Solid State Communications, Vol. 48, No. 7, pp. 601-603, 1983. Printed in Great Britain.

0038-1098/83 $3.00 + .00 Pergamon Press Ltd.

AMORPHOUS FILMS OF CuS

Joy George and K.S. Joseph

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

(Received 14July 1983 by H. Kawamura)

Thin films of CuS have been prepared by reactive evaporation of copper in a sulphur atmosphere. It is found that films deposited on to substrates kept below 315 K are amorphous in nature. The amorphous films have a resistivity o f ~ 10 s ohm cm and are n-type. The films are golden yellow in colour and are fairly transparent before the onset of band to band tran- sitions. Optical studies give a band gap of 1.60 eV at room temperature (295 K).

THE COPPER-SULPHUR SYSTEM is rich in phases and there are four stable phases at room temperature. These are, in the copper rich region, chalcocite (Cu2S), djur- leite (Cul.gsS), anilite (Cul.TsS), and in the sulphur rich region, covellite (CuS). Of these chalcocite and djurleite has been studied in some detail in bulk and thin film form and are p-type semiconductors. Interest in these films are due to its application in CdS : Cu2S solar cells. It has been shown that the efficiency of these solar cells depend on the phase and composition of the Cu2S layer [1 ]. The phase, covellite is the least studied in the Cu-S phase system. Okamoto et al. [2] concluded from elec- trical resistivity measurements that CuS is metallic in nature. Hullinger and Mooser [3] from theoretical con- siderations show that in intermetallic compounds where d-orbitals as well as s or p-orbitals concern to the valen- cies of the compositions, semiconductivity occurs if cation d electrons are assumed to be localized. Magnetic susceptibility data given in [2] indicates the presence of partially localized 3d electrons in CuS. Recently poly- crystalline films of CuS have been prepared by the authors [4] and optical studies indicated the presence of a band gap of 2.37 eV establishing that CuS is a semi- conductor. It can then be said that the concept of the semiconducting bond introduced by Mooser and Pearson [5] holds in the case of CuS also. Results of Okamoto et al. [2] may be due to the extreme degeneracy of the pressed samples used.

The films were prepared by the reactive evaporation technique and it was found that films prepared below a substrate temperature of 315 K are amorphous in nature. We here report data on the optical properties of these films in the range 300 to 2500 nm.

Direct evaporation of a compound often leads to non-stoichiometric films because of the decomposition of the components etc. This technique also involves the

problem of preparing the bulk material which is difficult if the volatile component is to be added in much excess than the stoichiometric ratio, to get high resistivity material. Reactive evaporation overcomes these diffi- culties and is practicable when one of the components of a compound has a high vapour pressure. The tech- nique consist of evaporating the less volatile element in an atmosphere of the more volatile element. In this fashion it is also possible to get any desired excess partial pressure of the more volatile component. Films obtained by this technique are usually amorphous in nature, when deposited on to room temperature substrates.

Evaporation was carried out in a conventional vacuum system with an oil diffusion pump. Two times electrolyzed copper and three times recrystallized sulphur were used as evaporants. A glass crucible placed in a conical basket of molybdenum wire was used as the sulphur source. A molybdenum boat was used to evaporate copper. The copper source was covered with stainless steel heat shields to minimise substrate heating. In this way we could control the temperature of the substrate within 3 K.

With the substrate temperatures and partial pressures used in these experiments, the supersaturation of sul- phur molecules will not be sufficient to form a film by itself. Only the metal/compound film form at this tem- perature. It is evident that if sufficient sulphur molecules are present for reaction, only the compound film is formed.

It has been found that the following deposition conditions give CuS films:

metal atom flux = 1 -2 x 1014 atomscm -2 sec -l

chalcogen flux = 1 -2 x 1016 molecules cm -2 sec -1

substrate temperature = 295-315 K amorphous ffdrns 325-440 K crystalline films.

601

102 101 1 110

HO

101102 ~ ~ _ b

I I I

30" ae 50"

Fig. 1. X-ray diffractograms of films prepared at various substrate temperature T s ; (a) 440 K, (b) 345 K, (c) 305 K. The amorphous nature of the film prepared at 305 K is evident. The background is due to the glass substrate.

8O

60

~4o

i I ) I 500 1500 Mnm)

Fig. 2. Transmission spectra of a film of thickness 390 nm.

2500

4x~o s

3

u

1

The amorphous films thus prepared show a golden yellow colour and the crystalline Films a deep green colour. The deposition rate of the films were 0.2-0.5 nm see -~ and the films used in this study was prepared at a substrate temperature of 305 K. The thickness of the films was around 400 nm.

