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Structural and conductivity type transitions in vapour phase grown (ZeTe)x(CdSe)1−x single crystals
0038-1098/93 $6.00 + .00 Pergamon Press Ltd
Solid State Communications, Vol. 86, No. 4, pp. 253-255, 1993. Printed in Great Britain.
STRUCTURAL AND CONDUCTIVITY TYPE TRANSITIONS IN VAPOUR PHASE GROWN (ZnTe)x(CdSe)~_x SINGLE CRYSTALS V.K.M. Rani, R.P.V. Lakshmi, R. Venugopal, D.R. Reddy and B.K. Reddy Department of Physics, S.V. University, Tirupati, India 517 502
(Received 12 October 1992 by P. Burlet) (ZnTe)x(CdSe)l- x single crystals have been prepared by vapour phase growth technique. ZnTe crystals are p-type with zincblende structure, whereas CdSe crystals are n-type with wurtzite structure. In mixed crystals of (ZnTe)x(CdSe)l_x n-type conductivity with wurtzite structure is observed for x < 0.4 whereas only p-type conductivity with zincbtende structure is observed for x >_ 0.5. The structural transition is obvious for 0.4 < x < 0.5 and the lattice parameters are found to vary linearly with composition within each structure. The structure and conductivity type transitions are attributed to a strong auto-compensation phenomenon in II-VI alloy systems.
1. INTRODUCTION
2. EXPERIMENTAL
ZnTe-CdSe ALLOY system offers wide scope for basic as well as applied research. The energy gap values of this alloy system fall in the visible range and hence this system promises wide applicability. Further, ZnTe and CdSe exhibit divergent structural and electrical properties under normal conditions. CdSe crystallizes in wurtzite structure whereas ZnTe crystallizes in zinc-blende structure. ZnTe is p-type whereas CdSe is always n-type. These compounds cannot be prepared in the conjugate types of conductivity due to a very high degree of autocompensation [1]. ZnS-CdS [2], CdTe-CdSe [3, 4], CdTe-CdS [4, 5], ZnTe-CdSe [6], ZnTe-CdS and ZnS-CdSe are the II-VI alloy systems where structural changes are imminent but change in conductivity type is expected only in ZnTe based systems. Of all these systems the last two have not been studied so far and studies on ZnTe-CdSe system are only sporadic [6]. Vitrikhovskii [6] reported structural ambiguity in the melt grown crystals in the range 0.35 < x < 0.4. Oleinik et al. [7] reported that at x = 0 . 5 , (ZnTe)x(CdSe)l_x system breaks down into (ZnSe)09(CdTe)0.t and (ZnSe)0.1(CdTe)0.9 phases whereas Vitrikhovskii [6] did not report any such phase split. In view of these contradicting results, structural and conductivity studies were carried out on (ZnTe)x(CdSe)t-x single crystals prepared by vapour phase growth technique for the first time and the results of these investigations are reported in this paper.
(ZnTe)x(CdSe)l- x single crystals were grown by a self-sealing vapour phase technique described elsewhere [8]. 99.9% pure CdSe and ZnTe obtained from Balzers (Switzerland) were used as source materials. Crystal growth was carried out in an argon atmosphere of 100-150torr at II00°C. Each growth run lasted for about 7-10 days. Chemical analysis of the crystals by atomic absorption spectroscopy showed that the actual composition (x) was the same as that of the target composition within 2%. Mixed crystals with x = 0.0, 0.2, 0.4, 0.5, 0+6, 0.8 and 1.0 were grown. X-ray back reflection photographs of samples of 2 x 2 x 2 mm 3 size cut from different portions of the as-grown crystal boules confirmed the single crystallinity of the grown crystals. However, lattice parameter studies were carried out using powdered samples prepared from the as-grown single crystals. X-ray powder diffractometer technique, Philips PW 1130 with Cu Ks radiation was used for obtaining XRDs at a scanning speed of 2° rain -1 in the range 10-70 °. The diffracting planes in CdSe and ZnTe were identified using standard ASTM tables. The diffracting planes in the mixed crystals of ZnTe-CdSe system have been indexed using analytical methods [9, 10]. The lattice parameters "a" and "a" and "c" of zincblende and wurtzite structures were calculated using the standard formulae [9]. The type of conductivity was determined by the hot probe test [11] and also by polarity of Hall voltage.
253
254
T R A N S I T I O N S IN V A P O U R PHASE G R O W N (ZnTe)x(CdSe)l-x qc
ch 7.06F
oh 4.38 -
6"15 V;t rikhovskii
. . . . .
