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Formation of GaN nanostructures with various morphologies by photo-assisted electroless chemical etching
Materials Letters 64 (2010) 2380–2383
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Formation of GaN nanostructures with various morphologies by photo-assisted electroless chemical etching Hongjun Wang a,b, Changwei Zou a, Canxin Tian a, Lin Zhou a, Ming Li a, Dejun Fu a,⁎, Taewon Kang b a b
Department of Physics and Key Laboratory of Acoustic Photonic Materials and Devices of Ministry of Education, Wuhan University, Wuhan 430072, China Quantum Functional Semiconductor Research Center, Dongguk University, Seoul 100-715, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 June 2010 Accepted 21 July 2010 Available online 24 July 2010 Keywords: Semiconductors Surfaces Nanomaterials
a b s t r a c t Interaction of GaN crystal faces with chemicals is crucial to understand why various nanostructures are formed during the etching process. We have prepared GaN nanostructures by a photo-assisted electroless chemical etching method in solutions containing KOH and K2S2O8. Morphology nanostructure GaN layers grown by molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE) were studied. For the GaN layers grown by MBE, the etching reaction process starts at grain boundaries and dislocation domains on the surface and inverted hexagonal pyramids are eventually formed. For the GaN layers grown by HVPE, scattered etch pits with well-defined hexagonal facets are observed after the etching process. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Nanostructures of the wide bandgap semiconductor GaN have attracted considerable attention because of their potential applications in photonic and electronic devices. For example, the arrays of GaN pyramids have a strong field emission at a low threshold of 70 KV/cm, which can be used in field emission display devices [1]. The high quality micrometer size GaN pyramids can be used to form efficient micro-cavities for multiple-mode optical resonance [2]. The inverted hexagonal pyramids with shaped air voids can improve the light extraction efficiency of InGaN light emission diodes [3]. GaN nanostructures have been successfully prepared by the chemical etching method previously mentioned. Unfortunately, the layers only can be deposited on the conductive substrate by this method, which is inconvenient in the production setting [4]. A photoassisted electroless chemical etching technique proved to be an effective method, which does not need a counter electrode and by adding sufficient oxidizing agent, photo-excited electrons in the aqueous solution are consumed [5]. However, there are few systematic works on the formation mechanism of the GaN nanostructures with different facet morphologies by the photo-assisted electroless chemical etching method. In this paper, we have prepared several GaN nanostructures by photo-assisted electroless chemical etching in KOH, with K2S2O8 being added in the etching solution as the oxidizing agent. The formation mechanism of different GaN nanostructures is addressed.
The GaN layers used in the present work were epilayers either grown by hydride vapor phase epitaxy (HVPE) or molecular beam epitaxy (MBE). All the GaN layers were grown on c-plane sapphires and undoped. Prior to the epilayer growth, an AlN buffer layer with a thickness of 20 nm was grown on the substrates at 600°C.The asgrown GaN epilayers were cleaned by standard processes and preetched in the solution of HCl:H2O=1:1 for 3 min in sequence. In the etching experiment, the sample was placed on a Teflon base and immersed in a KOH solution with a pH value of 11.8. To promote the oxide dissolution, 50 ml of 0.1 M K2S2O8 solution was added to the solution. All the reagents were analytically pure and used without further purification during the procedure. Ultrapure distilled water was used in all the reaction processes. The etching processes were agitated by using a magnetic stirrer at room temperature. At the same time, a 100 W Hg lamp was used as the ultraviolet light source at a radiation wavelength of 254 nm. The morphology of the etched layers was measured by field emission scanning electron microscopy operated at 25 kV and atomic force microscopy operated in the tapping mode.
⁎ Corresponding author. Accelerator Laboratory, Department of Physics, Wuhan University, Wuhan 4372, China. Tel./fax: +86 27 6875 3587. E-mail address: [email protected] (D. Fu). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.07.057
3. Results and discussion Fig. 1a shows the SEM image of the as-grown GaN layer grown by HVPE at 900 °C. It can be seen that the morphology of the as-grown GaN layer is much rougher than the layer grown by MBE at 600 °C (Fig. 2a). Fig. 1b shows the SEM image of the corresponding GaN layer after etching for 3 h in pH=11.8 KOH solution mixed with 0.1 M K2S2O8. Well-defined hexagonal etch pits are sporadically scattered on the plane. We can clearly see that some of the etch pits expose the
H. Wang et al. / Materials Letters 64 (2010) 2380–2383
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Table 1 Polarities of GaN films deposited by MBE and HVPE on c-plane sapphire substrates.
