Journal of Science: Advanced Materials and Devices (2017) 165e171 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article The effect of annealing on structural, optical and photosensitive properties of electrodeposited cadmium selenide thin films Somnath Mahato a, b, *, Asit Kumar Kar a a b Department of Applied Physics, Indian Institute of Technology (Indian School of Mines) Dhanbad, 826004 Jharkhand, India Saha Institute of Nuclear Physics (Surface Physics and Material Science Division), 1/AF Bidhannagar, Kolkata 700064, India a r t i c l e i n f o a b s t r a c t Article history: Received December 2016 Received in revised form April 2017 Accepted April 2017 Available online 15 April 2017 Cadmium selenide (CdSe) thin films have been deposited on indium tin oxide coated glass substrate by simple electrodeposition method X-ray Diffraction (XRD) studies identify that the as-deposited CdSe films are highly oriented to [002] direction and they belong to nanocrystalline hexagonal phase The films are changed to polycrystalline structure after annealing in air for temperatures up to 450 C and begin to degrade afterwards with the occurrence of oxidation and porosity CdSe completely ceases to exist at higher annealing temperatures CdSe films exhibit a maximum absorbance in the violet to bluegreen region of an optical spectrum The absorbance increases while the band gap decreases with increasing annealing temperature Surface morphology also shows that the increase of the annealing temperature caused the grain growth In addition, a number of distinct crystals is formed on top of the film surface Electrical characteristics show that the films are photosensitive with a maximum sensitivity at 350 C © 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: CdSe Thin film Electrodeposition XRD Photosensitivity Introduction Semiconductors are very important and interesting because of their technological applications in optoelectronics and microelectronic devices like photodiodes [1], sensors [2], light emitting diodes [3], solar cells [4], photoelectrochemical cells [5], photovoltaic cells [6] and photodetectors for optical communications etc Among them, Cadmium Selenide (CdSe) is a IIeVI group compound semiconducting material of the periodic table This compound is a highly photosensitive material in the visible region due to their suitable band gap (1.74 eV) Different processes such as chemical vapour deposition [7], physical vapour deposition [8], thermal evaporation technique [9], spray-pyrolysis [10], chemical bath deposition [11], dip coating [12] and electrodeposition [13] have been used for depositing cadmium selenide thin films However, the electrodeposition process is one of the simplest and low-cost techniques because it is easy to manage and it requires very simple arrangement Deposition rate is easily controlled by changing deposition potential, concentration and pH value of the electrolyte Many groups are working on cadmium selenide using the process of electrodeposition [5,7,14e16] The optoelectronic, microelectronic and other applications of cadmium selenide thin films depend on their structural and electronic properties affecting device performance These properties are strongly influenced by the deposition parameters such as deposition time, deposition potential, concentration of electrolytic solution, pH of the electrolyte and thermal annealing Thermal treatment is one of the important factors to enhance the efficiency and stability of photosensitive devices Thus, studies of the effect of annealing on structural, optical and electrical properties of thin films are very important in understanding and enhancing device sensitivity [17e19] The aim of this present work is to prepare cadmium selenide thin films by a simple electrodeposition process on indium tin oxide (ITO) coated glass substrates and to study the effect of annealing temperature (Ta) on films' photosensitivity The effect of annealing on crystallinity, morphology and optical absorbance of the films are also presented and discussed * Corresponding author Department of Applied Physics, Indian Institute of Technology (Indian School of Mines) Dhanbad, 826004 Jharkhand, India E-mail address: som.phy.ism@gmail.com (S Mahato) Peer review under responsibility of Vietnam National University, Hanoi http://dx.doi.org/10.1016/j.jsamd.2017.04.001 2468-2179/© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 166 S Mahato, A.K Kar / Journal of Science: Advanced Materials and Devices (2017) 165e171 Experimental 3.5 mW/cm2) tungsten bulb controlled by a dc power supply and it was placed 20 cm away from the sample during experiment 