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Investigations of optical and electrical properties of In-doped GaN films grown by gas-source molecular beam epitaxy
Journal of Crystal Growth 209 (2000) 396}400
Investigations of optical and electrical properties of In-doped GaN "lms grown by gas-source molecular beam epitaxy X.Q. Shen!,*, P. Ramvall!, P. Riblet!, Y. Aoyagi!, K. Hosi", S. Tanaka", I. Suemune" !Semiconductors Laboratory, The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako-Shi, Saitama 351-0198, Japan "Research Institute for Electronic Science, Hokkaido University, N 12}W 6, Kita-ku, Sapporo 060-0812, Japan
Abstract Optical and electrical properties of GaN "lms grown on a-Al O (0 0 0 1) substrates by GSMBE using In-doping 2 3 method were investigated. It was found that both of them were improved, compared to those of a nondoped GaN one. l-PL results at 20 K indicated that In-doped "lms emit luminescence more uniformly than that of a nondoped one. Furthermore, Hall e!ect measurements at 300 K showed higher electron mobility of In-doped samples than that of a nondoped one. It is suggested that the presence of In during the growth of GaN "lms plays a role in reducing the number of structural imperfections to improve the optical and electrical properties. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 73.61.Ey; 78.55.Cr; 81.05.Ea; 81.15.Hi Keywords: In-doping; GaN "lms; Photoluminescence; l-Photoluminescence; Gas-source molecular beam epitaxy
1. Introduction Wide band-gap GaN and related III}V nitride materials have attracted a great deal of attention due to its potential use in optic and electronic devices. However, GaN and AlGaN, which are suitable for ultraviolet LDs applications, have not been widely used yet. The main reason is thought to be the poor luminescence e$ciency of these materials. Recently, it has been reported that the photoluminescence (PL) intensity of GaN "lms at room * Corresponding author. Present address: Materials Science Division, Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan. Tel.: #81-298-54-3373; fax: #81-298-54-5434. E-mail address: [email protected] (X.Q. Shen)
temperature (RT) was greatly enhanced by In-doping method during the gas-source molecular beam epitaxy (GSMBE) growth, which could be more than 30 times stronger in PL magnitude than that of a nondoped sample [1]. Several research groups have also reported positive e!ects of In on the improvements of optical properties of GaN "lms grown by metalorganic chemical vapor deposition (MOCVD) [2] and molecular beam epitaxy (MBE) [3,4]. Both surfactant e!ect and incorporation effect of In during the MBE growth of GaN were suggested. To understand the In-doping mechanism more deeply, it is necessary to investigate optical properties, especially in the micrometer area, together with electrical properties. In this paper, we report on an investigation of optical and electrical properties of In-doped GaN
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 5 7 8 - 3
X.Q. Shen et al. / Journal of Crystal Growth 209 (2000) 396}400
397
"lms, which were grown by GSMBE. PL, l-PL (20 K) and Hall e!ect measurement (300 K) were used to characterize these GaN samples.
2. Experimental procedure In-doping method means that In #ux is supplied during the GaN growth at high temperature, which is suitable for GaN growth but is much higher for growth of In Ga N. Therefore, the incorporatx 1~x ing amount of In is very low, which is thought to be less than 0.1% in composition. The GaN "lms were grown on a-Al O (0 0 0 1) substrates by GSMBE. 2 3 Details of the experimental conditions and processes were reported in Ref. [1]. The growth temperature was "xed at 7253C, while In #uxes were changed for di!erent In-doped GaN samples. To investigate the optical properties of GaN "lms, PL (10}300 K) and l-PL (20 K) measurements were performed, respectively, using He}Cd laser as an excitation source. In case of l-PL measurements, the size of laser spot was focused to about 1 lm and the distance between two measuring points was about 10 lm. Hall e!ect measurement was carried out at 300 K by using Van der Pauw technique.
