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Hyperfine coupling constant in manganese-doped Cd1−xZnxTe
J. Phys. Chem. Solids Vol. 47, No. 8, pp. 789-793. Printed in Great Britam.
1986
OOZZ-3697186 $3.00 + 0.00 Pergamon Journals Ltd.
HYPERFINE COUPLING MANGANESE-DOPED
CONSTANT Cd, _ ,Zn,Te
IN
A. K. KOH, D. J. MILLER and C. T. GRAINGER School
of Physics,
University
of New South
(Received
Wales, P.O. Box I, Kensington Australia
13 January
1986; accepted 6 March
2033, New South
Wales,
1986)
Abstract-An E.P.R. study was made on Mn-doped Cd,_,Zn,Te using an X-band spectrometer. Hypertine coupling constant of the material was measured both at ambient and liquid nitrogen temperature. The results obtained were correlated to the ionicity of the constituent compounds. A comparison of the present results is also made with a theoretical model proposed by Huang. Keyworcis: Hyperfine semiconductors.
coupling
constant,
ionicity,
effective distance,
1. INTRODUCTION
CdTe and ZnTe are both II-VI semiconductors which crystallize in the zincblende structure with a lattice constant of 6.482 and 6.1024 A, respectively [l]. In recent years they have attracted much research interest primarily for their technological applications; ZnTe as a light-emitting material [2] and CdTe as a material for solar cells, infrared detectors, nuclear detectors and a substrate for Hg, _,Cd,Te [3]. When alloyed with each other, they form a continuous series of solid solutions Cd, _,Zn,Te which has a segregation coefficient of approximately 2 especially for a small value of x [4]. The mixed alloy Cd, _,Zn,Te also has technological importance in its own right as an efficient, visible electroluminescence material [5]. In this paper we report on an E.P.R. study of Mn-doped Cd,_,Zn,Te which to the best of our knowledge has scarcely been performed although similar studies on CdTe and ZnTe have already been done.
2. EXPERIMENTAL SET-UP The polycrystalline samples of Cd, _ .Zn,Te : Mn2+ (0 < x < 1) were prepared by mixing appropriate portions of the constituent semiconductors together with a trace of manganese (0.01 at.%). The mixtures were sealed off in a quartz ampoule under a vacuum of about 200 pm pressure. The samples were heated up slowly from ambient temperature and annealed at about 950°C for several days. Prolonged annealing is necessary in order to homogenize the charge and to enable the manganese ions to diffuse into the semiconductor matrix. An X-ray diffraction technique using k, copper radiation was used to assess the crystal quality and to determine the crystal structure. The lattice parameter of the alloy was also determined. The lattice constant against composition for 789
local axial symmetry,
semimagnetic
Cd, _,Zn,Te:Mn2+ was found to obey Vegard’s law. Its order of magnitude was also found to be in good agreement with other previously published data [6] (see Fig. 1). E.P.R. measurements were carried out using a conventional homodyne X-band spectrometer with a 100 kHz field modulation. The working temperatures were at ambient and 77 K. 3. EXPERIMENTAL RESULTS Those samples which were magnetically dilute showed the characteristic six lines corresponding to the main electronic transitions with AM = + 1 and Am = 0 where M and m are the electronic and nuclear magnetic quantum numbers, respectively. Forbidden transitions with AM = + 1 and Am = + 1 were also observed. Figure 2 shows the spectra of ZnTe: Mn2+ both at 300 and 77 K as representative examples. The spacing between each pair of the six hypertine lines is a measure of the hyperfme structure constant A. The samples into which manganese had been excessively introduced showed broad featureless lines which did not provide much useful information in the present context. The line broadening was due possibly to the dipolar interaction between the manganese ions which were in close proximity with each other. Instead of recording the hyperflne spectrum on conventional chart paper, we recorded it with a microcomputer. Each spectrum was digitized at 400 points and a program was developed to determine the separation between the lines. 4.
DISCUSSION
Figure 3.shows the hyperfine structure constant A of Cd,_.Zn,Te:Mn2+ as a function of composition at both 300 and 77 K. The E.P.R. signals were stronger particularly at 77 K and in most cases they were more well-resolved compared to that at 300 K.
