- Email: [email protected]
Growth of InGaN multiple quantum wells and GaN eplilayer on GaN substrate
ARTICLE IN PRESS
Physica B 376–377 (2006) 532–535 www.elsevier.com/locate/physb
Growth of InGaN multiple quantum wells and GaN eplilayer on GaN substrate Sung-Nam Leea,b,, H.S. Paeka, J.K. Sona, T. Sakonga, E. Yoonb, O.H. Nama, Y. Parka a
Photonics Laboratory, Samsung Advanced Institute of Technology, Suwon, Republic of Korea School of Materials Science and Engineering, Seoul National University, Seoul, Republic of Korea
b
Abstract We investigated that the surface morphology of GaN epilayers was significantly affected by the surface tilt orientation of GaN substrate. Surface morphologies of GaN epilayers on GaN substrates show three types: mirror, wavy, and hillock. These surface morphologies are dependent on the surface orientation of GaN substrates. It is found that the hillock morphology of GaN epilayer was formed on the GaN substrate with surface tilt orientation less than 0.11. As the surface tilt angle increased to 0.351, the surface morphology varied from hillock to wavy morphology. Above a surface tilt angle of 0.41, surface morphology changed to the mirror-like type morphology. Additionally, these three types of GaN surface morphology also affected the optical quality of GaN epilayers as well as InGaN multiple quantum wells on GaN substrates by non-uniform In incorporation on the different surface morphologies of GaN epilayers. r 2005 Elsevier B.V. All rights reserved. PACS: 685.55.Jk; 81.05.Ea; 81.15.Gh; 78.55.m Keywords: GaN; InGaN; Homoepitaxy; PL; Surface orientation
1. Introduction III-nitrides have attracted much attention for optoelectronic device applications whose emission wavelength ranges from green to ultraviolet light due to their wide band gaps. It is well known that the large lattice mismatch between GaN and sapphire substrate leads to high density of threading dislocations in nitride-based epitaxial layers [1]. However, since the GaN substrates have recently become commercially available, a lot of groups have studied the homoepitaxial growth to enhance the reliability of optoelectronic device by minimizing the dislocation density of GaN epilayers [2–4]. By homoepitaxy, twodimensional step flow growth of GaN can be achieved without the need for additional deposition steps such as surface nitridation, a thin low-temperature nucleation Corresponding author. Photonics Laboratory, Samsung Advanced Institute of Technology, Suwon, Republic of Korea. Tel.: +82 31 280 9133; fax: +82 31 280 9357. E-mail address: [email protected] (S.-N. Lee).
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.12.135
layer, or thick buffer layers [4]. It is well known that GaN substrates grown by hydride vapor-phase epitaxy (HVPE) can be mechanically polished, and subsequently reactive-ion etched to prepare epi-ready substrates [3–5]. However, we have often observed the non-uniform hexagonal hillock formation on the III-nitride epilayers grown on GaN substrates. For device applications, since the smooth surfaces are required, the formation of hexagonal hillocks on epilayers should be avoided. A common way to avoid such growth features on the surface of epilayer is the use of substrates with a slight surface tilt orientation with respect to the exact crystallographic plane [6–9]. Additionally, in the growth of InGaN epilayer as an active layer of light-emitting devices, it is well known that the In incorporation is significantly sensitive to the surface morphology of a GaN template [10]. Therefore, the In composition fluctuation of InGaN quantum well can originate from the surface morphology of a GaN template. In this study, we investigated the surface morphology of GaN templates on GaN substrates with different surface tilt orientation. It is found that the formation of hillock
ARTICLE IN PRESS S.-N. Lee et al. / Physica B 376–377 (2006) 532–535
and wavy morphology can be suppressed by controlling the surface tilt orientation of a GaN substrate. The different morphological development of InGaN quantum wells is found to have a surprisingly important consequence on the indium incorporation efficiency.
