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Si doping of metal-organic chemical vapor deposition grown gallium nitride using ditertiarybutyl silane metal-organic source
ARTICLE IN PRESS
Journal of Crystal Growth 298 (2007) 239–242 www.elsevier.com/locate/jcrysgro
Si doping of metal-organic chemical vapor deposition grown gallium nitride using ditertiarybutyl silane metal-organic source W.K. Fong, K.K. Leung, C. Surya Photonics Research Centre, Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hong Kong SAR, China Available online 16 November 2006
Abstract Liquid Si ditertiarybutyl silane (DTBSi) metal-organic source was used as the Si dopant source for the growth of n-type GaN by metalorganic chemical vapor deposition (MOCVD) for the first time to replace the conventional gaseous Si sources like silane SiH4 [K. Pakula, R. Bozek, J.M. Baranowski, J. Jasinski, Z. Liliental-Weber, J. Crystal Growth 267 (2004) 1] and disilane Si2H6 [L.B. Rowland, K. Doverspike, D.K. Gaskill, Appl. Phys. Lett. 66 (1995) 1495]. Electrical, structural, optical, and surface properties of the samples doped by DTBSi as well as an undoped control sample are determined by Hall, high resolution X-ray diffraction (HRXRD), photoluminescence (PL), and atomic force microscopy (AFM) measurements respectively. A constant doping efficiency for GaN is obtained with carrier concentration up to 1018 cm3. The typical HRXRD full-width at half-maximum values of symmetric (0 0 2) and asymmetric (1 0 2) planes are 284 and 482 arcsec, respectively. The near band edge PL intensity is found to be increased proportional to the doping concentration. Dark spot density is also determined from AFM measurement. r 2006 Elsevier B.V. All rights reserved. PACS: 68.55.Ln; 78.55.Cr; 73.61.Ey Keywords: A1. Doping; A3. Metal-organic chemical vapor deposition; B1. Nitrides
1. Introduction The fabrication of GaN-based devices requires a controllable and reproducible process for the doping of the materials. The most common n-type dopant for IIInitride materials is silicon. For metal-organic chemical vapor deposition (MOCVD) growth of III-nitride materials, hazardous gaseous Si sources such as silane SiH4 [1] and disilane Si2H6 [2] are commonly used. Liquid Sidoping source, liquid Si ditertiarybutyl silane (DTBSi) of chemical form t-(C4H9)2SiH2 is used as an alternative to the gaseous silane and disilane sources. The advantages of using DTBSi are the lower toxicity and the liquid phase of the source at room temperature. The use of liquid source growth process can simplify the MOCVD gas handling process and cost. To the best of our knowledge, the only report on the use of DTBSi as a dopant source is on the Corresponding author. Tel.: +852 27666198; fax: +852 23628439.
E-mail addresses: [email protected] (W.K. Fong), [email protected] (C. Surya). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.10.024
growth of n-type InP and GaAs by MOCVD [3]. There is no report on the growth of n-type GaN using DTBSi as dopant source. Therefore, this article represents the first report on the use of DTBSi as a doping source for GaN by MOCVD. The doping incorporation as a function of the growth conditions is investigated by Hall-effect measurement. In addition, the optical, structural, and surface properties of the n-type GaN layers are being reported. 2. Experimental procedure Gallium nitride thin films were grown on (0 0 0 1) sapphire substrates with a Thomas Swan close-coupledshowerhead MOCVD reactor using trimethylgallium (TMGa) and ammonia (NH3) as the source precursors for Ga and N. In this MOCVD system, in situ reflectance, with a probe wavelength of 635 nm, from the growth surface was measured to control and monitor the effects of the growth parameters on the film quality as well as the growth rate. First, thermal cleaning of the sapphire substrates was done by heating the wafers to 1085 1C for
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10 min under hydrogen ambient. A 30 nm thick nucleation layer was deposited at 520 1C, followed by 1.5 mm thick undoped GaN epilayer at an elevated temperature of 1035 1C. The reactor pressure was kept at 200 Torr during the epitaxial growth. Typical two-step growth methodology was used by varying the V/III ratio in the initial growth stage of the undoped GaN layer [4]. The V/III ratio changed from 1175 to 1470 during the first 20 min of growth. Then, all epilayers were grown with a constant V/III ratio of 1350. Finally, 1.2 mm thick Si-doped GaN layer was deposited using DTBSi as a doping source. The boiling and melting points of DTBSi, (C4H9)2SiH2, are 128 and 38 1C, respectively. The vapor pressure P of DTBSi as a function of temperature T is given by log P (Torr) ¼ 8.831–2321/T (K). For a growth rate of about 2 mm/h, the DTBSi source bath temperature was set to 10 1C to cover the full doping range with electron concentration varying from 2 1017 to 5 1018 cm3. A 2.7 mm undoped GaN sample was also grown as a control to match the total thickness of other samples consisting of both undoped and Si-doped GaN. The electrical, optical, structural, and surface properties of the samples were characterized by Hall-effect, photoluminescence (PL), high resolution X-ray diffraction (HRXRD), and atomic force microscopy (AFM), respectively.
