- Email: [email protected]
Strain effect in ZnSe epilayers grown on GaAs substrates
Journal of Crystal Growth 243 (2002) 389–395
Strain effect in ZnSe epilayers grown on GaAs substrates Y.-M. Yua, S. Nama, Byungsung Oa, K.-S. Leea, P.Y. Yub, Jongwon Leec, Y.D. Choid,* a
Department of Physics, Chungnam National University, Taejon 305-764, South Korea Department of Physics, Sunchon National University, Sunchon 540-742, South Korea c Department of Materials Engineering, Hanbat National University, Taejon 305-719, South Korea d Department of Optical and Electronic Physics, Mokwon University, 800 Doan-Dong, Seo-ku, Taejon 302-729, South Korea b
Received 25 March 2002; accepted 12 June 2002 Communicated by H. Ohno
Abstract The growth temperature dependence of strain in ZnSe epilayers on (1 0 0) GaAs substrates was investigated. The strain effect was confirmed using high-resolution X-ray diffraction, Raman scattering and photoluminescence. With the increasing growth temperature, the lattice constant decreased and the Raman frequency red-shifted. Also, the near band-edge emission peak energy decreased and the energy difference between the heavy hole- and the light hole-free exciton peak increased a little. This is well explained by the thermal tensile strain remaining in the epilayer. r 2002 Elsevier Science B.V. All rights reserved. PACS: 61.10.i; 78.30.j; 78.55.Et; 78.55.m Keywords: A1. Photoluminescence; A1. Raman scattering; A1. Strain; A3. Hot-wall epitaxy; B1. Zinc compounds
1. Introduction ZnSe has been studied extensively because of potential applications for opto-electronic devices operating in the blue–green spectral range, and the researches to grow the high-quality ZnSe epilayers have been well established by molecular-beam epitaxy (MBE) and metalorganic chemical vapor deposition [1–4]. Recently, many researches were carried out to grow ZnSe-based low-dimensional quantum structures making use of large strain *Corresponding author. Tel.: +82-42-829-7552; fax: +8242-823-0639. E-mail address: [email protected] (Y.D. Choi).
[5,6]. Not only the growth of high-quality crystals and epilayers but also the understanding of the strains is very important to apply to the high efficiency opto-electronic devices with the strained quantum structures. Generally, GaAs substrates which have a small lattice mismatch to the ZnSe epilayers are used to grow ZnSe-based lowdimensional quantum structures. In addition to a lattice mismatch, ZnSe and GaAs have different chemical properties and different thermal expansion coefficients, which may be the sources of crystal defects generated at the ZnSe epilayer/ GaAs substrate interface [7]. The effects of strain have been studied for ZnSe/GaAs epilayers grown by MBE and atomic layer epitaxy [8].
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 5 4 1 - 5
Y.-M. Yu et al. / Journal of Crystal Growth 243 (2002) 389–395
In this work, the growth temperature dependence of strain is investigated to study the strain effect in ZnSe-based semiconductor quantum structures grown on GaAs substrate. The strain effect was examined using high-resolution X-ray diffraction (HRXRD), Raman scattering, and photoluminescence (PL).
2. Experimental procedure ZnSe epilayers were grown on the semi-insulating (1 0 0) GaAs substrates by hot-wall epitaxy (HWE), and 5 N ZnSe powder was used as the source material. The GaAs substrates were ultrasonically cleaned by trichloroethylene, acetone and methanol in sequence for 5 min each. Then, they were chemically etched in 50–601C 3H2SO4:H2O2:H2O solution for 1 min and rinsed with methanol and deionized water. After having been dried with Ar gas, they were put on the substrate holder in HWE setup. Prior to the epilayer growth, the GaAs substrates were preheated for 20 min at 5801C to remove both the oxide layers and the impurities remaining on the substrate surface. Then, the substrates were slowly cooled to the growth temperature of 260–4001C. The source and wall temperatures were set to 6801C and 5301C, respectively. To get unstrained ZnSe epilayers, GaAs substrate was removed. 10H2O2:NH4OH solution was used as the selective etchant. The film thicknesses were 1–2 mm, which were determined by the reflectance spectra taken by a spectrophotometer. The HRXRD spectra, low temperature (10 K) PL spectra and room temperature Raman spectra were taken. For PL and Raman spectra a He–Cd laser (l ¼ 325 nm) and Ar-ion laser (l ¼ 514:5 nm) were used as the light sources and a f ¼ 0:85 m double monochromator with a charge-coupled device was used.
3. Results and discussion
G aA s (400)
H R -X R D intensity (a. u.)
