layer by using an inversion transfer technology; removing the Si substrate to the non-doped GaN buffer layer, and realizing etching self-termination on the AlN insertion layer by using the high etching selection ratio of the non-doped GaN buffer layer and the AlN insertion layer; and removing the AlN insertion layer to obtain the N-surface GaN-based epitaxial structure. According to the invention, the difficulty of directly growing N-side GaN can be avoided, the heat dissipation is improved, and the etching stop layer can realize the accurate regulation and control of the thickness of the residual material.

Description

Technical Field

[0001] The present invention belongs to the field of semiconductors, and in particular relates to an N-face GaN-based epitaxial structure based on an inversion transfer technology and a preparation method thereof. Background Art

[0002] With the development of the times, the growing demand for high data transmission rates has driven the frequency utilization of the millimeter wave band (30-300 GHz). Compared with material systems such as Si, GaAs, and InP, the larger band gap (Eg) and higher critical breakdown field strength make GaN and SiC more suitable for high-voltage and high-power applications. Compared with 4H-SiC, GaN not only exhibits a better electron saturation rate, but GaN-based heterostructures can also achieve higher electron mobility and carrier concentration. At the same time, GaN-based epitaxial structures can be grown on high-thermal conductivity substrates such as SiC and diamond, with excellent heat dissipation characteristics. Due to these advantages, GaN-based material systems exhibit excellent JFOM (Johnson Figure of Merit), BFoM (Baliga Figure of Merit), and BHFFoM (Baliga High-Frequency Figure of Merit), and therefore have wide applications in RF microwaves, power switches, 5G communications, satellite communications, and other civil and military radar fields.

[0003] As a III-V compound semiconductor, GaN has the most commonly used and stable structure, the wurtzite structure. Because the positive and negative charges in GaN crystals are non-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-polar, while GaN grown along the [000-1] direction is N-polar. Early research on GaN HEMTs focused on devices grown in the Ga-polar direction. However, with the increase in application frequency bands, the principle of proportional reduction has placed higher requirements on the design and preparation of small-sized device structures. Traditional Ga-polar GaN materials have almost reached their application bottleneck and are unlikely 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 incomparable excellent properties: (1) There is a natural back barrier in the N-polar GaN-based heterostructure, which can confine the wave function of 2DEG to the GaN / AlGaN interface, improve the carrier confinement, and make the device have 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 width, which is conducive to achieving lower ohmic contact resistance and lowering the knee voltage; (3) For ultra-high frequency applications, when realizing small-sized GaN-based HEMT devices, N-polar GaN-based HEMTs are based on a flip-chip structure, which effectively shortens the distance between the gate and the channel, making it more conducive to the realization of high-performance high-frequency power devices. Therefore, N-polar GaN-based epitaxial structures have greater potential for application in millimeter wave and higher frequency bands.

[0004] N-face 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-face GaN materials is difficult and expensive. Currently, only the University of California, Santa Barbara (UCSB) can obtain high-quality N-face GaN-based epitaxial materials suitable for RF applications through direct epitaxy. Moreover, in the existing technology, although the cost of growing GaN materials on Si substrates is low, their heat dissipation is poor, which limits their application in RF power.

[0005] The difficulty of directly growing N-polar GaN can be avoided by using the inverted transfer technique. The inverted transfer technique utilizes the 180° difference between the two polar directions of GaN and produces N-polar GaN through bonding and removal of the original substrate of Ga-polar GaN-based epitaxial material. However, in the process of removing the original material by etching, it is difficult to accurately control the thickness of the remaining GaN, resulting in uncertainty in the thickness of the remaining N-polar GaN material, and will cause significant etching damage and increase surface roughness.

