Photoelectric studies of gallium monosulfide single crystals

Article abstract of DOI:10.1016/j.jpcs.2004.06.011

Photoconductivity (PC) studies were carried out on GaS single crystals prepared from melt by directional solidification. We studied the effect of light intensity, applied voltage on both the PC and the lifetime of carriers. The V-I characteristics and the

Full text of DOI:10.1016/j.jpcs.2004.06.011

Journal of Physics and Chemistry of Solids 66 (2005) 5–10
www.elsevier.com/locate/jpcs
Photoelectric studies of gallium monosulfide single crystals
a,
G.A. Gamal , M.I. Azad
b
*
aChemical Engineering Department, Clarkson University, Postdeam, NY 13699, USA
bPhysics Department, State University of New York (SUNY), Potsdam, NY, USA
Received 26 November 2003; accepted 4 June 2004
Abstract
Photoconductivity (PC) studies were carried out on GaS single crystals prepared from melt by directional solidification. We studied the
effect of light intensity, applied voltage on both the PC and the lifetime of carriers. The V–I characteristics and the absorption spectra were
checked for different sample thickness. The present investigation was extended to study the spectral distribution of the photocurrent for GaS.
It was found that the photocurrent curves are practically independent of the bias voltage. The energy gap for GaS was found to be 2.5 eV.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: D. Phase transitions; D. Ferroelectricity
1. Introduction
the progressive interest in the material and in the possibility
of its optical applications, the work is presented.
The GaS compound is representative of AIII BVI crystal.
It crystallizes in the same hexagonal lattice as b-GaSe [1].
The lattice parameters are aZ0.3578 nm and cZ1.547 nm
and it was also reported [2] that the single layer is arranged
in the S-Ga-Ga-S chain along the hexagonal c-axis. GaS has
attracted many authors because it is a promising semi-
conductor. For example, intensive work has been performed
on GaS in the form of thin films [3–6]. Other publications
were about low temperature-photoluminescence [7], Raman
scattering [8] and thermal conductivity [9]. However, some
papers were focused on the optical studies in a limited wave
length range or on the glass form of the compound [10]. An
increase in the low frequency dielectric constant of GaS
under hydrostatic pressure was observed by Segura and
Chevy [11]. Thermoelectric power measurements of GaS
were previously published [12]. From the literature survey
there is no data concerning the influence of thickness on the
I–V characteristics or on the absorption spectra. From the
point of view of elucidation of the data and according to
2. Experimental details
The crystal growth and crystal identification work were
done in the Chemical Engineering Department of the
Clarkson University, NY. The eutectic GaS (68.5% Ga and
31.5% S) was produced from gallium (Aldrich 99.9999%
pure) and sulfur (BDH 99.999% pure). The weighed
components were sealed in a 10-mm inside diameter pointed
lower end quartz ampoule with 10K5 Pa. The materials were
melt at 1373 K. According to the phase diagram, this
temperature was reduced to 1188 K (i.e. the crystallization
temperature). Then, the melt was directionally solidified in a
vertical Bridgman–Stockbarger furnace. The temperature
profile was determined in a similar empty ampoule, yielding
a temperature gradient at the eutectic temperature of
w25 8C/cm, with the temperature increasing with height
throughout the molten region. So, the ampoule was translated
downwards at a constant rate of 2 mm/h. The product ingot
length was about 5.8 cm. It was of a layer nature with a bright
yellow color. The crystals were identified at the same
University by means of the following technique:
* Corresponding author. Present address: Physics Department, Faculty of
Science, South Valley University, Qena, Egypt.
– Scanning channeling electron microscopy (type JSM
6300 Jeol mark).
E-mail addresses: gamal@clarkson.edu (G.A. Gamal), gagamal99@
yahoo.com (G.A. Gamal).
0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2004.06.011