390
Fig. 1 shows the HRXRD spectra for ZnSe epilayers grown at the different temperatures. The
400oC 360oC 340oC 300oC 260oC
-1200 -900
-600
-300
0
300
600
∆ θ (arcsec)
Fig. 1. HRXRD spectra for different ZnSe/GaAs grown at different temperatures.
ZnSe (4 0 0) peaks were observed on the left side of the strong GaAs (4 0 0) peaks. With the increasing growth temperature, the full-width at half-maximum (FWHM) decreased but at 4001C. The angle separation between the GaAs substrate (4 0 0) peak and the ZnSe (4 0 0) peak of HRXRD with the ( [9] gives the lattice constant of GaAs, 5.6537 A lattice constant a> of ZnSe epilayers. Fig. 2(a) shows the growth temperature dependence of the lattice constant of ZnSe epilayers. As a whole, with the increasing growth temperature, the lattice constant decreased. And the lattice constant of the ZnSe epilayers is found to be ( [9], smaller than that of a bulk ZnSe, 5.6687 A which means that the tensile strain remains in the ZnSe epilayers. The dotted line in Fig. 2(a) is a guideline. In the heterostructures, the strain parallel to the epilayer interface e can be found by C11 e ¼ exx ¼ eyy ¼ ezz ; ð1Þ 2C12 ezz ¼
3.1. HRXRD measurements
ZnS e (400)
a> 1; aZnSe
ð2Þ
where a> is the strained lattice constant perpendicular to the interface and aZnSe the unstrained lattice constant. C11 ¼ 8:59 1011 dyn/cm2 and
Lattice constant, a⊥ (Å)
Y.-M. Yu et al. / Journal of Crystal Growth 243 (2002) 389–395 5.669
Bulk ZnSe, a Z nS e = 5.6687 Å 5.668
5.667
5.666
5.665
220 (a)
260
300
340
380
420
In-plane strain, -ε (x10 -3 )
Substrate temperature (˚C )
Fig. 2(b) shows the strain in the ZnSe epilayers as a function of growth temperature. Note that the y-axis is e: The data points are the strain from the lattice constant measured by HRXRD. The dotted line is the linear line fitted to the experimental data and the solid line is the calculated thermal strain. The experimental values seem to be a little smaller than the calculated ones since the X-ray penetration depth is very large and the compressive strain effect due to the lattice mismatch near the interface may be included.
0.0 -0.2
3.2. Raman scattering measurements
-0.4
Fig. 3 shows the Raman scattering spectra in the zone center of the ZnSe epilayers grown at different temperatures. The resolution was better than 0.3 cm1. The bottom spectrum in Fig. 3 is the spectrum of a free standing ZnSe epilayer and only the unstrained ZnSe LO phonon peak is observed at 252.65 cm1 without GaAs LO phonon peak, which is observed near 290 cm1 in the
-0.6 -0.8
experim ental data strain(fitted) therm al strain (calculated)
-1.0
220 (b)
391
260 300 340 380 Substrate temperature (˚C )
420
Fig. 2. Growth temperature dependence of (a) lattice constants and (b) in-plane strain in the ZnSe epilayers.
ZnSe LO 2
ethermal ¼ ðaZnSe aGaAs Þ DT;
400oC
Raman intensity (a.u.)
C12 ¼ 5:06 1011 dyn/cm are the elastic stiffness constants for ZnSe [9]. So the strain for the sample grown at 3201C is found to be +4.0 104 from ( the measured lattice constant, 5.6660 A. In general, the strain remaining in the epilayer will be misfit strain and/or thermal strain. The misfit strain due to the lattice mismatch can be considered to fully relax since the ZnSe epilayers in this study are thicker than 1 mm [10]. The thermal strain due to the difference of thermal expansion can be calculated by
340oC
300oC 260oC
ð3Þ
where aGaAs and aZnSe are the thermal expansion coefficients of GaAs (5.75 106/K) and ZnSe (7.4 106/K) [9], respectively, and DT is the difference between the growth temperature and the measurement temperature (300 K). For the ZnSe epilayer grown at 3201C, the thermal tensile strain is calculated as +5.0 104.
G aAs LO
Free standing ZnSe layer 245
250
255
290
295
300
-1
Raman shift (cm ) Fig. 3. Raman scattering spectra for ZnSe/GaAs grown at various temperatures.
