- Email: [email protected]
BeZnCdSe superlattice and quantum well structures on InP substrates
Journal of Crystal Growth 227–228 (2001) 660–664
MBE growth of BeZnCdSe quaternaries, MgSe/BeZnCdSe superlattice and quantum well structures on InP substrates Masatoshi Takizawa, Ichirou Nomura, Song-Bek Che, Akihiko Kikuchi, Kazuhiko Shimomura, Katsumi Kishino* Department of Electrical and Electronic Engineering, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan
Abstract BeZnCdSe quaternaries, MgSe/BeZnCdSe superlattice (SL) and BeZnCdSe quantum well (QW) structures were grown on InP substrates by molecular beam epitaxy for the first time. As for a lattice-matched Be0.08(Zn0.30Cd0.70)0.92Se sample, a single-peak photoluminescence (PL) spectrum was observed at the temperature range from 15 K to room temperature. The PL peak wavelength and the full-width at half-maximum (FWHM) value at 15 K were 572 nm and 8.8 meV, respectively. PL measurements of MgSe/BeZnCdSe SL samples were observed single-peak emissions in the green-to-blue color range from 525 to 470 nm at 15 K with changing MgSe layer-thickness ratio from 0.20 to 0.60. As for the QW sample consisting of four BeZnCdSe QWs with a different well width, PL peaks corresponding to each QW were observed in the wavelength range from 459 to 561 nm at 15 K. By fitting calculated transition energies in the QWs to the PL peak energies, the band offset ratio between the BeZnCdSe well and the MgSe/BeZnCdSe SL barrier was estimated to be DEc : DEv ¼ 9 : 1. r 2001 Elsevier Science B.V. All rights reserved. PACS: 71.20.Nr; 78.40.Fy; 78.66.Hf; 78.55.Et Keywords: A3. Molecular beam epitaxy; A3. Quantum wells; A3. Superlattices; B2. Semiconducting II–VI materials; B2. Semiconducting indium phosphide; B2. Semiconducting quaternary alloys
1. Introduction Beryllium chalcogenides such as BeSe and BeTe have higher cohesive energies due to their high degree of covalency, compared with other II–VI compounds such as ZnSe and ZnTe [1,2]. Therefore, incorporation of beryllium chalcogenides into conventional II–VI compounds has been noticed for enhancing the lattice hardening [2–4], *Corresponding author. Tel.: +81-3-3238-3323; fax: +81-33238-3321. E-mail address: [email protected] (K. Kishino).
which is expected to lead to lengthening device lifetimes of conventional II–VI laser diodes (LDs). Here, we will focus on BeZnCdSe quaternaries and MgSe/BeZnCdSe superlattice (SL) II–VI compounds on InP substrates, which are very attractive for wide-range visible optical devices such as laser diodes (LDs). The relationships between band gap energies and lattice constants of these II–VI compounds and InP are shown in Fig. 1. The band gap energies of BeZnCdSe quaternaries lie in the range from 2.1 to 2.5 eV keeping lattice-matching to InP (see a crossed area of a dashed line and a triangle area surrounded by BeSe, ZnSe and CdSe). The band gap energies
0022-0248/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 7 9 3 - X
M. Takizawa et al. / Journal of Crystal Growth 227–228 (2001) 660–664
661
2. BeZnCdSe quaternaries
Fig. 1. Relations between band gap energies and lattice constants of BeZnCdSe and MgSe II–VI compounds.
correspond to the wavelength range from orange (590 nm) to blue-green (500 nm). Furthermore, the band gap energies of MgSe/BeZnCdSe SLs can be widely controlled from 2.1 to 4 eV by only changing layer-thickness ratios of SLs keeping the lattice-matching. Therefore, BeZnCdSe and the SL materials are expected to be applied to highly reliable and wide range visible optical devices. In this study, BeZnCdSe quaternaries, MgSe/ BeZnCdSe SL and BeZnCdSe quantum well (QW) structures were grown on InP substrates by molecular beam epitaxy (MBE). The grown samples were characterized by photoluminescence (PL) measurements from 15 K to room temperature (RT). As for the lattice-matched Be0.08 (Zn0.30Cd0.70)0.92Se sample, a narrow single-peak PL spectrum was observed at 572 nm. The PL peak wavelength of MgSe/BeZnCdSe SL samples shortened from 525 to 470 nm with increasing the MgSe layer-thickness ratio. For the QW sample, PL peaks from the QWs were observed. By fitting calculated transition energies in the QWs to the PL peak energies, the band offset ratio between the BeZnCdSe well and the MgSe/BeZnCdSe SL barrier was estimated.
