Characterization of copper selenide thin films deposited

by chemical bath deposition technique

Al-Mamun, A.B.M.O. Islam*

Department of Physics, University of Dhaka, Dhaka 1000, Bangladesh

Available online 28 July 2004

Abstract

A low-cost chemical bath deposition (CBD) technique has been used for the preparation of Cu2�xSe thin films onto glass

substrates and deposited films were characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS),

atomic force microscopy (AFM) and UV–vis spectrophotometry. Good quality thin films of smooth surface of copper selenide

thin films were deposited using sodium selenosulfate as a source of selenide ions. The structural and optical behaviour of the

films are discussed in the light of the observed data.

# 2004 Elsevier B.V. All rights reserved.

Keywords: AFM; CBD; Copper selenide; XPS; XRD

1. Introduction

Most of the semiconducting metal chalcogenides

are important materials for applications in various

photoelectric and other kinds of devices. Thin films

of metal chalcogenides can be deposited on to glass,

metal, plastics and other substrates by a variety of

techniques, such as flash evaporation [1,2], vacuum

evaporation [3], electrodeposition [4], and electroless

deposition [5], to the simplest method of chemical

bath deposition (CBD) [6–16]. The optical and elec-

trical characteristics of the deposited materials often

depend on the deposition technique used. In this

experiment copper selenide thin films with various

ratios of copper and selenium were deposited onto

glass substrates using low-cost CBD method. This

paper describes the results observed by XRD, XPS,

AFM and optical investigations on the Cu2�xSe film

prepared by CBD.

2. Experimental details

The chemicals, used for the preparation of thin

films, were LR grade (Merck) cupric chloride dihy-

drate (CuCl2�2H2O), selenium powder (BDH) of

99.99% purity, sodium sulfite (Na2SO3) (Merck),

triethanol amine (TEA) (Merck) and ammonium

hydroxide (NH4OH) (Merck). At first, selenium was

used for the preparation of sodium selenosulfate.

Second, CuCl2�2H2O solution was mixed with

NaSeSO3 at constant stirring. Ten milliliters of TEA

(0.1 M) was added to this solution. Thirty percent of

NH4OH solution was used to adjust the pH of the

reaction bath. Microscope glass slides were used as

substrates and they were cleaned well with detergent

and distilled water, and were kept in H2SO4 for about

1 h. The substrates were then washed first under

Applied Surface Science 238 (2004) 184–188

* Corresponding author. Fax: þ880 2 8615583.

E-mail address: oislambd@yahoo.com (A.B.M.O. Islam).

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2004.05.208

running tap water to clean the acid off and then

cleaned with acetone. Just after cleaning with acetone,

they were washed with running tap water and finally

cleaned with distilled water and were dried in air prior

to film deposition. The substrates were then immersed

vertically into the deposition bath against the wall of

the beaker containing the reaction mixture. The

deposition was allowed to proceed at room tempera-

ture for different time durations from 15 to 180 min.

After deposition, the glass slides were taken out from

the bath, washed with distilled water and were dried in

blowing air. The thicknesses of the films for about 1 h

deposition were obtained in the range 0.12–0.18 mm.

Identification of the deposited film material was

carried out by a Philips X’Pert X-ray diffractometer.

XPS experiments were performed in a VG ESCALAB

MkII photoelectron spectrometer. Electrons were

excited with an X-ray source (Mg Ka: hn ¼1253.6 eV). A Thermomicroscopes AFM was used

for morphological study. The optical measurements

were performed using a UV-121V spectrophotometer,

Shimadzu, Japan.

3. Results and discussion

3.1. XRD measurement

Fig. 1 shows the XRD pattern of Cu2�xSe thin film.

Fig. 1(a) is for the as-deposited Cu2�xSe thin film and

Fig. 1(b) for the Cu2�xSe thin film followed by

annealing at 523 K in air. Lot of noise is observed

in the XRD pattern which may be due to the growth of

disorder film. From this pattern it shows that no well-

defined peak was found and no well-defined plane was

obtained in the case of as-deposited films, which

suggests that the as-deposited films were disorder.

