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Structural and optical characterization of InGaN nanoparticles synthesized at low temperature
Materials Letters 99 (2013) 128–130
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Structural and optical characterization of InGaN nanoparticles synthesized at low temperature M.A. Qaeed a,c,n, K. Ibrahim a, Ruchi Srivastava a, M.K.M. Ali a, A. Salhin b a
Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, Penang, Malaysia School of Chemical Sains, Universiti Sains Malaysia, 11800 Penang, Malaysia c Physics Department, Faculty of Education, Hodeidah University, Yemen b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 December 2012 Accepted 22 February 2013 Available online 4 March 2013
This study reports a simple and low cost chemical method for the synthesis of InGaN nanoparticles at low temperature. The nanoparticles were drop-casted on polyethylene terephthalate (PET) substrate and characterized via field emission scanning electron microscopy (FESEM). X-ray diffraction (XRD) showed a nano-crystalline InGaN with hexagonal phase and In mole fraction of 15. As confirmed by FESEM, the average diameter of In0.15Ga0.85N nanoparticles increased simultaneously with increase in the thickness of as-deposited films. The room temperature photoluminescence (PL) spectrum showed a blue emission at 2.4 eV and UV emission peak at 3.1 eV for In0.15Ga0.85N. & 2013 Elsevier B.V. All rights reserved.
Keywords: InGaN Nanoparticles Optical materials and properties Low temperature PET
1. Introduction InGaN has attracted large attention due to the ability of its wide band gap energies to be tuned by simply adjusting the In and Ga ratios, from 0.7 eV (InN) to 3.4 eV (GaN). This band gap can be applied as an active region in the fabrication of tunable color light emitting diodes and laser diode [1,2]. High breakdown field, high carrier mobility, high temperature and chemical stability are factors that make InGaN a significant candidate for applications such as high electron mobility transistor (HEMT) and hetero-structure field effect transistor (HFET) [3,4]. Furthermore, distinctive features like its direct wide band gap, strong interaction bonds and high thermal conductivity make InGaN an appealing option for optoelectronic applications [5]. This alloy has also been considered as a basic building block for photovoltaic applications because of its high carrier mobility and wide solar spectrum coverage [6]. The thin films of InGaN have been grown by variety of techniques such as Molecular Beam Epitaxy (MBE), Metal Organic Chemical Vapor Deposition (MOCVD) and Metalorganic Vapor Phase Epitaxy (MOVPE) [7,8,9]. Earlier studies reported the synthesis of InN and GaN nanoparticles using variety of methods such as the, solvothermal method [10], pyrolysis at high temperature and the combustion method [11,12]. However, these conversional methods are not suitable for the preparation of InGaN nanoparticles. There are few reports on the use of advanced alternative methods for the
n
Corresponding author. Tel.: þ60 104391995; fax: þ 60 46579150. E-mail address: [email protected] (M.A. Qaeed).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.02.090
synthesis of InGaN nanoparticles, although they require expensive chemicals, high temperature and the yield of prepared nanoparticles is relatively low [13,14]. This paper reports the synthesis of InGaN nanoparticle at 90 1C temperature and under ambient pressure using a chemical method. The process is highly suitable for large scale production of nanoparticles as it does not require high temperature condition, and vacuum/gas environment. The chemicals used in this process are relatively cheaper than those currently used in the industry.
2. Experimental procedure Gallium(III) acetylacetonate (Ga(acac)3; 99.99%) and indium(III) acetylacetonate (In(acac)3; 99.99%) were procured from Sigma-Aldrich, while methanol, oleylamine (approximate C18content 80–90%), toluene, HNO3 (34.5%) and NH4OH (28–30%) were purchased from Acros Organics. To prepare InGaN nanoparticles, 100 mg of Ga(acac)3 was mixed with 18 mg of In(acac)3 in 10 ml of oleylamine under ambient temperature and pressure. The mixture was then poured in rotary evaporator flask placed in a water bath at 90 1C. 10 ml NH4OH and 5 ml HNO3 were added to the solution, and the resulting mixture was kept at 90 1C for 12 h. The obtained solution was stirred for 1 h at 60 1C, and then allowed to cool at room temperature with 5 ml methanol added afterwards. The resultant precipitate was separated by centrifugation, and washed five times with a mixture of toluene and methanol to avoid any impurity. These synthesized nanoparticles were dispersed in a mixture of 10 ml toluene and 5 ml methanol.