(a)
533
0.02°
2. Experimental We have grown high-quality GaN epilayer and InGaN/ InGaN multiple quantum wells (MQWs) with smooth surface morphology on c-plane GaN substrates by metalorganic chemical vapor deposition (MOCVD) and the optical and surface analyses of epilayers grown on those substrates are examined with respect to the tilting angles. GaN substrates were tilted toward ½1 1 2¯ 0 direction by various angles of 0.021, 0.351, and 0.451. All samples were grown under the same growth condition that was optimized for the growth of GaN epilayer and InGaN/ InGaN MQWs on GaN substrates. Trimethylgallium (TMGa), trimethylindium (TMIn), and ammonia (NH3) were used as precursors for Ga, In and N, respectively. All sample structures were deposited on a 2:0 mm thick undoped GaN base layer without any buffer layer. The standard QWs structure consisted of 5-period of QWs with 3 nm thick undoped In0.06Ga0.94N wells and 7 nm thick Sidoped In0.02Ga0.98N barriers. The growth temperature of the GaN-based layer and the QW region were 1030 and 770 1C, respectively. Surface morphology was observed by a Nomarski optical microscope, and the optical qualities were characterized by micro-photoluminescence (PL) using the 325 nm line of a 1.5 mW He–Cd laser at room temperature. We found that the surface morphology and the optical qualities were very sensitive to surface tilt angles of c-plane GaN substrates.
100 µm (b)
0.35°
100 µm (c)
0.45°
100 µm Fig. 1. The images formed by a Nomarski optical microscope show the three types of GaN surface morphology grown on GaN substrates with different surface tilt angles of: (a) 0.021, (b) 0.351, and (c) 0.451.
3. Results and discussion Fig. 1 shows the three surface morphologies of GaN epilayers grown GaN substrates by a Nomarski optical microscope. Surface morphologies of GaN epilayers on GaN substrates show three type of surfaces: mirror surface, wavy, and hillock surface. These surface morphologies are dependent on the surface orientation of GaN substrate. The surface tilt angle of GaN substrate may originate from surface treatment process in fabricating the GaN substrate. It is found that the hillock morphology of GaN epilayer was formed on the GaN substrate with 0.021 surface tilt angle as shown in Fig. 1(a). As the surface tilt angle increased to 0.351, the surface morphology varied from hillock to wavy. Above the surface tilt angle of 0.41, the surface morphology of GaN epilayer shows the mirror-like surface. Additionally, the surface morphologies of InGaN MQWs on GaN epilayer also show the same surface morphologies as those of GaN epilayers. In the formation of hillock surface, it is well known that the pyramidal hillock growth is initiated by the nucleation of critical two-
dimensional nuclei at the terminal points of threading dislocations on the surface, having a screw component of the Burgers vector normal to the interface [9]. Therefore, based on our observation, we can speculate that the GaN substrate with a very low surface tilt angle has more dislocations with screw components than edge components. Fig. 2 shows micro-PL intensity mapping images ð40 40 mm2 Þ at the band edge emission wavelength of GaN epilayer grown on GaN substrates with different surface orientations. It basically exhibits the dark spots, which are the terminal points of threading dislocations originated from GaN substrates. In Fig. 2(a) at the surface tilt angle of 0.021, micro-PL intensity mapping image is very similar to the hillock shape of the GaN epilayer. The dark lines between facets are considered to result from the low-angle grain boundaries at which the PL intensity is reduced because of the crystal imperfection. However, the intensity mapping images of wavy and mirror-like surface
ARTICLE IN PRESS 534
S.-N. Lee et al. / Physica B 376–377 (2006) 532–535
Fig. 3. The PL spectra of InGaN/InGaN MQWs grown on three types of GaN epilayer morphology.
Fig. 2. Micro-PL intensity mapping images ð40 40 mm2 Þ at band edge emission wavelength of GaN epilayer grown on GaN substrates with different surface tilt angle of: (a) 0.021, (b) 0.351, and (c) 0.451.
