Solid State Communications, Vol. 49, No. 1, pp. 103-106, 1984. Printed in Great Britain.

0038-1098/84 $3.00 + .00 Pergamon Press Ltd.

ELECTRICAL CHARACTERIZATION OF TIN DISULPHIDE CRYSTALS

Joy George and C.K. Valsala Kumari

Solid State Physics Laboratory, Department of Physics, University of Cochin, Cochin 682022, Kerala, India

(Received 12 September 1983 by H. Kawamura)

Tin disulphide crystals have been grown by the physical vapour transport method and the electrical conduction mechanism in these crystals using MIM structures is reported. The conduction is found to be space charge limited. Trap concentration, trap depth, free carrier mobility and Fermi level etc. have been determined. Dependence of the current on tempera- ture in the ohmic region gives an activation energy of 0.40 + 0.05 eV.

1. INTRODUCTION

THE LAYERED CRYSTAL tin disulphide crystallizes in the CdI2 structure. In this compound each tin atom is surrounded by six sulphur atoms in an octahedral co- ordination. In the layers the atoms are held together by covalent bonding and in between the layers there is V~m der Waal's bonding. Because of this type of bond- ing, the crystals show highly anisotropic properties.

Tin disulphide single crystals are usually grown by chemical vapour transport method [ 1, 2]. Recently these crystals were grown using physical vapour trans- port by the present authors [3]. Only Said and Lee [4], Patil and Tredgold [5] have reported the electrical conduction mechanism of tin disulphide crystals and their investigations were on crystals grown using iodine as the transporting agent. Crystals grown using transport- ing agent are highly contaminated and this usually has a pronounced effect on the electrical properties. Resis- tivity and mobility generally decreased with increasing iodine content.

In the present work SnS2 crystals were grown by the physical vapour transport method and a detailed study was made on the electrical properties of these crystals. The current voltage characteristics of the crystals in the direction parallel to the c-axis have been investigated at different temperatures using MIM struc- tures and the trap concentration Art, trap depth Et, free carrier mobility #o and the Fermi level E F have been determined.

2. EXPERIMENTAL

Tin disulphide crystals were grown by the physical vapour transport method. Quartz ampoules of 17 cm length and 1.6 cm diameter were used. Stoichiometric proportions of tin (99.999%) and sulphur (three times recrystaUized from solution) were sealed in the quartz tube at a pressure of 10 -s Torr. The ampoules were

placed in a horizontal two-zone furnace (TI = 700°C, T2 = 630°C). Temperature of the zones were con- trolled to an accuracy of -+ 1 °C using temperature controllers specially fabricated in our laboratory for this purpose [6]. The growth time varied from 4 0 - 150 hours. The crystals grown were golden yellow in cOlour and of area 2 cm 2 and thickness ranging from 10 to 60 #m.

To study the current-voltage characteristics aluminium electrodes of known area were deposited by vacuum evaporation on opposite faces of the crystals. All measurements were carried out in an evacuated (pressure 10 -2 Torr) all-metal cell with provision for temperature measurement and heating. The temperature of the crystal during measurement was controlled to an accuracy of +- 0.5°C by controlling the current through the heater. The crystals were held at the desired temperature for a sufficiently long time before the measurements were made. Current was measured using a digital picoammeter (ECIL, Model EA5600) and the voltage using a digital multimeter (HP, Model 3465A). Current and voltage were measured with an accuracy of 0.5%. Sufficient time was given for the current reading to stabilize and extreme care was taken to avoid specimen heating. Very reproducable results could be obtained on all the specimens studied.

3. RESULTS AND DISCUSSIONS

The current-voltage characteristics of a typical MIM structure is shown in Fig. 1. It can be seen that at low voltages the current is proportional to voltage (ohmic law region) followed by a square law region at high fields. This region is called the shallow trap square law region. The square law region is followed by another region where current increases sharply with voltage (I oc V n; n >1 3) and this is due to the fdhng up of the traps. This region is followed by yet another square

103

104 ELECTRICAL CHARACTERIZATION OF TIN DISULPHIDE CRYSTALS Vol. 49, No. 1

16 2

I0 3

g :E <--,_4 HIO

_.~ 10 - l~<V

I I

10- I ' lo 0

2 I~V t

/ / / /P

i /

/ t 1 /

/

1 / /

kt•lFL I

I I

I I I

i I lo 1

V(VOLT)

I lo 2

Fig. 1. I - V characteristics for a typical AI-SnS2-A1 structure (thickness d = 20/am, T = 298 K electrode area = 0.125 cm2).

law region called trap free square law region. These observations, where an ohmic region is followed by a square law region indicate a charge injection into the semiconductor with trapping centers, which may be interpreted in terms of space charge limited conduction [7, 8] and the current obeys a law of the form

V 2 J = Ouoe-U, (1)

where J is the current density, 0is the fraction of total carriers which are free,/ao is the charge carrier mobility, e is the dielectric constant, Vis the voltage and d is the specimen thickness. The log I vs log d plot (Fig. 2) in the first square law region shows the d 3 dependence estab- lishing that the conduction mechanism is space charge limited.