2.1 Film deposition Cadmium selenide thin films have been deposited on indium tin oxide coated glass substrates by using a simple two-electrode electrodeposition process at room temperature (28 C) Sputter coated ITO/glass was procured from Macwin India, Delhi The substrate, having sheet resistance 10 U/sq, was used as a working electrode or cathode, and a flat graphite rod was used as an anode The size of both the electrodes submersed in electrolyte was about  cm2 and hence the area of deposition was about cm2 The electrodes were separated by a distance of about cm Substrates were cleaned in acetone within an ultrasonic bath for 15 and then cleaned in running distilled water for and, finally, they were dried in air for 15 before deposition For electrodeposition of the films, cadmium chloride (CdCl2) and selenous acid (H2SeO3) were used as the sources of cadmium and selenium, respectively in the electrolyte; the molar concentrations of cadmium chloride and selenous acid were 0.08 M and 0.005 M, respectively The electrolyte was continuously stirred for 15 in a beaker by using a Teflon coated magnetic paddle attached to a stirrer, in order to perfectly dissolve the ingredients in distilled water All the chemicals were procured from Sigma Aldrich and had 99.99% purity The total volume of the prepared electrolyte was 100 ml pH The value of the electrolyte was kept at 1.9 by using HNO3 solution The deposition was conducted for 15 with a fixed deposition potential of 1.80 V for all the films After deposition, the thin film coated substrates were taken out from the electrolyte, then rinsed in distilled water and dried in air The as-deposited films were annealed at 250, 350, 450, 550 and 650 C in the air for one hour in a muffle furnace with a ramp up rate of C/min followed by normal cooling to room temperature 2.2 Reaction mechanism The reaction mechanism of CdSe thin film is discussed as follows The deposition process is based on the slow release of Cd2ỵ ions and Se2 ions in the solution ion-by-ion basis and settling on the ITO coated glass substrates The deposition takes place when the ionic product of Cd2ỵ and Se2 is greater than the solubility product Cadmium selenide is deposited according to the following over-all net reaction [20] H2 SeO3 þ Cd2þ þ 6eÀ þ 4Hþ #CdSe þ 3H2 O The rate of the formation of CdSe is determined by the bath parameters such as pH, concentration and temperature of the electrolyte [21] Results and discussion 3.1 Crystallinity Cadmium selenide thin film grown on ITO coated glass substrate is found to be polycrystalline with hexagonal (wurtzite) crystal structure Fig 1(a) shows the XRD pattern of as-deposited or unannealed CdSe thin film The peak at 25.99 corresponds to the plane (002) which is much stronger than other peaks The intense peak at (002) suggests a dominant orientation of nanocrystalline phase of CdSe thin film within an otherwise amorphous or nearly amorphous matrix The small hump in the background is due partly to the amorphous nature of ITO coated glass substrate and also may be due to some amorphous phase presented in the CdSe thin film itself [22] Fig 1(b)e(f) shows the XRD patterns of annealed films Annealing at 250 C [Fig 1(b)] makes the film more oriented towards (002) plane The polycrystalline hexagonal CdSe phase is found after annealing at 350 C [Fig 1(c)] Intensity of the most intense peak is continuously found to decrease with the increase of annealing temperature It signifies a gradual change of a highly oriented nanocrystalline phase to a polycrystalline phase Further heat treatment from 450 C to 650 C shows that the CdSe phase gradually changes to CdO phase [Fig 1(d)e(f)] At the annealing temperature 550 C and above, CdSe completely disappears All the XRD patterns from Figs (d)e(f) show the characteristic diffraction peaks of (111) and (200) planes of polycrystalline hexagonal CdO phase Other peaks (211) at 21.88 , (222) at 30.91, (400) at 35.68 and (622) at 61.18 correspond to ITO This suggests that the after annealing of CdSe thin films in air at a higher temperature [Ta ! 