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GaN-DFB lasers: overgrown lasers and vertical modes
Materials Science and Engineering B59 (1999) 386 – 389
Optically pumped GaInN/GaN-DFB lasers: overgrown lasers and vertical modes R. Hofmann *, V. Wagner, M. Neuner, J. Off, F. Scholz, H. Schweizer 4. Physikalisches Institut, Uni6ersitat Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany
Abstract The laser operation of GaInN/GaN distributed feedback lasers (DFB) with separate confinement double hetero or multi quantum well structures on SiC is demonstrated. The separate confinement heterostructures were realized by overgrowing the dry etched DFB gratings. Optically excited lasing is observed with emission wavelengths between 402 and 421 nm, depending linearly on the grating period. The laser thresholds are compared to laser thresholds of non overgrown DFB lasers. Furthermore, the wavelengths selectivity of DFB lasers is used to study vertical modes in laser structures grown on sapphire. In a thick asymmetric laser structure, higher vertical modes can be distinguished because of their different effective refractive index heff. In thin heterostructures, the vertical ground mode is lasing. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Distributed feedback laser; GaInN/GaN laser diodes; Vertical modes; MOVPE
1. Introduction In the past two years, great progress has been achieved in the realization of III – V nitride based injection lasers which led to the demonstration of pulsed and continuous wave laser operation at room temperature [1,2]. While the first nitride lasers were demonstrated on sapphire, pulsed operation was also shown for laser structures grown on 6H – SiC [3]. One of the difficulties for the realization of laser diodes is still the fabrication of suitable laser mirrors due to the poor cleaving behaviour of the nitrides and their wurzite substrate materials. Therefore, many groups realize laser mirror facets by dry etching which is sometimes combined with a polishing procedure and high reflection coating [4]. The mirror problem can be avoided by using a distributed-feedback (DFB) laser design. Earlier optical experiments on index coupled DFB laser with dry etched gratings showed that similar laser thresholds can be obtained as for Fabry–Perot (FP) designs [5]. However, for an electrically pumped DFB laser, a second epitaxy is necessary to overgrow the etched grating. In this paper, we present the first studies on optically excited laser emission from over* Corresponding author. Tel.: +49-711-6854961; fax: + 49-7116855097. E-mail address: [email protected] (R. Hofmann)
grown DFB lasers and discuss their laser thresholds. Furthermore, the wavelength selectivity of DFB lasers according to the Bragg condition allows detailed investigations of some material and laser properties such as the dispersion of the effective refractive index and the wavelength dependence of the laser threshold [5]. In this paper, we use this property of DFB lasers to show that in thick GaInN/GaN structures on sapphire, several vertical modes exist (as already proposed from farfield measurements by Hofstetter [6]) and contribute to the laser action.
2. Experiment To realize the separate confinement heterostructure (SCH) DFB lasers, first the n-side Al0.18Ga0.82N:Si cladding (0.3–1 mm), the lower GaN waveguide (0.1 mm), the GaInN active layer, and the upper GaN waveguide (0.1 mm) were grown on SiC substrates by MOVPE. The GaInN active layer was either a 10 nm single layer with 15% indium or a multi quantum well layer with five wells 2.5 nm thick and separated by 5 nm thick GaN barriers. Second order gratings with grating periods between 150 and 190 nm were then defined by electron beam lithography using a positive resist (PMMA) and transferred into the upper GaN
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 3 5 0 - X
R. Hofmann et al. / Materials Science and Engineering B59 (1999) 386–389
waveguide layer by ECR-RIE dry etching. The etch depth was 50 nm. After removing the resist mask the laser structure was completed by growing the upper Al0.18Ga0.82N cladding layer in a second MOVPE step. Using an optical microscope, the overgrown AlGaN layer appeared rather smooth and only slightly rougher above the gratings then above the original surface. To control the profile of the overgrown gratings, a sample was cleaved perpendicular to the gratings and viewed from the side using a REM. By comparing grating profiles which were not overgrown and overgrown, we found that the gratings keep their original shape during overgrowth except for a slight flattening of the edges. Finally, a mesa was etched to provide lateral waveguiding. For the realisation of the double heterostructure (DH) DFB lasers on sapphire, see [7]. The DFB lasers were optically pumped with a pulsed XeCl excimer laser operating at 308 nm and with a pulse repetition rate of 10 Hz. The excimer beam was focused to a stripe orientated parallel to the resonator axis by a cylindrical lens in order to pump the DFB-cavity homogeneously. The laser activity was measured in backscattering geometry with a 0.85 m monochromator and a UV sensitive bialkali photomultiplier tube. The samples were kept at room temperature during the whole measurement.
