Applied Surface Science 218 (2003) 290–296

Preparation and characterization of copper telluride thin films by

modified chemical bath deposition (M-CBD) method
H.M. Pathana,*, C.D. Lokhandea,*, D.P. Amalnerkarb, T. Sethb
a
Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India
b
Centre for Materials for Electronic Technology (C-MET), Off Pashan Road, Panchavati, Pune 411008, India
Received 6 July 2002; received in revised form 21 April 2003; accepted 22 April 2003

Abstract

Copper telluride thin films were deposited using modified chemical method using copper(II) sulphate; pentahydrate
[CuSO4
5H2O] and sodium tellurite [Na2TeO3] as cationic and anionic sources, respectively. Modified chemical method is
based on the immersion of the substrate into separately placed cationic and anionic precursors. The preparative conditions such
as concentration, pH, immersion time, immersion cycles, etc. were optimized to get good quality copper telluride thin films at
room temperature. The films have been characterized for structural, compositional, optical and electrical transport properties by
means of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), Rutherford
back scattering (RBS), optical absorption/transmission, electrical resistivity and thermoemf measurement techniques.
# 2003 Elsevier B.V. All rights reserved.

Keywords: Modified chemical method; Copper telluride; Synthesis and properties

1. Introduction structures depending upon the value x (1 < x < 2)

[5]. The Cu2Te has an optical band gap 1.1 eV [6].
There has been increasing interest during the past Thin film deposition by modified chemical method,
few decades in semi-conducting copper chalcogenide has recently attracted interest, because the method
thin films because of their wide range of applications offers many advantages over the more established
in various fields of science and technology. Copper synthetic route to semiconductor materials. Factors
chalcogenide thin films have number of applications such as control of film thickness and deposition rate by
in various devices such as solar cells, super ionic varying pH, reagent concentration and complexing
conductors, photo-detectors, photothermal conversion, agent are allied with a potential to coat large area
electroconductive electrodes, microwave shielding with low cost. The modified chemical method can be
coating, etc. [1–4]. Copper telluride belongs to such used at room temperature for deposition from aqueous
copper chalcogenide (groups I–VI compound) materi- solutions. The basic difference between chemical bath
als. Copper telluride (CuxTe) has different crystal deposition (CBD) and modified chemical method
is the growth mode. In CBD, all the precursors are
* present at the same time in the reaction vessel, whereas
Corresponding authors. Tel.: þ91-231-2690571;
fax: þ91-231-2691533.
in the modified chemical method, the substrate is
E-mail addresses: habib_ist@rediffmail.com (H.M. Pathan), treated separately with each precursor and rinsing
l_chandrakant@yahoo.com (C.D. Lokhande). separates these treatments. As the rinsing isolates

0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0169-4332(03)00623-8
 H.M. Pathan et al. / Applied Surface Science 218 (2003) 290–296 291

the individual steps, the adsorbed species on the 25 mm 1 mm. The microslides were boiled in chro-
substrate surface control the reactions and film growth mic acid for 30 min, washed with detergent, rinsed in
takes place. The thickness of the films is controlled acetone, and finally ultrasonically cleaned with double
directly by the number of the deposition cycles. distilled water.
Modified chemical method is based on sequential Copper telluride films were deposited from an
reaction at the substrate surface. Rinsing follows each aqueous solution adopting the following procedure:
reaction, which enables heterogeneous reaction The cationic precursor for copper telluride was 0.1 M
between the solid phase and the solvated ions in the copper sulphate; pentahydrate [CuSO4
5H2O] with
solution. This is intended to grow thin films of water pH  5. The source for telluride ion was 0.05 M
insoluble ionic or ion covalent compounds of the sodium tellurite with pH  9. Cleaned glass substrate
KpAa type by heterogeneous chemical reaction at was immersed in the solution of copper sulphate
the solid solution interface between adsorbed cations solution for 20 s, where copper ions are adsorbed
pKaþ and anion aAp following the reaction on the surface of the glass substrate. The glass sub-
0 strate was rinsed in deionized water for 50 s to remove
pKaq aþ þ qXaq b þ b0 Yaq q þ þ aAp
0
loosely bound or excess copper ions from the sub-
! KpAsS # þ qXaq b þ b0 Yaq q þ strate. The glass substrate was then immersed in
with ap ¼ bq ¼ b0 q0 . Where K represents the cation anionic precursor (Na2TeO3) solution for 20 s where
(Cd2þ, Zn2þ, Fe3þ, Cuþ, etc.), p the number of the telluride ions were reacted with pre-adsorbed
cations, a the numerical value of charge on cation, copper ions on the glass substrate to form copper
X an ion in cationic precursors having negative charge telluride film on the surface of the glass. The opti-
(X ¼ SO4 2 , Cl2, NO32, etc.), q the number of X in mized deposition conditions for copper telluride as
cationic precursors, b the numerical value of charges tabulated in Table 1.
on X, b0 the number of Y in the anionic solutions, q0
the numerical value of charge on Y, Y the ion which 2.2. Characterization of the copper telluride films
is attached to chalcogen ion, A the anion (O, S, Se
and Te) and a the number of anions. Thickness of the copper telluride film was deter-
A number of attempts have been made on deposi- mined by weight difference method. X-ray diffraction
tion of CuxS and CuxSe thin films by modified CBD pattern of copper telluride films were recorded by
method; however, no report is available on the deposi- using a Philips model PW-1710 in the range of scan-
tion of copper telluride thin films. In the present ning angle 20–608 (2y) with Cu Ka radiations using
investigation, semi-conducting copper telluride thin the wavelength 1.5418 Å. The microstructure and
films have been deposited at room temperature onto chemical composition of the copper telluride thin
glass substrates using modified chemical method. The film were obtained using scanning electron micro-
films were characterized for their structural, surface scopy, and energy dispersive X-ray analysis using
morphological, compositional, optical and electrical Philips XL-30 SEM and EDAX analyzer, respectively.
properties by means of X-ray diffraction (XRD),
scanning electron microscopy (SEM), energy disper- Table 1
sive X-ray analysis (EDAX), Rutherford back scatter- Deposition conditions for copper telluride thin films by modified
ing (RBS), optical absorption, electrical resistivity and chemical method
thermoemf measurement techniques. Precursor CuSO4 (2N TEAa Na2TeO3
þ 2N HHb)

