Anomalous rise of photocurrent in amorphous thin films of Ge22Se78

So~'id State Communications, Vol. 64, No. 3, pp. 371-374, 1987. Printed in Great Britain.

0038-1098/87 $3.00 + .00 © 1987 Pergamon Journals Ltd.

A N O M A L O U S RISE O F P H O T O C U R R E N T IN A M O R P H O U S T H I N FILMS O F Ge22Se78* S. Goel and A. Kumar Department of Physics, Harcourt Butler Technological Institute, Kanpur - - 208 002, India

(Received 20 January 1987; in revised form 30 April 1987 by M. Balkanski) Transient photoconductivity measurements have been made in vacuum evaporated thin films of Gez2Se78. It has been observed that, under certain experimental conditions, the rise of photocurrent shows an anomalous behaviour. Before attaining the steady state, the photocurrent passes through a maximum. This anomalous behaviour has been studied at different temperatures by shining monochromatic light of different wavelengths and intensities. The results have been explained in terms of recombination mechanism in this material. 1. I N T R O D U C T I O N R E C E N T L Y , C H A L C O G E N I D E GLASSES have drawn great attention of scientists because of their basic and applied aspects in various solid state devices. A c o m m o n feature of these materials is the presence of localized states in the mobility gap due to inherent defects and absence o f long range order. As the photocurrent is controlled by carrier localization and delocalization processes, the transient photoconductivity measurements are expected to give informations about the localized states in these materials. The present Communication reports on the time rise of photocurrent in amorphous thin films of Ge22 Se78 prepared by vacuum evaporation. The measurements have been made using monochromatic light o f different wavelengths (420 to 660 nm) at various temperatures (302 to 352 K) and intensities [1 to 210 arbitrary units (a.u.)]. We find an anomalous behaviour in the rise of photocurrent under certain experimental conditions. The effect of temperature, intensity and the wavelength of the light on this anomalous behaviour has been studied in detail. 2. E X P E R I M E N T A L Glassy alloy of Ge22Se78 is prepared by quenching technique. High purity (99.999%) materials were weighed according to their atomic percentages and are sealed in quartz ampoule (length ~ 5 cm and internal dia ~ 8 mm) with a vacuum ,-~ 10 5 torr. The sealed ampoule is kept inside a furnace where the temperature israised to 950°C at a rate o f 3-4°C m i n - ' . * Work supported by University Grants Commission, New Delhi, India.

371

The ampoule is frequently rocked for 10 h at the maximum temperature to make the melt homogeneous. Quenching is done in air. Thin films of the glassy alloy are prepared at room temperature by vacuum evaporation at a base pressure ,-~ 10 -5 torr on well degassed glass substrates which had predeposited nichrome electrodes. The co-planar structures (length ,-~ 1.2cm and electrode gap ~ 0 . 5 m m ) are used for the photoconductivity measurements. To measure the rise of photocurrent, the light is shown on the sample through a transparent window of a metallic cryostat in which sample is mounted in a vacuum ~ 10 -3 torr. The current is measured as a function of time by a 3½ digit digital pico-ammeter (Achme, model SD-100). All the measurements are done after annealing the sample, in a vacuum ~ 10 3 torr, at 150°C for 2 h inside the same cryostat. The dark conductivity (ad) is measured as a function of temperature (302 to 450 K). The value of a d is ~ 5 x 10-9~-1cm-1 at 302K and is thermally activated with a single activation energy (0.9 eV) as also reported earlier by us [1]. The sample is found to be highly photoconducting in the steady state. At room temperature, the photosensitivity (Iph/Id) , a t the highest intensity used is ~ 100 in white light. The stability of the sample to the light exposure is also studied. No permanent change in the dark conductivity, activation energy for d.c. conduction and photosensitivity is observed after exposing the sample to white light (200 W tungsten lamp) for 8 h in a vacuum ~ l 0 -3 torr. The sample is found ohmic up to 30 V in dark as well as in presence of light. The present measurements are, however, made at very low voltage (1.5 V) which is applied by a dry cell.

P H O T O C U R R E N T IN AMORPHOUS THIN FILMS

372

3. RESULTS

E

Figures 1 and 2 show the rise ofphotocurrent with time at different temperatures for red (660 nm) and violet (420 nm) light respectively. These measurements have been taken at the maximum intensity of light (210 a.u.). It is clear from Fig. 1 that, in red light, photocurrent rises monotonically to the steady state value at all temperatures. The photocurrent takes about 120 sec to reach the steady state. Such type of rise of photocurrent is common in chalcogenide glasses [1-6] and we, therefore, call it a normal behaviour. In case of the violet light (see Fig. 2), the rise of photocurrent is similar to that in red light at 302 K. However, at higher temperatures (/> 322 K), the rise is found to be quite different. In this case, the photocurrent passes through a maximum value [Iph(max)] before attaining the steady state value [Iph(s0]" We call this behaviour anomalous as this type of behaviour is not common in chalcogenide glasses. Only in one more glass system (As2Se3), such type of anomalous behaviour in rise of photocurrent has recently been reported by Andriesh et al. [7]. They studied such behaviour at only one temperature 295 K and used white light for their measurements. To assess the relative variation of the anomalous effect at various temperatures, wavelengths and intensities we define a quantity .... 2.2.0

2.00

o--~ 302K 322K ~ 34/.K : : 3/.8K 352K Ge22Se78 210o.u.

