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Elastic strain effects dependence on the energy band gaps and the lattice parameters in CdTe epitaxial layers on GaAs (100) substrates grown by hot wall epitaxy
Solid State Communications, Voi. 84, No. 9, pp. 901-903, 1992. Printed in Great Britain.
0038-1098/92 $5.00 + .00 Pergamon Press Ltd
ELASTIC STRAIN EFFECTS DEPENDENCE ON THE ENERGY BAND GAPS AND THE LATTICE PARAMETERS IN CdTe EPITAXIAL LAYERS ON GaAs (1 00) SUBSTRATES GROWN BY HOT WALL EPITAXY J.H. Lee, S.B. Kim, B.J. Koo, I.H. Chung, M. Jung* and H.L. Park t Department of Physics, Yonsei University, Seoul 120-749, Korea and T.W. Kim Department of Physics, Kwangwoon University, 447-1 Wolgye-dong, Nowon-ku, Seoul 139-701, Korea
(Received 8 June 1992 by C.N.R. Rao) Electrolyte electroreflectance spectroscopy and X-ray diffraction measurements on CdTe/GaAs strained heterostructures grown by hot wall epitaxy were carried out to investigate the effect of the elastic strain dependence on the energy band gaps and the lattice parameters. Biaxial compressive strains existed in CdTe layers thinner than 3/zm. This critical thickness was the smallest value for any CdTe/GaAs heterostructures previously grown by hot wall epitaxy. The results from electrolyte electroreflectance spectroscopy and X-ray diffraction were in good qualitative agreement with each other. lattice mismatch between CdTe and GaAs. The stress induced by the lattice mismatch can be relaxed due to misfit dislocations. This communication reports an assessment of the strain in HWE-grown CdTe thin films on (1 00) GaAs using electrolyte electroreflectance (EER) spectroscopy and X-ray diffraction. The EER transition is particularly useful between critical points because the direct transitions can be clearly observed. Polycrystalline CdTe precleaned by repeated sublimation was used as a source material, and the substrate was n-type GaAs with a (1 0 0) orientation. The surface of the substrate was cleaned chemically with acetone, trichloroethylene, and deionized water, alternately. The final etching was done in a H2SO4 : H202 : H20 (5 : 1 : 1) solution before insertion into the hot wall apparatus. The source temperature was 480°C, and the substrate temperature was varied between 230°C and 330°C during the growth of CdTe. The optimum growth temperature for a CdTe layer was obtained from the broadening parameter determined by EER measurements, and the thickness of the grown CdTe layer was determined by scanning electron microscopy. The best * Permanent address: Department of Physics, Ewha quality CdTe epilayers were obtained at a substrate temperature of 300°C. The thicknesses of the CdTe /Tomen's University, Seoul 120-750, Korea. o whom all correspondence should be addressed. epilayers were between 0.4 and 3/~m. The crystallinity
IN RECENT YEARS, the growth of CdTe has attracted attention due to its novel physical properties and many device applications [1-5]. Although CdTe/GaAs heterostructures have inherent problems for obtaining epitaxial growth due to the large lattice mismatch (Aa/a = 14.6%), high quality single crystalline CdTe has been grown on (1 0 0) oriented GaAs substrates by molecular beam epitaxy [6], metalorganic chemical vapor deposition [7], and hot wall epitaxy (HWE) [3]. CdTe has been recognized as an important material for solar-cell, "r-ray detector, and optoelectronic applications. Recently, new attention has been paid to CdTe due to its close lattice match and chemical compatibility with HgCdTe because CdTe can be an ideal buffer layer for the growth of HgCdTe. The present lack of high quality and large area bulk CdTe substrate is generally recognized as the stumbling block to the development of an efficient HgCdTe detector. Many efforts have been made to obtain CdTe epitaxial films with high quality and large area which can be used as substrates for the growth of HgCdTe [8-12]. Thin films of CdTe are usually grown on GaAs substrates in spite of the high
