- Email: [email protected]
GaN MQW grown by molecular beam epitaxy
Materials Science and Engineering B93 (2002) 131 /134 www.elsevier.com/locate/mseb
Structural and optical characterization of thick InGaN layers and InGaN/GaN MQW grown by molecular beam epitaxy F.B. Naranjo a,*, S. Ferna´ndez a, M.A. Sa´nchez-Garcı´a a, F. Calle a, E. Calleja a, A. Trampert b, K.H. Ploog b a
ISOM and Departamento de Ingenierı´a Electro´nica, ETSI Telecomunicacio´n, Universidad Polite´cnica de Madrid, Ciudad Universitaria, 28040 Madrid, Spain b Paul-Drude-Institut fuer Festkoerperelektronik, Hausvogteiplatz 5-7, D-10117 Berlin, Germany
Abstract Thick Inx Ga1 x N (0.20B x B 0.27) layers and InGaN/GaN multiple quantum wells (MQWs) are grown by plasma-assisted molecular beam epitaxy on GaN/Al2O3 templates. The strain and In-content is estimated from high-resolution X-ray diffraction, showing that the bulk samples are not fully relaxed. A bowing parameter of 3.6 eV is obtained from absorption measurements of Inx Ga1 x N layers. Strong In-dependent excitonic localization is observed in these bulk layers, leading to an increase in the absorption band edge with the In content. Regarding the MQWs structures, high-resolution transmission electron microscopy reveals an increase in the interface roughness for high In content. The dominant PL emission of the MQWs shows a red-shift when increasing the well thickness for a given In-content, due to internal piezoelectric field. The excitonic localization is studied and compared between thick layers and MQWs structures. # 2002 Elsevier Science B.V. All rights reserved. Keywords: InGaN; Molecular beam epitaxy; Transmission electron microscopy; Localization; Strain
1. Introduction InGaN-based light emitting diodes (LED) and laser diodes (LD) grown on sapphire substrates by metal / organic chemical vapor deposition (MOCVD) techniques have been demonstrated in the recent years [1]. However, there are many aspects of the growth of this material that are still not well understood. In particular, the thermodynamical unstability of the GaN /InN alloy has been a subject of many reports [2]. This effect, and the segregation of In at the surface during growth, are some reasons that make the growth conditions of this material so critical [3]. Molecular beam epitaxy (MBE) has attracted much attention, due to the in situ control of the growth using reflection high energy electron diffraction (RHEED) and the possibility of achieving abrupt interfaces. The lower temperature decomposition
of InN (630 8C) compared with GaN (850 8C) [4] gives rise to the need of lower growth temperatures for InGaN layers (around 580 8C). In this aspect, MBE allows the use of these low temperatures. On the other hand, the quality of the GaN grown by MBE on sapphire is generally not as good as MOCVD-grown samples. With the use of MOCVD-GaN templates for MBE (homoepitaxy), this problem could be avoided [5]. The study of InGaN-based multiple quantum wells (MQW) is necessary to improve the performance of the devices. In this paper we report on the growth and characterization of bulk InGaN layers and InGaN/GaN MQW by MBE on GaN templates. The characterization is performed in order to compare the emission mechanism in both kinds of samples.
2. Experiment * Corresponding author. Tel.: 34-91-549-5700x420; fax: 34-91336-7323. E-mail address: [email protected] (F.B. Naranjo).
