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Identification of the crystallographic polarity of the {111} CdTe surfaces
Materials Science and Engineering, B18 ( 1993) 91-93
91
Identification of the crystallographic polarity of the {111 } CdTe surfaces S. C. Gupta, M. G a u t a m a n d A. K. S r e e d h a r Solid State Physics Laboratory, L u c k n o w Road, Delhi 110054 (India) (Received April 12, 1992)
Abstract A simple, non-destructive chemical method has been described for the determination of the crystallographic polarity of the {11 l} CdTe surfaces. The chemicals used are dilute HCI and zinc. The hydrogen gas evolved during the reaction preferentially gathers only on the Cd surface (as per Fewster's convention) of (111) CdTe, and can be used to identify the crystallographic polarity of (111) CdTe.
1. Introduction Cadmium telluride (CdTe) has zinc blende crystal structure with cubic non-centro-symmetric space group [1] F 4 3 m (Td~). Its (111) crystallographic axis is a polar axis. One of its {111} surfaces terminates with Cd atoms, called {111] A surface, while the other surface terminates with Te atoms, called {111} B surface. CdTe is used as a substrate for the growth of epilayers of mercury cadmium telluride (HgCdTe) by vapour phase epitaxy (VPE), liquid phase epitaxy (LPE), molecular beam epitaxy (MBE), cathodic sputtering, metal-organic chemical vapour deposition (MOCVD) etc., to be used for infrared devices in 1-3/~m, 3-5 # m and 8 - 1 4 / ~ m wavelength regions [2]. This is because the crystal structure of CdTe matches with that of HgCdTe. The lattice constant of CdTe fairly matches with that of HgCdTe and CdTe is transparent to infrared radiation of interest (viz., 1-14 k~m) [1-3]. The growth of an epilayer of HgCdTe on CdTe by LPE, MBE and cathodic sputtering techniques is very much dependent on whether it is grown on a Cd {111} surface or Te {111 } surface [2]. It is, therefore, essential to know the polarity of CdTe. Various methods have been used to determine and to distinguish the Cd and Te {111} surfaces of CdTe, such as X-ray diffraction [4, 5], transmission electron microscopy [6], reflection high energy electron diffraction [7], Auger electron spectroscopy [8] and chemical methods [9-13]. However, the results obtained by these methods are inconsistent. This inconsistency arises owing to the division of authors between the conventions of Warekois et al. [4] and Fewster and Whiffin [5]. These conventions are opposite to each other. Recently Brown et al. [14] have justified the con0921-5107/93/$6.00
vention of Fewster and Whiffin according to which the (111) A face is identified with the Cd ending surface, and the (111) B face is the Te ending surface on the basis of their work on electron microdiffraction. They have pointed out that the etchants of the Inoue et al. [10] and Nakagawa et al. [11] are not very reliable. According to them, the reliable discriminatory etchants called black-white etchants consisting of a 1 : 1 : 1 mixture of HF, HNO3 and either acetic or lactic acid are the most reliable in identifying the A or B faces of CdTe. Both these etchants leave the A face matt black and the B face bright. II-VI Incorporated USA, who supplies this material, used to follow the Warekois convention earlier. They have now adopted Fewster's convention. They identify the A and B faces using the Nakagawa etchant. In this paper, we have reported on a very simple discriminatory non-destructive chemical method which can also be used to identify the (111) polar faces of CdTe. This method is based upon the differences in chemical properties of A and B surfaces [ 15, 16].
2. Experimental details and results Single crystal (111) oriented wafers, of dimensions 10 mm × 10 mm x 1 mm, used in the experiment were obtained from II-VI Incorporated, USA. These wafers were lapped and chemo-mechanically polished with 0.2% Br-ethylene glycol, rinsed in IN KOH/methanol solution for a few minutes to remove the residual Te film [17], cleaned with methanol, washed in deionized water after each step and finally dried in nitrogen gas flow. The wafer was then suspended and dipped in a beaker containing powdered zinc (Zn) and dilute © 1993 - Elsevier Sequoia. All rights reserved
92
S. C. Gupta et al.
/
Identifying crystallographic polarity of CdTe surfaces
hydrochloric acid (diluted to 4% or below with deionized water, so that the reaction is slow and controlled). T h e hydrogen gas that is evolved during the chemical reaction between HC1 and Z n was observed to be attracted to and collected together on one of the surfaces of CdTe (Fig. 1). This surface, on which bubbles of hydrogen were deposited, appeared dull when viewed through the solution. T h e other surface
Fig. 1. Photograph of one of the surfaces (see text) of (111) CdTe to which hydrogen gas is attracted.