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

[0007] In order to solve the above problems existing in the prior art, the present invention provides an N-face GaN-based epitaxial structure based on inverted transfer technology and a preparation method thereof. The technical problem to be solved by the present invention is achieved through the following technical solutions:

[0008] In a first aspect, an embodiment of the present invention provides an N-face GaN-based epitaxial structure and a method for preparing the same based on an inverted transfer technique, the method comprising:

[0009] After obtaining a Si substrate and pre-treating it, an AlN nucleation layer and an undoped GaN buffer layer are sequentially grown thereon;

[0010] Growing an AlN insertion layer 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, a Si-doped graded AlN layer, and the like 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 inversion transfer technique to obtain a bonded structure;

[0013] Sequentially removing the Si substrate, the AlN nucleation layer, and the undoped GaN buffer layer from the bonded structure, and utilizing a high etching selectivity between the undoped GaN buffer layer and the AlN insertion layer to achieve etching self-termination in the AlN insertion layer;

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

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

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

[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 of Al and N, respectively, under set temperature and pressure conditions, by adjusting the flow rates of TMAl and NH3, an AlN nucleation layer is grown in multiple steps.

[0019] At 1150° C., trimethylgallium (TMGa) and NH 3 are used as Ga and N precursors, respectively, and hydrogen is used as a carrier gas. The flow rate of TMGa gas is set to 300 sccm to form an undoped GaN buffer layer on the AlN nucleation layer.

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

[0021] At 1100° C., TMAl and NH 3 were used as precursors of Al and N, respectively, and hydrogen was used as a 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, a Si-doped graded AlN layer, and a Si-doped graded AlN 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, including:

[0024] At 1100° C., using TMGa and NH 3 as Ga and N precursors, respectively, and hydrogen as a carrier gas, with a TMGa gas source flow rate set to 100 sccm, to form an undoped GaN layer on the AlN insertion layer;

[0025] At 1100° C., using TMAl, TMGa, and NH3 as precursors of Al, Ga, and N, respectively, and hydrogen as a carrier gas, the three react to form an AlGaN layer on the undoped GaN layer, wherein the aluminum content of the AlGaN layer is 0.22-0.27;

[0026] At 1100° C., using TMGa and NH 3 as Ga and N precursors, respectively, and hydrogen as a carrier gas, with a TMGa gas source flow rate set to 100 sccm, to form an undoped GaN channel layer on the AlGaN layer;

[0027] At 1100° C., using TMAl and NH 3 as Al and N precursors, respectively, and hydrogen as a carrier gas, with a flow rate of the TMAl gas source set to 260 sccm, to form an AlN spacer layer on the undoped GaN channel layer;

[0028] At 1100° C., TMAl, TMGa, and NH3 are reacted as precursors of Al, Ga, and N, respectively, with hydrogen as a carrier gas, to form an AlGaN barrier layer on the AlN spacer layer; wherein the aluminum content of the AlGaN barrier layer is 0.33-0.39;

[0029] At 1100°C, TMAl, TMGa and NH3 are used as precursors of Al, Ga and N respectively, and hydrogen is used as a carrier gas to react with each other to form a Si-doped graded Al layer 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 barrier layer, the aluminum component x gradually changes from the aluminum component of the AlGaN barrier layer to 0.05, and the Si doping concentration is 3×10 18 ~6×10 18 cm -3 ;

[0030] At 1100 ° C, TMGa and NH3 were used as Ga and N precursors respectively, and hydrogen was used as a carrier gas to form a gradient Al doped with Si. x Ga 1-x A Si-doped GaN spacer layer is formed on the N barrier layer; wherein the Si doping concentration in the Si-doped GaN spacer layer is 3×10 18 ~6×10 18 cm -3 ;

[0031] At 1100° C., TMGa and NH 3 are used as Ga and N precursors respectively, and hydrogen is used as a carrier gas, so that TMGa, NH 3 and ferrocene react simultaneously 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 inversion transfer technique to obtain a bonded structure, including:

[0033] performing 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;

[0034] Depositing a layer of silicon nitride on the surface of the obtained Fe-doped semi-insulating GaN layer by a PECVD process, and then forming a Ni / Cu / Au stacked metal on the surface of the deposited silicon nitride by electron beam evaporation, thereby obtaining a first material structure;

[0035] Depositing a layer of silicon nitride on the surface of the obtained SiC substrate using a PECVD process, and then forming a Ni / Cu / Au laminated metal on the deposited silicon nitride surface by electron beam evaporation, thereby obtaining a second material structure; wherein the thickness of each metal layer in the Ni / Cu / Au laminated metal is 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 by the Au-Au thermal compression bonding method, so that the second material structure is reversely stacked on the first material structure; then the whole structure is flipped 180° so that the SiC substrate is located at the bottom, thereby obtaining the bonded structure.