=

6h(max) -- 6h(st) [ph(max)

't2C

1.6C (

u

(l)

Using the data plotted in Fig. 2, it can be shown that the value of E goes on increasing as the temperature is increased from 322 to 352 K indicating that the anomalous effect increases with the increase of temperature. At the lowest temperature (302 K), anomalous effect is not seen at all [see Fig. 2]. To study the effect of wavelength on the anomalous behaviour, we have studied the rise of photocurrent at various wavelengths (420, 470, 530, 620 and 660 nm). These measurements are done at a particular temperature 352K and at the highest intensity (210 a.u.). The results of these measurements are shown in Fig. 3. It can be shown from this figure that the value of E goes on decreasing as the wavelength of light increases from 420 to 530 nm indicating that anomalous effect decreases as the wavelength of light increases. At 620 and 660 nm, anomalous behaviour is not observed at all (results not plotted in Fig. 3). To study the effect of intensity on the anomalous behaviour, we have measured the rise of photocurrent at a particular temperature 352 K for different intensities. Violet light (420 nm) is used for these measurements as the effect is maximum for this wavelength. The results of these measurements are shown in Fig. 4. At the lowest intensity (1 a.u.) used, the anomalous effect is not seen at all. As the intensity of light increases the anomalous effect appears and the value of 1./.•

1.8C ~

Vol. 64, Nb. 3

o---0302K

322K ~ 3/./.K -- ~-348K H 352K Ge22Se.78

1.0C

't/C

_...,,--.--~"~'~'~~

1.2c

~

1.00

0.60

(180

0.4(]

0.60

0./.G

|

0

0.2C

i

20

i

60

40 Time

i

EO

i

100

i

120

(sec)

Fig. 1. Rise of photocurrent with time at different temperatures in red light (660 nm) at an intensity of 210 arb. units.

Time~sec)

Fig. 2. Rise of photocurrent with time at different temperatures in violet light (420 nm) at an intensity of 210 arb. units.

Vo~ 64, No. 3

P H O T O C U R R E N T IN A M O R P H O U S THIN FILMS

4.0C

2.00~

S30nm 470 nm 420 nm

o~o

o---.e la.u. e---.* 20 a.u. ~ 75 o.u.

1,8C

Ge 22Se78 352K 210 a.u.

373

16C

=

130o.u. 190a.u. = 210a.u.

3.0C

~Z0C

0.60 100

/'

a

a

a

4t

0.4C

a2o i

J

/

i

30

60

90

120

0

30

60

Time(sec)

90

120

Time(sac)

Fig. 3. Rise of photocurrent with time at 352 K in the light of different wavelengths at an intensity of 210 arb. units.

Fig. 4. Rise of photocurrent with time at 352K in violet light (42Onto) of different intensities (1 to 210 arb. units).

E increases with the increase of intensity in the range 1 to 75 a.u. However, the value of E decreases again in the intensity range (75 a.u. to 210 a.u.). These results indicate that anomalous effect first increases and then decreases with the increase of light intensity.

present case, we have measured steady state photoconductivity as a function of light intensity. The results for violet light at 302 and 370 K are plotted in Fig. 5. This figure indicates that Iph(st) increases with light intensity as a power law where the power changes from 1.0 to 0.5 at higher intensities at 370 K. However, at room temperature (302K), no such transition is observed (see Fig. 5). Our steady state photoconductivity results also indicate that a transition from monomolecular to bimolecular recombination does not take place in red light (660 nm) at any temperature upto 370 K (results not shown here).