appears bright (Fig. 2). T h e surface on which hydrogen gas attracted was marked. T h e wafer was then rotated in situ a number of times and it was found that every time the same surface was covered with hydrogen gas bubbles. Next the sample was subjected to the black-white etchants mentioned earlier. It was observed that both the etchants leave the surface on which hydrogen gas settled/gathered matt black. T h e opposite surface, which did not exhibit a collection of hydrogen gas bubbles, did not stain with the black-white etchants. It is thus established that Cd end surface of (111) cut CdTe (as per Fewster's convention) is the surface on which hydrogen gas, evolved in the reaction of Z n with HCI, preferentially settles. T h e accumulation of hydrogen bubbles on one face of (111) cut CdTe can be explained on the basis of the chemical reactivity of atomic hydrogen evolved in the experiment. Differences in the chemical reactivity between {111} and {111} surfaces has also been observed in group III-V compounds in experiments of etching and on adsorption of oxygen and carbon dioxide gases [15, 16, 18]. T h e mechanism which could explain these differences are not sufficiently known [18] as the detailed mechanism involves knowledge of the nature of the dangling bonds and of the hybrid orbitals at the surfaces. However, accepting Fewster's convention in identifying the Cd and Te faces in CdTe, it is found that hydrogen bubbles accumulate on the Cd surface. T h e experiment described above gives an unambiguous and non-destructive method to discriminate between the {111 }A and B surfaces. References
Fig. 2. Photograph of the second surface (see text) of (111) CdTe to which hydrogen gas is not attracted.
1 K. Zanio, in R. K. Willardson and A. C. Beer (eds.), Cadmium Telluride in Semiconductors and Semi-metals, Vol. 13, Academic Press, New York, 1978, p. 53. 2 M.A. Herman and M. Pessa, J. Appl. Phys., 57(1985) 2671. 3 R. Dornhaus and G. Nimtz, in G. H6hler (ed.), Narrow gap semiconductors, Vol. 98, in Springer Tracts in Modern Physics, Springer Verlag, Berlin, 1983, p. 119. 4 E. P. Warekois, M. C. Lavine, A. N. Mariano and H. C. Gotos, J. Appl. Phys., 33(1962) 690. 5 P. F. Fewster and P. A. C. Whiffin, J. Appl. Phys., 54(1983) 4668. 6 G. Lu and D. J. H. Cockayne, Philos. Mag., A53 (1986) 307. 7 C, Hsu, S. Sivanathan, X. Chu and J. P. Faurie, Appl. Phys. Lett., 48(1986) 908. 8 Y. C. Lu, C. M. Stahle, J. Morimots, R. H. Bube and R. S. Feigelson, J. Appl. Phys., 61 (1987) 924. 9 T. H. Myers, J. E Schetzina, T. J. Magee and R. D. Orenonel, J. Vac. Sci. Technol., AI (1983) 1593. 10 M. Inoue, I. Teramota and S. Takayanaji, J. Appl. Phys., 33 (1962) 2579. 11 K. Nakagawa, K. Maeda and G. Takeuchi, Appl. Phys. Lett., 34(1979) 574.
S. C. Gupta et al.
/
Identifying crystallographic polarity of CdTe surfaces
12 Y. C. Lu, R. K. Route, D. Elwell and R. S. Feigelson, J. Vac. Sci. Technol., A3(1985) 269. 13 S. C. Gupta, F. R. Chavada, S. K. Koul and M. Gautam, J. Electrochem. Soc., 135 (1988) 2894. 14 R. D. Brown, K. Durose, G. J. Russell and J. Woods, J. Cryst. Growth, 101 (1990) 211. 15 H. C. Gatos and M. C. Lavine, J. Electrochem. Soc., 107
93
(1960) 427. 16 D. Hanemen, Phys. Rev., 121 (1962) 1093. 17 P. M. Amritraj and E Ht. Pollak, Appl. Phys. Lett., 45 (1984) 789. 18 D. Haneman, in R. K. Willardson and H. L. Goering (eds.), Compound Semiconductors, Vol. 1, Reinhold Publishing Corp., New York, 1962, p. 423.