BeZnCdSe quaternaries were grown by MBE on (1 0 0) InP substrates with 0.2-mm-thick InGaAs buffer layers which were grown by metal–organic chemical vapor deposition (MOCVD) prior to the MBE growth. Elemental Be(5N), Zn(7N), Cd(6N), Mg(6N) and Se(6N) were used for the MBE growth. Thermal cleaning of the substrates was performed at 5001C under As pressure in the III–V MBE chamber. After the substrates were transferred into the II–VI MBE chamber through the ultra-high vacuum (UHV) tube, BeZnCdSe layers were grown at 3001C. In order to control heterovalency at the II–VI/III–V interfaces, Zn-irradiations for 30 s were performed before the growth. The beam pressures of Zn, Cd and Se were fixed to be around 3 10@7, 7 10@7 and 2 10@6 Torr, respectively. Under the above pressure condition, Zn composition (y) in Bex(ZnyCd1@y)1@xSe was kept to be about 0.3. On the other hand, Be beam pressure was changed from 0.7 to 1.5 10@8 Torr to control Be composition (x). After the growth, all the samples showed (2 1) streaky reflection of high-energy electron diffraction (RHEED) patterns, which indicated layer-bylayer growth under VI-group (Se) rich conditions. The total thickness and growth rate of the BeZnCdSe layers were around 1 mm and 0.5 mm/ h, respectively. Each elemental composition of BeZnCdSe layers was estimated based on the electron probe micro analysis (EPMA) and X-ray diffraction (XRD) measurements. Zn (y) and Cd (1@y) compositions in Bex(ZnyCd1@y)1@xSe were estimated based on the EPMA, and Be composition (x) was estimated based on the Zn and Cd compositions and the lattice-mismatching value obtained by the XRD measurements. The PL measurements were performed from 15 K to RT using the 325 nm line of the He–Cd laser for excitation. Fig. 2 shows PL spectra at 15 K for Bex(ZnyCd1@y)1@xSe with various Be compositions (x), while with Zn composition (y) fixed to be the constant value (y ¼ 0:3). The PL peak wavelength shortened from 596 to 532 nm on increasing x from 0.003 to 0.21. For each sample, a single-peak spectrum was observed without any deep level
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M. Takizawa et al. / Journal of Crystal Growth 227–228 (2001) 660–664
From y22y diffraction curves measured by the four-crystal high-resolution XRD system, the low FWHM value of 46 arcsec was obtained for the nearly lattice-matched sample (x ¼ 0:08), which indicated a high crystalline quality. For this sample, single-peak band-edge emissions were observed even at RT. The PL peak wavelength and the FWHM value were 596 nm and 56.8 meV, respectively.
3. MgSe/BeZnCdSe superlattices
Fig. 2. PL spectra at 15 K for Bex(Zn0.3Cd0.7)1@xSe with various Be compositions (x). The PL peak wavelength shortened from 596 to 532 nm on increasing x from 0.003 to 0.21.
emission. Full-width at half-maximum (FWHM) values of the PL peaks depended on latticemismatching of the BeZnCdSe layers. In this experiment, since the samples were prepared with changing only Be composition and fixing Zn and Cd composition, the lattice-mismatching value changed from +1.23% to @1.75% on increasing x from 0.003 to 0.21. As Fig. 1 shows, a low FWHM value of 8.8 meV was obtained for a nearly lattice-matched sample with Da=a ¼ @0:057% (x ¼ 0:08, y ¼ 0:30). On the other hand, FWHM values increased according to the lattice-mismatching, which suggested degradation of a crystal quality. Especially, the broadening of the PL peaks was more remarkable on the tensile strain side (higher x) compared with the compressive strain side (lower x). For example, the FWHM value was 11.2 meV for Da=a ¼ þ1:23% (x ¼ 0:003), and 74.4 meV for Da=a ¼ @1:75% (x ¼ 0:21).