O-
ch
•ch t~ - o , h ~,-ah 4-34
-
Vol. 86, No. 4
h
-
c -
Hexagonat
Cubic
D -ctc
7.02
•
6.13
-aC
°4
v 4-30
;.11
6"9B
p-
i-
~
, 4"26 "
6.09
6"94 /
/ / 4.22
.
GI~ 0.4
6"90 0.2
MOLE
FRACTION
6.07
~ 0"6
OF
ZnTe
1"0
O.B
('x)
Fig. 1. Variation of lattice parameter with composition in (ZnTe)x(CdSe)]_~ powdered samples. 3. RESULTS A N D D I S C U S S I O N From X-ray studies it is observed that (ZnTe)x(CdSe)l _ ~ crystals crystallize in wurtzite structure for 0 < x < 0.4. For 0.5 < x < 1.0 the structure is cubic zinc-blende. Figure 1 shows the variation of lattice parameter with composition. A structural transition is obvious around x = 0.4 and the lattice parameter(s) are found to vary linearly with compositions within each structure. Vitrikhovskii [6] reported structural ambiguity in the melt grown crystals in the range 0.35 < x < 0.4. Such a transition is expected since the end compounds of the present system normally crystallize in different structures. Table 1 gives some such transitions reported earlier in other similar IIVI alloy systems [2-5]. Figures 2 and 3 show the X R D s of crystals with x = 0.4, 0.5 respectively. Below the transition composition regions the hexagonal structure is stable and above this only Table 1. Composition range of structural transition in I I - VI alloy systems
System
cubic structure exists. It is clear that the transition region is very sharp in the present system. Vitrikhovskii [6] reported cubic structure also for x = 0.4 in melt grown (ZnTe)x(CdSe) ] _ x crystals. However, in the present investigations vapour grown crystals with x = 0.4 are found to have hexagonal structure. An interesting observation made by Oleinik et al. [7] was that at x = 0.5, in (ZnTe)x(CdSe)l-x, the system breaks down into (ZnSe)0.9(CdTe)0j and (ZnSe)0j(CdTe)0.9 phases. But Vitrikhovskii [6] and the 100 (002}
80
z
60
J
(1101
>-
Structural transition Reference composition region X
(ZnS) (CdS), _x (CdTe)x(CdSe)l_~ (CdTe)x(CdS)~_x (ZnTe)x(CdSe)l-x
0.05 0.4 < x < 0.55 0.4 < x < 0.8 0.35 < x < 0.4
[2] [3, 4] [4, 5] [6]
OI 70
i
~
i
I
i
gO
50
40
30
20
10
20
Fig. 2. X-ray diffractogram of (ZnTe)o.4(CdSe)0.6 powdered sample.
Vol. 86, No. 4
TRANSITIONS IN VAPOUR PHASE G R O W N (ZnTe)x(CdSe)~_x
100
200)
(111) 80
6C
z 40
z w
(311)
(220)
20
0
70
6L0
i 50
i &O c
i 30
i 20
10
26
Fig. 3. X-ray diffractogram of (ZnTe)o.5(CdSe)o.5 powdered sample. present authors have not observed any such dissociation of the system near the middle compositions. The hot probe test shows that crystals with x _< 0.4 have n-type conductivity whereas crystals with x _> 0.5 have p-type conductivity. The conductivity type change is found to occur in a narrow range of composition between 0.4 and 0.5. Vitrikhovskii [6] also reported similar conductivity type transition in melt grown crystals near this composition. The conductivity in ZnTe-CdSe system may be understood in terms of strong autocompensation to the extent of second ionization attributed to effective ionic radii ratio. Table 2 shows the average radii ratio calculated by giving appropriate weightage to compositions of metallic and chalcogen ions and the observed type of conductivity in the present system for different compositions. The conductivity type in all these polar semiconductors is determined by autocompensation rather than the external dopings if any. According to the auto-compensation theory [1] p-type conductivity is possible only if the ionic radii ratio is
255
less than 1.06 and n-type conductivity is possible if this ratio is greater than 1.12. If the ratio lies between 1.06 and 1.12 both types of conductivity may be possible. Accordingly, in the present work crystals with 0 < x < 0.4 are found to be n-type whereas those with x = 0.8 and 1.0 are found to be p-type. In crystals with x = 0.6 which have the ionic radii ratio lying in the critical range (1.06-1.12) both n- and ptype conductivities should be possible. However, in the present work only p-type crystals are obtained for this particular composition. For x = 0.5, the ionic radii ratio is greater than 1.12, thus only n-type conductivity should be possible. But in the present work p-type conductivity is obtained in crystals of this composition. Further, our observation is in conformation with the observation of Vitrikhovskii [6]. Therefore, the compensation phenomenon in mixed crystals either may not entirely be attributed to second ionization or the concept of taking average metallic/chalcogen covalent radii ratio might be an over-simplification and as such may not be exact. Auto-compensation due to antisites and clusters of point defects may also be appreciable. Acknowledgements - The authors are grateful to the University Grants Commission, India, for providing financial support to this work, and to Dr S. Prabhakar, NML, Dr K.V. Reddy and Dr B.S.V. Gopalam, IIT, Madras, India for providing the AAS and XRD facilities. REFERENCES 1. 2. 3. 4. 5. 6.