Typical in situ reflectance plot obtained from sample grown by two-step growth methodology [4] is shown in Fig. 1. Reflectance intensity reaches the minimum after 725 s of growth of the undoped GaN. This indicates a rough surface which is attributed to larger islands in the early stage of growth. Larger island or lower island density is needed for lower crystalline defect epilayer when the islands coalesce
[5]. The initially isolated hexagonal islands coalesce in stage IV, corresponding to 1.5 mm thick undoped GaN. A long transition time is needed to transform from the 3D to 2D growth due to the rough surface of the initial growth stage. Si-doped GaN was grown when this undoped GaN template achieved the quasi-2D growth. The large reflectance oscillation amplitude and no amplitude damping indicate quasi-2D growth mode maintained during the growth of 1.2 mm Si-doped GaN and there was no significant thickness non-uniformity. The Si-doping characteristics for GaN using DTBSi were studied by varying the flow rate ratio of DTBSi to TMGa, which shows that the carrier concentration can be varied from 1 1017 to 5 1018 cm3 by adjusting the flow rate of DTBSi with a slope of unity as shown in Fig. 2. The roomtemperature carrier mobility was also determined from Bio-Rad HL5500 Hall-effect system as a function of DTBSi/TMGa flow rate as shown in Fig. 3. Roomtemperature electron mobility of 554 cm2 V1 s1 was obtained for electron concentrations in the low 1017 cm3 range, indicating excellent crystal quality when using DTBSi as a Si doping source. These values of electron mobility are equal to or greater than those obtained by other research groups using siliane [4] and disilane [2] Si doping source. Therefore, using DTBSi as a Si-doping source can produce films of equal or better quality with respect to electronic behavior. In addition, Hall mobility and carrier concentration can be reliably repeated using DTBSi as a Si-doping source. Run-to-run Hall mobility fluctuates between 466 and 467 cm2 V1 s1 while carrier concentration fluctuates between 5.60 and 5.97 1017 cm3 under the same growth conditions. AFM studies on the as-grown film surface were also conducted. All samples exhibit smooth surface with evenly spaced and aligned steps indicative of layer-by-layer growth. This is consistent with the in situ reflectance result
Fig. 1. Typical in situ reflectance spectrum of MOCVD grown GaN. I: Start of low-temperature buffer layer growth, II: start of high-temperature undoped GaN growth, III: start of the second stage of the undoped GaN growth, and IV: start of the Si-doped GaN growth.
Fig. 2. Carrier concentration of Si-doped GaN as a function of DTBSi/ TMGa flow rate ratio at 300 K.
3. Results and discussion
ARTICLE IN PRESS W.K. Fong et al. / Journal of Crystal Growth 298 (2007) 239–242
Fig. 3. Carrier mobility of Si-doped GaN as a function of DTBSi/TMGa flow rate ratio at 300 K.