InGaN layers and MQWs were grown on 2-mm thick MOCVD-GaN templates. The growth was performed in
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 0 3 2 - 6
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a commercial MECA2000 MBE system. Active nitrogen was provided using a RF plasma source from Oxford Applied Research while conventional Knudsen cells were employed for In, Ga and Al sources. More details about the system configuration can be found elsewhere [6]. The templates were Ti coated (back side) and cleaned with organic solvents before loading in the MBE system. Prior to the growth, the substrates were outgassed in the growth chamber at 650 8C during 30 min. The growth temperature was monitored using an optical pyrometer focused at the surface of the sample. A 300-nm thick MBE-GaN buffer layer was grown to avoid the template surface contamination at 700 8C. The temperature was then decreased for the growth of the InGaN/GaN MQW and bulk layers to 560/600 8C. Five periods of Inx Ga1 x N/GaN were grown with the In mole fraction (x ) in the range 0.06 /0.16. The quantum well and barrier thickness were changed between 3 /5 and 5 /9 nm, respectively. For comparison, a set of 300-nm InGaN bulk layers were grown on a similar GaN buffer layer. The amount of active nitrogen was kept constant using a RF power of 400 W and a nitrogen flow of 1.0 sccm for all the layers, quantum wells and barriers. For all the samples, a 2 2 surface reconstruction is observed after the growth of the initial GaN layer and cooling down of the sample, right before the InGaN growth, indicating a smooth (2D) GaN surface. High resolution X-ray diffraction (HRXRD) was employed to assess the amount of strain and the In content of the samples. For InGaN bulk layers, the symmetric (0002) and the asymmetric (105) reflections were used, assuming the validity of Vegard’s law for the lattice parameter [7] and taking the In-content dependent elastic coefficients C13 and C33 from Ambacher [4]. The quantum well and barrier thickness, the In content and the strain state were obtained in the MQWs by fitting the measured (0002) symmetric reflection using a dynamical simulation program. To investigate the interface roughness in the layers, transmission electron microscopy (TEM) measurements were carried out in a Jeol JEM 3010 microscope working at 300 kV accelerating voltage. The TEM cross-sectional samples were conventionally prepared by mechanical thinning and polishing followed by Arion beam milling using a cold stage at liquid nitrogen. Transmission data were obtained using a Perkin / Elmer Lambda 9 spectrophotometer, while photoluminescence (PL) measurements were performed in a double cycle He-flow cryostat using a He /Cd (325 nm) laser as excitation source with 1 mW. The emission was dispersed by a high-resolution THR1000 JobinYvon spectrometer, detected by a GaAs photomultiplier and processed using lock-in techniques.
3. Results 3.1. InGaN bulk layers Fig. 1 shows the low temperature (8 K) PL from different InGaN bulk layers, an emission energy increase is observed when decreasing the In content. Multiple peak emission is due to Fabry/Perot oscillations in the whole structure (InGaNGaN bufferGaN MOCVD 2.72 mm). To study the excitonic localization effect and its dependence with the In content, temperature-dependent PL measurements were carried out. The results were compared to absorption data, which were derived from the measured transmission using the relation a (E )8 ln(T ). The effective band gap, EG,eff, and an estimation of the band edge broadening, DE , were calculated within a sigmoidal absorption approximation, a(E) 1 exp
a0 EG;eff E
(1)
DE
Fig. 2a and b shows the PL emission evolution with temperature for two samples with 20 and 27% In, respectively. The layers are not fully relaxed with this thickness (300 nm) and a relaxation degree of 0.51 and 0.86, respectively, was estimated using HRXRD. In both samples, a clear red-shift is observed for temperatures below 200 and 150 K, respectively, while for higher temperatures the emission energy suffers a blue-shift. This behavior is explained in terms of carrier localization in a distribution of local minima formed in higher In content than the surrounding InGaN matrix [8]. At low temperatures, photo-generated carriers are randomly distributed among the different minima, but when increasing temperature, electron/hole pairs have enough energy to reach the lower energy minima, and the emission red-shifts. Temperature-induced blue-shift
Fig. 1. Low temperature PL spectra for different In content layers. The In content and strain degree is specified for each layer.