Table 2. Covalent radii ratio and type of conductivity in (Zn Te)x (CdSe) l _ x single crystals
7.
Composition x
Covalent radii ratio (RM/RN)
Type of conductivity
8.
0.0 0.2 0.4 0.5 0.6 0.8 1.0
1.30
n
9.
1.24 1.18 1.14 1.11 1.04 0.99
n n p p p p
10. 11.
G. Mandel, F.F. Morehead & P.R. Wanger, Phys. Rev. 136A, 826 (1964). P. Cherin, E.L. Lind & E.A. Davies, J. Electrochem. Soc. 117, 2 (1970). A.J. Strauss & Steiniger, J. Electrochem. Soc. 117, 1420 (1970). H. Tai, S. Nakashima & S. Hori, Phys. Status Solidi (a) 30, K115 (1975). K. Ohata, J. Saraie & T. Tanaka, Jpn J. Appl. Phys. 12, 1198 (1973). N.I. Vitrikhovskii, Neorg. Mater. 13, 437 (1977). G.S. Oleinik, V.N. Tomashik & I.B. Mizetskaya, Poluprovodn. Tekh and Mikrolektron (USSR) 28, 56 (1978). P. Chandrasekharam, D. Raja Reddy & B.K. Reddy, Phys. Status Solidi (a) 79, K105 (1983). L.V. Azaroff & M.J. Buerger, The Powder Method in X-ray Crystallography, McGrawHill, Tokyo (1958). C.N. Bunn, Chemical Crystallography, Oxford University, Oxford (1961 ). W.R. Runyan, Semiconductor Measurements and Instrumentation, McGraw-Hill, Tokyo (1975).
Solid State Communications, Vol. 86, No. 4, pp. 253-255, 1993. Printed in Great Britain.
STRUCTURAL AND CONDUCTIVITY TYPE TRANSITIONS IN VAPOUR PHASE GROWN (ZnTe)x(CdSe)~_x SINGLE CRYSTALS V.K.M. Rani, R.P.V. Lakshmi, R. Venugopal, D.R. Reddy and B.K. Reddy Department of Physics, S.V. University, Tirupati, India 517 502
(Received 12 October 1992 by P. Burlet) (ZnTe)x(CdSe)l- x single crystals have been prepared by vapour phase growth technique. ZnTe crystals are p-type with zincblende structure, whereas CdSe crystals are n-type with wurtzite structure. In mixed crystals of (ZnTe)x(CdSe)l_x n-type conductivity with wurtzite structure is observed for x < 0.4 whereas only p-type conductivity with zincbtende structure is observed for x >_ 0.5. The structural transition is obvious for 0.4 < x < 0.5 and the lattice parameters are found to vary linearly with composition within each structure. The structure and conductivity type transitions are attributed to a strong auto-compensation phenomenon in II-VI alloy systems.
1. INTRODUCTION
2. EXPERIMENTAL
ZnTe-CdSe ALLOY system offers wide scope for basic as well as applied research. The energy gap values of this alloy system fall in the visible range and hence this system promises wide applicability. Further, ZnTe and CdSe exhibit divergent structural and electrical properties under normal conditions. CdSe crystallizes in wurtzite structure whereas ZnTe crystallizes in zinc-blende structure. ZnTe is p-type whereas CdSe is always n-type. These compounds cannot be prepared in the conjugate types of conductivity due to a very high degree of autocompensation [1]. ZnS-CdS [2], CdTe-CdSe [3, 4], CdTe-CdS [4, 5], ZnTe-CdSe [6], ZnTe-CdS and ZnS-CdSe are the II-VI alloy systems where structural changes are imminent but change in conductivity type is expected only in ZnTe based systems. Of all these systems the last two have not been studied so far and studies on ZnTe-CdSe system are only sporadic [6]. Vitrikhovskii [6] reported structural ambiguity in the melt grown crystals in the range 0.35 < x < 0.4. Oleinik et al. [7] reported that at x = 0 . 5 , (ZnTe)x(CdSe)l_x system breaks down into (ZnSe)09(CdTe)0.t and (ZnSe)0.1(CdTe)0.9 phases whereas Vitrikhovskii [6] did not report any such phase split. In view of these contradicting results, structural and conductivity studies were carried out on (ZnTe)x(CdSe)t-x single crystals prepared by vapour phase growth technique for the first time and the results of these investigations are reported in this paper.
