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High-quality GaN nanowires grown on Si and porous silicon by thermal evaporation
Applied Surface Science 263 (2012) 50–53
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
High-quality GaN nanowires grown on Si and porous silicon by thermal evaporation L. Shekari ∗ , A. Ramizy, K. Omar, H. Abu Hassan, Z. Hassan Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 9 April 2012 Received in revised form 30 July 2012 Accepted 31 July 2012 Available online 31 August 2012 Keywords: Thermal evaporation growth GaN nanowires Porous silicon
a b s t r a c t Nanowires (NWs) of GaN thin films were prepared on as-grown Si (1 1 1) and porous silicon (PS) substrates using thermal evaporation method. The film growth produced high-quality wurtzite GaN NWs. The size, morphology, and nanostructures of the crystals were investigated through scanning electron microscopy, high-resolution X-ray diffraction and photoluminescence spectroscopy. The NWs grown on porous silicon were thinner, longer and denser compared with those on as-grown Si. The energy band gap of the NWs grown on PS was larger than that of NWs on as-grown Si. This is due to the greater quantum confinement effects of the crystalline structure of the NWs grown on PS. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The direct band gap semiconductor GaN (band gap of 3.4 eV) has great potential for use in optoelectronic devices over a wide range of wavelengths for III-nitrides, from the blue to the near ultraviolet wavelength. One-dimensional nanostructures such as NWs and nanorods have excellent electrical, optical, and mechanical properties, as well as great potential for various applications, such as probe microscopy tips and interconnections in nanoelectronics [1]. Processing techniques, especially those for crystal growth of III–V nitride nanostructures, have been successfully established. Plasma etching and reactive ion etching have been mainly used to etch III–V nitride crystals. However, the damage caused by the ion or plasma bombardment is a serious problem in these processes [2–4]. Vapor phase methods have also been used for NW production. These include physical methods such as laser-assisted catalytic growth, carbon-nanotube-confined reaction, catalytic reaction based on a vapor–liquid–solid mechanism and thermal chemical vapor deposition, and direct reaction of metal gallium with ammonia [5–10]. Wu et al. [13] synthesized GaN NWs on Si substrate by ammoniating Ga2 O3 /BN films under a flowing ammonia atmosphere. Their results demonstrate that NWs are hexagonal wurtzite GaN with smooth surfaces. They also discussed the growth mechanism of crystalline GaN NW. Popa et al. [14] demonstrated
∗ Corresponding author. Tel.: +006017 4884 613; fax: +60 604 6579150. E-mail address: lsg09 [email protected] (L. Shekari). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.164
controlled nanostructuring of GaN by focused-ion-beam treatment with subsequent photoelectrochemical (PEC) etching. The proposed mask-less approach based on direct writing of surface negative charge, which shields the material against PEC etching, allows the fabrication of GaN nanowalls and NWs with lateral dimensions as small as 100 nm. Thermal evaporation using a tube furnace and flowing Ar gas is a promising technology for the fabrication of GaN NWs and nanorods. It is inexpensive and can be easily controlled to obtain NWs of desired sizes [11,12]. The objective of the present study was to prepare high-quality single crystal wurtzite GaN NWs on PS on the polished and unpolished sides, and Si (1 1 1) substrate; using thermal evaporation technique, a novel procedure for preparing GaN semiconductors, without using ammonia gas. The porous substrates play important roles in fabricating GaN nanostructures due to the ability of the pore walls to interact with atoms and molecules together with the capacity of controllable pore space to capture or load gas molecules, and solid particles [15–18].