9~1
902
ELASTIC STRAIN EFFECTS IN CdTe EPITAXIAL LAYERS
Vol. 84,"No. 9
Table 1. The thickness dependence of the energy gaps and strains in CdTe epilayers Thickness (#m)
Eg (eV)
e (EER)
~ (X-ray)
0.4 0.6 1.0 3.0
1.5033 1.5078 1.5089 1.5114
-5.05 x 10-3 -2.24 × 10-3 -1.56 × 10-3 0
-2.77 x 10 -3 -1.95 × 10-3 -0.27 x 10 -3 0
of the obtained CdTe layers was verified using X-ray rocking curves. For the EER measurements, the sample was immersed in an electrolyte consisting of one part of 3% tartaric acid by weight in water and two parts of ethylene glycol. A gold wire was also immersed in the electrolyte to serve as the other electrode. A d.c. voltage source was applied for biasing the sample. Also, modulating voltages in the range between 0 and 2V were used. The electroreflectance spectra were analyzed using Aspens' theory [13] for low fields. The dependence of the band edge under strain has near extensively investigated for zincblende structures [5, 14]. Thus, the strain of the CdTe epilayer can be determined by measuring the band shift (AEg) using the following equation [5]:
AEg = 2ae Sll + 2S12 -4- be Sii - St2 Sll + Si2 Sll + Si2 '
(1)
where a and b are the deformation potentials of the hydrostatic and tetragonai components of the strain, respectively, c is the component of the strain in the epitaxial layer, and Sij is the elastic compliance constant. The AEg of CdTe under biaxial compression can be simplified by using a = - 2 . 7 4 e V ,
I
CdTe/Go~
,
I
"
~(~
.0 ~m
750
800
I 850
900
WAVELENGTH (nm) Fig. 1. EER spectra of CdTe epilayers with various thicknesses.
b=-l.4eV, S ~ l = 4 . 2 5 x 1 0 - 3 k b a r -I, and SI2 = -1.73 x 10-3 kbar -l [15], and AEg = 1.604eeV.
(2)
For confirming the EER results, the strains were also obtained from the X-ray diffraction profiles by using the following equation [16]: e-
Cll Cll+2Ci2
Aa± Sit + Si2 Aa± --, a0 Slt-Si2 a0
(3)
where C,7 is the elastic stiffness, a0 is the substrate lattice parameter, and Aa± is the lattice mismatch between the GaAs substrate and the grown CdTe epilayer. The EER spectra with various thickness CdTe epilayers are shown in Fig. 1. The thickness dependence of the lattice parameters of the grown CdTe epilayers are presented in Fig. 2. The lattice parameters of the CdTe epilayer are matched to those of bulk CdTe around 3 # m as shown in Fig. 2. Therefore, no strain existed for a thick CdTe epilayer on (1 0 0) GaAs. To the best of our knowledge, the 3 #m thickness is the smallest magnitude grown by the HWE until now [! 7]. The thickness dependence of the energy gaps (Eg) and strains CdTe on (100) GaAs are compiled in Table 1. Although the general trend between the EER and X-ray measurements is consistent with the X-ray data, the magnitudes of the strains determined from EER are larger than those obtained from X-ray measurements. 6.53 I I o< CdTelGoAs " " 6.52 - • I'~ 6.51 hff) Z 6.50 0(_) 6.49 IJ.I U ~_. 6.48 CdTe BULK }-
i
I 5
THICKNESS (IJm)
4
Fig. 2. Thickness dependence of the on lattice parameters of the CdTe epilayers.
Vo1.'84, No. 9
ELASTIC STRAIN EFFECTS IN CdTe EPITAXIAL LAYERS
In summary, the results show that the strain can be determined through optical means using EER spectroscopy and X-ray diffraction measurements. These results indicate the elastic strain effects are strongly dependent on the energy band gaps and lattice parameters. Biaxial compressive strains existed in CdTe layers thinner than 3 #m. This value is the smallest one for any CdTe/GaAs heterostructure previously grown by HWE.
Acknowledgements - This work was supported in part by the Korea Science and Engineering Foundation through SPRC at Junbuk National University. REFERENCES 1.
2. 3.
