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Effect of deposition temperature on structural, optical properties and configuration of CdSe nanocrystalline thin films deposited by chemical bath deposition
Materials Science in Semiconductor Processing 43 (2016) 177–181
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Effect of deposition temperature on structural, optical properties and configuration of CdSe nanocrystalline thin films deposited by chemical bath deposition E. Gholami Hatam n, N. Ghobadi Department of Physics, Faculty of Science, Malayer University, Malayer, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 3 September 2015 Received in revised form 22 November 2015 Accepted 15 December 2015
CdSe nanoparticle thin films were deposited on glass substrates by the chemical bath deposition (CBD) method at low deposition temperature ranging from room temperature up to 50 °C while the pH of the bath was kept constant at 12.1. The structural and morphological variation were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) technique. The energy band gap and optical properties were characterized by the absorbance spectra. Rutherford backscattering spectroscopy (RBS) analysis reveals the excess of Cd rather than Se in depth profile along the thin film thickness. The prepared CdSe nanoparticles have cubic structure and by increasing the temperature the deposited films become continues, homogeneous and tightly adherent. The results also revealed that by increasing the deposition temperature from room temperature up to 50 °C, the band gap decreases from 3.52 eV up to 1.84 eV. & 2015 Elsevier Ltd. All rights reserved.
Keywords: CdSe Thin film Chemical bath deposition Deposition temperature
1. Introduction There has been an increasing demand in deposition and characterization of II–IV group semiconductor compound thin films because of their wide applications in various fields of optoelectronic, sensors, detectors and biotechnology during the past few decades [1–4]. Among them, CdSe has high potential candidate in many applications such as solar cells, thin film transistor, lasers, gamma ray detectors and photoelectrochemical cells [5–9] because of high efficiency of radiative recombination, absorption coefficient, photosensitivity, direct band gap (1.74 eV) and quantum size effects. The energy band structure of CdSe nanocrystalline can be tailored by utilizing the unique size-dependent properties and thus influences the physical properties of material [10]. Controlling over the growth and shape of the CdSe nanocrystals is critical in realizing the design of novel functional devices [11,12]. Various techniques such as thermal evaporation [13], electrodeposition using thermally charged nanocrystals [14], spray pyrolysis [15], metalorganic chemical vapor deposition (CVD) [16] and chemical bath deposition (CBD) [17–18] have been employed to prepare thin films of CdSe. Among them, chemical bath deposition is the most convenient, inexpensive, suitable and reliable to produce CdSe thin films because of its large scale of capability [19]. A good understanding control over the complexing agents, n
Corresponding author. E-mail address: [email protected] (E. Gholami Hatam).
http://dx.doi.org/10.1016/j.mssp.2015.12.013 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
deposition time, temperature and pH can improve the size, shape, and quality of the crystallites [20–23]. Hereof, the present study deals with the preparation of CdSe thin film using chemical bath deposition technique to investigate the effect of low deposition temperature on optical properties, structure, morphology and homogeneity. Furthermore the powerful RBS technique was used to investigate the elemental profile of Cd and Se along the thickness of the prepared thin films.
2. Experimental procedures In this investigation, the CdSe thin films were grown on glass substrate (26 7.6 2 mm3) at different temperatures by the chemical bath deposition method. In the preparation procedure, the substrates were washed in detergent, rinsed in acetone, cleaned ultrasonically and afterwards were rinsed using a mixture of double distilled water and methanol. The preparation of the deposition solution is described by several researcher using the CBD technique [24,25]. Here, 50 ml of 0.25 M cadmium acetate (to provide Cd2 þ ions) was taken in a glass beaker under constant stirring and then 25% ammonia was slowly added to the solution. Next, the 50 ml of freshly prepared 0.25 M Na2SeSO3 was added to the solution. The glass substrates were vertically immersed in to the deposition solution with volume of (12 19 15 cm3) where the bath solution was covered. During the deposition, the reaction solutions were maintained constant at three temperatures of room temperature,
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40 °C and 50 °C. In order to control the pH at constant value of 12.1, ammonia was added to the solution which contains Cd2 þ ions. In the final stage, all the deposited substrates were removed from the chemical bath after 18 h. The deposited samples were then washed with deionized water and methanol to remove the loosely adhered CdSe nanoparticles on the films and finally dried in air. As follows, the steps of reaction mechanism involved in the deposition of CdSe thin films are:
D = Kλ /β cos θ
(2)
where K is constant value of 0.9 for spherical shape of crystallites, λ is the wavelength of the X-ray radiated, β is full width at half maximum (FWHM) in radian and θ is the diffraction angle. The full width at half maximum (FWHM) of thin films indicates that films have roughly the same crystallite size, while, for higher value of temperature FWHM decreases and hence the grain size increases. By increasing the bath temperature from room temperature to
(1)
Here, the surface morphological of the grown CdSe nanocrystalline thin films was investigated by scanning electron microscopy (SEM) and the optical absorption was measured by an UV– vis spectrometer. They were structurally characterized by X-ray diffraction (XRD) using a Philip analytical X-ray diffractometer in the 2θ geometry. In the following, nanostructured depth profile of the elemental composition of the Cd and Se was measured by the Rutherford backscattering spectrometry (RBS) method.
