Annealing temperature dependent structural and

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sensor device [4], solar cell [5], photovoltaic cell [6] as well as ... The present work deals with the the deposition of CdSe thin films using simple two electrode ... Cadmium selenide thin films were deposited on indium tin oxide coated glass ... electrode can be explained by the chemical reactions involved in the aqueous ...
Materials Science in Semiconductor Processing 39 (2015) 742–747

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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

Annealing temperature dependent structural and optical properties of electrodeposited CdSe thin films S. Mahato a,n, Nanda Shakti b, A.K. Kar a a b

Department of Applied Physics, Indian School of Mines, Dhanbad 826004, India Department of Physics, Central University of Rajasthan, India

art ic l e i nf o

a b s t r a c t

Article history: Received 20 May 2015 Received in revised form 6 June 2015 Accepted 7 June 2015 Available online 1 July 2015

Cadmium selenide films were synthesized using simple electrodeposition method on indium tin oxide coated glass substrates. The synthesized films were post annealed at 200 °C, 300 °C and 400 °C. X-ray diffraction of the films showed the hexagonal structure with crystallite size o3 nm for as deposited films and 3–25 nm for annealed films. The surface morphology of films using field emission scanning electron microscopy showed granular surface. The high resolution transmission electron microscopy of a crystallite of the film revealed lattice fringes which measured lattice spacing of 3.13 Å corresponding to (002) plane, indicating the lattice contraction effect, due to small size of CdSe nanocrystallite. The calculation of optical band gap using UV–visible absorption spectrum showed strong red-shift with increase in crystallite size, indicating to the charge confinement in CdSe nanocrystallite. & 2015 Elsevier Ltd. All rights reserved.

Keywords: CdSe Thin film Electrodeposition XRD HRTEM UV–vis spectroscopy

1. Introduction Cadmium selenide (CdSe) is well known II–VI group compound semiconductor material. It is an important material due to its applications in photoconductor [1,2], light emitting diode [3], sensor device [4], solar cell [5], photovoltaic cell [6] as well as photoelectrochemical cell (PEC) [7]. The material is highly photosensitive in the visible region because of its suitable band gap (1.74 eV). Various techniques including physical vapor deposition [8], chemical vapor deposition [9], thermal evaporation [10], spray-pyrolysis [11], dip coating [12], chemical bath deposition [13], and electrodeposition [14] have been used for depositing cadmium selenide thin films. The electrodeposition is one of the simplest and low-cost non-vacuum technique for the growth of CdSe thin films. The deposition rate is easily controlled by changing the deposition potential, deposition time, concentration of solution and pH of the electrolyte; this technique is also used for large area thin film deposition [15,16]. The present work deals with the the deposition of CdSe thin films using simple two electrode deposition process. The deposited films were annealed at three different temperatures viz. 200 °C, 300 °C, and 400 °C for 1 h. The crystallographic analysis of the films using X-ray diffraction (XRD) showed hexagonal structure. The surface morphology of the deposited films examined n

Corresponding author. E-mail address: [email protected] (S. Mahato).

http://dx.doi.org/10.1016/j.mssp.2015.06.019 1369-8001/& 2015 Elsevier Ltd. All rights reserved.

using Field emission scanning electron microscope (FESEM) showed nano-sized grains. The high resolution transmission electron microscope (TEM) measured lattice spacing of the nano crystallite for (002) plane, revealed lattice contraction. The optical band gap of the films were calculated using UV–vis spectroscopy showed red-shift with crystallite size, indicated charge confinement in nano crystallites.

