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SUST Journal of Science and Technology, Vol. 20, No. 6, 2012; P:1-7
Deposition and Characterization of p-Cu2O Thin Films
(Submitted: July 18, 2012; Accepted for Publication: November 29, 2012)
M. Rasadujjaman1*, M. Shahjahan2, M. K. R. Khan3 and M. M. Rahman3
1Department of Physics, Dhaka University of Engineering and Technology, Gazipur, Bangladesh; 2Department of
Applied Physics and Electronics, Bangabandhu Sheikh Mujibur Rahman Science & Technology University,
Gopalganj, Bangladesh; 3Department of Physics, University of Rajshahi, Rajshahi-6205, Bangladesh
*E-mail: rana1phyru@gmail.com
Abstract
In this work we have studied the p-type cuprous oxide (Cu2O) thin film fabricated with
thermal oxidation and spray pyrolysis techniques. Cuprous oxide layer was prepared by thermal
oxidation method inside a furnace in the temperature range of 930C - 1010C. X-ray diffraction
studies revealed the formation of single phase cubic Cu2O films. The best quality Cu2O films were
obtained at 970C. The electrical resistivity of the Cu2O layer varies with the variation of
temperature and found of the order ~103 Ωcm. Such films have also been prepared on glass
substrates by spray pyrolysis technique. Optical study showed that the Cu2O films were highly
absorbing in the visible range of electromagnetic radiation. The absorption coefficient, dielectric
susceptibility and optical conductivity are evaluated.
Keywords: Cuprous Oxide thin films, Glass substrates, Oxidation method, X-ray diffraction,
Spray pyrolysis, Band gap energy, Optical absorbance
1. Introduction
Thin film of cuprous oxide (Cu2O) is an interesting well-known p-type semiconductor with a direct band gap
of 2.0 eV. Furthermore, Cu2O is an abundant and economically available material with low toxicity. Even though
Cu2O is one of the earliest semiconducting materials known to physicists and materials scientists, not much
technological advancements has been achieved due to poor conversion efficiencies (2%) in solar cell applications
[1]. This is due to a very limited amount of work devoted to this semiconductor. Although the crystal structure of
cuprous oxide always creates difficulties in the understanding of its electronic conductivity mechanism the
deposition and characterization of cuprous oxide thin films via different techniques have attracted considerable
attention due to their potential application prospects in solar cells [2–5], magnetic devices [6], catalysis [7,8],
photocatalyst [9]. In spite of few studies regarding to the oxidation and spray pyrolysis method, the thermal
oxidation and spray pyrolysis method has some merits, such as the easy control of chemical components and
fabrication of thin film at a low cost to investigate structure and optical properties of Cu2O thin films.
In this paper, Cu2O films have been deposited by thermal oxidation and describe the structural and electrical
properties. Cuprous oxide films have been also deposited by spray pyrolysis method on glass substrate for optical
characterization.
2. Materials and Methods
2.1. Deposition of Cu2O thin film:
2.1.1. Thermal oxidation
The preparation of Cu2O film as reported in the literature is done usually by thermal oxidation technique. A
high purity copper sheet is heated at an elevated temperature in pure oxygen or in laboratory air. A black CuO is
2 M. Rasadujjaman, M. Shahjahan, M. K. R. Khan and M. M. Rahman
formed after a sufficiently long oxidation time, is removed either mechanically or chemically. Also the oxidation
process is followed by annealing the sample at lower temperature (800°C) and then stopping the process by
quenching in cold water [1]. Several oxidation procedures were developed in the past to oxidize copper in order to
obtain Cu2O films with particular characteristics.
The thermal oxidation procedure we used deviated from the previous one. In this technique, Cu2O films were
prepared on 0.8 mm copper sheets of rectangular shape of dimension 2.0 cm × 2.2 cm inside a furnace in the
temperature range of (930–1010)C. The temperature is raised with a rate of 20C/min. Properties of p-Cu2O
strongly depend on the details of the oxidation process. Here no oxygen flow environment was used. It has been
suggested that during oxidation, Cu2O is formed first and after a sufficient long oxidation time, CuO is formed.
