in-situ facile chemical bath deposition of cus thin film ... · rinsed rigorously with laboline...
TRANSCRIPT
In-situ Facile Chemical Bath Deposition of CuS Thin Film for
Photosensor Application
Bhushan B. Chaudhari1, Nitin P. Huse2, Nanasaheb P. Huse3,*, Ramphal B. Sharma4,* 1,3P. G. Department of Physics, NTVS’s G. T. Patil College, Nandurbar. (MS-India)- 425412.
2Department of Botany, Deogiri College, Aurangabad. (MS-India)- 421004. 4Thin Film and Nanotechnology Lab, Department of Physics, Dr. B. A. M. U., Aurangabad. (MS-India)- 421004.
[email protected] [email protected]
Abstract— Herein we report the successful synthesis of CuS thin film by in-situ facile chemical bath deposition technique on a silica glass substrate at low temperature. Reagents such as copper sulfate, thiourea, triethanolamine, and ammonia of AR grade have been used to prepare the precursor solutions for the deposition of the CuS thin film. The structural confirmation of as-grown CuS thin film has been confirmed by X-ray diffraction pattern with average crystallite size ~13.13 nm. Two distinct sharp peaks have been observed which confirms the single phase crystalline nature of the CuS thin film without any impurity. These peaks are assigned to the respective planes by comparing it with standard JCPDS card. XRD pattern shows Covellite phase of as-grown copper sulfide thin film when compared with standard JCPDS card # 06-0464. From the optical study, it has been found that the as-grown thin film has a higher absorbance in the visible region with a computed band gap of ~2.28 eV which is in very good agreement with the previous reports. I-V characteristics have been measured with the help of I-V source meter of Keithley interfaced with class AAA solar simulator in presence of dark and light illumination. From I-V characteristics Photosensing nature of the film has been observed as there is a drastic increase in current after light illumination which may be attributed to the breaking of some covalent bonds due to incident photon energy. The photosensitivity has been calculated for the 100 Watt light and was found to be ~56 % at bias voltage 2 V.
Keywords— Copper sulfide, Covellite, Thin Film, CBD and Photosensitivity.
I. INTRODUCTION
In the last few decades, semiconducting chalcogenides thin films have been studied immensely in the field of science and technology for various applications [1]. In this context, the research has been focused to find the materials which are abundant in earth’s crust and which shows stable behavior over an extended period of operation. One of such materials is Copper sulfide which is available in five stable phases at room temperature such as Covellite (CuS) in S-rich region which has higher electrical conductivity and stability up to ~500°C whereas anilite (Cu1.75S), digenite (Cu1.8S), djurleite (Cu1.95S) and chalcocite (Cu2S) in the Cu rich region [2]. The Copper sulfide has a wide range of bandgap ranging from 1.2 to 2.5 eV, which depends on the stoichiometric composition of CuxS (x=1-2) [3]. The Copper sulfide finds large application in various fields such as electroconductive coatings [4], solid-state solar cell [5], electrodes and catalysis [6], field emission [7] and switching [8] etc. A number of reports are available on physical and chemical synthesis techniques of copper sulfide thin film such as Chemical Vapor Deposition (CVD) [9], Thermal Co-evaporation [10], Radio Frequency Reactive Sputtering [11], Photochemical Deposition [12], Electrodeposition [13], Spray Pyrolysis [14], Successive Ionic Layer Adsorption and Reaction (SILAR) [15], and Chemical Bath Deposition (CBD) [16,17] etc. Among them, the chemical bath deposition (CBD) method is effective since it has enormous advantages over other conventional thin film deposition methods. Chemical bath deposition excessively used simple and low-cost techniques for the large-scale commercial production and uniform deposition of the thin film. Furthermore chemical bath deposition yields adherent, uniform, and homogenous thin films. The main advantages of chemical bath deposition are the growth of thin film can be controlled by numerous parameters like the temperature of the bath, pH of the solution, the concentration of the precursor solution and the deposition time.
Although there are many reports are available focused on different property study and their synthesis of copper sulfide thin film, but few have reported photosensing properties of the copper sulfide thin film [17,18]. With this motivation, we are reporting here the photosensitivity of copper sulfide thin film deposited by facile and economic chemical bath deposition method. Along with photosensing properties structural and optical properties of as-grown copper sulfide, thin films have been studied and reported here.
