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GaInP double heterostructures grown by organometallic vapor phase epitaxy
Physica B 308–310 (2001) 891–894
Luminescence properties of Er,O-codoped GaAs/GaInP double heterostructures grown by organometallic vapor phase epitaxy A. Koizumia,*, N. Watanabea, K. Inouea, Y. Fujiwaraa, Y. Takedaa,b a
Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan b CREST, JST (Japan Science and Technology), Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
Abstract Er,O-codoped GaAs/GaInP double heterostructures (DHS : Er,O) were grown by organometallic vapor phase epitaxy and the intra-4f shell transitions of Er ions were investigated by photoluminescence (PL) measurements. Er,Ocodoping was carried out with trisdipivaloylmethanatoerbium (Er(DPM)3) as an Er source and an addition of O2 diluted with Ar to a reactor. Er,O-codoped GaAs (GaAs : Er,O) and GaInP layers were all grown at 5501C. In-depth profiles showed a uniform distribution of Er and O along the growth direction in the GaAs : Er,O active layer. The Er concentration in the GaAs : Er,O active layer was evaluated to be about 5 1017 cm 3. In 4.2 K PL measurements, DHS : Er,O sample exhibited Er–2O lines and the intensity was approximately three times stronger than that of GaAs : Er,O sample. This suggests that photoexcited carriers confined in the GaAs : Er,O active layer contribute effectively to excitation of Er. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Erbium; Oxygen; Double heterostructures; Organometallic vapor phase epitaxy
1. Introduction Recently, there has been a wide scientific and technological interest in semiconductors doped with rare-earth (RE) impurities. RE isoelectronic impurity incorporated into semiconductors shows sharp and temperature-stable intra-4f-shell luminescence. This stability occurs because the filled outer 5s and 5p electron shells screen transitions within the inner 4f electron shell from interaction with the host. These properties are attractive for fabricating optical devices such as semiconductor lasers and optical amplifiers. Special attention has been paid to Er ions in III–V semiconductors. Er doping in III–V semiconductors produces sharp luminescence at around 1.54 mm due to *Corresponding author. Tel.: +81-52-789-3620; fax: +8152-789-3239. E-mail address: [email protected] (A. Koizumi).
intra-4f-shell transition from the first excited state (4I13/2) to the ground state (4I15/2) of Er ions. The emission is coincident with the minimum loss wavelength region of silica-based fibers. However, it has been found that Er ions doped in semiconductors show various fine structures, depending on doping conditions. This indicates that various kinds of Er centers with different atom configurations are formed simultaneously in a semiconductor because the fine structures of photoluminescence (PL) spectra reflect atomic configurations around Er ions. Selective formation of highly efficient Er centers for luminescence has been strongly desired for applications of Er-related luminescence to light-emitting devices. Oxygen (O) has been found to play an important role in the production of efficient Er-related luminescent centers in GaAs. Er,O-codoped GaAs (GaAs : Er,O) produces a strong emission intensity and simple PL spectrum predominantly from one kind of Er center [1]. The Er center has been identified as an Er atom located
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 9 5 1 - 6
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at the Ga sublattice with two adjacent O atoms (Er–2O center) [2]. The dependence of the PL spectra on growth temperature revealed the existence of a threshold growth temperature between 5601C and 5801C for Er incorporation into GaAs [3]. For optical device applications, Er should be introduced in confinement structures such as double heterostructures (DHS). In these structures, a GaAs : Er,O layer should be sandwiched between layers of a different semiconductor. The growth of Ga0.51In0.49P (hereafter GaInP) is very attractive as an aluminum-free alternative to the conventional AlxGa1 xAs-based materials. The aluminum-free GaAs/GaInP material system has advantages over the GaAs/AlxGa1 xAs material system, such as the low reactivity of GaInP to O. In this paper, we report the results on fabrication of GaAs : Er,O/GaInP double heterostructures (DHS : Er,O) grown by organometallic vapor phase epitaxy (OMVPE) and its luminescence properties.
