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Control of Zn composition (0 < x < 1) in Cd1−xZnxTe epitaxial layers on GaAs substrates grown by MOVPE
Applied Surface Science 244 (2005) 347–350 www.elsevier.com/locate/apsusc
Control of Zn composition (0 < x < 1) in Cd1 xZnxTe epitaxial layers on GaAs substrates grown by MOVPE K. Yasuda*, M. Niraula, H. Kusama, Y. Yamamoto, M. Tominaga, K. Takagi, Y. Aagata Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan Received 31 May 2004; accepted 22 September 2004 Available online 29 December 2004
Abstract Cd1 xZnxTe epitaxial layers over the entire composition range (x from 0 to 1) were grown on (1 0 0) GaAs substrates by atmospheric pressure metalorganic vapor phase epitaxy. A growth temperature of 560 8C, and the group VI/II precursor flow rate ratio of 2 or larger enabled us to control the Zn composition strictly on the grown epilayers. Epitaxial layers with high crystal quality were obtained in a wide range of Zn composition. The double crystal rocking curves (DCRC) full-width at half maximum (FWHM) values were between 260 and 670 arcsec at the end points of alloy range. The low-temperature PL measurements showed distinct bound-exciton emissions and weak deep-level emissions. # 2004 Elsevier B.V. All rights reserved. PACS: 73.61.Ga; 81.05.Dz; 81.15.Kk Keywords: CdZnTe; MOVPE; Optoelectronic devices
1. Introduction Cd1 xZnxTe crystals are considered promising in a variety of optoelectronic device applications like solar cells, light emitting diodes, nuclear radiation detectors because it has a direct bandgap that can be tuned from near infrared (1.5 eV) to visible (2.3 eV) by controlling the Zn composition in the grown crystals. Most of those * Corresponding author. Tel.: +81 52 735 5435; fax: +81 52 735 5578. E-mail address: [email protected] (K. Yasuda).
device applications require a large-area uniform single crystal wafer. Hence, there are increased interests in the growth of Cd1 xZnxTe crystals on GaAs or Si substrates, as these substrates are easily available in a large-area at low cost as well as robust. In addition, Cd1 xZnxTe epitaxial layers (x = 0.04) on GaAs or Si can be used as a lattice matched substrate for the HgCdTe layers growth for the infrared detector application [1,2]. Likewise, Cd1 xZnxTe crystals with x-values from 0 to 0.2 are finding increased applications in nuclear radiation detector fabrication, while x-value of 0.4 is considered suitable for the efficient tandem solar cells. In this
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.09.144
348
K. Yasuda et al. / Applied Surface Science 244 (2005) 347–350
regard, the ability to grow epitaxial Cd1 xZnxTe layers over the entire compositional range (x from 0 to 1) offers the device designer added flexibility, and opens new possibilities of this material in different optoelectronic applications [1–5]. However, only a few studies have been carried out in the past on the epitaxial growth of Cd1 xZnxTe layers for a wide range of Zn composition [3–6]. The controlled value of x from 0 to 0.06 has been reported in the past for the metalorganic vapor phase epitaxy (MOVPE) grown crystals, but the controllability and the uniformity at higher values of x have been problematic [5–7]. We have investigated an epitaxial growth of Cd1 xZnxTe layers on the GaAs substrates in an MOVPE system. The epitaxial growth was carried over the entire range of Zn composition from 0 (CdTe) to 1(ZnTe). In this paper, we report on the growth characteristics and the structural property of the Cd1 xZnxTe layers.
2. Experiment The epitaxial growth of Cd1 xZnxTe layers on (1 0 0) GaAs substrates was carried out in an atmospheric pressure vertical cell MOVPE system. The details about this growth system have been described elsewhere [8,9]. The growth was carried out at substrate temperatures of 450 and 560 8C, using dimethylcadmium (DMCd), dimethylzinc (DMZn) and diethyltellurium (DETe) precursors. The wall of the growth cell was heated to 200–300 8C (hot-wall condition). The supply ratio of DMZn, defined as DMZn/(DMCd + DMZn), was varied from 0 to 1, while keeping the total group II precursor (DMCd + DMZn) supply rate constant. Three different ratios of group VI precursor (DETe) to total group II precursor flow rate (VI/II ratio) of 1, 2 and 4 were studied. The growth orientation was controlled to be (1 0 0) direction, and the thickness of the epilayers studied was from 5 to 10 mm. The composition of grown layers was evaluated from the lattice parameter of the (4 0 0) plane using the X-ray diffraction. The crystal quality was evaluated by X-ray double crystal rocking curve (DCRC) measurement on the (4 0 0) plane, and by a low temperature (4.2 K) photoluminescence measurement using an Ar+-ion laser excitation at power density less than 1 W/cm2.