F.B. Naranjo et al. / Materials Science and Engineering B93 (2002) 131 /134
133
Fig. 2. PL emission energy and FWHM for samples with 20% In (a) and 27% In (b).
for temperatures above 200 K (20% In) and 150 K (27% In) is attributed to filling of higher-energy minima with electron and hole pairs due to the thermal energy. This effect is clearer for the sample with the 27% In, where the PL emission energy at room temperature is higher than at low temperature. The distribution of carriers at the different minima explains the behavior of the emission FWHM with the temperature. At room temperature, the PL FWHM and the absorption bandedge broadening is observed to increase with the In content. Absorption band edge from 153 to 210 meV were calculated for In contents from 20 to 27% In, respectively, which support the hypothesis of an In content localization present in these layers [9]. The calculated effective band-gap was extrapolated to the corresponding relaxed band-gap for each In content estimated from HRXRD measurements considering dEG/do 14.5 eV [10], leading to a bowing parameter for relaxed InGaN of 3.6 eV, in agreement with the data in the literature [11].
Fig. 3. HRXRD (0002) reflection for two different MQWs, fitting results are described in the text.
the XRD diffraction data, as it has been observed by other authors [12]. High-resolution TEM image is shown in Fig. 4 for sample (b), as it can be observed, the quantum well and barriers thicknesses are in good agreement with HRXRD simulations.
3.2. InGaN MQW The HRXRD (0002) symmetric reflection is shown for two different MQW structures in Fig. 3. Using the simulation program, the best fitting leads to: sample (a), with barrier and well thickness of 6 and 5 nm, respectively; and sample (b), with 5 and 3 nm of barrier and well thickness, respectively. The fully strained InGaN has an In content of 0.16 for sample (a) and 0.12 for sample (b). As it can be observed in Fig. 3, the satellite peaks are clearly visible in sample (b). For samples with high In content (as sample (a)), cross-section TEM images reveals an increase of the interface roughness, which is consistent with the broadening of the satellite peaks in
Fig. 4. HRTEM image of sample (b).
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Fig. 5. (a) Evolution of PL emission energy and FWHM of the emission for the sample (a). (b) Evolution of PL emission energy and FWHM of the emission for the sample (b).
Fig. 5 shows the temperature dependent photoluminescence evolution of samples (a) and (b). The S-shape of the PL emission energy with the temperature reveals the existence of localization, more important in the sample (b) (thinner wells), which has a blue-shift of 30 meV for temperatures above 250 K. For sample (a), with thicker wells, the effect of the localization is lower; it is well known that the piezoelectric field increases with the In content, and the total potential drop in the well is proportional to the well thickness and the polarization field. In fact, in sample (a) both mechanisms (localization and piezoelectric fields) are responsible for the emission energy, a decrease of the well thickness from 5 to 4 nm, maintaining the In content and barrier thickness, lead to an emission blue-shift of 165 meV (not shown). From this shift, a field of 1.6 MV cm 1 is estimated in the well, lower than the theoretical predictions [13] probably due to contribution by localization effect.
4. Conclusions InGaN bulk layers and MQWs were grown on GaN templates to study the effect of the In localization in the samples. HRXRD reveal that the thick layers (300 nm) are not fully relaxed within the range of In contents studied. From absorption measurements, an increase of the absorption band-edge was observed, being dependent on the In content. Localization is present in MQW and an increase of the PL emission energy is observed when decreasing the well thickness, showing an important effect of piezoelectric fields in thick quantum wells.
Acknowledgements The authors acknowledge the contribution of B. Beaumont for the MOCVD GaN templates. Thanks are due to B. Jenichen for his help in XRD measurements and to O. Brandt for the HRXRD simulation program. Partial financial support was provided by IST ESPRIT 1999-10292 ‘AGETHA’ Project.