M.A. Qaeed et al. / Materials Letters 99 (2013) 128–130
The solution was then drop-casted on PET substrates for three different thicknesses, and the obtained films were kept at 60 1C for 2–3 min.
3. Results and discussions Fig. 1 illustrates the XRD patterns of synthesized InGaN nanoparticles based on three thicknesses of PET substrate. XRD data shows that InxGa(1 x)N nanoparticles have peaks at 2y ¼31.25 and 34.00 values, revealing a low crystalline structure [15]. The first XRD peak of InxGa(1 x)N at 31.25 [16] was obtained between InN(100) peak at 29.1 [9] and GaN(100) peak at 32.38 [17]. The second XRD peak of InxGa(1 x)N nanoparticles at 34.00 [15] was obtained between GaN(002) peak at 36.8 [17,18] and InN(002) peak at 33.1 [9]. The origin of these two peaks of InxGa(1 x)N nanoparticles is characterized by (100) and (002) crystal planes with lattice ˚ which confirms the InGaN hexparameter a¼ 3.3 A˚ and c¼ 5.26 A, agonal structure [15]. The In composition (x) in InxGa(1 x)N nanoparticle was determined from the XRD results using the Vegard’s law, which is given as follows: x¼
C InGaN C GaN C InN C GaN
ð1Þ
CInGaN, CGaN and CInN are out-plane lattice parameters of InGaN, GaN and InN respectively. The mean particle size of synthesized nanoparticles (DP) can be calculated using Scherrer’s formula DP ¼
0:9 l b cos y
ð2Þ
where DP is the crystal size of nanoparticles, b is the full width at half maximum (FWHM) of broadened peak, l is the wavelength of incident X-ray¼1.5406 A˚ and y is the Bragg angle. The value of In composition(x) is calculated from Eq. (1) to be 0.15, while the average particle sizes of In0.15Ga0.85N nanoparticles calculated from Eq. (2) were 62.18 nm, 41.5 nm and 22.43 nm for the thicknesses 223 nm, 205 nm and 197 nm respectively. This confirms that thickness increases the agglomeration of nanoparticles due to the increase
Fig. 1. XRD pattern of InGaN nanoparticles of thickness (a) 223 nm, (b) 205 nm and (c) 197 nm on PET substrate.
129
in the van der Waals forces between the particles, thus increasing the crystalline grain size. The energy dispersive X-ray spectroscopy (EDX) spectra taken for In0.15Ga0.85N nanoparticles are shown in Fig. 2 for three thicknesses of the PET substrate. The EDX spectrum corresponding to synthesized nanoparticle consists of In, Ga, N, C and O with no other elements. However the peak of C and O in EDX originated from PET substrate. This indicates that the samples are free of any contamination. EDX spectra for different thicknesses reveal that chemical composition of material remains the same with changes in thickness. These results verify that the chemical purity of synthesized nanoparticle is consistent with XRD results. Fig. 2 shows the FESEM images of In0.15Ga0.85N nanoparticles based on three thicknesses—223 nm, 205 nm and 197 nm. The average diameter of grain size decreases as thickness decreases, which is also like the XRD results. Therefore, the optical properties of synthesized nanoparticles are important in understanding the quality of the material. Fig. 3 shows the photoluminescence (PL) spectrum of In0.15Ga0.85N nanoparticle based thin film of three thicknesses recorded at room temperature. The PL spectrum is characterized by two peaks. The first at 3.1 eV is attributed to the near band edge (NBE) of InGaN, while the second peak is a broad blue band emission centered at 2.4 eV (500 nm) [7,19,20], which can be ascribed to crystal defects such as VN-related complexes or as a result of In mole fraction [21,22] of the three thicknesses. Fig. 3 indicates that as the thickness increases, PL intensity also increases. The following equation is used for the calculation of band gap of InxGa1 xN systems: EðgÞ ¼ EGaN xðEGaN EInN Þbxð1xÞ
ð3Þ
where EGaN (3.48 eV) and EInN (0.7 eV) indicate band gap of GaN and InN respectively, and b (1.43 eV) is the Bowing parameter which is constant. The band gap of In0.15Ga0.85N nanoparticles
Fig. 2. FESEM images and EDX spectrum of InGaN nanoparticles deposited by drop casting with 3 different thicknesses: (a) 223 nm, (b) 205 nm and (c) 197 nm on PET substrate.
