Photoelectrical properties of layered GaS single crystals and related structures

Article abstract of DOI:10.1016/j.materresbull.2008.04.014

The photoconductivity of pure and Cu-doped layered gallium monosulfide (GaS) single crystals, and of Ni-GaS(Cu)-In and GaS(Cu)-ZnO structures, is investigated. The activation energies of surface states were found as 0.10 eV, 0.40 eV and 17 meV, ～400 meV for GaS and GaS(Cu), respectively. Cu acceptor levels are localized at 0.44 eV and 0.52 eV above the valence band of GaS. ZnO-GaS(Cu) heterojunctions show remarkable photosensitivity in the wavelength range of 250-700 nm.

Full text of DOI:10.1016/j.materresbull.2008.04.014

Available online at www.sciencedirect.com
Materials Research Bulletin 43 (2008) 3195–3201
www.elsevier.com/locate/matresbu
Photoelectrical properties of layered GaS single crystals
and related structures
a
M. Caraman , V. Chiricenco , L. Leontie , I.I. Rusu
b
c,
a
*
a
Bacau University, 157 Calea Marasesti, 600115 Bacau, Romania
b
Moldova State University, 60 A Mateevici Street, Chisinau MD-2009, Republic of Moldova
c
Faculty of Physics, ‘‘Al.I. Cuza’’ University, 11 Carol I Boulevard, 700506 Iasi, Romania
Received 25 March 2007; received in revised form 31 January 2008; accepted 17 April 2008
Available online 26 April 2008
Abstract
The photoconductivity of pure and Cu-doped layered gallium monosulfide (GaS) single crystals, and of Ni–GaS(Cu)–In and
GaS(Cu)–ZnO structures, is investigated. The activation energies of surface states were found as 0.10 eV, 0.40 eV and 17 meV,
ꢀ
400 meV for GaS and GaS(Cu), respectively. Cu acceptor levels are localized at 0.44 eV and 0.52 eV above the valence band of
GaS. ZnO–GaS(Cu) heterojunctions show remarkable photosensitivity in the wavelength range of 250–700 nm.
2008 Elsevier Ltd. All rights reserved.
#
Keywords: A. Layered compounds; A. Semiconductors; B. Crystal growth; D. Electrical properties
1
. Introduction
High-resistivity semiconductors in both single crystals and thin films are largely used in the technology of
microelectronic and optoelectronic devices [1]. The area of practical applications considerably enlarges if respective
materials possess also electrical and optical anisotropy. These characteristics are fully illustrated by III–VI layered
semiconductors, to whom gallium monosulfide (GaS) belongs. The unit hexagonal cell of GaS consists of two
packages of S–Ga–Ga–S type. Besides, its symmetry axis C is perpendicular to (0 0 0 1) planes.
6
Gallium monosulfide has proved a promising material for electronics, in chemical and electrical passivation of
GaAs surface [2,3]. It is also known as a parent substance for gallium–lanthanum–sulphide (GLS) chalcogenide
glasses, with important applications in the nonlinear integrated optics [4–9].
The practical utilization of GaS layers in optoelectronics is conditioned by the possibility of obtaining layers with
controlled electrical resistivity, without significant modification of optical and photoelectrical properties. This goal
can be achieved by doping GaS single crystals with groups II and I impurities, which in certain conditions leads to an
increase in majority carrier concentration.
The study of photoconduction in GaS was reported by a rather limited number of authors [10–12] so far.
In the present work, we investigate the photoconductivity (PC) characteristics of Ni–GaS–In, Ni–GaS(Cu)–In and
ZnO–GaS(Cu)–Ni structures in wide spectral and temperature ranges.
*
Corresponding author. Tel.: +40 232 201165; fax: +40 232 201150/201.
E-mail address: lleontie@uaic.ro (L. Leontie).
0025-5408/$ – see front matter # 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2008.04.014

