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Structural properties of zinc-blende MnTe layers grown by hot-wall epitaxy
Applied Surface Science 169±170 (2001) 325±330
Structural properties of zinc-blende MnTe layers grown by hot-wall epitaxy T. Matsumotoa, Y. Sounoa, H. Tatsuokaa,*, Y. Nakanishib, H. Kuwabaraa a
Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan
b
Received 30 July 1999; accepted 5 October 1999
Abstract Zinc-blende(zb) MnTe layers were grown by compositional conversion of ZnTe to MnTe, and structural property of the layers was examined. The growth rate and the thickness of the MnTe layers are much in¯uenced by growth temperature and substrate orientation. The interdiffusion between ZnTe and MnTe shows strong anisotropic reaction rate. The resultant zbMnTe layer, however, is found to be structurally and morphologically homogeneous. # 2001 Elsevier Science B.V. All rights reserved. Keywords: MnTe; ZnTe; MnSb; Epitaxy; Interdiffusion; Conversion
1. Introduction Zinc-blende(zb) MnTe has been previously regarded as a hypothetical material incorporated in semimagnetic semiconductors such as Cd1±xMnxTe, Zn1±xMnxTe, whereas the stable phase of the MnTe is the NiAs structure, and zb-MnTe has not been considered to be present in stable phase. Experimentally, epilayers of the zb-MnTe have been grown by molecular beam epitaxy (MBE) under nonequilibrium condition [1±3]. Recently, the growth and optical property of zb-MnTe by compositional conversion of ZnTe to MnTe, are also reported [4]. The details, however, of the growth mechanism and structural property of the layer have not been understood. In
*
Corresponding author. Tel.: 81-53-478-1099; fax: 81-53-478-1099. E-mail address: [email protected] (H. Tatsuoka).
this paper, the structural property of the zb-MnTe layers is reported and growth mechanism is discussed. 2. Experiments MnTe layers were grown by compositional conversion of ZnTe. The ZnTe layers were grown on GaAs substrates by hot-wall epitaxy (HWE) system, then was exposed to Mn and Sb ¯ux at substrate temperature ranging between 330 and 4508C. The Mn ¯ux used here was about 3 1013 at./cm2 s, and the Sb/Mn ¯ux ratio supplied to ZnTe surface was 10. The effects of growth temperature and growth orientation for zbMnTe growth were examined. The structural properties of the resultant layers were characterized using X-ray diffraction measurement (XRD), conventional transmission electron microscopy (TEM). Compositional analysis of the layers was performed using Auger electron spectroscopy
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 6 6 5 - 6
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T. Matsumoto et al. / Applied Surface Science 169±170 (2001) 325±330
(AES). Optical re¯ectance spectra were also measured to estimate the optical band gap. 3. Results and discussion Fig. 1(a)±(c) show the substrate temperature dependence of XRD spectra for the resultant layers grown on GaAs(0 01), (1 1 1)A and (1 1 1)B substrates, respectively. Fig. 1(a) indicates that, at a low growth temperature, Mn atoms are diffused into ZnTe layer to form Zn1-xMnxTe on GaAs(0 0 1). And at high growth temperature, the layer is completely converted to zbMnTe which will be con®rmed in the following sections. Hence, the high growth temperature is essential to promote the formation of zb-MnTe. Furthermore,
Fig. 1. Growth temperature dependence of XRD spectra for the resultant layers grown on (a) GaAs(0 0 1), (b) GaAs(1 1 1)A and (c) GaAs(1 1 1)B.
