Electrical conductivity, thermoelectric power and optical absorption studies of amorphous mercury selenide films

Thin Solid Films, 144 (1986) 41-48

41

ELECTRONICS AND OPTICS

ELECTRICAL CONDUCTIVITY, THERMOELECTRIC POWER AND O P T I C A L A B S O R P T I O N STUDIES O F A M O R P H O U S M E R C U R Y S E L E N I D E FILMS K. N. SHARMA AND K. BARUA

Department of Physics, Dibrugarh University, Dibrugarh 786004 (India) (Received October 10, 1985; revised March 26, 1986; accepted April 22, 1986)

Amorphous films of mercury selenide (a-HgSe) of various thicknesses (350-500 nm) were formed by vacuum evaporation of the compound onto glass slides maintained at 268-271 K. The aging effect at about 300 K was studied. The electrical conductivity of these films was measured after annealing in successive heating and cooling cycles. The thermoelectric power (TEP) of polycrystalline films varied from - 6 3 to - 9 4 ~ t V K -1 within the temperature range 259-383 K. Annealing of the a-HgSe films from 300 to 535 + 2 K resulted in a change in the T E P at about 300 K from - 0.4 mV K - 1 to - 60 ~tV K - 1. The activation energy for conduction and the optical energy gap were found to be 0.78 eV and 1.41-1.68 eV respectively.

1. INTRODUCTION

The electrical, optical and thermoelectric properties of amorphous chalcogenides have been discussed by Mott and Davis x. Elpat'evskaya and Regel 2 have reported that the red transparent films of HgSe prepared on glass substrates in the temperature range 283-353 K had low mobilities. From their diffuse X-ray diffraction patterns it was concluded that the films were almost amorphous with a crystalline size of about 10 nm. Lomas e t al. 3 have reported that red transparent films of HgSe formed by vacuum evaporation of the compound onto freshly cleaved mica substrates maintained at about 281 K were amorphous when examined using reflection electron diffraction and were of p-type conductivity. The galvanomagnetic properties of polycrystalline HgSe (c-HgSe) films have also been reported a-6. Thus it is seen that there has been no detailed study of the electrical, thermoelectric and optical properties of a-HgSe films. We therefore undertook a study of the conduction mechanism, the optical absorption in the visible region and the thermoelectric properties of amorphous HgSe (a-HgSe) films and we compare some of the data with those for c-HgSe films. We have previously reported 7 transmission electron micrographs and electron diffraction patterns on a-HgSe and c-HgSe films together with the crystallization kinetics. 0040-6090/86/$3.50

© ElsevierSequoia/Printedin The Netherlands

42 2.

K . N . SHARMA, K. BARUA

EXPERIMENTAL DETAILS

2.1. Preparation offilms Red transparent films of HgSe 380-500 nm thick were deposited by vacuum evaporation of the compound (99.999% pure; Koch-Light, U.K.) from a microconical silica basket onto ultrasonically cleaned glass substrates maintained at 268-271 K. Films of thickness less than 380nm were found to be electrically discontinuous, whilst films of thickness greater than 500nm always developed microcracks on heating above 338 + 2 K. The pressure during the deposition was 4 x 10 5 Torr. The temperature of the source was about 490 K and the rate of deposition was 15.6-18.6 nm min 1. The source-to-substrate distance was 15 cm. Samples of size 0.5 cm × 1 cm were obtained either by cutting a single large area (2.5 cm × 7 cm) of the film or by using masks of appropriate dimensions on smaller glass slides. Polycrystalline HgSe films of thickness 240-265 nm were prepared on glass slides maintained at 370 K. The source temperature was about 520 K and the rate of deposition was 7.8-9.0 nm min 1. Samples of size 3 c m x 1 cm were obtained using masks. The films were annealed in situ at 383 _+2 K for 40 min. The film thickness was determined using the multiple-beam interference method.

2.2. Aging effect To study the aging effect films were deposited to cover the gap between two predeposited aluminium electrodes to which the electrical contacts had been established using conducting silver paint. The films were kept in dry air at about 300 K and then the resistance was measured with time.

2.3. Conductivity measurement For the measurement of d.c. electrical resistance the potential difference across the film was measured using an electrometer amplifier (EA 813, ECIL, India), with an input resistance of 10 x4 ~, while the current was measured using an electronic volt-amp6re-ohmmeter (PK 2503, Philips). A maximum potential difference of 2 V was applied. The effective area of the film was 0.5 cm x 0.3-0.4 cm. The heating rate was 1.0-1.2 K m i n - 1 and the range of temperatures studied was 259-385 K. The electrical properties could be obtained reproducibly when the films were annealed at about 347 K. Thus the electrical resistance was measured only during the heating cycle. A chromel-alumel thermocouple was used to measure the temperature of the films. The tip of the thermocouple was placed so as to touch the film side of the substrate, and the thermo-e.m.f, developed was measured to an accuracy of _+2 ~tV.