Composition of the films were determined by taking the X-ray diffraction pattern of the films prepared at various substrate temperatures. For these measurements Films were not detached from the substrate. A Philips PW 1140/90 X-ray diffractometer with Bragg-Brentano geometry was used.

Optical measurements were carried out using a Cary 17D double beam spectrophotometer. Absorption coefficient ct, extinction coefficient k and refractive

1.5 2.0 2.5 3.0 tw(ev)

602 AMORPHOUS FILMS OF CuS Vol. 48, No. 7

Fig. 3. Plot of absorption coefficient a vs hv in the fundamental absorption region. The intercept gives an optical band gap of 1.60 -+ 0.02 eV. Temperature of measurement 295 K.

index n of the films were determined from the trans- mission spectra by the method of Manifacier et al. [6]. Thickness of the films were determined by multiple beam interferometry.

The X-ray diffraction pattern of the films prepared at various temperatures are shown in Fig. 1. It can be seen that the films prepared at 345 and 440 K are cry- stalline in nature and shows only the lines of covellite phase (CuS) [7]. Films prepared at room temperature (305 K) show the diffused lines of covellite phase which is indicative of the amorphous nature of these films. The diffused background is due to the glass substrate.

Electrical measurements show that the films are fairly resistive (p = l0 s ohm cm)and show negative temperature coefficient of resistance [8]. All the films prepared show n-type conductivity.

Figure 2 shows the transmission spectra of a film of thickness 390 nm. It can be seen that the film is fairly transparent before the onset of band to band transitions except for a slight decrease in transmission around 1200 nm. Electrical data given above and the transmission measurements establish that films are semiconducting in nature.

In many amorphous materials in the photon energy region where ~ "~ 10 s cm -1, the absorption coefficient is found to obey a law of the form [9]

, ~ , ~ ( m , - E ~ ) r . ( 1 )

Values of r between 1 and 3 have been observed. The constant Eg can be used to define an optical gap,

Vol. 48, No. 7 AMORPHOUS FILMS OF CuS 603

although it may represent an extrapolated rather than a real zero in the density of states.

The plot of ot vs hv is shown in Fig. 3. The value of E e obtained (1.60 eV) is in good agreement with the transmission data. The plot of (ahu) ~/2 versus hv, which is the more commonly observed relationship (r = 2), gives an energy gap of 1 eV which obviously is not in agreement with the transmission data. The unity value obtained for r indicates a sharp rise in the density of states at the band edges. This type of behaviour (ct (by --Ee) ) was earlier reported in the case of amor- phous selenium [ 10].

In amorphous materials when a law of the form given by equation (1) holds, transitions are believed to take place between the extended states of the valence band and the extended states of the conduction band and hence the band gap in the crystalline and amorphous phases must be approximately equal [11 ]. But it can be seen that in the case of covellite, the value of the band gap for the crystalline material is 0.77 eV higher than that for the amorphous material. This high value may be due to the top of the valence band being empty of elec- trons and transitions taking place from deeper lying levels in the valence band to the conduction band. This emptying of the valence band is highly probable because of the high p-type conductivity (/9 = 10 -4 ohm cm) exhibited by the crystalline samples.

Films of CuS can be prepared by the reactive evaporation of copper in a sulphur atmosphere. Films deposited on to substrates kept below 315 K are amor-

phous in nature. The amorphous f'rims show n-type conductivity and are semiconducting. The films are fairly transparent before the onset of band to band transitions. Absorption coefficient in the range l0 s cm -l obeys a law of the form ~ o: (hu -- Ee) which shows the presence of sharp band edges.

Acknowledgements - One of the authors (KS J) would like to thank C.S.I.R., New Delhi, for the award of a research fellowship.

REFERENCES

1. W. Palz, J. Besson & J. Vedel, Proc. lOth IEEE Photovoltaic Specialists Conference, Palo Alto 1973, IEEE, New York (1973).

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3. F. HuUinger & E. Mooser, J. Phys. Chem. Solids 24, 283 (1963).

4. J. George & K.S. Joseph, unpublished results. 5. E. Mooser & W.B. Pearson, Progress in Semicon-

ductors 5, 103 (1960). 6. J.C. Manifacier, J. Gasiot & J.P. Fillard, J. Phys.

E9, 1002 (1976). 7. JCPDS card No. 24-60 (1974). 8. J. George & K.S. Jospeh, unpublished results. 9. M. Zavetova & B. Velicky, Optical Properties o f

Solids New Developments, (Edited by B.O. Sera- phin) p. 386. North Holland, Amsterdam (1976).

10. E.A. Davies, J. Non. Cryst. Solids 4, 107 (1970). 11. J. Tauc, Optical Properties o f Solids, (Edited by

F. Abels) p. 279. North Holland, Amsterdam (1972).