Solid State Communications, Vol. 26, pp. 73-75. 0 Pergamon Press Ltd. 1978. Printed in Great Britain
ANISOTROPIC CONDUCTIVITY IN PLASTICALLY DEFORMED ZnS* P. Merchant and C. Elbaum Brown University, Providence, RI 02912, U.S.A. (Received 15 November 1977 by J. Taut) Plastic deformation of ZnS single crystals was found to induce large anisotropies in the d.c. electrical conductivity, as well as changes in the optical properties. The results are consistent with a dislocation band model proposed earlier . EARLIER STUDIES have shown that plastic deformation of compound semiconductor crystals may lead to large anisotropies in their d.c. electrical conductivity [ 1,2] . These anisotropies were attributed to the presence of dislocations which are believed to have pseudo-onedimensional energy bands in the fundamental gap (between the valence and conduction bands) of the semiconductor [l-4] . This is in agreement with theoretical predictions [ 1,5,6] . In these models, the conductivity along the dislocations may become comparable to that of normal band conduction if there is a sufficient population of carriers in the dislocation band, and if the Fermi energy lies inside this band [7 1. In this note we report the results of electical conductivity and optical transmission and reflectivity measurements on undeformed and plastically deformed samples of ZnS (hexagonal structure). Of particular interest is the anisotropy of the d.c. electrical conductivity, u, of one of the deformed samples of this material. Prior to deformation, samples of ZnS were cut from a large (- 20 cm3) single crystal and oriented with their 3,4 and 5 mm edges along the  , [OOOl] and  crystallographic directions, respectively. The samples were deformed by uniaxial compression applied along the [45iO] direction, at elevated temperatures, in slightly over one atmosphere of helium. The applied stresses ranged from 1.87-2.95 kgmms2 and the deformation was performed at various temperatures in the range 698-773°C. After deformation, all of the samples showed a decrease in the  dimensions, an increase in length along the [OOOl] direction and less than a 0.1% change in the  length. The deformations were found to range from 11% to 24% plastic strain. In all of the deformed samples varying amounts of cracking were found to occur. Iaue back-scattering X-ray patterns
0 0 0 0
0 0 0
hu(eV) :Fig. 1. Transmitted intensity of unpolarized light through undeformed ZnS, as a function of energy (R_oom temperature; incident beam normal to the (45 10) face). were taken of the deformed samples, on the faces corresponding to the (0001) planes prior to deformation. These patterns, along with shapes of the deformed samples, were found to be consistent with a -rotation of the [OOOl] axis about the  axis in the (i2iO) plane. In view of these observations the (2021) and (lOi1) planes are among the possible slip planes. Indium contacts were applied to both the deformed and undeformed samples and the temperature dependence of the d.c. electrical conductivity (in the dark) was measured with a Keithley 610A electrometer using the 2 point probe method. The temperature control over the range covered, 300-30 K, was provided by a Cryogenic Technology, Inc. refrigerator, and the temperatures were monitored with a Chromel-p Au +
* Research was supported in part by the National Science Foundation through the Materials Research Laboratory at Brown University and by the Advanced Research Projects Agency. 73
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ZnS T= 298°K
hu(eV) Fig. 2. Unpolarized room temperature reflectivities from the undeformed and deformed ZnS samples, as a function of energy. (Beam incident at an angle of 20” to the [45 lo] crystallographic direction). ZnS
IO‘T (OK) Fig. 3. Log d.c. electrical conductivity (dark) of the 19% strained ZnS sample along the [OOOl] direction, as a function of reciprocal temperature. 0.07 at.% Fe theromocouple mounted inside an OFHC copper block and separated from the sample by teflon tape. The optical measurements were made using a Cary model 17 spectrophotometer in the 3.1-3.9 eV region (4000-3200 A). For the optical measurements the samples were polished down to a 0.05~ finish with A&O3 powder. The undeformed samples were characterized by a nearly isotropic room temperature d.c. electrical