0 in equation (1) is the ratio between the free electrons no in the conduction band to the total elec- tron density (no + n t ) , n t being the density of the trapped electrons. Experimentally 0 is determined from the equation

no J1 0 = - - - , (2)

no + n t J2

10 -3

zr ,( v I . -4

lO . 4

lO -5

16 6

~ ~ ~ \ \~\\I'c °~ ~3

I I 10 100

cl (,4.trn)

Fig. 2. Current vs thickness in the space charge limited region, slope = - - 3 . ( V = 5 volt, T = 298 K).

where J1 and J2 are the current densities at the start and end of the sharp rise o f the current (Fig. 1). Substituting the experimentally determined value of 0 = 0.205 and e = 7.2 [9] in equation (1), we get the value of free carrier mobility tao as 6 cm 2 V -I sec -1. It may be men- tioned that for the crystals grown by chemical transport technique, mobility parallel to the c-axis is of the order of 10-4cm 2 V -l sec -1 [10].

From the threshold voltage Vx where the ohmic current cross over to space charge limited current in the absence of traps (obtained by extrapolating the trap free square law region to meet the ohmic region) enables us to evaluate the density of thermally generated free carriers no. The value of no is given by

0e no = 1.8 x 1 0 - 6 d 2 Vx" (3)

The density of thermally generated free carriers was found to be no = 2.3 x 1011 cm -3. Using the values o f no and 0 in equation (2), we get the density of trapped carriers n t = 9.1 x 1011 cm -3.

The conductivity e is given by

o = noelao, (4)

where e is the electronic charge. Using the values of no and #o, o was found to be equal to 2.2 x 10 -7 ~ - 1 cm-1 .

This calculated value of o agrees very well with the

Vol. 49, No. 1 ELECTRICAL CHARACTERIZATION OF TIN DISULPHIDE CRYSTALS 105

E < "~165

16 6

a b c d e

I

lo o d lo 2 V(VOLT)

Fig. 3. Current voltage characteristics at various tem- peratures T(K): (a) 424, (b) 402, (c) 382, (d) 358, (e) 341 : thickness of the crystal = 25/am.

io I

10 0

Io-I

I I 2.3 2.6 2.9

1/T(10-3 K "1 )

Fig. 4. log 0 vs 1/T, Et = 0.10 eV.

experimental value of 0 obtained from the ohmic regions. The increase in current after the shallow trap square law region was due to the filling of traps (Fig. 1). The trap filled limit voltage measures the fraction of the total concentration of traps that is empty in thermal equilibrium. The trap concentration is calculated using

1.1 x 106 e N t = d2 VTFL, (5)

where Va, Fr. is the trap filled limit voltage. Using the experimental value of VTrL, Nt was obtained as 2.3 x 1013 cm -a.

The temperature dependence of the I - V charac- teristics is shown in Fig. 3. It can be seen that the current increases with the increase of temperature for the same applied voltage. The temperature dependence of/9 is given by

0 = ~ t exp ~ , (6)

where Are is the effective density of states in the conduc- tion band, E t is the trap depth, k is the Boltzmann constant and T is the absolute temperature. Using equation (2), the values of 0 for various temperatures were calculated from the temperature dependence of I - V characteristics and log 0 vs 1/T was plotted (Fig. 4).

10 -~

lO -=-

< IG ~

1J-

I I 2.3 :).6 2.9

1/T(lO-3K -1)

Fig. 5. log Ivs l /T, in the ohmic region ( V = 800 mV) Ea = 0.40 eV.

106 ELECTRICAL CHARACTERIZATION OF TIN DISULPHIDE CRYSTALS Vol. 49, No. 1

From the slope of the straight line, the value of the trap depth E t was found to be equal to 0.10 + 0.01 eV. From the intercept at T -l = 0 K -1, N c was obtained to be equal to 5.1 x 1017 cm -3. Said and Lee [4] have also reported a trap depth at 0.14 eV in tin disulphide.

The Fermi level EF measured from the bottom of the conduction band is given by

n o

From the plot of log no vs I/T, EF was determined and was equal to 0.15 eV.

The thermally activated electrical conductivity is

o = o0 exp , (8)

where Oo is a constant and E a is the activation energy. The temperature dependence of current in the low field region (V = 800 mV) for different temperatures is shown in Fig. 5. From the slope, the value of activation energy was obtained as 0.40 --- 0.05 eV. Said and Lee [4] has reported an activation energy of 0.40 eV in SnS2 crystals, which is in agreement with the present value.

4. CONCLUSION

Electrical conduction mechanism is studied using MIM structures in single crystals of tin disulphide grown by physical vapour transport method. It is found that

the conduction is space charge limited. A trap level exists at a depth of 0.10 -+ 0.01 eV below the conduc- tion band and the trap concentration is found to be 2.3 x 1013cm -3.

Acknowledgements - The authors would like to thank the Council of Scientific and Industrial Research, New Delhi for financial support.

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