2. Experimental GaAs : Er,O and DHS : Er,O samples used in this study were fabricated by a low-pressure growth system with a cold-wall quartz 4-barrel reactor [4]. The reactor pressure was 0.1 atm. The substrates were semiinsulating and Si-doped GaAs was oriented in the (1 0 0) direction. Triethylgallium (TEGa), trimethylindium (TMIn), tertiarybutylarsine (TBAs) and tertiarybutylphosphine (TBP) were used as sources. Er doping was carried out with trisdipivaloylmethanatoerbium (Er(C11H19O2)3, Er(DPM)3) as an Er source which was introduced into the reactor by a H2 flow through the Er source cylinder. The rate of the H2 flow was kept at 125 sccm. Ar gas with 38.4 ppm of 18O2 was used as an additional O2 source [3]. The O2 content in growth ambient was set at 0.2 ppm. Growth sequence for GaAs–GaInP interface used in this work is as follows. At the beginning of the layer growth, group-V gas was switched on first for 1 s and then group-III gas was switched on. At the end of layer growth, the group-III gas was switched off and growth was interrupted under the group-V gas to change TEGa flow rate and substrate temperature. In the growth of GaAs/GaInP DHS, we used two growth sequences (sequences A and B). Variations of gas flow and growth temperature for the sequences are schematically drawn in Fig. 1. In sequence A, GaInP and GaAs layers were grown at 5801C and 5401C. The temperatures were optimum growth temperatures for GaInP and GaAs : Er,O, respectively. In sequence B, GaAs and GaInP layers were all grown at 5501C. The crystal quality of the DHS was characterized by PL measurements at 77 K using the DHS having 200 nm-thick GaAs without Er,Ocodoping and 200 nm-thick GaInP cladding layer.
Fig. 1. Variations of gas flow and growth temperature for (a) sequence A and (b) sequence B. In sequence A, GaInP and GaAs layers were grown at 5801C and 5401C, respectively. In sequence B, on the other hand, these layers were all grown at 5501C.
The measurements were carried out with a 514.5 nm beam of Ar-ion laser and an S1-type detector cooled by dry ice. The structure and growth conditions of DHS : Er,O samples are as follows. All the samples were initiated by the growth of a 150 nm thick GaAs buffer layer at 6101C. Then, 1 mm-thick GaInP cladding layer, 1 mmthick GaAs : Er,O active layer and 0.6 mm-thick GaInP cladding layer were grown with sequence B. These thicknesses were evaluated using undoped GaAs/GaInP DHS samples by a scanning electron microscope (SEM). As a reference sample, GaAs : Er,O was grown under the same doping conditions as the active layer of DHS : Er,O. PL measurements of Er-related luminescence were carried out mainly with the samples directly immersed in liquid He at 4.2 K. The photoexcitation source was a CW-mode semiconductor laser diode, operating at 660 nm, with an incident power of 30 mW. The photon energy is below the band-gap of the GaInP cladding
A. Koizumi et al. / Physica B 308–310 (2001) 891–894
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layer, which enabled us to excite the GaAs active layer directly. The luminescence of the sample was dispersed using a 0.91 m grating monochromator and detected with a liquid nitrogen-cooled Ge p–i–n photodiode using a chopper and a lock-in technique. For measurements of concentration profiles by secondary ion mass spectroscopy (SIMS), an O+ ion and a Cs ion were used as primary ions for Er and O, respectively.
corresponding to the near-band-edge emission from GaAs. Also, a broad peak is weakly observed at 850 nm. This result suggests that photoexcited carriers injected in GaAs active layer recombine effectively in GaAs layer. Therefore, we can conclude that samples grown with sequence B have good quality of GaAs–GaInP interface compared with sequence A.
