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ZnCdS QW structures on CdS(0001) and ZnCdS(0001) substrates grown by MOVPE
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Physica E 17 (2003) 516 – 517 www.elsevier.com/locate/physe
Hexagonal ZnCdS epilayers and CdSSe/ZnCdS QW structures on CdS(0001) and ZnCdS(0001) substrates grown by MOVPE P.I. Kuznetsova;∗ , V.A. Jitova , G.G. Yakushchevaa , B.S. Shchamkhalovaa , L.Yu. Zakharova , V.I. Kozlovskyb , Ya.K. Skasyrskyb , K.P. O’Donnellc , C. Trager-Cowanc a Institute
of Radioengineering and Electronics of RAS, 1 Vvedenskogo, Fryazino, Moscow, Reg. 141120, Russia b P.N. Lebedev Physical Institute of RAS, Moscow, Russia c Strathclyde University, Glasgow, UK
Abstract Optimization of growth conditions allows us to grow mirror-like hexagonal ZnCdS layers and CdSSe/ZnCdS quantum well (QW) structures on CdS(0001) and ZnCdS(0001) substrates by MOVPE. ZnCdS epilayers with high cathodoluminescence (CL) intensity at room temperature (RT) have also been obtained. An intense QW emission line has been observed in CdS/ZnCdS QW structures. With increasing excitation intensity the line shifted to shorter wavelength, which is a manifestation of an internal piezoelectric e?ect. An additional short-wavelength line appears in low-temperature CL spectra of CdSSe/ZnCdS QW structures due to localization of charge carriers by nonuniformity of alloy composition or QW thickness. ? 2002 Elsevier Science B.V. All rights reserved. Keywords: MOVPE; CdSSe/ZnCdS QW; CdZnS substrates
Hexagonal II–VI heterostructures are promising for long-lived II–VI lasers as they are more resistant to the formation of dark spot defects than cubic ones under high excitation levels [1,2]. To grow such structures, cubic A3 B5 (1 1 1) substrates are usually used [2–4]. Recently CdS layers and the Drst hexagonal CdS/ZnCdS structures on (0001)-oriented ZnCdS and CdSSe substrates were grown by MOVPE [5]. In this paper we present new data concerning an optimization of growth conditions to obtain mirror-like ZnCdS epilayers with high luminescence properties and the Drst CdSSe/ZnCdS structures. ∗ Corresponding author. Tel.: 7-095-526-92-54; fax: 7-095702-95-72. E-mail address: [email protected] (P.I. Kuznetsova).
Atmospheric pressure MOVPE growth was carried out at temperatures from 370◦ C to 460◦ C. The CdSSe/ZnCdS QW structures consist of a 0:4 m thick Znx Cd 1−x S bu?er layer with x ≈ 0:2, single or multiple 4 –16 nm thick CdS or CdSSe QWs separated by 60 nm thick Znx Cd 1−x S barriers and capping layer. The grown structures were characterised by AFM, PL and CL. The latter was carried out at 14 K, with an electron energy of 3–30 keV and current density from 0.1 to 50 mA cm−2 . The surface morphology varies dramatically with the growth temperature: at temperatures higher than 430◦ C the surface is rough due to active decomposition of CdZnS in H2 . But with decreasing temperature the growth rate and CL intensity decrease. An optimization of growth conditions was carried out to
1386-9477/03/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-9477(02)00856-1
P.I. Kuznetsov et al. / Physica E 17 (2003) 516 – 517 10 CdS sub.
5
(a)
ZnCdS bar .
0
(b)
DAP of sub.
5 0
Intensity (a.u.)
(c) 5 0
(d) 5 0 5
(e)
CdS sub. artifacts
0 je =0.1 mA/cm2
5
je =20
(f)
mA/cm2
0 450
500
550
600
650
700
Wavelength (nm) Fig. 1. CL spectra at 14 K of the CdS1−x Sex =Zn0:2 Cd 0:8 S structures: (a) 8 nm thick SQW (x = 0:28) grown on the Zn0:05 Cd 0:95 S(0001)B; (b) 10 nm thick SQW (x = 0:28) grown on the CdS(0001)B; (c) 5nm SQW (x = 0:4) grown on the CdS(0001)B; (d) 15 nm thick Dve QWs (x = 0) grown on the Zn0:05 Cd 0:95 S(0001); (e) 4 nm thick SQW (x = 0) grown on the Zn0:05 Cd 0:95 S(0001)B and (f) 5 nm thick SQW (x = 0) grown on the CdS(0001)B.