because there are no reflectance amplitude damping at the latter stage of growth. Well-aligned steps favor the high carrier mobility but anisotropic step structure indicates disordered step growth configurations, leading to low carrier mobility [6]. Therefore, well-aligned surface steps of our samples again reveal excellent electronic behavior of the films. Fig. 4 shows the typical AFM image of the sample of carrier concentration of 5.97 1017 cm3. Two types of dark spots are observed, namely large and small dark spot [7]. The dark spots of diameter approximately 55 nm are terminated at the growth step. This type of dark spots has either pure screw or mixed screw-edge character of threading dislocations. The small dark spots of diameter approximately 30 nm correspond to pure edge dislocation and do not terminate at the step. It is not easy to discriminate large spot from the small one in the AFM image, the dark spot density is counted regardless of its size and type. The dark spots on the image correspond to step terminations, with a density of 2.8 108 cm2. This value is of the same order of magnitude or even lower density than those obtained from samples grown using silane [6]. The Si-doped film quality was also determined by HRXRD. The film quality is good as revealed by the FWHM values. Typical FWHM of symmetric (0 0 2) and asymmetric (1 0 2) rocking curves are 288 and 475 arcsec respectively for the control sample, 2.7 mm undoped GaN. All the Si-doped samples exhibit similar FWHM values indicating that there is no significant material deterioration due to Si doping up to a carrier concentration of 5 1018 cm3. The influence of Si doping using DTBSi on the optical properties has also been studied and the results are summarized in Table 1. The room-temperature PL intensity increases with increasing doping concentration, attributed to the increase of carriers, while the PL peaks are broadened. The intensity ratio of yellow luminescence to
241
Fig. 4. AFM image of GaN doped by DTBSi of carrier concentration 5.97 1017 cm3.
Table 1 Room-temperature GaN PL normalized intensity and corresponding FWHM, and intensity ratio of YL to band edge at different DTBSi/ TMGa flow rate ratio. Room-temperature carrier concentrations are also included GaN PL normalized intensity
PL FWHM (nm)
YL/ UVGaN
[Si]/[Ga] ( 106)
Room-temperature carrier concentration ( 1017 cm3)
0.23 0.45 0.49 0.58 1
5.51 5.90 6.02 6.50 7.90
0.324 0.295 0.288 0.236 0.175
1.07 4.14 4.96 10.7 50.5
1.35 4.17 5.6 9.43 40
the band edge emission decreases with increasing doping level. It is generally accepted that YL are caused by gallium vacancies. Due to the substitution of Si dopants in gallium lattice sites, an increase in the Si-doping concentration can reduce the number of gallium vacancies. Similar phenomena are also been observed in previous reports [8,9].
4. Conclusions DTBSi has been investigated as a possible Si-doping source for the growth of GaN by MOCVD for the first time to replace the commonly used Si sources such as silane and disilane. A constant doping efficiency is obtained with carrier concentration up to 5 1018 cm3. Hall mobility, HRXRD, and PL measurements show that using DTBSi as a Si-doping source can produce GaN films of equal or better quality. AFM studies show that layer-by-layer growth mode can be attained with carrier concentration up to 1018 cm3. Dark spot density as low as 2.8 108 cm2 can also be obtained.
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Acknowledgments This work was supported in part by grants from the Research Grants Council of Hong Kong (Project no. PolyU 5134/02E and N_PolyU518/01) Further support is provided by a University Research Grant of the Hong Kong Polytechnic University. References [1] K. Pakula, R. Bozek, J.M. Baranowski, J. Jasinski, Z. Liliental-Weber, J. Crystal Growth 267 (2004) 1. [2] L.B. Rowland, K. Doverspike, D.K. Gaskill, Appl. Phys. Lett. 66 (1995) 1495.
[3] S. Leu, H. Protzmann, F. Ho¨hnsdorf, W. Stolz, J. Steinkirchner, E. Hufgard, J. Crystal Growth 195 (1998) 91. [4] S. Kim, J. Oh, J. Kang, D. Kim, J. Won, J.W. Kim, H.K. Cho, J. Crystal Growth 262 (2004) 7. [5] X.H. Wu, P. Fini, E.J. Tarsa, B. Heying, S. Keller, U.K. Mishra, S.P. DenBaars, J. Crystal Growth 189–190 (1998) 231. [6] N. Nakada, M. Mori, H. Ishikawa, T. Egawa, T. Jimbo, Jpn. J. Appl. Phys. 42 (2003) 2573. [7] P.J. Hansen, Y.E. Strausser, A.N. Erickson, E.J. Tarsa, P. Kozodoy, E.G. Brazel, J.P. Ibbetson, U. Mishra, V. Narayanamurti, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 72 (1998) 2247. [8] S.T. Li, C.L. Mo, L. Wang, C.B. Xiong, X.X. Peng, F.Y. Jiang, Z.B. Deng, D.W. Gong, J. Lumin 93 (2001) 321. [9] W. Van der Stricht, I. Moerman, P. Demeester, J.A. Crawley, E.J. Thrush, MRS Internet J. Nitride Semicond. Res. 1 (1996) 3.