3. Results and discussion 3.1. PL and k-PL characterizations of In-doped and nondoped GaN xlms Firstly, In-doped and nondoped GaN "lms were characterized by usual PL measurements. Measuring temperatures of PL were changed from 10 to 300 K. Within the limit of detection, none of the investigated samples exhibited any yellow luminescence. Fig. 1 shows the PL spectra of one nondoped GaN sample (Fig. 1a) and one In-doped sample (Fig. 1b), respectively. Above 140 K, both samples show single peak related to band-edge emissions. However, additional two peaks (around 3.418 and 3.315 eV) appear below 100 K, which are thought to be originated from defects or O related and a donor to acceptor (D}A) pair recombination, respectively [5}7]. The behaviors of these peaks are quite di!erent between the nondoped sample and
Fig. 1. PL spectra at various measuring temperatures of GaN "lms grown at the same growth conditions (a) without and (b) with an In supply during growth. In #ux is 1.05]1015 atoms/cm2 s.
the In-doped one as shown in Fig. 1. In case of nondoped sample, intensities of these two peaks become stronger with decreasing the measuring temperature, compared to the band-edge emission. However, In-doped sample shows very low emissions of these two peaks. Furthermore, the Indoped sample shows no shift of band-edge emission compared to the nondoped one, which indicates
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X.Q. Shen et al. / Journal of Crystal Growth 209 (2000) 396}400
that In incorporation is less than 0.1% in composition. We do not have direct evidence for the In incorporation, however, Foxon et al. [4] reported the SIMS results showing a small amount of In incorporation in the GaN "lms. The reduction of the PL peaks at 3.418 and 3.315 eV indicates that the presence of In during growth is responsible of the suppression of the defects or O-related PL emission and the DA-pair recombination. To investigate the optical properties in more detail, l-PL technique was applied to characterize the PL emission uniformity of grown samples at micrometer area. The spectra are similar to that shown in Fig. 1, but the dependence of peak intensity on measuring positions shows di!erent behaviors between the nondoped and the In-doped GaN samples. More uniform PL emission is obtained from the In-doped sample than from the nondoped one. Since PL intensity would be in#uenced by surface morphologies, it is more accurate to compare the peak intensity ratio than the peak intensity itself. In this case, the surface morphology e!ect should be removed. Fig. 2 shows the measuring position dependence of the intensity ratio of peaks (3.418 and 3.315 eV) to the band-edge emission peak (3.462 eV). It is clear that ratios of each peak depend on the measuring position in case of nondoped GaN sample; however, ratios almost do not change for the In-doped one. From this result, it suggests that the In-doped sample gives a more uniform PL emission than the nondoped sample does. 3.2. Electrical properties of In-doped GaN To further investigate the e!ects of In, Hall effect measurements at 300 K have been carried out. All samples were found to be n-type and the carrier concentration varied in the range of 1018}1019 cm~3. The Hall mobility and the carrier concentration are shown in Table 1. The mobility of a nondoped sample is only about 19 cm2/V s, while in case of In-doped samples, it could be increased up to about 80 cm2/V s. Even though the absolute values of the mobility in our experiments are not better than the reported results [8], the trend is clear: the mobility increases when In is supplied during growth of the GaN "lms. Since the
Fig. 2. The position dependence of the ratio of peak intensities (3.418 and 3.315 eV) to the band-edge emission peak one (3.462 eV) for GaN "lms (a) without and (b) with In supply during the growth, measured by l-PL results at 20 K. In #ux is 1.05]1015 atoms/cm2 s.
electron mobility depends on the In #ux but the concentration does not as shown in Table 1, we think that the compensation ratio of the crystals might be changed. This phenomenon has been previously observed for GaAs [9] and InP [10].
X.Q. Shen et al. / Journal of Crystal Growth 209 (2000) 396}400
399
Table 1 Sample name
In #ux (1014 atoms/cm2 s)
Carrier concentration (1018 cm~3)
Electron mobility (cm2/V s)
C1 C2 C3 C4 C5
0 4.1 6.2 10.5 15.3
7.3 19.1 35.8 8.2 1.5
18.7 26.2 33.5 52.3 79.9
3.3. Discussions As indicated above, both optical and electrical properties of In-doped GaN are improved. The strong PL peaks at 3.418 and 3.315 eV, which is prominent in the nondoped GaN sample but merely absent in the In-doped samples, have been attributed to O or structural defects and DA-pair, respectively [5}7]. It is known that O and N vacancy constitute donors [11,12], while the Gavacancy constitutes an acceptor [13,14]. The Ga-vacancy is also believed to cause yellow luminescence in GaN and, in addition, to act as a compensation center [15]. Since no systematic change in the electron concentration was observed regardless of the In-dose, it may be assumed that the improvement in crystal quality is obtained by eliminating both donors and acceptors by In thus leading to a reduced degree of compensation in the GaN crystal. Careful study suggests that the PL peak at 3.418 eV may not be caused by O, but by structural defects. From l-PL results, more uniform PL emission from In-doped sample than that from nondoped one was obtained. Since defects including vacancies in the GaN "lm are considered as nonradiative centers, the uniform PL emission from In-doped sample indicates that those nonradiative centers are greatly reduced or compensated by In incorporation. It is assumed that the distribution of nonradiative centers is random in conventional GaN "lms, therefore, it might cause the nonuniform PL emission, depending on the position. On the other hand, when In #ux is supplied, the In plays a role as surfactant and incorporation e!ects [3,4]. These e!ects contribute to the high crystal quality and the reduction of nonradiative centers (such as defects and vacancies),
resulting in an uniform PL emission. This assumption is also supported by usual PL and Hall measurement results. Therefore, although the exact physical mechanism governing the improvement in the crystal quality is presently not clear, an increase of the surface di!usion length by an In surfactant e!ect and an incorporation e!ect of In into Ga vacancy, may constitute a possible explanation.