References [1] S. Nakamura, Semicond. Sci. Technol. 14 (1999) R27. [2] T. Takayama, M. Yuri, K. Itoh, T. Baba, J.S. Harris, Jr., J. Appl. Phys. 88 (2000) 1104. [3] H. Chen, A.R. Smith, R.M. Feenstra, D.W. Greve, J.E. Northrup, MRS Internet J. Nitride Semicond. Res. 4S1 (1999) G9.5. [4] O. Ambacher, J. Phys. D: Appl. Phys. 31 (1998) 2653. [5] M.A. Sa´nchez-Garcı´a, F.B. Naranjo, J.L. Pau, A. Jime´nez, E. Calleja, E. Mun˜oz, S.I. Molina, A.M. Sa´nchez, F.J. Pacheco, R. Garcı´a, Phys. Status Solidi A176 (1999) 447. [6] M.A. Sa´nchez-Garcı´a, E. Calleja, E. Monroy, F.J. Sa´nchez, F. Calle, E. Mun˜oz, R. Beresford, J. Cryst. Growth 183 (1998) 23. [7] M. Schuster, P.O. Gervais, B. Jobst, W. Ho¨sler, R. Averbeck, H. Riechert, A. Iberl, R. Sto¨mmer, J. Phys. D: Appl. Phys. 32 (1999) A56. [8] L.P.D. Schenk, M. Leroux, P. de Mierry, J. Appl. Phys. 88 (2000) 1525. [9] R.W. Martin, P.G. Middlenton, K.P. O’Donnell, W. Van der stricht, Appl. Phys. Lett. 74 (1999) 263. [10] S. Pereira, M.R. Correia, T. Monteiro, E. Pereira, E. Alves, A.D. Sequeira, N. Franco, Appl. Phys. Lett. 78 (2001) 2137. [11] C. Wetzel, T. Takeuchi, S. Yamaguchi, H. Katoh, H. Amano, I. Akasaki, Appl. Phys. Lett. 73 (1998) 2566. [12] Ig-Hyeon Kim, Hyeong-Soo Park, Yong-Jo Park, Taeil Kim, Appl. Phys. Lett. 73 (1998) 1634. [13] F. Della Sala, A. Di Carlo, P. Lugli, F. Bernardini, V. Fiorentini, R. Scholz, Jean-Marc Jancu, Appl. Phys. Lett. 74 (1999) 2002.
Structural and optical characterization of thick InGaN layers and InGaN/GaN MQW grown by molecular beam epitaxy F.B. Naranjo a,*, S. Ferna´ndez a, M.A. Sa´nchez-Garcı´a a, F. Calle a, E. Calleja a, A. Trampert b, K.H. Ploog b a
ISOM and Departamento de Ingenierı´a Electro´nica, ETSI Telecomunicacio´n, Universidad Polite´cnica de Madrid, Ciudad Universitaria, 28040 Madrid, Spain b Paul-Drude-Institut fuer Festkoerperelektronik, Hausvogteiplatz 5-7, D-10117 Berlin, Germany
Abstract Thick Inx Ga1 x N (0.20B x B 0.27) layers and InGaN/GaN multiple quantum wells (MQWs) are grown by plasma-assisted molecular beam epitaxy on GaN/Al2O3 templates. The strain and In-content is estimated from high-resolution X-ray diffraction, showing that the bulk samples are not fully relaxed. A bowing parameter of 3.6 eV is obtained from absorption measurements of Inx Ga1 x N layers. Strong In-dependent excitonic localization is observed in these bulk layers, leading to an increase in the absorption band edge with the In content. Regarding the MQWs structures, high-resolution transmission electron microscopy reveals an increase in the interface roughness for high In content. The dominant PL emission of the MQWs shows a red-shift when increasing the well thickness for a given In-content, due to internal piezoelectric field. The excitonic localization is studied and compared between thick layers and MQWs structures. # 2002 Elsevier Science B.V. All rights reserved. Keywords: InGaN; Molecular beam epitaxy; Transmission electron microscopy; Localization; Strain
1. Introduction InGaN-based light emitting diodes (LED) and laser diodes (LD) grown on sapphire substrates by metal / organic chemical vapor deposition (MOCVD) techniques have been demonstrated in the recent years [1]. However, there are many aspects of the growth of this material that are still not well understood. In particular, the thermodynamical unstability of the GaN /InN alloy has been a subject of many reports [2]. This effect, and the segregation of In at the surface during growth, are some reasons that make the growth conditions of this material so critical [3]. Molecular beam epitaxy (MBE) has attracted much attention, due to the in situ control of the growth using reflection high energy electron diffraction (RHEED) and the possibility of achieving abrupt interfaces. The lower temperature decomposition
of InN (630 8C) compared with GaN (850 8C) [4] gives rise to the need of lower growth temperatures for InGaN layers (around 580 8C). In this aspect, MBE allows the use of these low temperatures. On the other hand, the quality of the GaN grown by MBE on sapphire is generally not as good as MOCVD-grown samples. With the use of MOCVD-GaN templates for MBE (homoepitaxy), this problem could be avoided [5]. The study of InGaN-based multiple quantum wells (MQW) is necessary to improve the performance of the devices. In this paper we report on the growth and characterization of bulk InGaN layers and InGaN/GaN MQW by MBE on GaN templates. The characterization is performed in order to compare the emission mechanism in both kinds of samples.