for the case of GaAs(1 1 1)A is similar to that for GaAs(0 0 1). On the other hand, two distinct diffraction peaks corresponding to ZnTe and zb-MnTe on GaAs(1 1 1)B substrate are observed. This result shows the coexistence of both zb-MnTe and ZnTe layers for the case of GaAs(1 1 1)B substrates. Fig. 2 shows cross-sectional TEM images and selected area diffraction pattern for the layers grown at the growth temperatures of 3908C with GaAs(0 0 1) substrate. It is observed that zb-MnTe layer is formed at ZnTe surface. The interface between ZnTe and zbMnTe is not abrupt which means that the composition of the layer is gradually changed from ZnTe to zbMnTe along the growth direction. This result agrees with XRD measurement (Fig. 1(a)). It should be noted that the interface between ZnTe and zb-MnTe tends to lie on {1 1 1} planes, and it is considered that the interdiffusion is favored on {1 1 1} planes. The defect structure of the layer is also shown in Fig. 2(b). It is observed that stacking faults lying on {1 1 1} planes in the ZnTe layer are diminished in the inter-diffused zbMnTe layer. The elimination of stacking faults in inter-diffused layer was also reported previously for the case of MnHgTe layers grown by inter-diffused multilayer process (IMP) [5]. It is considered that rearrangement of atomic con®guration by interdiffusion eliminates the grown-in defects in the layers. Fig. 3(a) and (b) show cross-sectional TEM images for the layers grown at the growth temperature of 4508C on GaAs(0 0 1) and GaAs(1 1 1)B substrates, respectively. As shown in Fig. 3(a), the layer is completely converted to zb-MnTe, and the selected area diffraction pattern con®rms that the crystalline structure of the layer is zinc-blende, for the case of GaAs(0 0 1) substrate. On the other hand, MnSb islands are deposited on the zb-MnTe layers on ZnTe/GaAs(1 1 1)B substrate. It is also observed that high density of stacking faults and twin domains in the ZnTe layer are eliminated in the zb-MnTe region. It is also found that the reaction rate depends on the substrate orientation. Assuming the diffusion controlled reaction, the interdiffuion coef®cients are derived as 6 10ÿ15 and 3 10ÿ16 cm2/s for the layers grown at 3908C on GaAs(0 0 1) and 4508C on GaAs(1 1 1)B, respectively. This anisotropic interdiffusion suggests that the compositional conversion takes place by replacement of Mn atoms to Zn sites. Because the surface is covered with Te atoms on
T. Matsumoto et al. / Applied Surface Science 169±170 (2001) 325±330
327
Fig. 2. (a) Cross-sectional TEM image for the layer grown under the substrate temperature of 3908C and (b) enlarged defect structure of the layer. The layer was grown on GaAs(0 0 1) and the growth time is 1 h.
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T. Matsumoto et al. / Applied Surface Science 169±170 (2001) 325±330
Fig. 3. Cross-sectional TEM images for the layers grown at the substrate temperature of 4508C, on (a) GaAs(0 0 1) and (b) GaAs(1 1 1)B substrates, respectively. The growth time is 1 h for both the cases.
T. Matsumoto et al. / Applied Surface Science 169±170 (2001) 325±330
ZnTe(1 1 1)B planes where it is dif®cult to exchange Mn atoms into Zn sites. The anisotropic diffusion was also reported for In-Ga-Sb ternary crystal growth [6]. At the optimum condition, zb-MnTe layers can be grown without any MnSb deposition, and no evidence for the existence of Zn and Sb-related compounds is observed for the layer grown on GaAs(0 0 1). It is considered that Mn atoms temporarily bind with Sb atoms to form MnSb, excess Sb and Zn atoms, however, are re-evaporated from the layer surface during the growth, and the formation of zb-MnTe is thermodynamically favored as compared with ZnTe and MnSb formation. During the interfacial reaction, the role of Sb atoms has not been understood. The formation mechanism of the resultant layer is in¯uenced by an interfacial energy and a heat of formation of the compounds. It is considered that the interdiffusion of Mn and Zn atoms effectively takes plase between MnSb-ZnTe diffusion couple with keeping the stoichiometric composition of the initial and resultant layers. In addition, it should be noted that Sb is known to be a surfactant material to obtain high-quality layers with smooth interface. It might be possible that Sb reacts with ZnTe layers as a surfactant for the zbMnTe growth in this study. Fig. 4 shows the chemical compositional pro®le of the resultant layer grown under optimum growth condition. The spectrum shows that the layer is mainly composed of Mn and Te atoms, and Zn and Sb are incorporated as impurity level. Thus, it is con®rmed that the layer is MnTe. Fig. 5 shows optical re¯ectance spectra for the ZnTe and resultant zb-MnTe layers measured at room temperature. Interference fringes in the spectra are due to multiple re¯ections in the grown layers, and the clear
329
Fig. 5. Optical re¯ectance spectra for the ZnTe layer and the resultant zb-MnTe layer.