4. Summary In conclusion, both optical and electrical properties of GaN "lms grown on a-Al O (0 0 0 1) sub2 3 strates by GSMBE using In-doping method were found to be improved, compared to those of a nondoped GaN one. PL measurements show that the band-edge-related emission was enhanced by more than one order of magnitude in the presence of In during growth, while the luminescence originating from D}A pair recombination and structural defects was reduced. l-PL results at 20 K indicated that In-doped "lms emit luminescence more uniformly than that of a nondoped one. Furthermore, Hall e!ect measurements at 300 K showed higher electron mobility of In-doped samples than that of a nondoped one. It is suggested that the presence of In during growth of GaN "lms plays a role in reducing the number of structural imperfections to improve the optical and electrical properties.
Acknowledgements The authors would like to thank H. Hirayama for useful discussion and help in this work. This work was supported by Grant-in-Aid for
400
X.Q. Shen et al. / Journal of Crystal Growth 209 (2000) 396}400
Developmental Scienti"c Research No. C09650374 from the Ministry of Education, Science, Sports and Culture, Japan and by the Science and Technology Agency (STA) of Japan.
References [1] X.Q. Shen, Y. Aoyagi, Jpn. J. Appl. Phys. 38 (1999) L14. [2] C.K. Shu, J. Ou, H.C. Lin, W.K. Chen, M.C. Lee, Appl. Phys. Lett. 73 (1998) 641. [3] F. Widmann, B. Daudin, G. Feuillet, N. Pelekanos, J.L. Rouviere, Appl. Phys. Lett. 73 (1998) 2642. [4] C.T. Foxon, S.E. Hooper, T.S. Cheng, J.W. Orton, G.B. Ren, B.Ya. Ber, A.V. Merkulov, S.V. Novikov, V.V. Tret'yakov, Semicond. Sci. Technol. 13 (1998) 1469. [5] B.-C. Chung, M. Gershenzon, J. Appl. Phys. 72 (1992) 651. [6] S. Fischer, C. Wetzel, W. Walukiewicz, E.E. Haller, Mater. Res. Soc. Symp. Proc. 395 (1996) 571.
[7] X.Q. Shen, P. Ramvall, P. Riblet, Y. Aoyagi, Jpn. J. Appl. Phys. 38 (1999) L411. [8] S. Nakamura, Y. Harada, M. Seno, Appl. Phys. Lett. 58 (1991) 2021. [9] W. Walukiewicz, J. Lagowski, H.C. Gatos, J. Appl. Phys. 53 (1982) 769. [10] W. Walukiewicz, J. Lagowski, L. Jastrzebski, P. Rava, M. Lichtensteiger, C.H. Gatos, H.C. Gatos, J. Appl. Phys. 51 (1980) 2659. [11] W. Seifert, R. Franzheld, E. Butter, H. Sobotta, V. Riede, Cryst. Res. Technol. 18 (1983) 383. [12] H.P. Maruska, J.J. Tietjen, Appl. Phys. Lett. 15 (1969) 327. [13] J. Neugebauer, C.G. Van de Walle, Appl. Phys. Lett. 69 (1996) 503. [14] K. Saarinen, T. Laine, S. Kuisma, J. NissilaK , P. HautojaK rvi, L. Dobrzynski, J.M. Baranowski, K. Pakula, R. Stepniewski, M. Wodjak, A. Wysmolek, T. Suski, M. Leszczynski, I. Grzegory, S. Porowski, Phys. Rev. Lett. 79 (1997) 3030. [15] K. Saarinen, P. SeppaK laK , J. Oila, P. HautojaK rvi, C. Corbel, O. Briot, R.L. Aulombard, Appl. Phys. Lett. 73 (1998) 3253.