2.4. Thermoelectric power measurements To establish good electrical contacts and to avoid any possible interaction of the film material with the contacts, platinum foils were placed at the film ends. Two brass blocks, one on each end, were then placed over the platinum foils and pressed by means of screws. The distance between the hot and cold junctions was about 3 cm. Thin silver wires were used as measuring leads. To establish a thermal gradient

PROPERTIES OF

a-HgSe FILMS

43

along the length of the film a microheater was attached to one of the brass blocks to be used as the hot end of the film. The temperatures of the junctions and that of the film were measured using chromel-alumel thermocouples. The temperature difference between the hot and the cold junctions was usually 10-15 K. The film temperature was recorded using the thermocouple placed at the middle of the sample. The thermo-e.m.f, of the film was measured using a microvoltmeter (pp 9004, Philips) and those of the thermocouples were measured using another microvoltmeter (Aplab India, model 5005). The assembly was kept inside a vacuum jacket and during the conductivity and T E P measurements the pressure was about 3 × 10- 3 Torr. For measurements at temperatures below r o o m temperature (300 K), the vacuum jacket was inserted into a freezing mixture and for measurements above r o o m temperature the vacuum jacket was kept inside a cylindrical furnace for which the temperature was controlled up to + 2 K.

2.5. Optical absorption measurements The transmittance at normal incidence was recorded using a spectrophotometer (Systronic, India, type 105) within the wavelength range 340-960 nm. The 100~o transmittance was adjusted using an uncoated glass slide, similar to the substrate, for each reading at a different wavelength. 3. RESULTS

3.1. Aging effect The film resistance increases, irrespective of the thickness, at a rapid rate within the first 10 days after deposition and then at a slower rate, approaching a constant value after about 26 days. Figure 1 shows a plot of the d.c. conductivity a against time for two typical films at about 300 K. Films aged in this manner were used for electrical measurements.

3.2. Electrical conductivity After natural aging the films were annealed at first for 30 min at different temperatures. The electrical conductivity a was recorded at about 300 K before and after annealing. Figure 2 shows plots of In a against 1/T for three typical films annealed at 383 ___2, 341 + 2 and 347__+2 K. After annealing, tr was found to decrease irreversibly and the conductivity data could be repeated if the measurements were carried out within the limit of the annealing temperatures. The plots show two regions of activation, the temperature ranges of which are from 259 to 343 __+2 K (region AB) and from 356 + 2 to 385 K (region CD). It may be mentioned that within these two regions the tr values can be repeated provided that the temperature does not exceed the limit mentioned above. The activation energies were calculated by equating the slopes to AE/k. It is seen that the activation energy in the region C D is constant and is about 0.78 eV. After heating the films beyond 385 K, their conductivities decreased irreversibly. Optical micrographs of such films showed the appearance of microcracks over the surfaces. Plots 1 and 2 are for two c-HgSe films after the first annealing cycle. It is seen that tr remains almost constant within the temperature range 259-383 K. The conductivity was found to be

44

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i

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S H A R M A , K. B A R U A

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,

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25

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)-

8 Time

~

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(days)

35

28

2.5

3.0

35 (X10-3 K 1)

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Fig. 1. Electrical conductivity cragainst time of aging at about 300 K for films of thicknesses 415 nm (O) and 786 nm ((3). Fig. 2. Plots of In a vs. lIT for films annealed at 338 + 2 K (O), 341 _ 2 K (©) and 347 + 2 K ( + ) for 30 min (film thickness about 417 nm) and for polycrystalline films of thickness 240 nm (~) and 265 nm (A). The activation energies for different regions are shown in the figure. thickness dependent. In subsequent heating cycles the value of cr did not change appreciably. 3.3. Thermoelectrzc power Figure 3 shows plots of the T E P Q versus 1/T for two a m o r p h o u s unannealed films aged for 10 days (regression lines 1 and 2) at a b o u t 300 K and for two polycrystalline films (curve 3). The a m o r p h o u s films were of average thicknesses 420 and 434 n m obtained in two independent evaporations, and the polycrystalline films were of average thicknesses 240 and 286 nm. Ten a m o r p h o u s films were studied and the regression lines for these films lie within lines 1 and 2. F o r all the a m o r p h o u s films, it is observed that when the temperature was less than 353 K the thermoe.m.f, values fluctuated and the values of Q appeared to be scattered. However, at higher temperatures the Q values were found to decrease steadily with increasing film temperature. F o r all the films the cold end was always negatively polarized. In case of c-HgSe films the T E P varies from - 6 3 to - 9 4 ~ t V K - 1 within the temperature range 2 5 9 - 3 8 3 K . The Q values increased with an increase in temperature, first at a lower rate for the temperature range 260-340 K, and then at a higher rate in the range 340-384 K. 3.4. Optical absorption The extinction coefficient kf was determined using the relation

