Method for preparing N-polarity GaN based on CMP, N-polarity GaN and its application

Abstract

The present invention discloses a method for preparing N-polarity GaN based on CMP, N-polarity GaN, and applications thereof. The method for preparing N-polarity GaN based on CMP comprises: exposing the N-polarity surface of a Ga-polarity GaN material; performing chemical mechanical polishing and thinning on the N-polarity surface of the Ga-polarity GaN material using an alkaline polishing liquid to expose the N-polarity GaN therein; wherein the alkaline polishing liquid contains abrasive polishing nanoparticles. The N-polarity GaN prepared by the method for preparing N-polarity GaN based on CMP provided in an embodiment of the present invention does not suffer from etching damage, can reduce ohmic contact resistance, and thus improve the performance of the N-polarity GaN and its devices, and the surface roughness of the prepared N-polarity GaN is smaller.

Description

Technical Field

[0001] The present invention particularly relates to a method for preparing N-polarity GaN based on CMP, N-polarity GaN and applications thereof, and belongs to the field of electronic science and technology. Background Art

[0002] Commonly used GaN materials have a wurtzite structure. Because center-inversion symmetry is absent along the c-axis

[0001] of the unit cell in the wurtzite GaN structure, GaN materials can be divided into Ga-polar GaN materials and N-polar GaN materials based on their polarity. Currently, conventional GaN materials are all prepared based on Ga-polar GaN materials. Compared to conventional Ga-polar GaN, N-polar GaN materials have completely opposite polarity and therefore possess many novel properties.

[0003] Leveraging the other properties of N-polarity GaN materials offers significant advantages in the fabrication of optoelectronic devices, detectors, switches, and radio frequency devices. In LED applications, the polarization electric field of an N-face LED structure aligns with the built-in electric field, thus reducing the width of the depletion region, facilitating a lower turn-on voltage and mitigating the Stark effect in quantum wells.

[0004] In sensor applications, research has found that H atoms have a higher affinity for the surface of N-face GaN materials than for Ga-face materials, thus showing great potential for the fabrication of hydrogen sensors. In the RF field of GaN HEMTs, N-polarity GaN HEMTs offer advantages such as a natural back barrier, low ohmic contact resistance, and improved scalability, resulting in higher current gain cutoff frequency (ft) and maximum oscillation frequency (fmax).

[0005] Although N-polar GaN and its devices possess a range of excellent properties, heteroepitaxial growth of N-polar GaN has been a challenge. The substrate transfer method is an alternative to MBE and MOCVD epitaxy for fabricating N-polar GaN HEMTs. Researchers have used this method to grow N-polar GaN, studying its ohmic contact properties and fabricating N-polar GaN HEMTs.

[0006] The process of preparing N-polarity GaN and its devices by the substrate transfer method can be divided into: bonding the Ga-polarity epitaxial wafer to the substrate to be transferred, removing the original substrate of the Ga-polarity epitaxial wafer to expose the N-polarity surface, thinning the N-polarity surface epitaxial layer to expose the N-polarity GaN, and then preparing the device.

[0007] Ga-polar GaN epitaxial wafers to be transferred onto substrates can be divided into two types depending on their substrates: sapphire-based epitaxial wafers and Si-based epitaxial wafers. For ease of bonding and removal of the original substrate, Ga-polar GaN epitaxial wafers on Si substrates are generally used for the preparation of N-polar GaN. Both the Ga-polar GaN on Si substrates and the GaN layer of the HEMT epitaxial structure require an AlGaN stress buffer layer and an AlN nucleation layer beneath them. To expose the N-polar GaN after substrate transfer, the AlN nucleation layer, multi-layer AlGaN stress buffer layer, and high-resistance GaN layer must be removed (referred to as N-polar epitaxial layer thinning). All currently reported and patented epitaxial layer thinning methods use dry etching.

[0008] However, the existing common dry etching technologies all have etching damage, which can be manifested as increased surface roughness, increased film defects, and increased ohmic contact resistance of the prepared material. The low-damage dry etching technology is extremely slow (calculated at 1nm / min, it takes 33 hours to etch a 2μm thinning thickness), resulting in extremely high costs. Summary of the Invention

[0009] The main purpose of the present invention is to provide a method for preparing N-polarity GaN based on CMP, N-polarity GaN and applications thereof, so as to overcome the deficiencies in the prior art.

