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AlAsSb heterostructures
ELS EVI E R
Thin Solid Films 380 (2000) 29-251
www.elsevier.com/locate/tsf
Interface control of InGaAs/AlAsSb heterostructures T. Mozume" , N. Georgiev Femtosecond TechnologyAssociation (FESTA),5-5 Tokodai, Tsukuba 300-2635, Japan
Abstract
We report here a photoluminescence (PL) study of AlAsSb/InGaAs/AlAsSb single quantum wells (SQWs) that are lattice-matched to InP substrates grown by molecular beam epitaxy (MBE). The group V species used, the interface termination procedures, and Si doping are shown to markedly influence the PL spectra of the SQWs: (1) the undoped SQWs grown using dimer group V species combined with As interface termination show sharp PL spectra, which correspond to a direct transition between the confined electrons and holes of InGaAs; (2) the PL peak is broadened and shifted towards a longer wavelength as a ~ ,PL spectra result of Sb interface termination; and (3) when the AlAsSb or InGaAs layers are highly doped to 1 X l O I 9 ~ m - the become very broad. These results are most probably attributable to the enhanced exchange reaction between As and Sb, and also to the diffusion of In and Ga at the interfaces resulting from Si high doping and the use of tetramer group V species. 0 2000 Elsevier Science B.V. All rights reserved. Keywords: Molecular beam epitaxy (MBE); InGaAs/AlAsSb; Photoluminescence (PL); Band alignment
1. Introduction
Ino,~,Gao,,7As/AlAso,~6Sbo,,, heterostructures lattice-matched to InP substrates are attracting an increasing focus of research interest, due to their unique physical properties and their potential for application in optoelectronic and electronic devices. The reported conduction band offset is 1.74 eV at the r-point, providing a high degree of electronic confinement. We have reported the near-infrared inter-sub-band transitions from this material system [1-3], and shown that this heterostructure is a promising candidate for use in high-speed switches in optical communication systems 141. Although the InGaAs/AlAsSb system has been reported to be Type I1 [5], we have recently found that this heterostructure shows a variety of photolumines-
* Corresponding author. Tel.: + 81-298-47-5181; fax: 4417. E-mail address: [email protected] (T. Mozume).
+ 81-298-47-
cence (PL) spectra including Type I and Type I1 (staggered) PL spectra [5,6]when the growth conditions are changed. In this paper, we report a detailed study of the photoluminescence of In 0.53Ga 0.47As/AlAs 0.56Sb0.44 single wells grown by molecular beam epitaxy (MBE). 2. Experimental
All the studied layers were grown using conventional MBE. Antimony and As were supplied using valved cracker cells, with and without cracking. Details of the growth procedures are reported in a separate paper [7]. The lattice mismatch A a / a of InGaAs and AlAsSb was found to be less than f 3 x l o p 3 by X-ray diffraction (XRD) measurements using a high resolution fourcrystal diffractometer (Philips Materials Research Diffractometer). Photoluminescence measurements were carried out at 77 K using an argon-ion laser emitting at 514.5 nm. The luminescence was dispersed in a 0.64-m monochromator and detected by means of a cooled Ge detector
0040-6090/00/$ - see front matter 0 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 ( 0 0 ) 0 1 5 1 7 - 0
T. Mozume,N. Geovgiev/ Thin Solid Films 380 (2000)249-251
250
using conventional lock-in detection. The diameter of the laser spot was approximately 1 mm. 3. Results and discussions
The PL spectrum was found to be very sensitive to the type of group V species, the interface termination procedure, and the doping density. In the following, we will first demonstrate the difference in PL spectra from samples grown by tetramer and dimer group V sources. We will then show the results of interface termination. Finally we will present the results of doping.
1.2
1.4
1.6
1.8
wavelength Qun)
3.1. As and Sb species dependence
Fig. 2. Group V species dependence of PL spectra. (a) shows a sample grown by dimer sources and (b) shows a sample grown by tetramer sources.