A. K. KOH et al.
790
Cd,_, Znx Te o PRESENT
WORK
. AFTER RADAUTSAN ET AL PHY STAT SOL. 37, Kg.1970
6./.0.z -
u w
6 38-
6.30-
: 6.28L! Q i
6-266. ZL6.226.20
-
6.18
-
6.16
-
6. 16 -
0
O-l
0.2
0.3 MOLAR
Fig. 1. Lattice
constant
of Cd
O-L
0.5
FRACTION
0.7
0.6 OF
0.8
o-9
1-o
ZnTe
, .Zn,Te: Mn*+ showing that Vegard’s law is well obeyed. m= %
m=-%
m=-‘/2
I
m=%
I
m=‘/,
Ill= 5/z
I
ZnTe: Mn 2* 77 K
DPPH
marker
s19’na.l
LOG
H,
Fig. 2. E.P.R.
spectra
of ZnTe:MnZ+,
(a) at 300 K and (b) at 77 K. In the figure m stands quantum number.
for magnetic
Hyperfine coupling constant in manganese-doped
63.2 63. 0
Cdl.x
Znx
Cd, _,Zn,Te
o
Te
l
791
77 K 300K
62.5 62.0 3 61.5 2 61.0
’ 60.5 60.01
59.5
z[, , , , , , , , , ] , , , , , , , , , t
0.1
o-2
0.3
0.5
0.L
0.6
0.7
0.8
0.9
1.0
x molar %
Fig. 3. Hyperfine constant as a function of composition for Cd, _ .Zn,Te: MnZ+ at room temperature and 77 K.
The magnitude of A is in the region of 62 gauss which can be considered as quite large. Several theories have been proposed in the past to account for the large hyperfine splitting of Mn2+ and the iso-electronic Fe3+. According to Abragam [7j, the large values of A were due to an admixture of the 3s4s3d5 configuration into the 3s33d5 configuration. However, subsequent theoretical calculations of the admixture by Abragam, Horowitz and Pryce [8] showed it to be too small to explain the observed hyperfine structure. A more plausible explanation that can account for the hyperfme structure has been given by Heine [9), Wood and Pratt [lo] and Watson and Freeman [1 11. The theory dwells on the fact that the exchange interaction between the d-electrons and paired s-electrons in the core is spin orientation dependent. Since the spins of the five d-electrons are parallel in the “S,,, state, these electrons will experience a different exchange interaction with the electron in an s2 configuration which has its spin parallel to that of the d-electrons than for that which is antiparallel. This results in a polarization of the s2 configuration which can then give rise to a magnetic field at the nucleus. Watson and Freeman [l l] have made detailed calculations of the polarizations of the Is’, 2s’ and 3s’ configurations. They have quantitatively been able to account for the observed hyperfine structure of Mn2+ and Fe3+ in ionic crystals. As is evidenced from Fig. 3, A at 77 K is higher than at 300 K for the whole range of alloy composition. Similar behaviour has also been observed in the analogous alloy systems Hg, _ .Cd, Se: Mn2+ [ 121 and Pb,_&Te:Mn2+ [13]. A monotonic increase of the hyperfine constant A in the E.P.R. spectra of Mn2+ with increasing ionicity was first noted by van Wieringen [ 141and Matumura [15]. Later, Henning [16] as well as Simanek and Muller [17] further extended the observations to a large number of systems and to other 3d-ions and discussed possible mechanisms in greater detail. On
the basis of Matumura’s scheme, CdTe is more ionic than ZnTe and similarly for CdTe-rich alloys. The magnitudes of A in CdTe and ZnTe are in reasonable agreement with those found by Matumura [15]. In terms of chemical affinity, it shows that there is a stronger affinity of Mn2+ ions with the Zn atoms than the Cd atoms although the distribution of Zn or Cd atoms around Mn2+ is quite random in nature. Recently, Lehmann [18] proposed that the hypefine splitting constant A in the E.P.R. spectra of the ds-ions of Mn2+ and Fe3+ as impurities in crystals increases with the size of the host ion. We attempted to test this theory on our system. For purposes of determining the size, we resorted to a scheme proposed by Inoue and Huang [19] using what is called case of the effective distance. In the Cd, _,Zn,Te: Mn2+ the effective distance d,(x) may be written as &(x)=(1
-x)(&&A +x(dz,,,(A
-B)-&x) -B)-&,+%,
(1)
where $,,(A - B) and d,,,(A -B) are the interatomic distances for CdTe and ZnTe, respectively; R is the ionic radius. By applying eqn (1) to the Cd, _,Zn,Te:Mn’+ system, we observe that den for CdTe is indeed larger than that for ZnTe indicating that there is indeed a correlation between a larger de, and an enhanced hyperfine splitting constant. Referring once again to Fig. 3 it is found that A increases monotonically for x = 1 to x = 0 although the increase is nonlinear. The hyperfine coupling constant on the whole is quite independent of composition especially for x < 0.25. The dependence of A on temperature is quite small and the increase in A with respect to temperature is rather uniform for all compositions. The temperature dependence of A has been advanced in a theory by Huang [20]. According to the