Y.-M. Yu et al. / Journal of Crystal Growth 243 (2002) 389–395
-1
LO phonon peak position (cm )
392
252.6
Free standing ZnSe, ω o = 252.65 cm
-1
252.5 252.4
and
252.3
DO ¼ as
252.2 252.1
220
260
300
340
380
420
o
(a)
S ubstrate tem perature ( C ) 0.0
-3
In-plane strain, - ε (x10 )
where oLO is the bulk ZnSe LO phonon frequency, and DOH and DO are given by S11 þ 2S12 DOH ¼ g ð5Þ oLO e S11 þ S12
252.7
(b)
-0.3
-0.9 experim ental data strain(fitted) therm al strain(calculated)
-1.5
220
260
300
340
380
420
o
S ubstrate tem perature ( C )
Fig. 4. (a) Raman shift and (b) in-plane strain in the ZnSe epilayers as a function of growth temperature.
other spectra. It can be seen that the ZnSe LO phonon peak red-shifts very small from that of the unstrained film as the growth temperature increases. This means that the biaxial tensile strain in the ZnSe epilayers increases with increasing growth temperature. Fig. 4(a) shows the growth temperature dependence of the LO phonon peak in the ZnSe epilayer. All the shifts are smaller than that of the free standing layer. Also, with the increasing growth temperature, the LO phonon frequency decreases. This agrees with the result from the lattice constants, which is considered to be due to the tensile strain. In the zinc-blende structure, the straininduced shift of the LO phonon frequency is given by [11] o ¼ oLO þ 2 DOH 23DO;
ð4Þ
ð6Þ
respectively. In case of ZnSe, the deformation potentials g and as are reported to be –0.9 and 0.62, respectively. With S11 ¼ 0:0230 GPa1 and S12 ¼ 0:0085 GPa1 [11], the expression for the ZnSe LO phonon frequency shift is found as follows: o oLO ¼ 415:042e:
-0.6
-1.2
S11 S12 oLO e; S11 þ S12
ð7Þ
Fig. 4(b) shows the growth temperature dependence of the strain calculated from the Raman scattering spectra with the free standing ZnSe epilayer frequency. Note again the y-axis is e: The strain increases with the increasing growth temperature, which is similar to the result obtained from HRXRD. However, the strain obtained from Raman shift is larger than that obtained from the X-ray diffraction spectra. This may be due to the penetration depth difference. The penetration depth of the laser light is much less than that of X-ray. So in Raman scattering, only the strain near the surface is reflected while XRD results include the deep interface effect. 3.3. Photoluminescence measurements Fig. 5 shows the typical near-edge emission spectrum of a 4-mm-thick ZnSe epilayer grown at 3201C. The heavy-hole exciton peak (Xhh ) at 2.8022 eV and the light-hole exciton peak (Xlh ) at 2.7935 eV split by the thermal tensile strain, and the neutral donor bound exciton peak I20 at 2.7963 eV and I2 at 2.7935 eV due to the diffused Ga atoms are observable [12,13]. The peak due to the neutral acceptor I1 appeared to be very weak, and two peaks at 2.7736 eV (Iov ) and 2.7660 eV (X-LO) may be identified to be nonexcitonic recombinations involving the Se-site-related defects and the first phonon replica of free excitonic peaks [14].
Y.-M. Yu et al. / Journal of Crystal Growth 243 (2002) 389–395
I2 '
PL intensity (a.u.)
I2
X lh X hh
I1 X -LO Iv
o
2.75 2.76 2.77 2.78 2.79 2.80 2.81 2.82 P hoton energy (eV )
Fig. 5. Typical PL spectrum for a 4-mm-thick ZnSe/GaAs.
SA
Y-band
P L intensity (a. u.)
400oC
360oC
340oC
300oC
D AP
260oC
2.0
2.2
2.4
2.6
2.8
3.0
P hoton energy (eV ) Fig. 6. PL spectra for the ZnSe epilayers grown at different temperatures.