2. Materials and methods The electrochemical cell was used to fabricate the porous silicon (PS) [19]. N-type Si wafer with a dimension of 1 cm × 1 cm × 283 m, (1 1 1) orientation, resistivity of 0.75 cm, and doping concentration of 1.8 × 1017 cm−3 was etched through an electrochemical process to produce the porous structure. The wafer was placed in an electrolyte solution (hydrofluoric acid (HF): ethanol, 1:4) with a current density of 60 mA/cm2 at an etching time of 15 min for each side. Before the etching process, the Si substrate was cleaned using the Radio Corporation of America
L. Shekari et al. / Applied Surface Science 263 (2012) 50–53
51
Fig. 1. SEM images of as-grown Si and PS formed on both sides of the crystalline Si wafer.
method (RCA) to remove the oxide layer, it was then immersed in HF acid to remove the native oxide. The electrochemical cell was made by Teflon and has a circular aperture with radius of 0.4 cm, and the silicon wafer is sealed below. The cell consists of two-electrode system with the Si wafer as the anode and platinum as the cathode. The process was carried out at room temperature. After etching, all samples were rinsed with ethanol and dried in air. The GaN NWs were prepared by heating GaN powder of purity 99.999% at ∼1000 ◦ C for 2 h under an Ar gas stream introduced at 0.5 cm3 /min. Subsequently, they were deposited on as-grown Si and PS substrates placed inside ceramic boats by the thermal evaporation with no template or catalyst used in the synthesis. Prior to heating, the furnace was cleaned using a stream of Ar. The surface morphology and structural properties of the nanostructures were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM) and high-resolution X-ray diffraction (HR-XRD). Photoluminescence (PL) was also performed at room temperature using He–Cd laser ( = 325 nm). 3. Results and discussion
Fig. 2. AFM images of PS: (a) as-grown; (b) polished front side; (c) unpolished back side.
The SEM images in Fig. 1 illustrate the smooth surface of asgrown Si and the uniformity of the surface of the PS for the polished and unpolished sides. The uniformity was due to the isotropic characteristic of HF/ethanol etching and the optimal conditions of the current density and etching time. The isotropic etching resulted in spherical pores. The etched surface formed on the polished side showed pores and small protrusions. Pores on the unpolished side were larger (around 110 nm) than those on the polished side (around 40 nm) due to greater etching of the initial surface with higher surface roughness than polished side. Fig. 2 demonstrates that the three dimensional topographic images of the PS etched surfaces with the pyramidal shape were distributed over the entire surface. The pyramidal shape indicated that the increase in the surface roughness is due to the effect of the etching parameters’ effect on the surface characterization. Fig. 3(a) shows the configuration of NWs formed on Si by extended high-temperature processing inside the furnace. The short and fluffy fibrous NWs with sharp tips are randomly oriented and less dense. The nanostructures formed on polished and unpolished sides of the PS are shown in Fig. 3(b) and (c), respectively. The nanostructures on both sides of the PS had corn-shaped structures. On closer inspection, the corn-shaped structure on the polished side consisted of bundles of long NWs which are dense and well-defined and preferentially oriented in one direction. The corn-shaped structure on the unpolished side had the same characters, however their NWs seem to be bundled closer together, thus creating a smoother image than those on the polished side.
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L. Shekari et al. / Applied Surface Science 263 (2012) 50–53
Fig. 3. SEM images of GaN NW grown on (a) as-grown Si; (b) PS formed on the polished side; (c) PS formed on the unpolished side.
280
spectrum of GaN NWs is related to the quality of the NWs that determines the number of luminescent centers, and relaxation of thermally charged carriers. The surface etching produced denser, longer and well-defined NW structures that increased the number of luminescent centers, which then increased the PL of the GaN NWs, thus serving as sequences to the thermal electrons.
Fig. 4. HR-XRD spectra of PS and GaN NWs on as-grown Si and on both sides of the crystalline Si wafer.
Fig. 6. PL GaN NWs on as-grown Si, and GaN NWs on PS.