130
M.A. Qaeed et al. / Materials Letters 99 (2013) 128–130
Acknowledgments I wish to thank the Ministry of Higher Education Malaysia for supporting this work under Grant no. 203/PSF/6721001 and Hodeidah University for awarding the scholarship that helped my pursuit of a Ph.D. abroad.
References
Fig. 3. Photoluminescence of InGaN nanoparticles of (a) 223 nm, (b) 205 nm and (c) 197 nm on PET substrate.
three
thicknesses:
calculated from Eq. (3) gave 2.9 eV, which is closer to 3.1 eV in the PL data.
4. Conclusions In summary, semiconducting hexagonal InxGa(1 x)N nanoparticles were synthesized at 90 1C using a simple chemical method. X-ray diffraction (XRD) results proved the presence of hexagonal InxGa(1 x)N nanoparticle with the In mole fraction of 0.15. The XRD analysis concludes that the thickness of In0.15Ga0.85N nanoparticle increases with increase in crystalline grain size, as shown by FESEM images. The PL spectrum exhibits a blue emission peak at 2.4 eV (500 nm) and a near band edge (NBE) of InGaN at 3.1 eV (400 nm). This makes it a potential candidate for optoelectronic device applications in the visible region. The band gap of In0.15Ga0.85N nanoparticle was calculated to be 2.9 eV which is closer to 3.1 eV in the PL when data comparison.
[1] Funato M, Hayashi K, Ueda M, Kawakami Y, Narukawa Y, Mukai T. Appl Phys Lett 2008;93021126-3. [2] Nakamura S. Science 1998;281:956–61. [3] Laboutin O, Cao Y, Johnson W, Wang R, Li G, Jena D, et al. Appl Phys Lett 2012;100121909-2. [4] Xie J, Leach JH, Ni X, Wu M, Shimada R, Ozgur U, et al. Appl Phys Lett 2007;91:262102–3. [5] Nakamura S, Pearton S, Fasol G. The blue laser diode. Berlin: Springer-Verlag; 2000. [6] Dahal R, Pantha B, Li J, Lin JY, Jiang HX. Appl Phys Lett 2009;94063505-3. [7] Li SF, As DJ, Lischka K, Salazar DGP, Scolfaro LMR, Leite JR, et al. Mater Res Soc Symp Proc 2005;83(1):E8.15.1–6. [8] Mo C, Fang W, Pu Y, Liu H, Jiang FJ. Cryst Growth 2005;285:312–5. [9] Park HJ, Kang SW, Kryliouk O, Anderson T. Mater Res Soc Symp Proc 2006;892: FF11-9. 1-6. [10] Bai YJ, Liu ZG, Xu XG, Cui DL, Hao X, Feng X, et al. J Cryst Growth 2002;241:189–94. [11] Pan G, Kordesch ME, Van Patten PG. Chem Mater 2006;18:5392–3. [12] Kudrawiec R, Nyk M, Syperek M, Podhorodecki A, Misiewicz J, Strek W. Appl Phys Lett 2006;88181916-3. [13] Gopalakrishnan M, Purushothaman V, Venkatesh PS, Ramakrishnan V, Jeganathan KJ. Mater Res Bull 2012;47:3323–7. [14] Schwenzer B, Meier C, Masala O, Seshadri R, DenBaars SP, Mishra UK. J Mater Chem 2005;15:1891–5. [15] Donnell KP, Mosselmans JFW, Martin RW, Pereira S, White MEJ. Phys: Condens Matter 2001;13:6977–91. [16] Kawai Y, Ohsuka S, Iwaya M, Kamiyama S, Amano H, Akasaki I. Proc SPIE 2008;6889:688904–8. [17] Xi HZ, Man1 BY, Chen CS, Liu M, Wei J, Yang SYJ. Semicond Sci Technol 2009;24:085024–7. [18] Shan XC, Jun LW, Feng S, Zhao ZAH, Fu GY, Ping CY, et al. J Chin Phys Lett 2010;27:038102–4. [19] Dartsch H, Tessarek C, Aschenbrenner T, Figge S, Kruse C, Schowalter M, et al. J Cryst Growth 2011;320:28–34. [20] Chang S, Kuo CH, Su YK, Wu LW, Sheu JK, Wen TC, et al. IEEE J Sel Top Quantum Electron 2002;8:744–5. [21] Saron KMA, Hashim MR. Superlattices Microstructures 2013;56:55–63. [22] Nakamura S, Senoh M, Iwasa N, Nagahama S. Jpn J Appl Phys 1995;34:L797–9.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Structural and optical characterization of InGaN nanoparticles synthesized at low temperature M.A. Qaeed a,c,n, K. Ibrahim a, Ruchi Srivastava a, M.K.M. Ali a, A. Salhin b a
Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, Penang, Malaysia School of Chemical Sains, Universiti Sains Malaysia, 11800 Penang, Malaysia c Physics Department, Faculty of Education, Hodeidah University, Yemen b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 December 2012 Accepted 22 February 2013 Available online 4 March 2013
This study reports a simple and low cost chemical method for the synthesis of InGaN nanoparticles at low temperature. The nanoparticles were drop-casted on polyethylene terephthalate (PET) substrate and characterized via field emission scanning electron microscopy (FESEM). X-ray diffraction (XRD) showed a nano-crystalline InGaN with hexagonal phase and In mole fraction of 15. As confirmed by FESEM, the average diameter of In0.15Ga0.85N nanoparticles increased simultaneously with increase in the thickness of as-deposited films. The room temperature photoluminescence (PL) spectrum showed a blue emission at 2.4 eV and UV emission peak at 3.1 eV for In0.15Ga0.85N. & 2013 Elsevier B.V. All rights reserved.