3. Results and discussion
3.1. O6ergrown DFB lasers Fig. 1 shows the laser emission of five overgrown SCH-DH DFB lasers. The emission wavelength depends linearly on the grating period. This is in agreement with the Bragg condition l = 2neffL/m, where neff is the
Fig. 1. Room temperature laser emission of separate confinement DH DFB lasers with etched and overgrown gratings. The grating periods L are indicated above the laser peaks.
387
Fig. 2. Dependence of the laser peak intensity on the pump power density. The emission intensity was determined by integrating the area under the laser peak.
effective refractive index of the heterostructure, m=2 is the order of the refraction, and L is the grating period which proves that the feedback necessary for the laser emission is still caused by the gratings after the overgrowth. The DFB lasers show a clear threshold behaviour which can be seen in Fig. 2. Typical values measured for the laser threshold of the SCH double heterostructure as well as for the SCH multi quantum well structure are 2–3 MW cm − 2. This seems to be large compared to the best values of 0.9 MW cm − 2 which we obtained for a non overgrown but otherwise identical DH structure on SiC. However, to compare the overgrown with the non overgrown lasers, one has to take into account that the upper Al0.8Ga0.82N cladding layer absorbs the exciting excimer pump pulse rather strongly. Assuming an absorption coefficient of 8× 104 cm − 1 at 308 nm [8], we estimated that only 45% of the incident pulse reaches the upper GaN waveguide and contributes to the generation of carriers contributing to the laser emission. To prove this, we etched different deep stripes in the upper AlGaN layer of a SCH structure and measured the luminescence intensity in dependence of the etch depth. This confirmed the estimated attenuation of the excitation. Therefore, for comparison we corrected the thresholds of the overgrown lasers by the obtained factor which are then in the range 1 and 1.5 MW cm − 2 and are the same for non overgrown lasers.
3.2. Vertical modes To investigate the vertical modes in GaN-based laser
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R. Hofmann et al. / Materials Science and Engineering B59 (1999) 386–389
Fig. 3. (a) Mode distribution of a thick GaInN/GaN double heterostructure on sapphire calculated at 390 nm by a transfer matrix program using the following refractive indices: nsapphire = 176, nGaN (390 nm) = 2.61, and nGalnN (390 nm) = 2.80. Three vertical modes are found with different confinement factors G; (b) Mode distribution of a thin GaInN/GaN double heterostructure on sapphire calculated at 410 nm by a transfer matrix program using following refractive indices: nsapphire = 176, nGaN (410 nm)=2.55, and nGalnN (410 nm)=2.71. Only the vertical ground mode is supported; (c,d) Typical emission spectra for the asymmetric DH (c) and symmetric DH (d) structure; (e,f) Emission wavelengths in dependence on the grating period for the asymmetric structure (e) and the symmetric structure (f). The dots indicate the measured values. The solid lines are calculated using the Bragg equation and effective refractive indices determined by a transfer matrix program. The dispersion of the refractive indices was taken into account for these calculations.