A. K. KOH er al.
192
4.5
-
2.0 0
I
I
I
0.1
0.2
0.3
I
MOLAR
Fig. 4. Experimental
curves
I
0.L
05
FRACTION
A(T)=Ao-BT,
(2)
where A, is a constant. B is a function of various parameters such as the charge, e, of the bonding electron of the paramagnetic ion, the effective charge ecR, of an anion, the density, p, the anion-cation distance, R, and the Debyk temperature, On, of the host crystal; Bcc(e .e,rr)2/pR8B&
(3)
The relation of eqn (2) and (3) has been found to be applicable to Mn2+ in II-VI semiconductors [19, and 211. We have attempted to see if this model fits our experimental results. According to the model, the weaker-temperature dependence of A shows that the
0.1
02
03
01 x
Fig. 5. Theoretical
I
I
I
0.7
0.6
09
ZnTe,
60.0 1.0
x
for A and B from eqn (2) in the case of Cd, _,Zn,Te:Mn’+.
model, for the case of Mn2+ in octahedral symmetry, in which the Van-Vleck orbit-lattice interaction is used to evaluate the phonon-induced hypertine structure constant in alkali halides the temperature dependence of A may be expressed as
0
I O-6
MOLAR
value of B in the mixed alloys becomes quite small compared to the two component semiconductors. By extension, we may assume that it is due to a decrease of the effective charge or an increase in the Debye temperature when the crystal is formed. By substituting experimental values of A into eqn (2) we can extract “experimental” values for B. Both the experimental values of A and B are shown as a function of composition in Fig. 4. By substituting theoretical values of the parameters in eqn (3), we can also obtain a theoretical curve for B as a function of composition (see Fig. 5). The experimental and theoretical curves for B do not agree showing that the model is not applicable in the Cd, _,ZnXTe:Mn2+ system. Similar breakdown has also occurred in Pb,Sn, _,Te [13] and CdS, _,Se,
Pa
4. CONCLUDING
REMARKS
The in hypertine structure constant Cd,_.ZnXTe:Mn2+ has been found to be related to
05
06
FRACTION
07
06
09
1.0
ZnTe
curve for B from eqn (3) in the case of Cd, _,Zn,Te:
Mn*+.
Hyperfine coupling constant in manganese-doped the effective distance of the host crystals, the greater the effective distance, the greater is the hyperfine splitting. The hyperfine structure constant is larger at 77 K than at 300 K for the whole alloy composition. The temperature-dependence of the hype&e structure constant in this alloy system is modest, about 1 gauss difference between 77 and 300 K. It may be of interest to know that our recent studies have shown that Mn2+ occupies a site with local axial symmetry in Cd, _.Zn,Te and the full results will be reported in a subsequent publication. Such observation of local axial symmetry has recently been reported for Hg, _,Cd,Te: Mn2+ [23]. It is interesting to note that the study of II-IV mixed alloys doped with manganese has taken on a new dimension in recent years with the advent of a new class of materials called semimagnetic semiconductors. These materials are synthesized from II-VI compounds with the cation sites being replaced randomly by a transition element, typically manganese. Examples of such materials are Cd, _ x Mn,Te, Hg, _ x Mn,Se and Zn, _ ,Mn,Te. Already they have been found to be suitable materials for infrared detectors tunable both by temperature and magnetic field [24]. This is reminiscent of Hg, _ $d,Te which is also a good material tunable by composition [25]. In fact Hg, _.Mn,Te and Hg, _,Cd,Te can be considered as allied to each other. A physical study of II-VI mixed alloys doped with manganese might help to shed light on the physical properties of semimagnetic semiconductors.