450 C], reaction occurs and chemically a new phase formation takes place; the polycrystalline phase of CdO gradually prevails over the polycrystalline phase of CdSe with increase in temperature XRD plots from (a) to (f) also exhibit gradual reduction in overall peak intensity and hence a rise in background intensity They also demonstrate the appearance of more ITO peaks with enhanced intensity at higher annealing temperature These facts might be related to the gradual loss of CdSe and later CdO [Figs (e) and (f)] from the surface of the thin films due to sublimation during annealing and the possibility of diffusion into the substrate may be ruled out Average crystallite size of CdSe films is found to vary from 16.8 nm to 21.9 nm This was calculated from Scherrer's formula using full width at half maximum (FWHM) b of the peaks of XRD profiles [23e25] D¼ 2.3 Film properties X-ray diffraction (XRD) patterns were recorded using XRD (BRUKER D8 FOCUS) system with the Cu Ka radiation (l ¼ 1.5406 Å) qe2q scan was taken for the range of 10 e80 with a speed of 0.20 / s and with a step size of 0.030 Optical absorption spectra were obtained for the region 300 nme900 nm using UVeviseNIR spectrophotometer The microstructure and composition of the CdSe thin films were studied using a scanning electron microscope (FESEM, Model: JEOL JSM-5800 Scanning Microscope) and energy dispersive analysis of X-ray (EDAX) module attached with the same SEM system respectively The electrical resistivity of the samples was measured by the two-point probe technique Currentevoltage measurements in dark and illumination were accomplished using a Keithley 2400 source metre The light source was a 100 W (intensity Kl bhkl cos q (1) where D ¼ crystallite size, K ¼ shape factor (0.9), and l ¼ wavelength of Cu Ka radiation The microstrain (ε) values have been calculated by using the following formula: ε¼ bhkl (2) tan q Assuming that, the particle size and strain are independent of each other, equations (1) and (2) may be combined to the following form: bhkl cos q ẳ Kl ỵ sin q D (3) S Mahato, A.K Kar / Journal of Science: Advanced Materials and Devices (2017) 165e171 167 Fig XRD patterns of CdSe/ITO thin films at different annealing temperatures: (a) As-deposited, (b) 250 C, (c) 350 C, (d) 450 C, (e) 550 C, and (f) 650 C This is known as WilliamsoneHall formula [26] The graph was plotted between bhkl cos q versus sin q as shown in Fig From the linear fit to the data, the crystallite size was estimated from the intercept along ordinate, and strain (ε) was found from the slope of the fit From WilliamsoneHall (WeH) method the average crystallite size is determined to be 31.5 nm for CdSe thin film annealed at 450 C The dislocation density d has been calculated by using the formula for the highly intense X-ray diffraction peaks d¼ 15ε : aD (4) All the calculated values are shown in Table As expected, increase in annealing temperature leads to increase in crystallite size, and decrease in strain and dislocation density of the films 3.2 UVeviseNIR spectroscopy Fig shows the variations of optical absorbance and transmittance (inset) with wavelength of the as-deposited and annealed CdSe thin films Absorption spectra is strong around the violet to visible region Afterwards, it continuously decreases with increase in wavelength and becomes almost constant at near infrared (NIR) region for the as-deposited film For the annealed films, however, the decrease in absorbance shows a sharp fall at around 700 nm and then it gradually saturates in the NIR region The absorbance 168 S Mahato, A.K Kar / Journal of Science: Advanced Materials and Devices (2017) 165e171 Fig WeH plot for a film annealed at 450 C increases and the broad peak shifts from violet to blue-green region with increasing annealing temperature It may be due to increased crystallite size in the thin films The colour of the film is found to change from red-orange to dark black after annealing The values of the band gap of the films have been determined from transmission spectra by using the following relation applicable to near edge optical absorption of semiconductors: a¼ Ãn K  hn À Eg hn (5) where a is absorption co-efficient, hn is the photon energy, K is a constant, Eg is the band gap and n is a constant which equals to ½ for allowed direct band-gap semiconductor in the present case [27,28] The band gap energy of CdSe/ITO thin film has been determined by Tauc plot based on the above formula as shown in Fig The optical band gaps are found to be 2.13 eV, 1.95 eV, 1.91 eV and 1.88 eV for thin films of as-deposited and annealed at 250, 350 and 450 C temperature respectively The band gap of as-deposited or unannealed film is higher compared to the annealed CdSe thin films because the deposition at room temperature gives rise to films with smaller crystallite size So the energy band gap of CdSe thin films tend to decrease as the annealing temperature is increased due to increased crystallite size of the films The value of the extinction coefficient (k) is calculated from the following relation [29]: k¼ Fig UVeviseNIR absorbance and transmittance (inset) spectra of CdSe/ITO thin films annealed at different temperatures al (6) 4p The graphical