Journal of Crystal Growth 298 (2007) 239–242 www.elsevier.com/locate/jcrysgro
Si doping of metal-organic chemical vapor deposition grown gallium nitride using ditertiarybutyl silane metal-organic source W.K. Fong, K.K. Leung, C. Surya Photonics Research Centre, Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hong Kong SAR, China Available online 16 November 2006
Abstract Liquid Si ditertiarybutyl silane (DTBSi) metal-organic source was used as the Si dopant source for the growth of n-type GaN by metalorganic chemical vapor deposition (MOCVD) for the first time to replace the conventional gaseous Si sources like silane SiH4 [K. Pakula, R. Bozek, J.M. Baranowski, J. Jasinski, Z. Liliental-Weber, J. Crystal Growth 267 (2004) 1] and disilane Si2H6 [L.B. Rowland, K. Doverspike, D.K. Gaskill, Appl. Phys. Lett. 66 (1995) 1495]. Electrical, structural, optical, and surface properties of the samples doped by DTBSi as well as an undoped control sample are determined by Hall, high resolution X-ray diffraction (HRXRD), photoluminescence (PL), and atomic force microscopy (AFM) measurements respectively. A constant doping efficiency for GaN is obtained with carrier concentration up to 1018 cm3. The typical HRXRD full-width at half-maximum values of symmetric (0 0 2) and asymmetric (1 0 2) planes are 284 and 482 arcsec, respectively. The near band edge PL intensity is found to be increased proportional to the doping concentration. Dark spot density is also determined from AFM measurement. r 2006 Elsevier B.V. All rights reserved. PACS: 68.55.Ln; 78.55.Cr; 73.61.Ey Keywords: A1. Doping; A3. Metal-organic chemical vapor deposition; B1. Nitrides
1. Introduction The fabrication of GaN-based devices requires a controllable and reproducible process for the doping of the materials. The most common n-type dopant for IIInitride materials is silicon. For metal-organic chemical vapor deposition (MOCVD) growth of III-nitride materials, hazardous gaseous Si sources such as silane SiH4 [1] and disilane Si2H6 [2] are commonly used. Liquid Sidoping source, liquid Si ditertiarybutyl silane (DTBSi) of chemical form t-(C4H9)2SiH2 is used as an alternative to the gaseous silane and disilane sources. The advantages of using DTBSi are the lower toxicity and the liquid phase of the source at room temperature. The use of liquid source growth process can simplify the MOCVD gas handling process and cost. To the best of our knowledge, the only report on the use of DTBSi as a dopant source is on the Corresponding author. Tel.: +852 27666198; fax: +852 23628439.
E-mail addresses: [email protected] (W.K. Fong), [email protected] (C. Surya). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.10.024
growth of n-type InP and GaAs by MOCVD [3]. There is no report on the growth of n-type GaN using DTBSi as dopant source. Therefore, this article represents the first report on the use of DTBSi as a doping source for GaN by MOCVD. The doping incorporation as a function of the growth conditions is investigated by Hall-effect measurement. In addition, the optical, structural, and surface properties of the n-type GaN layers are being reported. 2. Experimental procedure Gallium nitride thin films were grown on (0 0 0 1) sapphire substrates with a Thomas Swan close-coupledshowerhead MOCVD reactor using trimethylgallium (TMGa) and ammonia (NH3) as the source precursors for Ga and N. In this MOCVD system, in situ reflectance, with a probe wavelength of 635 nm, from the growth surface was measured to control and monitor the effects of the growth parameters on the film quality as well as the growth rate. First, thermal cleaning of the sapphire substrates was done by heating the wafers to 1085 1C for
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10 min under hydrogen ambient. A 30 nm thick nucleation layer was deposited at 520 1C, followed by 1.5 mm thick undoped GaN epilayer at an elevated temperature of 1035 1C. The reactor pressure was kept at 200 Torr during the epitaxial growth. Typical two-step growth methodology was used by varying the V/III ratio in the initial growth stage of the undoped GaN layer [4]. The V/III ratio changed from 1175 to 1470 during the first 20 min of growth. Then, all epilayers were grown with a constant V/III ratio of 1350. Finally, 1.2 mm thick Si-doped GaN layer was deposited using DTBSi as a doping source. The boiling and melting points of DTBSi, (C4H9)2SiH2, are 128 and 38 1C, respectively. The vapor pressure P of DTBSi as a function of temperature T is given by log P (Torr) ¼ 8.831–2321/T (K). For a growth rate of about 2 mm/h, the DTBSi source bath temperature was set to 10 1C to cover the full doping range with electron concentration varying from 2 1017 to 5 1018 cm3. A 2.7 mm undoped GaN sample was also grown as a control to match the total thickness of other samples consisting of both undoped and Si-doped GaN. The electrical, optical, structural, and surface properties of the samples were characterized by Hall-effect, photoluminescence (PL), high resolution X-ray diffraction (HRXRD), and atomic force microscopy (AFM), respectively.