MgSe/BeZnCdSe SL structures with various layer-thickness combinations of MgSe and BeZnCdSe layers were grown on InP substrates by the same way described in Section 2. The SL period was fixed to be 4.4 nm. Each elemental composition of Bex(ZnyCd1@y)1@xSe layers in the SLs was chosen to be x ¼ 0:08 and y ¼ 0:29 for lattice-matching to InP. The samples were prepared with changing MgSe layer-thickness ratio to the SL period (xMgSe) from 0.20 to 0.60. The xMgSe values were estimated based on EPMA measurements. The SL pair number and the total thickness were 182 and 0.8 mm, respectively. On the SL layers, 20-nm-thick BeZnCdSe cap layers were grown to prevent surface oxidization. From y22y diffraction curves of XRD measurements, satellite peaks were observed, accompanied with main peaks. From the main peak positions, the lattice-mismatching values were estimated to be from @0.37% to 0%. The SL periods estimated from the satellite peak positions were around 4.3 nm, which agreed with the values obtained by dividing the total SL thickness by the pair number. PL spectra 15 K MgSe/BeZnCdSe SLs with various xMgSe from 0.20 to 0.60 are shown in Fig. 3. For comparison, a spectrum of a bulk BeZnCdSe sample (i.e., xMgSe=0) is also shown. For each sample, a single-peak spectrum without any deep-level emission was observed. PL peak wavelength shortened from 572 to 470 nm on increasing xMgSe from 0 to 0.60. These PL peaks correspond to the color range from yellow to blue. FWHM values of the PL peaks increased from 29.6 to 83.3 meV on increasing xMgSe from 0.20 to
M. Takizawa et al. / Journal of Crystal Growth 227–228 (2001) 660–664
Fig. 3. PL spectra at 15 K for MgSe/BeZnCdSe SLs with various MgSe layer-thickness ratio to the SL period (xMgSe ) and a BeZnCdSe quaternary (i.e., xMgSe ¼ 0). PL peak wavelength shortened from 572 to 470 nm on increasing xMgSe from 0 to 0.60.
0.60, which may be due to the degradation of a crystal quality caused by increasing the layerthickness of MgSe. Broadening of the PL spectra for higher xMgSe may be improved by optimizing the growth condition of MgSe layers. For the SL samples, single-peak PL emissions were observed even at RT. For example, the PL peak wavelengths were 541 and 477 nm for the SL samples with xMgSe=0.20 and 0.60, respectively. These results indicate that MgSe/BeZnCdSe SLs give good emission properties in the visible light range from yellow to blue and are attractive materials for wide-range visible optical devices.
4. BeZnCdSe quantum wells QW structures consisting of BeZnCdSe QW and MgSe/BeZnCdSe SL barrier layers were
663
fabricated on InP substrates under the same growth condition in Section 2. Be and Zn compositions (x; y) in BeZnCdSe layers of QWs and SL barriers were 0.08 and 0.29, respectively. The SL barriers consisted of 0.69nm-thick MgSe and 0.45-nm-thick BeZnCdSe layers, which had MgSe layer-thickness ratio of 0.6. In this experiment, QW sample consisting of four kinds of single QW with a different well width in one wafer was prepared. The sample structure was as follows: from a substrate side, a 490-nm-thick SL barrier, 8.9-, 4.6-, 2.3-, 1.1-nmthick QW layers separated by 34-nm-thick SL barriers, 150-nm top SL barrier, finally a 5-nm BeZnCdSe cap layer. A PL spectrum at 15 K for the QW sample is shown in Fig. 4(a). PL peaks from the QWs and the SL barrier layer were clearly observed. The PL peak wavelength for the QWs varied from 459 to 561 nm on changing the well width from 1.1 to 8.9 nm. The PL peak from the barrier layer was 419 nm. A dependency of the PL peak energies for the QWs on the well width is shown in Fig. 4(b) by closed triangles. In addition, transition energies calculated for the QWs are also shown by a solid line. In this calculation, the band gap energies of the BeZnCdSe well and the MgSe/ BeZnCdSe SL barrier are 2.16 and 2.96 eV, respectively, which were obtained from the experimental PL peak energies shown in Figs. 2 and 4(a). Effective masses are assumed to be mc =mhh ¼ 0:13m0 =1:22m0 for the well and 0:19m0 =1:17m0 for the barrier. The calculation was performed by changing a band offset ratio between a well and a barrier as a fitting parameter. In this calculation, any excitonic effects such as the exciton binding energy in the QWs were not taken into account. As a result, the transition energies were calculated by using the band offset ratio of DEc : DEv ¼ 9 : 1 most approximate to the experimental PL peak energies as shown in Fig. 4(b). The band offset ratio agrees with that of ZnCdSe/MgZnCdSe QWs [5], which indicates that incorporation of BeSe into MgZnCdSe materials does not give a rapid change of the band lineup relations and that type-I heterostructures can be obtained by using MgSe/ BeZnCdSe materials.
