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Study of laser induced photoconductivity in thin films of amorphous Sbl5Ge5Se80 alloy
A. Maan, L. Sharma, H. Dahiya, D. Goyal
To cite this version:
A. Maan, L. Sharma, H. Dahiya, D. Goyal. Study of laser induced photoconductivity in thin films of amorphous Sbl5Ge5Se80 alloy. Journal de Physique III, EDP Sciences, 1993, 3 (6), pp.1211-1220.
�10.1051/jp3:1993112�. �jpa-00248994�
J. Phys. III France 3 (1993) 1211-1220 JUNE 1993, PAGE 1211
Classification
Physic-s Abstracts 72.20J 72.40
Study of laser induced photoconductivity in thin films of
amorphous SbisGesse~o alloy
A. S. Maan, L. R. Sharma, H. S. Dahiya and D. R. Goyal
Physics Department, Maharshi Dayanand University, Rohtak -124 001 (Haryana), India
(Received 12 January 1993, ret,ised J7 Marc-h 1993, accepted 26 Marc-h J993)
Abstract. Photoconductivity in thin films of amorphous Sbj~Ge~sesn using a He-Ne Laser is
investigated as a function of temperature and intensity. Temperature dependence of dark and photo
current (steady state) reveals that both processes are activated in nature in the observed temperature
range. The variation of photocurrent with incident excitation is indicative of a power law dependence. The values of the intensity exponent y suggest the bimolecular recombination in the present sample. To supplement the above studies, transient photoconductivity measurements have been made as a function of temperature and intensity. It is observed that the photocurrent decay in the sample has been exponential in the entire temperature and intensity range.
Introduction.
Amorphous solids are of considerable interest in solid state physics because of the intriguing question of the relation between electronic properties and disorder. Many laboratories are
conducting extensive investigations of chalcogenide glasses because of their basic and applied aspects. Among the experimental methods that can be used in such investigations, those based
on photoconductivity are very important and advantageous. It involves not only the optical
processes of carrier excitation, radiative as well as non-radiative recombination but also the various transport mechanisms of extended state conduction, multiple trapping and conduction
by hopping through a distribution of localized band tail states. As such photoconductivity
measurements are also relevant from the application point of view as well as to understand the
nature of localized states present in these materials. In amorphous chalcogenides, Ge-Sb-Se
alloys are attractive candidates for applications requiring low transmission losses, as they show transparency to infrared radiation fom 2-16 ~Lm [1, 2].
Photoconductivity measurements have been carried out on amorphous semiconductors in bulk form or in thin film form [3-11] and a number of models [12-14] have been proposed to
explain the results. The diverse nature of photoconducting properties in a-semiconductors is
visible from the presence of maxima with respect to temperature in many glasses [3-5].
Whereas it's absence has been reported by some others lo, I I]. Also, a linear dependence of photocurrent on incident flux has been observed in certain cases [4, 6] whereas many have
JOURNAL DE PHYSIQUE III T 1 N's JUNE1991 44
1212 JOURNAL DE PHYSIQUE HI N° 6
reported a square root depedence [6, 8, 10]. In a similar way the diversity in decay mechanisms
is clear from the fact that it has been reported as non-exponential in some cases lo, 14, 16], a power law decay [17, 18] in few others and also a sum of exponential decays [6, 19] depending
upon the nature of trapping and recombination processes taking place in the material.
Apart from the well known work of development and applications of Sb-Ge-Se alloys in the IR transmission fibres, a number of researchers have been actively carrying out various
investigations in these alloys. Studies like E.S.R. [20], elastic behaviour [2 II, thermodynamic properties [22] chemical ordering [23], optical and FIR absorptions [24-29] have been reported
in these glasses. Also investigated is the effect of irradiation [30] and pressure [31, 32] on the various electronic properties. Mehra et al. [33] and Sikka [34] have studied the electrical
conduction while Mann et al. [10] and Mathur and Kumar [35] have reported the photoconduc-
tion in a different composition of the glassy system using white light.
The present communication reports about photoconductivity studies in thin films of
amorphous Sbj5Ge5Seso alloy. The measurements include the temperature dependence of dark
current and also of steady state photocurrent, the latter at different excitation levels. Intensity
dependence of photocurrent has also been made at various fixed temperatures in the range (290-425 K). To supplement these studies and also to analyze the nature of decay, transient
photocurrent measurements are also included at various illumination levels and temperatures.
Experimental details.
To begin with, the glassy Sb,~Ge~seg~ alloy was prepared in bulk form by quenching technique. Elemental constituents of five nines purity in desired stoichiometric ratios were sealed in evacuated quartz ampoule. The sealed ampoule was further heated to l 000 °C in an
electric fumace with rocking arrangement. The ampoule was kept at the ambient temperature (1000 °C) for about lo h under continuous rocking and thereafter was quenched into ice cooled water. X-ray diffraction pattern of the sample was obtained which confirmed the glassy
nature of the material.