Cd, _,Zn,Te REFERENCES
Edina A. and Takahashi T., J. Cryst. Growrh 59, 51 (1982). 2. Nashio M., Nakamura Y. and Ogawa H., J. uppl. Phys. Japan 22, 1346 (1983). Semimetals 13, 39 (1978). 3. Zanio K., Semiconductors 4. Steininger J. M., J. uppl. Phys. 41, 2713 (1970). 5. Morehead F. F. and Mandel G., Appl. Phys. Letr. 5, 53 (1964). 6. Radautsan S. I., Tasurkan A. E. and Maksimova., Phys. Stat. Sol. 37, k9 (1970). 7. Abragam A., Phys. Rev. 79, 534 (1950). 8. Abragam A., Horowitz J. and Pryce M. H. L., Proc. R. Sot. (London) A230, 169 (1955). 9. Heine V., Phys. Rev. 107, 1002 (1957). 10. Wood J. H. and Pratt G. W. Jr., Phys. Rev. 107, 995
(1957). 11. Watson R. E. and Freeman A. J., Phys. Rev. 124, 1117 (1961). 12. Leibler K., Giriat W., Wilamowski Z. and Iwanowski R., Phys. Stat. Sol. (b) 47, 405 (1971). 13. Hejwowski T. and Subotowicz M., Phys. Stat. Sol. (b) 106, 373 (1981). 14. Van Wieringen J. S., Discuss. Faraday Sot. 19, 118 (1955). 15. Matumura O., J. phys. Sot. Japan 14, 108 (1959). 16. Henning J. C. M., Phys. Lett. 24A, 40 (1967). 17. Simanek E. and Muller K. A., J. Phys. Chem. Solids 31, 1027 (1970). 18. Lehmann G., J. Phys. Chem. Solids 41, 919 (1980). 19. Inoue M. and Huang C. Y., J. phys. Sot. Japan 32,763 (1972).
20. Huang C. Y., Phys. Rev. 158, 280 (1967). 21. Inoue M., J. phys. Sot. Japan 33, 1024 (1972). 22. Deigen M. F., Zevin V. Y., Maevskii V. M., Geifman I. N., Konovalov V. I. and Vitrikhovskii N. I., Soviet Phys.-Semiconductors
Acknowledgements-This work was supported by a grant from the Australian Research Grants Scheme. We thank M. Benton for technical support.
793
2, 923 (1969).
23 Koh A. K., Miller D. J. and Grainger C. T., Phys. Rev. 29B, 4904 (1984). 24. Furdyna J. K., J. appl. Phys. 53, 7637 (1982). 25. Montegu B., Laugier A. and Triboulet R., J. appl. Pkys. 56, 3061 (1984).
1986
OOZZ-3697186 $3.00 + 0.00 Pergamon Journals Ltd.
HYPERFINE COUPLING MANGANESE-DOPED
CONSTANT Cd, _ ,Zn,Te
IN
A. K. KOH, D. J. MILLER and C. T. GRAINGER School
of Physics,
University
of New South
(Received
Wales, P.O. Box I, Kensington Australia
13 January
1986; accepted 6 March
2033, New South
Wales,
1986)
Abstract-An E.P.R. study was made on Mn-doped Cd,_,Zn,Te using an X-band spectrometer. Hypertine coupling constant of the material was measured both at ambient and liquid nitrogen temperature. The results obtained were correlated to the ionicity of the constituent compounds. A comparison of the present results is also made with a theoretical model proposed by Huang. Keyworcis: Hyperfine semiconductors.
coupling
constant,
ionicity,
effective distance,
1. INTRODUCTION
CdTe and ZnTe are both II-VI semiconductors which crystallize in the zincblende structure with a lattice constant of 6.482 and 6.1024 A, respectively [l]. In recent years they have attracted much research interest primarily for their technological applications; ZnTe as a light-emitting material [2] and CdTe as a material for solar cells, infrared detectors, nuclear detectors and a substrate for Hg, _,Cd,Te [3]. When alloyed with each other, they form a continuous series of solid solutions Cd, _,Zn,Te which has a segregation coefficient of approximately 2 especially for a small value of x [4]. The mixed alloy Cd, _,Zn,Te also has technological importance in its own right as an efficient, visible electroluminescence material [5]. In this paper we report on an E.P.R. study of Mn-doped Cd,_,Zn,Te which to the best of our knowledge has scarcely been performed although similar studies on CdTe and ZnTe have already been done.