3. Results and discussion 3.1. Surface morphology study using SEM Scanning electron microscopy (SEM) was employed to investigate the particle size and morphologies of the thin films deposited at different deposition temperatures. From SEM studies, it was clarified that CdSe thin films homogenously cover the substrates and the film are composed from small nano-sized grains. Fig. 1(a) shows the SEM images of the CdSe nanostructure thin film at room temperature at 18 h deposition time. It gives a general morphology of the sample and represents that the substrate surface is in a large-scale array of high disordered nanowire. Fig. 1 (b) exhibits a large-scale array of high disordered quantum dot with average diameter of 33 nm at 40 °C. SEM image in Fig. 1 (c) represents that the substrate surface is composed of nanoparticles with an average diameter of 40 nm at 50 °C. The growth of grain size for the CdSe thin films with increasing the temperature of the chemical bath is due to the phenomenon of coalescence. The more increase in the bath temperature the bigger particles are formed. 3.2. Microstructural studies by XRD In order to investigate the crystal structure of CdSe thin films Cu-Kα radiation (λ ¼1.54 Å) was used to extract X-ray diffraction pattern. The obtained peaks were indexed with standard XRD power patterns with X′pert high score plus software. The major peak intensities was appeared around 25°, 42° and 49° corresponding to (111), (220) and (311) direction, are shown in (Fig. 2), which indicates that these samples are pure-phase compound. The peaks corresponding to the cubic CdSe (space group: F43/m) phase with cell constants h¼k ¼l ¼6.0770 Å are in good agreement with JCPDS 00-019-0191. The intensity and sharpness of diffracted peaks emphasized that the obtained samples are well crystallized. The average crystallite size of CdSe thin films (D) has been evaluated by Scherrer equation which are listed in Table 1,
Fig. 1. SEM micrograph of samples with scale size of 100 nm for temperature deposition of (a) room temperature, (b) 40 °C and (c) 50 °C.
E. Gholami Hatam, N. Ghobadi / Materials Science in Semiconductor Processing 43 (2016) 177–181
179
Fig. 2. XRD spectrum of CdSe thin films prepared at different deposition temperature deposited for 18 h.
Table 1 Parameters obtained from XRD for CdSe thin films deposited on glass. substrate at different bath temperatures. Temperature (°C)
Peak no.
β (2θ)
Peak pos. (2θ)
D (nm)
RT
1 2 3 1 2 3 1 2 3
3.55 4.20 4.44 2.50 1.98 3.08 1.83 2.80 2.2
26.24 43.15 49.95 25.50 42.60 49.84 25.53 42.60 49.85
2.3 2.0 2.0 3.3 4.3 2.8 4.5 3.0 4.0
40
50
The effective mass approximation describes the influence of the particle size on the band gap energy ER in quantum dot semiconductor nanocrystal
ER = Eg +
50 °C the intensity of peaks without changes in their positions increases which is the sign of reaching better quality of crystallites. 3.3. Optical absorption Optical absorption spectroscopy is the most commonly used technique in exploring the quantum effects in semiconductor nanoparticles. Through this technique, the development of discrete features in the spectra and the enlargement of the energy gap in semiconductor nanostructure can be revealed. The absorption data were analyzed using the following well known relation for near edge optical absorption of semiconductors [26],
α = A(hν − Eg )n /hν
Fig. 3. The absorbance spectra of CdSe thin films at 18 h deposition time prepared at room temperature, 40 °C and 50 °C.