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Volume VI, Issue I, January/2019
ISSN NO: 1076-5131
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II. EXPERIMENTAL To get the homogeneous and uniform deposition of thin films substrate cleaning plays a vital role. Silica glass slides have been
used as a substrate in the present work which was cleaned by using chromic acid, laboline solution, and distilled water. Glass slides were kept in a chromic acid at ~70 °C temperature for six hours and washed by distilled water. These glass slides further rinsed rigorously with laboline solution followed by distilled water and then dried in air.
To prepare the precursor solution for the deposition of Copper sulfide thin film 50 ml deionized water has been taken in separate beakers. The 0.2 M copper sulfate (CuSO4.8H2O) and 0.2 M Thiourea (CS(NH2)2) has been taken in the separate beakers and stirred simultaneously. In the beaker of copper sulfate, 3 drops of Triethanolamine (TEA) added with constant stirring followed by addition of ammonia (NH3) to maintain the pH of the solution up to ~11. The completely dissolved thiourea solution was then added to the copper sulfate solution with constant stirring. Then pre-cleaned glass substrates have been immersed in the solution vertically the final solution was kept in the water bath maintained at ~50 °C for one hour. After one hour uniform, adherent obtained thin films were rinsed with water and dried in air subsequently. The schematic representation of the chemical bath deposition method to deposit the CuS thin film is shown in Fig. 1.
Fig. 1: Schematic of chemical bath deposition to deposit CuS thin film.
III. CHARACTERIZATION The as-grown thin films have been carried out for structural, optical and electrical characterization. The structural analysis was
carried out by using X-ray Diffractometer (Bruker, AXS, D-8Advanced, Germany) in scanning range 20°-70° (2θ) using CuKα1 radiation with wavelength 1.5405 Å. The optical properties of the film have been recorded by using UV-Vis. Spectrophotometer (Perkin-Elmer Lambda-25 Spectrophotometer) in the wavelength range between 300 nm to 900 nm. The electrical properties of the as-grown thin film have been carried out by I-V characteristics using I-V source meter of Keithley model 2400 interfaced with class AAA solar simulator in dark and under light illumination in the voltage range of ±2 V.
TABLE I STRUCTURAL PARAMETERS OBTAINED FROM XRD PATTERN OF CUS THIN FILM.
(2θ)° (hkl)
planes Interplanar spacing (Å)
FWHM β (10-
3rad) Crystallite size D
(nm) (1015)
(lines/m2) ɛ
(10-3)
27.1 101 3.286 7.39 19.29 2.688 1.8
29.24 102 3.051 10.05 14.26 4.914 2.43
31.6 103 2.827 33.02 4.36 52.51 7.94
51.71 108 1.765 10.53 14.63 4.666 2.37
Average values 13.13 16.21 3.63
JASC: Journal of Applied Science and Computations
Volume VI, Issue I, January/2019
ISSN NO: 1076-5131
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IV.RESULTS AND DISCUSSION STRUCTURAL ANALYSIS
The structural analysis of the as-grown CuS thin film was done by using X-Ray Diffractometer (XRD) which is the most significant and most widely used technique for the purpose of identification of phase composition and structure of the material. Fig. 2 shows the typical XRD pattern obtained from the as-grown CuS thin film onto a glass substrate which indicates the formation of Covellite CuS which reveals Covellite hexagonal phase of the as-grown CuS thin film when compared with standard JCPDS card (JCPDS # 06-0464). Two distinct prominent peaks along with two others have been observed at (2θ) 27.1°, 29.2°, 31.6°, 51.7° corresponds to the diffraction from (101), (102), (103) and (108) planes respectively, exhibits the polycrystalline nature of the as-grown CuS thin film. From XRD pattern it is clear that no other impurity peaks have been observed which the evidence of the single-phase CuS formation without any peaks from any other possible phases of copper sulfide.