Investigations of optical and electrical properties of In-doped GaN "lms grown by gas-source molecular beam epitaxy X.Q. Shen!,*, P. Ramvall!, P. Riblet!, Y. Aoyagi!, K. Hosi", S. Tanaka", I. Suemune" !Semiconductors Laboratory, The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako-Shi, Saitama 351-0198, Japan "Research Institute for Electronic Science, Hokkaido University, N 12}W 6, Kita-ku, Sapporo 060-0812, Japan
Abstract Optical and electrical properties of GaN "lms grown on a-Al O (0 0 0 1) substrates by GSMBE using In-doping 2 3 method were investigated. It was found that both of them were improved, compared to those of a nondoped GaN one. l-PL results at 20 K indicated that In-doped "lms emit luminescence more uniformly than that of a nondoped one. Furthermore, Hall e!ect measurements at 300 K showed higher electron mobility of In-doped samples than that of a nondoped one. It is suggested that the presence of In during the growth of GaN "lms plays a role in reducing the number of structural imperfections to improve the optical and electrical properties. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 73.61.Ey; 78.55.Cr; 81.05.Ea; 81.15.Hi Keywords: In-doping; GaN "lms; Photoluminescence; l-Photoluminescence; Gas-source molecular beam epitaxy
1. Introduction Wide band-gap GaN and related III}V nitride materials have attracted a great deal of attention due to its potential use in optic and electronic devices. However, GaN and AlGaN, which are suitable for ultraviolet LDs applications, have not been widely used yet. The main reason is thought to be the poor luminescence e$ciency of these materials. Recently, it has been reported that the photoluminescence (PL) intensity of GaN "lms at room * Corresponding author. Present address: Materials Science Division, Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan. Tel.: #81-298-54-3373; fax: #81-298-54-5434. E-mail address: [email protected] (X.Q. Shen)
temperature (RT) was greatly enhanced by In-doping method during the gas-source molecular beam epitaxy (GSMBE) growth, which could be more than 30 times stronger in PL magnitude than that of a nondoped sample [1]. Several research groups have also reported positive e!ects of In on the improvements of optical properties of GaN "lms grown by metalorganic chemical vapor deposition (MOCVD) [2] and molecular beam epitaxy (MBE) [3,4]. Both surfactant e!ect and incorporation effect of In during the MBE growth of GaN were suggested. To understand the In-doping mechanism more deeply, it is necessary to investigate optical properties, especially in the micrometer area, together with electrical properties. In this paper, we report on an investigation of optical and electrical properties of In-doped GaN
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 5 7 8 - 3
X.Q. Shen et al. / Journal of Crystal Growth 209 (2000) 396}400
397
"lms, which were grown by GSMBE. PL, l-PL (20 K) and Hall e!ect measurement (300 K) were used to characterize these GaN samples.
2. Experimental procedure In-doping method means that In #ux is supplied during the GaN growth at high temperature, which is suitable for GaN growth but is much higher for growth of In Ga N. Therefore, the incorporatx 1~x ing amount of In is very low, which is thought to be less than 0.1% in composition. The GaN "lms were grown on a-Al O (0 0 0 1) substrates by GSMBE. 2 3 Details of the experimental conditions and processes were reported in Ref. [1]. The growth temperature was "xed at 7253C, while In #uxes were changed for di!erent In-doped GaN samples. To investigate the optical properties of GaN "lms, PL (10}300 K) and l-PL (20 K) measurements were performed, respectively, using He}Cd laser as an excitation source. In case of l-PL measurements, the size of laser spot was focused to about 1 lm and the distance between two measuring points was about 10 lm. Hall e!ect measurement was carried out at 300 K by using Van der Pauw technique.
3. Results and discussion 3.1. PL and k-PL characterizations of In-doped and nondoped GaN xlms Firstly, In-doped and nondoped GaN "lms were characterized by usual PL measurements. Measuring temperatures of PL were changed from 10 to 300 K. Within the limit of detection, none of the investigated samples exhibited any yellow luminescence. Fig. 1 shows the PL spectra of one nondoped GaN sample (Fig. 1a) and one In-doped sample (Fig. 1b), respectively. Above 140 K, both samples show single peak related to band-edge emissions. However, additional two peaks (around 3.418 and 3.315 eV) appear below 100 K, which are thought to be originated from defects or O related and a donor to acceptor (D}A) pair recombination, respectively [5}7]. The behaviors of these peaks are quite di!erent between the nondoped sample and
Fig. 1. PL spectra at various measuring temperatures of GaN "lms grown at the same growth conditions (a) without and (b) with an In supply during growth. In #ux is 1.05]1015 atoms/cm2 s.