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

[0038] performing chemical mechanical polishing on the uppermost surface of the Si substrate in the bonded structure;

[0039] The Si substrate in the bonded structure is removed by etching using an inductively coupled plasma etching process using a high etching selectivity ratio of sulfur hexafluoride SF6 plasma to Si / AlN;

[0040] For the surface of the N-polar AlN layer exposed after etching away the Si substrate, the AlN nucleation layer and part of the undoped GaN buffer layer are first removed by BCL3 / CL2 plasma etching; then the remaining undoped GaN buffer layer is removed by BCL3 / Ar plasma etching, and the high etching selectivity of GaN / AlN is utilized to achieve self-termination of etching in the AlN insertion layer, wherein 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 N-polarity undoped GaN layer exposed after the AlN insertion layer is removed is subjected to surface treatment to obtain an N-face GaN-based epitaxial structure, including:

[0042] The AlN insertion layer is removed by atomic layer etching using Cl2 / Ar plasma at a preset rate; wherein the Cl2 flow rate is 100 sccm, the Ar flow rate is 80 sccm, the upper electrode power of the etching equipment is 100 W, the lower electrode power is 10 W, and the pressure is 5 mTorr;

[0043] The surface of the exposed N-polarity undoped GaN layer is chemically mechanically polished and then cleaned to obtain an N-face GaN-based epitaxial structure.

[0044] In a second aspect, an embodiment of the present invention provides an N-face GaN-based epitaxial structure based on an inverted transfer technology, which is prepared according to the preparation method of the N-face GaN-based epitaxial structure based on an inverted transfer technology described in the first aspect.

[0045] The present invention uses GaN-based materials and adopts inversion transfer technology to prepare a high-quality N-face GaN-based epitaxial structure. It has the following beneficial effects:

[0046] 1. Avoiding the difficulty of directly growing N-face GaN-based epitaxial materials. Specifically, the present invention uses an inverted transfer technique to invert high-quality Ga-face GaN-based epitaxial materials by 180°. N-face GaN-based epitaxial materials are obtained through bonding, etching and other processes, effectively avoiding the difficulty of directly growing N-face GaN-based epitaxial materials.

[0047] 2. Improve the poor heat dissipation of Si substrates. Although the cost of growing GaN materials on existing Si substrates is low, their poor heat dissipation limits their application in RF power. This invention removes the original Si substrate and uses SiC as the substrate for N-face GaN-based epitaxial material through bonding, greatly improving the material's thermal conductivity.

[0048] 3. Based on the inverted transfer technology, the thickness of the remaining N-polarity GaN is precisely controlled to reduce surface roughness and process complexity. After the bonding is completed, the original material needs to be removed by etching. In this process, it is difficult to precisely control the thickness of the remaining GaN, which makes the thickness of the remaining N-polarity GaN material extremely difficult to control and will cause greater etching damage. The present invention inserts a layer of AlN as an etch stop layer between the undoped GaN buffer layer and the undoped GaN layer, and utilizes the high etching selectivity of the undoped GaN buffer layer and the AlN insertion layer (the etching selectivity of N-polarity GaN to AlN is above 60) to achieve self-termination of etching on the AlN insertion layer, so as to achieve precise control of the thickness of the remaining epitaxial material. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 A schematic flow chart of a method for preparing an N-face GaN-based epitaxial structure based on an inverted transfer technique provided by an embodiment of the present invention;

[0050] Figure 2a to Figure 2m A schematic diagram of the process for preparing an N-face GaN-based epitaxial structure based on the inverted transfer technology provided in an embodiment of the present invention. DETAILED DESCRIPTION

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

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

[0053] S1, after obtaining a Si substrate and pre-treating it, an AlN nucleation layer and an undoped GaN buffer layer are sequentially grown thereon;

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

[0055] The Si substrate is thermally cleaned in hydrogen to remove residual oxides on the surface. For pretreatment of the Si substrate, see Figure 2a understand.