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M. Takizawa et al. / Journal of Crystal Growth 227–228 (2001) 660–664
Fig. 4. (a) A PL spectrum at 15 K for the BeZnCdSe QW sample. (b) A dependency of the PL peak energies for the QWs on the well width (closed triangles) and transition energies calculated for the QWs (a solid line).
5. Summary
Acknowledgements
BeZnCdSe quaternaries, MgSe/BeZnCdSe SLs and BeZnCdSe QWs were grown on InP substrates by MBE. PL measurements at 15 K and RT for a lattice-matched Be0.08 (Zn0.30Cd0.70)0.92Se sample gave a yellow light single-peak spectrum without any deep-level emission at the peak wavelengths of 572 and 596 nm, respectively. FWHM values of the PL peak at 15 K were 8.8 meV. As for MgSe/ BeZnCdSe SLs, single-peak PL emissions were observed at 15 K in the green-to-blue color range from 525 to 470 nm on changing MgSe layerthickness ratio from 0.20 to 0.60. As for the QW sample consisting of four BeZnCdSe QWs with a different well width, PL peaks corresponding to each QW were observed in the wavelength range from 459 to 561 nm at 15 K. By fitting calculated transition energies in the QWs to the PL peak energies, the band offset ratio between the BeZnCdSe well and the MgSe/BeZnCdSe SL barrier was estimated to DEc : DEv ¼ 9 : 1.
This work was supported by the ‘‘Research for the Future’’ program of the Japan Society for the Promotion of Science No. JSPS-RFTF97P00102 and partly by a scientific research grant-in-aid from the Ministry of Education, Science, Sports and Culture of Japan No. #12750300.
References [1] C. V"eri!e, J. Crystal Growth 184/185 (1998) 1061. [2] A. Waag, Th. Litz, F. Fischer, H.-J. Lugauer, T. Baron, K. . Schull, U. Zehnder, T. Gerhard, U. Lunz, M. Keim, G. Reuscher, G. Landwehr, J. Crystal Growth 184/185 (1998) 1. [3] S. Che, I. Nomura, W. Shinozaki, A. Kikuchi, K. Shimomura, K. Kishino, J. Crystal Growth 214/215 (2000) 321. [4] T. Takada, S. Che, I. Nomura, A. Kikuchi, K. Shimomura, K. Kishino, Phys. Stat. Sol. A 180 (1) (2000) 37. [5] I. Nomura, T. Morita, M. Haraguchi, T. Nagano, H. Shinbo, A. Kikuchi, K. Kishino, Nonlinear Opt. 18 (2–4) (1997) 223.
MBE growth of BeZnCdSe quaternaries, MgSe/BeZnCdSe superlattice and quantum well structures on InP substrates Masatoshi Takizawa, Ichirou Nomura, Song-Bek Che, Akihiko Kikuchi, Kazuhiko Shimomura, Katsumi Kishino* Department of Electrical and Electronic Engineering, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan
Abstract BeZnCdSe quaternaries, MgSe/BeZnCdSe superlattice (SL) and BeZnCdSe quantum well (QW) structures were grown on InP substrates by molecular beam epitaxy for the first time. As for a lattice-matched Be0.08(Zn0.30Cd0.70)0.92Se sample, a single-peak photoluminescence (PL) spectrum was observed at the temperature range from 15 K to room temperature. The PL peak wavelength and the full-width at half-maximum (FWHM) value at 15 K were 572 nm and 8.8 meV, respectively. PL measurements of MgSe/BeZnCdSe SL samples were observed single-peak emissions in the green-to-blue color range from 525 to 470 nm at 15 K with changing MgSe layer-thickness ratio from 0.20 to 0.60. As for the QW sample consisting of four BeZnCdSe QWs with a different well width, PL peaks corresponding to each QW were observed in the wavelength range from 459 to 561 nm at 15 K. By fitting calculated transition energies in the QWs to the PL peak energies, the band offset ratio between the BeZnCdSe well and the MgSe/BeZnCdSe SL barrier was estimated to be DEc : DEv ¼ 9 : 1. r 2001 Elsevier Science B.V. All rights reserved. PACS: 71.20.Nr; 78.40.Fy; 78.66.Hf; 78.55.Et Keywords: A3. Molecular beam epitaxy; A3. Quantum wells; A3. Superlattices; B2. Semiconducting II–VI materials; B2. Semiconducting indium phosphide; B2. Semiconducting quaternary alloys
1. Introduction Beryllium chalcogenides such as BeSe and BeTe have higher cohesive energies due to their high degree of covalency, compared with other II–VI compounds such as ZnSe and ZnTe [1,2]. Therefore, incorporation of beryllium chalcogenides into conventional II–VI compounds has been noticed for enhancing the lattice hardening [2–4], *Corresponding author. Tel.: +81-3-3238-3323; fax: +81-33238-3321. E-mail address: [email protected] (K. Kishino).
