Surface morphology of polar, semipolar and nonpolar freestanding GaN after chemical etching

Journal Pre-proofs Full Length Article Surface morphology of polar, semipolar and nonpolar freestanding GaN after chemical etching Haijian Zhong, Chunyu Zhang, Wentao Song, Kebei Chen, Yaohuan Sheng, Gengzhao Xu, Zhenghui Liu PII: DOI: Reference:

S0169-4332(20)30280-4 https://doi.org/10.1016/j.apsusc.2020.145524 APSUSC 145524

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Applied Surface Science

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15 September 2019 22 January 2020 23 January 2020

Please cite this article as: H. Zhong, C. Zhang, W. Song, K. Chen, Y. Sheng, G. Xu, Z. Liu, Surface morphology of polar, semipolar and nonpolar freestanding GaN after chemical etching, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145524

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Surface morphology of polar, semipolar and nonpolar freestanding GaN after chemical etching Haijian Zhong1, Chunyu Zhang2, Wentao Song2, Kebei Chen2, Yaohuan Sheng1, Gengzhao Xu2, , Zhenghui Liu2,



1

School of Information Engineering, Gannan Medical University, Ganzhou 341000, P. R. China

2

Suzhou Institute of Nano-tech and Nano-Bionics, CAS, Suzhou 215123, P. R. China

Abstract: Surface etching methods were investigated for the removal of residues of chemical reagents and contaminants from polar (Ga- and N-plane), semipolar (r-plane) and nonpolar (a- and m-plane) freestanding GaN surfaces. Ga-, N-, r- and m-plane were etched in H3PO4, NaOH and (NH4)2S solutions. A-plane was treated in the solution of 10 wt% KOH in ethylene glycol (EG) at 80 °C. Different surface morphology were obtained and were characterized with atomic force microscope. The typical characteristics for the surface morphologies and interfacial angles in each etched GaN plane may help identify the type of crystal plane conveniently. Ga-polar GaN shows a clear and uniform step structure on surfaces. N-plane presented a step structure with tooth-like terraces. The (2021) r-plane, m-plane and a-plane show stripe-like structures, while the (202 1) r-plane is stable and hard to be etched to exhibit step structures. The



Corresponding authors at: Suzhou Institute of Nano-tech and Nano-Bionics, CAS, Suzhou 215123, P. R. China Email addresses: [email protected] (Gengzhao (Zhenghui Liu)

1

Xu); [email protected]

m-plane facets on a-plane surface caused by the etching method was found and discussed.

1. Introduction： For the last several decades, Gallium nitride (GaN) material, due to its excellent electric and optical properties, has undergone great development towards modern power electronic and optoelectronic devices, such as Schottky barrier diodes (SBDs), high electron mobility transistors (HEMTs), light emitting diodes (LEDs), laser diodes (LDs) and solar cells (SCs)[1-5]. Presently, these commercial nitride-based devices are mainly fabricated on cplane GaN substrates. However, it has been shown theoretically and experimentally that the internal spontaneous and piezoelectronic polarization effects of c-plane GaN can cause a strong electric field in the interface, which alters the properties of devices such as the so-called quantum confined Stark effect (QCSE) and efficiency droop in LEDs [6-9]. One possible solution to reduce or eliminate these effects is to use semipolar or nonpolar orientations of GaN to improve device performance [10-12]. The planes perpendicular to the c-plane such as (1010) (m-plane) and (1120) (a-plane) planes, are nonpolar planes, and crystal planes orientated between c-plane and nonpolar planes such as (2021) (r-plane), are semipolar planes, as shown in Fig. 1.

2

Fig. 1. A schematic representation of c-, r-, a- and m-planes of GaN

Compared with the growth of c-plane GaN, the direct growth processes of largesize and thick film high-quality semipolar and nonpolar GaN are still not spread widely by the metal organic chemical vapor deposition (MOCVD) or hydride vapor-phase epitaxy (HVPE)[13-16]. They are mainly made by slicing thick bulk GaN crystals grown by HVPE [17]. The visible surface damage and high roughness would be generated after slicing [18]. A surface with damage and high roughness should undergo further physical and chemical treatment for the next fabrication process of device such as the epitaxial growth of semiconductor heterostructure or the preparation of ohmic contact [19-22]. Thus, these semipolar and nonpolar GaN should be treated such as the lapping and polishing technology[23]. However, a smooth surface acquired from polishing treatments without any step and terrace structures is difficult to be used in the further growth process of GaN in which step-flow growth mode and island growth mode will be favored[24-31]. Therefore, it is necessary to study systematically the 3