Keywords: InGaN Nanoparticles Optical materials and properties Low temperature PET
1. Introduction InGaN has attracted large attention due to the ability of its wide band gap energies to be tuned by simply adjusting the In and Ga ratios, from 0.7 eV (InN) to 3.4 eV (GaN). This band gap can be applied as an active region in the fabrication of tunable color light emitting diodes and laser diode [1,2]. High breakdown field, high carrier mobility, high temperature and chemical stability are factors that make InGaN a significant candidate for applications such as high electron mobility transistor (HEMT) and hetero-structure field effect transistor (HFET) [3,4]. Furthermore, distinctive features like its direct wide band gap, strong interaction bonds and high thermal conductivity make InGaN an appealing option for optoelectronic applications [5]. This alloy has also been considered as a basic building block for photovoltaic applications because of its high carrier mobility and wide solar spectrum coverage [6]. The thin films of InGaN have been grown by variety of techniques such as Molecular Beam Epitaxy (MBE), Metal Organic Chemical Vapor Deposition (MOCVD) and Metalorganic Vapor Phase Epitaxy (MOVPE) [7,8,9]. Earlier studies reported the synthesis of InN and GaN nanoparticles using variety of methods such as the, solvothermal method [10], pyrolysis at high temperature and the combustion method [11,12]. However, these conversional methods are not suitable for the preparation of InGaN nanoparticles. There are few reports on the use of advanced alternative methods for the
n
Corresponding author. Tel.: þ60 104391995; fax: þ 60 46579150. E-mail address: [email protected] (M.A. Qaeed).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.02.090
synthesis of InGaN nanoparticles, although they require expensive chemicals, high temperature and the yield of prepared nanoparticles is relatively low [13,14]. This paper reports the synthesis of InGaN nanoparticle at 90 1C temperature and under ambient pressure using a chemical method. The process is highly suitable for large scale production of nanoparticles as it does not require high temperature condition, and vacuum/gas environment. The chemicals used in this process are relatively cheaper than those currently used in the industry.
2. Experimental procedure Gallium(III) acetylacetonate (Ga(acac)3; 99.99%) and indium(III) acetylacetonate (In(acac)3; 99.99%) were procured from Sigma-Aldrich, while methanol, oleylamine (approximate C18content 80–90%), toluene, HNO3 (34.5%) and NH4OH (28–30%) were purchased from Acros Organics. To prepare InGaN nanoparticles, 100 mg of Ga(acac)3 was mixed with 18 mg of In(acac)3 in 10 ml of oleylamine under ambient temperature and pressure. The mixture was then poured in rotary evaporator flask placed in a water bath at 90 1C. 10 ml NH4OH and 5 ml HNO3 were added to the solution, and the resulting mixture was kept at 90 1C for 12 h. The obtained solution was stirred for 1 h at 60 1C, and then allowed to cool at room temperature with 5 ml methanol added afterwards. The resultant precipitate was separated by centrifugation, and washed five times with a mixture of toluene and methanol to avoid any impurity. These synthesized nanoparticles were dispersed in a mixture of 10 ml toluene and 5 ml methanol.