[0056] The process of sequentially growing the AlN nucleation layer and the undoped GaN buffer layer on the obtained Si substrate includes the following two steps. The results are shown in Figure 2b As shown:

[0057] 1) Using trimethylaluminum (TMAl) and ammonia (NH3) as precursors of Al and N, respectively, under set temperature and pressure conditions, by setting the flow rates of TMAl and NH3, an AlN nucleation layer is grown in multiple steps;

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

[0059] 2) At 1150° C., trimethylgallium (TMGa) and NH 3 are used as Ga and N precursors, respectively, and hydrogen is used as a carrier gas. The flow rate of TMGa gas is 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 may be 1 to 1.3 μm.

[0061] S2, growing an AlN insertion layer 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 NH 3 were used as precursors of Al and N, respectively, and hydrogen was used as a 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 a subsequent etching stop layer.

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

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

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

[0068] In an optional implementation, see Figure 2d, S3 may include the following steps:

[0069] S31, at 1100° C., using TMGa and NH 3 as Ga and N precursors, respectively, and hydrogen as a carrier gas, with a flow rate of the TMGa gas source set to 100 sccm, to form an undoped GaN layer on the AlN insertion layer;

[0070] The thickness of the undoped GaN layer may be 20-50 nm.

[0071] S32, at 1100° C., using TMAl, TMGa, and NH 3 as precursors of Al, Ga, and N, respectively, and hydrogen as a carrier gas, to react the three to form an AlGaN layer on the undoped GaN layer.

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

[0073] S33, at 1100° C., using TMGa and NH 3 as Ga and N precursors, respectively, and hydrogen as a carrier gas, with a flow rate of the TMGa gas source set to 100 sccm, to form an undoped GaN channel layer on the AlGaN layer;

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

[0075] S34, at 1100° C., using TMAl and NH 3 as Al and N precursors, respectively, and hydrogen as a carrier gas, with a flow rate of the TMAl gas source set to 260 sccm, to form an AlN spacer layer on the undoped GaN channel layer;

[0076] The thickness of the AlN spacer layer may be 0.6-1 nm.

[0077] S35, at 1100° C., using TMAl, TMGa, and NH 3 as precursors of Al, Ga, and N, respectively, and hydrogen as a carrier gas, to react 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; and the thickness of the AlGaN barrier layer can be 10-20 nm.

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

[0080] Wherein, the Si-doped graded Al x Ga 1-x In the N barrier layer, the aluminum component x gradually changes from the aluminum component of the AlGaN barrier layer to 0.05, and the Si doping concentration is 3×10 18 ~6×10 18 cm -3 ; Si-doped graded Al x Ga 1-x The thickness of the N barrier layer may be 20 nm.

[0081] S37, at 1100 ° C, using TMGa and NH3 as Ga and N precursors respectively, and hydrogen as a carrier gas, to form a silicon-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 18 ~6×10 18 cm -3 ; The thickness of the Si-doped GaN spacer layer can be 8 to 12 nm.

[0083] S38, at 1100° C., using TMGa and NH 3 as Ga and N precursors respectively, and hydrogen as a carrier gas, so that TMGa, NH 3 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 may be 0.5 to 1.5 μm.

[0085] Through step S3, a Ga-polarity GaN-based epitaxial material is grown on the Si substrate.