[0010] To achieve the aforementioned object of the invention, the technical solutions adopted by the present invention include:

[0011] An embodiment of the present invention provides a method for preparing N-polarity GaN based on CMP, which includes:

[0012] Exposing the N-polar surface of the Ga-polar GaN material;

[0013] An alkaline polishing liquid is used to chemically mechanically polish and thin the N-polar surface of the Ga-polar GaN material after transfer from the substrate, so as to expose the N-polar GaN therein; wherein the alkaline polishing liquid contains abrasive polishing nanoparticles.

[0014] An embodiment of the present invention also provides N-polarity GaN prepared by the method.

[0015] In a specific embodiment, the surface roughness of the N-polar GaN is less than 1 nm, preferably less than 0.3 nm.

[0016] The embodiment of the present invention provides the use of the N-polar GaN in preparing GaN-based semiconductor devices.

[0017] Compared with the prior art, the advantages of the present invention include:

[0018] 1) The present invention provides a method for preparing N-polar GaN based on CMP, which is simple to operate, lower in cost, and higher in efficiency;

[0019] 2) The N-polarity GaN prepared by the CMP-based method provided in the embodiment of the present invention has no etching damage, can reduce the ohmic contact resistance, and thus improve the performance of the N-polarity GaN and its devices;

[0020] 3) The surface roughness of N-polar GaN prepared by the CMP-based method provided in the embodiment of the present invention is smaller;

[0021] 4) The embodiment of the present invention provides a method for preparing N-polar GaN based on CMP, which has a high thinning rate for N-polar AlN (AlGaN) and a lower thinning rate for N-polar GaN. Therefore, the polishing time can be controlled to the minute level, and high-quality N-polar GaN can be obtained efficiently. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a schematic diagram of a process flow for preparing an N-polarity GaN material and device provided in a typical embodiment of the present invention;

[0023] Figure 2 2. This is a schematic diagram of the structure of a wafer to be polished and thinned after substrate transfer and before CMP in a typical embodiment of the present invention;

[0024] Figure 3 Schematic diagram of a wafer structure after CMP in a typical embodiment of the present invention;

[0025] Figure 4 This is an AFM (atomic force microscope) image of an N-polar GaN surface obtained by CMP removal of N-polar AlN and AlGaN in a typical embodiment of the present invention. DETAILED DESCRIPTION

[0026] In view of the shortcomings of the prior art, the inventors of this case, after long-term research and extensive practice, have proposed the technical solution of the present invention. The following will further explain this technical solution, its implementation process and principles.

[0027] CMP (chemical mechanical polishing) is a common technology used for flattening or polishing semiconductor wafers. In CMP technology, the wafer is mounted on a wafer carrier, and the wafer carrier is mounted on a carrier assembly; the polishing head fixes the wafer and brings it into contact with the polishing layer of the polishing pad of the CMP device; the carrier assembly can provide controllable pressure between the wafer and the polishing pad. At the same time, the polishing liquid can be distributed onto the polishing pad and enter the gap between the wafer and the polishing pad. The relative movement and relative rotation of the polishing pad and the wafer can achieve polishing and flattening of the wafer surface.

[0028] An embodiment of the present invention provides a CMP method for preparing N-polar GaN materials and related device structures. Different from the currently commonly used dry etching process for preparing N-polar GaN materials, the alkaline polishing liquid used in the CMP method of the present invention can have a very high etching selectivity for N-polar AlN (AlGaN) and GaN, thereby efficiently removing N-polar AlN and AlGaN while retaining N-polar GaN, thereby obtaining high-quality N-polar GaN.

[0029] The inventors of this case discovered through experiments that compared with Ga polarity, N-polarity GaN, AlGaN, and AlN are more easily corroded in alkaline solutions, and there is a large selectivity ratio between N-polarity AlN and AlGaN and GaN during the CMP polishing process. Based on this, an embodiment of the present invention provides a method for preparing N-polarity GaN based on CMP, which utilizes alkaline polishing liquid to selectively chemically mechanically polish (CMP) N-polarity AlN, AlGaN, and GaN, thereby achieving the effect of efficiently removing AlN and AlGaN while retaining GaN, and thus obtaining high-quality N-polarity GaN.