PS polished side
260
PS unpolished side
240 220 200 180 160 140
Intensity(Counts)
Fig. 4 shows the high-resolution X-ray diffraction (HR-XRD) patterns of the PS, GaN NWs on the as-grown Si, and the NWs on both sides of the PS substrate. The sharp peaks can be indexed to the hexagonal wurtzite structure of GaN with lattice constants a = 3187 A˚ and c = 5185 A˚ (reference code 01-089-8937). These values are close to the reported values of bulk GaN crystals [20]. The sharp diffraction peaks suggest that the synthesized GaN NWs are of high quality [21]. More GaN NW peaks were produced on both sides of the PS substrate compared with those of GaN NWs on grown Si. This clearly indicates that the NWs are denser and of higher quality than those on Si as shown by Fig. 3. The PS has a surface structure which is more pliant than Si for greater growth of GaN NWs, thus producing more GaN peaks in the XRD patterns. The PL spectra of both sides of PS are shown in Fig. 5. The PL of PS on the polished side shows a peak at 685.5 nm (1.80 eV), with a full width at half maximum (FWHM) intensity of 300 meV. The PL of the PS formed on the unpolished side produced a blue shifted luminescence peak at 668.5 nm (1.85 eV) with a FWHM of ∼280 meV. This indicates that the porous structures are confined to nanometer dimensions. The energy gap of PS on the polished and unpolished sides increased to 1.80 and 1.85 eV, respectively. As shown in Fig. 6, the PL peak of GaN NWs on both sides of PS is at 340 nm (3.64 eV), with a FWHM of 60 meV, which attributed to electron–hole pairs recombination. The broadening of the energy band gap at 395 nm (3.13 eV) with a FWHM of 212 meV for GaN NWs on Si is due to a lesser quantum confinement effects of the as grown crystalline structure [22]. Thus, the peak shift in the PL
120 100 80
300
400
500
600
700
800
900
Wavelength(nm) Fig. 5. PL spectra of both sides of PS.
L. Shekari et al. / Applied Surface Science 263 (2012) 50–53
4. Conclusions Thermal evaporation technique is a promising, and inexpensive technology to produce GaN NWs, whose size and distribution can be controlled by changing the growth conditions. The morphology of the synthesized NWs reveals that porous substrates play important roles in fabricating dense and well-defined GaN nanostructures. Structural and optical characterizations by XRD and PL show that the growth of the NWs was a function of the porosity of the initial semiconducting substrate. The PL peak of GaN NWs on PS may be explained by the quantum confinement effects of its crystalline structure, and the radiative recombination of thermalized electrons and holes. The peak shift in the PL spectrum of GaN NWs on Si is related to the lesser quality of NWs grown with decreasing number of luminescent centers and the relaxation of hot charge carriers. Acknowledgment The authors gratefully acknowledge the financial support from Postgraduate Research Grant Scheme 1001/PFIZIK/822128 of Universiti Sains Malaysia. References [1] Y. San, L. Hong-dong, Y. Hai-bin, L. Dong-mei, S. Hai-ping, Z. Guang-tian, Preparation and microstructure of nanosized GaN crystals, Chin. Phys. Lett. 13 (1996) 444. [2] A. Ramizy, Z. Hassan, K. Omar, Nanostructured GaN on silicon fabricated by electrochemical and laser-induced etching, Mater. Lett. 66 (2010) 61–63. [3] K. Omar, Z. Hassan, K. Goh, H. Teh, H.A. Hassan, Synthesis porous GaN by using UV-assisted electrochemical etching and its optical studies, Mod. Appl. Sci. 3 (3) (2009) P132. [4] F. Yam, Z. Hassan, S. Ng, Porous GaN prepared by UV assisted electrochemical etching, Thin Solid Films 515 (7–8) (2007) 3469–3474. [5] C.J. Barrelet, A.B. Greytak, C.M. Lieber, Nanowire photonic circuit elements, Nano Lett. 4 (10) (2004) 1981–1985.