conductivity (dark) of - 10m9(S&cm)-‘. Temperature dependent conductivities are not presented since the contacts exhibited a considerable non-ohmic behavior for temperatures below about 240 K. However, it is apparent that there is a significant density of active impurity and/or defect states within the energy gap, since a conductivity of lo-’ (a-cm)-’ is much larger than that of pure ZnS, which has a band gap of 3.65 eV at 300K . The room temperature, unpolarized transmitted and reflected light intensities from the undeformed ZnS sample in the 3.1-3.9 eV region are shown in Figs. 1 and 2. Both the gradual approach of the transmission curve to the energy (hv) axis, Fig. 1, and the slow change in the reflectivity with hv, for hv < 3.5 eV, Fig. 2, indicate that there is a considerable density of impurity and/or defect levels in the gap, located near the band extrema. The 19% deformed sample showed the largest change in conductivity, compared to that of the undeformed sample. At room temperature it was found that the conductivities along the [OOOI ] and  directions had changed to 2 x lo-’ (a-cm)-’ and 1 x lo-” (S&cm)-‘,respectively, leading to an anisotropy in the conductivity of 2 x lo4 at room temperature. A plot of log u vs 103/To(), for the [OOOl] direction in the 19% deformed sample is shown in Fig. 3. An activation energy of 0.04 eV is apparent in the high temperature region. For 103/T(K) > 7, the contacts began to exhibit non-ohmic behavior. Such behavior was also found for measurements of conductivity along the [2 1301 directions, at all temperatures in the range covered.
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ANISOTROPIC CONDUCTIVITY IN PLASTICALLY DEFORMED ZnS
The reflectivities of some of the deformed ZnS samples are shown in Fig. 2. It is apparent that there is a significant decrease in the reflectivities for energies below the band gap. For energies above the gap, the decrease is probably not significant as some of the cracks in the samples intersected the surfaces. The error bars are Included to show the effects of inhomogeneities found in measuring the reflectivity of different sections of the sample surfaces. Transmission measurements could not be taken for the deformed samples because insufficient light was transmitted in this condition (%T < 1% for all hu in the measured region). This may be due to cracks. The results presented above show that in the 19% deformed sample, the conductivity along the [OOOl] direction has increased by about a factor of 1O*.compared to that of the undeformed sample and that along the  direction it has decreased by about a factor of lo*. An interpretation of these changes in d.c. conductivity and optical properties which is consistent with the dislocation band model of , is that, after deformation, dislocations which are oriented along a direction at a small angle to the [OOOl] direction act as acceptor sites. These sites compensate donor levels found in the undeformed sample. As a result of this compensation, the population of carriers in the dislocation band is increased to a level sufficient to increase the conductivity along this direction, compared to that found in the undeformed sample. The reduced conductivity
along the  direction is the result of emptying of the donor levels into the dislocation band. The reduced reflectivities of the deformed samples for hv < Ep may be a further indication of this compensation. It is also interesting to note that the largest anisotropy occurred for the sample strained to 19%, which was deformed at 773”C, the highest deformation temperature used here. The next highest deformation temperature, 75O”C, used fnr the 24% strained sample showed the next largest anisotropy in the conductivity, 6.5. Finally, it is noted that the large anisotropy seen in the 19% deformed sample cannot be caused by the cracks in this sample as the majority of these cracks run parallel to the [2 1301 direction. Thus their effect should be to cause anisotropy opposite to that observed, i.e. lowest conductivity along the [OOOl] direction. In conclusion, plastic deformation of ZnS is found to result in an anisotropic d.c. electrical conductivity, similar in nature to effects noted previously in CdS [2-4]. Thus ZnS is another compound semiconductor in which the effects of dislocation energy bands lead to significant changes in its electrical transport and optical properties.
Acknowledgement - The authors wish to thank
Mr. Yuko Okamoto for assistance in taking some of the optical measurements.
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