representation of the variation of extinction coefficient with wavelength is shown in Fig The graph shows that even for the photons having energy above band gap, the absorption coefficient is not constant and strongly depends on wavelength For photons which have energy very close to that of the band gap, the absorption is relatively low since only the electrons at the valence Fig Tauc plots for as-deposited and annealed CdSe/ITO thin films band edge can interact with the photon to cause absorption As the photon energy increases, not just the electrons already having energy close to that of the band gap can interact with the photons, a larger number of other electrons below band edge can also interact with the photons resulting in absorption Thus extinction coefficient has high values near the absorption edge and it has very small values at higher wavelengths 3.3 Surface morphology The surface morphology of as-deposited film and annealed films has been studied using FESEM as shown in Fig 6(a)e(f) Surface Table Structural parameters for as-deposited and annealed CdSe thin films calculated from their corresponding XRD profiles Ta ( C) Crystallite size (nm) Lattice parameters (Å) a c Unannealed 250 350 450 16.8 18.1 18.2 31.5a 4.24 4.24 4.24 4.24 6.93 6.93 6.93 6.93 a Calculated from WeH plot Strain (ε) Dislocation density d (Â1017/m2) 0.526 0.492 0.485 0.413a 11.09 9.62 9.43 6.68 S Mahato, A.K Kar / Journal of Science: Advanced Materials and Devices (2017) 165e171 Fig Dispersion curves of extinction coefficient (k) for as-deposited and annealed CdSe/ITO thin films topography of as-deposited film is shown in Fig 6(a) From the topograph, it is observed that the as-deposited films are continuous with homogeneous distribution of densely packed blister-like particles of nonuniform size varying from several tens of nanometre to about 250 nm Fig 6(b) shows a cross-sectional tilted view of the film annealed at 250 C; spherical nanosized grains of globule-like structure are observed with several 100 nm in size and the grains are closely packed with each other to form a crystalline matrix A wide view of the corresponding area has been presented in Fig 6(e), which covers parts of both cross-sectional and surface features It appears that after annealing, the particulate features 169 were more uniform in size, reducing the range of variation observed in as-deposited films At the annealing temperature 350 C [Fig 6(c)], it is found that the films become rougher with the development of some pebble-like crystalline surface features of size varying from about 50 nm to 300 nm Apparently the blisterlike features in figure (a) have played the role of growth centres and crystalline features are developed through the process of surface and volume diffusion with increase in temperature The SEM micrographs of the film annealed at 450 C are shown in Figs 6(d) and (f) where the latter represents a wide area view A drastic change in crystalline structural features is observed for 100 C increase in annealing temperature with respect to Fig 6(c) Excellent single crystalline structures of width as big as 1.5 mm with various polygon like [30] facets are noticed to evolve on the film surface but with very less in number compared to the film annealed at 350 C Some pores are also found to develop on the surface of the film of irregular shape appearing like crystalline voids Other than the crystals on the surface and the pores, the surface of the film appears to be smooth with clear demarcation of crystalline grains i.e grain boundaries Grain size varies from about 100 nm to 500 nm Top surfaces of the embedded crystalline grains are found to form a nice mosaic pattern Due to annealing, a number of smaller grains or crystals diffuse and coalesce together to effectively form larger crystalline grains with clear crystallographic faces Above mentioned results demonstrate that the process of annealing induces two parallel grain growth processes e one within the volume of the thin film matrix e a primary growth process, and the other over the thin film surface e a secondary growth process Crystalline nature of CdSe thin films is also indicated by XRD measurement Thickness of the films was found to be about mm by crosssectional imaging in SEM Energy dispersive analysis of X-rays (EDAX) confirms the presence of both Cd and Se in the films It also reveals that the thin films annealed at different temperatures are nonstoichiometric in nature Fig Scanning electron micrographs of CdSe thin films: (a) As-deposited, (b) annealed at 250 C (a tilted cross-sectional view), (c) annealed at 350 C, and (d) annealed at 450 C, all shown