R. Hofmann et al. / Materials Science and Engineering B59 (1999) 386–389
structures on sapphire, we compared two GaInN/GaN DH structures, one with a thick buffer (500 nm), which we want to call asymmetric structure, the other with a thin buffer (100 nm), which we want to call symmetric structure because the buffer has the same thickness as the upper GaN layer. Details of the DH structures are given in Table 1. In Fig. 3(c,d), typical emission spectra of DFB lasers processed on the asymmetric and on the symmetric structure are shown. While DFB lasers on the symmetric structure always show only one emission peak, the spectra of DFB lasers on the asymmetric structure contain two peaks for certain grating periods. The spectral distance between these peaks is typically several nanometers. The occurrence of these peaks in the DFB laser spectra can be explained by the existence of several vertical modes. These are caused by the large refractive index steps between GaN/air and GaN/sapphire which causes the epitaxial layers to act as a cavity for the lightwave. Such modes were already observed in the farfield of an optically pumped SCH laser structure by Hofstetter [6]. In Fig. 3(a,b), the calculated vertical mode distribution for the asymmetric and the symmetric structure are plotted. The calculation was carried out by a transfer matrix program for a wavelength of 390 nm for the asymmetric structure and 410 nm for the symmetric structure using the refractive indices indicated in the figure caption. As can be seen in Fig. 3(b), the symmetric structure guides only one vertical mode, the ground mode TE0 because higher vertical modes do not fit in the small optical cavity. The confinement factor for this structure is 8.0%. The asymmetric structure guides three vertical modes. While the ground mode TE0 has only a very low confinement factor in the active region (G(TE0)=2.4%), the confinement factors of the two higher modes are G(TE1) = 4.8% and G(TE2)= 4.5% and are nearly the same. This suggests that the lasing modes in the asymmetric structure are the first and second order vertical modes. To prove this, we plotted the emission wavelength of each observed laser peak against the grating period (dots in Fig. 3(e,f). We than calculated the effective refractive indices heff of each vertical mode with the transfer matrix program and determined the expected Table 1 Vertical structure of the symmetric and asymmetric double heterostructures on sapphire used to investigate vertical modes
Substrate Nucleation GaN-buffer GaInN GaN
Asymmetric
Symmetric
Sapphire AlN 500 nm 12 nm 100 nm
Sapphire AlN 100 nm 10 nm 100 nm
389
emission wavelength in dependence of the grating period using the calculated heff and the Bragg equation for second order gratings l= h calc eff L The calculated emission wavelengths are plotted as solid lines in Fig. 3(e,f). One sees that the measured emission wavelengths match with the calculated emission wavelengths for the first and second order vertical modes in the asymmetric structure. The fact that the measured values are systematically smaller than the calculated values is attributed to band filling effects under high excitation conditions in the experiment. For the symmetric structure, a good agreement can be reached for the zero order vertical mode (Fig. 3(f)). In conclusion, we have demonstrated room temperature laser operation of optically pumped separate confinement GaInN/GaN DFB lasers with overgrown gratings. The profile of the gratings did not change significantly during overgrowth. The laser thresholds of these lasers are comparable to non overgrown lasers. DFB lasers on sapphire with a total epitaxial layer thickness greater than 250 nm show higher vertical modes. Which of these vertical modes is finally lasing is selected by the DFB grating period due to the different effective refractive indices heff of the vertical modes. Vertical single mode emission of FP lasers on sapphire are therefore expected only for very thin epitaxial layers, but this usually reduces the crystal quality.
Acknowledgements The authors wish to thank H. Gra¨beldinger, P. Burkard, K. Schleyer for technical support. This work was supported by the Deutsche Forschungsgesellschaft.
References [1] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsuchita, H. Kiyoku, Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) 217. [2] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsuchita, Y. Sugimoto, H. Kiyoku, Appl. Phys. Lett. 70 (1997) 616. [3] A. Kuramata, K. Domen, R. Soejima, K. Horino, S. Kubota, T. Tanahashi, Jap. J. Appl. Phys. 36 (1997) 1130 – 1132. [4] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsuchita, H. Kiyoku, Y. Sugimoto, Appl. Phys. Lett. 68 (1996) 2105. [5] R. Hofmann, V. Wagner, H.-P. Gauggel, H. Bolay, F. Scholz, H. Schweizer, IEEE J. Sel. Top. Quantum Electron. 3 (1997) 456. [6] D. Hofstetter, D.P. Bour, R.L. Thornton, N.M. Johnson, Appl. Phys. Lett. 70 (1997) 1650. [7] R. Hofmann, H.-P. Gauggel, U.A. Griesinger, H. Gra¨beldinger, F. Adler, P. Ernst, H. Bolay, V. Ha¨rle, F. Scholz, H. Schweizer, M.H. Pilkuhn, Appl. Phys. Lett. 69 (1996) 2068. [8] O. Ambacher, M. Arzberger, D. Brunner, H. Angerer, F. Freudenberg, N. Esser, T. Wethkamp, K. Wilmers, W. Richter, M. Stutzmann, MRS Internet J. Nitride Semicond. Res. 2 (1997) 22.