6
G.A. Gamal, M.I. Azad / Journal of Physics and Chemistry of Solids 66 (2005) 5–10
switching phenomenon is observed during the work of the
current–voltage characteristics (CVC) of the virgin GaS
sample.
– Computer program (voyager), which is a microanalysis
and digital imaging system for electron microscope.
– Transmission electron microscopy with energy
dispersive X-ray spectroscopy by NORAN instrument
(USA) [13,14].
The dependence of the switching behavior of the active
thickness between the electrodes is shown in Fig. 1 at room
temperature. The curves present a characteristic shape for
each different thickness of the sample, consisting of two
sections, a pre-switching portion (off state) and a region of
sharp current growth with subsequent switching to the low
resistance state. The CVC as a whole is shifted towards
higher potentials with reducing the thickness of the sample.
After switching to the low resistivity state, the samples
remain in this state under zero bias for some hours. This
effect is known as switching with memory. The thickness
of the sample varied from 0.6 to 3.75 mm. It was observed
during the work and from the figure that the holding
current and the voltage are affected with the thickness of
the crystal. This was also noticed in a previous work done
for ZnS crystal [15]. It is evident from Fig. 1 that the
threshold potential required for striking, increases with the
thickness. This result can be explained clearly in terms of
Ovshinsky model [16] in which injection and impact
ionization are believed to play important roles in such
process. This was previously observed in other III–VI
single crystals [17,18], and also in the ternary ZnIn2S4
crystal [19]. The linear dependence of Vth on the thickness
d shows that switching is a bulk effect and a critical field is
needed. The threshold current ith also increases with
increasing sample thickness. So the threshold power,
calculated as EthZith$Vth, increases with the sample
thickness too, according to the law Ethwd0.73. This relation
The V–I characteristics of the compound at four different
thickness of the sample, i.e. GaS were examined. The
thickness was measured by means of highly precision
traveling microscope, Man Company (Burlington). We
selected the following thickness values: 0.60, 1.22, 2.51,
and 3.75 mm. In the whole experiment, the samples were
cleaved from the virgin ingot and silver conducting paste
was employed as an Ohmic contact. A Keithley electro-
meter (610C) was used for recording the current values. For
the AC photoconductivity (PC), the samples were connected
in series with a highly stable DC power supply and a load
resistance equal to 12!104 U. The variation of the voltage
across the load resistance due to the modulated (PC) was
observed on a double beam oscillograph. The light source
was a 1000 W tungsten lamp. Also, a chopper controller
(SR520) used in the AC (PC) measurements has a square
pulse of monochromatic light. The applied light intensity
doses were evaluated by means of a Lx-101 Luxmeter. A
monochromator Hilger & Watts 6186.3 has been used for
the absorption spectra, the spectral distribution and the
photocurrent variation measurements.
3. Results and discussions
In the present investigation, we studied M-GaS-M
structure in a sandwich form. The bistable, or memory
Fig. 1. Current–voltage characteristics of GaS at different sample thickness.

G.A. Gamal, M.I. Azad / Journal of Physics and Chemistry of Solids 66 (2005) 5–10
7
A symmetrical square monochromatic light pulse for PC
excitation was used in case of AC PC measurements. The
chopper frequency varied from 20 to 400 Hz. The PC, as a
function of chopping frequency of the modulation of the
incident light, was registered directly at different values of
light intensity. Fig. 3 represents the dependence of PC, as a
function of chopping frequency F, at a bias voltage of 60 V
and sample thickness of 3.73 mm. The selected values of
light intensities were 500, 750 and 1000 Lux. From the
obtained dependence we observe that the behavior of the
curves in all the figures is similar and they have a general
shape in which the PC decreases with increasing frequency
in the whole range of investigated frequencies. The behavior
of these curves obeys the relation [20]:
Dsw=Dsst Z tanhð1=4FtÞ
According to this relation, and making use of such curve, we
can derive the lifetime of the carriers at different light
intensities. So, the relation between the carrier lifetime t
and the light intensity can be obtained. The lifetime t is
inversely proportional to the illumination intensity. The
value of t was found to be of the order of 10K3 s. In view of
the high density of the vacant acceptor levels, photocarriers
generated by illumination were captured almost immedi-
ately by these levels. Therefore, phototransport could occur
either by jumps between random centers or could involve
capture by traps.
Fig. 2. Dependence of threshold current, threshold voltage and threshold
power on sample thickness for GaS single crystal.
shows a linear proportionality between the active thickness
and the required power for switching; see the inset in
Fig. 2 (top left). As the thickness of the device increases, a
large power for switching is required. This work is also
important because it enables us to select the proper
thickness and bias of the corresponding voltage for
studying the PC of the crystal.
The same work was repeated whilst changing the voltage
in order to establish the effect of the applied bias voltage on
the photoconduction behavior of the GaS single crystals.
This work was done at bias voltage values of 60, 80 and
100 V. The ratio Dsw/Dsst decreases with increasing
frequency as shown in Fig. 4. At high chopping frequencies,
Fig. 3. Variation of AC photoconductivity of GaS with chopping frequency at different light intensities.