which is expected to lead to lengthening device lifetimes of conventional II–VI laser diodes (LDs). Here, we will focus on BeZnCdSe quaternaries and MgSe/BeZnCdSe superlattice (SL) II–VI compounds on InP substrates, which are very attractive for wide-range visible optical devices such as laser diodes (LDs). The relationships between band gap energies and lattice constants of these II–VI compounds and InP are shown in Fig. 1. The band gap energies of BeZnCdSe quaternaries lie in the range from 2.1 to 2.5 eV keeping lattice-matching to InP (see a crossed area of a dashed line and a triangle area surrounded by BeSe, ZnSe and CdSe). The band gap energies
0022-0248/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 7 9 3 - X
M. Takizawa et al. / Journal of Crystal Growth 227–228 (2001) 660–664
661
2. BeZnCdSe quaternaries
Fig. 1. Relations between band gap energies and lattice constants of BeZnCdSe and MgSe II–VI compounds.
correspond to the wavelength range from orange (590 nm) to blue-green (500 nm). Furthermore, the band gap energies of MgSe/BeZnCdSe SLs can be widely controlled from 2.1 to 4 eV by only changing layer-thickness ratios of SLs keeping the lattice-matching. Therefore, BeZnCdSe and the SL materials are expected to be applied to highly reliable and wide range visible optical devices. In this study, BeZnCdSe quaternaries, MgSe/ BeZnCdSe SL and BeZnCdSe quantum well (QW) structures were grown on InP substrates by molecular beam epitaxy (MBE). The grown samples were characterized by photoluminescence (PL) measurements from 15 K to room temperature (RT). As for the lattice-matched Be0.08 (Zn0.30Cd0.70)0.92Se sample, a narrow single-peak PL spectrum was observed at 572 nm. The PL peak wavelength of MgSe/BeZnCdSe SL samples shortened from 525 to 470 nm with increasing the MgSe layer-thickness ratio. For the QW sample, PL peaks from the QWs were observed. By fitting calculated transition energies in the QWs to the PL peak energies, the band offset ratio between the BeZnCdSe well and the MgSe/BeZnCdSe SL barrier was estimated.
BeZnCdSe quaternaries were grown by MBE on (1 0 0) InP substrates with 0.2-mm-thick InGaAs buffer layers which were grown by metal–organic chemical vapor deposition (MOCVD) prior to the MBE growth. Elemental Be(5N), Zn(7N), Cd(6N), Mg(6N) and Se(6N) were used for the MBE growth. Thermal cleaning of the substrates was performed at 5001C under As pressure in the III–V MBE chamber. After the substrates were transferred into the II–VI MBE chamber through the ultra-high vacuum (UHV) tube, BeZnCdSe layers were grown at 3001C. In order to control heterovalency at the II–VI/III–V interfaces, Zn-irradiations for 30 s were performed before the growth. The beam pressures of Zn, Cd and Se were fixed to be around 3 10@7, 7 10@7 and 2 10@6 Torr, respectively. Under the above pressure condition, Zn composition (y) in Bex(ZnyCd1@y)1@xSe was kept to be about 0.3. On the other hand, Be beam pressure was changed from 0.7 to 1.5 10@8 Torr to control Be composition (x). After the growth, all the samples showed (2 1) streaky reflection of high-energy electron diffraction (RHEED) patterns, which indicated layer-bylayer growth under VI-group (Se) rich conditions. The total thickness and growth rate of the BeZnCdSe layers were around 1 mm and 0.5 mm/ h, respectively. Each elemental composition of BeZnCdSe layers was estimated based on the electron probe micro analysis (EPMA) and X-ray diffraction (XRD) measurements. Zn (y) and Cd (1@y) compositions in Bex(ZnyCd1@y)1@xSe were estimated based on the EPMA, and Be composition (x) was estimated based on the Zn and Cd compositions and the lattice-mismatching value obtained by the XRD measurements. The PL measurements were performed from 15 K to RT using the 325 nm line of the He–Cd laser for excitation. Fig. 2 shows PL spectra at 15 K for Bex(ZnyCd1@y)1@xSe with various Be compositions (x), while with Zn composition (y) fixed to be the constant value (y ¼ 0:3). The PL peak wavelength shortened from 596 to 532 nm on increasing x from 0.003 to 0.21. For each sample, a single-peak spectrum was observed without any deep level
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M. Takizawa et al. / Journal of Crystal Growth 227–228 (2001) 660–664
From y22y diffraction curves measured by the four-crystal high-resolution XRD system, the low FWHM value of 46 arcsec was obtained for the nearly lattice-matched sample (x ¼ 0:08), which indicated a high crystalline quality. For this sample, single-peak band-edge emissions were observed even at RT. The PL peak wavelength and the FWHM value were 596 nm and 56.8 meV, respectively.