morphologies of GaN epilayers were featureless except for dark spots, and were more uniform than that of a GaN epilayer with hillock surface morphology. These surface morphologies of GaN epilayers grown on GaN substrates with different surface tilt angles also affected the optical quality of InGaN MQWs as shown in Fig. 3. The PL emission wavelength of InGaN/InGaN
MQWs is decreased with increasing the surface tilt angle of GaN substrates. It shows that the In incorporation rate of InGaN MQWs on GaN epilayer with the hillock surface is more than that of InGaN QWs with mirror-like GaN morphology. Additionally, full-width at half-maximum (FWHM) of PL is also decreased with increasing the surface tilt angle by changing surface morphology from hillock to mirror-like surface. It shows that the In distribution of InGaN QWs grown on the GaN with low surface tilt angle is less uniform than that with relatively high tilt angle. In order to confirm the In-phase separation with different surface tilt angles, we also measured the micro-PL intensity mapping ð40 40 mm2 Þ at the emission wavelength of InGaN QWs with different surface morphologies, as shown in Fig. 4. The density of dark spots in intensity mapping images for InGaN MQWs is significantly decreased, compared to that of GaN epilayers as shown in Fig. 2. Due to the relatively fast radiative recombination and the short carrier diffusion length, the emission efficiency of InGaN is not significantly affected by dislocation density so that the dislocation related dark spots are normally not observed in InGaN films on GaN substrates. However, mapping images for each sample exhibits their own features with dark and bright patterns. We conclude that these images are similar to the surface morphology of GaN templates for each sample. From these results, it indicated that the indium incorporation efficiency is drastically higher on the facets than on a smooth surface [10,11]. We speculate that the deposition of the In adatoms are attracted at facets rather than on a smooth surface to reduce the surface energy due to many dangling bonds at facets. Therefore, the In phase separation in InGaN QWs layer can easily occur on the rough surface of GaN epilayer grown on free-standing GaN substrates.
ARTICLE IN PRESS S.-N. Lee et al. / Physica B 376–377 (2006) 532–535
535
the surface tilt angle of GaN substrates. By increasing the surface tilt angle above 0.41, the surfaces of GaN epilayer on GaN substrate becomes mirror like. Additionally, the surface tilt angle of GaN substrates also affect the optical quality of InGaN MQWs by non-uniform incorporation of indium on the hillock and wavy surfaces of GaN epilayers grown on GaN substrates with surface tilt angle below 0.41. Acknowledgements The authors would like to thank the other members of BD-LD team for their fruitful discussions and encouragements. References [1] S.N. Lee, J.K. Son, H.S. Paek, T. Sakong, W. Lee, K.H. Kim, S.S. Kim, Y.J. Lee, D.Y. Noh, E. Yoon, O.H. Nam, Y. Park, Phys. Stat. Sol. (c) 1 (2004) 2458. [2] X. Xu, R.P. Vaudo, J. Flynn, J. Dion, G.R. Brandes, Phys. Stat. Sol. (a) (2005) 727. [3] A.R.A. Zauner, J.L. Weyher, M. Plomp, V. Kirilyuk, I. Grzegory, W.J.P. van Enckevort, J.J. Schermer, P.R. Hageman, P.K. Larsen, J. Cryst. Growth 210 (2000) 435. [4] C.R. Miskys, M.K. Kelly, O. Ambacher, G. Martinez-Criado, M. Stutzmann, Appl. Phys. Lett. 77 (2000) 1858. [5] M. Schauler, F. Eberhard, C. Kirchner, V. Schwegler, A. Pelzmann, M. Kamp, K.J. Ebeling, F. Bertram, T. Riemann, J. Christen, P. Prystawko, M. Leszczynski, I. Grzegory, S. Porowski, Appl. Phys. Lett. 74 (1999) 365. [6] W.J.P. van Enckevort, G. Janssen, W. Vollenberg, L.J. Giling, J. Cryst. Growth 148 (1995) 365. [7] X.R. Huang, J. Bai, M. Dudley, R.D. Dupis, U. Chowdhury, Appl. Phys. Lett. 86 (2005) 211946. [8] T. Someya, K. Hoshino, Y. Arakawa, Appl. Phys. Lett. 79 (2001) 1992. [9] J. Zhou, Y.J. Tang, G.Y. Zhang, Solid State Commun. 121 (2002) 381. [10] H. Tang, S. Haffouz, A. Powell, J.A. Bardwell, J. Webb, Appl. Phys. Lett. 86 (2005) 121110. [11] K. Kawasaki, I. Nakamatsu, H. Hirayama, K. Tsutsui, Y. Aoyagi, J. Cryst. Growth 243 (2002) 129.
Fig. 4. Micro-PL intensity mapping images ð40 40 mm2 Þ at band edge emission wavelength of InGaN QWs grown on the GaN epilayer with different surface morphologies like (a) hillock, (b) wavy, and (c) mirror.