further surface treatment methods of the polar, semipolar and nonpolar surface of GaN after polishing so as to display the atomic step structure on the GaN surfaces, which will help investigate the surface structures of different planes of GaN and further epitaxial growth mechanism. Cleaning or wet etching has been a main surface treatment method for GaN samples and investigated by a number of groups. Surface cleaning is to remove oxides and other contaminations on the surfaces, which has a significant influence on epitaxial defects, metal contact resistance/stability, and overall device quality[32]. Cleaning processes does not change much on the surface roughness of the GaN sample, such as HCl or HF-based solution treatments for the removal of oxides [33]. However, wet etching will have obvious effects on the GaN surfaces, which can generally be divided into two categories, namely electrochemical etching and chemical etching. Wet etching has a variety of applications, including the identification of GaN polarity, defect evaluation and device fabrication on surfaces[34]. Generally, the processes of wet etching of GaN in either acid or base solutions contain oxidation of the GaN surface and following dissolution of the resulting oxides (Ga2O3)[34-38]. The acid or base etchant acts as the catalyst and dissolves gallium oxide into solution. However, we cannot randomly choose acidic or basic substances to etch GaN surface because of polarity selective etching. The mechanism of polarity selective etching has been interpreted by many published reports[38-40]. In addition, it is noted that etching process could influence the surface states due to the changes of dangling bonds or the chemical absorbed layers[41-43], such as an increase in the surface band bending[444

46].The etching rate of GaN surfaces can be controlled effectively through adjusting temperatures, concentrations of the solutions, and etching time[47-51]. In this paper, we investigated the surface etching methods to obtain the atomic step or terrace structure of on the surface of the lapped and polished Ga- & N-plane (polar), r-plane (semipolar) and a- & m-plane (nonpolar) GaN. The surface structures of GaN were characterized with AFM. The typical characteristics for the surface morphologies and interfacial angles in each etched GaN plane may help identify the type of crystal plane conveniently. Furthermore, we explored the surface stability of GaN after the etching processes.

2. Experiments： A bulk GaN crystal with about 350 μm thickness was grown by hydride vapor phase epitaxy (HVPE). Ga source was generated by a reaction between metallic Ga and gaseous HCl at 850℃, and NH3 was used as N source. The growth was heated to 1040 °C in a mixed carrier gas flow (H2: N2=1:1) at atmospheric pressure. The typical growth rate approximately was 100 μm·h−1. Then the high quality of the GaN with low dislocation densities (less than 5×10-6 cm-1) was obtained. The semi-polar or non-polar GaN was cutted from bulk GaN at a certain angle and all of samples were polished to remove surface damage. The dimensions of Ga- and N-face c-plane GaN were 10mm×10mm×1mm. The dimensions of the r-plane and a- & m-plane samples were all 10mm×5mm×1mm. The semipolar r-plane (2021) GaN samples were prepared through the wire cutting process with an inclined angle of 75° to the c-axis along the [1014] 5

direction. Nonpolar GaN samples of a-plane (1120) and m-plane (1010) were also provided by the wire cutting vertically in the c-plane with the wire oriented along the [1 100] and [1210] direction, respectively. Lapping and polishing processes were employed to the sample surfaces. The orientation of the crystal axes of all the GaN samples used in our experiments were determined by XRD. The topography and surface roughness of samples were characterized by using AFM in tapping mode. Scanning directions in the AFM measurements were parallel to the identified orientation flat. The etching methods used are summarized in Table 1. All chemical reagents used in the etching methods were analytical grade quality. All water used were deionized water with resistivity higher than 18 MΩ · cm. Ultrasonic rinse was applied to ensure the interactions between chemical reagents and GaN surface, or the removal of all residues of chemical reagents and contaminants on the GaN surface. All containers made of pure quartz glass used were pre-cleaned. The use of ammonium sulfide ((NH4)2S) was to prevent re-oxidation of GaN surfaces[33]. All samples were finally blown dry with high pure nitrogen gas. AFM used in this work was a commercial instrument model (Bruker, Dimension ICON) under air conditions. The measurement error of the following root-mean-square (RMS) roughness for AFM images is 0.08 nm. The value is determined by the expanded uncertainty, suggested symbol 𝑈, and is obtained by multiplying the standard uncertainty, suggested symbol 𝑢𝑐, by a coverage factor, suggested symbol 𝑘. Thus 𝑈 = 𝑘 × 𝑢𝑐. The standard uncertainty 𝑢𝑐 is calculated by the instrument noise (0.04 nm, the RMS roughness of AFM image with 0 nm measuring scale). The coverage factor k is 2, which 6

defines an interval having a level of confidence of approximately 95%.