Fig. la,b show the PL spectra of 20-nm InGaAs SQW samples grown by tetramer and dimer for As and Sb, respectively. At low excitation power, clear double peaks are observed from the sample grown using tetramer sources. With increasing excitation power, the longer wavelength (lower energy) peak (hereafter denoted as ‘peak 1’) shows a blue shift, whereas the shorter-wavelength (1.54 pm) peak (hereafter denoted as ‘peak 11’) does not show any shift over the entire excitation power range. At an excitation power of 9.2 mW, the lower-energy peak (peak I) coincides with peak 11. The energy of peak I is 60 meV lower than that of peak I1 when the excitation power is low, and is smaller than the InGaAs band gap. Peak I1 has a narrower spectral half-width of approximately 25 meV compared to that of peak I, which is approximately 50 meV. The PL spectra of samples grown using dimer As and Sb are sharp (Fig. lb), and the peak energy is higher than the InGaAs bandgap. No peak shift is observed when the excitation power is increased. Fig. 2 shows the PL spectra of 11- and 10-nm InGaAs SQWs grown from tetramer and dimer sources, respectively. Arsenic interface termination is also employed for the 10-nm sample. The use of dimer As and
Sb sources combined with the interface termination produces a fairly sharp PL spectrum. This is consistent with the 20-nm SQW shown in Fig. lb. Judging from the PL spectra shown in Fig. l b and Fig. 2, the InGaAs/AIAsSb system is not a Type I1 system as reported by Nakata et al. [5], but is a Type I system. Because: (1) the PL spectra become very sharp when the interface is controlled properly; (2) the PL peak energy of 20- and 10-nm InGaAs well samples (Fig. l b and peak (a) of Fig. 2) are higher than that of InGaAs band gap and correspond well to the direct transition energy between the confined energy level and valence band in the InGaAs well; and (3) these peaks show no excitaion power dependence. Band alignment calculation provides additional evidence that it is a Type I system [8]. Peak I in Fig. l a observed in the sample grown by tetramer group V sources, in spite of showing the typical PL excitation power dependence of a Type I1 system, can be attributed to the interfacerelated peak caused by interface disorder. Details of the effects of interface termination will be reported in a separate paper. 3.2. Inteface termination dependence
1.2
1.4
1.6
wavelength (pn)
1.8
1
1.2
.
. 1.4
......
1.6 wavelength (pm)
.
I 1.8
Fig. 1. Excitation power dependence of the PL spectra of 20-nm InGaAs/AlAsSb SQWs. (a) shows the PLS of the sample grown by tetrameric group V sources and (b) shows the sample grown from dimeric group V sources.
Interface termination procedure has a profound influence on the QW property of InGaAs/AlAsSb. We have already reported that the PL peak energy of an Sb-terminated short period superlattice is approximately 60 meV lower than that of the equivalent Asterminated version. A similar shift of the PL peak is observed from 10-nm InGaAs SQW grown using dimeric As and Sb (Fig. 3). By terminating the upper interface of InGaAs with Sb, the PL peak broadens and shifts to approximately 1.58 pm, depending on excitation power. This shift appears to be caused by the replacement of Sb with As during Sb termination, since Sb incorporation into InGaAs shrinks the band gap. If we suppose that not only the group V species exchange,
T. Mozume,N. Georgiev / Thin Solid Films 380 (2000)249-251
251
but also the group I11 species interdiffuse, we will obtain the Type I1 band alignment. The evidence of the group I11 species interdiffusion is presented in the next section.
3.3. Doping-induced changes in PL spectra Dramatic changes in PL spectra are observed when samples are highly doped (Fig. 4). When the doping density is increased to 1x 1019 cmP3,the PL spectrum broadens markedly, whereas the sample doped to 1x lo1' cmP3with an undoped 10-nm spacer layer between InGaAs shows a sharp spectrum similar to that of an undoped sample, although the shoulder on the longer wavelength side of the peak remains. These changes in PL spectra can be attributed to the doping-induced interdiffusion of host materials. Indium and Ga diffusion into upper and lower AlAsSb layers has been confirmed by recent SIMS results. Details of the doping-induced interface disorder will be reported in a separate paper. 4. Summary
PL measurements were performed to gain an understanding of the growth parameter dependence of InGaAs/AIAsSb SQW properties. As a result of using dimeric group V species instead of tetramer versions, the PL peak becomes very sharp, suggesting the band
1.0
1.2
1.4
1.6
1.8
wavelength (pn) Fig. 4. PL spectra of InGaAs/AlAsSb SQW. (a) shows the 1X 1019 cm-3 barrier-doped sample; (b) shows the 1 X 10l8 cm-3 barrierdoped sample with a 10-nm undoped spacer layer between the InGaAs layers; and (c) illustrates an undoped sample.