2. Experimental Cadmium selenide thin films were deposited on indium tin oxide coated glass substrates by using the electrodeposition process. For deposition of CdSe thin films, the reaction mixture consisted of 0.02 M cadmium chloride (CdCl2), 0.13 M ammonium chloride (NH4Cl) and 0.2 M sodium selenosulphite (Na2SeSO3). All the chemicals used are of high purity (Aldrich, 99.9%). Indium tin oxide (ITO) coated glass plate with sheet resistance 10 Ω/cm2 was used as cathode and a high purity graphite rod was used as anode. A stock solution of 0.2 M selenosulphite (Na2SeSO3) was prepared by stirring an aqueous solution of 0.5 M Na2SO3 and 0.2 M elemental selenium at about 50 °C for 24 h [17]. The electrolyte was continuously stirred by using a Teflon coated magnetic paddle to dissolve perfectly in distilled water. The pH of the electrolyte was adjusted to 8 using ammonia solution. The total volume of electrolyte was taken 100 ml and the temperature was maintained at room temperature. During electrodeposition, deposition potential

S. Mahato et al. / Materials Science in Semiconductor Processing 39 (2015) 742–747

was fixed at  1.60 V and deposition time was maintained for 10 min. After deposition, the CdSe thin film on ITO substrate was taken out from the electrolyte and dried in air for few minutes. The as-deposited films were annealed at three different temperatures viz. 200 °C, 300 °C, and 400 °C in the air for 60 min. The structural properties of the films were studied using X-ray diffraction (XRD) (BRUKER D8-FOCUS) using Cu Kα radiation (λ ¼ 1.5406 Å).The 2θ scan was taken from the range of 10–80° with a scan speed of 0.20°/s and step of size of 0.030°. Optical properties of the films were studied using absorption spectroscopy in the region (200–1100 nm) with UV–vis–NIR spectrophotometer at room temperature. Field emission scanning electron microscope (FESEM) (JEOL JSM-5800) and High-resolution transmission electron microscope (TEM) (FEI Tecnai-G20 with a LaB6 filament, operated at 200 KV ) were used to examine the surface morphology and crystallites of the films respectively.

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cadmium tetraammonium ion [Cd(NH3)4]2 þ into the same solution as CdCl2-Cd2 þ þ 2Cl  NH4Cl-NH3 þHCl Cd2 þ þ4NH3-[Cd(NH3)4]2 þ The produced cadmium tetraammonium ion reacts with the selenium ion and form cadmium selenide thin film at ITO electrode [Cd(NH3)4]2 þ þSe2  -CdSe þ4NH3

3. Results and discussion

Sodium selenosulfate (Na2SeSO3) hydrolyzes in solution to give Se2  ions which may result in the possibility of simultaneous formation of CdSe as well as deposition of elemental selenium. In fact, the relative rates of formation of CdSe and Se are decided by the parameters such as pH, concentration and temperature of electrolyte [18,19].

3.1. Growth process

3.2. Structural property

The growth mechanism of CdSe thin film on ITO coated glass electrode can be explained by the chemical reactions involved in the aqueous phase. Firstly, the reduction of sodium selenosulfate (Na2SeSO3) into the selenion ions (Se2  ) in the aqueous ammonium solution takes place according to the reactions

To determine the structural properties of “as-deposited” and annealed CdSe thin films, X-ray diffraction (XRD) patterns were analyzed. Fig. 1 shows XRD patterns of as-deposited (a) and annealed CdSe thin films (b)–(d) on ITO coated glass substrates. The XRD pattern of as-deposited CdSe thin film [Fig. 1(a)] shows lack of well-defined peaks indicating its amorphous like nature. Figs. 1(b), (c) and (d) shows the XRD pattern of CdSe film annealed at 200 °C, 300 °C and 400 °C respectively. From the figure we notice that as the annealing temperature of films is increased from 200 °C to 400 °C, the peak intensity starts to increase and becomes well resolved indicating considerable improvement in the crystallinity

Na2SeSO3 þ OH  -Na2SO4 þHSe  HSe  þOH  -H2O þSe2  The cadmium chloride (CdCl2) and ammonium chloride (NH4Cl) present in the electrolytic solution form a complex

Fig. 1. X-ray diffraction spectra of CdSe/ITO thin films: (a) as-deposited, (b) annealed at 200 °C, (c) annealed at 300 °C and (d) annealed at 400 °C.