3. Results and discussions： Fig. 2 shows optical microscope images of the GaN films of different planes after wet etching processes. For the Ga-plane, r-plane, m-plane, and a-plane, the obvious changes in surface morphology are not observed by the optical microscope as shown in Figs. 2(a), (c), (d), (e), and (f), respectively. Therefore, it indicates that the wet etching processes do not noticeably damage film surfaces and maintain their smooth. In contrast, the surface morphology is dramatically varied for N-polar GaN films as shown in Fig. 2(b). The hexagonal facets, which are a feature of N-polar GaN, appear after the wet etching processes and the surface morphology becomes rough.

Table 1 The summation of the etching methods Sample

Method

Etching results

Ga-plane

Ultrasonic rinse in 85% H3PO4 for 20 min Ultrasonic rinse in deionized water for 3 times and 1 min each Ultrasonic rinse in 1M NaOH for 20 min Ultrasonic rinse in deionized water for 3 times and 1 min each Ultrasonic rinse in (NH4)2S for 30 seconds Ultrasonic rinse in deionized water for 3 times and 1 min each One rinse in ethanol 5 min Blow dry with high purity N2

A clear and uniform step structure on surfaces

N-plane

Ultrasonic rinse in 1M NaOH for 5 min Ultrasonic rinse in deionized water for 3 times and 1 min each Ultrasonic rinse in 85% H3PO4 for 5 min Ultrasonic rinse in deionized water for 3 times and 1 min each

An inhomogeneous structure with big terraces on surfaces

7

Ultrasonic rinse in (NH4)2S for 30 seconds Ultrasonic rinse in deionized water for 3 times and 1 min each One rinse in ethanol 5 min Blow dry with high purity N2 r-plane (2021)

The same to Ga-plane

A dense stripe-like structure

r-plane (202 1)

Ⅰ. Acidic and basic etching method similar to Ga-plane with 5M NaOH instead of 1M NaOH. II. 98% H2SO4 etching method similar to Gaplane with 98% H2SO4 instead of 85% H3PO4 and without NaOH treatment. III. ICP treatment: Cl2 (25 sccm)/BCl3 (10 sccm), 6 min. Then apply acidic and basic etching method similar to Ga-plane with 5M NaOH instead of 1M NaOH.

No obvious changes after each step (I, II and III).

m-plane

The same to Ga-plane

A stripe-like structure

a-plane

10min etching in the solution of 10wt% KOH in ethylene glycol (EG) at 80 °C Ultrasonic rinse in deionized water for 3 times and 1 min each One rinse in ethanol 5 min Blow dry with high purity N2

A stripe-like structure

Fig. 2. Optical microscope images of GaN samples after wet etching processes: (a) Ga-plane, (b) N-plane, (c) r-plane (2021), (d) r-plane (202 1), (e)m-plane, and (f) a-plane. Surface 8

morphologies of Ga-plane, r-plane, m-plane, and a-plane have no obvious change. N-polar surface varies drastically with the hexagonal facets emerging.

3.1 The polar plane (c-plane) Ga-plane Owing to the surface charge of Ga-Polar GaN induced by polarization is negative[45.52.53], in the clean process, H3PO4 was firstly employed to etch the GaN surface, and NaOH to remove the residues of H3PO4. The sample was measured by AFM method before and after etching as shown in Fig. 3. Before etching, there were intensive particles on the GaN surface and the terraces may be seen indistinctly in Fig. 3(a). However, after etching, the GaN surface can only be found few particles within 5×5 μm region in Fig. 3(b). The RMS roughness for Fig. 3(a) and 3(b) are 0.10 nm and 0.07 nm. Fig. 3(c) is a zoom-in image acquired by AFM with 500×500 nm area in Fig. 3(b), which shows a clear and uniform step structure. As shown in Fig. 3(c) and 3(d), the step height is about 0.28±0.03 nm corresponding to the height of a Ga-N bilayer that is the so-called one-monolayer (∼0.26 nm)[54]. The surface shows long, straight, parallel, and evenly spaced monatomic steps. Moreover, the etched GaN sample exhibited a good surface chemical stability under atmospheric environment, which was measured with AFM a week after etching and showed a similar clear surface.

