- Email: [email protected]
GaN dots with varying GaN cap thickness
Journal of Crystal Growth 362 (2013) 353–356
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Optical and structural studies of InN/GaN dots with varying GaN cap thickness Wen-Cheng Ke n, Chia-Yu Kao, Wei-Chung Houng, Chih-An Wei Department of Mechanical Engineering, Yuan Ze University, Chung-Li 320, Taiwan, ROC
a r t i c l e i n f o
abstract
Available online 19 October 2011
This paper reported a structural and optical study of single InN/GaN dots with varying GaN cap thickness. The surface morphology of the 10-nm thick GaN capping layer shows a truncated pyramidal shape and changed to a dome shape when the thickness of the capping layer increases to 20-nm. The increase in compressive strain of InN dots with the increase in the cap thickness was analyzed by high resolution x-ray diffraction (HRXRD). A redshift was observed in the photoluminescence (PL) peak energy of the InN dots with increasing GaN capping thickness. The redshift of the PL peak energy of the 20-nm thick GaN capping layer sample was believed to be due to the GaN capping avoids InN decomposition and decreases the surface defect density. In addition, the maximum PL intensity of the 20-nm thick GaN capping layer sample was 2.5 times higher than that of the uncapped sample. & 2011 Elsevier B.V. All rights reserved.
Keywords: A1. Nanostructures A3. Metalorganic chemical vapor deposition B1. Nitrides B2. Semiconducting indium compounds
1. Introduction The revision of the InN band gap energy to 0.7 eV [1,2], has allowed for many additional applications of optoelectronic devices working in the infrared range, including high efficiency solar cells, etc. In recent years, the benefits of multiple exciton generation (MEG) have been demonstrated in colloidal suspensions of PbSe, PbS, PbTe, CdSe and GaAs quantum dots (QDs) by achieving high-efficiency nanostructured solar cells [3,4]. In order to increase the absorption of nanostructured solar cells, it is necessary to increase the volume of the nanostructure. In other words, the growth technique for high density dots [5,6] or for the stacking of dots layers plays an important role in increasing the density of the dot volume. The InN dots are formed through self-assembly in the highly mismatched InN/GaN system (i.e., with a lattice mismatch of 11%) by means of the strain-induced two-dimensional (2D) to threedimensional (3D) reorganization of the InN layer, similar to a Stranski–Krastanov (SK) growth mode [7]. Since the formation of InN dots on a GaN surface is strain driven, multiple dots layers can introduce defects and deteriorate the optical properties. These defects act as carrier trapping centers and limit the light-emitting and photovoltaic efficiency of optoelectronic devices. In addition, the spontaneous polarization and strain-induced piezoelectric polarization fields play an important role in carrier recombination in nitride materials [8]. Thus, it is essential that a systemic investigation of the strain distribution in InN dots is carried out for high performance
n
Corresponding author. E-mail address: [email protected] (W.-C. Ke).
0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.10.017
optoelectronic devices. To this end, this paper presents a systematic study of the effect of cap thickness on the structural and optical properties of single layer of InN/GaN dots structures.
2. Experimental details The InN/GaN nanodot structures were grown on 1 mm thick GaN/(0001) sapphire substrates by low pressure metal organic chemical vapor deposition (MOCVD). The InN dots were grown by pulsed mode method at a temperature of 650 1C to achieve higher density InN dots [5]. The gas flow sequence for the PM method basically consists of four steps: 20 s TMInþNH3 growth step, 20 s NH3 source step with 10 s purge steps in between. During the growth step, the mole flow rates of TMIn and NH3 are 1.53 101 and 4.46 105 mmole/min, respectively. The InN dots layer is then covered by a series of GaN capping layers with a thickness of 10 nm and 20 nm at 650 1C, respectively. The PL measurements were performed using the 488-nm line of an argon-ion laser as the excitation source. The PL signals were dispersed by ARC Pro 500 monochromator and were detected by a cooled InGaAs photodiode with a cutoff wavelength of 2.05 mm. 3. Results and discussion Fig. 1 shows the atomic force microscopy (AFM) images of the uncapped InN dots and the single-layer InN dots. The dot density, height and diameter were 1.5 109 cm 3, 20.2 nm and 96.5 nm, respectively, for the uncapped InN dots. The GaN capping morphology plays an important role in achieving a well stacked
354
W.-C. Ke et al. / Journal of Crystal Growth 362 (2013) 353–356
1 fim
Height (nm)
100 10 nm 20 nm 50
0 -100
-500
50 Width (nm)
10
0
Fig. 1. AFM images of (a) un-capped InN dots, single-layer InN dots covered by (b) a 10-nm thick and (c) a 20-nm thick GaN capping layer. (d) The AFM line scan profiles of the GaN capping layers with various thicknesses.