91
Identification of the crystallographic polarity of the {111 } CdTe surfaces S. C. Gupta, M. G a u t a m a n d A. K. S r e e d h a r Solid State Physics Laboratory, L u c k n o w Road, Delhi 110054 (India) (Received April 12, 1992)
Abstract A simple, non-destructive chemical method has been described for the determination of the crystallographic polarity of the {11 l} CdTe surfaces. The chemicals used are dilute HCI and zinc. The hydrogen gas evolved during the reaction preferentially gathers only on the Cd surface (as per Fewster's convention) of (111) CdTe, and can be used to identify the crystallographic polarity of (111) CdTe.
1. Introduction Cadmium telluride (CdTe) has zinc blende crystal structure with cubic non-centro-symmetric space group [1] F 4 3 m (Td~). Its (111) crystallographic axis is a polar axis. One of its {111} surfaces terminates with Cd atoms, called {111] A surface, while the other surface terminates with Te atoms, called {111} B surface. CdTe is used as a substrate for the growth of epilayers of mercury cadmium telluride (HgCdTe) by vapour phase epitaxy (VPE), liquid phase epitaxy (LPE), molecular beam epitaxy (MBE), cathodic sputtering, metal-organic chemical vapour deposition (MOCVD) etc., to be used for infrared devices in 1-3/~m, 3-5 # m and 8 - 1 4 / ~ m wavelength regions [2]. This is because the crystal structure of CdTe matches with that of HgCdTe. The lattice constant of CdTe fairly matches with that of HgCdTe and CdTe is transparent to infrared radiation of interest (viz., 1-14 k~m) [1-3]. The growth of an epilayer of HgCdTe on CdTe by LPE, MBE and cathodic sputtering techniques is very much dependent on whether it is grown on a Cd {111} surface or Te {111 } surface [2]. It is, therefore, essential to know the polarity of CdTe. Various methods have been used to determine and to distinguish the Cd and Te {111} surfaces of CdTe, such as X-ray diffraction [4, 5], transmission electron microscopy [6], reflection high energy electron diffraction [7], Auger electron spectroscopy [8] and chemical methods [9-13]. However, the results obtained by these methods are inconsistent. This inconsistency arises owing to the division of authors between the conventions of Warekois et al. [4] and Fewster and Whiffin [5]. These conventions are opposite to each other. Recently Brown et al. [14] have justified the con0921-5107/93/$6.00
vention of Fewster and Whiffin according to which the (111) A face is identified with the Cd ending surface, and the (111) B face is the Te ending surface on the basis of their work on electron microdiffraction. They have pointed out that the etchants of the Inoue et al. [10] and Nakagawa et al. [11] are not very reliable. According to them, the reliable discriminatory etchants called black-white etchants consisting of a 1 : 1 : 1 mixture of HF, HNO3 and either acetic or lactic acid are the most reliable in identifying the A or B faces of CdTe. Both these etchants leave the A face matt black and the B face bright. II-VI Incorporated USA, who supplies this material, used to follow the Warekois convention earlier. They have now adopted Fewster's convention. They identify the A and B faces using the Nakagawa etchant. In this paper, we have reported on a very simple discriminatory non-destructive chemical method which can also be used to identify the (111) polar faces of CdTe. This method is based upon the differences in chemical properties of A and B surfaces [ 15, 16].
2. Experimental details and results Single crystal (111) oriented wafers, of dimensions 10 mm × 10 mm x 1 mm, used in the experiment were obtained from II-VI Incorporated, USA. These wafers were lapped and chemo-mechanically polished with 0.2% Br-ethylene glycol, rinsed in IN KOH/methanol solution for a few minutes to remove the residual Te film [17], cleaned with methanol, washed in deionized water after each step and finally dried in nitrogen gas flow. The wafer was then suspended and dipped in a beaker containing powdered zinc (Zn) and dilute © 1993 - Elsevier Sequoia. All rights reserved
92
S. C. Gupta et al.
/
Identifying crystallographic polarity of CdTe surfaces
hydrochloric acid (diluted to 4% or below with deionized water, so that the reaction is slow and controlled). T h e hydrogen gas that is evolved during the chemical reaction between HC1 and Z n was observed to be attracted to and collected together on one of the surfaces of CdTe (Fig. 1). This surface, on which bubbles of hydrogen were deposited, appeared dull when viewed through the solution. T h e other surface
Fig. 1. Photograph of one of the surfaces (see text) of (111) CdTe to which hydrogen gas is attracted.