Fig. 1: XRD pattern of as-grown CuS thin film
The average crystallite size (D) has been calculated by using the Scherer formula given in Eq. 1, which was found to be ~13.13 nm. The dislocation density (δ), strain (ε) have been calculated by using Eq. 2 & 3 along with few other crystalline parameters and the calculated values are reported in Table 1. The minor broadening of the diffraction peaks have been observed which may be attributed to the strain and dislocation density remain in the film while the growth of the thin film [19].
D�hkl� � �
�� � (1)
δ � �
�� (2)
ε � �� �
� (3)
Where D represents the crystallite size estimated for the (hkl) reflection planes, K is a dimensionless shape factor and has the value 0.91; β is the full width at half maximum (FWHM) calculated for all peaks and λ is the wavelength of the X-rays used.
JASC: Journal of Applied Science and Computations
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ISSN NO: 1076-5131
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OPTICAL STUDY The optical absorption spectra have been recorded with the help of UV-Vis. Spectrophotometer to study the optical properties
of the as-grown CuS thin film. Fig. 3 shows the absorption spectra plotted with the absorbance ‘αt’ as a function of wavelength whereas the inset in Fig. 3 represents the Tauc’s plot of the as-grown CuS thin film to compute the band gap value. The absorption spectrum shows a blue shift in the absorption with a band edge at ~600 nm which can be attributed to the relatively smaller crystallite size [20]. The absorption data were further used to compute the band gap of the as-grown CuS thin film. The band gap has been computed by extrapolating the straight line of the curve to intersect the energy axis in Tauc’s plot and the obtained value is 2.28 eV which is in good agreement with the earlier reports [2,17,21]. The Tauc’s relation used to determine the band gap value is given in Eq. 4.
� ����������
�
�� (4)
Where α is the absorption coefficient, h stands for Plank's constant and ν is the frequency of incident light, Eg stands for optical band gap and n has the value of 1/2 and 2 for indirect and direct transitions respectively.
Fig. 3: UV-Vis. Absorption spectra and Tauc’s plot of as-grown CuS thin film.
ELECTRICAL STUDY
The electrical properties have been studied by I-V characteristics curve in dark under light illumination of 100 W light in the voltage range of ±2 V. The unit cm2 area of the as-grown CuS thin film was outlined with silver paste contacts at two corners of the film to make sure the good neutral electrical contact. Fig. 4 shows the typical I-V characteristics curve in which both the curves shows straight line nature passing through the origin having a high current which may be due to the ohmic nature of the metal-semiconducting contact. Also, the light illumination results in a drastic increase in current which may be attributed to the production of free charge carriers due to the incident photon energy [2]. From the I-V characteristics curve the photosensor efficiency (P), photoresponsivity (R) and photosensitivity (S) has been calculated from the Eq. (5), (6) and (7) and the obtained values are 1.41 %, 56 %, and 7 µA/cm2 which shows its potential candidature in various optoelectronic devices [2,22,23].
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P �!"
!# (5)
R �!"�!#
%&' (6)
S �%� � +#�+,
+#× 100 (7)
Where, Rd & Rl stands for resistance in dark and light respectively, Ip & Id stands for photonic current and dark current respectively, S is the effective film area, and Pi stands for the power of the incident light per unit area.
Fig. 1: I-V characteristics of as-grown CuS thin film.
V. CONCLUSION Here in this report, we have successfully synthesized CuS thin film by facile chemical bath deposition at a relatively low
temperature. The single phase Covellite structure of the as-grown CuS thin film has been confirmed from X-ray diffraction. The optical study of the as-grown CuS thin film shows a blue shift in absorbance which results in 2.28 eV band gap which is comparable to the earlier reports. The electrical properties have been studied and which shows high current both in dark and under light illumination of 100 W. The obtained values of photosensor efficiency (P), photoresponsivity (R) and photosensitivity (S) shows its potential candidature for various optoelectronic devices.
ACKNOWLEDGMENT
Authors are thankful to the Principal, NTVS’s G. T. Patil College, Nandurbar for providing necessary lab facilities to carry out the research work. We are also thankful to the Head, Department of Physics and Department of Nanotechnology, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad for the support and characterization facilities. We are also thankful to the Director, School of Physical science, North Maharashtra University, Jalgaon for XRD characterization facilities. REFERENCES
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ISSN NO: 1076-5131
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