2. Experiment * Corresponding author. Tel.: 34-91-549-5700x420; fax: 34-91336-7323. E-mail address: [email protected] (F.B. Naranjo).
InGaN layers and MQWs were grown on 2-mm thick MOCVD-GaN templates. The growth was performed in
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 0 3 2 - 6
132
F.B. Naranjo et al. / Materials Science and Engineering B93 (2002) 131 /134
a commercial MECA2000 MBE system. Active nitrogen was provided using a RF plasma source from Oxford Applied Research while conventional Knudsen cells were employed for In, Ga and Al sources. More details about the system configuration can be found elsewhere [6]. The templates were Ti coated (back side) and cleaned with organic solvents before loading in the MBE system. Prior to the growth, the substrates were outgassed in the growth chamber at 650 8C during 30 min. The growth temperature was monitored using an optical pyrometer focused at the surface of the sample. A 300-nm thick MBE-GaN buffer layer was grown to avoid the template surface contamination at 700 8C. The temperature was then decreased for the growth of the InGaN/GaN MQW and bulk layers to 560/600 8C. Five periods of Inx Ga1 x N/GaN were grown with the In mole fraction (x ) in the range 0.06 /0.16. The quantum well and barrier thickness were changed between 3 /5 and 5 /9 nm, respectively. For comparison, a set of 300-nm InGaN bulk layers were grown on a similar GaN buffer layer. The amount of active nitrogen was kept constant using a RF power of 400 W and a nitrogen flow of 1.0 sccm for all the layers, quantum wells and barriers. For all the samples, a 2 2 surface reconstruction is observed after the growth of the initial GaN layer and cooling down of the sample, right before the InGaN growth, indicating a smooth (2D) GaN surface. High resolution X-ray diffraction (HRXRD) was employed to assess the amount of strain and the In content of the samples. For InGaN bulk layers, the symmetric (0002) and the asymmetric (105) reflections were used, assuming the validity of Vegard’s law for the lattice parameter [7] and taking the In-content dependent elastic coefficients C13 and C33 from Ambacher [4]. The quantum well and barrier thickness, the In content and the strain state were obtained in the MQWs by fitting the measured (0002) symmetric reflection using a dynamical simulation program. To investigate the interface roughness in the layers, transmission electron microscopy (TEM) measurements were carried out in a Jeol JEM 3010 microscope working at 300 kV accelerating voltage. The TEM cross-sectional samples were conventionally prepared by mechanical thinning and polishing followed by Arion beam milling using a cold stage at liquid nitrogen. Transmission data were obtained using a Perkin / Elmer Lambda 9 spectrophotometer, while photoluminescence (PL) measurements were performed in a double cycle He-flow cryostat using a He /Cd (325 nm) laser as excitation source with 1 mW. The emission was dispersed by a high-resolution THR1000 JobinYvon spectrometer, detected by a GaAs photomultiplier and processed using lock-in techniques.