fringes show that the resultant layer is morphologically homogeneous. In the ®gure fundamental optical absorption begins at photon energy interference fringes disappear (ZnTe: 2.4 eV, zb-MnTe: 3.2 eV). Each of arrows in the ®gure corresponds to respective optical band gap energy. The growth of GaAsP, InGaAs and MnSi using compositional conversion has been reported, and the growth mechanism has been investigated [7±9]. It is found in this study that this growth procedure can be applied for the growth of new types of materials which do not exist in nature such as zb-MnTe. 4. Conclusion The zb-MnTe layers were grown by compositional conversion of ZnTe to MnTe, and structural property of the layers was examined. The growth of zb-MnTe is con®rmed by XRD and TEM. The growth rate and the thickness of the MnTe layers are much in¯uenced by growth temperature and substrate orientation. It is also found that the interdiffusion between ZnTe and MnTe shows strong anisotropic reaction rate. The reaction rate along ZnTeh1 1 1i is much slower than that along ZnTeh0 0 1i. The resultant zb-MnTe layers are found to be structurally and morphologically homogeneous. References
Fig. 4. The AES spectra for the resultant zb-MnTe layer grown at 4508C for 1 h on GaAs(0 0 1) substrate.
[1] S.M. Durbin, J. Han, O. Sungki, M. Kobayashi, D.R. Menke, R.L. Gunshor, Q. Fu, N. Pelekanos, A.V. Nurmikko, D. Li, J. Gonsalves, N. Otsuka, Appl. Phys. Lett. 55 (1989) 2087. [2] P. Klosowski, T.M. Giebultowicz, J.J. Rhyne, N. Samarth, H. Luo, J.K. Furdyna, J. Appl. Phys. 70 (1991) 6221.
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T. Matsumoto et al. / Applied Surface Science 169±170 (2001) 325±330
[3] J.K. Furdyna, J. Appl. Phys. 64 (1998) R29. [4] T. Matsumoto, K. Isaji, H. Tatsuoka, Y. Nakanishi, H. Kuwabara, in: Proceedings of the 4th Symposium on the Physics and Application of Spin-Related Phenomena in Semiconductors, Sendai, 1998, p. 141. [5] H. Tatsuoka, K. Durose, M. Funaki, in: A.G. Cullis, A.E. Station-Bevan, J.I. Hutchison (Eds.), Proceedings of the Institute of Physics Conference on Microscopy of Semiconducting Materials, Series 134, Institute of Physics, Bristol, 1993, p. 421.
[6] M. Kumagawa, H. Ohtsu, E. Hamakawa, T. Koyama, M. Masaki, K. Takahashi, V.G. Lifshits, Y. Hayakawa, Mater. Sci. Eng. B44 (1997) 301. [7] M. Kimura, Z. Qin, H. Udono, S. Dost, A. Tanaka, T. Sukegawa, Mater. Cryst. Eng. B44 (1997) 16. [8] Y. Hayakawa, E. Hamakawa, T. Koyama, M. Kumagawa, J. Cryst. Growth 163 (1996) 220. [9] K. Matsuda, H. Tatsuoka, K. Matsunaga, K. Isaji, H. Kuwabara, P.D. Brown, Y. Xin, R.D. Borkowski, C.J. Humphreys, Jpn. J. Appl. Phys. 37 (1998) 6556.