4. DISCUSSIONS An analysis [8] of the steady state photoconductivity [Oph(st)],in case of a semiconductor having single trap level, shows that O'ph(st)increases with light intensity (F) as a power law [O'ph(st)~F:' ] where the power 7 = 1 for monomolecular recombination and 7 = 0.5 for bimolecular recombination. Rose [9] has, however, pointed out that, in case of a semiconductor containing a distribution of localized states in the band gap, 7 may be between 0.5 and 1.0 even in case of bimolecular recombination. The monomolecular recombination is expected to take place [8] when excess carrier density (An) is much smaller than the thermally generated carrier density (no), i.e., An ~ no. On the other hand, when An >> no, bimolecular recombination is expected to take place [8]. A transition from monomolecular to bimolecular is, therefore, possible as the intensity of light increases from low level to high level. Such type of transition has indeed been observed in various kinds ofchalcogenide glasses [10-12]. To understand the nature of recombination in the

n

-20

Ge 225e78 /.20 nm o---o 302K

-21

zs-~s 370 K

-22

~ - 23

c -24

-25 -26

= 2

I 3

i 4

i S

fi

tn F (o.u)

Fig. 5. Intensity dependence of steady state photocurrent in violet light (420 nm).

374

PHOTOCURRENT IN AMORPHOUS THIN FILMS

From the above discussion it is clear that bimolecular recombination in the present case is observed at higher temperatures and higher energy of light only. The observation of bimolecular recombination at higher temperatures and higher energy of light may be associated with the increase in the value of An near the surface due to the increase in the absorption coefficient at higher temperatures and higher energy of light. Andriesh et al. [7] have recently presented a theory which predicts the presence of maxima in the rise of photocurrent with time. According to them, such a maximum occurs due to nonequilibrium recombination in the high intensity region (An >> no) where bimolecular recombination takes place. At lower intensities (An ,~ no) where monomolecular recombination takes place, this maximum may be absent and the photocurrent rises monotonically to the steady state. At intermediate intensities, a transition from monomolecular to bimolecular recombination may take place as time increases from low to high values. Due to the quasi-stationary state near the maxima, the maxima may not be very sharp at intermediate intensities [7]. Following Andriesh et al. [7], the maxima in the rise curve in the present case may be explained in terms of bimolecular recombination at larger intensities. At lower intensities, the absence of this maximum may be due to the absence of bimolecular recombination as monomolecular recombination may be the only process in the whole time interval at these low intensities as observed by us in violet light at high temperatures (see Fig. 5). This is further supported by the fact that anomalous behaviour is not observed at 302K in violet light where only monomolecular recombination is observed as discussed earlier (see Fig. 5). The absence of the anomalous behaviour in red light (660 nm) may also be associated with the absence of bimolecular recombination at all temperatures. The above discussion shows that our transient photocurrent results can be understood on the same theoretical ground which has been used by Andriesh et al. [7] to explain their results in case of amorphous thin films of A s 2 S e 3 . However, the decay of photocurrent after the maxima does not follow a power law in the present case. This is in contrast with the theory of Andriesh et al. [7]. Moreover, the value of E starts

Vol. 64, No. 3

decreasing in the high intensity range (75 a.u. to 210 a.u.) indicating that anomalous effect decreases with the increase of light intensity in this range. However, in the low intensity range, the value of E increases with the increase of light intensity. The reasons of such discrepancies are difficult to understand as the exact distribution and the nature of localized states is not known in the present material. From the present study, we thus conclude that the anomalous behaviour in the rise of photocurrent is observed in amorphous thin films of Ge22Se78 as observed by Andriesh et al. [7] in amorphous thin films of As2Se3. In the present case, the anomalous behaviour is observed in certain experimental conditions only. The anomalous effect increases at higher temperatures and lower wavelengths of light. This anomalous behaviour can be understood in terms of non-equilibrium recombination at higher intensities as suggested by Andriesh et al. [7]. The reason for not observing this behaviour at all temperatures and wavelengths may be associated with the absence of bimolecular recombination in the intensity range used for the present measurements. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. l l. 12.

R. Mathur & A. Kumar, Solid State Commun. 59, 163 (1986). R. Mathur & A. Kumar, Rev. Phys. Appl. 21, 579 (1986). R. Mathur & A. Kumar, Solid State Commun. 61, 785 (1987). K. Shimakawa, A. Yoshida & T. Arizumi, J. Non-cryst. Solids 16, 258 (1974). E.A. Fagen & H. Fritzsche, J. Non-cryst. Solids 2, 180 (1970). E.A. Fagen & H. Fritzsche, J. Non-cryst. Solids 4, 480 (1970). A.M. Andriesch, V.I. Arkhipov, M.S. Iovu, A.I. Rudenko & S.D. Shutov, Solid State Commun. 48, 1041 (1983). P. Nagels, Amorphous Semiconductors (Edited by M.H. Brodsky) Springer-Verlag, Berlin (1979). A. Rose, Concepts in Photoconductivity and Allied Problems, Interscience, New York (1963). T.C. Arnoldussen, R.H. Bube, E.A. Fagen & S. Holmberg, J. Non-cryst Solids 8-10, 933 (1972). J.G. Simmons & G.W. Taylor, J. Non-cryst. Solids 8-10, 947 (1972). K. Weiser, J. Non-cryst. Solids 8-10, 922 (1972).