3. MgSe/BeZnCdSe superlattices
Fig. 2. PL spectra at 15 K for Bex(Zn0.3Cd0.7)1@xSe with various Be compositions (x). The PL peak wavelength shortened from 596 to 532 nm on increasing x from 0.003 to 0.21.
emission. Full-width at half-maximum (FWHM) values of the PL peaks depended on latticemismatching of the BeZnCdSe layers. In this experiment, since the samples were prepared with changing only Be composition and fixing Zn and Cd composition, the lattice-mismatching value changed from +1.23% to @1.75% on increasing x from 0.003 to 0.21. As Fig. 1 shows, a low FWHM value of 8.8 meV was obtained for a nearly lattice-matched sample with Da=a ¼ @0:057% (x ¼ 0:08, y ¼ 0:30). On the other hand, FWHM values increased according to the lattice-mismatching, which suggested degradation of a crystal quality. Especially, the broadening of the PL peaks was more remarkable on the tensile strain side (higher x) compared with the compressive strain side (lower x). For example, the FWHM value was 11.2 meV for Da=a ¼ þ1:23% (x ¼ 0:003), and 74.4 meV for Da=a ¼ @1:75% (x ¼ 0:21).
MgSe/BeZnCdSe SL structures with various layer-thickness combinations of MgSe and BeZnCdSe layers were grown on InP substrates by the same way described in Section 2. The SL period was fixed to be 4.4 nm. Each elemental composition of Bex(ZnyCd1@y)1@xSe layers in the SLs was chosen to be x ¼ 0:08 and y ¼ 0:29 for lattice-matching to InP. The samples were prepared with changing MgSe layer-thickness ratio to the SL period (xMgSe) from 0.20 to 0.60. The xMgSe values were estimated based on EPMA measurements. The SL pair number and the total thickness were 182 and 0.8 mm, respectively. On the SL layers, 20-nm-thick BeZnCdSe cap layers were grown to prevent surface oxidization. From y22y diffraction curves of XRD measurements, satellite peaks were observed, accompanied with main peaks. From the main peak positions, the lattice-mismatching values were estimated to be from @0.37% to 0%. The SL periods estimated from the satellite peak positions were around 4.3 nm, which agreed with the values obtained by dividing the total SL thickness by the pair number. PL spectra 15 K MgSe/BeZnCdSe SLs with various xMgSe from 0.20 to 0.60 are shown in Fig. 3. For comparison, a spectrum of a bulk BeZnCdSe sample (i.e., xMgSe=0) is also shown. For each sample, a single-peak spectrum without any deep-level emission was observed. PL peak wavelength shortened from 572 to 470 nm on increasing xMgSe from 0 to 0.60. These PL peaks correspond to the color range from yellow to blue. FWHM values of the PL peaks increased from 29.6 to 83.3 meV on increasing xMgSe from 0.20 to
M. Takizawa et al. / Journal of Crystal Growth 227–228 (2001) 660–664
Fig. 3. PL spectra at 15 K for MgSe/BeZnCdSe SLs with various MgSe layer-thickness ratio to the SL period (xMgSe ) and a BeZnCdSe quaternary (i.e., xMgSe ¼ 0). PL peak wavelength shortened from 572 to 470 nm on increasing xMgSe from 0 to 0.60.