Structural properties of zinc-blende MnTe layers grown by hot-wall epitaxy T. Matsumotoa, Y. Sounoa, H. Tatsuokaa,*, Y. Nakanishib, H. Kuwabaraa a
Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan
b
Received 30 July 1999; accepted 5 October 1999
Abstract Zinc-blende(zb) MnTe layers were grown by compositional conversion of ZnTe to MnTe, and structural property of the layers was examined. The growth rate and the thickness of the MnTe layers are much in¯uenced by growth temperature and substrate orientation. The interdiffusion between ZnTe and MnTe shows strong anisotropic reaction rate. The resultant zbMnTe layer, however, is found to be structurally and morphologically homogeneous. # 2001 Elsevier Science B.V. All rights reserved. Keywords: MnTe; ZnTe; MnSb; Epitaxy; Interdiffusion; Conversion
1. Introduction Zinc-blende(zb) MnTe has been previously regarded as a hypothetical material incorporated in semimagnetic semiconductors such as Cd1±xMnxTe, Zn1±xMnxTe, whereas the stable phase of the MnTe is the NiAs structure, and zb-MnTe has not been considered to be present in stable phase. Experimentally, epilayers of the zb-MnTe have been grown by molecular beam epitaxy (MBE) under nonequilibrium condition [1±3]. Recently, the growth and optical property of zb-MnTe by compositional conversion of ZnTe to MnTe, are also reported [4]. The details, however, of the growth mechanism and structural property of the layer have not been understood. In
*
Corresponding author. Tel.: 81-53-478-1099; fax: 81-53-478-1099. E-mail address: [email protected] (H. Tatsuoka).
this paper, the structural property of the zb-MnTe layers is reported and growth mechanism is discussed. 2. Experiments MnTe layers were grown by compositional conversion of ZnTe. The ZnTe layers were grown on GaAs substrates by hot-wall epitaxy (HWE) system, then was exposed to Mn and Sb ¯ux at substrate temperature ranging between 330 and 4508C. The Mn ¯ux used here was about 3 1013 at./cm2 s, and the Sb/Mn ¯ux ratio supplied to ZnTe surface was 10. The effects of growth temperature and growth orientation for zbMnTe growth were examined. The structural properties of the resultant layers were characterized using X-ray diffraction measurement (XRD), conventional transmission electron microscopy (TEM). Compositional analysis of the layers was performed using Auger electron spectroscopy
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 6 6 5 - 6
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T. Matsumoto et al. / Applied Surface Science 169±170 (2001) 325±330
(AES). Optical re¯ectance spectra were also measured to estimate the optical band gap. 3. Results and discussion Fig. 1(a)±(c) show the substrate temperature dependence of XRD spectra for the resultant layers grown on GaAs(0 01), (1 1 1)A and (1 1 1)B substrates, respectively. Fig. 1(a) indicates that, at a low growth temperature, Mn atoms are diffused into ZnTe layer to form Zn1-xMnxTe on GaAs(0 0 1). And at high growth temperature, the layer is completely converted to zbMnTe which will be con®rmed in the following sections. Hence, the high growth temperature is essential to promote the formation of zb-MnTe. Furthermore,
Fig. 1. Growth temperature dependence of XRD spectra for the resultant layers grown on (a) GaAs(0 0 1), (b) GaAs(1 1 1)A and (c) GaAs(1 1 1)B.