multilayered structure because the flat surface of the GaN capping layer is helpful for the InN dots growth in the next layer. The AFM line scan profiles of the GaN capping layers with various thicknesses are shown in Fig. 1(d). It shows that the surface morphology of the 10-nm thick GaN capping layer shows a truncated pyramid shape and changes to a dome shape when the thickness of the capping layer increases to 20-nm. The transmission electron microscopy (TEM) images of singlelayer InN dots with a 20 nm capping layer are shown in Fig. 2. The TEM image clearly shows that the threading dislocations were terminated in the truncated shape of the InN dots. A very thin InN wetting layer is also clearly evident at the interface between the InN dots and the GaN buffer layer. The aspect ratio of the uncapped InN dots changes from 1/5.3 (19.6 nm/103 nm) to 1/5.6 (19.7 nm/110 nm) in the 20 nm thick GaN capping sample. The change in morphology and the size changes of the QDs during capping for many material systems have been widely reported [9–11], and are usually explained as being the result of intermixing or the atomic exchanges process. In this study, the decreased aspect ratio was believed due to a strain in the InN dots after the GaN capping process. In order to study the strain level in the InN dots, we performed an x-ray diffraction (XRD) measurement. Fig. 3 shows the (002) o/2y scans of uncapped InN dots, and of the single-layer buried InN dots with various thicknesses of the GaN capping layer. The XRD peak position of InN dots were 31.2641, 31.3181 and 31.4231 for the uncapped InN dots, single-layer InN dots covered by a 10-nm thick and a 20-nm thick GaN capping layer. Combining Bragg’s law with the expression for the inter-planar spacing in hexagonal structures, the lattice constant in the growth direction is found to be c ¼l l/(2sinyB), for any allowed (00l) reflection of x-rays of the wavelength l. For this calculation, the lattice ˚ and the lattice constant (c axis) of the GaN buffer layer is 5.185 A, ˚ The values constant (c axis) of the uncapped InN dots is 5.717 A. of the lattice constants for the case of stain-free InN are estimated to be in the range of c¼5.703 A˚ [12]. The larger lattice constant, c, is attributed to the uniaxial tensile stress in the growth direction of the uncapped InN dots. In contrast, the lattice constant is
20 nm Fig. 2. TEM image of a single-layer of InN dots covered by a 20-nm thick GaN capping layer.
5.689 A˚ for InN dots covered by a 20-nm thick GaN capping layer. The experimental results indicated that the residual strain relaxation in the InN dots was dependent on the thickness of GaN capping layer. In Fig. 3(b), it is shown that the lattice constant c decreased from 5.717 to 5.689 A˚ when the thickness of GaN capping layer increased from 0 to 20 nm. In order to study the residual strain relaxation in the InN dots with different thickness of GaN capping layer. The strain (ezz) along z axis was estimated from the relation,
ezz ¼
CC 0 100% C0
ð1Þ
where C denotes the lattice constant of the InN dots and C0 is strainfree InN films. In Fig.3 (b), the residual strain is reduced from 0.245% to 0.07% as the GaN cap thickness increased from 0 to 10 nm. However, the residual strain of InN dots reduced to 0.245% for 20-nm thick GaN capping layer. Fig. 4 shows the 17 K-PL spectra of the un-capped InN dots, as well as the single-layer InN dots with various GaN cap thickness.
W.-C. Ke et al. / Journal of Crystal Growth 362 (2013) 353–356
355
0.3 Uncapped 10 nm 20 nm
5.72
5.71
0.1
5.70
0.0
Strain (%)
XRD intensity (a.u.)
Lattice constant (A)
0.2
-0.1
5.69
-0.2 5.68 31.0
31.5 32.0 θ-2θ (arcsec)
34
-0.3
35
0
5 10 15 Cap thickness (nm)
20
Fig. 3. (a) The (002) XRD measurement of un-capped and single-layer InN dots with various GaN cap thickness. (b) The relationship between the lattice constant/strain and GaN cap thickness.
Normalized PL intensity (a.u.)