(ZnTe)x(CdSe)l- x single crystals were grown by a self-sealing vapour phase technique described elsewhere [8]. 99.9% pure CdSe and ZnTe obtained from Balzers (Switzerland) were used as source materials. Crystal growth was carried out in an argon atmosphere of 100-150torr at II00°C. Each growth run lasted for about 7-10 days. Chemical analysis of the crystals by atomic absorption spectroscopy showed that the actual composition (x) was the same as that of the target composition within 2%. Mixed crystals with x = 0.0, 0.2, 0.4, 0.5, 0+6, 0.8 and 1.0 were grown. X-ray back reflection photographs of samples of 2 x 2 x 2 mm 3 size cut from different portions of the as-grown crystal boules confirmed the single crystallinity of the grown crystals. However, lattice parameter studies were carried out using powdered samples prepared from the as-grown single crystals. X-ray powder diffractometer technique, Philips PW 1130 with Cu Ks radiation was used for obtaining XRDs at a scanning speed of 2° rain -1 in the range 10-70 °. The diffracting planes in CdSe and ZnTe were identified using standard ASTM tables. The diffracting planes in the mixed crystals of ZnTe-CdSe system have been indexed using analytical methods [9, 10]. The lattice parameters "a" and "a" and "c" of zincblende and wurtzite structures were calculated using the standard formulae [9]. The type of conductivity was determined by the hot probe test [11] and also by polarity of Hall voltage.
253
254
T R A N S I T I O N S IN V A P O U R PHASE G R O W N (ZnTe)x(CdSe)l-x qc
ch 7.06F
oh 4.38 -
6"15 V;t rikhovskii
. . . . .
O-
ch
•ch t~ - o , h ~,-ah 4-34
-
Vol. 86, No. 4
h
-
c -
Hexagonat
Cubic
D -ctc
7.02
•
6.13
-aC
°4
v 4-30
;.11
6"9B
p-
i-
~
, 4"26 "
6.09
6"94 /
/ / 4.22
.
GI~ 0.4
6"90 0.2
MOLE
FRACTION
6.07
~ 0"6
OF
ZnTe
1"0
O.B
('x)
Fig. 1. Variation of lattice parameter with composition in (ZnTe)x(CdSe)]_~ powdered samples. 3. RESULTS A N D D I S C U S S I O N From X-ray studies it is observed that (ZnTe)x(CdSe)l _ ~ crystals crystallize in wurtzite structure for 0 < x < 0.4. For 0.5 < x < 1.0 the structure is cubic zinc-blende. Figure 1 shows the variation of lattice parameter with composition. A structural transition is obvious around x = 0.4 and the lattice parameter(s) are found to vary linearly with compositions within each structure. Vitrikhovskii [6] reported structural ambiguity in the melt grown crystals in the range 0.35 < x < 0.4. Such a transition is expected since the end compounds of the present system normally crystallize in different structures. Table 1 gives some such transitions reported earlier in other similar IIVI alloy systems [2-5]. Figures 2 and 3 show the X R D s of crystals with x = 0.4, 0.5 respectively. Below the transition composition regions the hexagonal structure is stable and above this only Table 1. Composition range of structural transition in I I - VI alloy systems
System
cubic structure exists. It is clear that the transition region is very sharp in the present system. Vitrikhovskii [6] reported cubic structure also for x = 0.4 in melt grown (ZnTe)x(CdSe) ] _ x crystals. However, in the present investigations vapour grown crystals with x = 0.4 are found to have hexagonal structure. An interesting observation made by Oleinik et al. [7] was that at x = 0.5, in (ZnTe)x(CdSe)l-x, the system breaks down into (ZnSe)0.9(CdTe)0j and (ZnSe)0j(CdTe)0.9 phases. But Vitrikhovskii [6] and the 100 (002}
80
z
60
J
(1101
>-
Structural transition Reference composition region X
(ZnS) (CdS), _x (CdTe)x(CdSe)l_~ (CdTe)x(CdS)~_x (ZnTe)x(CdSe)l-x
0.05 0.4 < x < 0.55 0.4 < x < 0.8 0.35 < x < 0.4
[2] [3, 4] [4, 5] [6]
OI 70
i
~
i
I
i
gO
50
40
30
20
10
20
Fig. 2. X-ray diffractogram of (ZnTe)o.4(CdSe)0.6 powdered sample.