Fig. 1. SEM images of (a) the GaN layer grown by HVPE, and (b) the corresponding layer after etching for 3 h in pH=11.8 KOH solution mixed with 50 ml of 0.1 M K2S2O8. The inset shows the magnification image of a well-defined hexagonal etch pit.
substrate, whereas the others do not reveal the substrate; this is related to the etching time. Except for hexagonal etched pits and some unintentional contaminations, the etched GaN has a rather smooth top surface. The inset of Fig. 1b gives a magnified image of a welldefined hexagonal etch pit, which also reveals the substrate. It is shown that the sidewalls of the etched pits are also rather smooth. As the GaN layers grown by HVPE at 900 °C exhibit Ga-face polarity, which is resistant to most chemical etchants except the defects on the surface [6]. The polarities of the GaN films deposited by
Growth technique
Sapphire substrate
Polarity
MBE MBE HVPE
Without buffer layer With AlN buffer layer Both with and without buffer layer
N-face Ga-face Ga-face
MBE and HVPE on the c-plane sapphire substrate are summarized in Table 1, discussed by Stutzmann and Li et al. [6–8]. Therefore, the formation processes of the hexagonal etch pits involve two steps. At the beginning, a thin film of the top GaN surface is etched out, revealing a much smooth top surface compared with the as-grown layers (Fig. 1b). Similar phenomenon was also found for the GaN layers grown by MOVCD, which was treated by a wet etching method in an alkaline solution at a temperature above 160 °C [9]. The hexagonal etch pits are eventually formed when the etching time is longer, and are sporadically scattered on the smooth surface, as a result of selective etching. As discussed by Hong et al. [10], the tunnellike nanopipes have been observed on the GaN films, which were aligned along the growth direction of the crystal and penetrate the entire layer. It is believed the hexagonal etch pits are formed at the nanopipe defects by etching the wall of the nanopipes, which distributes on the as-grown GaN surface. When this reaction proceeds until the exposure of the substrate, hexagonal pits are eventually formed. The agent K2S2O8 plays a key role in the etching process. In this process, the photo-generated holes oxidize GaN to Ga2O3, which then dissolves in the alkaline solution. The excited electrons are 2− consumed through the reduction of S2O2− 8 to SO4 , without changing the pH value of the electrolyte [11]. Fig. 2a shows the SEM image of the as-grown GaN layers by MBE at 600°C. Grain boundaries and dislocations at the corner of the grains can be clearly seen on the surface [12]. Fig. 2b shows the SEM image of this epilayer after etching for 1 h in pH=11.8 KOH solution mixed with 0.1 M K2S2O8. In this case inverted hexagonal pyramids are observed by the etching processes. The SEM image shows that the top surface of the inverted hexagonal pyramids is flat. The magnification of the inverted hexagonal pyramids is shown in Fig. 2c. The
Fig. 2. SEM images of (a) the as-grown GaN layer grown by MBE, (b) the corresponding layer after etching for 1 h in pH=11.8 KOH solution mixed with 50 ml of 0.1 M K2S2O8, and (c) the enlarged magnification of inverted hexagonal pyramids. (d) Schematic diagram of the inverted hexagonal pyramid formation process.
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H. Wang et al. / Materials Letters 64 (2010) 2380–2383
Fig. 3. SEM images of (a) an isolated pyramid and (b) the adjacent pyramids of the GaN layers after etching for 200 min in pH=11.8 KOH solution mixed with 50 ml of 0.1 M K2S2O8. Architecture of the (c) isolated pyramid and (d) adjacent pyramids.