alignment of this system is type I, not type I1 as previously reported. Misleading band alignment is caused by interface disorder when grown by tetramer group V sources or under conditions of high doping. Acknowledgements
This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) within the framework of the Femtosecond Technology Project. References
1.0
1.2
1.4
1.6
1.8
wavelength (pm) Fig. 3. Interface termination dependence of the PL spectra of 10-nm SQWs. (a) illustrates the As-terminated sample and (b) shows the Sb-terminated sample.
[l] T. Mozume, H. Yoshida, A. Neogi, M. Kudo, Jpn. J. Appl. Phys. 38 (1999) 126-1289. [2] T. Mozume, H. Yoshida, A. Neogi, M. Kudo, J. Cryst. Growth 201/202 (1999) 1077-1080. [3] T. Mozume, N. Georgiev, H. Yoshida, A. Neogi, M. Kudo, Institute Phys. Conf. Ser. 162 (1998) 131-136. [4] H. Yoshida, T. Mozume, T. Nishimura, 0. Wada, Technical Digest of CLEO/Pacific RIM '97, (1997) 127. [5] Y. Nakata, Y. Sugiyama, T. Inata, 0. Ueda, S. Sasa, S. Muto, T. Fujii, Mater. Res. SOC.Symp. Proc. 198 (1990) 29-294. [6] T. Mozume, N. Georgiev, T. Nishimura, H. Yoshida, S. Nishikawa, A. Neogi, J. Cryst. Growth 209 (2000) 445-449. [7] N. Georgiev, T. Mozume, J. Cryst. Growth 209 (2000) 247-251. [8] C.G. Van de Walle, Phys. Rev. B39 (1989) 1871-1883.
Thin Solid Films 380 (2000) 29-251
www.elsevier.com/locate/tsf
Interface control of InGaAs/AlAsSb heterostructures T. Mozume" , N. Georgiev Femtosecond TechnologyAssociation (FESTA),5-5 Tokodai, Tsukuba 300-2635, Japan
Abstract
We report here a photoluminescence (PL) study of AlAsSb/InGaAs/AlAsSb single quantum wells (SQWs) that are lattice-matched to InP substrates grown by molecular beam epitaxy (MBE). The group V species used, the interface termination procedures, and Si doping are shown to markedly influence the PL spectra of the SQWs: (1) the undoped SQWs grown using dimer group V species combined with As interface termination show sharp PL spectra, which correspond to a direct transition between the confined electrons and holes of InGaAs; (2) the PL peak is broadened and shifted towards a longer wavelength as a ~ ,PL spectra result of Sb interface termination; and (3) when the AlAsSb or InGaAs layers are highly doped to 1 X l O I 9 ~ m - the become very broad. These results are most probably attributable to the enhanced exchange reaction between As and Sb, and also to the diffusion of In and Ga at the interfaces resulting from Si high doping and the use of tetramer group V species. 0 2000 Elsevier Science B.V. All rights reserved. Keywords: Molecular beam epitaxy (MBE); InGaAs/AlAsSb; Photoluminescence (PL); Band alignment
1. Introduction
Ino,~,Gao,,7As/AlAso,~6Sbo,,, heterostructures lattice-matched to InP substrates are attracting an increasing focus of research interest, due to their unique physical properties and their potential for application in optoelectronic and electronic devices. The reported conduction band offset is 1.74 eV at the r-point, providing a high degree of electronic confinement. We have reported the near-infrared inter-sub-band transitions from this material system [1-3], and shown that this heterostructure is a promising candidate for use in high-speed switches in optical communication systems 141. Although the InGaAs/AlAsSb system has been reported to be Type I1 [5], we have recently found that this heterostructure shows a variety of photolumines-
* Corresponding author. Tel.: + 81-298-47-5181; fax: 4417. E-mail address: [email protected] (T. Mozume).