Fig. 6 shows the growth temperature dependence of the PL spectra in the ZnSe/GaAs epilayers. When the growth temperature is low, the strong edge emission and donor–acceptor (DAP) transition are observed and the deep level emission peak is hardly observable. However, with increasing growth temperature the edge emission
393
peaks increase and become dominant while the peaks due to DAP rapidly decrease. And the deep level emissions such as Y-band (near 2.6 eV) associated with the structural defects [15] and the self-activated (SA) emission near 2.2 eV [16] increase. The energy dispersive X-ray spectrometry showed that the ZnSe surface is Zn-rich at low growth temperature and Se-rich at high growth temperature. It is reported that with the increasing VI/II ratio, the intensity ratio of the DAP emission to the band-edge emission decreased [17]. And it is reported that in the PL characteristic study of Gadoped ZnSe epilayers, Zn-rich epilayers exhibited the strong DAP peaks and related-LO phonon replicas, which were hardly observed in the Se-rich epilayers. Also, it is reported that the intensity of the deep level peak near 2.2 eV appeared to be considerably weak in the Zn-rich epilayers [18]. Therefore, the change of the PL spectra may be due to the variation of the VI/II ratio. Fig. 7(a) shows the growth temperature dependence of PL emission energies such as Xhh and Xlh : As the growth temperature increased, all the peaks shifted to the lower energy side, and the energy difference between Xhh and Xlh peaks increased little by little because the Xhh peaks slowly shift to lower energy than the Xlh peaks [8]. Also, these peaks were observed at the lower energy than free exciton peak X1s at 2.806 eV for the free standing ZnSe epilayer. This is well explained by the fourfold degenerated-valence bands (k ¼ 0) being split into the light-hole subbands (mJ ¼ 73=2) and the heavy-hole subbands (mJ ¼ 71=2) due to the thermal tensile strains in the ZnSe epilayers. Then, the relation of the energy difference between Xhh and Xlh peaks, Ehh Elh ; and the strains may be given by [8] C11 þ 2C12 Ehh Elh ¼ 2b e; ð8Þ C11 where b is the shear deformation potential. For a 1–2 mm-thick ZnSe epilayer grown at 3201C, the energy difference between Xhh and Xlh is 4.5 meV. Therefore, the strain is found to be e ¼ þ8:6 104 with b ¼ 1:2 eV [19]. Fig. 7(b) shows the growth temperature dependence of the strain (e) found from the PL spectra.
Y.-M. Yu et al. / Journal of Crystal Growth 243 (2002) 389–395
394
P eak energy (eV )
2.808 X hh X lh
2.804
2.800
2.796 240
280
320
360
400
o
S ubstrate tem perature ( C )
(a)
ZnSe epilayers was found to be smaller than that of its bulk, and it decreased linearly with the increasing growth temperature. The ZnSe LO phonon peak energy was less than that of the unstrained LO phonon peak energy, and it redshifted as the growth temperature increased. Also, with the increasing growth temperature, the near band-edge emission peak energy decreased, and the energy difference between Xhh and Xlh peaks increased little by little. These could be well explained by the thermal tensile strain in the epilayers.
experim ental data therm al strain
-3
In-plane strain, - ε (x10 )
0.0
-0.4
Acknowledgements This work was supported by Korea Research Foundation Grant (KRF-2000-015-DP0158).
-0.8
References -1.2 240
(b)
280
320
360
400
o
S ubstrate tem perature C
Fig. 7. (a) PL peak energies and (b) strain for ZnSe/GaAs as a function of growth temperature.
The solid line shown in Fig. 7(b) indicates the thermal strain represented as the Eq. (3). At low temperature (o70 K), the thermal expansion coefficients are small and their difference is also very small [9,20]. Accordingly, the difference in the thermal expansion coefficients was taken into account from the growth temperature to 70 K. It was confirmed that the temperature dependence of the thermal strains was well consistent with that of the strains found from the PL spectra.
4. Conclusion ZnSe epilayers were grown on GaAs (1 0 0) substrates from 2601C to 4001C by HWE. The growth temperature dependence of the strain was confirmed through HRXRD, Raman scattering, and PL measurements. The lattice constant of the
[1] T. Yao, M. Ogura, S. Matsuoka, T. Morishita, Appl. Phys. Lett. 43 (1983) 499. [2] A. Yoshikawa, S. Yamaga, K. Yanaka, H. Kasai, J. Crystal Growth 72 (1985) 13. [3] K.P. Giapis, D.-C. Lu, K.F. Jensen, Appl. Phys. Lett. 54 (1989) 353. [4] M.A. Haase, J. Qiu, J.M. DePuydt, H. Cheng, Appl. Phys. Lett. 59 (1991) 1272. [5] J.L. Merz, S. Lee, J.K. Furdyna, J. Crystal Growth 184/ 185 (1985) 228. [6] B.P. Zhang, D.D. Manh, K. Wakatsuki, Y. Segawa, J. Crystal Growth 227/228 (2001) 645. [7] J. Luyo-Alvarado, M. Melendez-Lira, M. Lopez-Lopez, I. Hernandez-Calderon, M.E. Constantino, H. NavarroContreras, M.A. Vidal, Y. Takagi, K. Samonji, H. Yonezu, J. Appl. Phys. 84 (1998) 1551. [8] T. Yao, Y. Okada, S. Matsui, K. Ishida, I. Fujimoto, J. Crystal Growth 81 (1987) 518. [9] O. Madelung (Ed.), Numerical Data and Function relationships in Science and Technology, Vol. 17, Springer, Berlin, 1982. [10] K. Wolf, S. Jilka, H. Sahin, H. Stanzl, T. Reisinger, A. Naumov, W. Gebhardt, J. Crystal Growth 152 (1995) 34. [11] D.J. Olego, K. Shahzad, J. Petruzzello, D. Cammack, Phys. Rev. B 36 (1987) 7674. [12] H.A. Mar, R.M. Park, J. Appl. Phys. 60 (1986) 1229. [13] G. Kudlek, N. Presser, J. Gutowski, K. Hingerl, H. Sitter, S.M. Durbin, D.R. Menke, M. Kobayashi, R.L. Gunshor, J. Appl. Phys. 68 (1990) 5630. [14] K. Shahzad, D.J. Olego, D.A. Cammack, Phys. Rev. B 39 (1989) 13016.