(3)
ℏ2π 2 (1/me* + 1/mh*) − 1.78e2 /εR 2R 2
(4)
in terms of the bulk band gap Eg, the radius of the quantum dot R, relative dielectric constant ɛ, as well as the effective mass of the excited electron me* and of the electron hole mh*. Where the last term arises from Coulomb interaction and reduces the energy of CdSe by about 0.1 eV. The value of Eg for bulk CdSe is 1.74 eV, me*/me=0.13, mh*/me=0.45 and ɛ ¼5.8. The optical gap for deposited thin films at different deposition temperature (Fig. 4) indicates that the optical gap has been decreased by increasing the deposition temperature at deposition time of 18 h. The optical band gap Eg was obtained by the intercept at the x-axis in the plot of (αhν)2 versus hν. The optical band gap decreases from ‘3.52 eV’ to ‘1.84 eV’ as the deposition temperature changes from room temperature to 50 °C. By substituting in Eq. (4), the evaluated size of prepared CdSe nanoparticles increases from 1.8 nm to 4.1 nm when the deposition temperature varies from room temperature to 50 °C. There is a good agreement with the crystallite sizes obtained from XRD. This may be the case where the nanoparticle are single crystalline. The nanocrystalline films present very strong confinement effects with either reduction in grain size or increasing the band gap energy. Here, the
α is the absorption coefficient, A is a constant, hν is the photon energy, Eg is the optical gap and n is constant value of 1 for direct 2
semiconductors. In order to investigate the optical properties of CdSe thin films we used UV–vis spectrometry (Perkin-Elmer, UV/ VIS Spectrometer Lambda 25-USA) in the wavelength region between 300 and 1100 nm. Through this measurement, the absorption spectra of the CdSe thin films with different deposition temperatures are presented in Fig. 3. The absorption spectra revealed a blueshift with decreasing deposition temperature from 50 °C to room temperature. Based on the fact that the absorption edge of small nanoparticles size are in low wavelengths, the observed blueshifts are most possibly due to decreases in nanoparticles size. Dependence of nanoparticle size on the temperature of the chemical bath is governed by the reaction in solution and on the substrate as a heterogeneous site for nucleation. In other words, the growth of crystals through precipitation via primary and secondary nucleation in a supersaturated solution will be different in the solution and on the substrate.
Fig. 4. The optical gap for deposited thin films at room temperature, 40 °C and 50 °C.
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Fig. 5. Experimental RBS spectra (black color) and simulations by SIMNRA (red color) for the thin film of CdSe on a SiO2 substrate. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
prepared CdSe thin films exhibit size-dependent electronic band gap energies. 3.4. Rutherford backscattering study Rutherford Backscattering Spectrometry (RBS) is an ion scattering technique used for compositional thin film analysis. RBS is unique in that it allows quantification without the use of reference standards. During an RBS analysis, high-energy (MeV) He2 þ ions (i.e. alpha particles) are directed onto the sample and yield of the backscattered He2 þ ions for each channel is measured at a given angle. Since the backscattering cross-section for each element is known, it is possible to obtain a quantitative compositional depth profile from the RBS spectrum for films that are less than 1 μm thick. C. Jeynes et al. have demonstrated %1 of accuracy in RBS measurements [27]. In RBS spectrum the energy (E) of the detected particle is linearly proportional to the channel number (ch). Two channel positions of known energy values are sufficient to calibrate the analyzer, but it is better to have more and calculate the two coefficients by linear regression. From the position of the surface peaks and the corresponding energy values the calibration coefficient are evaluated through linear regression. Individual peaks were assigned to specific elements by fitting the channel numbers to a line equation using tabulated kinematics factor. The peaks appear in order of atomic number and lighter element appears at lower channel number. With the use of a standard, such as bismuth implanted, one can determine the areal density Nx (atoms/cm2) which is multiples of mass density ρ (g/cm3), Avogadro’s Number (atoms) and average thickness t (cm) divided by molecular mass M (g), Nx ¼(ρNA/M)t, of the sample through this ratio,
σ YQ Nx = Bi x Bi σxYBiQ x NBi
(5)
In above equation Yx is the peak area of detected element x, Qx the amount of collected current during sample acquisition. sx is the non-Rutherford cross-section and is determined by following equation,
σx = 1 − ((0.049)Z1(Z2)4/3/Elab)
(6)
where Z1 is the atomic number of detected element, Z2 the atomic number of the gas used to create ion beam and Elab the beam energy in keV.