obtain mirror-like ZnCdS with high enough CL intensity at RT and an acceptable growth rate. For Zn0:2 Cd 0:8 S optimum growth temperature and II/VI ratio in Lows were 370◦ C and 0.2, respectively. AFM images show that the ZnCdS epilayer is Latter than CdS. RMS roughness of the former is about 1:2 nm while it is 5 nm for the CdS epilayer. Fig. 1 shows low temperature CL spectra of different QW structures. At low excitation levels, the
517
CL spectrum of the 5 nm thick CdS/ZnCdS SQW grown on the ZnCdS(0001)B contains emission lines with maxima at 460 and 520 nm which correspond to the ZnCdS bu?er and the CdS QW, respectively. With increasing excitation level, the QW emission line moves to short-wave side, a manifestation of an internal piezoelectric e?ect [3]. A wide red emission band due to deep levels does not change with excitation level. The CL spectra of the CdSSe/ZnCdS QW structures have intense lines in red range and an additional intense line with maximum at 490 nm. Red lines are narrower than the red deep level band and they move to short-wave side with excitation level. Therefore we suggest that these lines are due to the CdSSe QWs. The 490 nm peak line is likely to be due to emission of excitons localized by nonuniformity of alloy composition or QW thickness. In conclusion, the presented results show that hexagonal ZnCdS epilayers and CdSSe/ZnCdS QW structures may be grown by MOVPE on CdS(0001) and ZnCdS(0001) substrates. Additional investigations are required to study the structural defects and their e?ect on luminescence properties. The work was supported by RFBR, Grants 00-15-96624 and 01-02-16409, RAS Program “Low-dimension quantum structures”, Russian Federation Program “Physics of solid-state nanostructures” and the NATO SfP under Grant 974355. References [1] M.C. Tamargo et al., Proceedings of the 2nd International Symposium Blue Laser & Light Emit. Diodes, Chiba, 1998, p. 703. [2] A. Jia, et al., J. Crystal Growth 214/215 (2000) 1085. [3] I.V. Bradley, J.P. Creasey, K.P. O’Donnell, J. Crystal Growth 184/185 (1998) 728. [4] N. Matsumura, J. Ueda, J. Saraie. Jpn. J. Appl. Phys. 39 (2000) L1026. [5] V.I. Kozlovsky et al., J. Crystal Growth 248 (2002) 62.
Physica E 17 (2003) 516 – 517 www.elsevier.com/locate/physe
Hexagonal ZnCdS epilayers and CdSSe/ZnCdS QW structures on CdS(0001) and ZnCdS(0001) substrates grown by MOVPE P.I. Kuznetsova;∗ , V.A. Jitova , G.G. Yakushchevaa , B.S. Shchamkhalovaa , L.Yu. Zakharova , V.I. Kozlovskyb , Ya.K. Skasyrskyb , K.P. O’Donnellc , C. Trager-Cowanc a Institute
of Radioengineering and Electronics of RAS, 1 Vvedenskogo, Fryazino, Moscow, Reg. 141120, Russia b P.N. Lebedev Physical Institute of RAS, Moscow, Russia c Strathclyde University, Glasgow, UK
Abstract Optimization of growth conditions allows us to grow mirror-like hexagonal ZnCdS layers and CdSSe/ZnCdS quantum well (QW) structures on CdS(0001) and ZnCdS(0001) substrates by MOVPE. ZnCdS epilayers with high cathodoluminescence (CL) intensity at room temperature (RT) have also been obtained. An intense QW emission line has been observed in CdS/ZnCdS QW structures. With increasing excitation intensity the line shifted to shorter wavelength, which is a manifestation of an internal piezoelectric e?ect. An additional short-wavelength line appears in low-temperature CL spectra of CdSSe/ZnCdS QW structures due to localization of charge carriers by nonuniformity of alloy composition or QW thickness. ? 2002 Elsevier Science B.V. All rights reserved. Keywords: MOVPE; CdSSe/ZnCdS QW; CdZnS substrates
Hexagonal II–VI heterostructures are promising for long-lived II–VI lasers as they are more resistant to the formation of dark spot defects than cubic ones under high excitation levels [1,2]. To grow such structures, cubic A3 B5 (1 1 1) substrates are usually used [2–4]. Recently CdS layers and the Drst hexagonal CdS/ZnCdS structures on (0001)-oriented ZnCdS and CdSSe substrates were grown by MOVPE [5]. In this paper we present new data concerning an optimization of growth conditions to obtain mirror-like ZnCdS epilayers with high luminescence properties and the Drst CdSSe/ZnCdS structures. ∗ Corresponding author. Tel.: 7-095-526-92-54; fax: 7-095702-95-72. E-mail address: [email protected] (P.I. Kuznetsova).