3. Results and discussion
DHS : Er,O samples were prepared with sequence B. In-depth profiles of Er, O, Ga and As for DHS : Er,O are shown in Fig. 3. For the DHS : Er,O sample, a distinct GaAs/GaInP DHS is seen in the SIMS profile. The profile reveals a uniform distribution of Er and O along the growth direction in the GaAs : Er,O active layer. Furthermore, in GaInP cladding layers, Er concentration is below the SIMS detection limit and O concentration is at the background level. Er concentration in GaAs : Er,O active layer, calibrated by using an Erimplanted GaAs sample, was evaluated to be about 5 1017 cm 3. This Er concentration was the same as GaAs : Er,O reference sample.
3.1. PL characterization of undoped GaAs/GaInP DHS It is well known that making a high-quality GaInP– GaAs interface is very difficult via OMVPE growth because of the problems in As–P substitution and indium memory effects in reactor gas switching [5–13]. Characteristics of GaInP–GaAs interface strongly depend on the growth sequence for the interface. Fig. 2(a) shows 77 K PL spectrum of undoped DHS grown with sequence A. The spectrum has one peak at wavelength of 884 nm with the full-width at half-maximum (FWHM) of 54 meV. The wavelength of 884 nm is longer than those of the near-band-edge emissions from GaAs. Several researchers have also reported a similar broad peak at wavelengths between 850 and 930 nm in low-temperature PL spectra of GaAs/GaInP heterostructures [10,12]. Fig. 2(b) shows PL spectrum of a sample grown with sequence B. The spectrum is dominated by a peak
Fig. 2. 77 K PL spectra in undoped GaAs/GaInP DHS grown with (a) sequence A and (b) sequence B. An interfacerelated peak at 884 nm disappears in the sample grown with sequence B.
3.2. SIMS profiles in DHS : Er,O sample
Fig. 3. In-depth profiles of Er, O, Ga and As for DHS : Er,O. Er and O are introduced selectively in the GaAs : Er,O active layer.
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are selectively formed in GaAs : Er,O active layer. The intensity of Er-related luminescence was about three times higher than the GaAs : Er,O sample grown under the same doping conditions and Er concentration as in the GaAs : Er,O active layer of DHS : Er,O. These results are interpreted easily by the advantage of the DHS; confinement of photoexcited carriers and elimination of surface recombination.
Acknowledgements
Fig. 4. 4.2 K Er-related PL spectra in (a) DHS : Er,O and (b) GaAs : Er,O. As shown by arrows, the spectrum is dominated by the Er–2O lines, indicating that an Er–2O center is selectively formed in GaAs : Er,O active layer of DHS : Er,O.
3.3. Er-related luminescence in DHS : Er,O In Fig. 4, the 4.2 K I13/2- I15/2 Er-related luminescence from DHS : Er,O sample is compared with the luminescence from the GaAs : Er,O reference sample. The spectrum of DHS : Er,O shows identical positions of Er-related PL lines as in the GaAs : Er,O. Vertical arrows denote the Er–2O lines. This indicates that Er–2O centers are selectively formed in GaAs : Er,O active layers of DHS : Er,O. The intensity of the Errelated luminescence is approximately three times stronger than that of GaAs : Er,O sample. These results suggest that carriers photoexcited in the GaAs : Er,O region contribute effectively to excitation of Er because GaInP cladding layers confine the carriers and eliminate the surface recombination. 4
4
4. Conclusion We have successfully grown Er,O-codoped GaAs/ GaInP double heterostructures. In-depth profiles of Er and O exhibited a uniform distribution along the growth direction in the GaAs : Er,O active layer. The Er-related luminescence indicated that Er–2O centers
The authors would like to thank Hitachi Cable Ltd. for GaAs substrates. This work was supported in part by Grant-in-Aids for Scientific Research (A)(2) No. 13305022, for Scientific Research (B)(2) No. 11450119 and No. 13555002, for Exploration Research No. 13875062 from the Ministry of Education, Science, Sports and Culture.