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of CdSe films [20]. All the diffraction peaks were indexed to the hexagonal (würtzite) phase of CdSe for the diffraction pattern corresponding to annealing temperature 400 °C. The relative intensities of the peaks are in good agreement with standard data [JCPDS File no. 08-459]. It may be noted that the XRD patterns did not show peaks corresponding to CdO or SeO2 for any annealing temperature. This confirmed that the CdSe thin films were thermally stable up to annealing temperature 400 °C, indicating the high purity of product [21]. The average crystallite size (D) of the CdSe films were determined using the Scherrer's formula using full width at half maximum (FWHM) β of the intense peak [22].

D=

0.94λ β cos θ

(1)

where λ is the wavelength of X-ray (λ ¼1.5406 Å) and θ is the diffraction angle. The average crystallite size on calculation gave values 2.4 nm, 3.8 nm, 21.4 nm and 23.9 nm for as-deposited and annealed films at 200 °C, 300 °C and 400 °C respectively. Thus the particle size was found to increase with increase of annealing temperature which indicates to the diffusion of grain boundaries of smaller grains to form larger grains on annealing. Moreover increase in crystallite size of the film on annealing also indicates to the fact that a minimum crystallite size is required for the formation of the stable hexagonal crystal structure. 3.3. Morphological property The surface morphology of as-deposited and annealed CdSe films were studied using Field emission scanning electron microscopy (FESEM) as shown in Fig. 2(a)–(d). The surface morphology

of as-deposited film is shown in Fig. 2(a). From the micrograph, it is observed that the as-deposited films are smooth, continuous, and uniform without cracks or holes. The film has homogeneous distribution of particles with average size of ≲10 nm. From micrographs of annealed films [Fig. 2(b)–(d)], spherical nanosized grains of globule-like structure are observed which indicate the nanocrystalline nature of CdSe thin films as indicated by XRD measurement. The films are composed of CdSe nanocrystallites with average size of  10 nm,  25 nm and  100 nm for films asdeposited and annealed at 200 °C, 300 °C and 400 °C respectively. Comparing Fig. 2(b), (c) and (d) it appears that after annealing the nanocrystallites undergoes interdiffusion which results in increase in crystallite size. Transmission electron microscopy was used to study the structural properties of nanocrystallites of the CdSe films. Fig. 3(a) shows the TEM micrograph of CdSe film annealed at 300 °C. From the micrograph it is observed that the average crystallite size is 25 75 nm, in agreement with the value observed in surface morphological characterization using FESEM. The inset in Fig. 3(a) shows the selected area electron diffraction (SAED) pattern of the crystallites of the film. Since SAED gives information about the orientation of crystallographic planes in a selected local area of the sample, the diffuse ring pattern with spots indicate the polycrystalline nature due to the overlapping of number of crystallites in a small area. Fig. 3(b) shows the Highresolution transmission electron microscope (HRTEM) image of a CdSe nano crystallite of the film. The micrograph shows the presence of lattice fringes, indicating its single crystalline nature which is free from defects and dislocations. The lattice fringes over a chosen area as shown by circle, was used to measure the lattice spacing d. The lattice spacing was found to be d¼ 3.13 Å, which corresponds to (002) plane of hexagonal CdSe. This value of lattice

Fig. 2. Field emission scanning electron micrographs of CdSe thin films annealed at various temperatures: (a) As-deposited, (b) annealed at 200 °C, (c) annealed at 300 °C and (d) annealed at 400 °C.

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Fig. 3. (a) Transmission electron micrograph of CdSe thin film annealed at 300 °C; inset showing the selected area electron diffraction pattern, (b) High-resolution transmission electron micrograph of CdSe film annealed at 300 °C; inset showing the lattice spacing corresponding to (002) plane of CdSe crystallite.