Optically pumped GaInN/GaN-DFB lasers: overgrown lasers and vertical modes R. Hofmann *, V. Wagner, M. Neuner, J. Off, F. Scholz, H. Schweizer 4. Physikalisches Institut, Uni6ersitat Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany
Abstract The laser operation of GaInN/GaN distributed feedback lasers (DFB) with separate confinement double hetero or multi quantum well structures on SiC is demonstrated. The separate confinement heterostructures were realized by overgrowing the dry etched DFB gratings. Optically excited lasing is observed with emission wavelengths between 402 and 421 nm, depending linearly on the grating period. The laser thresholds are compared to laser thresholds of non overgrown DFB lasers. Furthermore, the wavelengths selectivity of DFB lasers is used to study vertical modes in laser structures grown on sapphire. In a thick asymmetric laser structure, higher vertical modes can be distinguished because of their different effective refractive index heff. In thin heterostructures, the vertical ground mode is lasing. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Distributed feedback laser; GaInN/GaN laser diodes; Vertical modes; MOVPE
1. Introduction In the past two years, great progress has been achieved in the realization of III – V nitride based injection lasers which led to the demonstration of pulsed and continuous wave laser operation at room temperature [1,2]. While the first nitride lasers were demonstrated on sapphire, pulsed operation was also shown for laser structures grown on 6H – SiC [3]. One of the difficulties for the realization of laser diodes is still the fabrication of suitable laser mirrors due to the poor cleaving behaviour of the nitrides and their wurzite substrate materials. Therefore, many groups realize laser mirror facets by dry etching which is sometimes combined with a polishing procedure and high reflection coating [4]. The mirror problem can be avoided by using a distributed-feedback (DFB) laser design. Earlier optical experiments on index coupled DFB laser with dry etched gratings showed that similar laser thresholds can be obtained as for Fabry–Perot (FP) designs [5]. However, for an electrically pumped DFB laser, a second epitaxy is necessary to overgrow the etched grating. In this paper, we present the first studies on optically excited laser emission from over* Corresponding author. Tel.: +49-711-6854961; fax: + 49-7116855097. E-mail address: [email protected] (R. Hofmann)
grown DFB lasers and discuss their laser thresholds. Furthermore, the wavelength selectivity of DFB lasers according to the Bragg condition allows detailed investigations of some material and laser properties such as the dispersion of the effective refractive index and the wavelength dependence of the laser threshold [5]. In this paper, we use this property of DFB lasers to show that in thick GaInN/GaN structures on sapphire, several vertical modes exist (as already proposed from farfield measurements by Hofstetter [6]) and contribute to the laser action.
2. Experiment To realize the separate confinement heterostructure (SCH) DFB lasers, first the n-side Al0.18Ga0.82N:Si cladding (0.3–1 mm), the lower GaN waveguide (0.1 mm), the GaInN active layer, and the upper GaN waveguide (0.1 mm) were grown on SiC substrates by MOVPE. The GaInN active layer was either a 10 nm single layer with 15% indium or a multi quantum well layer with five wells 2.5 nm thick and separated by 5 nm thick GaN barriers. Second order gratings with grating periods between 150 and 190 nm were then defined by electron beam lithography using a positive resist (PMMA) and transferred into the upper GaN
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 3 5 0 - X
R. Hofmann et al. / Materials Science and Engineering B59 (1999) 386–389
waveguide layer by ECR-RIE dry etching. The etch depth was 50 nm. After removing the resist mask the laser structure was completed by growing the upper Al0.18Ga0.82N cladding layer in a second MOVPE step. Using an optical microscope, the overgrown AlGaN layer appeared rather smooth and only slightly rougher above the gratings then above the original surface. To control the profile of the overgrown gratings, a sample was cleaved perpendicular to the gratings and viewed from the side using a REM. By comparing grating profiles which were not overgrown and overgrown, we found that the gratings keep their original shape during overgrowth except for a slight flattening of the edges. Finally, a mesa was etched to provide lateral waveguiding. For the realisation of the double heterostructure (DH) DFB lasers on sapphire, see [7]. The DFB lasers were optically pumped with a pulsed XeCl excimer laser operating at 308 nm and with a pulse repetition rate of 10 Hz. The excimer beam was focused to a stripe orientated parallel to the resonator axis by a cylindrical lens in order to pump the DFB-cavity homogeneously. The laser activity was measured in backscattering geometry with a 0.85 m monochromator and a UV sensitive bialkali photomultiplier tube. The samples were kept at room temperature during the whole measurement.