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[6] G.D. Yuan, W.J. Zhang, J.S. Jie, X. Fan, J.A. Zapien, Y.H. Leung, L.B. Luo, P.F. Wang, C.S. Lee, S.T. Lee, p-Type ZnO nanowire arrays, Nano Lett. 8 (April (8)) (2008) 7. [7] J. Halpern, A. Bello, J. Gilcrease, G. Harris, M. He, Biphasic GaN nanowires: growth mechanism and properties, Microelectron. J. 40 (2) (2009) 316–318. [8] J. Li, J. Liu, L.S. Wang, R.P.H. Chang, Physical and electrical properties of chemical vapor grown GaN nano/microstructures, Inorg. Chem. 47 (22) (2008) 10325–10329. [9] W. Shi, Y. Zheng, N. Wang, C. Lee, S. Lee, Microstructures of gallium nitride nanowires synthesized by oxide-assisted method, Chem. Phys. Lett. 345 (5–6) (2001) 377–380. [10] Z. Dong, C. Xue, H. Zhuang, S. Wang, H. Gao, D. Tian, Y. Wu, J. He, Y. Liu, Synthesis of three kinds of GaN nanowires through Ga2 O3 films’ reaction with ammonia, Phys. E: Low-dimens. Syst. Nanostruct. 27 (1–2) (2005) 32–37. [11] K. Al-Heuseen, M. Hashim, N. Ali, Enhanced optical properties of porous GaN by using UV-assisted electrochemical etching, Phys. B: Condens. Matter 405 (15) (2010) 3176–3179. [12] V. Ulin, S. Konnikov, Electrochemical pore formation mechanism in III–V crystals (part I), Semiconductors 41 (7) (2007) 832–844. [13] Y. Wu, C. Xue, H. Zhuang, D. Tian, Y. Liu, J. He, F. Wang, L. Sun, Y. Ai, Synthesis of single-crystal GaN nanowires on Si (1 1 1) substrate by ammonification technology, Mater. Lett. 60 (25–26) (2006) 3076–3078. [14] V. Popa, I. Tiginyanu, O. Volciuc, A. Sarua, M. Kuball, P. Heard, Fabrication of GaN nanowalls and nanowires using surface charge lithography, Mater. Lett. 62 (30) (2008) 4576–4578. [15] A.R. Boccaccini, J.L. Spino, V. Cannillo (Eds.), Hermetic Glass Bodies with Controlled Porosity: Processing and Properties, Wiley Online Library, 2002. [16] O. Bretcanu, C. Samaille, A.R. Boccaccini, Simple methods to fabricate Bioglass® derived glass–ceramic scaffolds exhibiting porosity gradient, J. Mater. Sci. 43 (12) (2008) 4127–4134. [17] F.G. Torres, A.R. Boccaccini, O.P. Troncoso, Microwave processing of starch based porous structures for tissue engineering scaffolds, J. Appl. Polym. Sci. 103 (2) (2007) 1332–1339. [18] X. Zhao, F. Su, Q. Yan, W. Guo, X.Y. Bao, L. Lv, Z. Zhou, Templating methods for preparation of porous structures, J. Mater. Chem. 16 (7) (2006) 637–648. [19] A. Ramizy, Z. Hassan, K. Omar, Y. Al-Douri, M. Mahdi, New optical features to enhance solar cell performance based on porous silicon surfaces, Appl. Surf. Sci. 267 (2011) 6112–6117. [20] P. Perlin, C. Jauberthie-Carillon, J.P. Itie, A. San Miguel, I. Grzegory, A. Polian, Raman scattering and X-ray-absorption spectroscopy in gallium nitride under high pressure, Phys. Rev. B 45 (1) (1992) 83. [21] J. Wang, D. Yu, C. Li, J. Zhou, Y. Wang, S. Feng, Large-scale synthesis of singlephase, high-quality GaN nanocrystallites, Appl. Phys. A: Mater. Sci. Process. 78 (5) (2004) 753–755. [22] A. Ramizy, Z. Hassan, K. Omar, Porous GaN on Si (1 1 1) and its application to hydrogen gas sensor, Sens. Actuators B: Chem. 155 (2011) 699–708.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
High-quality GaN nanowires grown on Si and porous silicon by thermal evaporation L. Shekari ∗ , A. Ramizy, K. Omar, H. Abu Hassan, Z. Hassan Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 9 April 2012 Received in revised form 30 July 2012 Accepted 31 July 2012 Available online 31 August 2012 Keywords: Thermal evaporation growth GaN nanowires Porous silicon