+ 81-298-47-
cence (PL) spectra including Type I and Type I1 (staggered) PL spectra [5,6]when the growth conditions are changed. In this paper, we report a detailed study of the photoluminescence of In 0.53Ga 0.47As/AlAs 0.56Sb0.44 single wells grown by molecular beam epitaxy (MBE). 2. Experimental
All the studied layers were grown using conventional MBE. Antimony and As were supplied using valved cracker cells, with and without cracking. Details of the growth procedures are reported in a separate paper [7]. The lattice mismatch A a / a of InGaAs and AlAsSb was found to be less than f 3 x l o p 3 by X-ray diffraction (XRD) measurements using a high resolution fourcrystal diffractometer (Philips Materials Research Diffractometer). Photoluminescence measurements were carried out at 77 K using an argon-ion laser emitting at 514.5 nm. The luminescence was dispersed in a 0.64-m monochromator and detected by means of a cooled Ge detector
0040-6090/00/$ - see front matter 0 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 ( 0 0 ) 0 1 5 1 7 - 0
T. Mozume,N. Geovgiev/ Thin Solid Films 380 (2000)249-251
250
using conventional lock-in detection. The diameter of the laser spot was approximately 1 mm. 3. Results and discussions
The PL spectrum was found to be very sensitive to the type of group V species, the interface termination procedure, and the doping density. In the following, we will first demonstrate the difference in PL spectra from samples grown by tetramer and dimer group V sources. We will then show the results of interface termination. Finally we will present the results of doping.
1.2
1.4
1.6
1.8
wavelength Qun)
3.1. As and Sb species dependence
Fig. 2. Group V species dependence of PL spectra. (a) shows a sample grown by dimer sources and (b) shows a sample grown by tetramer sources.
Fig. la,b show the PL spectra of 20-nm InGaAs SQW samples grown by tetramer and dimer for As and Sb, respectively. At low excitation power, clear double peaks are observed from the sample grown using tetramer sources. With increasing excitation power, the longer wavelength (lower energy) peak (hereafter denoted as ‘peak 1’) shows a blue shift, whereas the shorter-wavelength (1.54 pm) peak (hereafter denoted as ‘peak 11’) does not show any shift over the entire excitation power range. At an excitation power of 9.2 mW, the lower-energy peak (peak I) coincides with peak 11. The energy of peak I is 60 meV lower than that of peak I1 when the excitation power is low, and is smaller than the InGaAs band gap. Peak I1 has a narrower spectral half-width of approximately 25 meV compared to that of peak I, which is approximately 50 meV. The PL spectra of samples grown using dimer As and Sb are sharp (Fig. lb), and the peak energy is higher than the InGaAs bandgap. No peak shift is observed when the excitation power is increased. Fig. 2 shows the PL spectra of 11- and 10-nm InGaAs SQWs grown from tetramer and dimer sources, respectively. Arsenic interface termination is also employed for the 10-nm sample. The use of dimer As and
Sb sources combined with the interface termination produces a fairly sharp PL spectrum. This is consistent with the 20-nm SQW shown in Fig. lb. Judging from the PL spectra shown in Fig. l b and Fig. 2, the InGaAs/AIAsSb system is not a Type I1 system as reported by Nakata et al. [5], but is a Type I system. Because: (1) the PL spectra become very sharp when the interface is controlled properly; (2) the PL peak energy of 20- and 10-nm InGaAs well samples (Fig. l b and peak (a) of Fig. 2) are higher than that of InGaAs band gap and correspond well to the direct transition energy between the confined energy level and valence band in the InGaAs well; and (3) these peaks show no excitaion power dependence. Band alignment calculation provides additional evidence that it is a Type I system [8]. Peak I in Fig. l a observed in the sample grown by tetramer group V sources, in spite of showing the typical PL excitation power dependence of a Type I1 system, can be attributed to the interfacerelated peak caused by interface disorder. Details of the effects of interface termination will be reported in a separate paper. 3.2. Inteface termination dependence
1.2
1.4
1.6
wavelength (pn)
1.8
1
1.2
.