Bulk as obtained was further used for the preparation of the planar thin film samples by
Vacuum evaporation technique. Indium electrodes onto a well degassed Corning glass slide
Were deposited by vacuum evaporation technique prior to the deposition of the sample under
investigation. The thickness of the films was monitored on a VICO digital thickness monitor
using a quartz crystal and was adjusted to about 0.5 ~Lm. The other details are the same as
reported elsewhere [10]. Thin film samples so obtained were annealed under vacuum at 373 K,
for about 2 h. A Helium Neon Laser with 25 mW power output and beam diameter 2 mm, was
used to irradiate the samples. Intensity variation of the incident light was obtained by using glass slides and relative intensity was calculated by calibrating with an energy meter. Using the values of energy at various illumination levels, value of photon flux was obtained for a
particular intensity. The current measurements were made by using a 617 Keithley programmable electrometer with memory and data storage facility. A d.c. bias of 5 V was
provided by a stabilized power supply. The sample was found to be ohmic up to 30 V d.c. The
temperature measurements were made using a Chromel Alumel thermocouple and a
3 1/2 digit temperature indicator with an accuracy of 0.I °C.
Results and discussion.
Temperature dependence of d-c- conductivity in the present sample has been observed in the temperature range 296 to 421 K. Figure I shows the variation of dark current with 000/T in the observed temperature range. The In I~ vs. 000/T plot is a straight line indicating that the d.c. conductivity is an activated process with a single activation energy. Using the data in the
N° 6 LASER INDUCED PHOTOCONDUCTION IN A Sb,5Ge5Sego ALLOY 1213
-I
-I N
~ ~ ~~ ~ ~20
-I 9 . I .53 x i o~°
x g-I 5 xi oi 9
j v 5~B xi oi 9
I AE = O.30eV
f -2 I ~~
/
c
-2
-24
i~ eV.
2 5
~~~.o 2.4 2.8 3.2
3.5
I COO / T /
Fig. I. Variation of dark and steady state photocurrent (at four intensities indicated by the values ofN, number of photons/m~ s) with temperature.
figure, a value of activation energy (AE for dark carriers has been calculated which comes out to be 0.77 eV.
Also included in the figure I is the variation of steady state photocurrent with temperature at four different intensity levels represented by the value of N, number of photons/(m~ s). The fact that the semilogarithmic plots of I~~ vs. 000/T are straight lines at all the illumination
levels means that photoconductivity too is an activated process with a single activation energy AE~~ (0.30 eV) almost independent of intensity in the entire explored temperature range.
As shown in the figure the present sample is quite a good photoconductor with
photosensitivity (I~~/I~ 30) at room temperature under present experimental conditions. The ratio I~~/I~ starts decreasing with increasing temperature and becomes quite small in the high temperature range. According to Mott and Davis [36], in chalcogenide glasses the temperature dependence of photoconductivity has three regions and maximum must be observed due to
change in the recombination mechanisms. This is quite understandable from the fact that as the photo carrier concentration becomes equal to the dark carrier concentration (I~ = I~~) at the
crossover of the Id and I~h curves, the recombination changes from that of bimolecular to
monomolecular. It is due to this change that photocurrent decreases with increasing
temperature and the plots of Inl~~ i,s. 1000/T are expected to pass through a maximum.
1214 JOURNAL DE PHYSIQUE III N° 6
However, no maximum in the photocurrent could be observed in present temperature range.
Our results of photocurrent at different photon flux (Fig. 2) also do not indicate any change of recombination kinetics and the slope of the curves remains the same in the entire range of
photon flux studied. This is in agreement with the absence of maxima in the I~~ vs. 000/T plots (Fig. I). Similar absence of maxima has been observed by many authors lo, II, 37] in various glassy alloys. Chamberlain and Moseley [38] have attributed this absence of maxima
in their Ge-Se alloys to the two carrier conduction as suggested by others [39] for the
germanium chilcogenides.
,
V
-2i ~ ~
. 293 K 0~8)
o 313
.> 048)
6 334>.( o.55)
? ° 378>, o.571
~~'r
Fig.
emperatures.
N° 6 LASER INDUCED PHOTOCONDUCTION IN A Sbj~Ge~se~o ALLOY 1215
N
a 2.24xij~
. I.s3xio~~
~
90
x g.i5xiO~~
<
ff 19
o 5.48XiO q 75
wf
j
/fo
15
I 2 3 4 5 6 7 8
T,me (Sec.)
Fig. 3. Rise and decay of photocurrent with time at four different intensities (N represents number of photons/m2 s) at a fixed temperature (295 K).
second optical pulse duration was selected to cause the optical excitation. The data of rise and
decay of photocurrent was stored in the memory of the programmable electrometer amplifier at
a data sampling rate of 3 data per second. Figure 4 depicts the transient photocurrent
measurements in the present sample at four different temperatures at the highest intensity IN
=
2.24 x 10~° photons/m~.s). As is clear from the figures 3 and 4, the rise and decay of photocurrent are relatively fast. Although the photocurrent reaches near zero during decay, certain amount of current still remains. This type of current called persistent current [9] has been subtracted from the observed values during decay and for further analysis the corrected values of I~~ vs, t are plotted in figures 5 and 6 at different intensities and temperatures respectively.