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M. Caraman et al. / Materials Research Bulletin 43 (2008) 3195–3201
2
. Experimental
ꢁ
5
ꢁ6
ꢁ6
Gallium monosulfide was synthesized from its component elements. Ga (99.999), containing 10 % Al, <10
ꢁ
%
%
6
ꢁ5
ꢁ7
ꢁ6
ꢁ6
ꢁ6
ꢁ5
Bi, <10 % Cd, 10 % Mg, 6 ꢂ 10 % Mn, 10 % Cu, 2 ꢂ 10 % Ni, 2 ꢂ 10 % Pb, 5 ꢂ 10 % Zn, 3 ꢂ 10
ꢁ
5
ꢁ5
ꢁ4
C, 5 ꢂ 10 % Fe, 6 ꢂ 10 % Si, <10 % Sn and S in stoichiometric proportions were used. The synthesis was
performed in a vacuum furnace at temperature 1120 8C, for 6 h. The melt was mixed by a electromechanical buzzer
(n = 70 Hz).
The investigated GaS single crystals were grown by Bridgman method. From bulk single crystal two types of layers
have been splitted, with crystal axis (C ) perpendicular and parallel to single crystal surface, respectively. Both
6
undoped and Cu-doped single crystalline GaS layers with thickness in the range (12–20 mm), showing n-type
electrical conductivity have been investigated. Cu doping was accomplished by thermal diffusion (at 720 K, for
ꢁ
5
3
0 min, under a vacuum of 2 ꢂ 10 Torr) of metal, which was formerly deposited onto single crystal lateral surface
by thermal vacuum evaporation. As measured from atomic emission spectra, Cu concentration in investigated samples
was 0.11 at.%.
The Ni and In electrodes were deposited onto freshly splitted crystal surfaces by thermal evaporation under vacuum
ꢁ
5
of ꢀ5 ꢂ 10 Torr. The thin films of ZnO have been grown onto (0 0 0 1) surface of GaS(Cu) single crystals by d.c.
magnetron sputtering method, in an atmosphere of 90% O and 10% Ar. The current density in the gas discharge was
2
ꢁ
2
up to 10 mA cm . The optical transmittance in temperature range of 78–293 K was measured by using a SPECORD
M-40 spectrophotometer, with an energy resolution of 0.5 meV. The sample temperature was measured with a copper–
constantan thermocouple. The optical transmittance (at 500 nm) of as-deposited ZnO films, with thickness between
ꢁ
2
1
.50 mm and 5.00 mm, was ꢀ73%, while their resistivity laid in the range (250–1300 V cm ) at room temperature.
The d.c. current–voltage (I–V) characteristics were measured in both dark and illumination conditions. The electric
field was directed along the C crystal axis. The ohmicity of the electrodes was verified and confirmed by linearity of I–
6
V characteristics over a wide bias range.
Awolfram filament lamp at temperature 3270 K was used for illumination. The light from the lamp passed through
2
a 3 mm thick blue-green filter SZS-22. The power density of the light beam was about 22 mW/cm . The integral
illuminance (without absorptional light filter) was about 1000 lx.
The spectral dependences of short-circuit (SC) photocurrent and of photoconductivity [13] were measured by
means of a spectrometric installation based on a MDR-2 type monochromator. The signal on the charging resistor was
registered by a selective I2-8 amplifier, tuned on the frequency of sinusoidal light pulses, 42 Hz. A Xe-arc lamp of
DKCS-250 type was used as light source with cvasi-continuous spectrum. The energy of monochromatic incident
beam was measured by means of a Vth-1 type thermoelement with quartz window. The photoresponse relaxation
curves were registered by means of a S8-14 memory oscilloscope.
3
. Results and discussion
Fig. 1 presents the I–V characteristics for the single crystalline layers of GaS and GaS(Cu), with thickness of 9 mm
and 12 mm, respectively. As one can easily see, the shape of current–voltage dependences does not depends on the
4
polarity of applied voltage and they are linear (regions a) for electric fields up to ꢀ10 V/cm. The linear aspect of the
above I–V curves, as well as their form at reverse bias, indicate that in the presence of an external electric field, Ni and
In form electron injecting contacts into GaS and GaS(Cu).
The linearity of I(V) dependences for a wide range of voltages leads to conclusion that the concentration of injected
electrons is small as compared to that of thermally generated carriers from donor shallow levels. In this case the current
density is determined by the relation:
V
J ¼ enm d
;
(1)
where n is the concentration of thermally generated carriers, m their mobility, V the applied voltage and d is the gap
between electrodes.
4
At field intensities E ꢃ 10 V/cm, the I(V) dependence of Ni–GaS–In structures becomes parabolic in both dark and
illumination conditions (regions b, Fig. 1). This attests the presence of the space-charge-limited current (SCLC), due to
the volumic charge localized in the vicinity of metal electrodes [14].