References [1] K. Takahei, A. Taguchi, J. Appl. Phys. 74 (1993) 1979. [2] K. Takahei, A. Taguchi, Y. Horikoshi, J. Nakata, J. Appl. Phys. 76 (1994) 4332. [3] Y. Fujiwara, T. Kawamoto, T. Koide, Y. Takeda, Physica B 273–274 (1999) 770. [4] Y. Fujiwara, S. Furuta, K. Makita, Y. Ito, Y. Nonogaki, Y. Takeda, J. Crystal Growth 146 (1995) 544. [5] T. Nittono, S. Sugitani, F. Hyuga, J. Appl. Phys. 78 (1995) 5387. [6] M. Razeghi, Ph. Maurel, F. Omnes, M. Defour, C. Boothroyd, W.M. Stobbs, M. Kelly, J. Appl. Phys. 63 (1988) 4511. [7] F. Omnes, M. Razeghi, Appl. Phys. Lett. 59 (1991) 1034. [8] X. He, M. Razeghi, Appl. Phys. Lett. 61 (1992) 1703. [9] D.P. Bour, J.R. Shealy, S. Mckernan, J. Appl. Phys. 63 (1988) 1241. [10] F.E.G. Guimaraes, B. Elsner, R. Westphalen, B. Spangenberg, H.J. Geelen, P. Balk, K. Heime, J. Crystal Growth 124 (1992) 199. [11] R. Bhat, M.A. Koza, M.J.S.P. Brasil, R.E. Nahory, C.J. Palmstorm, B.J. Wilkens, J. Crystal Growth 124 (1992) 576. [12] C.Y. Tsai, M. Moser, C. Geng, V. Harle, T. Forner, P. Michler, A. Hangleiter, F. Scholz, J. Crystal Growth 145 (1994) 786. [13] W. Seifert, N. Carlsson, M.E. Pistol, L. Samuelson, J. Crystal Growth 145 (1994) 758.
Luminescence properties of Er,O-codoped GaAs/GaInP double heterostructures grown by organometallic vapor phase epitaxy A. Koizumia,*, N. Watanabea, K. Inouea, Y. Fujiwaraa, Y. Takedaa,b a
Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan b CREST, JST (Japan Science and Technology), Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
Abstract Er,O-codoped GaAs/GaInP double heterostructures (DHS : Er,O) were grown by organometallic vapor phase epitaxy and the intra-4f shell transitions of Er ions were investigated by photoluminescence (PL) measurements. Er,Ocodoping was carried out with trisdipivaloylmethanatoerbium (Er(DPM)3) as an Er source and an addition of O2 diluted with Ar to a reactor. Er,O-codoped GaAs (GaAs : Er,O) and GaInP layers were all grown at 5501C. In-depth profiles showed a uniform distribution of Er and O along the growth direction in the GaAs : Er,O active layer. The Er concentration in the GaAs : Er,O active layer was evaluated to be about 5 1017 cm 3. In 4.2 K PL measurements, DHS : Er,O sample exhibited Er–2O lines and the intensity was approximately three times stronger than that of GaAs : Er,O sample. This suggests that photoexcited carriers confined in the GaAs : Er,O active layer contribute effectively to excitation of Er. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Erbium; Oxygen; Double heterostructures; Organometallic vapor phase epitaxy
1. Introduction Recently, there has been a wide scientific and technological interest in semiconductors doped with rare-earth (RE) impurities. RE isoelectronic impurity incorporated into semiconductors shows sharp and temperature-stable intra-4f-shell luminescence. This stability occurs because the filled outer 5s and 5p electron shells screen transitions within the inner 4f electron shell from interaction with the host. These properties are attractive for fabricating optical devices such as semiconductor lasers and optical amplifiers. Special attention has been paid to Er ions in III–V semiconductors. Er doping in III–V semiconductors produces sharp luminescence at around 1.54 mm due to *Corresponding author. Tel.: +81-52-789-3620; fax: +8152-789-3239. E-mail address: [email protected] (A. Koizumi).