Uncapped
105 0.82
20 nm
95 90
0.80
85
FWHM (meV)
10 nm
Peak energy (eV)
100
80 0.78 75
0.7
0.8 0.9 Energy (eV)
0
5 10 15 20 Cap thickness (nm)
Fig. 4. (a) The 17 K PL spectra of un-capped and single-layer InN dots with various GaN cap thickness. (b) The relationship between PL peak energy/FWHM and GaN cap thickness.
It should be noted that the PL intensity of the 20-nm thick GaN capping layer sample was 2.5 times higher than that of the uncapped sample. The PL spectrum of the un-capped InN dots shows peak energy at 0.807 eV, with a FWHM of 104 meV. The emission energy of the un-capped InN dots is higher than the reported bandgap energy of 0.69 eV [13], and indicates a strong Burstein–Moss (BM) effect due to the presence of a high electron concentration in the InN dots. Cimalla et al. [14] reported that the high surface electron concentration generated by the high surface density of states was due to the high ratio of surface area to the volume of dots. It is well known that the thermal instability due to the low InN dissociation temperature and high equilibrium N2 vapor pressure over the InN implies nitrogen desorption from the dots. Thus, we believe that GaN capping avoids InN decomposition and decreases the surface defect density. In Fig. 4(b), the experimental results also indicated that the peak energy and the FWHM of the PL
spectra of the single-layer InN dots decreases when the GaN capping thickness increases from 10-nm to 20-nm.
4. Conclusions In summary, we have investigated the structural and PL properties of InN/GaN dots. By varying the GaN cap thickness, it is possible to control the strain level. A redshift was observed in the photoluminescence (PL) peak energy of the InN dots with increasing GaN capping thickness. The redshift of the PL peak energy of the 20-nm thick GaN capping layer sample was believed to be due to the GaN capping avoids InN decomposition and decreases the surface defect density. The GaN cap thickness is not only very important for the dots strain but also for the emission properties of the InN dots.
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W.-C. Ke et al. / Journal of Crystal Growth 362 (2013) 353–356
Acknowledgments The authors gratefully acknowledge the financial support from the National Science Council of Taiwan, ROC under Contract no. NSC-98-2112-M155-001-MY3.
References [1] J. Wu, W. Walukiewicz, K.M. Yu, J.W. Ager III, E.E. Haller, H. Lu, W.J. Schaff, Y. Saito, Y. Nanishi, Applied Physics Letters 80 (2002) 3967. [2] T. Matsuoka, H. Okamoto, M. Nakao, H. Harima, E. Kurimoto, Applied Physics Letters 81 (2002) 1246. [3] O. Jani, I. Ferguson, C. Honsberg, S. Kurtz, Applied Physics Letters 91 (2007) 132117. [4] C.J. Neufeld, N.G. Toledo, S.C. Cruz, M. Iza, S.P. DenBaars, U.K. Mishra, Applied Physics Letters 93 (2007) 143502.
[5] W.C. Ke, S.J. Lee, C.Y. Kao, W.K. Chen, W.C. Chou, M.C. Lee, W.H. Chang, W.J. Lin, Y.C. Cheng, T.C. Lee, J.C. Lin, Journal of Crystal Growth 312 (2010) 3209. [6] S. Ruffenach, O. Briot, M. Moret, B. Gil, Applied Physics Letters 90 (2007) 153102. [7] Y.F. Ng, Y.G. Cao, M.H. Xie, X.L. Xie, X.L. Wang, S.Y. Tong, Applied Physics Letters 81 (2002) 3960. [8] E. Sarigiannidou, E. Monroy, B. Daudin, J.L. Rouvie re, A.D. Andreev, Applied Physics Letters 87 (2005) 203112. ¨ [9] Z. Zhong, J. Stangl, F. Schaffler, G. Bauer, Applied Physics Letters 83 (2003) 3695. [10] A. Hesse, J. Stangl, V. Holy, T. Roch, G. Bauer, O.G. Schmidt, U. Denker, B. Struth, Physical Review B 66 (2002) 085321. [11] J.G. Lozano, A.M. Sa´nchez, R. Garcı´a, D. Gonzalez, O. Briot, S. Ruffenach, Applied Physics Letters 88 (2006) 151913. [12] I. Vurgaftman, J.R. Meyer, Journal of Applied Physics 94 (2003) 3675. [13] F. Chen, A.N. Cartwright, H. Lu, W.J. Schaff, Journal of Crystal Growth 269 (2004) 10. [14] V. Cimalla, V. Lebedev, F.M. Morales, R. Goldhahn, O. Ambacher, Applied Physics Letters 89 (2006) 172109.