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Fig. 3. AFM images of a Ga-Polar GaN sample before (a) and after (b) wet etching processes with 5×5 μm scanning area. The RMS roughness values measured were 0.10 nm and 0.07 nm for (a) and (b). (c) A zoom-in image AFM image of the area in (b) with 500×500 nm scanning range. The white line across the step of GaN indicated the profile for the heights shown in (d) at approximately 0.28±0.03 nm corresponding to one bilayer high steps (∼0.26 nm) in cdirection.

N-plane As shown in Fig. 4(a), there are few particle contaminations and a number of scratches on the sample surface after polishing process. The surface charge of N-polar GaN induced by polarization is positive, which is opposite to Ga-polar GaN. Therefore, we firstly etched the N-polar GaN surface using NaOH solution for the step structure exposing. Then H3PO4 was employed to remove the residues of NaOH. Fig. 4(b) shows the surface morphology after the etching procedures. It is found that the scratches disappear and the surface displays tooth-like structures with short unparalleled step lines and irregular spaces between them. The step lines form an angle of 120° between 10

them. The step heights of big terraces were measured to be from about 1.0 nm to 4.0 nm. The small terraces as shown in the zoomed image Fig. 4(c) and the line profile in Fig. 4(d) show the step height with approximately 0.53±0.04 nm corresponding to the unit cell height for c-GaN of 0.52 nm. This may be originated from the two adjacent steps merging into a higher step during HVPE growth, which indicates doublemonolayer-height steps would be a lower energy state and a tendency for step merging. These results demonstrate a higher chemical activity for N-polar GaN, compared with Ga-plane for its uniform one-monolayer high step. As mentioned above, this would be mainly attributed to a lower repulsive force on N-plane GaN surface than that of Gaplane during wet etching process.

Fig. 4. AFM images of an N-plane GaN sample before (a) and after (b) wet etching processes with 10×10 μm scanning area. The RMS roughness values measured were 1.94 nm and 2.14 nm for (a) and (b). (c) A zoom-in image AFM image of the area in (b) with 1×1 μm scanning range. The white line across the step of GaN indicated the profile for the heights shown in (d) at approximately 0.53±0.04 nm corresponding to the unit cell height for c-GaN of ~0.52 nm. 11

3.2 The semipolar plane (r-plane) Fig. 5(a) is the AFM image of the (2021) r-plane GaN surface before etching, which shows a very smooth surface with no crystal structure, such as steps. After etching processes, as shown in Fig. 5(b) and 5(c), dense stripe-like structures can be found on the surface, which may be related to semipolar surface nano-faceting that results in the formation of (1010) and (1011) facets [55, 56]. The RMS roughness parameter for Fig. 5(a) and 5(b) is 0.11 nm and 0.18 nm, respectively, which demonstrates a little change in surface roughness. Fig. 5(d) shows the profile for the heights corresponding to the white dot line in Fig. 5(c). Two typical angles of two adjacent planes were measured to be close to 180° with 177.5° and 178°, respectively. These experiment results indicate that the etching method can effectively eliminate the influence of the polishing processes and reveal the (2021) r-plane GaN surface with crystal structure features. However, (202 1) r-plane GaN, which is the other side of (2021) plane, exhibited unusual behaviors during wet etching. A (202 1) r-plane GaN sample obtained by same polishing processes were treated with wet etching the same to Ga-plane and 85% H3PO4 under ultrasonic rinse. As shown in Fig. 6(a), 6(b) and 6(c), the surface morphologies of the sample were not changed obviously with a little variations of RMS roughness and no steps or terraces emerge on the surfaces. Strong acidity and oxidizing played a dispensable role in the etching process. In order to completely eliminate the influence of chemical substances in lapping and polishing processes, the (202 1) r-plane GaN sample was further dry-etched 600 nm deep using a Cl2/BCl3based inductively coupled plasma (ICP) treatment to acquire a pure GaN surface without any contaminations. After ICP treatment, the wet etching was carried on the 12

sample through wet etching the same to Ga-plane again. As shown in Fig. 6(d), the surface morphology also has no substantial changes and RMS roughness is reduced slightly. Similarly, there are not any step or terrace structures emerging on the surface yet. The same experiments have been repeated twice using other samples the same batch and we got similar results. Our studies show that (202 1) r-plane GaN would have a high stability under strongly acidic, basic and oxidizing conditions. At present, it is difficult for us to theoretically explain why (202 1) r-plane GaN is so chemically stable. In our future study, we will aim to seek appropriate etching methods and reasonable theoretical explanations for the stability.