at the same scale for better comparison [5mm  mm]; (e) a tilted cross-sectional wide view of (b) [40mm  40 mm] and (f) a large area view of (d) [15mm  15 mm] 170 S Mahato, A.K Kar / Journal of Science: Advanced Materials and Devices (2017) 165e171 Fig (a) Currentevoltage characteristics of CdSe thin film annealed at 350 C under dark and illumination conditions; Inset: Schematic diagram of IeV measurement set-up; (b) Variation of photosensitivity of CdSe thin films with annealing temperature area, low cost, and good quality CdSe thin films for photodiode and photovoltaic applications 3.4 Electrical property The electrical resistivity of CdSe/ITO thin film has been measured by using dc two probe methods It is determined by loading a direct current I and measuring a voltage drop V between two probes which are placed at a distance (s) of mm, using the following equation [31,32]: r ¼ 2ps V I (7) At room temperature the specific conductance was found to be of the order of 10À4 (UÀ1 cmÀ1) For photoconductivity measurement of CdSe thin films, area of the film exposed to light was  cm2 The dark and illumination IeV characteristics of CdSe thin films were recorded as shown in Fig 7(a) as an example All the films under dark conditions showed good rectifying nature They also responded to illumination giving rise to photocurrent with again rectifying nature or asymmetric semiconducting nature The characteristic curves demonstrated that the photo response was sensitive to annealing temperature The photosensitivity S of the films was calculated using the following formula: Conclusion The CdSe thin films have been successfully deposited by a simple two electrode electrodeposition method on ITO coated glass substrates The process of annealing in air has been found to change the crystallinity of films from highly oriented nanocrystalline (hexagonal wurtzite) structure to polycrystalline form With annealing globular nanocrystalline grains become bigger and a number of distinct micro-crystals are developed on top of the film surface; the crystals grow to a maximum in size at 450 C having clear crystallographic faces on their surface For annealing temperatures higher than 450 C, CdSe is chemically degraded and is converted to CdO The CdSe films exhibit strong absorbance in the violet to blue-green region With increase in the annealing temperature, the band gap decreases from 2.13 eV to 1.88 eV for the asdeposited and 450 C films The CdSe films are photosensitive; the sensitivity increases with annealing temperature up to 350 C and then decreases Acknowledgements slight À sdark s¼ sdark (8) where sphoto is the photoconductivity and sdark is the dark conductivity [9] The as-deposited CdSe thin films show weak photoconductivity and its sensitivity is less (S ~ 3) Annealing at 250 C reveals increased photoconductivity and its sensitivity increases to ~12 The photoconductivity is found to dramatically improve (S ~ 64) at 350 C annealing temperature So it is observed that the photosensitivity is increased with the increase of annealing temperature as shown in Fig 7(b) The reason is associated with the increased absorbance of the incident light in visible region with increase in annealing temperature Enhancement in the photoconductivity is due to the generation of more electron-hole pairs excited by the incident light Annealing at 450 C leads the photoconductivity to fall to zero because of the phase change and accompanying degradation of CdSe thin film At this temperature, microstructural defects like pores and formation of secondary phase like CdO impair and saturate the conduction of charge carriers even after their enhanced generation due to higher absorbance The result may be beneficial to the development of large Authors are grateful to Dr B Pandey, Dr N Das, Dr D Roy and Mr A Jana of the Department of 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transport, optical properties and photocurrent measurements in two-dimensional arrays of silicon nanocrystals embedded in SiO2, Nanoscale Res Lett (2011) 227 ... as follows The deposition process is based on the slow release of Cd2ỵ ions and Se2À ions in the solution ion-by-ion basis and settling on the ITO coated glass substrates The deposition takes place... O The rate of the formation of CdSe is determined by the bath parameters such as pH, concentration and temperature of the electrolyte [21] Results and discussion 3.1 Crystallinity Cadmium selenide. .. related to the gradual loss of CdSe and later CdO [Figs (e) and (f)] from the surface of the thin films due to sublimation during annealing and the possibility of diffusion into the substrate may be