appears bright (Fig. 2). T h e surface on which hydrogen gas attracted was marked. T h e wafer was then rotated in situ a number of times and it was found that every time the same surface was covered with hydrogen gas bubbles. Next the sample was subjected to the black-white etchants mentioned earlier. It was observed that both the etchants leave the surface on which hydrogen gas settled/gathered matt black. T h e opposite surface, which did not exhibit a collection of hydrogen gas bubbles, did not stain with the black-white etchants. It is thus established that Cd end surface of (111) cut CdTe (as per Fewster's convention) is the surface on which hydrogen gas, evolved in the reaction of Z n with HCI, preferentially settles. T h e accumulation of hydrogen bubbles on one face of (111) cut CdTe can be explained on the basis of the chemical reactivity of atomic hydrogen evolved in the experiment. Differences in the chemical reactivity between {111} and {111} surfaces has also been observed in group III-V compounds in experiments of etching and on adsorption of oxygen and carbon dioxide gases [15, 16, 18]. T h e mechanism which could explain these differences are not sufficiently known [18] as the detailed mechanism involves knowledge of the nature of the dangling bonds and of the hybrid orbitals at the surfaces. However, accepting Fewster's convention in identifying the Cd and Te faces in CdTe, it is found that hydrogen bubbles accumulate on the Cd surface. T h e experiment described above gives an unambiguous and non-destructive method to discriminate between the {111 }A and B surfaces. References
Fig. 2. Photograph of the second surface (see text) of (111) CdTe to which hydrogen gas is not attracted.
1 K. Zanio, in R. K. Willardson and A. C. Beer (eds.), Cadmium Telluride in Semiconductors and Semi-metals, Vol. 13, Academic Press, New York, 1978, p. 53. 2 M.A. Herman and M. Pessa, J. Appl. Phys., 57(1985) 2671. 3 R. Dornhaus and G. Nimtz, in G. H6hler (ed.), Narrow gap semiconductors, Vol. 98, in Springer Tracts in Modern Physics, Springer Verlag, Berlin, 1983, p. 119. 4 E. P. Warekois, M. C. Lavine, A. N. Mariano and H. C. Gotos, J. Appl. Phys., 33(1962) 690. 5 P. F. Fewster and P. A. C. Whiffin, J. Appl. Phys., 54(1983) 4668. 6 G. Lu and D. J. H. Cockayne, Philos. Mag., A53 (1986) 307. 7 C, Hsu, S. Sivanathan, X. Chu and J. P. Faurie, Appl. Phys. Lett., 48(1986) 908. 8 Y. C. Lu, C. M. Stahle, J. Morimots, R. H. Bube and R. S. Feigelson, J. Appl. Phys., 61 (1987) 924. 9 T. H. Myers, J. E Schetzina, T. J. Magee and R. D. Orenonel, J. Vac. Sci. Technol., AI (1983) 1593. 10 M. Inoue, I. Teramota and S. Takayanaji, J. Appl. Phys., 33 (1962) 2579. 11 K. Nakagawa, K. Maeda and G. Takeuchi, Appl. Phys. Lett., 34(1979) 574.
S. C. Gupta et al.
/
Identifying crystallographic polarity of CdTe surfaces
12 Y. C. Lu, R. K. Route, D. Elwell and R. S. Feigelson, J. Vac. Sci. Technol., A3(1985) 269. 13 S. C. Gupta, F. R. Chavada, S. K. Koul and M. Gautam, J. Electrochem. Soc., 135 (1988) 2894. 14 R. D. Brown, K. Durose, G. J. Russell and J. Woods, J. Cryst. Growth, 101 (1990) 211. 15 H. C. Gatos and M. C. Lavine, J. Electrochem. Soc., 107
93
(1960) 427. 16 D. Hanemen, Phys. Rev., 121 (1962) 1093. 17 P. M. Amritraj and E Ht. Pollak, Appl. Phys. Lett., 45 (1984) 789. 18 D. Haneman, in R. K. Willardson and H. L. Goering (eds.), Compound Semiconductors, Vol. 1, Reinhold Publishing Corp., New York, 1962, p. 423.