THE VARIATION OF ELECTRICAL CONDUCTIVITY WITH TEMPERATURE FOR CU-DOPED ZnS ' ALLOY

The variation of the electrical conductivity of copper (Cu) doped zinc sulphide (ZnS) alloy with temperature has been in9estigated. The electrical conductivity of the samples increases with temperature and obeys the Arrhenius relation, o = uo exp (Ed2kT) which is characteristic of semiconductors. The energy gaps determined from this empirical relation are 3.91, 3.82, 3.74, 3.60 and 3.58 eV for the various samples. The result shows that the energy gaps of the samples decrease with increase in the incorporation of Cu in the intrinsic ZnS. The narrowing of the band gaps facilitates the ease of electronic transition from the valence band to the conduction band thereby enhancing the conductivity of the samples. All the samples investigated are characterized by wide band gaps which make them invaluable for the fabrication of optoelectronic devices that utilize wide band gap materials.


INTRODUCTION MATERIALS AND METHODS
The properties of the materials used for the fabrication of semiconductor devices are' essential in determining the characteristic of the completed devices.The vallje of conduclivily is important in many devices.Hence, the electron transport property of thin films.may be characterized by the conductivity of the film.Therefore, a study of the temperature dependence of the conductivity is essential in the un~erstanding of the carrier-scaitering mechanism responsible for the transport property.Furthermore, the present study was undertaken to find out the effect of varying temperatures on the, electrical conductivity of Cu-doped ZnS samples in order to determine their suitability for the fabrication of semiconductor devices.
Zinc sulphide is of considerable interest because of its application in optoelectronic devices such as electroluminescence or light emitting diodes (Hiroshi and Koji 19C5;Masakazu e l al 1985;Richard and Frank 1985;Fen~andez and Sebastian 1993).Some of the properties which m2.Ke it an attractive material for optoelectronic device applications are its direct band gap and it is transparcnt over a wide range of the visible spectrum (Berg and Dean ,1976;Thonias 1981;Koppensteiner et al 1993).Also, ZnS is used in the production of fluorescent and luminous paints (Berg and Dean 1976).Furthermore, it is a prospective material for the passivation of the surfzces of some semiconductors and for the modulation of optoelectronic device (Osasona et al 1997).In addition.it can be used as a cathodoluminescent material for coating the screens of cathode ray tubes (Berg and Dean 19i6: Sybil 1982).The present work was therefore partly inspired by these applications and the need to search for more applications for ZnS and Cu-doped ZnS.
Copper is an important impurity in wide band gap zinc chalcogenides.In ZnS and ZnSe, the, incorporation of copper results in a variety of characteristic visible bands (Stringfellow and Bube 1968;Satoh and lgaki 1983).However, there is a long-standing uncertainty about the role of copper in the aforementioned materials.Hence.there is need to develop new experimental techniques for incorporating copper into zinc chalcogenides in order to provide additional information on the behaviour of this important impurity.It is necessary to gain a clear understanding of the effect of incorporating Lopper in ZnS on the electrical conductivity of ZnS.a.
Compounding of the Cu-doped ZnS alloy.
The materials used in this investigation were 99.99% pure copper II nitrate trihydrate (CuIN03)z.3H20) anu zinc sulphide (ZnS) powder obtained from the British Drug House (BDH).5 ml of different concentrations of aqueous solution of Cu(N03)2.3H20 were prepared and added drop by drop' to 100ml of ZnS suspension prepared in four different beakers and the stoichiometric composition of the samples is presented in Table 1.
Thereafter, the mixture was stirred continuously and precipitates were formed.The precipitates obtained were then filtered and air-dried overnight.The samples were later annealed in a stream of argon gas at a temperature of300°c for 5 hours and at an argon flow rate of 20ml min".After the annealing, the samples were cooled in an argon gas at room temperature.
Subsequently, they were crushed with a mortar and pestle and sieved through a mesh to obtain fine ground powders.Then thin pellets of the samples were formed from the finely ground powders of the synthesized materials by powder compression method using a vacuum pump aided powder presser.The pellets were then sintered.a t a temperature of 3 0 0 ' ~ for 4 hours in an electric-furnace to correct the imperfections that might have resulted from voids in the materials.The prepared pellet has a diameter of m m m and thickness of 0.38mm.Silver paste was used to make contacts on the samples.b.
Investigation . of the variation of Electrical Conductivity with Temperature.Each sample with contacts was inserted in a thin walled test.tube.The lower part of the test tube was immersed in a lagged heatable water bath.The water bath Was maintained at the desired temperature with the aid of a temperature controller, while uniformity of temperature was ensured with the aid of a magnetic stirrer immersed in the bath.The insulated electrical leads from the contacts were taken out of the test tube via ports which were vacuum sealed with araldite.They were connected to a digital electrometer   q is the electronic charge.
V is the applied voltage and 4 n is the ideality factor of the contact.temperatures for sample D These then move with higher velocities than their cooler neighbours.Therefore, the majority carriers at the hot probe diffuse out to the cold probe.This results in the hot region becoming slightly depleted of majority carriers and acquiring the potential of the ionized impurities there while the vicinity of the cold probe remains neutral.Current flows in the galvanometer, the direction of which depends on the sign of the charge of the ionized impurity.Since the hot probe was more negative with respect to the cold probe, it shows that the sample investigated behaves like a ptype semiconductor (Bar-Lev 1993).