[0030] The inventors of this case also found through experiments that the high selectivity of chemical mechanical polishing can be attributed to the different corrosion resistance of N-polar AlN (AlGaN) and N-polar GaN in alkaline polishing liquid. The Al atoms in N-polar AlN are more susceptible to corrosion by hydroxide ions in the polishing liquid, and AlO rapidly forms on the N-polar surface. x (OH) y , dissolving in the solution leads to a higher polishing rate. x Ga 1-x The corrosion rate of N is between that of AlN and GaN, and decreases with decreasing Al concentration. Therefore, during the CMP process of substrate transfer samples, N-polar AlN and high-Al content AlGaN layers quickly form oxides, which are then removed by the rotation of the polishing pad and abrasive particles, resulting in a higher removal rate. When the N-polar GaN layer is exposed, the polishing thinning rate decreases due to the difficulty in oxide formation, thereby showing a large selectivity.

[0031] An embodiment of the present invention provides a method for preparing N-polarity GaN based on CMP, which includes:

[0032] Exposing the N-polar surface of the Ga-polar GaN material;

[0033] An alkaline polishing solution is used to chemically mechanically polish and thin the N-polar surface of the Ga-polar GaN material to expose the N-polar GaN therein; wherein the alkaline polishing solution contains abrasive polishing nanoparticles.

[0034] In one embodiment, the content of polishing and grinding nanoparticles in the alkaline polishing solution is 2-20%.

[0035] In one embodiment, the pH value of the alkaline polishing solution is 9-12.

[0036] In one embodiment, the alkaline polishing solution includes a pH adjuster, polishing abrasive nanoparticles and deionized water.

[0037] In one embodiment, the particle size of the grinding and polishing nanoparticles is 1 to 100 nm.

[0038] In a specific embodiment, the material of the grinding and polishing nanoparticles includes any one or more combinations of silicon oxide, aluminum oxide, and cerium oxide.

[0039] In one embodiment, the pH adjuster comprises hydroxides or carbonates.

[0040] In a specific embodiment, during the chemical mechanical polishing and thinning, the pressure applied between the N-polar surface of the Ga-polar GaN material and the polishing pad is 0-100N.

[0041] In a specific embodiment, the rotation speed of the polishing pad is 10 to 50 rpm, the rotation speed of the Ga-polar GaN material is 10 to 50 rpm, and the rotation directions of the polishing pad and the Ga-polar GaN material are opposite.

[0042] In one embodiment, the Ga-polar GaN material includes N-polar Al x Ga 1-x N and N-polar GaN, 0<x≦1, and the N-polar Al x Ga 1-x The chemical mechanical polishing thinning rate selectivity ratio of N and N-polar GaN reaches above 30:1.

[0043] In a specific embodiment, the method specifically includes:

[0044] The Ga polar surface of the Ga polar GaN material grown on the first substrate is bonded to a second substrate, and then the first substrate is removed to expose the N polar surface of the Ga polar GaN material.

[0045] In a specific embodiment, the first substrate includes a Si substrate or a sapphire substrate, and the second substrate includes a Si substrate, a sapphire substrate, a SiC substrate or a diamond substrate.

[0046] In a specific embodiment, the method specifically includes: removing the first substrate by any one of mechanical grinding, dry etching, and wet etching.

[0047] In a specific embodiment, the method further includes: cleaning the N-polarity GaN material after the chemical mechanical polishing and thinning is completed.

[0048] An embodiment of the present invention also provides N-polarity GaN prepared by the method.

[0049] In a specific embodiment, the surface roughness of the N-polar GaN is less than 1 nm, preferably less than 0.3 nm.

[0050] The embodiment of the present invention provides the use of the N-polar GaN in preparing GaN-based semiconductor devices.

[0051] In a specific embodiment, the GaN-based semiconductor devices include optoelectronic devices, detectors, switching devices, and radio frequency devices.

[0052] An embodiment of the present invention provides a hydrogen sensor, which includes the N-polar GaN.

[0053] The technical solution, its implementation process and principles will be further explained below in conjunction with the accompanying drawings and specific implementation cases. Unless otherwise specified, the epitaxial growth process and chemical polishing equipment used in the embodiments of the present invention may be known to those skilled in the art.