Fig. 5. AFM images of an (2021) r-plane GaN sample before (a) and after (b) wet etching processes with 10×10 μm scanning area. The RMS roughness values measured were 0.11 nm and 0.18 nm for (a) and (b). (c) A zoom-in image AFM image of the area in (b) with 1×1 μm scanning range. The profile for the heights corresponding to the white dot line in (c) was shown in (d). Two typical angles of two adjacent planes were measured to be 177.5° and 178°, respectively.

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Fig. 6. 10×10 μm AFM images of a (202 1) r-plane GaN before etching (a) and after wet etching by 85% H3PO4 (20min) and 5M NaOH (20min) (b), 98% H2SO4 (20min) (c), and treated by ICP and then etched by 85% H3PO4 (20min) and 5M NaOH (20min) (d), respectively, under ultrasonic rinse. The corresponding RMS roughness is 0.15 nm (a), 0.14 nm (b), 0.09 nm (c), and 0.30 nm (d), respectively.

3.3 The nonpolar plane m-plane The surface treatment method of the m-plane GaN is the same as that of the Gapolar GaN. Fig. 7(a) and 7(b) are the AFM images of GaN before and after etching, whose RMS roughness changed slightly with 0.66 nm and 0.50 nm, respectively. The sample had a slight unintentional miscut oriented primarily towards the [0001] direction and a small component toward the [1210] direction, which resulted in the terraces of different size on m-plane GaN surface as shown Fig. 7(b) and 7(c). The width of terraces tends to be elongated in the [0001] direction, forming a stripe-like morphology, similar to the reported topography by Marta Sawicka et al.[57]. Fig. 7(d) shows two typical step heights measured to be about 0.30 nm and1.89 nm, respectively. As previously reported, one monolayer of m-plane GaN is 0.2762 nm[58-59]. Therefore, the step heights of 0.30 nm and 1.89 nm are almost corresponding to one monolayer 14

and seven monolayers, respectively. This may attribute to little inhomogeneity in the morphology of m-plane surface. However, the clean surface of m-plane GaN exposing in the atmospheric environment showed a poor chemical durability. The steps on the surface would get blurred in a week. In order to preserve m-plane GaN for long-term stable conservation, a series of experiments were carried out to evaluate the influences of the oxygen concentration and environmental humidity. The experimental results showed that m-plane GaN would suffer oxidation at room temperature even in dry air. While the sample was stored in the container filled with inert nitrogen gas, the surface morphology of m-plane GaN remained stable for long.

Fig. 7. AFM images of an m-plane GaN sample before (a) and after (b) wet etching processes with 10×10 μm scanning area. The RMS roughness values measured were 0.66 nm and 0.50 nm for (a) and (b), respectively. (c) A zoom-in image AFM image of the area in (b) with 500×500 nm scanning range. (d) The profile for the heights corresponding to the white dot line in (c). Two typical step heights were measured to be ~0.30 nm (one monolayer) and ~1.89 nm (seven monolayers), 15

respectively.

a-plane The a-plane sample firstly treated through the etching method of Ga-polar GaN. The surface morphology was changed dramatically during the etching processes according to Fig. 8(a) and 8(b). Compared with a 0.67 nm RMS roughness before etching, the RMS roughness of a-plane after etching increased sharply up to 12.70 nm and coarse stripe-like crystal structures were found ubiquitously. As is shown in Fig. 8(c) and 8(d), the typical angle of two adjacent planes were measured to be 120°. The results suggest an aggressive etching of the a-plane GaN, leading to an anisotropic etching profile and emergence of m-plane. It is due to a lower surface energy for aplane GaN. The etching rate is directly related to the surface energy. The surface energy is believed to be determined by the density of dangling bonds per unit area on the surface. The densities of the dangling bonds (DB) for a-plane and m-plane were reported to be 14.0 nm-2 and 12.1 nm-2, respectively[60-62]. The surface energies of aplane and m-plane were reported to be 123 meV/Å2 and 118 meV/Å2 calculated within the local-density approximation respectively[63]. Therefore, m-GaN is more energetically favored due to a lower surface energy and possess a higher stability in wet etching process. The angle between a-plane and m-plane is 30° with a-plane as the horizontal reference surface of the sample as shown in Fig. 1. Thus, undulations were observed on a-plane GaN with a 120° angle of intersection. In order to find a truly isotropic etch and obtain the a-plane surface with crystal 16