CONCLUSIONS
The results obtained from this investigation show that: 1.The electrical conductivity of the Cu-doped ZnS alloy increases with temperature and obeys the Arrhenius relation, o = uo exp (-EdZkT) which is characteristic of semiconductors.

2.
The gradual decrease in the band gaps of intrinsic ZnS as a result of the incorporation of copper, facilitates the ease of electronic transition f r ~m the valence band to the conduction band 'which consequently enhances the conductivity of the material.
1000rr (K") Fig. 6.In a versus 1/T for pure ZnS and the various samples

(
Keithley 160B) and a digital millivoltmeter (Hewlett -Packard 3465A) which measured the current and voltage respectively.The actual sample temperature and that of the water bath were determined with copperconstantan thermocouples whose cold junctions were maintained at O' C. Effective A. I. MUKOLU, Department of Physics., University d;f~do-~kiti, Ado-Ekiti, Nigeria.K. A. ADULOJU, Department of Physics, University of Ado-Ekiti, Ado-Ekiti, Nigeria.
achieved by connecting both the sample thermocouple and ihe heater I'eads to the temperature A controller.All the measurements were carried out at a number of temperatures between 303 and 343K.At any desired temperature, the currents were $0.0 measured by varying the voltage from 0.01 to 0.10V.From the currentvoltage (I-V) data, the electrical conductivity of the sample was determined.This process was repeated for the va-;ous samples.c.Determination of the carrier type of Cu-doped ZnS 8.0.sample with the Hot Probe Method.The surface of each sample was touched by two between which a galvanometer was connected.One of the probes was heated whife the other was s at room temperature.Thereafter, the gal'vanometer was observed for the direction of current flow which determined the type of carrier.RESULTS ANDPISCLJ_S.SIQN.The result of measurements of the currentvoltr.gecharacteristics of the Cu-doped ZnS samples ar various 4.0 .temperatures are summarized in Figures 1 to 5. The I -V characteristics of the samples obey the following relation given 303 by Bethe (1942), Henisch (1957), Padovani and Stratton 313 (1966), Krupanidhi et al (1983): the effective Richardson's constant.ObLTAGE k is the Boltzmann constant.S is the.area of the contact;.

Fig. 1 .
Fig. 1.I -V characteristics of Pure ZnS at various temperatures.is the temperature.25.00 @b is the barrier height of the contact.x to4q is the electronic charge.V is the applied voltage and Fig. 2. I-V characteristics of Cu-doped ~nS'at various temperatures for sample A

Figure 6
Figure 6 shows the variation log of conductivity (In a: of .hesample with inverse of temperature (Ill).This plo displays a linear variation between In o and 1/T.indicating tha the different plots obey the Arrhenius relation: a = a0 exp (-E$2kT)

Table 1 :
Stoichiometric composition of the various samples of Cu-doped ZnS alloy.

Table 2 :
The Energy Gaps of pure ZnS and the various.samples of Cu-doped ZnS.