M. Caraman et al. / Materials Research Bulletin 43 (2008) 3195–3201
3197
Fig. 1. Current–voltage characteristics of Ni–GaS(Cu)–In (1 and 3) and Ni–GaS–In (2 and 4) single crystalline layers, at temperature 293 K. 1 and 2:
dark characteristics; 3 and 4: light characteristics. Structure bias: (*) Ni(+), In(ꢁ); (+) Ni(ꢁ), In(+).
In the SCLC domain, the current density is described by the quadratic, Mott type law [15]:
ꢀ
ꢁ
Et
2
J ꢀ V exp ꢁ
;
(2)
kT
where E denotes trap energy (measured relative to the bottom of conduction band, CB), k the Boltzmann’s constant, V
t
the applied voltage across the sample and T is the absolute temperature.
As experiments demonstrated, the I(L) dependence for GaS(Cu) single crystals is linear for illuminance (L) values
3
from ꢀ10 lx up to (1–2)ꢂ 10 lx, at both room temperature and 78 K.
By studying the temperature dependence of photocurrent, was demonstrated that in temperature range from 78 K
up to almost room temperature, the thermal activation of photoconductivity for both GaS and GaS(Cu) samples takes
place (Fig. 2).
For GaS single crystals in temperature interval 78–200 K a slow photocurrent increase is observed, and at
temperatures T ꢃ 200 K this rise becomes exponential, showing two slopes. Such photocurrent increase with
temperature can be due to both transformation of deep trapping levels into recombination levels with large lifetimes for
nonequilibrium charge carriers, and release of charge carriers trapped on surface levels.
The temperature dependence (e.g. rise) of photocurrent obeys the law [14]:
ꢂ
ꢃ
Et
n ¼ B exp ꢁ 2
;
(3)
kT
2
Fig. 2. Photocurrent versus reciprocal temperature for GaS (1) and GaS(Cu) (2) crystals. Exciting level: 50 mW/cm .