2. EXPERIMENTAL SET-UP The polycrystalline samples of Cd, _ .Zn,Te : Mn2+ (0 < x < 1) were prepared by mixing appropriate portions of the constituent semiconductors together with a trace of manganese (0.01 at.%). The mixtures were sealed off in a quartz ampoule under a vacuum of about 200 pm pressure. The samples were heated up slowly from ambient temperature and annealed at about 950°C for several days. Prolonged annealing is necessary in order to homogenize the charge and to enable the manganese ions to diffuse into the semiconductor matrix. An X-ray diffraction technique using k, copper radiation was used to assess the crystal quality and to determine the crystal structure. The lattice parameter of the alloy was also determined. The lattice constant against composition for 789
local axial symmetry,
semimagnetic
Cd, _,Zn,Te:Mn2+ was found to obey Vegard’s law. Its order of magnitude was also found to be in good agreement with other previously published data [6] (see Fig. 1). E.P.R. measurements were carried out using a conventional homodyne X-band spectrometer with a 100 kHz field modulation. The working temperatures were at ambient and 77 K. 3. EXPERIMENTAL RESULTS Those samples which were magnetically dilute showed the characteristic six lines corresponding to the main electronic transitions with AM = + 1 and Am = 0 where M and m are the electronic and nuclear magnetic quantum numbers, respectively. Forbidden transitions with AM = + 1 and Am = + 1 were also observed. Figure 2 shows the spectra of ZnTe: Mn2+ both at 300 and 77 K as representative examples. The spacing between each pair of the six hypertine lines is a measure of the hyperfme structure constant A. The samples into which manganese had been excessively introduced showed broad featureless lines which did not provide much useful information in the present context. The line broadening was due possibly to the dipolar interaction between the manganese ions which were in close proximity with each other. Instead of recording the hyperflne spectrum on conventional chart paper, we recorded it with a microcomputer. Each spectrum was digitized at 400 points and a program was developed to determine the separation between the lines. 4.
DISCUSSION
Figure 3.shows the hyperfine structure constant A of Cd,_.Zn,Te:Mn2+ as a function of composition at both 300 and 77 K. The E.P.R. signals were stronger particularly at 77 K and in most cases they were more well-resolved compared to that at 300 K.
A. K. KOH et al.
790
Cd,_, Znx Te o PRESENT
WORK
. AFTER RADAUTSAN ET AL PHY STAT SOL. 37, Kg.1970
6./.0.z -
u w
6 38-
6.30-
: 6.28L! Q i
6-266. ZL6.226.20
-
6.18
-
6.16
-
6. 16 -
0
O-l
0.2
0.3 MOLAR
Fig. 1. Lattice
constant
of Cd
O-L
0.5
FRACTION
0.7
0.6 OF
0.8
o-9
1-o
ZnTe
, .Zn,Te: Mn*+ showing that Vegard’s law is well obeyed. m= %
m=-%
m=-‘/2
I
m=%
I
m=‘/,
Ill= 5/z
I
ZnTe: Mn 2* 77 K
DPPH
marker
s19’na.l
LOG
H,
Fig. 2. E.P.R.
spectra
of ZnTe:MnZ+,
(a) at 300 K and (b) at 77 K. In the figure m stands quantum number.
for magnetic
Hyperfine coupling constant in manganese-doped
63.2 63. 0
Cdl.x
Znx
Cd, _,Zn,Te
o
Te
l
791
77 K 300K
62.5 62.0 3 61.5 2 61.0
’ 60.5 60.01
59.5
z[, , , , , , , , , ] , , , , , , , , , t
0.1
o-2
0.3
0.5
0.L
0.6
0.7
0.8
0.9
1.0
x molar %
Fig. 3. Hyperfine constant as a function of composition for Cd, _ .Zn,Te: MnZ+ at room temperature and 77 K.