3. Results and discussion
3.1. O6ergrown DFB lasers Fig. 1 shows the laser emission of five overgrown SCH-DH DFB lasers. The emission wavelength depends linearly on the grating period. This is in agreement with the Bragg condition l = 2neffL/m, where neff is the
Fig. 1. Room temperature laser emission of separate confinement DH DFB lasers with etched and overgrown gratings. The grating periods L are indicated above the laser peaks.
387
Fig. 2. Dependence of the laser peak intensity on the pump power density. The emission intensity was determined by integrating the area under the laser peak.
effective refractive index of the heterostructure, m=2 is the order of the refraction, and L is the grating period which proves that the feedback necessary for the laser emission is still caused by the gratings after the overgrowth. The DFB lasers show a clear threshold behaviour which can be seen in Fig. 2. Typical values measured for the laser threshold of the SCH double heterostructure as well as for the SCH multi quantum well structure are 2–3 MW cm − 2. This seems to be large compared to the best values of 0.9 MW cm − 2 which we obtained for a non overgrown but otherwise identical DH structure on SiC. However, to compare the overgrown with the non overgrown lasers, one has to take into account that the upper Al0.8Ga0.82N cladding layer absorbs the exciting excimer pump pulse rather strongly. Assuming an absorption coefficient of 8× 104 cm − 1 at 308 nm [8], we estimated that only 45% of the incident pulse reaches the upper GaN waveguide and contributes to the generation of carriers contributing to the laser emission. To prove this, we etched different deep stripes in the upper AlGaN layer of a SCH structure and measured the luminescence intensity in dependence of the etch depth. This confirmed the estimated attenuation of the excitation. Therefore, for comparison we corrected the thresholds of the overgrown lasers by the obtained factor which are then in the range 1 and 1.5 MW cm − 2 and are the same for non overgrown lasers.
3.2. Vertical modes To investigate the vertical modes in GaN-based laser
388
R. Hofmann et al. / Materials Science and Engineering B59 (1999) 386–389
Fig. 3. (a) Mode distribution of a thick GaInN/GaN double heterostructure on sapphire calculated at 390 nm by a transfer matrix program using the following refractive indices: nsapphire = 176, nGaN (390 nm) = 2.61, and nGalnN (390 nm) = 2.80. Three vertical modes are found with different confinement factors G; (b) Mode distribution of a thin GaInN/GaN double heterostructure on sapphire calculated at 410 nm by a transfer matrix program using following refractive indices: nsapphire = 176, nGaN (410 nm)=2.55, and nGalnN (410 nm)=2.71. Only the vertical ground mode is supported; (c,d) Typical emission spectra for the asymmetric DH (c) and symmetric DH (d) structure; (e,f) Emission wavelengths in dependence on the grating period for the asymmetric structure (e) and the symmetric structure (f). The dots indicate the measured values. The solid lines are calculated using the Bragg equation and effective refractive indices determined by a transfer matrix program. The dispersion of the refractive indices was taken into account for these calculations.