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M. Caraman et al. / Materials Research Bulletin 43 (2008) 3195–3201
where n denotes the concentration of photogenerated charge carriers, B is a parameter including concentration of
surface levels, E the mean energy of trapping level and k is the Boltzmann’s constant.
t
3
The registered two slopes in dependence log I = f(10 /T) from Fig. 2 attests that thermal activation of photocurrent
f
in undoped GaS single crystals is conditioned by two levels with thermal activation energies of 0.10 eV and 0.40 eV,
respectively.
An exponential photocurrent increase in temperature range of 110–350 K was also evidenced in the case of Cu-
doped GaS crystals (Fig. 2). For temperatures lying between 110 K and 200 K, the photocurrent activation is
determined by the ionization of low energy surface states (E ꢄ 17 meV), while at temperatures T > 250 K, by the
t
level with an activation energy of about 0.40 eV (0.39 eV, more precisely).
The deep surface levels (E ꢄ 400 meV) in both undoped and Cu-doped GaS crystals are of the same nature and are
t
probably determined by structural defects located at interface between S–Ga–Ga–S packages, since, as was stated in
[
16], the impurity ionized atoms are localized just on the surface of respective stratified packs.
For a wide temperature range, photocurrent activation in Cu-doped GaS crystals is determined by ionization of low
energy surface centres (E ꢄ 17 meV); these centres do not become manifest in undoped GaS and therefore one can
t
assume that they are created by Cu impurity atoms.
The diminution of photosensitivity in case of GaS and GaS(Cu) samples at temperatures over 400 K (Fig. 2) can be
determined by complete ionization of trapping levels, as well by exponential increase of majority carrier concentration
as a result of thermal electron transitions from valence band (VB) into the conduction band.
The spectral distribution of photoconductivity in the case of Cu-doped GaS crystals, in the photon energy range
i
g
i
g
hn ꢃ E (E is the width of forbidden band at respective temperature), actually coincides to photoconductivity
spectrum of undoped crystals [17]. Comparison between PC spectra of undoped [17] and Cu-doped GaS (Fig. 3)
shows that presence of Cu impurities can be attested by two particularities located at 2.06 eV and 1.98 eV, in
the domain of absorption impurity band. At temperature 78 K the band from the impurity photoconductivity
spectrum is located in the energy domain 2.20–2.60 eV and also is manifest in the absorption spectrum [18].
The concordance between absorption and photoconductivity spectra allows us to assume that Cu impurity atoms
form localized states in the vicinity of valence band limit of GaS crystals from which, as a result of indirect
optical transitions, nonequilibrium charge carriers are generated into the conduction band. Since at room
temperature (Fig. 3, curve 2) the energy corresponding to most probable transitions from impurity states is equal
to 2.06 eV and 1.98 eV and taking into account that the indirect gap of GaS is equal to 2.5 eV [10] at 293 K, we
can determine the energy of localized Cu states as 0.44 eV and 0.52 eV, with respect to the top of the valence
band.
Fig. 4 shows the spectral curves of short-circuit current (I ) for untreated and thermally treated GaS(Cu)–ZnO
SC
heterojunctions. Respective structures were illuminated through ZnO layer, which is optically transparent in a wide
wavelength range. As one can see, in the case of untreated heterojunctions (curve 1), the spectrum shows an outlined
Fig. 3. Spectral distribution of photoconductivity for Cu-doped and undoped GaS crystals in the domain of fundamental absorption threshold. 1 and
3: T = 78 K; 2 and 4: T = 293 K.

M. Caraman et al. / Materials Research Bulletin 43 (2008) 3195–3201
3199
Fig. 4. Spectral distribution of short-circuit current in ZnO–GaS(Cu) heterojunctions. 1: untreated; 2: heat-treated in air, 30 min at T = 650 K.
maximum, with an energy position (photon energy) hn = 2.8 eV, which surpasses by 0.3 eV the indirect gap. Besides,
i
this spectrum evidences a continuous photoresponse in the range hn < E , which is determined, as results from the
comparison with photoconductivity spectra of GaS(Cu) crystals (Fig. 3), by indirect electron transitions via Cu
localized states.
g
The maximum at 2.8 eV (Fig. 4, curve 1) attests that the generation of nonequilibrium charge carriers takes place in
a thin GaS layer by the vicinity of ZnO–GaS junction. The rapid decrease of ISC at increasing photon energy indicates a
large density of recombination centres localized in respective layer by junction interface.
The heat treatment for 30 min at ꢀ650 K in air, applied to GaS(Cu)–ZnO heterojunctions, results in a rise in the
photocurrent through the junction of 2–3 orders of magnitude and, as one can see from Fig. 4 (curve 2), the shape of
ISC(hn) dependence changes essentially. Thus, the plateau by 2.15–2.45 eV, existing in the case of untreated junctions
(
curve 1), turns up into a predominant band with maximum (b) at 2.3 eV.
In the UV range a wide maximum (a) located at ꢀ3.5 eV is evidenced, followed by an I fall as photon energy
SC
increases. The rapid drop of photocurrent for exciting (photon) energies hn > 3.5 eV can be mainly explained by
taking into account the small value of mean free path of nonequilibrium charge carriers from polycrystalline ZnO
layer, as well as the large recombination rate of photogenerated electrons in single crystalline GaS(Cu) layer.
Fig. 5 presents I–V characteristics of ZnO–GaS(Cu)–Ni structures at room temperature in dark conditions (curve 1),
as well as under illumination (curve 2) in the range (330–620 nm) using a halogen lamp with a wide band filter of SZS-
2
2
1 type. The power density of incident beam was ꢀ50 mW/cm . The illumination was performed through
polycrystalline ZnO layer, whose transmittance at l > 400 nm was ꢀ80%. The shape of these characteristics is quite
2
= 50 mW/cm . Dotted curve-calculated
Fig. 5. Current–voltage characteristics of ZnO–GaS(Cu)–Ni heterostructures. Irradiance: (1) L
e
= 0; (2) L
e
characteristics, according to Eq. (4); fitting parameters: I
bias, V < 0.20 V.
f s
= 30.0 mA, I = 29.5 mA and A = 21. Inset: dark current–voltage characteristics, forward