Fig. 6. The elemental depth profile of the Cd and Se concentration in percentage with respect to the thin film thickness.
The Van de Graff accelerator was set-up to 2 MeV with He ion flux of 2.1 1010 particles/Sr. The surface barrier detector is positioned at 165° with respect to the beam direction and current of around 10 nA on the Faraday cap is adequate for characterization of thin films. Fig. 5 exhibits the RBS spectrum of CdSe nanocrystal. An acceptable fit between experimental and simulated RBS data using SIMNRA software was obtained by considering 5 layer of homogeneous CdSe deposited on SiO2 as the substrate. The lighter Se appears at lower energies than Cd because of lower kinematic factor and the substrate appears as a step edge followed by a sloping continuum of counts. This is because of the loss of scattering energy and increasing in cross-section as the beam penetrates deeper into the substrate. The fitted elemental profile data for Cd and Se concentration in depth is plotted in Fig. 6. The concentration scale assumes that no light element except than O and Si exist in the film. Also, in order to convert the areal density (atoms/cm2) of layer to the thickness (nm) a weighted average of bulk elemental densities (g/cm3) for Cd and Se was considered. This is because of that the density of atoms in thin film is less or equal than bulk density, as the atoms are generally less dense packed in thin films. The reason is surface phenomena as well as specifies in film growth. The presence of excess Cd is not due to contamination from unreacted starting material as RBS analysis of the supernatants used in the washing procedure verified that unreacted Cd and Se are completely removed with the third methanol wash. It is therefore consistent here that excess Cd would passivate surface Se atoms.
4. Conclusions In this study, the CdSe nanocrystalline thin films were prepared using the chemical bath deposition technique onto glass substrate at different deposition temperature. Using the various investigation techniques it was concluded that the deposition temperature has a characteristic role for fabrication of the thin layers with high quality and desired features. The optical gap studies for deposited thin films after 18 h indicated that the optical gap decreases by increasing the deposition temperature from 3.52 eV to 1.86 eV ones temperature changes from room temperature to 50 °C. The XRD results showed that CdSe are well crystallized with cubic structure. It was also found that the crystallite size diminish when deposition temperature decreases from 50 °C to room temperature. The RBS analysis revealed intriguing insight in to the structure of these nanocrystals by depth profile viewing the concentration of thin films. The presence of excess Cd is independent from initial amount of Cd in reaction mixture.
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Effect of deposition temperature on structural, optical properties and configuration of CdSe nanocrystalline thin films deposited by chemical bath deposition E. Gholami Hatam n, N. Ghobadi Department of Physics, Faculty of Science, Malayer University, Malayer, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 3 September 2015 Received in revised form 22 November 2015 Accepted 15 December 2015
CdSe nanoparticle thin films were deposited on glass substrates by the chemical bath deposition (CBD) method at low deposition temperature ranging from room temperature up to 50 °C while the pH of the bath was kept constant at 12.1. The structural and morphological variation were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) technique. The energy band gap and optical properties were characterized by the absorbance spectra. Rutherford backscattering spectroscopy (RBS) analysis reveals the excess of Cd rather than Se in depth profile along the thin film thickness. The prepared CdSe nanoparticles have cubic structure and by increasing the temperature the deposited films become continues, homogeneous and tightly adherent. The results also revealed that by increasing the deposition temperature from room temperature up to 50 °C, the band gap decreases from 3.52 eV up to 1.84 eV. & 2015 Elsevier Ltd. All rights reserved.