a b s t r a c t Nanowires (NWs) of GaN thin films were prepared on as-grown Si (1 1 1) and porous silicon (PS) substrates using thermal evaporation method. The film growth produced high-quality wurtzite GaN NWs. The size, morphology, and nanostructures of the crystals were investigated through scanning electron microscopy, high-resolution X-ray diffraction and photoluminescence spectroscopy. The NWs grown on porous silicon were thinner, longer and denser compared with those on as-grown Si. The energy band gap of the NWs grown on PS was larger than that of NWs on as-grown Si. This is due to the greater quantum confinement effects of the crystalline structure of the NWs grown on PS. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The direct band gap semiconductor GaN (band gap of 3.4 eV) has great potential for use in optoelectronic devices over a wide range of wavelengths for III-nitrides, from the blue to the near ultraviolet wavelength. One-dimensional nanostructures such as NWs and nanorods have excellent electrical, optical, and mechanical properties, as well as great potential for various applications, such as probe microscopy tips and interconnections in nanoelectronics [1]. Processing techniques, especially those for crystal growth of III–V nitride nanostructures, have been successfully established. Plasma etching and reactive ion etching have been mainly used to etch III–V nitride crystals. However, the damage caused by the ion or plasma bombardment is a serious problem in these processes [2–4]. Vapor phase methods have also been used for NW production. These include physical methods such as laser-assisted catalytic growth, carbon-nanotube-confined reaction, catalytic reaction based on a vapor–liquid–solid mechanism and thermal chemical vapor deposition, and direct reaction of metal gallium with ammonia [5–10]. Wu et al. [13] synthesized GaN NWs on Si substrate by ammoniating Ga2 O3 /BN films under a flowing ammonia atmosphere. Their results demonstrate that NWs are hexagonal wurtzite GaN with smooth surfaces. They also discussed the growth mechanism of crystalline GaN NW. Popa et al. [14] demonstrated
∗ Corresponding author. Tel.: +006017 4884 613; fax: +60 604 6579150. E-mail address: lsg09 [email protected] (L. Shekari). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.164
controlled nanostructuring of GaN by focused-ion-beam treatment with subsequent photoelectrochemical (PEC) etching. The proposed mask-less approach based on direct writing of surface negative charge, which shields the material against PEC etching, allows the fabrication of GaN nanowalls and NWs with lateral dimensions as small as 100 nm. Thermal evaporation using a tube furnace and flowing Ar gas is a promising technology for the fabrication of GaN NWs and nanorods. It is inexpensive and can be easily controlled to obtain NWs of desired sizes [11,12]. The objective of the present study was to prepare high-quality single crystal wurtzite GaN NWs on PS on the polished and unpolished sides, and Si (1 1 1) substrate; using thermal evaporation technique, a novel procedure for preparing GaN semiconductors, without using ammonia gas. The porous substrates play important roles in fabricating GaN nanostructures due to the ability of the pore walls to interact with atoms and molecules together with the capacity of controllable pore space to capture or load gas molecules, and solid particles [15–18].