T.W. Kim, Y.H. Chang, Y.D. Zheng, A.A. Reeder, B.D. McCombe, R.F.C. Farrow, T. Temfonte, F.A. Shirland & A. Noreika, J. Vac. Sci. Technol. B5, 980 (1987). T.W. Kim, B.J. Koo, M. Jung, S.B. Kim, H.L. Park, H. Lim, J.I. Lee & K.N. Kang, J. Appl. Phys. 71, 1049 (1992). K. Lischka, E.J. Fantner, T.W. Ryan & H. Sitter, Appl. Phys. Lett. 55, 1309 (1989).
4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
903
P. Sircar, Appl. Phys. Lett. 53, 1184 (1988). D.J. Olego, J. Petruzzello, S.K. Ghandhi, N.R. Taskar & I.B. Baht, Appl Phys. Lett. 51, 127 (1987). J.L. Reno, P.L. Gourley, G. Monfroy & J.P. Faurie, Appl. Phys. Lett. 53, 1747 (1988). W.E. Hoke, P.L. Lemonias & R. Traczewski, Appl. Phys. Lett. 44, 1046 (1984). C.H. Wang, K.Y. Cheng & S.J. Yang, Appl. Phys. Lett. 46, 962 (1985). K. Nishitami, O. Ihkawa & T. Murotani, J. Electron Mat. 12, 619 (1983). R. Srinivasa, M.B. Panish & H. Temkin, Appl. Phys. Lett. 50, 1441 (1987). H. Sitter, K. Lischka, W. Faschinger, J. Wolfrum, H. Pascher & J.L. Pantrat, J. Crystal Growth 86, 377 (1988). K.S. Choi, B.J. Koo, M. Jung, H.J. Shin, J.H. Lee, S.B. Kim, S.K. Chang & H.L. Park, Phys. Status Solidi (a) 122, K135 (1990). D.E. Aspens, Surf Sci. 37, 418 (1973). F.H. Pollak, Surf Sci. 37, 863 (1973). J. Allegre, J. Calatayud, B. Gil, H. Tufligo, G. Lentz, N. Magnea & H. Mariette, Phys. Rev. 1341, 8195 (1990). H. Asai & K. Oe, J. Appl. Phys. 54, 2052 (1983). H. Tatsuoka, H. Kuwabara, H. Fujiyasu & Y. Nakanishi, J. Appl. Phys. 65, 2073 (1989).
0038-1098/92 $5.00 + .00 Pergamon Press Ltd
ELASTIC STRAIN EFFECTS DEPENDENCE ON THE ENERGY BAND GAPS AND THE LATTICE PARAMETERS IN CdTe EPITAXIAL LAYERS ON GaAs (1 00) SUBSTRATES GROWN BY HOT WALL EPITAXY J.H. Lee, S.B. Kim, B.J. Koo, I.H. Chung, M. Jung* and H.L. Park t Department of Physics, Yonsei University, Seoul 120-749, Korea and T.W. Kim Department of Physics, Kwangwoon University, 447-1 Wolgye-dong, Nowon-ku, Seoul 139-701, Korea
(Received 8 June 1992 by C.N.R. Rao) Electrolyte electroreflectance spectroscopy and X-ray diffraction measurements on CdTe/GaAs strained heterostructures grown by hot wall epitaxy were carried out to investigate the effect of the elastic strain dependence on the energy band gaps and the lattice parameters. Biaxial compressive strains existed in CdTe layers thinner than 3/zm. This critical thickness was the smallest value for any CdTe/GaAs heterostructures previously grown by hot wall epitaxy. The results from electrolyte electroreflectance spectroscopy and X-ray diffraction were in good qualitative agreement with each other. lattice mismatch between CdTe and GaAs. The stress induced by the lattice mismatch can be relaxed due to misfit dislocations. This communication reports an assessment of the strain in HWE-grown CdTe thin films on (1 00) GaAs using electrolyte electroreflectance (EER) spectroscopy and X-ray diffraction. The EER transition is particularly useful between critical points because the direct transitions can be clearly observed. Polycrystalline CdTe precleaned by repeated sublimation was used as a source material, and the substrate was n-type GaAs with a (1 0 0) orientation. The surface of the substrate was cleaned chemically with acetone, trichloroethylene, and deionized water, alternately. The final etching was done in a H2SO4 : H202 : H20 (5 : 1 : 1) solution before insertion into the hot wall apparatus. The source temperature was 480°C, and the substrate temperature