Y.-M. Yu et al. / Journal of Crystal Growth 243 (2002) 389–395 [15] T. Taguchi, T. Kusao, A. Hiraki, J. Crystal Growth 72 (1985) 46. [16] D. Curie, J.S. Prener, in: M. Aven, J. S. Prener (Eds.), Physics and Chemistry of II–VI Compounds, NorthHolland, Amsterdam, 1967, Chapter 9. [17] T. Asano, K. Funato, F. Nakamura, A. Ishibashi, J. Crystal Growth 156 (1995) 373.
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[18] B.J. Skromme, S.M. Shibli, J.L. de Miguel, M.C. Tamargo, J. Appl. Phys. 65 (1989) 3999. [19] T. Suski, W. Paul, High Pressure in Semiconductor Physics II, Vol. 55, Academic Press, New York, 1998, p. 247. [20] V. Grobmann, M. Behringer, H. Heinke, D. Hommel, J. Crystal Growth 184/185 (1998) 100.
Strain effect in ZnSe epilayers grown on GaAs substrates Y.-M. Yua, S. Nama, Byungsung Oa, K.-S. Leea, P.Y. Yub, Jongwon Leec, Y.D. Choid,* a
Department of Physics, Chungnam National University, Taejon 305-764, South Korea Department of Physics, Sunchon National University, Sunchon 540-742, South Korea c Department of Materials Engineering, Hanbat National University, Taejon 305-719, South Korea d Department of Optical and Electronic Physics, Mokwon University, 800 Doan-Dong, Seo-ku, Taejon 302-729, South Korea b
Received 25 March 2002; accepted 12 June 2002 Communicated by H. Ohno
Abstract The growth temperature dependence of strain in ZnSe epilayers on (1 0 0) GaAs substrates was investigated. The strain effect was confirmed using high-resolution X-ray diffraction, Raman scattering and photoluminescence. With the increasing growth temperature, the lattice constant decreased and the Raman frequency red-shifted. Also, the near band-edge emission peak energy decreased and the energy difference between the heavy hole- and the light hole-free exciton peak increased a little. This is well explained by the thermal tensile strain remaining in the epilayer. r 2002 Elsevier Science B.V. All rights reserved. PACS: 61.10.i; 78.30.j; 78.55.Et; 78.55.m Keywords: A1. Photoluminescence; A1. Raman scattering; A1. Strain; A3. Hot-wall epitaxy; B1. Zinc compounds
1. Introduction ZnSe has been studied extensively because of potential applications for opto-electronic devices operating in the blue–green spectral range, and the researches to grow the high-quality ZnSe epilayers have been well established by molecular-beam epitaxy (MBE) and metalorganic chemical vapor deposition [1–4]. Recently, many researches were carried out to grow ZnSe-based low-dimensional quantum structures making use of large strain *Corresponding author. Tel.: +82-42-829-7552; fax: +8242-823-0639. E-mail address: [email protected] (Y.D. Choi).
[5,6]. Not only the growth of high-quality crystals and epilayers but also the understanding of the strains is very important to apply to the high efficiency opto-electronic devices with the strained quantum structures. Generally, GaAs substrates which have a small lattice mismatch to the ZnSe epilayers are used to grow ZnSe-based lowdimensional quantum structures. In addition to a lattice mismatch, ZnSe and GaAs have different chemical properties and different thermal expansion coefficients, which may be the sources of crystal defects generated at the ZnSe epilayer/ GaAs substrate interface [7]. The effects of strain have been studied for ZnSe/GaAs epilayers grown by MBE and atomic layer epitaxy [8].
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 5 4 1 - 5
Y.-M. Yu et al. / Journal of Crystal Growth 243 (2002) 389–395
In this work, the growth temperature dependence of strain is investigated to study the strain effect in ZnSe-based semiconductor quantum structures grown on GaAs substrate. The strain effect was examined using high-resolution X-ray diffraction (HRXRD), Raman scattering, and photoluminescence (PL).