2. Materials and methods The electrochemical cell was used to fabricate the porous silicon (PS) [19]. N-type Si wafer with a dimension of 1 cm × 1 cm × 283 m, (1 1 1) orientation, resistivity of 0.75 cm, and doping concentration of 1.8 × 1017 cm−3 was etched through an electrochemical process to produce the porous structure. The wafer was placed in an electrolyte solution (hydrofluoric acid (HF): ethanol, 1:4) with a current density of 60 mA/cm2 at an etching time of 15 min for each side. Before the etching process, the Si substrate was cleaned using the Radio Corporation of America
L. Shekari et al. / Applied Surface Science 263 (2012) 50–53
51
Fig. 1. SEM images of as-grown Si and PS formed on both sides of the crystalline Si wafer.
method (RCA) to remove the oxide layer, it was then immersed in HF acid to remove the native oxide. The electrochemical cell was made by Teflon and has a circular aperture with radius of 0.4 cm, and the silicon wafer is sealed below. The cell consists of two-electrode system with the Si wafer as the anode and platinum as the cathode. The process was carried out at room temperature. After etching, all samples were rinsed with ethanol and dried in air. The GaN NWs were prepared by heating GaN powder of purity 99.999% at ∼1000 ◦ C for 2 h under an Ar gas stream introduced at 0.5 cm3 /min. Subsequently, they were deposited on as-grown Si and PS substrates placed inside ceramic boats by the thermal evaporation with no template or catalyst used in the synthesis. Prior to heating, the furnace was cleaned using a stream of Ar. The surface morphology and structural properties of the nanostructures were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM) and high-resolution X-ray diffraction (HR-XRD). Photoluminescence (PL) was also performed at room temperature using He–Cd laser ( = 325 nm). 3. Results and discussion
Fig. 2. AFM images of PS: (a) as-grown; (b) polished front side; (c) unpolished back side.
The SEM images in Fig. 1 illustrate the smooth surface of asgrown Si and the uniformity of the surface of the PS for the polished and unpolished sides. The uniformity was due to the isotropic characteristic of HF/ethanol etching and the optimal conditions of the current density and etching time. The isotropic etching resulted in spherical pores. The etched surface formed on the polished side showed pores and small protrusions. Pores on the unpolished side were larger (around 110 nm) than those on the polished side (around 40 nm) due to greater etching of the initial surface with higher surface roughness than polished side. Fig. 2 demonstrates that the three dimensional topographic images of the PS etched surfaces with the pyramidal shape were distributed over the entire surface. The pyramidal shape indicated that the increase in the surface roughness is due to the effect of the etching parameters’ effect on the surface characterization. Fig. 3(a) shows the configuration of NWs formed on Si by extended high-temperature processing inside the furnace. The short and fluffy fibrous NWs with sharp tips are randomly oriented and less dense. The nanostructures formed on polished and unpolished sides of the PS are shown in Fig. 3(b) and (c), respectively. The nanostructures on both sides of the PS had corn-shaped structures. On closer inspection, the corn-shaped structure on the polished side consisted of bundles of long NWs which are dense and well-defined and preferentially oriented in one direction. The corn-shaped structure on the unpolished side had the same characters, however their NWs seem to be bundled closer together, thus creating a smoother image than those on the polished side.
52
L. Shekari et al. / Applied Surface Science 263 (2012) 50–53
Fig. 3. SEM images of GaN NW grown on (a) as-grown Si; (b) PS formed on the polished side; (c) PS formed on the unpolished side.
280
spectrum of GaN NWs is related to the quality of the NWs that determines the number of luminescent centers, and relaxation of thermally charged carriers. The surface etching produced denser, longer and well-defined NW structures that increased the number of luminescent centers, which then increased the PL of the GaN NWs, thus serving as sequences to the thermal electrons.
Fig. 4. HR-XRD spectra of PS and GaN NWs on as-grown Si and on both sides of the crystalline Si wafer.
Fig. 6. PL GaN NWs on as-grown Si, and GaN NWs on PS.