Fig. 1. Variation of Zn composition of the Cd1 xZnxTe layers with DMZn supply ratio at different growth temperatures and different VI/II precursor ratios.
3. Results and discussion Fig. 1 shows the variation of the Zn composition with the DMZn supply ratio at two different substrate temperatures, and VI/II precursor ratios. At a substrate temperature of 450 8C, and a VI/II precursor ratio of 2, the composition of Zn increased slightly from 0 to 0.04 with the increase of DMZn supply ratio from 0 to 0.8, and afterward the Zn composition increased abruptly towards 1.0. At the VI/II ratio of 4, on the other hand, the Zn composition increased gradually to 1.0 with an increase of the DMZn supply ratio from 0.6 to 1.0. In order to control the Zn composition further strictly we increased the growth temperature to 560 8C. This resulted in full control of Zn composition even at higher compositional region as in Fig. 1, where a VI/II ratio of 2 was used. The results in Fig. 1 indicate that Cd incorporation occurs more preferentially at low substrate temperatures and at small VI/II ratios, while the Zn incorporation increases with increasing substrate temperature and the VI/II ratio. The dependence of the Cd1 xZnxTe layers growth rate on the DMZn supply ratio was studied at the substrate of 560 8C, at two different hot-wall temperatures of 200 and 300 8C, as shown in Fig. 2, where the VI/II ratio used was 1. At the hot-wall temperature of 200 8C, the growth rate decreased monotonically with the increase of the DMZn supply ratio from 0 to 1.0, whereas the growth rate remained somewhat constant for the hot-wall temperature of 300 8C. It is considered that formation of ZnTe is small at 200 8C, and the growth rate of Cd1 xZnxTe layers is dominated by the formation of CdTe on the
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Fig. 2. Dependence of Cd1 xZnxTe layers growth rate on DMZn supply ratio.
growth surface, which decreases with the increase of the DMZn supply ratio (DMCd flow rate decreases when DMZn supply ratio is increased)[5]. On the other hand, when the hot-wall temperature is increased to 300 8C, the decreasing tendency of CdTe formation on the growth surface remains unchanged, however, the formation rate of ZnTe increases with the DMZn supply ratio, which balances the decreasing tendency of the growth, and hence the Cd1 xZnxTe layers growth rate remains same [5]. Fig. 3 shows the dependence of DCRC full-width at half-maximum (FWHM) values of the Cd1 xZnxTe (4 0 0) peaks with the Zn-composition. The epilayers were grown at 560 8C, with a VI/II ratio of 2, and the thickness was typically 5 mm. The FWHM values of the Cd1 xZnxTe layers ranged between 260 and 670 arcsec, except at the middle portion of the alloy range,
Fig. 4. Low-temperature (4.2 K) PL spectra of Cd1 xZnxTe layers with different Zn compositions.
where FWHM values increased to about 900 arcsec. Similar results have been reported in the past and it is considered due to the alloy disordering [6,10,11]. The results from the PL measurement of the Cd1 xZnxTe layers with different Zn compositions are shown in Fig. 4. The epilayers were grown at 560 8C, and a VI/II precursor flow ratio of 2. The results show distinct bound-exciton emission, which shifts to the higher energy with the Zn composition, but the deep level emission, which is generally related to crystal defects, was weak. A broadening of the bound-exciton emission was observed for the Zn composition of 0.53 and 0.75. This was considered due to the alloy disordering on the growth surface [10,11]. A distinct donor-acceptor emission (DAP) was also observed for Zn composition of 0.29. The distinct bound-exciton emission and weak deep-level emission on the PL measurement indicate grown layers are of high structural quality.
4. Conclusion
Fig. 3. DCRC FWHM values of Cd1 xZnxTe layers as a function of Zn composition.
Cd1 xZnxTe layers were grown over the entire composition range (x from 0 to 1) by MOVPE on (1 0 0) GaAs substrates. At a substrate temperature of 450 8C and a VI/II precursor flow ratio of 1, the Zn composition control was rather difficult at higher compositional region, however, it could be controlled fully by increasing the growth temperature to 560 8C
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K. Yasuda et al. / Applied Surface Science 244 (2005) 347–350
and/or increasing the VI/II precursor flow ratio more than 2. Epitaxial layers with high crystalline quality were obtained in a wide range of Zn composition. The DCRC FWHM values of the layers were between 260 and 670 arcsec at the end points of the alloy range. The low-temperature PL measurements showed distinct bound-exciton emission, and weak deep-level emission, indicating the grown layers were of high quality.