T=

To e x p (

4 ~ rd-)

(1)

a-HgSe

PROPERTIES OF

FILMS

45

where k is the wavelength of light used and d is the film thickness. TOwas taken as 100 and Twas taken as the reading on the instrument. The absorption coefficient ct was then calculated using the relation (2)

o~ = 4 n k r / 2

F o r the determination of the optical energy gap Eop, (cthco) 1/2 is plotted against h~. The value of Eop was determined by extrapolating the steep part to (othco)1/2 = 0. Figure 4 shows such plots for two typical a m o r p h o u s films of thicknesses 386 and 420 nm. The value of Eop was between 1.41 and 1.68 eV for all the films studied.

'tOO -1-(

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Fig. 3. Regression line plots of Q vs. 1/T for two unannealed a-HgSe films of thickness 402 nm (curve 1) and 434 nm (curve 2) and for polycrystalline films of thickness 240 nm ( + ) and 286 nm (A) (curves 3). Fig. 4. Plot of (htoct)z/2 vs. hto for a-HgSe films of thickness 386 nm (O) and 420 nm (0). 4. DISCUSSION A decrease in tr with time of aging has been observed s for a-ZnTe films kept in an oxygen atmosphere at a b o u t 300 K. This was attributed to a reduction in the n u m b e r of "dangling bonds", which are generally t h o u g h t to be present in most a m o r p h o u s films. Thus in the present investigation the decrease in tr m a y be partly due to aging and partly due to the exposure of the films to the atmosphere during the process of handling. The decrease in tr on annealing could be due to the reduction in the n u m b e r of dangling bonds as well as to the annealing of defects in the as-deposited films, whereas the increase in a with an increase in the annealing temperature above 338 K could be attributed to recrystallization effects. The conductivity varies exponentially with temperature and m a y be fitted to the relation tr = tr 1 exp --

(3)

where AE is the conductivity activation energy and tr 1 is the pre-exponential factor. The plots in Fig. 2 show that the activation energy is temperature dependent. This is in agreement with results obtained for a-ZnTe 8, a - G a A s 9 and a - G a S b 10 films. Thus the c o n d u c t i o n mechanism at relatively low temperatures below 345 K m a y be of the

46

K. N. SHARMA, K. BARUA

hopping type. The linearity in the plot of log a versus 1/T at temperatures higher than 356 K suggests band-to-band intrinsic conduction 9 1 The nearly temperature-independent nature of the conductivity for c-HgSe films can be attributed to the semimetallic nature of the deposits 4'6. The thickness dependence of tr for single-crystal films may be attributed to the varying effective carrier concentration that arises during the deposition process, even though the bulk starting material was the same for all the films. Since the T E P was negative for all the films, the predominent carriers were electrons. This is in agreement with previously published results on bulk single crystals 2'12-14 and for polycrystalline films of HgSe 5,6. Using the relation 15

k/AEq A)

where q is the electronic charge, k is the Boltzmann constant, A is a constant, AEq is the T E P activation energy, and assuming the dominant carriers to be the electrons, the values of AEq and A can be calculated. Figure 3 (lines 1 and 2) shows the leastsquares plots for films of thickness 402 and 434 nm. The values of A were - 20.12 and - 18.57 and those of AEq were 0.64 eV and 0.60 eV for films of thickness 402 nm and 434 nm respectively. The plots of Q against l I T appear to be similar in nature to those for a m o r p h o u s glasses such as As2S3 ~1 within the temperature range 300-400 K. The conductivity activation energy AE was found to be smaller than the T E P activation energy AEq of a-HgSe films. This can be attributed to the thermally activated hopping of the carriers at the band edge 11. Although the predominant charge carriers were electrons, the presence of some acceptor levels due to holes could not be entirely ruled out. Since the electron mobility is greater than the hole mobility, the net effect due to the presence of holes is a rapid decrease in the T E P as the temperature increases. Moreover, in such ambipolar conduction, the absolute value of Q may very well become low with an increase in the film temperature. This explains the results for unannealed a-HgSe films observed experimentally in this work. The annealing of a-HgSe films up to a temperature of 535 __+2 K in the present investigation resulted in a change in Q at about 300 K from - 0 . 4 mV K - 1 to 60 gV K - 1. Beyer and Stuke 9 have observed similar effects in a-Ge and a-GaAs. The gradual decrease in the value of Q at about 300 K with increasing annealing temperature can be attributed to a reduction in the number of metastable states. These energy states arise because of the technique of preparation of the films, which gives rise to a large number of dangling bonds. It therefore seems possible to assume that the hopping states are created by these dangling bonds and also by the disorder of the film itself. As shown in Fig. 3 (plot 3), the variation in Q with temperature for c-HgSe films was from - 63 to - 94 laV K - 1 for a total change in temperature from 259 to 383 K. A similar temperature dependence of the T E P for c-HgSe films prepared on mica has been reported 6. However, these researchers observed a change in Q from - 4 0 to - 160 g v K 1 for a total change in temperature from 77 to 500 K. The difference between the value of Q in the present investigation and the earlier value can be -