Keywords: CdSe Thin film Chemical bath deposition Deposition temperature
1. Introduction There has been an increasing demand in deposition and characterization of II–IV group semiconductor compound thin films because of their wide applications in various fields of optoelectronic, sensors, detectors and biotechnology during the past few decades [1–4]. Among them, CdSe has high potential candidate in many applications such as solar cells, thin film transistor, lasers, gamma ray detectors and photoelectrochemical cells [5–9] because of high efficiency of radiative recombination, absorption coefficient, photosensitivity, direct band gap (1.74 eV) and quantum size effects. The energy band structure of CdSe nanocrystalline can be tailored by utilizing the unique size-dependent properties and thus influences the physical properties of material [10]. Controlling over the growth and shape of the CdSe nanocrystals is critical in realizing the design of novel functional devices [11,12]. Various techniques such as thermal evaporation [13], electrodeposition using thermally charged nanocrystals [14], spray pyrolysis [15], metalorganic chemical vapor deposition (CVD) [16] and chemical bath deposition (CBD) [17–18] have been employed to prepare thin films of CdSe. Among them, chemical bath deposition is the most convenient, inexpensive, suitable and reliable to produce CdSe thin films because of its large scale of capability [19]. A good understanding control over the complexing agents, n
Corresponding author. E-mail address: [email protected] (E. Gholami Hatam).
http://dx.doi.org/10.1016/j.mssp.2015.12.013 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
deposition time, temperature and pH can improve the size, shape, and quality of the crystallites [20–23]. Hereof, the present study deals with the preparation of CdSe thin film using chemical bath deposition technique to investigate the effect of low deposition temperature on optical properties, structure, morphology and homogeneity. Furthermore the powerful RBS technique was used to investigate the elemental profile of Cd and Se along the thickness of the prepared thin films.
2. Experimental procedures In this investigation, the CdSe thin films were grown on glass substrate (26 7.6 2 mm3) at different temperatures by the chemical bath deposition method. In the preparation procedure, the substrates were washed in detergent, rinsed in acetone, cleaned ultrasonically and afterwards were rinsed using a mixture of double distilled water and methanol. The preparation of the deposition solution is described by several researcher using the CBD technique [24,25]. Here, 50 ml of 0.25 M cadmium acetate (to provide Cd2 þ ions) was taken in a glass beaker under constant stirring and then 25% ammonia was slowly added to the solution. Next, the 50 ml of freshly prepared 0.25 M Na2SeSO3 was added to the solution. The glass substrates were vertically immersed in to the deposition solution with volume of (12 19 15 cm3) where the bath solution was covered. During the deposition, the reaction solutions were maintained constant at three temperatures of room temperature,
178
E. Gholami Hatam, N. Ghobadi / Materials Science in Semiconductor Processing 43 (2016) 177–181
40 °C and 50 °C. In order to control the pH at constant value of 12.1, ammonia was added to the solution which contains Cd2 þ ions. In the final stage, all the deposited substrates were removed from the chemical bath after 18 h. The deposited samples were then washed with deionized water and methanol to remove the loosely adhered CdSe nanoparticles on the films and finally dried in air. As follows, the steps of reaction mechanism involved in the deposition of CdSe thin films are:
D = Kλ /β cos θ
(2)
where K is constant value of 0.9 for spherical shape of crystallites, λ is the wavelength of the X-ray radiated, β is full width at half maximum (FWHM) in radian and θ is the diffraction angle. The full width at half maximum (FWHM) of thin films indicates that films have roughly the same crystallite size, while, for higher value of temperature FWHM decreases and hence the grain size increases. By increasing the bath temperature from room temperature to
(1)
Here, the surface morphological of the grown CdSe nanocrystalline thin films was investigated by scanning electron microscopy (SEM) and the optical absorption was measured by an UV– vis spectrometer. They were structurally characterized by X-ray diffraction (XRD) using a Philip analytical X-ray diffractometer in the 2θ geometry. In the following, nanostructured depth profile of the elemental composition of the Cd and Se was measured by the Rutherford backscattering spectrometry (RBS) method.