the In-doped one as shown in Fig. 1. In case of nondoped sample, intensities of these two peaks become stronger with decreasing the measuring temperature, compared to the band-edge emission. However, In-doped sample shows very low emissions of these two peaks. Furthermore, the Indoped sample shows no shift of band-edge emission compared to the nondoped one, which indicates
398
X.Q. Shen et al. / Journal of Crystal Growth 209 (2000) 396}400
that In incorporation is less than 0.1% in composition. We do not have direct evidence for the In incorporation, however, Foxon et al. [4] reported the SIMS results showing a small amount of In incorporation in the GaN "lms. The reduction of the PL peaks at 3.418 and 3.315 eV indicates that the presence of In during growth is responsible of the suppression of the defects or O-related PL emission and the DA-pair recombination. To investigate the optical properties in more detail, l-PL technique was applied to characterize the PL emission uniformity of grown samples at micrometer area. The spectra are similar to that shown in Fig. 1, but the dependence of peak intensity on measuring positions shows di!erent behaviors between the nondoped and the In-doped GaN samples. More uniform PL emission is obtained from the In-doped sample than from the nondoped one. Since PL intensity would be in#uenced by surface morphologies, it is more accurate to compare the peak intensity ratio than the peak intensity itself. In this case, the surface morphology e!ect should be removed. Fig. 2 shows the measuring position dependence of the intensity ratio of peaks (3.418 and 3.315 eV) to the band-edge emission peak (3.462 eV). It is clear that ratios of each peak depend on the measuring position in case of nondoped GaN sample; however, ratios almost do not change for the In-doped one. From this result, it suggests that the In-doped sample gives a more uniform PL emission than the nondoped sample does. 3.2. Electrical properties of In-doped GaN To further investigate the e!ects of In, Hall effect measurements at 300 K have been carried out. All samples were found to be n-type and the carrier concentration varied in the range of 1018}1019 cm~3. The Hall mobility and the carrier concentration are shown in Table 1. The mobility of a nondoped sample is only about 19 cm2/V s, while in case of In-doped samples, it could be increased up to about 80 cm2/V s. Even though the absolute values of the mobility in our experiments are not better than the reported results [8], the trend is clear: the mobility increases when In is supplied during growth of the GaN "lms. Since the
Fig. 2. The position dependence of the ratio of peak intensities (3.418 and 3.315 eV) to the band-edge emission peak one (3.462 eV) for GaN "lms (a) without and (b) with In supply during the growth, measured by l-PL results at 20 K. In #ux is 1.05]1015 atoms/cm2 s.
electron mobility depends on the In #ux but the concentration does not as shown in Table 1, we think that the compensation ratio of the crystals might be changed. This phenomenon has been previously observed for GaAs [9] and InP [10].
X.Q. Shen et al. / Journal of Crystal Growth 209 (2000) 396}400
399
Table 1 Sample name
In #ux (1014 atoms/cm2 s)
Carrier concentration (1018 cm~3)
Electron mobility (cm2/V s)
C1 C2 C3 C4 C5
0 4.1 6.2 10.5 15.3
7.3 19.1 35.8 8.2 1.5
18.7 26.2 33.5 52.3 79.9
3.3. Discussions As indicated above, both optical and electrical properties of In-doped GaN are improved. The strong PL peaks at 3.418 and 3.315 eV, which is prominent in the nondoped GaN sample but merely absent in the In-doped samples, have been attributed to O or structural defects and DA-pair, respectively [5}7]. It is known that O and N vacancy constitute donors [11,12], while the Gavacancy constitutes an acceptor [13,14]. The Ga-vacancy is also believed to cause yellow luminescence in GaN and, in addition, to act as a compensation center [15]. Since no systematic change in the electron concentration was observed regardless of the In-dose, it may be assumed that the improvement in crystal quality is obtained by eliminating both donors and acceptors by In thus leading to a reduced degree of compensation in the GaN crystal. Careful study suggests that the PL peak at 3.418 eV may not be caused by O, but by structural defects. From l-PL results, more uniform PL emission from In-doped sample than that from nondoped one was obtained. Since defects including vacancies in the GaN "lm are considered as nonradiative centers, the uniform PL emission from In-doped sample indicates that those nonradiative centers are greatly reduced or compensated by In incorporation. It is assumed that the distribution of nonradiative centers is random in conventional GaN "lms, therefore, it might cause the nonuniform PL emission, depending on the position. On the other hand, when In #ux is supplied, the In plays a role as surfactant and incorporation e!ects [3,4]. These e!ects contribute to the high crystal quality and the reduction of nonradiative centers (such as defects and vacancies),
resulting in an uniform PL emission. This assumption is also supported by usual PL and Hall measurement results. Therefore, although the exact physical mechanism governing the improvement in the crystal quality is presently not clear, an increase of the surface di!usion length by an In surfactant e!ect and an incorporation e!ect of In into Ga vacancy, may constitute a possible explanation.