8
G.A. Gamal, M.I. Azad / Journal of Physics and Chemistry of Solids 66 (2005) 5–10
Fig. 4. Relation between AC photoconductivity of GaS and chopping frequency at different bias voltages.
the photoconductivity ratio decreases more slowly than in the
low-frequency range. The effect of the bias voltage on the PC
ratio Dsw/Dsst is evident from these curves. The derivation
of the carrier lifetime leads to the fact that t is inversely
proportional to the bias voltage. This can be accepted only if
we consider the trapping processes of the carriers, which are
activated as a result of increasing the bias voltage. Fig. 5
depicts the PC spectral distribution of p-type GaS single
crystal in the wavelength range (350–750 nm) at room
temperature. This study was done without a change of the
incident light intensity. The shape of the spectral distribution
was practically independent of the applied bias voltage. An
upward shift towards higher values of the photocurrent with
the increasing of the applied bias voltage is observed.
Fig. 5. The spectral distribution of the photocurrent for GaS at different bias voltages.

G.A. Gamal, M.I. Azad / Journal of Physics and Chemistry of Solids 66 (2005) 5–10
9
Fig. 6. The absorption spectra of GaS single crystal at 300 K at different sample thickness.
The photocurrent rises continuously with photon energy, and
reaches a certain maximum value at 460 nm, then a steep fall
is found at high photon energy. The spectral dependence of
the photocurrent agreed with previous measurements of the
DC PC of similar compound [21]. By applying the half-
maximum value [22], we could evaluate the average energy
gap as 2.5 eV.
to band states. For photon energies larger than the gap,
the absorption coefficient increases rapidly. Therefore, the
cross-section for transition involving impurities becomes
negligible and the dominant transitions are band-to-band
transitions. Regarding Fig. 6, the following definite
comments can be considered:
– The absorption edges extend over a considerable range of
energies, which leads us to believe that this arises from
indirect transitions.
The absorption spectra of gallium sulfide at 300 K is
shown in Fig. 6 for different sample thickness (0.6, 1.22,
2.51 and 3.73 mm). The samples for these measurements
were obtained by cleavage of the large single crystals
perpendicular to the c-axis.
– Near the fundamental absorption edge, there are absorp-
tion lines, after that the absorption sharply decreases.
– The large absorption coefficient of the line, its position in
the neighborhood of the fundamental absorption edge
The absorption coefficient is extremely low at hn!Eg
and the excitation involves transitions from impurity states