[0054] See also Figure 1 The fabrication process of an N-polarity GaN device mainly includes: obtaining Ga-polarity GaN material, bonding with the substrate to be transferred, removing the original substrate, CMP material thinning, and device fabrication. The specific process is as follows:

[0055] 1) Obtaining Ga-polar GaN materials:

[0056] It can be a GaN material or a related device epitaxial structure on a Si substrate or a sapphire substrate (i.e., a first substrate), such as a GaN HEMT epitaxial structure, a GaN LED epitaxial structure, etc.;

[0057] 2) Bonding with the substrate to be transferred (i.e., the second substrate, the same below):

[0058] The bonding method can be eutectic bonding (such as Au-Sn bonding), diffusion bonding (such as Au-Au, Cu-Cu bonding, Al-Al bonding), direct bonding, adhesive bonding, etc. The substrate to be transferred can be Si wafer, sapphire wafer, SiC wafer, diamond wafer, etc.

[0059] 3) Removal of the original substrate (i.e., the first substrate):

[0060] The original substrate can be removed by mechanical grinding, dry etching, wet etching, etc.

[0061] 4) CMP epitaxial layer thinning:

[0062] During the CMP polishing process, an alkaline polishing solution containing nano-scale polishing abrasive particles is used. Taking advantage of the large CMP polishing thinning rate selectivity of N-polar AlN, AlGaN, and GaN (AlN (AlGaN): GaN ≥ 30:1), the flipped epitaxial structure after the original substrate is removed is chemically mechanically polished and thinned, thereby achieving the preparation of N-polar GaN. The specific process is as follows:

[0063] 4.1) First, prepare a polishing solution with a pH of 9-12. The polishing solution comprises a pH adjuster, polishing abrasive particles, and deionized water. The pH adjuster may be a hydroxide (e.g., KOH, NaOH, etc.) or a carbonate (e.g., Na2CO3, etc.). The alkaline polishing solution contains 2-20% polishing abrasive nanoparticles. The polishing abrasive particles may be silicon oxide, aluminum oxide, cerium oxide, etc., and have a particle size of 1-100 nm.

[0064] 4.2) The flipped epitaxial structure, after removal of the original substrate, is secured to a wafer carrier, which is mounted on a carrier assembly, with the surface of the flipped epitaxial structure in contact with a polishing pad. The carrier assembly applies a pressure of 0 to 100 N, a polishing liquid is applied to the polishing surface at a flow rate of 10 ml / min, and the polishing pad and wafer carrier rotate relative to each other, wherein the polishing pad and wafer carrier rotate at a speed of 10 to 50 rpm in opposite directions, thereby achieving chemical mechanical polishing. Under these polishing conditions, the polishing reduction rate ratio of Al(Ga)N to GaN is maximized, thereby accurately obtaining N-polarity GaN.

[0065] It should be noted that the polishing time can be selected according to the thickness of AlN, AlGaN, and GaN to expose the N-polarity GaN. After the polishing is completed, the surface of the obtained wafer is cleaned to remove the abrasive particles;

[0066] 5) Prepare related devices, and the preparation process can adopt the existing device manufacturing process.

[0067] The present invention will be further described below with reference to specific embodiments. In the following embodiments, an AP-380F polishing machine is used. The polishing wafer is a 1 cm × 1 cm, flipped Si substrate GaN epitaxial structure that has been bonded and peeled off from the original substrate, i.e., a thinned wafer to be polished. Figure 2As shown, the GaN epitaxial structure includes a Si substrate, a bonding layer, a 2μm GaN layer, a 2μm AlGaN stress buffer layer, and a 200nm AlN nucleation layer from bottom to top; after polishing, the cross-section is observed using a SEM (scanning electron microscope) to determine the remaining thickness after polishing, and the polishing rate is the ratio of the thickness removed by polishing to the polishing time; after polishing, the surface quality is detected using an atomic force microscope, and the surface roughness (Rq) is calculated.

[0068] Example 1

[0069] A method for preparing N-polar GaN based on CMP, comprising:

[0070] A polishing solution with a pH of 10 is prepared using KOH as a pH adjuster and SiO2 with a particle size of 10 to 60 nm as polishing abrasive particles;

[0071] A wafer to be polished and thinned is fixed in the center of a wafer carrier, which is mounted on a carrier assembly. The surface of the wafer to be polished and thinned contacts the polishing pad, and the carrier assembly applies a pressure of 8 N to the wafer to be polished and thinned. The polishing pad rotates at 30 rpm, the polishing liquid flows at a rate of 10 ml / min, the wafer carrier rotates at 50 rpm, and the polishing time is 10 minutes. Selective polishing is performed utilizing the high selectivity ratio between the polishing and thinning rates of N-polar AlN (AlGaN) and N-polar GaN, rapidly removing a 200 nm AlN nucleation layer and a 2 μm AlGaN stress buffer layer, and slowly removing the GaN layer until the surface of the N-polar GaN channel layer is exposed.