2. Experimental procedure ZnSe epilayers were grown on the semi-insulating (1 0 0) GaAs substrates by hot-wall epitaxy (HWE), and 5 N ZnSe powder was used as the source material. The GaAs substrates were ultrasonically cleaned by trichloroethylene, acetone and methanol in sequence for 5 min each. Then, they were chemically etched in 50–601C 3H2SO4:H2O2:H2O solution for 1 min and rinsed with methanol and deionized water. After having been dried with Ar gas, they were put on the substrate holder in HWE setup. Prior to the epilayer growth, the GaAs substrates were preheated for 20 min at 5801C to remove both the oxide layers and the impurities remaining on the substrate surface. Then, the substrates were slowly cooled to the growth temperature of 260–4001C. The source and wall temperatures were set to 6801C and 5301C, respectively. To get unstrained ZnSe epilayers, GaAs substrate was removed. 10H2O2:NH4OH solution was used as the selective etchant. The film thicknesses were 1–2 mm, which were determined by the reflectance spectra taken by a spectrophotometer. The HRXRD spectra, low temperature (10 K) PL spectra and room temperature Raman spectra were taken. For PL and Raman spectra a He–Cd laser (l ¼ 325 nm) and Ar-ion laser (l ¼ 514:5 nm) were used as the light sources and a f ¼ 0:85 m double monochromator with a charge-coupled device was used.
3. Results and discussion
G aA s (400)
H R -X R D intensity (a. u.)
390
Fig. 1 shows the HRXRD spectra for ZnSe epilayers grown at the different temperatures. The
400oC 360oC 340oC 300oC 260oC
-1200 -900
-600
-300
0
300
600
∆ θ (arcsec)
Fig. 1. HRXRD spectra for different ZnSe/GaAs grown at different temperatures.
ZnSe (4 0 0) peaks were observed on the left side of the strong GaAs (4 0 0) peaks. With the increasing growth temperature, the full-width at half-maximum (FWHM) decreased but at 4001C. The angle separation between the GaAs substrate (4 0 0) peak and the ZnSe (4 0 0) peak of HRXRD with the ( [9] gives the lattice constant of GaAs, 5.6537 A lattice constant a> of ZnSe epilayers. Fig. 2(a) shows the growth temperature dependence of the lattice constant of ZnSe epilayers. As a whole, with the increasing growth temperature, the lattice constant decreased. And the lattice constant of the ZnSe epilayers is found to be ( [9], smaller than that of a bulk ZnSe, 5.6687 A which means that the tensile strain remains in the ZnSe epilayers. The dotted line in Fig. 2(a) is a guideline. In the heterostructures, the strain parallel to the epilayer interface e can be found by C11 e ¼ exx ¼ eyy ¼ ezz ; ð1Þ 2C12 ezz ¼
3.1. HRXRD measurements
ZnS e (400)
a> 1; aZnSe
ð2Þ
where a> is the strained lattice constant perpendicular to the interface and aZnSe the unstrained lattice constant. C11 ¼ 8:59 1011 dyn/cm2 and
Lattice constant, a⊥ (Å)
Y.-M. Yu et al. / Journal of Crystal Growth 243 (2002) 389–395 5.669
Bulk ZnSe, a Z nS e = 5.6687 Å 5.668
5.667
5.666
5.665
220 (a)
260
300
340
380
420
In-plane strain, -ε (x10 -3 )
Substrate temperature (˚C )
Fig. 2(b) shows the strain in the ZnSe epilayers as a function of growth temperature. Note that the y-axis is e: The data points are the strain from the lattice constant measured by HRXRD. The dotted line is the linear line fitted to the experimental data and the solid line is the calculated thermal strain. The experimental values seem to be a little smaller than the calculated ones since the X-ray penetration depth is very large and the compressive strain effect due to the lattice mismatch near the interface may be included.
0.0 -0.2
3.2. Raman scattering measurements
-0.4
Fig. 3 shows the Raman scattering spectra in the zone center of the ZnSe epilayers grown at different temperatures. The resolution was better than 0.3 cm1. The bottom spectrum in Fig. 3 is the spectrum of a free standing ZnSe epilayer and only the unstrained ZnSe LO phonon peak is observed at 252.65 cm1 without GaAs LO phonon peak, which is observed near 290 cm1 in the
-0.6 -0.8
experim ental data strain(fitted) therm al strain (calculated)
-1.0
220 (b)
391
260 300 340 380 Substrate temperature (˚C )
420
Fig. 2. Growth temperature dependence of (a) lattice constants and (b) in-plane strain in the ZnSe epilayers.