10
G.A. Gamal, M.I. Azad / Journal of Physics and Chemistry of Solids 66 (2005) 5–10
References
and the reproducibility of the results for all samples,
permit one to assume that this line is due to exciton
absorption.
[1] M.A. Alzhdanov, M.D. Nadzhafzad, Z.Y. Seidov, Phys. Solid State
20–21 (1) (1999) 41.
[2] O.Z. Alekperov, M.Z. Zarabakiev, Inorg. Mater. 10 (1998) 971–975.
[3] X.Y. Chen, X.Y. Hou, X.A. Cao, X.M. Ding, L.Y. Chen, G.Q. Zhao,
X. Wang, J. Cryst. Growth 1-2 (1997) 151–156.
[4] N. Okamoto, H. Tanka, Mater. Sci. Semi-Cond. Process. 1 (1999)
13–18.
The fundamental absorption edge in most semiconduc-
tors follows an exponential law. Above the exponential tail,
the absorption coefficient of semiconductor has been
observed to obey the equation
[5] M.R. Lezell, P. O’Brien, D.J. Otway, J.H. Park, Chem. Mater. 12
(1999) 3430.
n
aZu Z Bðhu KEgÞ
[6] S. Suh, D.M. Hoffman, Chem. Mater. 9 (2000) 2794.
[7] A. Aydinli, N.M. Gasanly, K. Goksen, J. Appl. Phys. 12 (2000)
7144–7149.
where aZu is the absorption coefficient of an angular
frequency of uZ2py, B is a constant, and n is an index
which can assume the values 1/2, 3/2, 2, and 3, depending
on the nature of the electronic transition responsible for
the absorption. It is well established that nZ1/2 corresponds
to the direct allowed transition (high-energy part of the
spectra), nZ3/2 to a forbidden direct transition, nZ2 to an
indirect allowed transition (low-energy part of the spectra)
and nZ3 to a forbidden direct transitions [22,23].
[8] N.M. Gasanly, A. Aydinli, H. Ozkan, C. Kocabas, Solid State
Commun. 3 (2000) 147–151.
[9] J.D. Beasley, Appl. Optics 6 (1994) 1000–1003.
[10] L.S. Griscom, J.L. Adam, K. Binnemans, J. Non-Cryst. Solids 257
(1999) 383–389.
[11] A. Segura, A. Chevy, Phys. Rev. B—Condens. Matter 7 (1994)
4601–4604.
[12] G.A. Gamal, M.M. Nassary, S.A. Hussein, Cryst. Res. Technol. 27 (7)
(1992) 997–1002.
For the determination of the energy gaps, Eg, of the GaS
[13] S. McDevitt, B.Em. Deam, D.G. Ryding, F.J. Scheltens, S. Mahajan,
Mater. Lett. 4 (1986) 451.
substance from the absorption spectra, we have plotted K1/2
,
K and K2 (where K is the experimental absorption
coefficient at 300 K), against the photon energy EZhn.
We only found that K1/2(E) approaches a straight line over
much the largest range of E values. When this line is
extrapolated to zero, which corresponds to the thicknesses:
3.73, 2.51, 1.22 and 0.6 mm and these yields’ energy gap
values: 2.49, 2.56, 2.55, 2.58 eV, respectively.
[14] Y.Y. Loginov, P.D. Brown, N. Thompson, G.L. Russell, J. Wood,
Microscopy of semiconducting meterials in: A.G. Gullis,
I.L. Hutchison (Eds.),, Institute of Physics Conference Series 100,
Institute of Physics, Bristal, 1989, p. 433.
[15] R.L. Hargreaves, P.R. Mason, J.C. Anderson, J. Phys. D: Appl. Phys.
7 (1974) 85.
[16] S.R. Ovshinsky, Phys. Rev. Lett. 21 (1968) 1450.
[17] A.T. Nagat, S.A. Hussein, Y.H. Gameel, G.A. Gamal, Phys. State Sol.
(a) 121 (1990) K201.
[18] M.M. Nassary, S.A. Hussein, A.E. Belal, H.A. El-Sheirh, Phys. State
Sol. (a) 145 (1994) 151.
Acknowledgements
[19] V.F. Zhitor, N.A. Moldovyan, S.I. Redautson, Sov. Phys. Semicond.
13 (15) (1979) 495.
[20] S.M.S. Ryvkin, Potenitiometric Effects Semicond. (1964).
[21] A.T. Nagat, G.A. Gamal, Y.H. Gameel, N.M. Mohamed, Phys. State
Sol. (a) 119 (1990) K47.
Because all this work was done in the State University of
New York (SUNY), one of the authors (Prof. Gamal GA)
wishes to thank {Prof. John A. Fallon} (president of SUNY)
for his limitless support during the work.
[22] T.S. Moss, Photoconductivity Electrons (1951).
[23] R.A. Smith, Philos. Mag. Suppl. 2 (1953) 81.