3. Results 3.1. InGaN bulk layers Fig. 1 shows the low temperature (8 K) PL from different InGaN bulk layers, an emission energy increase is observed when decreasing the In content. Multiple peak emission is due to Fabry/Perot oscillations in the whole structure (InGaNGaN bufferGaN MOCVD 2.72 mm). To study the excitonic localization effect and its dependence with the In content, temperature-dependent PL measurements were carried out. The results were compared to absorption data, which were derived from the measured transmission using the relation a (E )8 ln(T ). The effective band gap, EG,eff, and an estimation of the band edge broadening, DE , were calculated within a sigmoidal absorption approximation, a(E) 1 exp
a0 EG;eff E
(1)
DE
Fig. 2a and b shows the PL emission evolution with temperature for two samples with 20 and 27% In, respectively. The layers are not fully relaxed with this thickness (300 nm) and a relaxation degree of 0.51 and 0.86, respectively, was estimated using HRXRD. In both samples, a clear red-shift is observed for temperatures below 200 and 150 K, respectively, while for higher temperatures the emission energy suffers a blue-shift. This behavior is explained in terms of carrier localization in a distribution of local minima formed in higher In content than the surrounding InGaN matrix [8]. At low temperatures, photo-generated carriers are randomly distributed among the different minima, but when increasing temperature, electron/hole pairs have enough energy to reach the lower energy minima, and the emission red-shifts. Temperature-induced blue-shift
Fig. 1. Low temperature PL spectra for different In content layers. The In content and strain degree is specified for each layer.
F.B. Naranjo et al. / Materials Science and Engineering B93 (2002) 131 /134
133
Fig. 2. PL emission energy and FWHM for samples with 20% In (a) and 27% In (b).
for temperatures above 200 K (20% In) and 150 K (27% In) is attributed to filling of higher-energy minima with electron and hole pairs due to the thermal energy. This effect is clearer for the sample with the 27% In, where the PL emission energy at room temperature is higher than at low temperature. The distribution of carriers at the different minima explains the behavior of the emission FWHM with the temperature. At room temperature, the PL FWHM and the absorption bandedge broadening is observed to increase with the In content. Absorption band edge from 153 to 210 meV were calculated for In contents from 20 to 27% In, respectively, which support the hypothesis of an In content localization present in these layers [9]. The calculated effective band-gap was extrapolated to the corresponding relaxed band-gap for each In content estimated from HRXRD measurements considering dEG/do 14.5 eV [10], leading to a bowing parameter for relaxed InGaN of 3.6 eV, in agreement with the data in the literature [11].
Fig. 3. HRXRD (0002) reflection for two different MQWs, fitting results are described in the text.
the XRD diffraction data, as it has been observed by other authors [12]. High-resolution TEM image is shown in Fig. 4 for sample (b), as it can be observed, the quantum well and barriers thicknesses are in good agreement with HRXRD simulations.
3.2. InGaN MQW The HRXRD (0002) symmetric reflection is shown for two different MQW structures in Fig. 3. Using the simulation program, the best fitting leads to: sample (a), with barrier and well thickness of 6 and 5 nm, respectively; and sample (b), with 5 and 3 nm of barrier and well thickness, respectively. The fully strained InGaN has an In content of 0.16 for sample (a) and 0.12 for sample (b). As it can be observed in Fig. 3, the satellite peaks are clearly visible in sample (b). For samples with high In content (as sample (a)), cross-section TEM images reveals an increase of the interface roughness, which is consistent with the broadening of the satellite peaks in
Fig. 4. HRTEM image of sample (b).
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Fig. 5. (a) Evolution of PL emission energy and FWHM of the emission for the sample (a). (b) Evolution of PL emission energy and FWHM of the emission for the sample (b).