The magnitude of A is in the region of 62 gauss which can be considered as quite large. Several theories have been proposed in the past to account for the large hyperfine splitting of Mn2+ and the iso-electronic Fe3+. According to Abragam [7j, the large values of A were due to an admixture of the 3s4s3d5 configuration into the 3s33d5 configuration. However, subsequent theoretical calculations of the admixture by Abragam, Horowitz and Pryce [8] showed it to be too small to explain the observed hyperfine structure. A more plausible explanation that can account for the hyperfme structure has been given by Heine [9), Wood and Pratt [lo] and Watson and Freeman [1 11. The theory dwells on the fact that the exchange interaction between the d-electrons and paired s-electrons in the core is spin orientation dependent. Since the spins of the five d-electrons are parallel in the “S,,, state, these electrons will experience a different exchange interaction with the electron in an s2 configuration which has its spin parallel to that of the d-electrons than for that which is antiparallel. This results in a polarization of the s2 configuration which can then give rise to a magnetic field at the nucleus. Watson and Freeman [l l] have made detailed calculations of the polarizations of the Is’, 2s’ and 3s’ configurations. They have quantitatively been able to account for the observed hyperfine structure of Mn2+ and Fe3+ in ionic crystals. As is evidenced from Fig. 3, A at 77 K is higher than at 300 K for the whole range of alloy composition. Similar behaviour has also been observed in the analogous alloy systems Hg, _ .Cd, Se: Mn2+ [ 121 and Pb,_&Te:Mn2+ [13]. A monotonic increase of the hyperfine constant A in the E.P.R. spectra of Mn2+ with increasing ionicity was first noted by van Wieringen [ 141and Matumura [15]. Later, Henning [16] as well as Simanek and Muller [17] further extended the observations to a large number of systems and to other 3d-ions and discussed possible mechanisms in greater detail. On
the basis of Matumura’s scheme, CdTe is more ionic than ZnTe and similarly for CdTe-rich alloys. The magnitudes of A in CdTe and ZnTe are in reasonable agreement with those found by Matumura [15]. In terms of chemical affinity, it shows that there is a stronger affinity of Mn2+ ions with the Zn atoms than the Cd atoms although the distribution of Zn or Cd atoms around Mn2+ is quite random in nature. Recently, Lehmann [18] proposed that the hypefine splitting constant A in the E.P.R. spectra of the ds-ions of Mn2+ and Fe3+ as impurities in crystals increases with the size of the host ion. We attempted to test this theory on our system. For purposes of determining the size, we resorted to a scheme proposed by Inoue and Huang [19] using what is called case of the effective distance. In the Cd, _,Zn,Te: Mn2+ the effective distance d,(x) may be written as &(x)=(1
-x)(&&A +x(dz,,,(A
-B)-&x) -B)-&,+%,
(1)
where $,,(A - B) and d,,,(A -B) are the interatomic distances for CdTe and ZnTe, respectively; R is the ionic radius. By applying eqn (1) to the Cd, _,Zn,Te:Mn’+ system, we observe that den for CdTe is indeed larger than that for ZnTe indicating that there is indeed a correlation between a larger de, and an enhanced hyperfine splitting constant. Referring once again to Fig. 3 it is found that A increases monotonically for x = 1 to x = 0 although the increase is nonlinear. The hyperfine coupling constant on the whole is quite independent of composition especially for x < 0.25. The dependence of A on temperature is quite small and the increase in A with respect to temperature is rather uniform for all compositions. The temperature dependence of A has been advanced in a theory by Huang [20]. According to the
A. K. KOH er al.
192
4.5
-
2.0 0
I
I
I
0.1
0.2
0.3
I
MOLAR
Fig. 4. Experimental
curves
I
0.L
05
FRACTION
A(T)=Ao-BT,
(2)
where A, is a constant. B is a function of various parameters such as the charge, e, of the bonding electron of the paramagnetic ion, the effective charge ecR, of an anion, the density, p, the anion-cation distance, R, and the Debyk temperature, On, of the host crystal; Bcc(e .e,rr)2/pR8B&
(3)
The relation of eqn (2) and (3) has been found to be applicable to Mn2+ in II-VI semiconductors [19, and 211. We have attempted to see if this model fits our experimental results. According to the model, the weaker-temperature dependence of A shows that the
0.1
02
03
01 x
Fig. 5. Theoretical
I
I
I
0.7
0.6
09
ZnTe,
60.0 1.0
x
for A and B from eqn (2) in the case of Cd, _,Zn,Te:Mn’+.