Atmospheric pressure MOVPE growth was carried out at temperatures from 370◦ C to 460◦ C. The CdSSe/ZnCdS QW structures consist of a 0:4 m thick Znx Cd 1−x S bu?er layer with x ≈ 0:2, single or multiple 4 –16 nm thick CdS or CdSSe QWs separated by 60 nm thick Znx Cd 1−x S barriers and capping layer. The grown structures were characterised by AFM, PL and CL. The latter was carried out at 14 K, with an electron energy of 3–30 keV and current density from 0.1 to 50 mA cm−2 . The surface morphology varies dramatically with the growth temperature: at temperatures higher than 430◦ C the surface is rough due to active decomposition of CdZnS in H2 . But with decreasing temperature the growth rate and CL intensity decrease. An optimization of growth conditions was carried out to
1386-9477/03/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-9477(02)00856-1
P.I. Kuznetsov et al. / Physica E 17 (2003) 516 – 517 10 CdS sub.
5
(a)
ZnCdS bar .
0
(b)
DAP of sub.
5 0
Intensity (a.u.)
(c) 5 0
(d) 5 0 5
(e)
CdS sub. artifacts
0 je =0.1 mA/cm2
5
je =20
(f)
mA/cm2
0 450
500
550
600
650
700
Wavelength (nm) Fig. 1. CL spectra at 14 K of the CdS1−x Sex =Zn0:2 Cd 0:8 S structures: (a) 8 nm thick SQW (x = 0:28) grown on the Zn0:05 Cd 0:95 S(0001)B; (b) 10 nm thick SQW (x = 0:28) grown on the CdS(0001)B; (c) 5nm SQW (x = 0:4) grown on the CdS(0001)B; (d) 15 nm thick Dve QWs (x = 0) grown on the Zn0:05 Cd 0:95 S(0001); (e) 4 nm thick SQW (x = 0) grown on the Zn0:05 Cd 0:95 S(0001)B and (f) 5 nm thick SQW (x = 0) grown on the CdS(0001)B.
obtain mirror-like ZnCdS with high enough CL intensity at RT and an acceptable growth rate. For Zn0:2 Cd 0:8 S optimum growth temperature and II/VI ratio in Lows were 370◦ C and 0.2, respectively. AFM images show that the ZnCdS epilayer is Latter than CdS. RMS roughness of the former is about 1:2 nm while it is 5 nm for the CdS epilayer. Fig. 1 shows low temperature CL spectra of different QW structures. At low excitation levels, the
517
CL spectrum of the 5 nm thick CdS/ZnCdS SQW grown on the ZnCdS(0001)B contains emission lines with maxima at 460 and 520 nm which correspond to the ZnCdS bu?er and the CdS QW, respectively. With increasing excitation level, the QW emission line moves to short-wave side, a manifestation of an internal piezoelectric e?ect [3]. A wide red emission band due to deep levels does not change with excitation level. The CL spectra of the CdSSe/ZnCdS QW structures have intense lines in red range and an additional intense line with maximum at 490 nm. Red lines are narrower than the red deep level band and they move to short-wave side with excitation level. Therefore we suggest that these lines are due to the CdSSe QWs. The 490 nm peak line is likely to be due to emission of excitons localized by nonuniformity of alloy composition or QW thickness. In conclusion, the presented results show that hexagonal ZnCdS epilayers and CdSSe/ZnCdS QW structures may be grown by MOVPE on CdS(0001) and ZnCdS(0001) substrates. Additional investigations are required to study the structural defects and their e?ect on luminescence properties. The work was supported by RFBR, Grants 00-15-96624 and 01-02-16409, RAS Program “Low-dimension quantum structures”, Russian Federation Program “Physics of solid-state nanostructures” and the NATO SfP under Grant 974355. References [1] M.C. Tamargo et al., Proceedings of the 2nd International Symposium Blue Laser & Light Emit. Diodes, Chiba, 1998, p. 703. [2] A. Jia, et al., J. Crystal Growth 214/215 (2000) 1085. [3] I.V. Bradley, J.P. Creasey, K.P. O’Donnell, J. Crystal Growth 184/185 (1998) 728. [4] N. Matsumura, J. Ueda, J. Saraie. Jpn. J. Appl. Phys. 39 (2000) L1026. [5] V.I. Kozlovsky et al., J. Crystal Growth 248 (2002) 62.