distribution of the inverted hexagonal pyramids is uniform and their density is 8×106 cm−1 estimated from the SEM image. As shown in Table 1, the GaN layers exhibit Ga-face polarity when they are deposited on the sapphire substrate with the AlN buffer layer of a thickness of 20 nm by MBE [9]. As a result, the GaN layers grown by MBE are resistant to the alkaline etching solutions. From the SEM images we can see that the etching process appears to start at grain boundaries and dislocations and when the process reaches the lateral terminal facets, inverted hexagonal pyramids are formed, as illustrated in Fig. 2d. A similar phenomenon was observed for GaN:Mg films etched by the photoelectrochemical wet etching method [13]. The (0001) plane of the GaN surface with Ga-polarity is chemically stable and maintains its original geometry during the etching process. In comparison, the lateral face group is lower in surface energy [14]. As a result, the anisotropic etching phenomenon occurs and the lateral terminal surfaces are gradually etched, forming planes in a decline, as shown in Fig. 2d. A few well-defined hexagonal pyramids, either isolated or adjacent, are also found for the MBE-grown GaN layers after etching for 200 min in pH=11.8 KOH solution (Figs. 3a–b). The formation mechanism of the hexagonal pyramids is due to the inversion domains of N-face polarity on the surface, which can be easily etched in the alkaline aqueous solution. The N-face polarity GaN plane has an etched rate much higher than the other facets [7]. The lateral size of the isolated pyramid is 150 nm in diameter and consists of six identical triangles with extremely smooth faces. Both of the isolated and adjacent hexagonal pyramids show symmetric hexagonal structure and well-defined crystallographic planes. Fig. 3c shows a schematic architecture of an isolated hexagonal pyramid, plotted by using an AFM section scan picture. The heightover-base ratio of the triangle is 0.20, corresponding to the angle of 22.0° between the base and inclined facets. The facet is b1014N in orientation, indicating that the {1014} planes are etched preferentially. According to the Wulff theory [15], the equilibrium shape of a crystal is determined by both the surface energy and area to achieve the lowest total energy. This means that the {1014} planes of the isolate hexagonal pyramid have the lowest surface energy with respect to the corresponding surface area. The architecture of the
adjacent hexagonal pyramids is shown in Fig. 3d. The height-overbase ratios for the triangles of the adjacent pyramids are 0.27 and 0.26, corresponding to the angles of 27.1o and 28.6°, respectively. Both facets are close to b1013N in orientation. This means that the {1013} planes have the lowest surface energy with respect to the corresponding surface area. As a result, the {1013} terminal planes are eventually exposed for the adjacent hexagonal pyramids. 4. Conclusion We have studied the features and formation mechanisms of typical GaN nanostructures hexagonal pyramids, etch pits, and inverted hexagonal pyramids observed on epilayers processed by means of photo-assisted electroless chemical etching. We found that the etching process started at grain boundaries and dislocations for the GaN layers grown by MBE and inverted hexagonal pyramids were eventually formed. Hexagonal pyramids were formed on the MBEgrown GaN layers when the etching time was longer. For the GaN layers grown by HVPE, hexagonal etch pits were formed, with a sporadical distribution. The analysis of the results shows that the photo-assisted electroless chemical etching is selective for surface properties, especially the polarity, of GaN layers. Acknowledgements This work was supported by the Natural Science Foundation of China under contract no. 10875090 and the Korean Science and Engineering Foundation through QSRC at Dongguk University. References [1] Ward BL, Nam OH, Hartman JD, English SL, McCarson BL, Schlesser R, et al. Electron emission characteristics of GaN pyramid arrays grown via organometallic vapor phase epitaxy. J Appl Phys 1998;84:5238. [2] Jiang HX, Lin JY, Zeng KC, Yang W. Optical resonance modes in GaN pyramid microcavities. Appl Phys Lett 1999;75:763. [3] Park Eun-Hyun, Jang Jin, Gupta Shalini, Ferguson Ian, Kim Cheol-Hoi, Jeon SooKun, et al. Air-voids embedded high efficiency InGaN-light emitting diode. Appl Phys Lett 2008;93:191103.