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Optical and structural studies of InN/GaN dots with varying GaN cap thickness Wen-Cheng Ke n, Chia-Yu Kao, Wei-Chung Houng, Chih-An Wei Department of Mechanical Engineering, Yuan Ze University, Chung-Li 320, Taiwan, ROC
a r t i c l e i n f o
abstract
Available online 19 October 2011
This paper reported a structural and optical study of single InN/GaN dots with varying GaN cap thickness. The surface morphology of the 10-nm thick GaN capping layer shows a truncated pyramidal shape and changed to a dome shape when the thickness of the capping layer increases to 20-nm. The increase in compressive strain of InN dots with the increase in the cap thickness was analyzed by high resolution x-ray diffraction (HRXRD). A redshift was observed in the photoluminescence (PL) peak energy of the InN dots with increasing GaN capping thickness. The redshift of the PL peak energy of the 20-nm thick GaN capping layer sample was believed to be due to the GaN capping avoids InN decomposition and decreases the surface defect density. In addition, the maximum PL intensity of the 20-nm thick GaN capping layer sample was 2.5 times higher than that of the uncapped sample. & 2011 Elsevier B.V. All rights reserved.
Keywords: A1. Nanostructures A3. Metalorganic chemical vapor deposition B1. Nitrides B2. Semiconducting indium compounds
1. Introduction The revision of the InN band gap energy to 0.7 eV [1,2], has allowed for many additional applications of optoelectronic devices working in the infrared range, including high efficiency solar cells, etc. In recent years, the benefits of multiple exciton generation (MEG) have been demonstrated in colloidal suspensions of PbSe, PbS, PbTe, CdSe and GaAs quantum dots (QDs) by achieving high-efficiency nanostructured solar cells [3,4]. In order to increase the absorption of nanostructured solar cells, it is necessary to increase the volume of the nanostructure. In other words, the growth technique for high density dots [5,6] or for the stacking of dots layers plays an important role in increasing the density of the dot volume. The InN dots are formed through self-assembly in the highly mismatched InN/GaN system (i.e., with a lattice mismatch of 11%) by means of the strain-induced two-dimensional (2D) to threedimensional (3D) reorganization of the InN layer, similar to a Stranski–Krastanov (SK) growth mode [7]. Since the formation of InN dots on a GaN surface is strain driven, multiple dots layers can introduce defects and deteriorate the optical properties. These defects act as carrier trapping centers and limit the light-emitting and photovoltaic efficiency of optoelectronic devices. In addition, the spontaneous polarization and strain-induced piezoelectric polarization fields play an important role in carrier recombination in nitride materials [8]. Thus, it is essential that a systemic investigation of the strain distribution in InN dots is carried out for high performance
n
Corresponding author. E-mail address: [email protected] (W.-C. Ke).
0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.10.017
optoelectronic devices. To this end, this paper presents a systematic study of the effect of cap thickness on the structural and optical properties of single layer of InN/GaN dots structures.
2. Experimental details The InN/GaN nanodot structures were grown on 1 mm thick GaN/(0001) sapphire substrates by low pressure metal organic chemical vapor deposition (MOCVD). The InN dots were grown by pulsed mode method at a temperature of 650 1C to achieve higher density InN dots [5]. The gas flow sequence for the PM method basically consists of four steps: 20 s TMInþNH3 growth step, 20 s NH3 source step with 10 s purge steps in between. During the growth step, the mole flow rates of TMIn and NH3 are 1.53 101 and 4.46 105 mmole/min, respectively. The InN dots layer is then covered by a series of GaN capping layers with a thickness of 10 nm and 20 nm at 650 1C, respectively. The PL measurements were performed using the 488-nm line of an argon-ion laser as the excitation source. The PL signals were dispersed by ARC Pro 500 monochromator and were detected by a cooled InGaAs photodiode with a cutoff wavelength of 2.05 mm. 3. Results and discussion Fig. 1 shows the atomic force microscopy (AFM) images of the uncapped InN dots and the single-layer InN dots. The dot density, height and diameter were 1.5 109 cm 3, 20.2 nm and 96.5 nm, respectively, for the uncapped InN dots. The GaN capping morphology plays an important role in achieving a well stacked
354
W.-C. Ke et al. / Journal of Crystal Growth 362 (2013) 353–356
1 fim
Height (nm)
100 10 nm 20 nm 50
0 -100
-500
50 Width (nm)
10
0
Fig. 1. AFM images of (a) un-capped InN dots, single-layer InN dots covered by (b) a 10-nm thick and (c) a 20-nm thick GaN capping layer. (d) The AFM line scan profiles of the GaN capping layers with various thicknesses.