4. Conclusion Surface morphologies of GaN epilayers on GaN substrates showed three types. The mirror-like, wavy, and hillock surfaces of GaN epilayer is developed by changing
Physica B 376–377 (2006) 532–535 www.elsevier.com/locate/physb
Growth of InGaN multiple quantum wells and GaN eplilayer on GaN substrate Sung-Nam Leea,b,, H.S. Paeka, J.K. Sona, T. Sakonga, E. Yoonb, O.H. Nama, Y. Parka a
Photonics Laboratory, Samsung Advanced Institute of Technology, Suwon, Republic of Korea School of Materials Science and Engineering, Seoul National University, Seoul, Republic of Korea
b
Abstract We investigated that the surface morphology of GaN epilayers was significantly affected by the surface tilt orientation of GaN substrate. Surface morphologies of GaN epilayers on GaN substrates show three types: mirror, wavy, and hillock. These surface morphologies are dependent on the surface orientation of GaN substrates. It is found that the hillock morphology of GaN epilayer was formed on the GaN substrate with surface tilt orientation less than 0.11. As the surface tilt angle increased to 0.351, the surface morphology varied from hillock to wavy morphology. Above a surface tilt angle of 0.41, surface morphology changed to the mirror-like type morphology. Additionally, these three types of GaN surface morphology also affected the optical quality of GaN epilayers as well as InGaN multiple quantum wells on GaN substrates by non-uniform In incorporation on the different surface morphologies of GaN epilayers. r 2005 Elsevier B.V. All rights reserved. PACS: 685.55.Jk; 81.05.Ea; 81.15.Gh; 78.55.m Keywords: GaN; InGaN; Homoepitaxy; PL; Surface orientation
1. Introduction III-nitrides have attracted much attention for optoelectronic device applications whose emission wavelength ranges from green to ultraviolet light due to their wide band gaps. It is well known that the large lattice mismatch between GaN and sapphire substrate leads to high density of threading dislocations in nitride-based epitaxial layers [1]. However, since the GaN substrates have recently become commercially available, a lot of groups have studied the homoepitaxial growth to enhance the reliability of optoelectronic device by minimizing the dislocation density of GaN epilayers [2–4]. By homoepitaxy, twodimensional step flow growth of GaN can be achieved without the need for additional deposition steps such as surface nitridation, a thin low-temperature nucleation Corresponding author. Photonics Laboratory, Samsung Advanced Institute of Technology, Suwon, Republic of Korea. Tel.: +82 31 280 9133; fax: +82 31 280 9357. E-mail address: [email protected] (S.-N. Lee).
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.12.135
layer, or thick buffer layers [4]. It is well known that GaN substrates grown by hydride vapor-phase epitaxy (HVPE) can be mechanically polished, and subsequently reactive-ion etched to prepare epi-ready substrates [3–5]. However, we have often observed the non-uniform hexagonal hillock formation on the III-nitride epilayers grown on GaN substrates. For device applications, since the smooth surfaces are required, the formation of hexagonal hillocks on epilayers should be avoided. A common way to avoid such growth features on the surface of epilayer is the use of substrates with a slight surface tilt orientation with respect to the exact crystallographic plane [6–9]. Additionally, in the growth of InGaN epilayer as an active layer of light-emitting devices, it is well known that the In incorporation is significantly sensitive to the surface morphology of a GaN template [10]. Therefore, the In composition fluctuation of InGaN quantum well can originate from the surface morphology of a GaN template. In this study, we investigated the surface morphology of GaN templates on GaN substrates with different surface tilt orientation. It is found that the formation of hillock
ARTICLE IN PRESS S.-N. Lee et al. / Physica B 376–377 (2006) 532–535
and wavy morphology can be suppressed by controlling the surface tilt orientation of a GaN substrate. The different morphological development of InGaN quantum wells is found to have a surprisingly important consequence on the indium incorporation efficiency.