intra-4f-shell transition from the first excited state (4I13/2) to the ground state (4I15/2) of Er ions. The emission is coincident with the minimum loss wavelength region of silica-based fibers. However, it has been found that Er ions doped in semiconductors show various fine structures, depending on doping conditions. This indicates that various kinds of Er centers with different atom configurations are formed simultaneously in a semiconductor because the fine structures of photoluminescence (PL) spectra reflect atomic configurations around Er ions. Selective formation of highly efficient Er centers for luminescence has been strongly desired for applications of Er-related luminescence to light-emitting devices. Oxygen (O) has been found to play an important role in the production of efficient Er-related luminescent centers in GaAs. Er,O-codoped GaAs (GaAs : Er,O) produces a strong emission intensity and simple PL spectrum predominantly from one kind of Er center [1]. The Er center has been identified as an Er atom located
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 9 5 1 - 6
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A. Koizumi et al. / Physica B 308–310 (2001) 891–894
at the Ga sublattice with two adjacent O atoms (Er–2O center) [2]. The dependence of the PL spectra on growth temperature revealed the existence of a threshold growth temperature between 5601C and 5801C for Er incorporation into GaAs [3]. For optical device applications, Er should be introduced in confinement structures such as double heterostructures (DHS). In these structures, a GaAs : Er,O layer should be sandwiched between layers of a different semiconductor. The growth of Ga0.51In0.49P (hereafter GaInP) is very attractive as an aluminum-free alternative to the conventional AlxGa1 xAs-based materials. The aluminum-free GaAs/GaInP material system has advantages over the GaAs/AlxGa1 xAs material system, such as the low reactivity of GaInP to O. In this paper, we report the results on fabrication of GaAs : Er,O/GaInP double heterostructures (DHS : Er,O) grown by organometallic vapor phase epitaxy (OMVPE) and its luminescence properties.
2. Experimental GaAs : Er,O and DHS : Er,O samples used in this study were fabricated by a low-pressure growth system with a cold-wall quartz 4-barrel reactor [4]. The reactor pressure was 0.1 atm. The substrates were semiinsulating and Si-doped GaAs was oriented in the (1 0 0) direction. Triethylgallium (TEGa), trimethylindium (TMIn), tertiarybutylarsine (TBAs) and tertiarybutylphosphine (TBP) were used as sources. Er doping was carried out with trisdipivaloylmethanatoerbium (Er(C11H19O2)3, Er(DPM)3) as an Er source which was introduced into the reactor by a H2 flow through the Er source cylinder. The rate of the H2 flow was kept at 125 sccm. Ar gas with 38.4 ppm of 18O2 was used as an additional O2 source [3]. The O2 content in growth ambient was set at 0.2 ppm. Growth sequence for GaAs–GaInP interface used in this work is as follows. At the beginning of the layer growth, group-V gas was switched on first for 1 s and then group-III gas was switched on. At the end of layer growth, the group-III gas was switched off and growth was interrupted under the group-V gas to change TEGa flow rate and substrate temperature. In the growth of GaAs/GaInP DHS, we used two growth sequences (sequences A and B). Variations of gas flow and growth temperature for the sequences are schematically drawn in Fig. 1. In sequence A, GaInP and GaAs layers were grown at 5801C and 5401C. The temperatures were optimum growth temperatures for GaInP and GaAs : Er,O, respectively. In sequence B, GaAs and GaInP layers were all grown at 5501C. The crystal quality of the DHS was characterized by PL measurements at 77 K using the DHS having 200 nm-thick GaAs without Er,Ocodoping and 200 nm-thick GaInP cladding layer.