The oxidation process includes two steps:
4Cu + O2 → 2 Cu2O
2Cu2O + O2 → 4CuO
The oxidation temperature was optimized at 970°C, which was found to be the optimum condition to deposit
a single phase of Cu2O. Since the films were deposited at higher temperature, there is a possibility of creating
oxygen vacancies or defects in the film. As a result, resistivity of the film will be increased subsequently charge
carrier mobility will be decreased. To reduce resistivity and to maximize the charge carrier mobility post deposition
annealing was performed at 500°C for 2 hours.
2.1.1. Spray pyrolysis technique
The spray pyrolysis is a simple and least expensive for the preparation of the films compared with other
methods. The deposition of Cu2O films was performed by the spray pyrolysis technique. Before deposition, the
substrates was initially cleaned with detergent soap in tap water then rinsed with distilled water, acetone and ethyl
alcohol and allowed to air dry. Cuprous oxide thin films were deposited on glass substrates using the spray pyrolysis
method. The solution was prepared by dissolving appropriate amount of 0.1M copper (II) chloride (CuCl2.2H2O)
and distilled water in a beaker at room temperature with continuous stirring for 10 minutes. Then the solution was
sprayed on the heated substrate. Films of different thicknesses were deposited by varying deposition time from 10 to
30 minutes, keeping molar concentration of copper (II) chloride at 0.1M. Thicknesses of the films measured by
Newton’s ring method using Na-light, were found to increase with the increase of deposition time.
2.2. Characterization of Cu2O thin film:
The crystal structures of the oxidized Cu2O films were analyzed with CuKα line from a Shimadzu X-ray
diffractometer (XRD-6000) and the patterns were recorded in a range of 25–80° (2θ). DC electrical resistivity and
Hall Effect studies were performed by Van der Pauw’s method. Optical properties of the spray deposited Cu2O thin
films in the wavelength range (380–1100) nm were measured using a UV–Spectrophotometer (UV-1601PC
Shimadzu, Japan).
3. Results and Discussion
3.1. Structural studies
Figure 1 represents the XRD spectra of typical as-deposited Cu2O films (oxidized at 970C) and annealed
Cu2O film (annealed at 500°C for 2 hours). The films consists of single phase of Cu2O [JCPDS 05-0667] depending
on the preparation condition [10]. The crystallinity of the films increased after annealing and four XRD peaks at 2θ
= 29.74°, 36.56°, 61.44° and 73.74° appeared, owing to the diffraction of the (110), (111), (220) and (311) planes of
Cu2O.
Deposition and Characterization of p-Cu2O Thin Films 3
It is also found that the peak intensity of annealed sample along (111) plane is increased sharply indicating a
preferred orientation along this plane. After annealing a new peak is observed at 2θ = 42.52°, which resembles the
plane (200) of Cu2O.
The full width at half maximum (FWHM) and crystalline sizes of as-deposited and annealed films were
calculated from the diffraction peaks using the Debye-Scherrer formula [11],
cos
9.0
where, λ is the wavelength of the X-ray used, θ is the Bragg angle and β is the FWHM of a diffraction peak
expressed in radians. The cryatallite sizes for as-deposited and annealed samples along (111) planes are estimated to
be 18 nm and 24 nm, respectively. The result of XRD patterns indicates that if the oxidation temperature is
maintained at 970°C, the pure Cu2O films of increased crystallinity can be produced. From the XRD analyses it was
concluded that annealing plays a significant role in the microstructure and structural properties of the copper oxide
films.
3.2 Electrical properties
DC electrical resistivity measurements were performed in air ambient for freshly deposited films in the
temperature range of room temperature (RT) to 523 K by Van der Pauw’s method.