ZnSe LO 2
ethermal ¼ ðaZnSe aGaAs Þ DT;
400oC
Raman intensity (a.u.)
C12 ¼ 5:06 1011 dyn/cm are the elastic stiffness constants for ZnSe [9]. So the strain for the sample grown at 3201C is found to be +4.0 104 from ( the measured lattice constant, 5.6660 A. In general, the strain remaining in the epilayer will be misfit strain and/or thermal strain. The misfit strain due to the lattice mismatch can be considered to fully relax since the ZnSe epilayers in this study are thicker than 1 mm [10]. The thermal strain due to the difference of thermal expansion can be calculated by
340oC
300oC 260oC
ð3Þ
where aGaAs and aZnSe are the thermal expansion coefficients of GaAs (5.75 106/K) and ZnSe (7.4 106/K) [9], respectively, and DT is the difference between the growth temperature and the measurement temperature (300 K). For the ZnSe epilayer grown at 3201C, the thermal tensile strain is calculated as +5.0 104.
G aAs LO
Free standing ZnSe layer 245
250
255
290
295
300
-1
Raman shift (cm ) Fig. 3. Raman scattering spectra for ZnSe/GaAs grown at various temperatures.
Y.-M. Yu et al. / Journal of Crystal Growth 243 (2002) 389–395
-1
LO phonon peak position (cm )
392
252.6
Free standing ZnSe, ω o = 252.65 cm
-1
252.5 252.4
and
252.3
DO ¼ as
252.2 252.1
220
260
300
340
380
420
o
(a)
S ubstrate tem perature ( C ) 0.0
-3
In-plane strain, - ε (x10 )
where oLO is the bulk ZnSe LO phonon frequency, and DOH and DO are given by S11 þ 2S12 DOH ¼ g ð5Þ oLO e S11 þ S12
252.7
(b)
-0.3
-0.9 experim ental data strain(fitted) therm al strain(calculated)
-1.5
220
260
300
340
380
420
o
S ubstrate tem perature ( C )
Fig. 4. (a) Raman shift and (b) in-plane strain in the ZnSe epilayers as a function of growth temperature.
other spectra. It can be seen that the ZnSe LO phonon peak red-shifts very small from that of the unstrained film as the growth temperature increases. This means that the biaxial tensile strain in the ZnSe epilayers increases with increasing growth temperature. Fig. 4(a) shows the growth temperature dependence of the LO phonon peak in the ZnSe epilayer. All the shifts are smaller than that of the free standing layer. Also, with the increasing growth temperature, the LO phonon frequency decreases. This agrees with the result from the lattice constants, which is considered to be due to the tensile strain. In the zinc-blende structure, the straininduced shift of the LO phonon frequency is given by [11] o ¼ oLO þ 2 DOH 23DO;
ð4Þ
ð6Þ
respectively. In case of ZnSe, the deformation potentials g and as are reported to be –0.9 and 0.62, respectively. With S11 ¼ 0:0230 GPa1 and S12 ¼ 0:0085 GPa1 [11], the expression for the ZnSe LO phonon frequency shift is found as follows: o oLO ¼ 415:042e:
-0.6
-1.2
S11 S12 oLO e; S11 þ S12
ð7Þ
Fig. 4(b) shows the growth temperature dependence of the strain calculated from the Raman scattering spectra with the free standing ZnSe epilayer frequency. Note again the y-axis is e: The strain increases with the increasing growth temperature, which is similar to the result obtained from HRXRD. However, the strain obtained from Raman shift is larger than that obtained from the X-ray diffraction spectra. This may be due to the penetration depth difference. The penetration depth of the laser light is much less than that of X-ray. So in Raman scattering, only the strain near the surface is reflected while XRD results include the deep interface effect. 3.3. Photoluminescence measurements Fig. 5 shows the typical near-edge emission spectrum of a 4-mm-thick ZnSe epilayer grown at 3201C. The heavy-hole exciton peak (Xhh ) at 2.8022 eV and the light-hole exciton peak (Xlh ) at 2.7935 eV split by the thermal tensile strain, and the neutral donor bound exciton peak I20 at 2.7963 eV and I2 at 2.7935 eV due to the diffused Ga atoms are observable [12,13]. The peak due to the neutral acceptor I1 appeared to be very weak, and two peaks at 2.7736 eV (Iov ) and 2.7660 eV (X-LO) may be identified to be nonexcitonic recombinations involving the Se-site-related defects and the first phonon replica of free excitonic peaks [14].
Y.-M. Yu et al. / Journal of Crystal Growth 243 (2002) 389–395
I2 '
PL intensity (a.u.)