Fig. 1. Variations of gas flow and growth temperature for (a) sequence A and (b) sequence B. In sequence A, GaInP and GaAs layers were grown at 5801C and 5401C, respectively. In sequence B, on the other hand, these layers were all grown at 5501C.
The measurements were carried out with a 514.5 nm beam of Ar-ion laser and an S1-type detector cooled by dry ice. The structure and growth conditions of DHS : Er,O samples are as follows. All the samples were initiated by the growth of a 150 nm thick GaAs buffer layer at 6101C. Then, 1 mm-thick GaInP cladding layer, 1 mmthick GaAs : Er,O active layer and 0.6 mm-thick GaInP cladding layer were grown with sequence B. These thicknesses were evaluated using undoped GaAs/GaInP DHS samples by a scanning electron microscope (SEM). As a reference sample, GaAs : Er,O was grown under the same doping conditions as the active layer of DHS : Er,O. PL measurements of Er-related luminescence were carried out mainly with the samples directly immersed in liquid He at 4.2 K. The photoexcitation source was a CW-mode semiconductor laser diode, operating at 660 nm, with an incident power of 30 mW. The photon energy is below the band-gap of the GaInP cladding
A. Koizumi et al. / Physica B 308–310 (2001) 891–894
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layer, which enabled us to excite the GaAs active layer directly. The luminescence of the sample was dispersed using a 0.91 m grating monochromator and detected with a liquid nitrogen-cooled Ge p–i–n photodiode using a chopper and a lock-in technique. For measurements of concentration profiles by secondary ion mass spectroscopy (SIMS), an O+ ion and a Cs ion were used as primary ions for Er and O, respectively.
corresponding to the near-band-edge emission from GaAs. Also, a broad peak is weakly observed at 850 nm. This result suggests that photoexcited carriers injected in GaAs active layer recombine effectively in GaAs layer. Therefore, we can conclude that samples grown with sequence B have good quality of GaAs–GaInP interface compared with sequence A.
3. Results and discussion
DHS : Er,O samples were prepared with sequence B. In-depth profiles of Er, O, Ga and As for DHS : Er,O are shown in Fig. 3. For the DHS : Er,O sample, a distinct GaAs/GaInP DHS is seen in the SIMS profile. The profile reveals a uniform distribution of Er and O along the growth direction in the GaAs : Er,O active layer. Furthermore, in GaInP cladding layers, Er concentration is below the SIMS detection limit and O concentration is at the background level. Er concentration in GaAs : Er,O active layer, calibrated by using an Erimplanted GaAs sample, was evaluated to be about 5 1017 cm 3. This Er concentration was the same as GaAs : Er,O reference sample.
3.1. PL characterization of undoped GaAs/GaInP DHS It is well known that making a high-quality GaInP– GaAs interface is very difficult via OMVPE growth because of the problems in As–P substitution and indium memory effects in reactor gas switching [5–13]. Characteristics of GaInP–GaAs interface strongly depend on the growth sequence for the interface. Fig. 2(a) shows 77 K PL spectrum of undoped DHS grown with sequence A. The spectrum has one peak at wavelength of 884 nm with the full-width at half-maximum (FWHM) of 54 meV. The wavelength of 884 nm is longer than those of the near-band-edge emissions from GaAs. Several researchers have also reported a similar broad peak at wavelengths between 850 and 930 nm in low-temperature PL spectra of GaAs/GaInP heterostructures [10,12]. Fig. 2(b) shows PL spectrum of a sample grown with sequence B. The spectrum is dominated by a peak
Fig. 2. 77 K PL spectra in undoped GaAs/GaInP DHS grown with (a) sequence A and (b) sequence B. An interfacerelated peak at 884 nm disappears in the sample grown with sequence B.
3.2. SIMS profiles in DHS : Er,O sample
Fig. 3. In-depth profiles of Er, O, Ga and As for DHS : Er,O. Er and O are introduced selectively in the GaAs : Er,O active layer.