H. Wang et al. / Materials Letters 64 (2010) 2380–2383 [4] Zhuang D, Edgar JH. Wet etching of GaN, AlN, and SiC: a review. Mater Sci Eng, 2005;48:1–46. [5] Maher H, DiSanto DW, Soerensen G, Bologuesia CR, Tang H, Webb JB. Smooth wet etching by ultraviolet-assisted photoetching and its application to the fabrication of AlGaNÕGaN heterostructure field-effect transistors. Appl Phys Lett 2000;77:23. [6] Stutzmann M, Ambacher O, Eickhoff M, Karrer U, Lima Pimenta, Neuberger R, et al. Playing with polarity. Phys Status Solidi B 2001;228(2):505–12. [7] Li Dongsheng, Sumiy M, Fuke S, Yang Deren, Que Duanlin, Suzuki Y, et al. Selective etching of GaN polar surface in potassium hydroxide solution studied by x-ray photoelectron spectroscopy. J Appl Phys 2001;90(8). [8] Ng HM, Cho AY. Investigation of Si doping and impurity incorporation dependence on the polarity of GaN by molecular beam epitaxy. J Vac Sci Technol B 2002;20(3): 1217–20. [9] Kim BJ, Lee JW, Park HS, Park Y, Kim TI. Wet Etching of (0001) GaN/Al2O3 Grown by MOVPE. J Electronic Materials 1998;27(5).
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[10] Hong SK, Kim BJ, Park HS, Park Y, Yoon SY, Kim TI. Evaluation of nanopipes in MOCVD grown (0 0 0 1) GaN/Al2O3 by wet chemical etching. J Crystal Growth 1998;1991:275–8. [11] Bardwell JA, Webb JB, Tang H, Fraser J, Moisa S. Ultraviolet photoenhanced wet etching of GaN in K2S2O8 solution. J Appl Phys 2001;89(7). [12] Weimann NG, Eastman LF, Doppalapudi D, Ng HM, Moustakas TD. Scattering of electrons at threading dislocations in GaN. J Appl Phys 1998;83:3656. [13] Lin Chiafeng, Yang Zhong-Jie, Dai Jing-Jie, Zheng Jing-Hui, Chang Shou-Yi. The selfassemble GaN:Mg inverted hexagonal pyramids formatted by photoelectrochemical wet-etching process. Thin Solid Films 2007;515:4492–5. [14] Ng HM, Weimann NG, Chowdhury A. GaN nanotip pyramids formed by anisotropic etching. J Appl Phys 2003;94(1). [15] Porter DA, Easterling KE. Phase transformations in metals and alloys. New York: Chapman & Hall; 1991.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Formation of GaN nanostructures with various morphologies by photo-assisted electroless chemical etching Hongjun Wang a,b, Changwei Zou a, Canxin Tian a, Lin Zhou a, Ming Li a, Dejun Fu a,⁎, Taewon Kang b a b
Department of Physics and Key Laboratory of Acoustic Photonic Materials and Devices of Ministry of Education, Wuhan University, Wuhan 430072, China Quantum Functional Semiconductor Research Center, Dongguk University, Seoul 100-715, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 June 2010 Accepted 21 July 2010 Available online 24 July 2010 Keywords: Semiconductors Surfaces Nanomaterials
a b s t r a c t Interaction of GaN crystal faces with chemicals is crucial to understand why various nanostructures are formed during the etching process. We have prepared GaN nanostructures by a photo-assisted electroless chemical etching method in solutions containing KOH and K2S2O8. Morphology nanostructure GaN layers grown by molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE) were studied. For the GaN layers grown by MBE, the etching reaction process starts at grain boundaries and dislocation domains on the surface and inverted hexagonal pyramids are eventually formed. For the GaN layers grown by HVPE, scattered etch pits with well-defined hexagonal facets are observed after the etching process. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Nanostructures of the wide bandgap semiconductor GaN have attracted considerable attention because of their potential applications in photonic and electronic devices. For example, the arrays of GaN pyramids have a strong field emission at a low threshold of 70 KV/cm, which can be used in field emission display devices [1]. The high quality micrometer size GaN pyramids can be used to form efficient micro-cavities for multiple-mode optical resonance [2]. The inverted hexagonal pyramids with shaped air voids can improve the light extraction efficiency of InGaN light emission diodes [3]. GaN nanostructures have been successfully prepared by the chemical etching method previously mentioned. Unfortunately, the layers only can be deposited on the conductive substrate by this method, which is inconvenient in the production setting [4]. A photoassisted electroless chemical etching technique proved to be an effective method, which does not need a counter electrode and by adding sufficient oxidizing agent, photo-excited electrons in the aqueous solution are consumed [5]. However, there are few systematic works on the formation mechanism of the GaN nanostructures with different facet morphologies by the photo-assisted electroless chemical etching method. In this paper, we have prepared several GaN nanostructures by photo-assisted electroless chemical etching in KOH, with K2S2O8 being added in the etching solution as the oxidizing agent. The formation mechanism of different GaN nanostructures is addressed.