multilayered structure because the flat surface of the GaN capping layer is helpful for the InN dots growth in the next layer. The AFM line scan profiles of the GaN capping layers with various thicknesses are shown in Fig. 1(d). It shows that the surface morphology of the 10-nm thick GaN capping layer shows a truncated pyramid shape and changes to a dome shape when the thickness of the capping layer increases to 20-nm. The transmission electron microscopy (TEM) images of singlelayer InN dots with a 20 nm capping layer are shown in Fig. 2. The TEM image clearly shows that the threading dislocations were terminated in the truncated shape of the InN dots. A very thin InN wetting layer is also clearly evident at the interface between the InN dots and the GaN buffer layer. The aspect ratio of the uncapped InN dots changes from 1/5.3 (19.6 nm/103 nm) to 1/5.6 (19.7 nm/110 nm) in the 20 nm thick GaN capping sample. The change in morphology and the size changes of the QDs during capping for many material systems have been widely reported [9–11], and are usually explained as being the result of intermixing or the atomic exchanges process. In this study, the decreased aspect ratio was believed due to a strain in the InN dots after the GaN capping process. In order to study the strain level in the InN dots, we performed an x-ray diffraction (XRD) measurement. Fig. 3 shows the (002) o/2y scans of uncapped InN dots, and of the single-layer buried InN dots with various thicknesses of the GaN capping layer. The XRD peak position of InN dots were 31.2641, 31.3181 and 31.4231 for the uncapped InN dots, single-layer InN dots covered by a 10-nm thick and a 20-nm thick GaN capping layer. Combining Bragg’s law with the expression for the inter-planar spacing in hexagonal structures, the lattice constant in the growth direction is found to be c ¼l l/(2sinyB), for any allowed (00l) reflection of x-rays of the wavelength l. For this calculation, the lattice ˚ and the lattice constant (c axis) of the GaN buffer layer is 5.185 A, ˚ The values constant (c axis) of the uncapped InN dots is 5.717 A. of the lattice constants for the case of stain-free InN are estimated to be in the range of c¼5.703 A˚ [12]. The larger lattice constant, c, is attributed to the uniaxial tensile stress in the growth direction of the uncapped InN dots. In contrast, the lattice constant is
20 nm Fig. 2. TEM image of a single-layer of InN dots covered by a 20-nm thick GaN capping layer.
5.689 A˚ for InN dots covered by a 20-nm thick GaN capping layer. The experimental results indicated that the residual strain relaxation in the InN dots was dependent on the thickness of GaN capping layer. In Fig. 3(b), it is shown that the lattice constant c decreased from 5.717 to 5.689 A˚ when the thickness of GaN capping layer increased from 0 to 20 nm. In order to study the residual strain relaxation in the InN dots with different thickness of GaN capping layer. The strain (ezz) along z axis was estimated from the relation,
ezz ¼
CC 0 100% C0
ð1Þ
where C denotes the lattice constant of the InN dots and C0 is strainfree InN films. In Fig.3 (b), the residual strain is reduced from 0.245% to 0.07% as the GaN cap thickness increased from 0 to 10 nm. However, the residual strain of InN dots reduced to 0.245% for 20-nm thick GaN capping layer. Fig. 4 shows the 17 K-PL spectra of the un-capped InN dots, as well as the single-layer InN dots with various GaN cap thickness.
W.-C. Ke et al. / Journal of Crystal Growth 362 (2013) 353–356
355
0.3 Uncapped 10 nm 20 nm
5.72
5.71
0.1
5.70
0.0
Strain (%)
XRD intensity (a.u.)
Lattice constant (A)
0.2
-0.1
5.69
-0.2 5.68 31.0
31.5 32.0 θ-2θ (arcsec)
34
-0.3
35
0
5 10 15 Cap thickness (nm)
20
Fig. 3. (a) The (002) XRD measurement of un-capped and single-layer InN dots with various GaN cap thickness. (b) The relationship between the lattice constant/strain and GaN cap thickness.
Normalized PL intensity (a.u.)