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M. Caraman et al. / Materials Research Bulletin 43 (2008) 3195–3201
similar to that of typical p–n junctions. The current–voltage dependence of illuminated heterojunction is described by
19]:
[
ꢀ
ꢂ
ꢃ
ꢁ
eV
I ¼ I exp
ꢁ 1 ꢁ I ;
(4)
s
f
AkT
where I denotes the current of minority charge carriers through heterojunction, I the photocurrent and A is the quality
s
f
factor of junction. Values of three indicated parameters have been provided by the best-fit curve also indicated (dotted
curve) in Fig. 5: I = 30.0 mA, I = 29.5 mA and A = 21.
f
s
From the last figure it can be observed that dark and light I–V characteristics differ own to the value of coefficient A,
which is lower in the case of darkness. A more pronounced current increase at forward polarity can be due to (i)
modification of the energy spectrum of localized (in the forbidden band of GaS(Cu) crystals) states, which is directly
related to the concentration variation of recombination centres as a result of light absorption, and (ii) photoinjection,
since the majority carrier concentration in ZnO and GaS(Cu) under illumination is larger than that of equilibrium
2
charge carriers produced by junction. When illuminating with a calibrated beam of 50 mW/cm , the
photoelectromotive force reaches ꢀ0.4 V, and I is equal to 38 mA.
SC
The inset of Fig. 5 shows the dark I–V characteristics in forward bias, for voltage range V < 0.20 V. In region I, for
V < 0.06 V, diode quality factor is n ꢄ 1.2, indicating a predominant diffusion of minority charge carriers through the
heterojunction. At voltages V > 0.08 V (region II, n ꢄ 2.1), moreover recombination of minority carriers within
junction region becomes important, probably owing to structure defects in the surface layer of GaS.
In the case of reversed polarity, the current through junction is determined by two competitive phenomena, charge
leakage through surface states and tunneling through junction.
4
. Conclusions
4
For electric fields of intensity E ꢅ 10 V/cm the current–voltage characteristics in both GaS and GaS(Cu) single
4
crystals are linear. At intensities E > 10 V/cm, I(V) dependences turn out into quadratic and have been interpreted on
the basis of Mott model.
By analysing the temperature dependence of integral photocurrent, the activation energies of surface states have
been determined as 17 meV and about 400 meV for GaS(Cu), and 0.10 eV and 0.40 eV for undoped GaS crystals.
i
g
The spectral characteristics of photocurrent for GaS(Cu) single crystals in the energy range hn < E is determined
by two acceptor levels due to Cu impurity atoms, localized within energy gap of GaS at 0.44 eVand 0.52 eVabove the
valence band.
ZnO–GaS(Cu) heterojonctions show remarkable sensitivity in the energy range of 2–4 eV. Their annealing at
ꢀ400 K, in air, leads to an increase in sensitivity by 2–3 orders of magnitude by comparison to untreated
junctions.
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