The GaN layers used in the present work were epilayers either grown by hydride vapor phase epitaxy (HVPE) or molecular beam epitaxy (MBE). All the GaN layers were grown on c-plane sapphires and undoped. Prior to the epilayer growth, an AlN buffer layer with a thickness of 20 nm was grown on the substrates at 600°C.The asgrown GaN epilayers were cleaned by standard processes and preetched in the solution of HCl:H2O=1:1 for 3 min in sequence. In the etching experiment, the sample was placed on a Teflon base and immersed in a KOH solution with a pH value of 11.8. To promote the oxide dissolution, 50 ml of 0.1 M K2S2O8 solution was added to the solution. All the reagents were analytically pure and used without further purification during the procedure. Ultrapure distilled water was used in all the reaction processes. The etching processes were agitated by using a magnetic stirrer at room temperature. At the same time, a 100 W Hg lamp was used as the ultraviolet light source at a radiation wavelength of 254 nm. The morphology of the etched layers was measured by field emission scanning electron microscopy operated at 25 kV and atomic force microscopy operated in the tapping mode.
⁎ Corresponding author. Accelerator Laboratory, Department of Physics, Wuhan University, Wuhan 4372, China. Tel./fax: +86 27 6875 3587. E-mail address: [email protected] (D. Fu). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.07.057
3. Results and discussion Fig. 1a shows the SEM image of the as-grown GaN layer grown by HVPE at 900 °C. It can be seen that the morphology of the as-grown GaN layer is much rougher than the layer grown by MBE at 600 °C (Fig. 2a). Fig. 1b shows the SEM image of the corresponding GaN layer after etching for 3 h in pH=11.8 KOH solution mixed with 0.1 M K2S2O8. Well-defined hexagonal etch pits are sporadically scattered on the plane. We can clearly see that some of the etch pits expose the
H. Wang et al. / Materials Letters 64 (2010) 2380–2383
2381
Table 1 Polarities of GaN films deposited by MBE and HVPE on c-plane sapphire substrates.
Fig. 1. SEM images of (a) the GaN layer grown by HVPE, and (b) the corresponding layer after etching for 3 h in pH=11.8 KOH solution mixed with 50 ml of 0.1 M K2S2O8. The inset shows the magnification image of a well-defined hexagonal etch pit.
substrate, whereas the others do not reveal the substrate; this is related to the etching time. Except for hexagonal etched pits and some unintentional contaminations, the etched GaN has a rather smooth top surface. The inset of Fig. 1b gives a magnified image of a welldefined hexagonal etch pit, which also reveals the substrate. It is shown that the sidewalls of the etched pits are also rather smooth. As the GaN layers grown by HVPE at 900 °C exhibit Ga-face polarity, which is resistant to most chemical etchants except the defects on the surface [6]. The polarities of the GaN films deposited by
Growth technique
Sapphire substrate
Polarity
MBE MBE HVPE
Without buffer layer With AlN buffer layer Both with and without buffer layer
N-face Ga-face Ga-face
MBE and HVPE on the c-plane sapphire substrate are summarized in Table 1, discussed by Stutzmann and Li et al. [6–8]. Therefore, the formation processes of the hexagonal etch pits involve two steps. At the beginning, a thin film of the top GaN surface is etched out, revealing a much smooth top surface compared with the as-grown layers (Fig. 1b). Similar phenomenon was also found for the GaN layers grown by MOVCD, which was treated by a wet etching method in an alkaline solution at a temperature above 160 °C [9]. The hexagonal etch pits are eventually formed when the etching time is longer, and are sporadically scattered on the smooth surface, as a result of selective etching. As discussed by Hong et al. [10], the tunnellike nanopipes have been observed on the GaN films, which were aligned along the growth direction of the crystal and penetrate the entire layer. It is believed the hexagonal etch pits are formed at the nanopipe defects by etching the wall of the