0
10
20
30
40
50
250 300 350 400 450 500 550
Heating
Cooling
Re-heating
Re-cooling
Resistivity, (ohm-cm)
Temperature, T (K)
Heating
Cooling
Re-heating
Re-cooling
For thickness, t = 9.0 m
Fig. 2: Variation of resistivity with
temperature for Cu2O films of thickness 9.0
μm
-20
0
20
40
60
80
100
120
250 300 350 400 450 500 550
t = 8.5 m
t = 8.7 m
t = 9.0 m
Resistivity, (ohm-cm)
Temperature, T (K)
Fig. 3: Variation of resistivity with temperature
for Cu2O films of different thickness.
Fig. 1: X-ray diffraction spectra of typical (a) as-deposited and (b) annealed Cu2O film at 500°C for 2
hours.
30 40 50 60 70 80
Counts, (a.u.)
Position, 2(deg.)
(a)
As-deposited
(b)
Annealed
Cu2O (110)
Cu2O (111)
Cu2O (200)
Cu2O (220)
Cu2O (311)
25
Cu2O (110)
Cu2O (111)
Cu2O (220)
Cu2O (311)
4 M. Rasadujjaman, M. Shahjahan, M. K. R. Khan and M. M. Rahman
From Fig. 2, it is observed that resistivity shows almost reversible behavior between first heating and cooling
cycles as well as reheating-recooling cycles. However, the order of magnitude of resistivity decreased in RT region.
This may be due to the fact that during successive heating and cooling cycles the compactness of the films increases
and the defect density decreases and due to the removal of metastable phases present if any. Fig. 3 also shows the
electrical resistivity of as-deposited Cu2O films on Cu-substrates at different film thicknesses and is found to be of
the order of 103 ohm-cm. The resistivities are in the range of values reported for Cu2O thin films prepared by
chemical deposition and thermal oxidation methods [1, 12]. The lower resistivity values are required for solar cell
applications. Measured resistivity values at 310K for three different thicknesses are presented in table 1 below:
Table 1: Variation of resistivity with thickness
Thickness (µm)
Resistivity (Ωcm)
8.5
0.01 × 103
8.7
0.02 × 103
9.0
0.10 × 103
It is observed that the resistivity decreases with increasing temperature and this confirms the semiconducting
nature of Cu2O. The decrease in the electrical resistivity is considered to be caused by desorption of oxygen from
the film surface. It is also observed that the resistivity of Cu2O depends on the film thickness. With the increase of
film thickness, resistivity is found to be increased in the measured temperature range.
Conductivity values are determined for as-deposited Cu2O films in the temperature range of room
temperature (RT) to 523K. The variation of conductivity with inverse absolute temperature is found to be linear in
Fig. 4. The activation energy is estimated for the linear curve using the equation
)exp(
0kT
Eac
Measured activation energy values are presented in Table 2
Table 2: Variation of activation energy with thickness
Thickness (µm)
Activation energy (eV)
8.5
1.21
8.7
0.88
9.0
0.39
The activation energy is in good agreement with the reported value for Cu2O thin film [13] which is ascribed
to the hole conduction associated with a single acceptor impurity.
Fig. 4: Variation of lnσ with inverse temperature for Cu2O films of different thickness.
-6
-4
-2
0
2
4
6
8
2.2 2.4 2.6 2.8 3 3.2
t = 8.5 m
t = 8.7 m
t = 9.0 m
ln (mho-cm)-1
Inverse temperature, 1000/T (K-1)
Deposition and Characterization of p-Cu2O Thin Films 5
0
5
10
15
20
25
30
35
1.0 1.5 2.0 2.5 33
A 30 min
B 25 min
C 20 min
D 15 min
E 10 min
Photon energy h (eV)
Absorption Coefficient, x 104(cm-1)
A
B
C
D
E
From Hall Effect study, the Hall coefficients were found to be 120.61 coul/cm3 and 128.71 coul/cm3 for as-
deposited and annealed Cu2O films, respectively. The corresponding hole carrier concentrations were found to be of
the order of 1016cm-3. It was found that in both cases, as-deposited and annealed, Cu2O films were p-type in nature.