for the case of GaAs(1 1 1)A is similar to that for GaAs(0 0 1). On the other hand, two distinct diffraction peaks corresponding to ZnTe and zb-MnTe on GaAs(1 1 1)B substrate are observed. This result shows the coexistence of both zb-MnTe and ZnTe layers for the case of GaAs(1 1 1)B substrates. Fig. 2 shows cross-sectional TEM images and selected area diffraction pattern for the layers grown at the growth temperatures of 3908C with GaAs(0 0 1) substrate. It is observed that zb-MnTe layer is formed at ZnTe surface. The interface between ZnTe and zbMnTe is not abrupt which means that the composition of the layer is gradually changed from ZnTe to zbMnTe along the growth direction. This result agrees with XRD measurement (Fig. 1(a)). It should be noted that the interface between ZnTe and zb-MnTe tends to lie on {1 1 1} planes, and it is considered that the interdiffusion is favored on {1 1 1} planes. The defect structure of the layer is also shown in Fig. 2(b). It is observed that stacking faults lying on {1 1 1} planes in the ZnTe layer are diminished in the inter-diffused zbMnTe layer. The elimination of stacking faults in inter-diffused layer was also reported previously for the case of MnHgTe layers grown by inter-diffused multilayer process (IMP) [5]. It is considered that rearrangement of atomic con®guration by interdiffusion eliminates the grown-in defects in the layers. Fig. 3(a) and (b) show cross-sectional TEM images for the layers grown at the growth temperature of 4508C on GaAs(0 0 1) and GaAs(1 1 1)B substrates, respectively. As shown in Fig. 3(a), the layer is completely converted to zb-MnTe, and the selected area diffraction pattern con®rms that the crystalline structure of the layer is zinc-blende, for the case of GaAs(0 0 1) substrate. On the other hand, MnSb islands are deposited on the zb-MnTe layers on ZnTe/GaAs(1 1 1)B substrate. It is also observed that high density of stacking faults and twin domains in the ZnTe layer are eliminated in the zb-MnTe region. It is also found that the reaction rate depends on the substrate orientation. Assuming the diffusion controlled reaction, the interdiffuion coef®cients are derived as 6 10ÿ15 and 3 10ÿ16 cm2/s for the layers grown at 3908C on GaAs(0 0 1) and 4508C on GaAs(1 1 1)B, respectively. This anisotropic interdiffusion suggests that the compositional conversion takes place by replacement of Mn atoms to Zn sites. Because the surface is covered with Te atoms on
T. Matsumoto et al. / Applied Surface Science 169±170 (2001) 325±330
327
Fig. 2. (a) Cross-sectional TEM image for the layer grown under the substrate temperature of 3908C and (b) enlarged defect structure of the layer. The layer was grown on GaAs(0 0 1) and the growth time is 1 h.
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T. Matsumoto et al. / Applied Surface Science 169±170 (2001) 325±330
Fig. 3. Cross-sectional TEM images for the layers grown at the substrate temperature of 4508C, on (a) GaAs(0 0 1) and (b) GaAs(1 1 1)B substrates, respectively. The growth time is 1 h for both the cases.
T. Matsumoto et al. / Applied Surface Science 169±170 (2001) 325±330
ZnTe(1 1 1)B planes where it is dif®cult to exchange Mn atoms into Zn sites. The anisotropic diffusion was also reported for In-Ga-Sb ternary crystal growth [6]. At the optimum condition, zb-MnTe layers can be grown without any MnSb deposition, and no evidence for the existence of Zn and Sb-related compounds is observed for the layer grown on GaAs(0 0 1). It is considered that Mn atoms temporarily bind with Sb atoms to form MnSb, excess Sb and Zn atoms, however, are re-evaporated from the layer surface during the growth, and the formation of zb-MnTe is thermodynamically favored as compared with ZnTe and MnSb formation. During the interfacial reaction, the role of Sb atoms has not been understood. The formation mechanism of the resultant layer is in¯uenced by an interfacial energy and a heat of formation of the compounds. It is considered that the interdiffusion of Mn and Zn atoms effectively takes plase between MnSb-ZnTe diffusion couple with keeping the stoichiometric composition of the initial and resultant layers. In addition, it should be noted that Sb is known to be a surfactant material to obtain high-quality layers with smooth interface. It might be possible that Sb reacts with ZnTe layers as a surfactant for the zbMnTe growth in this study. Fig. 4 shows the chemical compositional pro®le of the resultant layer grown under optimum growth condition. The spectrum shows that the layer is mainly composed of Mn and Te atoms, and Zn and Sb are incorporated as impurity level. Thus, it is con®rmed that the layer is MnTe. Fig. 5 shows optical re¯ectance spectra for the ZnTe and resultant zb-MnTe layers measured at room temperature. Interference fringes in the spectra are due to multiple re¯ections in the grown layers, and the clear
329
Fig. 5. Optical re¯ectance spectra for the ZnTe layer and the resultant zb-MnTe layer.