was varied between 230°C and 330°C during the growth of CdTe. The optimum growth temperature for a CdTe layer was obtained from the broadening parameter determined by EER measurements, and the thickness of the grown CdTe layer was determined by scanning electron microscopy. The best * Permanent address: Department of Physics, Ewha quality CdTe epilayers were obtained at a substrate temperature of 300°C. The thicknesses of the CdTe /Tomen's University, Seoul 120-750, Korea. o whom all correspondence should be addressed. epilayers were between 0.4 and 3/~m. The crystallinity
IN RECENT YEARS, the growth of CdTe has attracted attention due to its novel physical properties and many device applications [1-5]. Although CdTe/GaAs heterostructures have inherent problems for obtaining epitaxial growth due to the large lattice mismatch (Aa/a = 14.6%), high quality single crystalline CdTe has been grown on (1 0 0) oriented GaAs substrates by molecular beam epitaxy [6], metalorganic chemical vapor deposition [7], and hot wall epitaxy (HWE) [3]. CdTe has been recognized as an important material for solar-cell, "r-ray detector, and optoelectronic applications. Recently, new attention has been paid to CdTe due to its close lattice match and chemical compatibility with HgCdTe because CdTe can be an ideal buffer layer for the growth of HgCdTe. The present lack of high quality and large area bulk CdTe substrate is generally recognized as the stumbling block to the development of an efficient HgCdTe detector. Many efforts have been made to obtain CdTe epitaxial films with high quality and large area which can be used as substrates for the growth of HgCdTe [8-12]. Thin films of CdTe are usually grown on GaAs substrates in spite of the high
9~1
902
ELASTIC STRAIN EFFECTS IN CdTe EPITAXIAL LAYERS
Vol. 84,"No. 9
Table 1. The thickness dependence of the energy gaps and strains in CdTe epilayers Thickness (#m)
Eg (eV)
e (EER)
~ (X-ray)
0.4 0.6 1.0 3.0
1.5033 1.5078 1.5089 1.5114
-5.05 x 10-3 -2.24 × 10-3 -1.56 × 10-3 0
-2.77 x 10 -3 -1.95 × 10-3 -0.27 x 10 -3 0
of the obtained CdTe layers was verified using X-ray rocking curves. For the EER measurements, the sample was immersed in an electrolyte consisting of one part of 3% tartaric acid by weight in water and two parts of ethylene glycol. A gold wire was also immersed in the electrolyte to serve as the other electrode. A d.c. voltage source was applied for biasing the sample. Also, modulating voltages in the range between 0 and 2V were used. The electroreflectance spectra were analyzed using Aspens' theory [13] for low fields. The dependence of the band edge under strain has near extensively investigated for zincblende structures [5, 14]. Thus, the strain of the CdTe epilayer can be determined by measuring the band shift (AEg) using the following equation [5]:
AEg = 2ae Sll + 2S12 -4- be Sii - St2 Sll + Si2 Sll + Si2 '
(1)
where a and b are the deformation potentials of the hydrostatic and tetragonai components of the strain, respectively, c is the component of the strain in the epitaxial layer, and Sij is the elastic compliance constant. The AEg of CdTe under biaxial compression can be simplified by using a = - 2 . 7 4 e V ,
I
CdTe/Go~
,
I
"
~(~
.0 ~m
750
800
I 850
900
WAVELENGTH (nm) Fig. 1. EER spectra of CdTe epilayers with various thicknesses.
b=-l.4eV, S ~ l = 4 . 2 5 x 1 0 - 3 k b a r -I, and SI2 = -1.73 x 10-3 kbar -l [15], and AEg = 1.604eeV.