nanopipes, which distributes on the as-grown GaN surface. When this reaction proceeds until the exposure of the substrate, hexagonal pits are eventually formed. The agent K2S2O8 plays a key role in the etching process. In this process, the photo-generated holes oxidize GaN to Ga2O3, which then dissolves in the alkaline solution. The excited electrons are 2− consumed through the reduction of S2O2− 8 to SO4 , without changing the pH value of the electrolyte [11]. Fig. 2a shows the SEM image of the as-grown GaN layers by MBE at 600°C. Grain boundaries and dislocations at the corner of the grains can be clearly seen on the surface [12]. Fig. 2b shows the SEM image of this epilayer after etching for 1 h in pH=11.8 KOH solution mixed with 0.1 M K2S2O8. In this case inverted hexagonal pyramids are observed by the etching processes. The SEM image shows that the top surface of the inverted hexagonal pyramids is flat. The magnification of the inverted hexagonal pyramids is shown in Fig. 2c. The
Fig. 2. SEM images of (a) the as-grown GaN layer grown by MBE, (b) the corresponding layer after etching for 1 h in pH=11.8 KOH solution mixed with 50 ml of 0.1 M K2S2O8, and (c) the enlarged magnification of inverted hexagonal pyramids. (d) Schematic diagram of the inverted hexagonal pyramid formation process.
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H. Wang et al. / Materials Letters 64 (2010) 2380–2383
Fig. 3. SEM images of (a) an isolated pyramid and (b) the adjacent pyramids of the GaN layers after etching for 200 min in pH=11.8 KOH solution mixed with 50 ml of 0.1 M K2S2O8. Architecture of the (c) isolated pyramid and (d) adjacent pyramids.
distribution of the inverted hexagonal pyramids is uniform and their density is 8×106 cm−1 estimated from the SEM image. As shown in Table 1, the GaN layers exhibit Ga-face polarity when they are deposited on the sapphire substrate with the AlN buffer layer of a thickness of 20 nm by MBE [9]. As a result, the GaN layers grown by MBE are resistant to the alkaline etching solutions. From the SEM images we can see that the etching process appears to start at grain boundaries and dislocations and when the process reaches the lateral terminal facets, inverted hexagonal pyramids are formed, as illustrated in Fig. 2d. A similar phenomenon was observed for GaN:Mg films etched by the photoelectrochemical wet etching method [13]. The (0001) plane of the GaN surface with Ga-polarity is chemically stable and maintains its original geometry during the etching process. In comparison, the lateral face group is lower in surface energy [14]. As a result, the anisotropic etching phenomenon occurs and the lateral terminal surfaces are gradually etched, forming planes in a decline, as shown in Fig. 2d. A few well-defined hexagonal pyramids, either isolated or adjacent, are also found for the MBE-grown GaN layers after etching for 200 min in pH=11.8 KOH solution (Figs. 3a–b). The formation mechanism of the hexagonal pyramids is due to the inversion domains of N-face polarity on the surface, which can be easily etched in the alkaline aqueous solution. The N-face polarity GaN plane has an etched rate much higher than the other facets [7]. The lateral size of the isolated pyramid is 150 nm in diameter and consists of six identical triangles with extremely smooth faces. Both of the isolated and adjacent hexagonal pyramids show symmetric hexagonal structure and well-defined crystallographic planes. Fig. 3c shows a schematic architecture of an isolated hexagonal pyramid, plotted by using an AFM section scan picture. The heightover-base ratio of the triangle is 0.20, corresponding to the angle of 22.0° between the base and inclined facets. The facet is b1014N in orientation, indicating that the {1014} planes are etched preferentially. According to the Wulff theory [15], the equilibrium shape of a crystal is determined by both the surface energy and area to achieve the lowest total energy. This means that the {1014} planes of the isolate hexagonal pyramid have the lowest surface energy with respect to the corresponding surface area. The architecture of the