When a material is subjected to an exciting radiation, a certain fraction of generated free carriers will be captured by traps. The traps filled during excitation will be emptied after cessation of illumination at a rate depending upon their cross section and ionization energy. If the retrapping of carriers freed from traps can be neglected then an exponential decay can be
expected for monoenergetic traps [40]. The decay in such a case is guided by the relation 1,
= I~ exp. (- t/T ). (i)
Where Ij is the value of the current at time It) after the cessation of excitation,
lo corresponds to the photocurrent at t
=
0 and T is the decay time constant.
Figures 5 and 6 show the decay of photocurrent with time at different excitation levels and temperatures respectively. It is clear from the plots in figure 5, the decay of photocurrent is
1216 JOURNAL DE PHYSIQUE III N° 6
T
I
+ 375 K
T x 352
>,
( o 335
f a 3ii ;
>,
( . 295,,
I
O I 2 3 4 5 6 7 8
Tim.15eG)
Fig. 4.- Rise and decay of photocurrent at five different temperatures. at a fixed intensity
(N 2.24
x 10~°).
-22
_~~ N ~
o 2.24xl0~~ 042 SW-
A 1.53x10~° (352
"
-24
v 9.lsx10~~ (060
w ~48x10~~ (0.58
~i
~r -25
~f
~ -~6
-27
-2fi
0 2 3 4 5
Time (Sec.)
Fig. 5. Decay of photocurrent (InI~~, corrected) with time at four different intensities (values taken from data of Fig. 3).
N° 6 LASER INDUCED PHOTOCONDUCTION IN A Sbj5Ge5Se~o ALLOY 1217
-2i
T c
. 295 K 0.62 Sec.)
~ 3il ,,( 0.73
; )
° 335
., o.29
>.
X 352
.. ( 0.21
< ? 376,,( 0.27 »
# ,,
~
l 2 3 4 5
Tlme(sec
Fig. 6. Decay of photocurrent (In I~h, corrected) with time at five different temperatures (values taken from data of Fig. 4),
exponential and with a single time constant at each excitation level. Using the plots, values of
decay time constant
r have been calculated and are indicated with each plot. To analyze the
dependence of decay time constant on photon flux, figure 7a shows the variation of In r with In N using the data from figure 5. One can clearly see from the figure that with
increasing N, little influence could be observed on the
r values. In figure 7b same data have been plotted versus photocurrent lo
= I~~ (t = 0 and here also r is almost independent of the initial value of photocurrent from where the decay of photocurrent sets in. Similar type of dependence of decay time constant on photocurrent and incident flux has been observed in a-
As~Se~ [15] at 306 K.
Now to examine the effect of temperature on decay mechanism let us consider the plots in
figure 6. The decay is exponential at all the observed temperatures but with a difference that the slope (r ) changes with change in temperature. The values of In r vs, 1000/T has been
plotted in figure 7c. As one can see from the plots that r has got a weak temperature dependence. The decay is perhaps resulting from the swamping of thermally produced carriers and occupied recombination centres by photogenerated densities (14). Such type of recombi-
nation is known to have weak temperature dependence.
Conclusions.
In the present work photoconductivity has been investigated using a He-Ne laser as a function of temperature and incident photon flux. Temperature dependence of photocurrent at different
1218 JOURNAL DE PHYSIQUE III N° 6
025
/ ~0~50
o
~ o
u1~
~
~ ~0'75
-1.00
~~5 ~&0 4&5 ~7.0
Ln N (no/m§~5ec.) a)
-.25
-~d l~
~ -.50
~
24© -23~ -23~ -22.5
LnI~(A) b)
U~ o
w d~
~' ~l'0
w
~ o
-i.8
36 2.B 30 3.2 3,4
I 000/T (K
c)
Fig. 7. a) Variation of decay time constant (r with intensity at a fixed temperature, b) Variation of decay time constant with photocurrent at the onset of decay (t = 0 at a fixed temperature, c) Variation of decay time constant (r with temperature at a fixed intensity,
excitation levels suggests that photoconductivity is an activated process with a single
activation energy in the present sample with the absence of maxima. Similar activated nature has been observed in the d-c- conductivity although the activation energy for dark conduction is
much greater than that of photoconduction. Steady state photocurrent measurement as a
function of photon flux suggests a power law dependence on incident intensity: Value of
intensity exponent y slightly increases with increasing temperature and indicates that bimolecular recombination is present.
N° 6 LASER INDUCED PHOTOCONDUCTION IN A Sbj5Ge5Se80 ALLOY 1219
Transient photoconductivity measurements at different temperatures and intensities were made. The decay of photocurrent with time has been analysed and comes out to be exponential.
Value of decay time constant (r) were calculated and show that decay remains almost
unaffected with increasing photon flux. The decay has been found to have a weak temperature dependence and r slightly decreases with temperature.
Acknowledgments.
Financial support from the U.G.C. during the course of work is gratefully acknowledged.
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