Uncapped
105 0.82
20 nm
95 90
0.80
85
FWHM (meV)
10 nm
Peak energy (eV)
100
80 0.78 75
0.7
0.8 0.9 Energy (eV)
0
5 10 15 20 Cap thickness (nm)
Fig. 4. (a) The 17 K PL spectra of un-capped and single-layer InN dots with various GaN cap thickness. (b) The relationship between PL peak energy/FWHM and GaN cap thickness.
It should be noted that the PL intensity of the 20-nm thick GaN capping layer sample was 2.5 times higher than that of the uncapped sample. The PL spectrum of the un-capped InN dots shows peak energy at 0.807 eV, with a FWHM of 104 meV. The emission energy of the un-capped InN dots is higher than the reported bandgap energy of 0.69 eV [13], and indicates a strong Burstein–Moss (BM) effect due to the presence of a high electron concentration in the InN dots. Cimalla et al. [14] reported that the high surface electron concentration generated by the high surface density of states was due to the high ratio of surface area to the volume of dots. It is well known that the thermal instability due to the low InN dissociation temperature and high equilibrium N2 vapor pressure over the InN implies nitrogen desorption from the dots. Thus, we believe that GaN capping avoids InN decomposition and decreases the surface defect density. In Fig. 4(b), the experimental results also indicated that the peak energy and the FWHM of the PL
spectra of the single-layer InN dots decreases when the GaN capping thickness increases from 10-nm to 20-nm.
4. Conclusions In summary, we have investigated the structural and PL properties of InN/GaN dots. By varying the GaN cap thickness, it is possible to control the strain level. A redshift was observed in the photoluminescence (PL) peak energy of the InN dots with increasing GaN capping thickness. The redshift of the PL peak energy of the 20-nm thick GaN capping layer sample was believed to be due to the GaN capping avoids InN decomposition and decreases the surface defect density. The GaN cap thickness is not only very important for the dots strain but also for the emission properties of the InN dots.
356
W.-C. Ke et al. / Journal of Crystal Growth 362 (2013) 353–356
Acknowledgments The authors gratefully acknowledge the financial support from the National Science Council of Taiwan, ROC under Contract no. NSC-98-2112-M155-001-MY3.
References [1] J. Wu, W. Walukiewicz, K.M. Yu, J.W. Ager III, E.E. Haller, H. Lu, W.J. Schaff, Y. Saito, Y. Nanishi, Applied Physics Letters 80 (2002) 3967. [2] T. Matsuoka, H. Okamoto, M. Nakao, H. Harima, E. Kurimoto, Applied Physics Letters 81 (2002) 1246. [3] O. Jani, I. Ferguson, C. Honsberg, S. Kurtz, Applied Physics Letters 91 (2007) 132117. [4] C.J. Neufeld, N.G. Toledo, S.C. Cruz, M. Iza, S.P. DenBaars, U.K. Mishra, Applied Physics Letters 93 (2007) 143502.
[5] W.C. Ke, S.J. Lee, C.Y. Kao, W.K. Chen, W.C. Chou, M.C. Lee, W.H. Chang, W.J. Lin, Y.C. Cheng, T.C. Lee, J.C. Lin, Journal of Crystal Growth 312 (2010) 3209. [6] S. Ruffenach, O. Briot, M. Moret, B. Gil, Applied Physics Letters 90 (2007) 153102. [7] Y.F. Ng, Y.G. Cao, M.H. Xie, X.L. Xie, X.L. Wang, S.Y. Tong, Applied Physics Letters 81 (2002) 3960. [8] E. Sarigiannidou, E. Monroy, B. Daudin, J.L. Rouvie re, A.D. Andreev, Applied Physics Letters 87 (2005) 203112. ¨ [9] Z. Zhong, J. Stangl, F. Schaffler, G. Bauer, Applied Physics Letters 83 (2003) 3695. [10] A. Hesse, J. Stangl, V. Holy, T. Roch, G. Bauer, O.G. Schmidt, U. Denker, B. Struth, Physical Review B 66 (2002) 085321. [11] J.G. Lozano, A.M. Sa´nchez, R. Garcı´a, D. Gonzalez, O. Briot, S. Ruffenach, Applied Physics Letters 88 (2006) 151913. [12] I. Vurgaftman, J.R. Meyer, Journal of Applied Physics 94 (2003) 3675. [13] F. Chen, A.N. Cartwright, H. Lu, W.J. Schaff, Journal of Crystal Growth 269 (2004) 10. [14] V. Cimalla, V. Lebedev, F.M. Morales, R. Goldhahn, O. Ambacher, Applied Physics Letters 89 (2006) 172109.