0.60, which may be due to the degradation of a crystal quality caused by increasing the layerthickness of MgSe. Broadening of the PL spectra for higher xMgSe may be improved by optimizing the growth condition of MgSe layers. For the SL samples, single-peak PL emissions were observed even at RT. For example, the PL peak wavelengths were 541 and 477 nm for the SL samples with xMgSe=0.20 and 0.60, respectively. These results indicate that MgSe/BeZnCdSe SLs give good emission properties in the visible light range from yellow to blue and are attractive materials for wide-range visible optical devices.
4. BeZnCdSe quantum wells QW structures consisting of BeZnCdSe QW and MgSe/BeZnCdSe SL barrier layers were
663
fabricated on InP substrates under the same growth condition in Section 2. Be and Zn compositions (x; y) in BeZnCdSe layers of QWs and SL barriers were 0.08 and 0.29, respectively. The SL barriers consisted of 0.69nm-thick MgSe and 0.45-nm-thick BeZnCdSe layers, which had MgSe layer-thickness ratio of 0.6. In this experiment, QW sample consisting of four kinds of single QW with a different well width in one wafer was prepared. The sample structure was as follows: from a substrate side, a 490-nm-thick SL barrier, 8.9-, 4.6-, 2.3-, 1.1-nmthick QW layers separated by 34-nm-thick SL barriers, 150-nm top SL barrier, finally a 5-nm BeZnCdSe cap layer. A PL spectrum at 15 K for the QW sample is shown in Fig. 4(a). PL peaks from the QWs and the SL barrier layer were clearly observed. The PL peak wavelength for the QWs varied from 459 to 561 nm on changing the well width from 1.1 to 8.9 nm. The PL peak from the barrier layer was 419 nm. A dependency of the PL peak energies for the QWs on the well width is shown in Fig. 4(b) by closed triangles. In addition, transition energies calculated for the QWs are also shown by a solid line. In this calculation, the band gap energies of the BeZnCdSe well and the MgSe/ BeZnCdSe SL barrier are 2.16 and 2.96 eV, respectively, which were obtained from the experimental PL peak energies shown in Figs. 2 and 4(a). Effective masses are assumed to be mc =mhh ¼ 0:13m0 =1:22m0 for the well and 0:19m0 =1:17m0 for the barrier. The calculation was performed by changing a band offset ratio between a well and a barrier as a fitting parameter. In this calculation, any excitonic effects such as the exciton binding energy in the QWs were not taken into account. As a result, the transition energies were calculated by using the band offset ratio of DEc : DEv ¼ 9 : 1 most approximate to the experimental PL peak energies as shown in Fig. 4(b). The band offset ratio agrees with that of ZnCdSe/MgZnCdSe QWs [5], which indicates that incorporation of BeSe into MgZnCdSe materials does not give a rapid change of the band lineup relations and that type-I heterostructures can be obtained by using MgSe/ BeZnCdSe materials.
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Fig. 4. (a) A PL spectrum at 15 K for the BeZnCdSe QW sample. (b) A dependency of the PL peak energies for the QWs on the well width (closed triangles) and transition energies calculated for the QWs (a solid line).
5. Summary
Acknowledgements
BeZnCdSe quaternaries, MgSe/BeZnCdSe SLs and BeZnCdSe QWs were grown on InP substrates by MBE. PL measurements at 15 K and RT for a lattice-matched Be0.08 (Zn0.30Cd0.70)0.92Se sample gave a yellow light single-peak spectrum without any deep-level emission at the peak wavelengths of 572 and 596 nm, respectively. FWHM values of the PL peak at 15 K were 8.8 meV. As for MgSe/ BeZnCdSe SLs, single-peak PL emissions were observed at 15 K in the green-to-blue color range from 525 to 470 nm on changing MgSe layerthickness ratio from 0.20 to 0.60. As for the QW sample consisting of four BeZnCdSe QWs with a different well width, PL peaks corresponding to each QW were observed in the wavelength range from 459 to 561 nm at 15 K. By fitting calculated transition energies in the QWs to the PL peak energies, the band offset ratio between the BeZnCdSe well and the MgSe/BeZnCdSe SL barrier was estimated to DEc : DEv ¼ 9 : 1.
This work was supported by the ‘‘Research for the Future’’ program of the Japan Society for the Promotion of Science No. JSPS-RFTF97P00102 and partly by a scientific research grant-in-aid from the Ministry of Education, Science, Sports and Culture of Japan No. #12750300.
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