PS polished side
260
PS unpolished side
240 220 200 180 160 140
Intensity(Counts)
Fig. 4 shows the high-resolution X-ray diffraction (HR-XRD) patterns of the PS, GaN NWs on the as-grown Si, and the NWs on both sides of the PS substrate. The sharp peaks can be indexed to the hexagonal wurtzite structure of GaN with lattice constants a = 3187 A˚ and c = 5185 A˚ (reference code 01-089-8937). These values are close to the reported values of bulk GaN crystals [20]. The sharp diffraction peaks suggest that the synthesized GaN NWs are of high quality [21]. More GaN NW peaks were produced on both sides of the PS substrate compared with those of GaN NWs on grown Si. This clearly indicates that the NWs are denser and of higher quality than those on Si as shown by Fig. 3. The PS has a surface structure which is more pliant than Si for greater growth of GaN NWs, thus producing more GaN peaks in the XRD patterns. The PL spectra of both sides of PS are shown in Fig. 5. The PL of PS on the polished side shows a peak at 685.5 nm (1.80 eV), with a full width at half maximum (FWHM) intensity of 300 meV. The PL of the PS formed on the unpolished side produced a blue shifted luminescence peak at 668.5 nm (1.85 eV) with a FWHM of ∼280 meV. This indicates that the porous structures are confined to nanometer dimensions. The energy gap of PS on the polished and unpolished sides increased to 1.80 and 1.85 eV, respectively. As shown in Fig. 6, the PL peak of GaN NWs on both sides of PS is at 340 nm (3.64 eV), with a FWHM of 60 meV, which attributed to electron–hole pairs recombination. The broadening of the energy band gap at 395 nm (3.13 eV) with a FWHM of 212 meV for GaN NWs on Si is due to a lesser quantum confinement effects of the as grown crystalline structure [22]. Thus, the peak shift in the PL
120 100 80
300
400
500
600
700
800
900
Wavelength(nm) Fig. 5. PL spectra of both sides of PS.
L. Shekari et al. / Applied Surface Science 263 (2012) 50–53
4. Conclusions Thermal evaporation technique is a promising, and inexpensive technology to produce GaN NWs, whose size and distribution can be controlled by changing the growth conditions. The morphology of the synthesized NWs reveals that porous substrates play important roles in fabricating dense and well-defined GaN nanostructures. Structural and optical characterizations by XRD and PL show that the growth of the NWs was a function of the porosity of the initial semiconducting substrate. The PL peak of GaN NWs on PS may be explained by the quantum confinement effects of its crystalline structure, and the radiative recombination of thermalized electrons and holes. The peak shift in the PL spectrum of GaN NWs on Si is related to the lesser quality of NWs grown with decreasing number of luminescent centers and the relaxation of hot charge carriers. Acknowledgment The authors gratefully acknowledge the financial support from Postgraduate Research Grant Scheme 1001/PFIZIK/822128 of Universiti Sains Malaysia. References [1] Y. San, L. Hong-dong, Y. Hai-bin, L. Dong-mei, S. Hai-ping, Z. Guang-tian, Preparation and microstructure of nanosized GaN crystals, Chin. Phys. Lett. 13 (1996) 444. [2] A. Ramizy, Z. Hassan, K. Omar, Nanostructured GaN on silicon fabricated by electrochemical and laser-induced etching, Mater. Lett. 66 (2010) 61–63. [3] K. Omar, Z. Hassan, K. Goh, H. Teh, H.A. Hassan, Synthesis porous GaN by using UV-assisted electrochemical etching and its optical studies, Mod. Appl. Sci. 3 (3) (2009) P132. [4] F. Yam, Z. Hassan, S. Ng, Porous GaN prepared by UV assisted electrochemical etching, Thin Solid Films 515 (7–8) (2007) 3469–3474. [5] C.J. Barrelet, A.B. Greytak, C.M. Lieber, Nanowire photonic circuit elements, Nano Lett. 4 (10) (2004) 1981–1985.