model, for the case of Mn2+ in octahedral symmetry, in which the Van-Vleck orbit-lattice interaction is used to evaluate the phonon-induced hypertine structure constant in alkali halides the temperature dependence of A may be expressed as
0
I O-6
MOLAR
value of B in the mixed alloys becomes quite small compared to the two component semiconductors. By extension, we may assume that it is due to a decrease of the effective charge or an increase in the Debye temperature when the crystal is formed. By substituting experimental values of A into eqn (2) we can extract “experimental” values for B. Both the experimental values of A and B are shown as a function of composition in Fig. 4. By substituting theoretical values of the parameters in eqn (3), we can also obtain a theoretical curve for B as a function of composition (see Fig. 5). The experimental and theoretical curves for B do not agree showing that the model is not applicable in the Cd, _,ZnXTe:Mn2+ system. Similar breakdown has also occurred in Pb,Sn, _,Te [13] and CdS, _,Se,
Pa
4. CONCLUDING
REMARKS
The in hypertine structure constant Cd,_.ZnXTe:Mn2+ has been found to be related to
05
06
FRACTION
07
06
09
1.0
ZnTe
curve for B from eqn (3) in the case of Cd, _,Zn,Te:
Mn*+.
Hyperfine coupling constant in manganese-doped the effective distance of the host crystals, the greater the effective distance, the greater is the hyperfine splitting. The hyperfine structure constant is larger at 77 K than at 300 K for the whole alloy composition. The temperature-dependence of the hype&e structure constant in this alloy system is modest, about 1 gauss difference between 77 and 300 K. It may be of interest to know that our recent studies have shown that Mn2+ occupies a site with local axial symmetry in Cd, _.Zn,Te and the full results will be reported in a subsequent publication. Such observation of local axial symmetry has recently been reported for Hg, _,Cd,Te: Mn2+ [23]. It is interesting to note that the study of II-IV mixed alloys doped with manganese has taken on a new dimension in recent years with the advent of a new class of materials called semimagnetic semiconductors. These materials are synthesized from II-VI compounds with the cation sites being replaced randomly by a transition element, typically manganese. Examples of such materials are Cd, _ x Mn,Te, Hg, _ x Mn,Se and Zn, _ ,Mn,Te. Already they have been found to be suitable materials for infrared detectors tunable both by temperature and magnetic field [24]. This is reminiscent of Hg, _ $d,Te which is also a good material tunable by composition [25]. In fact Hg, _.Mn,Te and Hg, _,Cd,Te can be considered as allied to each other. A physical study of II-VI mixed alloys doped with manganese might help to shed light on the physical properties of semimagnetic semiconductors.
Cd, _,Zn,Te REFERENCES
Edina A. and Takahashi T., J. Cryst. Growrh 59, 51 (1982). 2. Nashio M., Nakamura Y. and Ogawa H., J. uppl. Phys. Japan 22, 1346 (1983). Semimetals 13, 39 (1978). 3. Zanio K., Semiconductors 4. Steininger J. M., J. uppl. Phys. 41, 2713 (1970). 5. Morehead F. F. and Mandel G., Appl. Phys. Letr. 5, 53 (1964). 6. Radautsan S. I., Tasurkan A. E. and Maksimova., Phys. Stat. Sol. 37, k9 (1970). 7. Abragam A., Phys. Rev. 79, 534 (1950). 8. Abragam A., Horowitz J. and Pryce M. H. L., Proc. R. Sot. (London) A230, 169 (1955). 9. Heine V., Phys. Rev. 107, 1002 (1957). 10. Wood J. H. and Pratt G. W. Jr., Phys. Rev. 107, 995
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