spacing differs with that of the value for bulk CdSe (d ¼3.51 Å) [23]. The difference in lattice spacing may be due to the lattice contraction that takes place at smaller size of crystallites as reported by Zhang et. al. [24] for free standing CdSe nanocrystals. 3.4. Optical property The optical transmittance spectra for CdSe thin films are shown in Fig. 4. The CdSe film annealed at 400 °C shows maximum transmittance of  75% and the transmittance of the films decreases with decrease in annealing temperature. The spectra shows interference fringe pattern in the near infrared (NIR) region. These interference fringes are generated due to the constructive and destructive interference of the wave fronts from two interfaces viz. air/CdSe film and CdSe film/ITO substrate, defining its sinusoidal nature. The optical absorbance spectra of as-deposited and annealed CdSe thin films are shown in Fig. 5. From the spectra we observe that as the energy of the incident photon increases from 1.2 eV (1000 nm) to 1.8 eV (700 nm), the absorbance of the CdSe films shows constant low values. When the energy of the incident photon increases above 1.8 eV (700 nm), the absorbance of all the samples of CdSe film starts to increase, indicating the region of band edge absorption together with the absorption due to localized tail states inside the band gap of CdSe semiconductor. Moreover the absorption edge around 700 nm shows increase with rise in annealing temperature. This may be due to increased crystallite size and decrease in number of defects, on annealing. The color of the film was found to change from red-orange to dark black after annealing. Similar trend in optical absorption were reported by Kale et.al. [25] for chemical bath deposited CdSe thin films. The absorbance spectra were used to determine optical band

Fig. 5. Optical absorbance spectra of CdSe/ITO thin films: as-deposited and various annealing temperatures. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

gap energies of CdSe thin films by using the following relation for near band edge optical absorption of semiconductors.

⎛K ⎞ α = ⎜ ⎟ hν − Eg ⎝ hν ⎠

(

n

)

(2)

where α is absorption co-efficient, hν is the photon energy, K is a constant, Eg is the optical band gap and n is a constant (n equals 1/2 for allowed direct transitions) [26,27]. Fig. 6 shows the plot of (αhν)2 versus hν for as-deposited and annealed CdSe thin films; the linear portion of the curve is extrapolated to the energy (hν) axis and the intercept gives the value of optical band gap Eg of the film. The optical band gap values for as-deposited and annealed CdSe thin films, determined from Fig. 6 along with the crystallite size as a function of annealing temperature is plotted in Fig. 7. Fig. 7 indicates that as the annealing temperature of CdSe film increases, the optical band gap of the films decreases. Eg shows largest value of 2.2 eV for as-deposited film. Thus the annealing of films results in strong red-shift of optical band gap with respect to crystallite size. This red-shift of optical band gap is the result of the localization of charge carrier in individual nanocrystallites of the CdSe films. Moreover, the energy band gap for film annealed at 400 °C approaches to the value of bulk CdSe ( Eg,bulk ¼1.7 eV).

4. Conclusion

Fig. 4. Optical transmittance spectra of CdSe/ITO thin films: as-deposited and various annealing temperatures.

CdSe thin films were deposited from aqueous alkaline solution using the electrodeposition method on ITO substrate. The postdeposition air annealing of the films were carried out at 200 °C, 300 °C and 400 °C for 1 h. The XRD of films showed hexagonal

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Fig. 6. Tauc's plot for as-deposited and various annealed CdSe/ITO thin films.

to 2.2 eV. The films showed strong red-shift in optical band gap with increase in annealing temperature, indicating the charge confinement effect in nano crystallites of the film. In summary, the structural and opto-electronic properties of electrodeposited CdSe films can be tuned with annealing for their novel device applications.

References

Fig. 7. Plot of crystallite size and band gap with annealing temperature for CdSe/ ITO thin films.

(wurtzite) phase for annealed films. The crystallite size of film increased with annealing temperature, indicating the diffusion of smaller grains to form larger grains on annealing. The surface morphology of the CdSe films using FESEM showed grains of size o10 nm for as-deposited film and  10 to 100 nm for annealed films. The selected area electron diffraction of annealed film at 300 °C showed polycrystalline nature. High resolution transmission electron microscopy of the film revealed the d spacing of 3.13 Å for (002) plane which was found less than the value for bulk CdSe. This difference in d spacing was indicated to the contraction of CdSe lattice in nano crystallites of the film. The optical band gap of CdSe films was calculated using UV–vis spectroscopy were 1.9

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