structures, the etching solution of 10%wt KOH in EG was reported to be effective to achieve isotropic etching for a-plane GaN surface[64]. The EG molecules will adhere to a-plane, and its more complex molecular spatial configuration will hinder the access of etching agent (KOH) to some extent and slow down the etching rate of a-plane. We finally chose the method of 10wt% KOH in EG at 80 °C for 10 min based on the results of examination of the dependence of etching appearance on etching time and etching temperature. The AFM image after etching by the solution of 10wt% KOH in EG is shown in Fig. 9(a) with RMS roughness of 0.18 nm. Compared with the previous etching results of 12.70 nm, the RMS roughness of a-plane GaN etched by 10%wt KOH-EG declined about 70 times. Similar to published results[65], the stripe-like structures can also be observed from Fig. 9(b), which indicated that a certain anisotropic etching still occurred using the etching method of 10%wt KOH-EG. The AFM profile in Fig. 9(c) shows a peak-to-peak undulation of about ±2 nm and two typical angles of two adjacent planes with 174° and 171°, respectively, indicating a slightly anisotropic etching for a-plane GaN.

Fig. 8. AFM images of an a-plane GaN sample before (a) and after (b) wet etching processes with 17

10×10 μm scanning area. The RMS roughness values measured were 0.67 nm and 12.70 nm for (a) and (b), respectively. (c) A zoom-in image AFM image of the area in (b) with 500×500 nm scanning range. (d) The profile for the heights corresponding to the white dot line in (c). A typical angle of two adjacent m-planes were measured to be 120°.

Fig. 9. (a) AFM image of the a-plane GaN sample after 10%wt KOH-EG etching processes with 10×5 μm scanning area. The RMS roughness values measured was 0.18 nm for (a). (b) A zoom-in image AFM image of the area in (b) with 500×250 nm scanning range. (c) The profile for the heights corresponding to the white dot line in (b). Two typical angles of two adjacent planes were measured to be 174° and 171°, respectively.

From AFM results, it can be found that each plane shows a unique morphology after wet etching process. Ga-plane has long, straight, parallel, and evenly spaced monatomic steps. N-plane displays tooth-like structures with short unparalleled step lines and irregular spaces between them. r-plane shows dense stripe-like steps with relatively uniform distribution. m-plane then shows a non-uniformly distributed stripelike morphology and various step heights. a-plane demonstrates a surface instability after wet etching process, such as a great change in surface roughness and an angle of 18

120 ° for two adjacent planes. Conclusions： In this study, we explored the different etching methods for Ga-plane, N-plane, rplane, a-plane and m-plane GaN surface. AFM measurement was employed to characterize the surface morphology. Our experimental results showed that the etching method corresponding to the GaN plane could be effective to remove the residues of chemical reagents and contaminants during previous polishing processes and obtain clean surfaces with step structures. Interestingly, (202 1) r-plane GaN demonstrated a high stability under strongly acidic, basic and oxidizing conditions, and was difficult to obtain step-structure surface using the etching methods here. Note that m-plane GaN need to be stored in an inert atmosphere for its instability in the air. Moreover, AFM can be served to help quickly and nondestructively identify the Ga-plane, N-plane, rplane, a-plane and m-plane GaN surface for their unique crystal morphology after wetprocess etching method.

Acknowledgments First two authors contributed equally to this work. The authors gratefully acknowledge the National Key Technologies R&D Program of China (Nos. 2016YFB0400101), the Natural Science Foundation of China (Nos. 61674164, 61475184 and 11604368), the National Key Scientific Instrument and Equipment Development Project of China (No. 11327804), the Instrument Developing Project of the Chinese Academy of Sciences (No. YZ201341), and the China National Funds for 19

Distinguished Young Scientists (No. 61325022).

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Highlights 1) Residues might exist in a smooth surface acquired from polishing treatments. 2) Surface etching methods to remove the residues for different GaN planes are proposed. 3) AFM help identify the type of crystal surface of GaN.

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Declaration of Interest Statement No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my coauthors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. It is not being submitted to any other journal.

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Author contributions Haijian Zhong conceptualized the study, analyzed the data, and led the writing of the manuscript. Chunyu Zhang performed all the sample cleaning, etching and AFM experiments, and collected the data and assisted in data analysis. Gengzhao Xu and Zhenghui Liu verified the experimental processes and results, and reviewed and edited the manuscript. Wentao Song, Kebei Chen and Yaohuan Sheng provided valuable critical revisions of the manuscript. All authors read and approved the manuscript.

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