Fig. 5 shows the temperature dependent photoluminescence evolution of samples (a) and (b). The S-shape of the PL emission energy with the temperature reveals the existence of localization, more important in the sample (b) (thinner wells), which has a blue-shift of 30 meV for temperatures above 250 K. For sample (a), with thicker wells, the effect of the localization is lower; it is well known that the piezoelectric field increases with the In content, and the total potential drop in the well is proportional to the well thickness and the polarization field. In fact, in sample (a) both mechanisms (localization and piezoelectric fields) are responsible for the emission energy, a decrease of the well thickness from 5 to 4 nm, maintaining the In content and barrier thickness, lead to an emission blue-shift of 165 meV (not shown). From this shift, a field of 1.6 MV cm 1 is estimated in the well, lower than the theoretical predictions [13] probably due to contribution by localization effect.
4. Conclusions InGaN bulk layers and MQWs were grown on GaN templates to study the effect of the In localization in the samples. HRXRD reveal that the thick layers (300 nm) are not fully relaxed within the range of In contents studied. From absorption measurements, an increase of the absorption band-edge was observed, being dependent on the In content. Localization is present in MQW and an increase of the PL emission energy is observed when decreasing the well thickness, showing an important effect of piezoelectric fields in thick quantum wells.
Acknowledgements The authors acknowledge the contribution of B. Beaumont for the MOCVD GaN templates. Thanks are due to B. Jenichen for his help in XRD measurements and to O. Brandt for the HRXRD simulation program. Partial financial support was provided by IST ESPRIT 1999-10292 ‘AGETHA’ Project.
References [1] S. Nakamura, Semicond. Sci. Technol. 14 (1999) R27. [2] T. Takayama, M. Yuri, K. Itoh, T. Baba, J.S. Harris, Jr., J. Appl. Phys. 88 (2000) 1104. [3] H. Chen, A.R. Smith, R.M. Feenstra, D.W. Greve, J.E. Northrup, MRS Internet J. Nitride Semicond. Res. 4S1 (1999) G9.5. [4] O. Ambacher, J. Phys. D: Appl. Phys. 31 (1998) 2653. [5] M.A. Sa´nchez-Garcı´a, F.B. Naranjo, J.L. Pau, A. Jime´nez, E. Calleja, E. Mun˜oz, S.I. Molina, A.M. Sa´nchez, F.J. Pacheco, R. Garcı´a, Phys. Status Solidi A176 (1999) 447. [6] M.A. Sa´nchez-Garcı´a, E. Calleja, E. Monroy, F.J. Sa´nchez, F. Calle, E. Mun˜oz, R. Beresford, J. Cryst. Growth 183 (1998) 23. [7] M. Schuster, P.O. Gervais, B. Jobst, W. Ho¨sler, R. Averbeck, H. Riechert, A. Iberl, R. Sto¨mmer, J. Phys. D: Appl. Phys. 32 (1999) A56. [8] L.P.D. Schenk, M. Leroux, P. de Mierry, J. Appl. Phys. 88 (2000) 1525. [9] R.W. Martin, P.G. Middlenton, K.P. O’Donnell, W. Van der stricht, Appl. Phys. Lett. 74 (1999) 263. [10] S. Pereira, M.R. Correia, T. Monteiro, E. Pereira, E. Alves, A.D. Sequeira, N. Franco, Appl. Phys. Lett. 78 (2001) 2137. [11] C. Wetzel, T. Takeuchi, S. Yamaguchi, H. Katoh, H. Amano, I. Akasaki, Appl. Phys. Lett. 73 (1998) 2566. [12] Ig-Hyeon Kim, Hyeong-Soo Park, Yong-Jo Park, Taeil Kim, Appl. Phys. Lett. 73 (1998) 1634. [13] F. Della Sala, A. Di Carlo, P. Lugli, F. Bernardini, V. Fiorentini, R. Scholz, Jean-Marc Jancu, Appl. Phys. Lett. 74 (1999) 2002.