(a)
533
0.02°
2. Experimental We have grown high-quality GaN epilayer and InGaN/ InGaN multiple quantum wells (MQWs) with smooth surface morphology on c-plane GaN substrates by metalorganic chemical vapor deposition (MOCVD) and the optical and surface analyses of epilayers grown on those substrates are examined with respect to the tilting angles. GaN substrates were tilted toward ½1 1 2¯ 0 direction by various angles of 0.021, 0.351, and 0.451. All samples were grown under the same growth condition that was optimized for the growth of GaN epilayer and InGaN/ InGaN MQWs on GaN substrates. Trimethylgallium (TMGa), trimethylindium (TMIn), and ammonia (NH3) were used as precursors for Ga, In and N, respectively. All sample structures were deposited on a 2:0 mm thick undoped GaN base layer without any buffer layer. The standard QWs structure consisted of 5-period of QWs with 3 nm thick undoped In0.06Ga0.94N wells and 7 nm thick Sidoped In0.02Ga0.98N barriers. The growth temperature of the GaN-based layer and the QW region were 1030 and 770 1C, respectively. Surface morphology was observed by a Nomarski optical microscope, and the optical qualities were characterized by micro-photoluminescence (PL) using the 325 nm line of a 1.5 mW He–Cd laser at room temperature. We found that the surface morphology and the optical qualities were very sensitive to surface tilt angles of c-plane GaN substrates.
100 µm (b)
0.35°
100 µm (c)
0.45°
100 µm Fig. 1. The images formed by a Nomarski optical microscope show the three types of GaN surface morphology grown on GaN substrates with different surface tilt angles of: (a) 0.021, (b) 0.351, and (c) 0.451.
3. Results and discussion Fig. 1 shows the three surface morphologies of GaN epilayers grown GaN substrates by a Nomarski optical microscope. Surface morphologies of GaN epilayers on GaN substrates show three type of surfaces: mirror surface, wavy, and hillock surface. These surface morphologies are dependent on the surface orientation of GaN substrate. The surface tilt angle of GaN substrate may originate from surface treatment process in fabricating the GaN substrate. It is found that the hillock morphology of GaN epilayer was formed on the GaN substrate with 0.021 surface tilt angle as shown in Fig. 1(a). As the surface tilt angle increased to 0.351, the surface morphology varied from hillock to wavy. Above the surface tilt angle of 0.41, the surface morphology of GaN epilayer shows the mirror-like surface. Additionally, the surface morphologies of InGaN MQWs on GaN epilayer also show the same surface morphologies as those of GaN epilayers. In the formation of hillock surface, it is well known that the pyramidal hillock growth is initiated by the nucleation of critical two-
dimensional nuclei at the terminal points of threading dislocations on the surface, having a screw component of the Burgers vector normal to the interface [9]. Therefore, based on our observation, we can speculate that the GaN substrate with a very low surface tilt angle has more dislocations with screw components than edge components. Fig. 2 shows micro-PL intensity mapping images ð40 40 mm2 Þ at the band edge emission wavelength of GaN epilayer grown on GaN substrates with different surface orientations. It basically exhibits the dark spots, which are the terminal points of threading dislocations originated from GaN substrates. In Fig. 2(a) at the surface tilt angle of 0.021, micro-PL intensity mapping image is very similar to the hillock shape of the GaN epilayer. The dark lines between facets are considered to result from the low-angle grain boundaries at which the PL intensity is reduced because of the crystal imperfection. However, the intensity mapping images of wavy and mirror-like surface
ARTICLE IN PRESS 534
S.-N. Lee et al. / Physica B 376–377 (2006) 532–535
Fig. 3. The PL spectra of InGaN/InGaN MQWs grown on three types of GaN epilayer morphology.
Fig. 2. Micro-PL intensity mapping images ð40 40 mm2 Þ at band edge emission wavelength of GaN epilayer grown on GaN substrates with different surface tilt angle of: (a) 0.021, (b) 0.351, and (c) 0.451.