A little tendency of growing peaks is found at the

angles 2y ¼ 27.308, 45.358 and 52.788. The intensity

of the observed peaks is very low, which become

stronger due to annealing at 250 8C. Fig. 1(b) shows

well-defined peaks suggesting the formation of crys-

talline film due to annealing at higher temperatures. A

comparison of the observed pattern with the standard

JCPDS cards shows that the annealed samples with

above condition possess a structure matching the

mineral spherulites (JCPDS 26-512) [17], Cu2�xSe

with x ¼ 0.2 belongs to the cubic system with

a ¼ 5.697 A. Garcıa et al. observed the crystalline

structure in case of as-deposited Cu2�xSe (x ¼ 0.15)

films and possessed the structure matching the mineral

berelianite (JCPDS 6-680) [13]. In our case, the films

may not be thick enough to give sufficient intensity for

distinguishing the peaks from the noise. We have tried

to deposit thicker film, but the powder formation

occurs in case of thicker film. However, the observed

peak positions of the annealed sample are in well

agreement with those due to reflection from (1 1 1),

(2 2 0) and (3 1 1) planes of the reported structure. The

same reflection plane was observed for as-deposited

Cu2�xSe thin film prepared by CBD method using

CuSO4 and trisodium citrate solution [13,15,16]. The

average grain diameter for as-deposited sample was

found to be 0.025 nm that increases for annealed

samples to 0.724 nm. Very low grain size is observed

for as-deposited samples, which was increased about

30% owing to annealing. It is necessary to mention

here that the crystallite grain size in the films was

calculated using Scherrer formula [18].

3.2. XPS measurements

The XP spectra of different copper selenide thin

film of as-deposited sample presented in Fig. 2. In the

Fig. 1. XRD pattern of Cu2�xSe thin film: (a) as-deposited and (b)

after annealing at 523 K for 1 h in air.

Al-Mamun, A.B.M.O. Islam / Applied Surface Science 238 (2004) 184–188 185

figure, spectrum a represents for as-deposited sample,

while spectrum b for the sample annealed at 473 K for

30 min in UHV and spectrum c for the sample

annealed at 523 K for 30 min in UHV. It is observed

that the as-deposited film is mostly covered with

oxygen (at 531 eV) and carbon (at 285 eV) (spectrum

a). Whereas, the peak intensity of Cu and Se are very

weak. It is usual that the chemically deposited sample

may be contaminated with oxygen and carbon from

the environment as the sample is exposed to environ-

ment during preparation. It is seen that the peaks

corresponding to Cu and Se become prominent in

the sample annealed at 473 K (spectrum b) and after

annealing at 523 K, the intensity of Cu and Se peaks

become more prominent, thereby indicating lowering

of contamination owing to annealing at 523 K (spec-

trum c). Further annealing at around 573 K, Se starts to

desorb from the surface (data are not shown), which

means that copper selenide film remains stable below

573 K. The amount of oxygen decreases significantly

but Se is also started to decrease due to annealing at

above 573 K. The core level spectra of Cu 2p and Se

3d were also measured (data are not shown). The peak

area of Cu 2p and Se 3d has been calculated. Cu/Se

mole ratio has been calculated from the height ratio of

Cu 2p and Se 3d. It was also confirmed using the

formula

Ni

Nj

¼ Ii=Si

Ij=Sj

� �(1)

where N’s are the concentrations, I’s are the intensities

of photoelectron emission peaks after removing the

background and S’s are the ASFs (atomic sensitivity

factor) of the respective elements. The Cu/Se ratio is

estimated to be 1.8 for sample. Similarly different

samples have been deposited with different Cu and Se

ratio. The depth profile of the Cu2�xSe films by XPS

showed compositional uniformity along the depth.

3.3. AFM measurements

AFM image of as-deposited copper selenide thin

film is shown in Fig. 3 for the scan area of 2 mm�2 mm. It seems that the overall film surface is almost

smooth. The film surfaces contained small islands, and

many continuous islands are overlapping on the glass

substrate. Itwasobservedfromtheprofilediagramof the

AFM image that the mean roughness and mean height

are in the range 2.9–3.4 and 11–15 A, respectively.

3.4. Analysis of the optical absorption

The variations of transmittance T (%) of Cu2�xSe

thin film with wavelength l for as-deposited samples,

Fig. 2. XP spectra of Cu2�xSe thin film: (a) as-deposited, (b) after

annealing at 473 K for 30 min in UHV and (c) after annealing at

523 K for 30 min in UHV.

Fig. 3. AFM image of as-deposited Cu2�xSe thin film for the scan

area of 2 mm � 2 mm.