[0086] S4, after processing the Fe-doped semi-insulating GaN layer and the obtained SiC substrate, bonding the SiC substrate to the Fe-doped semi-insulating GaN layer using an inversion transfer technique to obtain a bonded structure;

[0087] See Figure 2e The upper and lower figures indicate 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, chemically mechanically polishing the surface of the Fe-doped semi-insulating GaN layer and the surface of the SiC substrate to be bonded, and then performing surface cleaning;

[0089] Among them, chemical mechanical polishing is to polish the surface through CMP (Chemical Mechanical Planarization / Polishing) to reduce the surface roughness.

[0090] The surface is cleaned with a sulfuric acid-hydrogen peroxide mixed solution.

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

[0092] The thickness of the deposited silicon nitride can be 10 to 20 nm. Figure 2f gesture.

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

[0094] The thickness of the deposited silicon nitride may be 10-20 nm.

[0095] In the Ni / Cu / Au stacked metal, the thickness of each metal layer is 50nm, 500nm, and 20nm, respectively;

[0096] Second material structure see Figure 2g gesture.

[0097] S44, bonding the Fe-doped semi-insulating GaN layer in the first material structure to the SiC substrate in the second material structure by the Au-Au thermal compression bonding method, so that the second material structure is reversely stacked on the first material structure; then flipping the whole structure 180° so that the SiC substrate is located at the bottom, to obtain a bonded structure.

[0098] The bonding process is shown in Figure 2h The bonded structure obtained after the overall flip is shown in Figure 2i It can be seen that the SiC substrate is at the bottom and the Si substrate is at the top.

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

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

[0101] S51, performing chemical mechanical polishing on the uppermost Si substrate surface in the bonded structure;

[0102] Among them, chemical mechanical polishing is to polish the surface through CMP to reduce the surface roughness.

[0103] S52, using a high etching selectivity of sulfur hexafluoride SF6 plasma to Si / AlN, etching and removing the Si substrate in the bonded structure through an inductively coupled plasma etching process;

[0104] See Figure 2j Since sulfur hexafluoride SF6 plasma has a high etching selectivity ratio for Si / AlN, it can etch Si but cannot etch AlN. Therefore, Si substrate etching can be achieved while retaining the AlN layer.

[0105] S53, for the surface of the N-polar AlN layer exposed after etching away the Si substrate, first remove the AlN nucleation layer and part of the undoped GaN buffer layer by BCL3 / CL2 plasma etching; then remove the remaining undoped GaN buffer layer by BCL3 / Ar plasma etching, and utilize the high etching selectivity of GaN / AlN to achieve self-termination of etching in the AlN insertion layer, wherein 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.

[0106] In S53, the AlN nucleation layer and a portion of the undoped GaN buffer layer are first removed by rapid etching using BCL3 / CL2 plasma at an etching rate of 25 nm / min. The remaining undoped GaN buffer layer is then removed by slow etching using BCL3 / Ar plasma at an etching rate of 6 nm / min. During the etching process, due to the high etching 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, removing the AlN insertion layer by ALE (Atomic Layer Etching) etching technology, and performing surface treatment on the N-polarity undoped GaN layer exposed after the AlN insertion layer is removed to obtain an N-face GaN-based epitaxial structure.

[0108] See Figure 2k In an optional implementation, S6 may include:

[0109] S61, using Cl2 / Ar plasma to etch away the AlN insertion layer at a preset rate using atomic layer etching technology; wherein the Cl2 flow rate is 100 sccm, the Ar flow rate is 80 sccm, the upper electrode power of the etching equipment is 100 W, the lower electrode power is 10 W, and the pressure is 5 mTorr;

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

[0111] After the AlN insertion layer is removed, the undoped GaN layer is exposed, which has N polarity.

[0112] S62, performing chemical mechanical polishing on the surface of the exposed N-polarity undoped GaN layer, and then performing surface cleaning to obtain an N-face GaN-based epitaxial structure.

[0113] Among them, chemical mechanical polishing is to polish the surface through CMP to reduce the surface roughness.