The details processes of electrical conductivity and Hall Effect in cuprous oxide are reported elsewhere [14].
3.3 Optical properties
By fixing the better optical properties of Cu2O film for use of solar absorber, we prepared Cu2O films by
spray pyrolysis technique using different spray time. The as-deposited Cu2O films were found to have a very high
optical absorption in the visible spectra. It was determined that all films behaved as absorber materials at about 400-
800 nm wavelength range and absorbance of between 68% to 98% for the films prepared at different deposition
time. The absorbance values of the films decreased sharply at about wavelengths greater than 800 nm because of
their transmittance properties as shown in Fig. 5. We also observed that, the transmittance increases in the
wavelength range greater than 800 nm.
Besides, as seen in Fig. 6, the optical transmissions of the Cu2O films were approximately less than 48% in
the visible region and these transmission values decreased with deposition time was increased.
40
50
60
70
80
90
100
400 500 600 700 800 900 1000 1100
A 30 min
B 25 min
C 20 min
D 15 min
E 10 min
A
B
C
D
E
Wavelength, (nm)
Absorbance, A (%)
Fig. 5: Variation of absorbance with wavelength
for Cu2O films on glass substrate.
0
10
20
30
40
50
400 500 600 700 800 900 1000 1100
A 30 min
B 25 min
C 20 min
D 15 min
E 10 min
Wavelength, (nm)
Transmittance, T (%)
A
B
C
D
E
Fig. 6: Variation of transmittance with
wavelength for Cu2O films on glass substrate.
Fig. 8: Variation of direct band gap with photon
energy for Cu2O films of different thicknesses.
Fig. 7: Variation of absorption coefficient as a function of
photon energy for Cu2O films of different thicknesses.
0
500
1000
1500
2000
1 1.2 1.4 1.6 1.8 2 2.2 2.4
t = 150 nm
t = 160 nm
t = 170 nm
t = 180 nm
t = 190 nm
Photon energy h (eV)
(h)2 x 108 (cm-1eV)2
6 M. Rasadujjaman, M. Shahjahan, M. K. R. Khan and M. M. Rahman
A typical relationship between the thickness and the absorption co-efficient (
) for the prepared films is
shown in Fig. 7. The absorption
co-efficient is found to increase with an increase in deposition time or thickness.
This indicates a likely dependence of the optical absorbance of the deposited films on the thickness during
deposition. In order to better understanding the changes in optical absorbance as a function of deposition parameters,
we computed the direct optical band gap, Eg values using results based on the optical spectrum. For determination of
the optical band gap energy Eg; the method based on the relation
2/
)( n
g
EhAh
was used [15] where n is a number that depends on the nature of the transition. In this case its value was found to be
1 (which corresponds to direct band to band transition) because that value of n yields the best linear graph of
(
h
)2
vs. hυ. Fig. 8 shows
(
h
)2
vs. hυ for the Cu2O film. The intersection of the straight line with the hυ-axis determines
the optical band gap energy Eg. It was found to vary with thickness.
Depending on the film thickness (150 nm to
190 nm) of the Cu2O films direct band gap varies from 1.90 eV to 1.64 eV. The band gap for film
thickness
150 nm
is found to be
1.90
eV,
which is low given in the earlier report [16]
. The direct band gaps
obtained for the Cu2O films are given in the table-3 below:
Table 3:
Variation of direct band gap with thickness
Thickness, t (nm)
Direct band gap, Eg (eV)
150
1.90
160
1.82
170
1.77
180
1.67
190
1.64
The studies indicate that the band gap of spray deposited Cu2O film is affected by film thickness. Other optical
parameters such as absorption coefficient (
), dielectric susceptibility (χ), and optical conductivity (
) of Cu2O
films are estimated using the relations [17].
tR
T
2
)1(
ln
1
41
22 kn
4nc
The values of α and χ estimated for a typical Cu2O film of thickness 160 nm are 17.17 × 104 cm-1, 0.04 and
respectively at hv = 1.7 eV. The positive susceptibility of the film indicates the paramagnetic nature of the material.