I2
X lh X hh
I1 X -LO Iv
o
2.75 2.76 2.77 2.78 2.79 2.80 2.81 2.82 P hoton energy (eV )
Fig. 5. Typical PL spectrum for a 4-mm-thick ZnSe/GaAs.
SA
Y-band
P L intensity (a. u.)
400oC
360oC
340oC
300oC
D AP
260oC
2.0
2.2
2.4
2.6
2.8
3.0
P hoton energy (eV ) Fig. 6. PL spectra for the ZnSe epilayers grown at different temperatures.
Fig. 6 shows the growth temperature dependence of the PL spectra in the ZnSe/GaAs epilayers. When the growth temperature is low, the strong edge emission and donor–acceptor (DAP) transition are observed and the deep level emission peak is hardly observable. However, with increasing growth temperature the edge emission
393
peaks increase and become dominant while the peaks due to DAP rapidly decrease. And the deep level emissions such as Y-band (near 2.6 eV) associated with the structural defects [15] and the self-activated (SA) emission near 2.2 eV [16] increase. The energy dispersive X-ray spectrometry showed that the ZnSe surface is Zn-rich at low growth temperature and Se-rich at high growth temperature. It is reported that with the increasing VI/II ratio, the intensity ratio of the DAP emission to the band-edge emission decreased [17]. And it is reported that in the PL characteristic study of Gadoped ZnSe epilayers, Zn-rich epilayers exhibited the strong DAP peaks and related-LO phonon replicas, which were hardly observed in the Se-rich epilayers. Also, it is reported that the intensity of the deep level peak near 2.2 eV appeared to be considerably weak in the Zn-rich epilayers [18]. Therefore, the change of the PL spectra may be due to the variation of the VI/II ratio. Fig. 7(a) shows the growth temperature dependence of PL emission energies such as Xhh and Xlh : As the growth temperature increased, all the peaks shifted to the lower energy side, and the energy difference between Xhh and Xlh peaks increased little by little because the Xhh peaks slowly shift to lower energy than the Xlh peaks [8]. Also, these peaks were observed at the lower energy than free exciton peak X1s at 2.806 eV for the free standing ZnSe epilayer. This is well explained by the fourfold degenerated-valence bands (k ¼ 0) being split into the light-hole subbands (mJ ¼ 73=2) and the heavy-hole subbands (mJ ¼ 71=2) due to the thermal tensile strains in the ZnSe epilayers. Then, the relation of the energy difference between Xhh and Xlh peaks, Ehh Elh ; and the strains may be given by [8] C11 þ 2C12 Ehh Elh ¼ 2b e; ð8Þ C11 where b is the shear deformation potential. For a 1–2 mm-thick ZnSe epilayer grown at 3201C, the energy difference between Xhh and Xlh is 4.5 meV. Therefore, the strain is found to be e ¼ þ8:6 104 with b ¼ 1:2 eV [19]. Fig. 7(b) shows the growth temperature dependence of the strain (e) found from the PL spectra.
Y.-M. Yu et al. / Journal of Crystal Growth 243 (2002) 389–395
394
P eak energy (eV )
2.808 X hh X lh
2.804
2.800
2.796 240
280
320
360
400
o
S ubstrate tem perature ( C )
(a)
ZnSe epilayers was found to be smaller than that of its bulk, and it decreased linearly with the increasing growth temperature. The ZnSe LO phonon peak energy was less than that of the unstrained LO phonon peak energy, and it redshifted as the growth temperature increased. Also, with the increasing growth temperature, the near band-edge emission peak energy decreased, and the energy difference between Xhh and Xlh peaks increased little by little. These could be well explained by the thermal tensile strain in the epilayers.
experim ental data therm al strain
-3
In-plane strain, - ε (x10 )
0.0
-0.4
Acknowledgements This work was supported by Korea Research Foundation Grant (KRF-2000-015-DP0158).
-0.8
References -1.2 240
(b)
280
320
360
400
o
S ubstrate tem perature C
Fig. 7. (a) PL peak energies and (b) strain for ZnSe/GaAs as a function of growth temperature.
The solid line shown in Fig. 7(b) indicates the thermal strain represented as the Eq. (3). At low temperature (o70 K), the thermal expansion coefficients are small and their difference is also very small [9,20]. Accordingly, the difference in the thermal expansion coefficients was taken into account from the growth temperature to 70 K. It was confirmed that the temperature dependence of the thermal strains was well consistent with that of the strains found from the PL spectra.
4. Conclusion ZnSe epilayers were grown on GaAs (1 0 0) substrates from 2601C to 4001C by HWE. The growth temperature dependence of the strain was confirmed through HRXRD, Raman scattering, and PL measurements. The lattice constant of the
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