Acknowledgements This work was supported from the New Energy and Industrial Technology Development Organization (NEDO) Grant (No. 03A47020), and also in parts from the Japan Society for the Promotion of Science through the Grant-in-Aid for Scientific Research (B15300181) and from the Suzuken Memorial Foundation. The authors wish to acknowledge these supports.
Reference [1] G. Brill, Y. Chen, N.K. Dhar, R. Singh, J. Elect. Mater. 32 (2003) 717. [2] W.L. Ahlgren, S.M. Johnson, et al. J. Vac. Sci. Technol. A 7 (1989) 331. [3] D.J. Olego, J.P. Faurie, S. Sivananthan, P.M. Raccah, Appl. Phys. Lett. 47 (1985) 1172. [4] R.D. Feldman, R.F. Austin, A.H. Dayem, H. Westerwick, Appl. Phys. Lett. 49 (1986) 797. [5] K. Yasuda, K. Mori, Y. Kubota, K. Kojima, F. Inukai, Y. Asai, T. Nimura, J. Elect. Mater. 27 (1998) 948. [6] J.L. Reno, E.D. Jones, Phys. Rev. B 45 (1992) 1440. [7] D.W. Kisker, J. Cryst. Growth 98 (1989) 127. [8] K. Yasuda, H. Hatano, M. Minamide, T. Maejima, K. Kawamoto, J. Cryst. Growth 166 (1996) 612. [9] K. Yasuda, Y. Tomita, Y. Masuda, T. Ishiguro, Y. Kawauchi, H. Morishita, Y. Agata, J. Elect. Mater. 31 (2002) 785. [10] Y.P. Chen, G. Brill, N.K. Dhar, J. Cryst. Growth 252 (2003) 270. [11] K. Yasuda, K. Kojima, K. Mori, Y. Kubota, T. Nimura, F. Inukai, Y. Asai, J. Elect. Mater. 27 (1998) 527.
Control of Zn composition (0 < x < 1) in Cd1 xZnxTe epitaxial layers on GaAs substrates grown by MOVPE K. Yasuda*, M. Niraula, H. Kusama, Y. Yamamoto, M. Tominaga, K. Takagi, Y. Aagata Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan Received 31 May 2004; accepted 22 September 2004 Available online 29 December 2004
Abstract Cd1 xZnxTe epitaxial layers over the entire composition range (x from 0 to 1) were grown on (1 0 0) GaAs substrates by atmospheric pressure metalorganic vapor phase epitaxy. A growth temperature of 560 8C, and the group VI/II precursor flow rate ratio of 2 or larger enabled us to control the Zn composition strictly on the grown epilayers. Epitaxial layers with high crystal quality were obtained in a wide range of Zn composition. The double crystal rocking curves (DCRC) full-width at half maximum (FWHM) values were between 260 and 670 arcsec at the end points of alloy range. The low-temperature PL measurements showed distinct bound-exciton emissions and weak deep-level emissions. # 2004 Elsevier B.V. All rights reserved. PACS: 73.61.Ga; 81.05.Dz; 81.15.Kk Keywords: CdZnTe; MOVPE; Optoelectronic devices
1. Introduction Cd1 xZnxTe crystals are considered promising in a variety of optoelectronic device applications like solar cells, light emitting diodes, nuclear radiation detectors because it has a direct bandgap that can be tuned from near infrared (1.5 eV) to visible (2.3 eV) by controlling the Zn composition in the grown crystals. Most of those * Corresponding author. Tel.: +81 52 735 5435; fax: +81 52 735 5578. E-mail address: [email protected] (K. Yasuda).