. 1.4
......
1.6 wavelength (pm)
.
I 1.8
Fig. 1. Excitation power dependence of the PL spectra of 20-nm InGaAs/AlAsSb SQWs. (a) shows the PLS of the sample grown by tetrameric group V sources and (b) shows the sample grown from dimeric group V sources.
Interface termination procedure has a profound influence on the QW property of InGaAs/AlAsSb. We have already reported that the PL peak energy of an Sb-terminated short period superlattice is approximately 60 meV lower than that of the equivalent Asterminated version. A similar shift of the PL peak is observed from 10-nm InGaAs SQW grown using dimeric As and Sb (Fig. 3). By terminating the upper interface of InGaAs with Sb, the PL peak broadens and shifts to approximately 1.58 pm, depending on excitation power. This shift appears to be caused by the replacement of Sb with As during Sb termination, since Sb incorporation into InGaAs shrinks the band gap. If we suppose that not only the group V species exchange,
T. Mozume,N. Georgiev / Thin Solid Films 380 (2000)249-251
251
but also the group I11 species interdiffuse, we will obtain the Type I1 band alignment. The evidence of the group I11 species interdiffusion is presented in the next section.
3.3. Doping-induced changes in PL spectra Dramatic changes in PL spectra are observed when samples are highly doped (Fig. 4). When the doping density is increased to 1x 1019 cmP3,the PL spectrum broadens markedly, whereas the sample doped to 1x lo1' cmP3with an undoped 10-nm spacer layer between InGaAs shows a sharp spectrum similar to that of an undoped sample, although the shoulder on the longer wavelength side of the peak remains. These changes in PL spectra can be attributed to the doping-induced interdiffusion of host materials. Indium and Ga diffusion into upper and lower AlAsSb layers has been confirmed by recent SIMS results. Details of the doping-induced interface disorder will be reported in a separate paper. 4. Summary
PL measurements were performed to gain an understanding of the growth parameter dependence of InGaAs/AIAsSb SQW properties. As a result of using dimeric group V species instead of tetramer versions, the PL peak becomes very sharp, suggesting the band
1.0
1.2
1.4
1.6
1.8
wavelength (pn) Fig. 4. PL spectra of InGaAs/AlAsSb SQW. (a) shows the 1X 1019 cm-3 barrier-doped sample; (b) shows the 1 X 10l8 cm-3 barrierdoped sample with a 10-nm undoped spacer layer between the InGaAs layers; and (c) illustrates an undoped sample.
alignment of this system is type I, not type I1 as previously reported. Misleading band alignment is caused by interface disorder when grown by tetramer group V sources or under conditions of high doping. Acknowledgements
This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) within the framework of the Femtosecond Technology Project. References
1.0
1.2
1.4
1.6
1.8
wavelength (pm) Fig. 3. Interface termination dependence of the PL spectra of 10-nm SQWs. (a) illustrates the As-terminated sample and (b) shows the Sb-terminated sample.
[l] T. Mozume, H. Yoshida, A. Neogi, M. Kudo, Jpn. J. Appl. Phys. 38 (1999) 126-1289. [2] T. Mozume, H. Yoshida, A. Neogi, M. Kudo, J. Cryst. Growth 201/202 (1999) 1077-1080. [3] T. Mozume, N. Georgiev, H. Yoshida, A. Neogi, M. Kudo, Institute Phys. Conf. Ser. 162 (1998) 131-136. [4] H. Yoshida, T. Mozume, T. Nishimura, 0. Wada, Technical Digest of CLEO/Pacific RIM '97, (1997) 127. [5] Y. Nakata, Y. Sugiyama, T. Inata, 0. Ueda, S. Sasa, S. Muto, T. Fujii, Mater. Res. SOC.Symp. Proc. 198 (1990) 29-294. [6] T. Mozume, N. Georgiev, T. Nishimura, H. Yoshida, S. Nishikawa, A. Neogi, J. Cryst. Growth 209 (2000) 445-449. [7] N. Georgiev, T. Mozume, J. Cryst. Growth 209 (2000) 247-251. [8] C.G. Van de Walle, Phys. Rev. B39 (1989) 1871-1883.