(2)
For confirming the EER results, the strains were also obtained from the X-ray diffraction profiles by using the following equation [16]: e-
Cll Cll+2Ci2
Aa± Sit + Si2 Aa± --, a0 Slt-Si2 a0
(3)
where C,7 is the elastic stiffness, a0 is the substrate lattice parameter, and Aa± is the lattice mismatch between the GaAs substrate and the grown CdTe epilayer. The EER spectra with various thickness CdTe epilayers are shown in Fig. 1. The thickness dependence of the lattice parameters of the grown CdTe epilayers are presented in Fig. 2. The lattice parameters of the CdTe epilayer are matched to those of bulk CdTe around 3 # m as shown in Fig. 2. Therefore, no strain existed for a thick CdTe epilayer on (1 0 0) GaAs. To the best of our knowledge, the 3 #m thickness is the smallest magnitude grown by the HWE until now [! 7]. The thickness dependence of the energy gaps (Eg) and strains CdTe on (100) GaAs are compiled in Table 1. Although the general trend between the EER and X-ray measurements is consistent with the X-ray data, the magnitudes of the strains determined from EER are larger than those obtained from X-ray measurements. 6.53 I I o< CdTelGoAs " " 6.52 - • I'~ 6.51 hff) Z 6.50 0(_) 6.49 IJ.I U ~_. 6.48 CdTe BULK }-
i
I 5
THICKNESS (IJm)
4
Fig. 2. Thickness dependence of the on lattice parameters of the CdTe epilayers.
Vo1.'84, No. 9
ELASTIC STRAIN EFFECTS IN CdTe EPITAXIAL LAYERS
In summary, the results show that the strain can be determined through optical means using EER spectroscopy and X-ray diffraction measurements. These results indicate the elastic strain effects are strongly dependent on the energy band gaps and lattice parameters. Biaxial compressive strains existed in CdTe layers thinner than 3 #m. This value is the smallest one for any CdTe/GaAs heterostructure previously grown by HWE.
Acknowledgements - This work was supported in part by the Korea Science and Engineering Foundation through SPRC at Junbuk National University. REFERENCES 1.
2. 3.
T.W. Kim, Y.H. Chang, Y.D. Zheng, A.A. Reeder, B.D. McCombe, R.F.C. Farrow, T. Temfonte, F.A. Shirland & A. Noreika, J. Vac. Sci. Technol. B5, 980 (1987). T.W. Kim, B.J. Koo, M. Jung, S.B. Kim, H.L. Park, H. Lim, J.I. Lee & K.N. Kang, J. Appl. Phys. 71, 1049 (1992). K. Lischka, E.J. Fantner, T.W. Ryan & H. Sitter, Appl. Phys. Lett. 55, 1309 (1989).
4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
903
P. Sircar, Appl. Phys. Lett. 53, 1184 (1988). D.J. Olego, J. Petruzzello, S.K. Ghandhi, N.R. Taskar & I.B. Baht, Appl Phys. Lett. 51, 127 (1987). J.L. Reno, P.L. Gourley, G. Monfroy & J.P. Faurie, Appl. Phys. Lett. 53, 1747 (1988). W.E. Hoke, P.L. Lemonias & R. Traczewski, Appl. Phys. Lett. 44, 1046 (1984). C.H. Wang, K.Y. Cheng & S.J. Yang, Appl. Phys. Lett. 46, 962 (1985). K. Nishitami, O. Ihkawa & T. Murotani, J. Electron Mat. 12, 619 (1983). R. Srinivasa, M.B. Panish & H. Temkin, Appl. Phys. Lett. 50, 1441 (1987). H. Sitter, K. Lischka, W. Faschinger, J. Wolfrum, H. Pascher & J.L. Pantrat, J. Crystal Growth 86, 377 (1988). K.S. Choi, B.J. Koo, M. Jung, H.J. Shin, J.H. Lee, S.B. Kim, S.K. Chang & H.L. Park, Phys. Status Solidi (a) 122, K135 (1990). D.E. Aspens, Surf Sci. 37, 418 (1973). F.H. Pollak, Surf Sci. 37, 863 (1973). J. Allegre, J. Calatayud, B. Gil, H. Tufligo, G. Lentz, N. Magnea & H. Mariette, Phys. Rev. 1341, 8195 (1990). H. Asai & K. Oe, J. Appl. Phys. 54, 2052 (1983). H. Tatsuoka, H. Kuwabara, H. Fujiyasu & Y. Nakanishi, J. Appl. Phys. 65, 2073 (1989).