morphologies of GaN epilayers were featureless except for dark spots, and were more uniform than that of a GaN epilayer with hillock surface morphology. These surface morphologies of GaN epilayers grown on GaN substrates with different surface tilt angles also affected the optical quality of InGaN MQWs as shown in Fig. 3. The PL emission wavelength of InGaN/InGaN
MQWs is decreased with increasing the surface tilt angle of GaN substrates. It shows that the In incorporation rate of InGaN MQWs on GaN epilayer with the hillock surface is more than that of InGaN QWs with mirror-like GaN morphology. Additionally, full-width at half-maximum (FWHM) of PL is also decreased with increasing the surface tilt angle by changing surface morphology from hillock to mirror-like surface. It shows that the In distribution of InGaN QWs grown on the GaN with low surface tilt angle is less uniform than that with relatively high tilt angle. In order to confirm the In-phase separation with different surface tilt angles, we also measured the micro-PL intensity mapping ð40 40 mm2 Þ at the emission wavelength of InGaN QWs with different surface morphologies, as shown in Fig. 4. The density of dark spots in intensity mapping images for InGaN MQWs is significantly decreased, compared to that of GaN epilayers as shown in Fig. 2. Due to the relatively fast radiative recombination and the short carrier diffusion length, the emission efficiency of InGaN is not significantly affected by dislocation density so that the dislocation related dark spots are normally not observed in InGaN films on GaN substrates. However, mapping images for each sample exhibits their own features with dark and bright patterns. We conclude that these images are similar to the surface morphology of GaN templates for each sample. From these results, it indicated that the indium incorporation efficiency is drastically higher on the facets than on a smooth surface [10,11]. We speculate that the deposition of the In adatoms are attracted at facets rather than on a smooth surface to reduce the surface energy due to many dangling bonds at facets. Therefore, the In phase separation in InGaN QWs layer can easily occur on the rough surface of GaN epilayer grown on free-standing GaN substrates.
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535
the surface tilt angle of GaN substrates. By increasing the surface tilt angle above 0.41, the surfaces of GaN epilayer on GaN substrate becomes mirror like. Additionally, the surface tilt angle of GaN substrates also affect the optical quality of InGaN MQWs by non-uniform incorporation of indium on the hillock and wavy surfaces of GaN epilayers grown on GaN substrates with surface tilt angle below 0.41. Acknowledgements The authors would like to thank the other members of BD-LD team for their fruitful discussions and encouragements. References [1] S.N. Lee, J.K. Son, H.S. Paek, T. Sakong, W. Lee, K.H. Kim, S.S. Kim, Y.J. Lee, D.Y. Noh, E. Yoon, O.H. Nam, Y. Park, Phys. Stat. Sol. (c) 1 (2004) 2458. [2] X. Xu, R.P. Vaudo, J. Flynn, J. Dion, G.R. Brandes, Phys. Stat. Sol. (a) (2005) 727. [3] A.R.A. Zauner, J.L. Weyher, M. Plomp, V. Kirilyuk, I. Grzegory, W.J.P. van Enckevort, J.J. Schermer, P.R. Hageman, P.K. Larsen, J. Cryst. Growth 210 (2000) 435. [4] C.R. Miskys, M.K. Kelly, O. Ambacher, G. Martinez-Criado, M. Stutzmann, Appl. Phys. Lett. 77 (2000) 1858. [5] M. Schauler, F. Eberhard, C. Kirchner, V. Schwegler, A. Pelzmann, M. Kamp, K.J. Ebeling, F. Bertram, T. Riemann, J. Christen, P. Prystawko, M. Leszczynski, I. Grzegory, S. Porowski, Appl. Phys. Lett. 74 (1999) 365. [6] W.J.P. van Enckevort, G. Janssen, W. Vollenberg, L.J. Giling, J. Cryst. Growth 148 (1995) 365. [7] X.R. Huang, J. Bai, M. Dudley, R.D. Dupis, U. Chowdhury, Appl. Phys. Lett. 86 (2005) 211946. [8] T. Someya, K. Hoshino, Y. Arakawa, Appl. Phys. Lett. 79 (2001) 1992. [9] J. Zhou, Y.J. Tang, G.Y. Zhang, Solid State Commun. 121 (2002) 381. [10] H. Tang, S. Haffouz, A. Powell, J.A. Bardwell, J. Webb, Appl. Phys. Lett. 86 (2005) 121110. [11] K. Kawasaki, I. Nakamatsu, H. Hirayama, K. Tsutsui, Y. Aoyagi, J. Cryst. Growth 243 (2002) 129.
Fig. 4. Micro-PL intensity mapping images ð40 40 mm2 Þ at band edge emission wavelength of InGaN QWs grown on the GaN epilayer with different surface morphologies like (a) hillock, (b) wavy, and (c) mirror.
4. Conclusion Surface morphologies of GaN epilayers on GaN substrates showed three types. The mirror-like, wavy, and hillock surfaces of GaN epilayer is developed by changing