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[6] G.D. Yuan, W.J. Zhang, J.S. Jie, X. Fan, J.A. Zapien, Y.H. Leung, L.B. Luo, P.F. Wang, C.S. Lee, S.T. Lee, p-Type ZnO nanowire arrays, Nano Lett. 8 (April (8)) (2008) 7. [7] J. Halpern, A. Bello, J. Gilcrease, G. Harris, M. He, Biphasic GaN nanowires: growth mechanism and properties, Microelectron. J. 40 (2) (2009) 316–318. [8] J. Li, J. Liu, L.S. Wang, R.P.H. Chang, Physical and electrical properties of chemical vapor grown GaN nano/microstructures, Inorg. Chem. 47 (22) (2008) 10325–10329. [9] W. Shi, Y. Zheng, N. Wang, C. Lee, S. Lee, Microstructures of gallium nitride nanowires synthesized by oxide-assisted method, Chem. Phys. Lett. 345 (5–6) (2001) 377–380. [10] Z. Dong, C. Xue, H. Zhuang, S. Wang, H. Gao, D. Tian, Y. Wu, J. He, Y. Liu, Synthesis of three kinds of GaN nanowires through Ga2 O3 films’ reaction with ammonia, Phys. E: Low-dimens. Syst. Nanostruct. 27 (1–2) (2005) 32–37. [11] K. Al-Heuseen, M. Hashim, N. Ali, Enhanced optical properties of porous GaN by using UV-assisted electrochemical etching, Phys. B: Condens. Matter 405 (15) (2010) 3176–3179. [12] V. Ulin, S. Konnikov, Electrochemical pore formation mechanism in III–V crystals (part I), Semiconductors 41 (7) (2007) 832–844. [13] Y. Wu, C. Xue, H. Zhuang, D. Tian, Y. Liu, J. He, F. Wang, L. Sun, Y. Ai, Synthesis of single-crystal GaN nanowires on Si (1 1 1) substrate by ammonification technology, Mater. Lett. 60 (25–26) (2006) 3076–3078. [14] V. Popa, I. Tiginyanu, O. Volciuc, A. Sarua, M. Kuball, P. Heard, Fabrication of GaN nanowalls and nanowires using surface charge lithography, Mater. Lett. 62 (30) (2008) 4576–4578. [15] A.R. Boccaccini, J.L. Spino, V. Cannillo (Eds.), Hermetic Glass Bodies with Controlled Porosity: Processing and Properties, Wiley Online Library, 2002. [16] O. Bretcanu, C. Samaille, A.R. Boccaccini, Simple methods to fabricate Bioglass® derived glass–ceramic scaffolds exhibiting porosity gradient, J. Mater. Sci. 43 (12) (2008) 4127–4134. [17] F.G. Torres, A.R. Boccaccini, O.P. Troncoso, Microwave processing of starch based porous structures for tissue engineering scaffolds, J. Appl. Polym. Sci. 103 (2) (2007) 1332–1339. [18] X. Zhao, F. Su, Q. Yan, W. Guo, X.Y. Bao, L. Lv, Z. Zhou, Templating methods for preparation of porous structures, J. Mater. Chem. 16 (7) (2006) 637–648. [19] A. Ramizy, Z. Hassan, K. Omar, Y. Al-Douri, M. Mahdi, New optical features to enhance solar cell performance based on porous silicon surfaces, Appl. Surf. Sci. 267 (2011) 6112–6117. [20] P. Perlin, C. Jauberthie-Carillon, J.P. Itie, A. San Miguel, I. Grzegory, A. Polian, Raman scattering and X-ray-absorption spectroscopy in gallium nitride under high pressure, Phys. Rev. B 45 (1) (1992) 83. [21] J. Wang, D. Yu, C. Li, J. Zhou, Y. Wang, S. Feng, Large-scale synthesis of singlephase, high-quality GaN nanocrystallites, Appl. Phys. A: Mater. Sci. Process. 78 (5) (2004) 753–755. [22] A. Ramizy, Z. Hassan, K. Omar, Porous GaN on Si (1 1 1) and its application to hydrogen gas sensor, Sens. Actuators B: Chem. 155 (2011) 699–708.