PROPERTIES OF

a-HgSe FILMS

47

explained as due to (1) the difference in the range of temperatures of observation and (2) the difference in the nature of the substrates. Gobrecht et al. 12 and Blue and Druse 13 have found that c-HgSe films behave as n-type degenerate semiconductors and that Q should vary linearly with temperature according to the relation 16

~2 k k T Q-

3 Iql ~ ( l + Y )

(5)

where y is defined as the parameter describing the dependence of the mean free path l of the carriers on their energy and the other symbols have their used meanings, y is given by the relation 1= E~

(6)

Ideally, ~ = 0 and 2 for lattice and ionized impurity scattering respectively. However, ionized impurity scattering cannot account for the increase in T E P with temperature above 340 K, because this type of scattering becomes stronger at low temperatures, much lower than the lowest limit of the present observation. Furthermore, the value of Q should have been larger than the observed values. Thus the predominant scattering may be due to lattice vibrations together with a parallel mechanism of enhanced scattering. Davis and Mott 17 have observed that the mobility gap of most amorphous material is greater than twice the electrical activation energy AE obtained from a plot of In a against l i T . This criterion is found to be satisfied in the present investigation since AE in the high temperature region is about 0.78 eV, while the optical energy gap Eop is 1.41-1.68 eV. It may be noted that Sorokin ~8 has suggested that the band gap of HgSe is about 0.7 eV. A similar relation for the optical band gap (Eop >~ 2AE) has also been observed for a-GeSe films 19. 5. CONCLUSION

The transport in a-HgSe films occurs by hopping at temperatures below 345 K and by band-to-band intrinsic conduction at temperatures higher than 356 K. The type of conduction is electronic. The temperature coefficient of resistance of annealed c-HgSe films on glass is nearly zero in the temperature range 259-370 K; thus these films may be used as thin film resistors. The optical energy gap of a-HgSe films is nearly twice the thermal activation energy. ACKNOWLEDGMENT

One of us (K.N.S.) is thankful to the University Grants Commission, New Delhi, for the award of a Teacher Research Fellowship. REFERENCES 1 N . F . Mott and E. A. Davis, Electronic Processes in Non-crystalline Materials, Clarendon, Oxford, 1979. 2 O . D . Elpat'evskaya and A. R. Regel, Soy. Phys. Tech. Phys., 2 (1957) 35. 3 R.A. Lomas, R. D. Tomlinson and M. J. Hampshire, Thin Solid Films, 13 (1972) 52.

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K.N.

SHARMA, K. BARUA

D.C. Barua, Ph.D. Thesis, Dibrugarh University, Assam, 1974. V.D. Deokar and A. Goswami, Ind. J. Pure App. Phys., 8 (1970) 93. A. Goswami and S. M. Ojha, Indian J. Pure Appl. Phys., 13 (1975) 721. K.N. Sharma and K. Barua, Indian J. Pure Appl. Phys., 21 (1983) 116. J.B. Webb and D. E. Brodie, Can. J. Phys., 52 (1974) 2240. W. Beyer and J. Stuke, J. Non-Cryst. Solids, 8 (1972) 321. B.S. Naidu and P. J. Reddy, Thin Solid Films, 61 (1979) 379. M. Datta, P. N. Banerjee, D. L. Bhattacharya, S. S. Prasad and K. P. Srivastava, Indian J. Pure Appl. Phys., 18 (1980)568. H. Gobrecht, U. Gerhardt, B. Reinemann and A. Tausend, J. Appl. Phys., Suppl., 32 (1962) 2246. M.D. Blue and P. W. Druse, J. Phys. Chem. Solids, 23 (1962) 577. S.L. Lehoczky, J. G. Broczman, D. A. Nelson and C. R. Whitsett, Phys. Rev. B, 9 (1974) 1598. H. Fritzche, SolidState Commun., 9 (1971) 1813. R.R. Heikes, in R. R. Heikes and R. R. Ura (eds.), Thermoelectricity, Interscience, New York, 1961. E.A. Davis and N. F. Mott, Philos. Mag., 22 (1970) 903. O.M. Sorokin, Zh. Tekh. Fiz.,28(1958) 1413. V.R. Katti, P. A. Govindacharyulu and D. N. Bose, Thin Solid Films, 14 (1972) 143.