1 Concentration (M) 0.1 0.05

2. Experimental
2 pH 5 9
3 Immersion time (s) 20 20
2.1. Copper telluride thin film formation 4 Number of immersions 60 60
5 Deposition temperature (K) 300 300
The deposition was carried out onto commercially a
TEA: triethanolamine.
available glass microslides of the size 75 mm b
HH: hydrazine hydrate.
292 H.M. Pathan et al. / Applied Surface Science 218 (2003) 290–296

The Rutherford back scattering analysis of films onto was determined from a polarity of the thermally
glass substrate was carried out using 1.8 MeV Heþ generated voltage at the hot end of the film.
ions. The ion beam was incident on the film surface
and semiconductor detector (implanted Si detector,
solid angle of 3:4 103 Sr) was mounted at a 3. Results and discussion
scattering angle 1708. The energy resolution was
better than 20 keV. About ð13Þ 105 counts per 3.1. Thickness measurement
peak were sampled to ensure statistical error of 1%.
The composition of the film can be determined simu- Thickness of the copper telluride films for different
lating the Rutherford back scattering spectra of the number of deposition cycles was measured using
films using software and fitting with experimental weight difference method given by
data. In order to study the optical properties, optical m
absorption and transmission were studied in the wave- t¼ (1)
Ar
length range of 350–850 nm with Hitachi 330 spectro-
photometer. The electrical resistivity of the film was where m is the mass of the film, A is the area of the
measured using a dc two-point probe method in the film and r (7.303 g/cm3) the mean density (Cu2Te:
temperature range of 300–500 K. A brass block was 7.27 g/cm3; CuTe: 7.10 g/cm3; Cu1.4Te: 7.54 g/cm3)
used as a sample holder, and chromel alumel thermo- of deposited material, as discussed in Section 3.2.
couple was used to measure the temperature differ- Fig. 1 shows the variation of copper telluride
ence. The type of electrical conductivity of the film film thickness for number of deposition cycles with

Fig. 1. Plot of thickness against number of deposition cycles for copper telluride thin film.
 H.M. Pathan et al. / Applied Surface Science 218 (2003) 290–296 293

Fig. 2. XRD pattern of copper telluride thin film deposited on glass substrate.