M.A. Qaeed et al. / Materials Letters 99 (2013) 128–130
The solution was then drop-casted on PET substrates for three different thicknesses, and the obtained films were kept at 60 1C for 2–3 min.
3. Results and discussions Fig. 1 illustrates the XRD patterns of synthesized InGaN nanoparticles based on three thicknesses of PET substrate. XRD data shows that InxGa(1 x)N nanoparticles have peaks at 2y ¼31.25 and 34.00 values, revealing a low crystalline structure [15]. The first XRD peak of InxGa(1 x)N at 31.25 [16] was obtained between InN(100) peak at 29.1 [9] and GaN(100) peak at 32.38 [17]. The second XRD peak of InxGa(1 x)N nanoparticles at 34.00 [15] was obtained between GaN(002) peak at 36.8 [17,18] and InN(002) peak at 33.1 [9]. The origin of these two peaks of InxGa(1 x)N nanoparticles is characterized by (100) and (002) crystal planes with lattice ˚ which confirms the InGaN hexparameter a¼ 3.3 A˚ and c¼ 5.26 A, agonal structure [15]. The In composition (x) in InxGa(1 x)N nanoparticle was determined from the XRD results using the Vegard’s law, which is given as follows: x¼
C InGaN C GaN C InN C GaN
ð1Þ
CInGaN, CGaN and CInN are out-plane lattice parameters of InGaN, GaN and InN respectively. The mean particle size of synthesized nanoparticles (DP) can be calculated using Scherrer’s formula DP ¼
0:9 l b cos y
ð2Þ
where DP is the crystal size of nanoparticles, b is the full width at half maximum (FWHM) of broadened peak, l is the wavelength of incident X-ray¼1.5406 A˚ and y is the Bragg angle. The value of In composition(x) is calculated from Eq. (1) to be 0.15, while the average particle sizes of In0.15Ga0.85N nanoparticles calculated from Eq. (2) were 62.18 nm, 41.5 nm and 22.43 nm for the thicknesses 223 nm, 205 nm and 197 nm respectively. This confirms that thickness increases the agglomeration of nanoparticles due to the increase
Fig. 1. XRD pattern of InGaN nanoparticles of thickness (a) 223 nm, (b) 205 nm and (c) 197 nm on PET substrate.
129
in the van der Waals forces between the particles, thus increasing the crystalline grain size. The energy dispersive X-ray spectroscopy (EDX) spectra taken for In0.15Ga0.85N nanoparticles are shown in Fig. 2 for three thicknesses of the PET substrate. The EDX spectrum corresponding to synthesized nanoparticle consists of In, Ga, N, C and O with no other elements. However the peak of C and O in EDX originated from PET substrate. This indicates that the samples are free of any contamination. EDX spectra for different thicknesses reveal that chemical composition of material remains the same with changes in thickness. These results verify that the chemical purity of synthesized nanoparticle is consistent with XRD results. Fig. 2 shows the FESEM images of In0.15Ga0.85N nanoparticles based on three thicknesses—223 nm, 205 nm and 197 nm. The average diameter of grain size decreases as thickness decreases, which is also like the XRD results. Therefore, the optical properties of synthesized nanoparticles are important in understanding the quality of the material. Fig. 3 shows the photoluminescence (PL) spectrum of In0.15Ga0.85N nanoparticle based thin film of three thicknesses recorded at room temperature. The PL spectrum is characterized by two peaks. The first at 3.1 eV is attributed to the near band edge (NBE) of InGaN, while the second peak is a broad blue band emission centered at 2.4 eV (500 nm) [7,19,20], which can be ascribed to crystal defects such as VN-related complexes or as a result of In mole fraction [21,22] of the three thicknesses. Fig. 3 indicates that as the thickness increases, PL intensity also increases. The following equation is used for the calculation of band gap of InxGa1 xN systems: EðgÞ ¼ EGaN xðEGaN EInN Þbxð1xÞ
ð3Þ
where EGaN (3.48 eV) and EInN (0.7 eV) indicate band gap of GaN and InN respectively, and b (1.43 eV) is the Bowing parameter which is constant. The band gap of In0.15Ga0.85N nanoparticles
Fig. 2. FESEM images and EDX spectrum of InGaN nanoparticles deposited by drop casting with 3 different thicknesses: (a) 223 nm, (b) 205 nm and (c) 197 nm on PET substrate.