186 Al-Mamun, A.B.M.O. Islam / Applied Surface Science 238 (2004) 184–188

annealed at different temperatures are shown in

Fig. 4(a). Transmittance is obtained to be about 5–

50% in the wavelength range 400–1100 nm. A gradual

decrease in transmittance on annealing is observed in

the lower wavelength region, which may be due to

absorption by free carrier in the degenerate films. The

peak values of transmission spectra are seen at around

820–980 nm means that the absorption started around

the same wavelength and the transmittance becomes

very low at l < 500 nm. In the near-infrared, the

transmittance decreases with the increase of wave-

length.

The variation of reflectance (R%) with l is shown in

Fig. 4(b). Reflectance is found to be about 2–20% at

the wavelength range 400–1100 nm. It is observed in

the R (%) versus l curves that there are two peaks

around 560–600 and 820–940 nm. Both the peaks shift

to higher wavelength with annealing temperature. The

second R (%) peak appears at the same wavelength

region where the transmittance peak appears in

Fig. 4(a). These behaviours are in well agreement

with the published results of Cu2�xSe thin film pre-

pared by CBD technique with CuSO4�5H2O at 298 K

for 8 h [14]. Noticeable change of reflectance is

observed due to annealing at different temperatures.

It is also observed the noticeable change of color of the

films due to annealing at different temperatures under

reflected and transmitted daylight condition. It is

observed that color of the film was changed from

greenish to orange due to annealing. It means that

as-deposited film is contaminated by oxygen, and

oxygen contamination starts to remove due to anneal-

ing. The color becomes greenish orange on annealing

at 473 K, and at 523 K, the color changes to fully

orange, which does not change on annealing above

573 K. Lakshmi et al. reported that the color of the as-

deposited Cu2�xSe film was reddish brown [15,16].

The direct and indirect bandgap values are obtained

from plots of a2 and a1/2, against the corresponding

value of the photon energy hn (eV), respectively

(figures are not shown). It is observed that the direct

bandgap varies in the range 1.9–2.3 eVand the indirect

bandgap is in the range 1.2–1.7 eV from as-deposited

sample to all annealed samples up to 523 K. The larger

bandgap values in the as-deposited samples compared

with that of the annealed samples are arising from

smaller grain size in the former [11]. All these optical

bandgap values are close to that Cu2�xSe thin film

prepared by CBD technique using CuSO4 and N,N-

dimethylselenourea [8,13,15,16]. It is observed that

both direct and indirect bandgap values of as-depos-

ited samples are higher compared with those of the

annealed samples. The decrease of bandgap due to

annealing may be understood by the improvement of

crystallinity of the as-deposited film on annealing as

observed in XRD.

4. Conclusion

Good quality thin films of smooth surface of copper

selenide thin films were deposited using sodium sele-

nosulfate as a source of selenide ions. The crystallinity

is very low in as-deposited samples, but that improves

Fig. 4. The optical transmittance (a) and reflectance (b) spectra of Cu2�xSe thin film: as-deposited films were annealed at different

temperature in the range 303–523 K.

Al-Mamun, A.B.M.O. Islam / Applied Surface Science 238 (2004) 184–188 187

on annealing in air at 523 K. The grain size of the as-

deposited samples was very small which is observed to

be increase about 30% owing to annealing in air at

523 K. Transmittance and reflectance were found in

the range 5–50 and 2–20%, respectively. The bandgap

for direct transitions varies in the range 1.9–2.3 eVand

that for indirect transition is in the range 1.2–1.7 eV

from as-deposited to annealed sample.

Acknowledgements

The authors are grateful to the Director and staff of

Semiconductor Technology Research Center (STRC),

University of Dhaka, for providing laboratory facil-

ities, and to the Bose Center for Advanced Study and

Research in Natural Sciences, University of Dhaka,

for financial support. The authors also like to acknowl-

edge Prof. W. Jaegermann, TU Darmstadt, Germany,

for allowing his laboratory to do the XPS and AFM

measurements during the research stay in Germany of

A.B.M.O.I.

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