R. Hofmann et al. / Materials Science and Engineering B59 (1999) 386–389
structures on sapphire, we compared two GaInN/GaN DH structures, one with a thick buffer (500 nm), which we want to call asymmetric structure, the other with a thin buffer (100 nm), which we want to call symmetric structure because the buffer has the same thickness as the upper GaN layer. Details of the DH structures are given in Table 1. In Fig. 3(c,d), typical emission spectra of DFB lasers processed on the asymmetric and on the symmetric structure are shown. While DFB lasers on the symmetric structure always show only one emission peak, the spectra of DFB lasers on the asymmetric structure contain two peaks for certain grating periods. The spectral distance between these peaks is typically several nanometers. The occurrence of these peaks in the DFB laser spectra can be explained by the existence of several vertical modes. These are caused by the large refractive index steps between GaN/air and GaN/sapphire which causes the epitaxial layers to act as a cavity for the lightwave. Such modes were already observed in the farfield of an optically pumped SCH laser structure by Hofstetter [6]. In Fig. 3(a,b), the calculated vertical mode distribution for the asymmetric and the symmetric structure are plotted. The calculation was carried out by a transfer matrix program for a wavelength of 390 nm for the asymmetric structure and 410 nm for the symmetric structure using the refractive indices indicated in the figure caption. As can be seen in Fig. 3(b), the symmetric structure guides only one vertical mode, the ground mode TE0 because higher vertical modes do not fit in the small optical cavity. The confinement factor for this structure is 8.0%. The asymmetric structure guides three vertical modes. While the ground mode TE0 has only a very low confinement factor in the active region (G(TE0)=2.4%), the confinement factors of the two higher modes are G(TE1) = 4.8% and G(TE2)= 4.5% and are nearly the same. This suggests that the lasing modes in the asymmetric structure are the first and second order vertical modes. To prove this, we plotted the emission wavelength of each observed laser peak against the grating period (dots in Fig. 3(e,f). We than calculated the effective refractive indices heff of each vertical mode with the transfer matrix program and determined the expected Table 1 Vertical structure of the symmetric and asymmetric double heterostructures on sapphire used to investigate vertical modes
Substrate Nucleation GaN-buffer GaInN GaN
Asymmetric
Symmetric
Sapphire AlN 500 nm 12 nm 100 nm
Sapphire AlN 100 nm 10 nm 100 nm
389
emission wavelength in dependence of the grating period using the calculated heff and the Bragg equation for second order gratings l= h calc eff L The calculated emission wavelengths are plotted as solid lines in Fig. 3(e,f). One sees that the measured emission wavelengths match with the calculated emission wavelengths for the first and second order vertical modes in the asymmetric structure. The fact that the measured values are systematically smaller than the calculated values is attributed to band filling effects under high excitation conditions in the experiment. For the symmetric structure, a good agreement can be reached for the zero order vertical mode (Fig. 3(f)). In conclusion, we have demonstrated room temperature laser operation of optically pumped separate confinement GaInN/GaN DFB lasers with overgrown gratings. The profile of the gratings did not change significantly during overgrowth. The laser thresholds of these lasers are comparable to non overgrown lasers. DFB lasers on sapphire with a total epitaxial layer thickness greater than 250 nm show higher vertical modes. Which of these vertical modes is finally lasing is selected by the DFB grating period due to the different effective refractive indices heff of the vertical modes. Vertical single mode emission of FP lasers on sapphire are therefore expected only for very thin epitaxial layers, but this usually reduces the crystal quality.
Acknowledgements The authors wish to thank H. Gra¨beldinger, P. Burkard, K. Schleyer for technical support. This work was supported by the Deutsche Forschungsgesellschaft.
References [1] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsuchita, H. Kiyoku, Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) 217. [2] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsuchita, Y. Sugimoto, H. Kiyoku, Appl. Phys. Lett. 70 (1997) 616. [3] A. Kuramata, K. Domen, R. Soejima, K. Horino, S. Kubota, T. Tanahashi, Jap. J. Appl. Phys. 36 (1997) 1130 – 1132. [4] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsuchita, H. Kiyoku, Y. Sugimoto, Appl. Phys. Lett. 68 (1996) 2105. [5] R. Hofmann, V. Wagner, H.-P. Gauggel, H. Bolay, F. Scholz, H. Schweizer, IEEE J. Sel. Top. Quantum Electron. 3 (1997) 456. [6] D. Hofstetter, D.P. Bour, R.L. Thornton, N.M. Johnson, Appl. Phys. Lett. 70 (1997) 1650. [7] R. Hofmann, H.-P. Gauggel, U.A. Griesinger, H. Gra¨beldinger, F. Adler, P. Ernst, H. Bolay, V. Ha¨rle, F. Scholz, H. Schweizer, M.H. Pilkuhn, Appl. Phys. Lett. 69 (1996) 2068. [8] O. Ambacher, M. Arzberger, D. Brunner, H. Angerer, F. Freudenberg, N. Esser, T. Wethkamp, K. Wilmers, W. Richter, M. Stutzmann, MRS Internet J. Nitride Semicond. Res. 2 (1997) 22.