Typical in situ reflectance plot obtained from sample grown by two-step growth methodology [4] is shown in Fig. 1. Reflectance intensity reaches the minimum after 725 s of growth of the undoped GaN. This indicates a rough surface which is attributed to larger islands in the early stage of growth. Larger island or lower island density is needed for lower crystalline defect epilayer when the islands coalesce
[5]. The initially isolated hexagonal islands coalesce in stage IV, corresponding to 1.5 mm thick undoped GaN. A long transition time is needed to transform from the 3D to 2D growth due to the rough surface of the initial growth stage. Si-doped GaN was grown when this undoped GaN template achieved the quasi-2D growth. The large reflectance oscillation amplitude and no amplitude damping indicate quasi-2D growth mode maintained during the growth of 1.2 mm Si-doped GaN and there was no significant thickness non-uniformity. The Si-doping characteristics for GaN using DTBSi were studied by varying the flow rate ratio of DTBSi to TMGa, which shows that the carrier concentration can be varied from 1 1017 to 5 1018 cm3 by adjusting the flow rate of DTBSi with a slope of unity as shown in Fig. 2. The roomtemperature carrier mobility was also determined from Bio-Rad HL5500 Hall-effect system as a function of DTBSi/TMGa flow rate as shown in Fig. 3. Roomtemperature electron mobility of 554 cm2 V1 s1 was obtained for electron concentrations in the low 1017 cm3 range, indicating excellent crystal quality when using DTBSi as a Si doping source. These values of electron mobility are equal to or greater than those obtained by other research groups using siliane [4] and disilane [2] Si doping source. Therefore, using DTBSi as a Si-doping source can produce films of equal or better quality with respect to electronic behavior. In addition, Hall mobility and carrier concentration can be reliably repeated using DTBSi as a Si-doping source. Run-to-run Hall mobility fluctuates between 466 and 467 cm2 V1 s1 while carrier concentration fluctuates between 5.60 and 5.97 1017 cm3 under the same growth conditions. AFM studies on the as-grown film surface were also conducted. All samples exhibit smooth surface with evenly spaced and aligned steps indicative of layer-by-layer growth. This is consistent with the in situ reflectance result
Fig. 1. Typical in situ reflectance spectrum of MOCVD grown GaN. I: Start of low-temperature buffer layer growth, II: start of high-temperature undoped GaN growth, III: start of the second stage of the undoped GaN growth, and IV: start of the Si-doped GaN growth.
Fig. 2. Carrier concentration of Si-doped GaN as a function of DTBSi/ TMGa flow rate ratio at 300 K.
3. Results and discussion
ARTICLE IN PRESS W.K. Fong et al. / Journal of Crystal Growth 298 (2007) 239–242
Fig. 3. Carrier mobility of Si-doped GaN as a function of DTBSi/TMGa flow rate ratio at 300 K.