adjacent hexagonal pyramids is shown in Fig. 3d. The height-overbase ratios for the triangles of the adjacent pyramids are 0.27 and 0.26, corresponding to the angles of 27.1o and 28.6°, respectively. Both facets are close to b1013N in orientation. This means that the {1013} planes have the lowest surface energy with respect to the corresponding surface area. As a result, the {1013} terminal planes are eventually exposed for the adjacent hexagonal pyramids. 4. Conclusion We have studied the features and formation mechanisms of typical GaN nanostructures hexagonal pyramids, etch pits, and inverted hexagonal pyramids observed on epilayers processed by means of photo-assisted electroless chemical etching. We found that the etching process started at grain boundaries and dislocations for the GaN layers grown by MBE and inverted hexagonal pyramids were eventually formed. Hexagonal pyramids were formed on the MBEgrown GaN layers when the etching time was longer. For the GaN layers grown by HVPE, hexagonal etch pits were formed, with a sporadical distribution. The analysis of the results shows that the photo-assisted electroless chemical etching is selective for surface properties, especially the polarity, of GaN layers. Acknowledgements This work was supported by the Natural Science Foundation of China under contract no. 10875090 and the Korean Science and Engineering Foundation through QSRC at Dongguk University. References [1] Ward BL, Nam OH, Hartman JD, English SL, McCarson BL, Schlesser R, et al. Electron emission characteristics of GaN pyramid arrays grown via organometallic vapor phase epitaxy. J Appl Phys 1998;84:5238. [2] Jiang HX, Lin JY, Zeng KC, Yang W. Optical resonance modes in GaN pyramid microcavities. Appl Phys Lett 1999;75:763. [3] Park Eun-Hyun, Jang Jin, Gupta Shalini, Ferguson Ian, Kim Cheol-Hoi, Jeon SooKun, et al. Air-voids embedded high efficiency InGaN-light emitting diode. Appl Phys Lett 2008;93:191103.
H. Wang et al. / Materials Letters 64 (2010) 2380–2383 [4] Zhuang D, Edgar JH. Wet etching of GaN, AlN, and SiC: a review. Mater Sci Eng, 2005;48:1–46. [5] Maher H, DiSanto DW, Soerensen G, Bologuesia CR, Tang H, Webb JB. Smooth wet etching by ultraviolet-assisted photoetching and its application to the fabrication of AlGaNÕGaN heterostructure field-effect transistors. Appl Phys Lett 2000;77:23. [6] Stutzmann M, Ambacher O, Eickhoff M, Karrer U, Lima Pimenta, Neuberger R, et al. Playing with polarity. Phys Status Solidi B 2001;228(2):505–12. [7] Li Dongsheng, Sumiy M, Fuke S, Yang Deren, Que Duanlin, Suzuki Y, et al. Selective etching of GaN polar surface in potassium hydroxide solution studied by x-ray photoelectron spectroscopy. J Appl Phys 2001;90(8). [8] Ng HM, Cho AY. Investigation of Si doping and impurity incorporation dependence on the polarity of GaN by molecular beam epitaxy. J Vac Sci Technol B 2002;20(3): 1217–20. [9] Kim BJ, Lee JW, Park HS, Park Y, Kim TI. Wet Etching of (0001) GaN/Al2O3 Grown by MOVPE. J Electronic Materials 1998;27(5).
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[10] Hong SK, Kim BJ, Park HS, Park Y, Yoon SY, Kim TI. Evaluation of nanopipes in MOCVD grown (0 0 0 1) GaN/Al2O3 by wet chemical etching. J Crystal Growth 1998;1991:275–8. [11] Bardwell JA, Webb JB, Tang H, Fraser J, Moisa S. Ultraviolet photoenhanced wet etching of GaN in K2S2O8 solution. J Appl Phys 2001;89(7). [12] Weimann NG, Eastman LF, Doppalapudi D, Ng HM, Moustakas TD. Scattering of electrons at threading dislocations in GaN. J Appl Phys 1998;83:3656. [13] Lin Chiafeng, Yang Zhong-Jie, Dai Jing-Jie, Zheng Jing-Hui, Chang Shou-Yi. The selfassemble GaN:Mg inverted hexagonal pyramids formatted by photoelectrochemical wet-etching process. Thin Solid Films 2007;515:4492–5. [14] Ng HM, Weimann NG, Chowdhury A. GaN nanotip pyramids formed by anisotropic etching. J Appl Phys 2003;94(1). [15] Porter DA, Easterling KE. Phase transformations in metals and alloys. New York: Chapman & Hall; 1991.