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are selectively formed in GaAs : Er,O active layer. The intensity of Er-related luminescence was about three times higher than the GaAs : Er,O sample grown under the same doping conditions and Er concentration as in the GaAs : Er,O active layer of DHS : Er,O. These results are interpreted easily by the advantage of the DHS; confinement of photoexcited carriers and elimination of surface recombination.
Acknowledgements
Fig. 4. 4.2 K Er-related PL spectra in (a) DHS : Er,O and (b) GaAs : Er,O. As shown by arrows, the spectrum is dominated by the Er–2O lines, indicating that an Er–2O center is selectively formed in GaAs : Er,O active layer of DHS : Er,O.
3.3. Er-related luminescence in DHS : Er,O In Fig. 4, the 4.2 K I13/2- I15/2 Er-related luminescence from DHS : Er,O sample is compared with the luminescence from the GaAs : Er,O reference sample. The spectrum of DHS : Er,O shows identical positions of Er-related PL lines as in the GaAs : Er,O. Vertical arrows denote the Er–2O lines. This indicates that Er–2O centers are selectively formed in GaAs : Er,O active layers of DHS : Er,O. The intensity of the Errelated luminescence is approximately three times stronger than that of GaAs : Er,O sample. These results suggest that carriers photoexcited in the GaAs : Er,O region contribute effectively to excitation of Er because GaInP cladding layers confine the carriers and eliminate the surface recombination. 4
4
4. Conclusion We have successfully grown Er,O-codoped GaAs/ GaInP double heterostructures. In-depth profiles of Er and O exhibited a uniform distribution along the growth direction in the GaAs : Er,O active layer. The Er-related luminescence indicated that Er–2O centers
The authors would like to thank Hitachi Cable Ltd. for GaAs substrates. This work was supported in part by Grant-in-Aids for Scientific Research (A)(2) No. 13305022, for Scientific Research (B)(2) No. 11450119 and No. 13555002, for Exploration Research No. 13875062 from the Ministry of Education, Science, Sports and Culture.
References [1] K. Takahei, A. Taguchi, J. Appl. Phys. 74 (1993) 1979. [2] K. Takahei, A. Taguchi, Y. Horikoshi, J. Nakata, J. Appl. Phys. 76 (1994) 4332. [3] Y. Fujiwara, T. Kawamoto, T. Koide, Y. Takeda, Physica B 273–274 (1999) 770. [4] Y. Fujiwara, S. Furuta, K. Makita, Y. Ito, Y. Nonogaki, Y. Takeda, J. Crystal Growth 146 (1995) 544. [5] T. Nittono, S. Sugitani, F. Hyuga, J. Appl. Phys. 78 (1995) 5387. [6] M. Razeghi, Ph. Maurel, F. Omnes, M. Defour, C. Boothroyd, W.M. Stobbs, M. Kelly, J. Appl. Phys. 63 (1988) 4511. [7] F. Omnes, M. Razeghi, Appl. Phys. Lett. 59 (1991) 1034. [8] X. He, M. Razeghi, Appl. Phys. Lett. 61 (1992) 1703. [9] D.P. Bour, J.R. Shealy, S. Mckernan, J. Appl. Phys. 63 (1988) 1241. [10] F.E.G. Guimaraes, B. Elsner, R. Westphalen, B. Spangenberg, H.J. Geelen, P. Balk, K. Heime, J. Crystal Growth 124 (1992) 199. [11] R. Bhat, M.A. Koza, M.J.S.P. Brasil, R.E. Nahory, C.J. Palmstorm, B.J. Wilkens, J. Crystal Growth 124 (1992) 576. [12] C.Y. Tsai, M. Moser, C. Geng, V. Harle, T. Forner, P. Michler, A. Hangleiter, F. Scholz, J. Crystal Growth 145 (1994) 786. [13] W. Seifert, N. Carlsson, M.E. Pistol, L. Samuelson, J. Crystal Growth 145 (1994) 758.