Vol. 86, No. 4
TRANSITIONS IN VAPOUR PHASE G R O W N (ZnTe)x(CdSe)~_x
100
200)
(111) 80
6C
z 40
z w
(311)
(220)
20
0
70
6L0
i 50
i &O c
i 30
i 20
10
26
Fig. 3. X-ray diffractogram of (ZnTe)o.5(CdSe)o.5 powdered sample. present authors have not observed any such dissociation of the system near the middle compositions. The hot probe test shows that crystals with x _< 0.4 have n-type conductivity whereas crystals with x _> 0.5 have p-type conductivity. The conductivity type change is found to occur in a narrow range of composition between 0.4 and 0.5. Vitrikhovskii [6] also reported similar conductivity type transition in melt grown crystals near this composition. The conductivity in ZnTe-CdSe system may be understood in terms of strong autocompensation to the extent of second ionization attributed to effective ionic radii ratio. Table 2 shows the average radii ratio calculated by giving appropriate weightage to compositions of metallic and chalcogen ions and the observed type of conductivity in the present system for different compositions. The conductivity type in all these polar semiconductors is determined by autocompensation rather than the external dopings if any. According to the auto-compensation theory [1] p-type conductivity is possible only if the ionic radii ratio is
255
less than 1.06 and n-type conductivity is possible if this ratio is greater than 1.12. If the ratio lies between 1.06 and 1.12 both types of conductivity may be possible. Accordingly, in the present work crystals with 0 < x < 0.4 are found to be n-type whereas those with x = 0.8 and 1.0 are found to be p-type. In crystals with x = 0.6 which have the ionic radii ratio lying in the critical range (1.06-1.12) both n- and ptype conductivities should be possible. However, in the present work only p-type crystals are obtained for this particular composition. For x = 0.5, the ionic radii ratio is greater than 1.12, thus only n-type conductivity should be possible. But in the present work p-type conductivity is obtained in crystals of this composition. Further, our observation is in conformation with the observation of Vitrikhovskii [6]. Therefore, the compensation phenomenon in mixed crystals either may not entirely be attributed to second ionization or the concept of taking average metallic/chalcogen covalent radii ratio might be an over-simplification and as such may not be exact. Auto-compensation due to antisites and clusters of point defects may also be appreciable. Acknowledgements - The authors are grateful to the University Grants Commission, India, for providing financial support to this work, and to Dr S. Prabhakar, NML, Dr K.V. Reddy and Dr B.S.V. Gopalam, IIT, Madras, India for providing the AAS and XRD facilities. REFERENCES 1. 2. 3. 4. 5. 6.
Table 2. Covalent radii ratio and type of conductivity in (Zn Te)x (CdSe) l _ x single crystals
7.
Composition x
Covalent radii ratio (RM/RN)
Type of conductivity
8.
0.0 0.2 0.4 0.5 0.6 0.8 1.0
1.30
n
9.
1.24 1.18 1.14 1.11 1.04 0.99
n n p p p p
10. 11.
G. Mandel, F.F. Morehead & P.R. Wanger, Phys. Rev. 136A, 826 (1964). P. Cherin, E.L. Lind & E.A. Davies, J. Electrochem. Soc. 117, 2 (1970). A.J. Strauss & Steiniger, J. Electrochem. Soc. 117, 1420 (1970). H. Tai, S. Nakashima & S. Hori, Phys. Status Solidi (a) 30, K115 (1975). K. Ohata, J. Saraie & T. Tanaka, Jpn J. Appl. Phys. 12, 1198 (1973). N.I. Vitrikhovskii, Neorg. Mater. 13, 437 (1977). G.S. Oleinik, V.N. Tomashik & I.B. Mizetskaya, Poluprovodn. Tekh and Mikrolektron (USSR) 28, 56 (1978). P. Chandrasekharam, D. Raja Reddy & B.K. Reddy, Phys. Status Solidi (a) 79, K105 (1983). L.V. Azaroff & M.J. Buerger, The Powder Method in X-ray Crystallography, McGrawHill, Tokyo (1958). C.N. Bunn, Chemical Crystallography, Oxford University, Oxford (1961 ). W.R. Runyan, Semiconductor Measurements and Instrumentation, McGraw-Hill, Tokyo (1975).