130
M.A. Qaeed et al. / Materials Letters 99 (2013) 128–130
Acknowledgments I wish to thank the Ministry of Higher Education Malaysia for supporting this work under Grant no. 203/PSF/6721001 and Hodeidah University for awarding the scholarship that helped my pursuit of a Ph.D. abroad.
References
Fig. 3. Photoluminescence of InGaN nanoparticles of (a) 223 nm, (b) 205 nm and (c) 197 nm on PET substrate.
three
thicknesses:
calculated from Eq. (3) gave 2.9 eV, which is closer to 3.1 eV in the PL data.
4. Conclusions In summary, semiconducting hexagonal InxGa(1 x)N nanoparticles were synthesized at 90 1C using a simple chemical method. X-ray diffraction (XRD) results proved the presence of hexagonal InxGa(1 x)N nanoparticle with the In mole fraction of 0.15. The XRD analysis concludes that the thickness of In0.15Ga0.85N nanoparticle increases with increase in crystalline grain size, as shown by FESEM images. The PL spectrum exhibits a blue emission peak at 2.4 eV (500 nm) and a near band edge (NBE) of InGaN at 3.1 eV (400 nm). This makes it a potential candidate for optoelectronic device applications in the visible region. The band gap of In0.15Ga0.85N nanoparticle was calculated to be 2.9 eV which is closer to 3.1 eV in the PL when data comparison.
[1] Funato M, Hayashi K, Ueda M, Kawakami Y, Narukawa Y, Mukai T. Appl Phys Lett 2008;93021126-3. [2] Nakamura S. Science 1998;281:956–61. [3] Laboutin O, Cao Y, Johnson W, Wang R, Li G, Jena D, et al. Appl Phys Lett 2012;100121909-2. [4] Xie J, Leach JH, Ni X, Wu M, Shimada R, Ozgur U, et al. Appl Phys Lett 2007;91:262102–3. [5] Nakamura S, Pearton S, Fasol G. The blue laser diode. Berlin: Springer-Verlag; 2000. [6] Dahal R, Pantha B, Li J, Lin JY, Jiang HX. Appl Phys Lett 2009;94063505-3. [7] Li SF, As DJ, Lischka K, Salazar DGP, Scolfaro LMR, Leite JR, et al. Mater Res Soc Symp Proc 2005;83(1):E8.15.1–6. [8] Mo C, Fang W, Pu Y, Liu H, Jiang FJ. Cryst Growth 2005;285:312–5. [9] Park HJ, Kang SW, Kryliouk O, Anderson T. Mater Res Soc Symp Proc 2006;892: FF11-9. 1-6. [10] Bai YJ, Liu ZG, Xu XG, Cui DL, Hao X, Feng X, et al. J Cryst Growth 2002;241:189–94. [11] Pan G, Kordesch ME, Van Patten PG. Chem Mater 2006;18:5392–3. [12] Kudrawiec R, Nyk M, Syperek M, Podhorodecki A, Misiewicz J, Strek W. Appl Phys Lett 2006;88181916-3. [13] Gopalakrishnan M, Purushothaman V, Venkatesh PS, Ramakrishnan V, Jeganathan KJ. Mater Res Bull 2012;47:3323–7. [14] Schwenzer B, Meier C, Masala O, Seshadri R, DenBaars SP, Mishra UK. J Mater Chem 2005;15:1891–5. [15] Donnell KP, Mosselmans JFW, Martin RW, Pereira S, White MEJ. Phys: Condens Matter 2001;13:6977–91. [16] Kawai Y, Ohsuka S, Iwaya M, Kamiyama S, Amano H, Akasaki I. Proc SPIE 2008;6889:688904–8. [17] Xi HZ, Man1 BY, Chen CS, Liu M, Wei J, Yang SYJ. Semicond Sci Technol 2009;24:085024–7. [18] Shan XC, Jun LW, Feng S, Zhao ZAH, Fu GY, Ping CY, et al. J Chin Phys Lett 2010;27:038102–4. [19] Dartsch H, Tessarek C, Aschenbrenner T, Figge S, Kruse C, Schowalter M, et al. J Cryst Growth 2011;320:28–34. [20] Chang S, Kuo CH, Su YK, Wu LW, Sheu JK, Wen TC, et al. IEEE J Sel Top Quantum Electron 2002;8:744–5. [21] Saron KMA, Hashim MR. Superlattices Microstructures 2013;56:55–63. [22] Nakamura S, Senoh M, Iwasa N, Nagahama S. Jpn J Appl Phys 1995;34:L797–9.