The value of σ is 2.16 × 1012 s-1 at 1.5 eV.
4. Conclusion
In the present work, we studied thin films of cuprous oxide grown by oxidation method. For the best quality of Cu2O
films, oxidation temperature was optimized to 970°C. Both the as-deposited and annealed films carry the crystalline
character of the Cu2O phase. The Cu2O film has mainly (111) crystalline orientations. The grain size of crystallites
was found to be 25 nm for annealed film. The electrical resistivity of the Cu2O films varies with the variation of
temperature and thickness. The activation energy of the Cu2O films of different thicknesses were in the ranges of
(0.39-1.21) eV. Hall effect measurement confirms the p-type nature of the Cu2O films. Cuprous oxide thin films
with different thicknesses were also prepared on a glass substrate by spray pyrolysis. From optical study it is
observed that Cu2O films are highly absorbing in the visible range of electromagnetic radiation. The band gap
energy decreases with the increasing thickness for Cu2O films. It is thought that because of these properties, Cu2O
thin films can be used as absorber material in photovoltaic applications.
Deposition and Characterization of p-Cu2O Thin Films 7
Acknowledgements
Dr. M. Faruk Hossain, Department of Electrical and Electronic Engineering, University of Toyama, Japan, is
gratefully acknowledged for the X-ray diffraction experiment.
References
[1] A. Mittiga, E. Salza, F. Sarto, M. Tucci, and R. Vasanthi, Appl. Phys. Lett. 88 163502 (2006).
[2] S. Ishizuka, S. Kato, Y. Okamoto, T. Sakurai, K. Akimoto, N. Fujiwara and H. Kobayashi, Appl. Surf. Sci.,
216 94 (2003).
[3] T. Mahalingam, J. S. P. Chitra, J. P. Chu and P. J. Sebastian, Mater. Lett. 58 1802 (2004).
[4] B. P. RaI, Sol. Cells, 25 265 (1988).
[5] K. Nakaoka, J. Ueyama and K. Ogura, J. Electro Chem. Soc., 151 C661 (2004).
[6] R. A. Borzi, S. J. Stewart, R. C. Mercader, G. Punte and F. Garcia, J. Magn. Magn. Mater., 1513 226–230
(2001).
[7] A. Kellersohn, E. Knozinger, W. Langel and M. Giersig, Adv. Mater., 652 7 (1995).
[8] K. Borgohain, J. B. Singh, M. V. Rama Rao, T. Shripathi and S. Mahamuni Phy. Rev. B 11093 61 (2000).
[9] M. Hara, T. Kondo, M. Komod, S. Ikeda, K. Shinohara, A. Tanaka, J. N. Kondoa and K. Domen, Chem.
Commun., 357-358 (1998).
[10] International Centre for Diffraction Data-Copyright JCPDS (1999), cards: JCPDS 05-0667 for Cu2O.
[11] B. D. Cullity, Elements of X-ray diffraction, Addition-Wesley Publishing Co. Inc, 262 (1967).
[12] Y. Hames, S. E. San, Solar Energy, 77 291–294 (2004).
[13] K. Nakaoka and K. Ogura, J. Electrochem. Soc., 149 C579 (2002).
[14] W. Feldman, Physical review, 64 3-4 (1943).
[15] A. E. Rakhshani, Solid-State Elect., 29 7 (1986).
[16] G. P. Pollack, D. Trivich, J. Appl. Phys. 46 63–172 (19751).
[17] T. Mahalingam, J. S. P. Chitra, J. P. Chu, H. Moon, H. J. Kwon, Y. D. Kim, J. Mater Sci: Mater Electron, 17
519–523 (2006).