[0114] The surface is cleaned with a sulfuric acid-hydrogen peroxide mixture. After surface polishing and cleaning, a smooth N-polarity GaN surface is finally obtained.

[0115] The final N-face GaN-based epitaxial structure (i.e., N-polarity GaN-based epitaxial structure) is obtained, see Figure 2m shown.

[0116] N-polarity GaN materials are generally obtained by direct epitaxial growth techniques such as MOCVD and MBE, but it is difficult and costly to directly grow N-polarity GaN materials. The use of inverted transfer technology can avoid the problem of difficulty in directly growing N-polarity GaN. The inverted transfer technology utilizes the characteristic that the two polarity directions of GaN differ by 180°, and obtains N-polarity GaN through bonding and removing the original substrate of Ga-polarity GaN-based epitaxial material. However, in the process of removing the original material by etching, it is difficult to accurately control the thickness of the remaining GaN, which leads to uncertainty in the thickness of the remaining N-polarity GaN channel layer, and will bring greater etching damage and increase the surface roughness.

[0117] Specifically, in order to ensure the growth quality of conventional Ga-polar GaN materials, a GaN buffer layer of more than 1 μm is usually required to be grown on the substrate. In order to obtain N-polar GaN through the inversion transfer technology, this part of the buffer layer needs to be etched away, and a part of the GaN needs to be left as the channel layer, and the thickness of the remaining GaN is about 10-20 nm. Since the thickness of this part of GaN is relatively large, it is necessary to balance the etching rate and the thickness of the remaining GaN material. In the absence of an effective etch stop layer, it is difficult to achieve the unity of efficiency and precision at the same time. In addition, the prepared N-polar GaN-based epitaxial material has a simple structure and average quality, and is not suitable for application in RF power devices.

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

[0119] In summary, the embodiment of the present invention uses GaN-based materials and adopts the inversion transfer technology to prepare a high-quality N-face GaN-based epitaxial structure, which has the following beneficial effects:

[0120] 1. Avoiding the difficulty of directly growing N-face GaN-based epitaxial materials. Specifically, the present invention uses an inverted transfer technique to invert high-quality Ga-face GaN-based epitaxial materials by 180°. N-face GaN-based epitaxial materials are obtained through bonding, etching and other processes, effectively avoiding the difficulty of directly growing N-face GaN-based epitaxial materials.

[0121] 2. Improve the poor heat dissipation of Si substrates. Although the cost of growing GaN materials on existing Si substrates is low, their poor heat dissipation limits their application in RF power. This invention removes the original Si substrate and uses SiC as the substrate for N-face GaN-based epitaxial material through bonding, greatly improving the material's thermal conductivity.

[0122] 3. Accurately control the thickness of the remaining N-polar GaN to reduce surface roughness and process complexity. After bonding is completed, the original material needs to be removed by etching. In this process, it is difficult to accurately control the thickness of the remaining GaN, which makes the thickness of the remaining N-polar GaN material extremely difficult to control and will cause large etching damage. The present invention inserts a layer of AlN as an etch stop layer between the undoped GaN buffer layer and the undoped GaN layer, and utilizes the high etching selectivity of the undoped GaN buffer layer and the AlN insertion layer (the etching selectivity of N-polar GaN to AlN is above 60) to achieve self-termination of etching on the AlN insertion layer, so as to achieve precise control of the thickness of the remaining epitaxial material.

[0123] In the second aspect, corresponding to the above method embodiment, the embodiment of the present invention further provides an N-face GaN-based epitaxial structure based on the inverted transfer technology, which is prepared using the preparation method of the N-face GaN-based epitaxial structure based on the inverted transfer technology described in the first aspect. Figure 2m As shown, from bottom to top, it includes:

[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 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 may be 10 to 20 nm.