fringes show that the resultant layer is morphologically homogeneous. In the ®gure fundamental optical absorption begins at photon energy interference fringes disappear (ZnTe: 2.4 eV, zb-MnTe: 3.2 eV). Each of arrows in the ®gure corresponds to respective optical band gap energy. The growth of GaAsP, InGaAs and MnSi using compositional conversion has been reported, and the growth mechanism has been investigated [7±9]. It is found in this study that this growth procedure can be applied for the growth of new types of materials which do not exist in nature such as zb-MnTe. 4. Conclusion The zb-MnTe layers were grown by compositional conversion of ZnTe to MnTe, and structural property of the layers was examined. The growth of zb-MnTe is con®rmed by XRD and TEM. The growth rate and the thickness of the MnTe layers are much in¯uenced by growth temperature and substrate orientation. It is also found that the interdiffusion between ZnTe and MnTe shows strong anisotropic reaction rate. The reaction rate along ZnTeh1 1 1i is much slower than that along ZnTeh0 0 1i. The resultant zb-MnTe layers are found to be structurally and morphologically homogeneous. References
Fig. 4. The AES spectra for the resultant zb-MnTe layer grown at 4508C for 1 h on GaAs(0 0 1) substrate.
[1] S.M. Durbin, J. Han, O. Sungki, M. Kobayashi, D.R. Menke, R.L. Gunshor, Q. Fu, N. Pelekanos, A.V. Nurmikko, D. Li, J. Gonsalves, N. Otsuka, Appl. Phys. Lett. 55 (1989) 2087. [2] P. Klosowski, T.M. Giebultowicz, J.J. Rhyne, N. Samarth, H. Luo, J.K. Furdyna, J. Appl. Phys. 70 (1991) 6221.
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T. Matsumoto et al. / Applied Surface Science 169±170 (2001) 325±330
[3] J.K. Furdyna, J. Appl. Phys. 64 (1998) R29. [4] T. Matsumoto, K. Isaji, H. Tatsuoka, Y. Nakanishi, H. Kuwabara, in: Proceedings of the 4th Symposium on the Physics and Application of Spin-Related Phenomena in Semiconductors, Sendai, 1998, p. 141. [5] H. Tatsuoka, K. Durose, M. Funaki, in: A.G. Cullis, A.E. Station-Bevan, J.I. Hutchison (Eds.), Proceedings of the Institute of Physics Conference on Microscopy of Semiconducting Materials, Series 134, Institute of Physics, Bristol, 1993, p. 421.
[6] M. Kumagawa, H. Ohtsu, E. Hamakawa, T. Koyama, M. Masaki, K. Takahashi, V.G. Lifshits, Y. Hayakawa, Mater. Sci. Eng. B44 (1997) 301. [7] M. Kimura, Z. Qin, H. Udono, S. Dost, A. Tanaka, T. Sukegawa, Mater. Cryst. Eng. B44 (1997) 16. [8] Y. Hayakawa, E. Hamakawa, T. Koyama, M. Kumagawa, J. Cryst. Growth 163 (1996) 220. [9] K. Matsuda, H. Tatsuoka, K. Matsunaga, K. Isaji, H. Kuwabara, P.D. Brown, Y. Xin, R.D. Borkowski, C.J. Humphreys, Jpn. J. Appl. Phys. 37 (1998) 6556.