device applications require a large-area uniform single crystal wafer. Hence, there are increased interests in the growth of Cd1 xZnxTe crystals on GaAs or Si substrates, as these substrates are easily available in a large-area at low cost as well as robust. In addition, Cd1 xZnxTe epitaxial layers (x = 0.04) on GaAs or Si can be used as a lattice matched substrate for the HgCdTe layers growth for the infrared detector application [1,2]. Likewise, Cd1 xZnxTe crystals with x-values from 0 to 0.2 are finding increased applications in nuclear radiation detector fabrication, while x-value of 0.4 is considered suitable for the efficient tandem solar cells. In this
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.09.144
348
K. Yasuda et al. / Applied Surface Science 244 (2005) 347–350
regard, the ability to grow epitaxial Cd1 xZnxTe layers over the entire compositional range (x from 0 to 1) offers the device designer added flexibility, and opens new possibilities of this material in different optoelectronic applications [1–5]. However, only a few studies have been carried out in the past on the epitaxial growth of Cd1 xZnxTe layers for a wide range of Zn composition [3–6]. The controlled value of x from 0 to 0.06 has been reported in the past for the metalorganic vapor phase epitaxy (MOVPE) grown crystals, but the controllability and the uniformity at higher values of x have been problematic [5–7]. We have investigated an epitaxial growth of Cd1 xZnxTe layers on the GaAs substrates in an MOVPE system. The epitaxial growth was carried over the entire range of Zn composition from 0 (CdTe) to 1(ZnTe). In this paper, we report on the growth characteristics and the structural property of the Cd1 xZnxTe layers.
2. Experiment The epitaxial growth of Cd1 xZnxTe layers on (1 0 0) GaAs substrates was carried out in an atmospheric pressure vertical cell MOVPE system. The details about this growth system have been described elsewhere [8,9]. The growth was carried out at substrate temperatures of 450 and 560 8C, using dimethylcadmium (DMCd), dimethylzinc (DMZn) and diethyltellurium (DETe) precursors. The wall of the growth cell was heated to 200–300 8C (hot-wall condition). The supply ratio of DMZn, defined as DMZn/(DMCd + DMZn), was varied from 0 to 1, while keeping the total group II precursor (DMCd + DMZn) supply rate constant. Three different ratios of group VI precursor (DETe) to total group II precursor flow rate (VI/II ratio) of 1, 2 and 4 were studied. The growth orientation was controlled to be (1 0 0) direction, and the thickness of the epilayers studied was from 5 to 10 mm. The composition of grown layers was evaluated from the lattice parameter of the (4 0 0) plane using the X-ray diffraction. The crystal quality was evaluated by X-ray double crystal rocking curve (DCRC) measurement on the (4 0 0) plane, and by a low temperature (4.2 K) photoluminescence measurement using an Ar+-ion laser excitation at power density less than 1 W/cm2.
Fig. 1. Variation of Zn composition of the Cd1 xZnxTe layers with DMZn supply ratio at different growth temperatures and different VI/II precursor ratios.
3. Results and discussion Fig. 1 shows the variation of the Zn composition with the DMZn supply ratio at two different substrate temperatures, and VI/II precursor ratios. At a substrate temperature of 450 8C, and a VI/II precursor ratio of 2, the composition of Zn increased slightly from 0 to 0.04 with the increase of DMZn supply ratio from 0 to 0.8, and afterward the Zn composition increased abruptly towards 1.0. At the VI/II ratio of 4, on the other hand, the Zn composition increased gradually to 1.0 with an increase of the DMZn supply ratio from 0.6 to 1.0. In order to control the Zn composition further strictly we increased the growth temperature to 560 8C. This resulted in full control of Zn composition even at higher compositional region as in Fig. 1, where a VI/II ratio of 2 was used. The results in Fig. 1 indicate that Cd incorporation occurs more preferentially at low substrate temperatures and at small VI/II ratios, while the Zn incorporation increases with increasing substrate temperature and the VI/II ratio. The dependence of the Cd1 xZnxTe layers growth rate on the DMZn supply ratio was studied at the substrate of 560 8C, at two different hot-wall temperatures of 200 and 300 8C, as shown in Fig. 2, where the VI/II ratio used was 1. At the hot-wall temperature of 200 8C, the growth rate decreased monotonically with the increase of the DMZn supply ratio from 0 to 1.0, whereas the growth rate remained somewhat constant for the hot-wall temperature of 300 8C. It is considered that formation of ZnTe is small at 200 8C, and the growth rate of Cd1 xZnxTe layers is dominated by the formation of CdTe on the
K. Yasuda et al. / Applied Surface Science 244 (2005) 347–350
349
Fig. 2. Dependence of Cd1 xZnxTe layers growth rate on DMZn supply ratio.