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

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

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

[0130] The Si-doped graded Al x Ga 1-x In the N barrier layer, the aluminum component x gradually changes from the aluminum component of the AlGaN barrier layer to 0.05, and the Si doping concentration is 3×1018 ~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 content in the AlGaN barrier layer is 0.33-0.39; the thickness of the AlGaN barrier layer may be 10-20 nm;

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

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

[0134] The aluminum content of the AlGaN layer is 0.22 to 0.27; the thickness of the AlGaN layer can be 2 to 3 nm;

[0135] The thickness of the undoped GaN layer may be 20 to 50 nm.

[0136] For specific details about the N-face GaN-based epitaxial structure based on the inverted transfer technology, please refer to the relevant content of the first aspect and will not be repeated here.

[0137] The embodiment of the present invention provides a high-quality N-polarity GaN-based epitaxial material, which is obtained by an inverted transfer technology. Specifically, through bonding, etching, chemical mechanical polishing and other processes, the high-quality Ga-polarity GaN-based epitaxial material is converted into a high-quality N-polarity GaN-based epitaxial material, avoiding the problem of difficulty in directly growing N-face GaN-based epitaxial material. In the present invention, the substrate of the initial epitaxial Ga-polarity GaN-based epitaxial material is a Si substrate, and the substrate to be bonded is a SiC substrate. By utilizing the advantage of the high thermal conductivity of the SiC material, the thermal conductivity of the final epitaxial material can be greatly improved, and the problem of poor heat dissipation of the Si substrate can be improved, so that the device can operate at a higher bias point, and ultimately a higher output power can be obtained. In addition, the AlN insertion layer is selected as the etching stop layer (the etching selectivity ratio of N-polarity GaN to AlN is above 60), which can meet the precise control of the thickness of the remaining epitaxial material, thereby being able to accurately control the thickness of the remaining N-polarity GaN, reducing surface roughness and process complexity.

[0138] It should be noted that, in the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and the like to indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

[0139] In the description of this specification, the reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification.

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

Claims

1. A method for preparing an N-face GaN-based epitaxial structure based on an inverted transfer technique, characterized in that: include: After obtaining a Si substrate and pre-treating it, an AlN nucleation layer and an undoped GaN buffer layer are sequentially grown thereon; Growing an AlN insertion layer 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, a Si-doped graded AlN layer, and the like 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 inversion transfer technique to obtain a bonded structure; Sequentially removing the Si substrate, the AlN nucleation layer, and the undoped GaN buffer layer from the bonded structure, and utilizing a high etching selectivity between the undoped GaN buffer layer and the AlN insertion layer to achieve etching self-termination in the AlN insertion layer; According to the obtained material structure, the AlN insertion layer is removed by ALE etching technology, and the N-polarity undoped GaN layer exposed after the AlN insertion layer is removed is surface treated to obtain an N-face GaN-based epitaxial structure.