because there are no reflectance amplitude damping at the latter stage of growth. Well-aligned steps favor the high carrier mobility but anisotropic step structure indicates disordered step growth configurations, leading to low carrier mobility [6]. Therefore, well-aligned surface steps of our samples again reveal excellent electronic behavior of the films. Fig. 4 shows the typical AFM image of the sample of carrier concentration of 5.97 1017 cm3. Two types of dark spots are observed, namely large and small dark spot [7]. The dark spots of diameter approximately 55 nm are terminated at the growth step. This type of dark spots has either pure screw or mixed screw-edge character of threading dislocations. The small dark spots of diameter approximately 30 nm correspond to pure edge dislocation and do not terminate at the step. It is not easy to discriminate large spot from the small one in the AFM image, the dark spot density is counted regardless of its size and type. The dark spots on the image correspond to step terminations, with a density of 2.8 108 cm2. This value is of the same order of magnitude or even lower density than those obtained from samples grown using silane [6]. The Si-doped film quality was also determined by HRXRD. The film quality is good as revealed by the FWHM values. Typical FWHM of symmetric (0 0 2) and asymmetric (1 0 2) rocking curves are 288 and 475 arcsec respectively for the control sample, 2.7 mm undoped GaN. All the Si-doped samples exhibit similar FWHM values indicating that there is no significant material deterioration due to Si doping up to a carrier concentration of 5 1018 cm3. The influence of Si doping using DTBSi on the optical properties has also been studied and the results are summarized in Table 1. The room-temperature PL intensity increases with increasing doping concentration, attributed to the increase of carriers, while the PL peaks are broadened. The intensity ratio of yellow luminescence to
241
Fig. 4. AFM image of GaN doped by DTBSi of carrier concentration 5.97 1017 cm3.
Table 1 Room-temperature GaN PL normalized intensity and corresponding FWHM, and intensity ratio of YL to band edge at different DTBSi/ TMGa flow rate ratio. Room-temperature carrier concentrations are also included GaN PL normalized intensity
PL FWHM (nm)
YL/ UVGaN
[Si]/[Ga] ( 106)
Room-temperature carrier concentration ( 1017 cm3)
0.23 0.45 0.49 0.58 1
5.51 5.90 6.02 6.50 7.90
0.324 0.295 0.288 0.236 0.175
1.07 4.14 4.96 10.7 50.5
1.35 4.17 5.6 9.43 40
the band edge emission decreases with increasing doping level. It is generally accepted that YL are caused by gallium vacancies. Due to the substitution of Si dopants in gallium lattice sites, an increase in the Si-doping concentration can reduce the number of gallium vacancies. Similar phenomena are also been observed in previous reports [8,9].
4. Conclusions DTBSi has been investigated as a possible Si-doping source for the growth of GaN by MOCVD for the first time to replace the commonly used Si sources such as silane and disilane. A constant doping efficiency is obtained with carrier concentration up to 5 1018 cm3. Hall mobility, HRXRD, and PL measurements show that using DTBSi as a Si-doping source can produce GaN films of equal or better quality. AFM studies show that layer-by-layer growth mode can be attained with carrier concentration up to 1018 cm3. Dark spot density as low as 2.8 108 cm2 can also be obtained.
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Acknowledgments This work was supported in part by grants from the Research Grants Council of Hong Kong (Project no. PolyU 5134/02E and N_PolyU518/01) Further support is provided by a University Research Grant of the Hong Kong Polytechnic University. References [1] K. Pakula, R. Bozek, J.M. Baranowski, J. Jasinski, Z. Liliental-Weber, J. Crystal Growth 267 (2004) 1. [2] L.B. Rowland, K. Doverspike, D.K. Gaskill, Appl. Phys. Lett. 66 (1995) 1495.
[3] S. Leu, H. Protzmann, F. Ho¨hnsdorf, W. Stolz, J. Steinkirchner, E. Hufgard, J. Crystal Growth 195 (1998) 91. [4] S. Kim, J. Oh, J. Kang, D. Kim, J. Won, J.W. Kim, H.K. Cho, J. Crystal Growth 262 (2004) 7. [5] X.H. Wu, P. Fini, E.J. Tarsa, B. Heying, S. Keller, U.K. Mishra, S.P. DenBaars, J. Crystal Growth 189–190 (1998) 231. [6] N. Nakada, M. Mori, H. Ishikawa, T. Egawa, T. Jimbo, Jpn. J. Appl. Phys. 42 (2003) 2573. [7] P.J. Hansen, Y.E. Strausser, A.N. Erickson, E.J. Tarsa, P. Kozodoy, E.G. Brazel, J.P. Ibbetson, U. Mishra, V. Narayanamurti, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 72 (1998) 2247. [8] S.T. Li, C.L. Mo, L. Wang, C.B. Xiong, X.X. Peng, F.Y. Jiang, Z.B. Deng, D.W. Gong, J. Lumin 93 (2001) 321. [9] W. Van der Stricht, I. Moerman, P. Demeester, J.A. Crawley, E.J. Thrush, MRS Internet J. Nitride Semicond. Res. 1 (1996) 3.