growth surface, which decreases with the increase of the DMZn supply ratio (DMCd flow rate decreases when DMZn supply ratio is increased)[5]. On the other hand, when the hot-wall temperature is increased to 300 8C, the decreasing tendency of CdTe formation on the growth surface remains unchanged, however, the formation rate of ZnTe increases with the DMZn supply ratio, which balances the decreasing tendency of the growth, and hence the Cd1 xZnxTe layers growth rate remains same [5]. Fig. 3 shows the dependence of DCRC full-width at half-maximum (FWHM) values of the Cd1 xZnxTe (4 0 0) peaks with the Zn-composition. The epilayers were grown at 560 8C, with a VI/II ratio of 2, and the thickness was typically 5 mm. The FWHM values of the Cd1 xZnxTe layers ranged between 260 and 670 arcsec, except at the middle portion of the alloy range,
Fig. 4. Low-temperature (4.2 K) PL spectra of Cd1 xZnxTe layers with different Zn compositions.
where FWHM values increased to about 900 arcsec. Similar results have been reported in the past and it is considered due to the alloy disordering [6,10,11]. The results from the PL measurement of the Cd1 xZnxTe layers with different Zn compositions are shown in Fig. 4. The epilayers were grown at 560 8C, and a VI/II precursor flow ratio of 2. The results show distinct bound-exciton emission, which shifts to the higher energy with the Zn composition, but the deep level emission, which is generally related to crystal defects, was weak. A broadening of the bound-exciton emission was observed for the Zn composition of 0.53 and 0.75. This was considered due to the alloy disordering on the growth surface [10,11]. A distinct donor-acceptor emission (DAP) was also observed for Zn composition of 0.29. The distinct bound-exciton emission and weak deep-level emission on the PL measurement indicate grown layers are of high structural quality.
4. Conclusion
Fig. 3. DCRC FWHM values of Cd1 xZnxTe layers as a function of Zn composition.
Cd1 xZnxTe layers were grown over the entire composition range (x from 0 to 1) by MOVPE on (1 0 0) GaAs substrates. At a substrate temperature of 450 8C and a VI/II precursor flow ratio of 1, the Zn composition control was rather difficult at higher compositional region, however, it could be controlled fully by increasing the growth temperature to 560 8C
350
K. Yasuda et al. / Applied Surface Science 244 (2005) 347–350
and/or increasing the VI/II precursor flow ratio more than 2. Epitaxial layers with high crystalline quality were obtained in a wide range of Zn composition. The DCRC FWHM values of the layers were between 260 and 670 arcsec at the end points of the alloy range. The low-temperature PL measurements showed distinct bound-exciton emission, and weak deep-level emission, indicating the grown layers were of high quality.
Acknowledgements This work was supported from the New Energy and Industrial Technology Development Organization (NEDO) Grant (No. 03A47020), and also in parts from the Japan Society for the Promotion of Science through the Grant-in-Aid for Scientific Research (B15300181) and from the Suzuken Memorial Foundation. The authors wish to acknowledge these supports.
Reference [1] G. Brill, Y. Chen, N.K. Dhar, R. Singh, J. Elect. Mater. 32 (2003) 717. [2] W.L. Ahlgren, S.M. Johnson, et al. J. Vac. Sci. Technol. A 7 (1989) 331. [3] D.J. Olego, J.P. Faurie, S. Sivananthan, P.M. Raccah, Appl. Phys. Lett. 47 (1985) 1172. [4] R.D. Feldman, R.F. Austin, A.H. Dayem, H. Westerwick, Appl. Phys. Lett. 49 (1986) 797. [5] K. Yasuda, K. Mori, Y. Kubota, K. Kojima, F. Inukai, Y. Asai, T. Nimura, J. Elect. Mater. 27 (1998) 948. [6] J.L. Reno, E.D. Jones, Phys. Rev. B 45 (1992) 1440. [7] D.W. Kisker, J. Cryst. Growth 98 (1989) 127. [8] K. Yasuda, H. Hatano, M. Minamide, T. Maejima, K. Kawamoto, J. Cryst. Growth 166 (1996) 612. [9] K. Yasuda, Y. Tomita, Y. Masuda, T. Ishiguro, Y. Kawauchi, H. Morishita, Y. Agata, J. Elect. Mater. 31 (2002) 785. [10] Y.P. Chen, G. Brill, N.K. Dhar, J. Cryst. Growth 252 (2003) 270. [11] K. Yasuda, K. Kojima, K. Mori, Y. Kubota, T. Nimura, F. Inukai, Y. Asai, J. Elect. Mater. 27 (1998) 527.