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

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

4. The method according to claim 1, wherein Growing an AlN insertion layer on the undoped GaN buffer layer as an etch stop layer, comprising: At 1100° C., TMAl and NH 3 were used as precursors of Al and N, respectively, and hydrogen was used as a 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, wherein An undoped GaN layer, an AlGaN layer, an undoped GaN channel layer, an AlN spacer layer, an AlGaN barrier layer, a Si-doped graded AlN layer, and the like 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, including: At 1100° C., using TMGa and NH 3 as Ga and N precursors, respectively, and hydrogen as a carrier gas, with a TMGa gas source flow rate set to 100 sccm, to form an undoped GaN layer on the AlN insertion layer; At 1100° C., using TMAl, TMGa, and NH3 as precursors of Al, Ga, and N, respectively, and hydrogen as a carrier gas, the three react to form an AlGaN layer on the undoped GaN layer, wherein the aluminum content of the AlGaN layer is 0.22-0.27; At 1100° C., using TMGa and NH 3 as Ga and N precursors, respectively, and hydrogen as a carrier gas, with a TMGa gas source flow rate set to 100 sccm, to form an undoped GaN channel layer on the AlGaN layer; At 1100° C., using TMAl and NH 3 as Al and N precursors, respectively, and hydrogen as a carrier gas, with a flow rate of the TMAl gas source set to 260 sccm, to form an AlN spacer layer on the undoped GaN channel layer; At 1100° C., TMAl, TMGa, and NH3 are reacted as precursors of Al, Ga, and N, respectively, with hydrogen as a carrier gas, to form an AlGaN barrier layer on the AlN spacer layer; wherein the aluminum content of the AlGaN barrier layer is 0.33-0.39; At 1100°C, TMAl, TMGa and NH3 are used as precursors of Al, Ga and N respectively, and hydrogen is used as a carrier gas to react with each other to form a Si-doped graded Al layer 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 barrier layer, the aluminum component x gradually changes from the aluminum component of the AlGaN barrier layer to 0.05, and the Si doping concentration is 3×10 18 ~6×10 18 cm -3 ; At 1100 ° C, TMGa and NH3 were used as Ga and N precursors respectively, and hydrogen was used as a carrier gas to form a gradient Al doped with Si. x Ga 1-x A Si-doped GaN spacer layer is formed on the N barrier layer; wherein the Si-doped concentration in the Si-doped GaN spacer layer is 3×10 18 ~6×10 18 cm -3 ; At 1100° C., TMGa and NH 3 are used as Ga and N precursors respectively, and hydrogen is used as a carrier gas, so that TMGa, NH 3 and ferrocene react simultaneously 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 inversion transfer technique to obtain a bonded structure, including: performing 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; Depositing a layer of silicon nitride on the surface of the obtained Fe-doped semi-insulating GaN layer by a PECVD process, and then forming a Ni / Cu / Au stacked metal on the surface of the deposited silicon nitride by electron beam evaporation, thereby obtaining a first material structure; Depositing a layer of silicon nitride on the surface of the obtained SiC substrate using a PECVD process, and then forming a Ni / Cu / Au laminated metal on the deposited silicon nitride surface by electron beam evaporation, thereby obtaining a second material structure; wherein the thickness of each metal layer in the Ni / Cu / Au laminated metal is 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 by the Au-Au thermal compression bonding method, so that the second material structure is reversely stacked on the first material structure; then the whole structure is flipped 180° so that the SiC substrate is located at the bottom, thereby obtaining the bonded structure.

8. The method according to claim 7, characterized in that The Si substrate, the AlN nucleation layer, and the undoped GaN buffer layer in the bonded structure are sequentially removed, and etching self-termination is achieved in the AlN insertion layer by utilizing a high etching selectivity between the undoped GaN buffer layer and the AlN insertion layer, comprising: performing chemical mechanical polishing on the uppermost surface of the Si substrate in the bonded structure; The Si substrate in the bonded structure is removed by etching using an inductively coupled plasma etching process using a high etching selectivity ratio of sulfur hexafluoride SF6 plasma to Si / AlN; For the surface of the N-polar AlN layer exposed after etching away the Si substrate, the AlN nucleation layer and part of the undoped GaN buffer layer are first removed by BCL3 / CL2 plasma etching; then the remaining undoped GaN buffer layer is removed by BCL3 / Ar plasma etching, and the high etching selectivity of GaN / AlN is utilized to achieve self-termination of etching in the AlN insertion layer, wherein 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 by ALE etching technology, and the N-polarity undoped GaN layer exposed after the AlN insertion layer is removed is subjected to surface treatment to obtain an N-face GaN-based epitaxial structure, including: The AlN insertion layer is removed by atomic layer etching using Cl2 / Ar plasma at a preset rate; wherein the Cl2 flow rate is 100 sccm, the Ar flow rate is 80 sccm, the upper electrode power of the etching equipment is 100 W, the lower electrode power is 10 W, and the pressure is 5 mTorr; The surface of the exposed N-polarity undoped GaN layer is chemically mechanically polished and then cleaned to obtain an N-face GaN-based epitaxial structure.

10. An N-face GaN-based epitaxial structure based on inversion transfer technology, characterized in that: It is prepared according to the method for preparing an N-face GaN-based epitaxial structure based on the inverted transfer technology according to any one of claims 1-9.

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