optimized deposition conditions. The terminal thick- 3.3. Scanning electron microscopy (SEM) and
ness of 0.36 mm was found for 50 deposition cycles. energy dispersive X-ray analysis (EDAX)
The growth rate was found to be 7.2 nm per cycle.
After 50 cycles, the films became powdery and peeled Scanning electron microscopy is a convenient tech-
off from the glass substrate. Similar behaviour has nique for surface microstructure studies of thin films.
been reported earlier for Bi2S3 thin films deposited by Fig. 3a and b shows scanning electron micrographs of
modified chemical bath deposition (M-CBD) [7]. copper telluride thin films deposited on glass substrate
at 1000 and 10,000 magnifications, respectively.
3.2. Structural studies
Table 2
The XRD pattern of copper telluride film on the glass Composition of XRD data with standard ASTM data of copper
substrate is as shown in Fig. 2. The comparison of telluride thin films deposited by modified chemical method
observed interplaner distance ‘d’ values with standard Observed Standard Plane Phase
‘d’ value from ASTM data [5] are made in Table 2. By ‘d’ ( ) ‘d’ ( ) (h k l)
comparison of observed ‘d’ values with standard ‘d’
1 3.61 3.61 3 0 0, 0 0 6 Cu2Te
values, it is concluded that the deposited material is 2 3.52 3.51 111 Cu2Te
copper telluride with mixed cubic and tetragonal crystal 3 3.47 3.47 002 CuTe
structure [mixed phases of Cu2Te, CuTe and Cu2xTex 4 2.94 2.99 107 Cu2Te
0.6]. No other phases of Cu and Te, such as CuxO, TeO 5 2.04 2.04 020 CuTe
was observed in XRD pattern, may be due to the 6 1.91 1.939 020 Cu2xTe
7 1.51 1.52 104 CuTe
amorphous structure or small percentage of the same.
294 H.M. Pathan et al. / Applied Surface Science 218 (2003) 290–296

From Fig. 3a, it is observed that copper telluride thin

film is uniform, smooth and homogeneous and well
covered to the substrate. Fig. 3b shows fine-grained
surface particles of copper telluride film. Energy dis-
persive X-ray analysis of copper telluride thin film
onto glass substrate was carried out. From the analyses
of the average at.% composition of film was found to
be Cu:Te as 67:33.

3.4. Rutherford back scattering

Fig. 4 shows Rutherford back scattering spectrum of

copper telluride film onto glass substrate. It displays the
number of detected back-scattered He ions as a function
of their energy, which is plotted together with simulated
spectra of copper telluride. The solid curve shows the
simulated RBS spectra of Cu–Te onto glass substrate.
For calculation it was assumed that Cu and Te are
homogeneously distributed in thin film. A small peak,
with the edge energy of 650 keValso shows the presence
of oxygen. The presence of oxygen and Si peaks in RBS
spectra is from the amorphous glass (SiO2) substrate.
The presence of oxygen has been detected by RBS
technique in many chemically deposited films [8,9].
The integrated numbers of count under curves (peak and
Fig. 3. Scanning electron microscopy (SEM) of copper telluride
tail) agree with the number of count under simulation
thin film at (a) 1000 and (b) 10,000 magnifications.
peaks, indicating that the film consists of Cu–Te.

Fig. 4. RBS spectra for copper telluride thin film onto glass substrate.
Fig. 5. Plots of absorption (at) and percentage transmittance (T%) against wavelength (l) for copper telluride thin film.

Fig. 6. Plot of log r vs. 1000/T for copper telluride thin film.
296 H.M. Pathan et al. / Applied Surface Science 218 (2003) 290–296

3.5. Optical studies The film consists of different phases of copper tel-
luride (CuTe, Cu2Te and Cu2xTe). No presence of
Optical absorption and transmission spectra (Fig. 5) CuxO, TeO, etc. were observed may be due the amor-
of the copper telluride thin film deposited on glass phous or small percentage of it. The room temperature
substrate (thickness 0.2 mm) was carried out by electrical resistivity was found to be of the order of
using spectrophotometer (Hitachi Model 330) in the 101 O cm. M-CBD deposited CuxTe films show p-type
wavelength range 350–850 nm. These spectra reveal electrical conductivity.
that film has high absorbance (104 cm1) indicating
direct band gap transition. However, percentage trans-
mission (90%) is maximum at l ¼ 830 nm. Acknowledgements

3.6. Electrical resistivity and thermoemf We thank the UGC, New Delhi (India) for the
measurement financial support through departmental research
scheme UGC-DRS-SAP (1999–2004). One of the
Dark electrical resistivity (r) of the copper telluride co-authors, CDL, wishes to acknowledge Humboldt
film was measured by using a dc two-point probe foundation, Germany, for the award of fellowship to
method. The variation of log r versus 1000/T for the carry out the research work and other co-authors,
film is shown in Fig. 6. The resistivity decreases with HMP, acknowledges to Shivaji University, Kolhapur
increase in temperature indicating the semi-conducting for the Award of Department research fellowship
behaviour of the copper telluride film. The dark elec- (DRF).
trical resistivity at room temperature was of the order
of 101 O cm which is same as the value (101 O cm)
reported earlier for electrodeposited CuxTe thin films References
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