4. Summary In conclusion, both optical and electrical properties of GaN "lms grown on a-Al O (0 0 0 1) sub2 3 strates by GSMBE using In-doping method were found to be improved, compared to those of a nondoped GaN one. PL measurements show that the band-edge-related emission was enhanced by more than one order of magnitude in the presence of In during growth, while the luminescence originating from D}A pair recombination and structural defects was reduced. l-PL results at 20 K indicated that In-doped "lms emit luminescence more uniformly than that of a nondoped one. Furthermore, Hall e!ect measurements at 300 K showed higher electron mobility of In-doped samples than that of a nondoped one. It is suggested that the presence of In during growth of GaN "lms plays a role in reducing the number of structural imperfections to improve the optical and electrical properties.
Acknowledgements The authors would like to thank H. Hirayama for useful discussion and help in this work. This work was supported by Grant-in-Aid for
400
X.Q. Shen et al. / Journal of Crystal Growth 209 (2000) 396}400
Developmental Scienti"c Research No. C09650374 from the Ministry of Education, Science, Sports and Culture, Japan and by the Science and Technology Agency (STA) of Japan.
References [1] X.Q. Shen, Y. Aoyagi, Jpn. J. Appl. Phys. 38 (1999) L14. [2] C.K. Shu, J. Ou, H.C. Lin, W.K. Chen, M.C. Lee, Appl. Phys. Lett. 73 (1998) 641. [3] F. Widmann, B. Daudin, G. Feuillet, N. Pelekanos, J.L. Rouviere, Appl. Phys. Lett. 73 (1998) 2642. [4] C.T. Foxon, S.E. Hooper, T.S. Cheng, J.W. Orton, G.B. Ren, B.Ya. Ber, A.V. Merkulov, S.V. Novikov, V.V. Tret'yakov, Semicond. Sci. Technol. 13 (1998) 1469. [5] B.-C. Chung, M. Gershenzon, J. Appl. Phys. 72 (1992) 651. [6] S. Fischer, C. Wetzel, W. Walukiewicz, E.E. Haller, Mater. Res. Soc. Symp. Proc. 395 (1996) 571.
[7] X.Q. Shen, P. Ramvall, P. Riblet, Y. Aoyagi, Jpn. J. Appl. Phys. 38 (1999) L411. [8] S. Nakamura, Y. Harada, M. Seno, Appl. Phys. Lett. 58 (1991) 2021. [9] W. Walukiewicz, J. Lagowski, H.C. Gatos, J. Appl. Phys. 53 (1982) 769. [10] W. Walukiewicz, J. Lagowski, L. Jastrzebski, P. Rava, M. Lichtensteiger, C.H. Gatos, H.C. Gatos, J. Appl. Phys. 51 (1980) 2659. [11] W. Seifert, R. Franzheld, E. Butter, H. Sobotta, V. Riede, Cryst. Res. Technol. 18 (1983) 383. [12] H.P. Maruska, J.J. Tietjen, Appl. Phys. Lett. 15 (1969) 327. [13] J. Neugebauer, C.G. Van de Walle, Appl. Phys. Lett. 69 (1996) 503. [14] K. Saarinen, T. Laine, S. Kuisma, J. NissilaK , P. HautojaK rvi, L. Dobrzynski, J.M. Baranowski, K. Pakula, R. Stepniewski, M. Wodjak, A. Wysmolek, T. Suski, M. Leszczynski, I. Grzegory, S. Porowski, Phys. Rev. Lett. 79 (1997) 3030. [15] K. Saarinen, P. SeppaK laK , J. Oila, P. HautojaK rvi, C. Corbel, O. Briot, R.L. Aulombard, Appl. Phys. Lett. 73 (1998) 3253.