[0072] After polishing and thinning, the wafer was removed and cleaned with deionized water, and then ultrasonically cleaned in acetone and isopropyl alcohol for 3 minutes. The wafer structure obtained after polishing and thinning is as follows: Figure 3 shown.

[0073] Example 2

[0074] A method for preparing N-polar GaN based on CMP, comprising:

[0075] A polishing solution with a pH of 12 was prepared using NaOH as a pH regulator and SiO2 with a particle size of 20 to 30 nm as polishing abrasive particles.

[0076] A wafer to be polished and thinned is fixed in the center of a wafer carrier, which is mounted on a carrier assembly so that the surface of the wafer to be polished and thinned contacts the polishing pad. The carrier assembly applies a pressure of 15 N to the wafer to be polished and thinned. The polishing pad rotates at 20 rpm, the polishing liquid flows at a rate of 10 ml / min, the wafer carrier rotates at 30 rpm, and the polishing time is 6 minutes. Selective polishing is performed utilizing the high selectivity ratio between the polishing and thinning rates of N-polar AlN (AlGaN) and N-polar GaN, rapidly removing a 200 nm AlN nucleation layer and a 2 μm AlGaN stress buffer layer, and slowly removing the GaN layer until the surface of the N-polar GaN channel layer is exposed.

[0077] After polishing and thinning, the wafer was removed and cleaned with deionized water, and then ultrasonically cleaned in acetone and isopropyl alcohol for 3 minutes. The wafer structure obtained after polishing and thinning is as follows: Figure 3 shown.

[0078] The surface quality of the wafer after polishing and thinning after the treatment of Example 1 was detected by atomic force microscopy, wherein the AFM (atomic force microscope) of the N-polar GaN surface obtained by chemical mechanical polishing to remove N-polar AlN and AlGaN was obtained. Figure 4 As shown, the scanning range is 5*5 μm. After calculation, the roughness (Rq) of the N-polar GaN surface obtained after polishing and thinning in Example 1 is as low as 0.3 nm.

[0079] Comparative Example 1

[0080] In Comparative Example 1, the conventional dry etching process is used to thin the wafer to be polished and thinned to remove N-polar AlN and AlGaN, thereby obtaining N-polar GaN. After AFM (atomic force microscope) testing, the roughness Rq of the N-polar GaN surface is about 3 nm.

[0081] Through comparative research, the inventors of this case found that the surface roughness of N-polar GaN prepared by CMP thinning in a method for preparing N-polar GaN based on CMP provided by an embodiment of the present invention is smaller; after testing, the surface roughness of N-polar GaN prepared by dry etching after conventional substrate transfer is relatively large. After AFM (atomic force microscope) testing, it was found that the minimum roughness is about 1nm, while the surface roughness of N-polar GaN prepared by CMP after substrate transfer can be as low as 0.3nm.

[0082] Comparative Example 1 uses a dry etching process to thin the wafer to be polished and thinned. However, dry etching technology has etching damage, which can be manifested as increased surface roughness, increased film defects, and increased ohmic contact resistance of the prepared material or device. The low-damage dry etching technology is extremely slow (calculated at 1nm / min, it takes 33h to etch and thin the thickness of 2μm), resulting in extremely high cost. In contrast, the CMP amplification intersection in the present invention has the advantages of high efficiency and low damage compared to ICP etching.

[0083] It should be noted that the GaN material targeted by CMP polishing is a GaN substrate with a thickness of hundreds of microns, while the thickness of the N-polar GaN material after substrate transfer in the present invention is 2-3 μm. As is generally recognized by those skilled in the art, when the thickness of the material being thinned is too thin, the CMP process is difficult to control, so an ICP etching process is usually used for thinning. However, the inventors of this case have discovered that the CMP polishing thinning rate selectivity ratio of N-polar AlN, AlGaN, and GaN is large (AlN (AlGaN): GaN = 30:1), which enables the present invention to more accurately control the polishing process and produce high-quality N-polar GaN.

[0084] The N-polarity GaN prepared by the CMP-based N-polarity GaN method provided in the embodiment of the present invention has no etching damage, can reduce the ohmic contact resistance, and thus improve the performance of the N-polarity GaN and its devices.

[0085] An embodiment of the present invention provides a method for preparing N-polar GaN based on CMP, which is simple to operate, lower in cost, and more efficient. In addition, since the alkaline polishing solution has a high etching selectivity ratio between N-polar AlN (AlGaN) and N-polar GaN, CMP has a high thinning rate for N-polar AlN (AlGaN) and a lower thinning rate for N-polar GaN. Therefore, the polishing time can be controlled to the minute level, and high-quality N-polar GaN can be obtained efficiently. In contrast, in traditional dry etching after substrate transfer, etching quality and etching speed cannot be achieved at the same time. A faster etching speed results in a rough surface and greater damage, while a very low etching speed is required to obtain a high-quality etched sample, thus requiring a longer etching time.

[0086] It should be understood that the above embodiments are merely illustrative of the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent variations or modifications made in accordance with the spirit and substance of the present invention are intended to be encompassed within the scope of protection of the present invention.

Claims

1. A method for preparing N-polar GaN based on CMP, characterized in that include: Exposing the N-polar surface of the Ga-polar GaN material; The N-polar surface of the Ga-polar GaN material is thinned by chemical mechanical polishing using an alkaline polishing solution to expose the N-polar GaN therein; wherein the Ga-polar GaN material contains N-polar Al x Ga 1-x N and N-polar GaN, 0<x≤1, the N-polar Al x Ga 1-x The chemical mechanical polishing thinning rate selectivity ratio of N and N-polar GaN reaches 30:1 or more, and the alkaline polishing solution contains abrasive polishing nanoparticles.

2. The method according to claim 1, wherein: The pH value of the alkaline polishing liquid is 9-12.

3. The method according to claim 1, wherein: The alkaline polishing liquid comprises a pH regulator, polishing and grinding nanoparticles and deionized water.

4. The method according to claim 3, wherein: The content of polishing and grinding nanoparticles in the alkaline polishing liquid is 2-20%.

5. The method according to claim 3, wherein: The particle size of the grinding and polishing nanoparticles is 1-100 nm.

6. The method according to claim 3, wherein: The material of the grinding and polishing nanoparticles includes any one or more combinations of silicon oxide, aluminum oxide, and cerium oxide.

7. The method according to claim 3, wherein: The pH adjuster includes hydroxides or carbonates.

8. The method according to claim 1, wherein: During the chemical mechanical polishing and thinning process, the pressure applied between the N-polar surface of the Ga-polar GaN material and the polishing pad is 0-100N.

9. The method according to claim 8, characterized in that: The rotation speed of the polishing pad is 10 to 50 rpm, the rotation speed of the Ga-polarity GaN material is 10 to 50 rpm, and the rotation directions of the polishing pad and the Ga-polarity GaN material are opposite.

10. The method according to claim 1, characterized in that Specifically include: The Ga polar surface of the Ga polar GaN material grown on the first substrate is bonded to a second substrate, and then the first substrate is removed to expose the N polar surface of the Ga polar GaN material.

11. The method according to claim 10, characterized in that Specifically include: The first substrate includes a Si substrate or a sapphire substrate, and the second substrate includes a Si substrate, a sapphire substrate, a SiC substrate or a diamond substrate.

12. The method according to claim 11, wherein: The method specifically includes: removing the first substrate by using any one of mechanical grinding, dry etching, and wet etching.

13. The method according to claim 1, characterized in that Also includes: After the chemical mechanical polishing and thinning is completed, the N-polarity GaN material is cleaned.

14. N-polar GaN prepared by the method according to any one of claims 1 to 13.

15. The N-polar GaN according to claim 14, characterized in that: The surface roughness of the N-polar GaN is less than 1 nm.

16. The N-polar GaN according to claim 15, characterized in that: The surface roughness of the N-polar GaN is less than 0.3 nm.

17. Use of the N-polar GaN according to claim 14, 15 or 16 in preparing a GaN-based semiconductor device.

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