3. Results and discussion 3.1. Surface morphology study using SEM Scanning electron microscopy (SEM) was employed to investigate the particle size and morphologies of the thin films deposited at different deposition temperatures. From SEM studies, it was clarified that CdSe thin films homogenously cover the substrates and the film are composed from small nano-sized grains. Fig. 1(a) shows the SEM images of the CdSe nanostructure thin film at room temperature at 18 h deposition time. It gives a general morphology of the sample and represents that the substrate surface is in a large-scale array of high disordered nanowire. Fig. 1 (b) exhibits a large-scale array of high disordered quantum dot with average diameter of 33 nm at 40 °C. SEM image in Fig. 1 (c) represents that the substrate surface is composed of nanoparticles with an average diameter of 40 nm at 50 °C. The growth of grain size for the CdSe thin films with increasing the temperature of the chemical bath is due to the phenomenon of coalescence. The more increase in the bath temperature the bigger particles are formed. 3.2. Microstructural studies by XRD In order to investigate the crystal structure of CdSe thin films Cu-Kα radiation (λ ¼1.54 Å) was used to extract X-ray diffraction pattern. The obtained peaks were indexed with standard XRD power patterns with X′pert high score plus software. The major peak intensities was appeared around 25°, 42° and 49° corresponding to (111), (220) and (311) direction, are shown in (Fig. 2), which indicates that these samples are pure-phase compound. The peaks corresponding to the cubic CdSe (space group: F43/m) phase with cell constants h¼k ¼l ¼6.0770 Å are in good agreement with JCPDS 00-019-0191. The intensity and sharpness of diffracted peaks emphasized that the obtained samples are well crystallized. The average crystallite size of CdSe thin films (D) has been evaluated by Scherrer equation which are listed in Table 1,
Fig. 1. SEM micrograph of samples with scale size of 100 nm for temperature deposition of (a) room temperature, (b) 40 °C and (c) 50 °C.
E. Gholami Hatam, N. Ghobadi / Materials Science in Semiconductor Processing 43 (2016) 177–181
179
Fig. 2. XRD spectrum of CdSe thin films prepared at different deposition temperature deposited for 18 h.
Table 1 Parameters obtained from XRD for CdSe thin films deposited on glass. substrate at different bath temperatures. Temperature (°C)
Peak no.
β (2θ)
Peak pos. (2θ)
D (nm)
RT
1 2 3 1 2 3 1 2 3
3.55 4.20 4.44 2.50 1.98 3.08 1.83 2.80 2.2
26.24 43.15 49.95 25.50 42.60 49.84 25.53 42.60 49.85
2.3 2.0 2.0 3.3 4.3 2.8 4.5 3.0 4.0
40
50
The effective mass approximation describes the influence of the particle size on the band gap energy ER in quantum dot semiconductor nanocrystal
ER = Eg +
50 °C the intensity of peaks without changes in their positions increases which is the sign of reaching better quality of crystallites. 3.3. Optical absorption Optical absorption spectroscopy is the most commonly used technique in exploring the quantum effects in semiconductor nanoparticles. Through this technique, the development of discrete features in the spectra and the enlargement of the energy gap in semiconductor nanostructure can be revealed. The absorption data were analyzed using the following well known relation for near edge optical absorption of semiconductors [26],
α = A(hν − Eg )n /hν
Fig. 3. The absorbance spectra of CdSe thin films at 18 h deposition time prepared at room temperature, 40 °C and 50 °C.
(3)
ℏ2π 2 (1/me* + 1/mh*) − 1.78e2 /εR 2R 2
(4)
in terms of the bulk band gap Eg, the radius of the quantum dot R, relative dielectric constant ɛ, as well as the effective mass of the excited electron me* and of the electron hole mh*. Where the last term arises from Coulomb interaction and reduces the energy of CdSe by about 0.1 eV. The value of Eg for bulk CdSe is 1.74 eV, me*/me=0.13, mh*/me=0.45 and ɛ ¼5.8. The optical gap for deposited thin films at different deposition temperature (Fig. 4) indicates that the optical gap has been decreased by increasing the deposition temperature at deposition time of 18 h. The optical band gap Eg was obtained by the intercept at the x-axis in the plot of (αhν)2 versus hν. The optical band gap decreases from ‘3.52 eV’ to ‘1.84 eV’ as the deposition temperature changes from room temperature to 50 °C. By substituting in Eq. (4), the evaluated size of prepared CdSe nanoparticles increases from 1.8 nm to 4.1 nm when the deposition temperature varies from room temperature to 50 °C. There is a good agreement with the crystallite sizes obtained from XRD. This may be the case where the nanoparticle are single crystalline. The nanocrystalline films present very strong confinement effects with either reduction in grain size or increasing the band gap energy. Here, the
α is the absorption coefficient, A is a constant, hν is the photon energy, Eg is the optical gap and n is constant value of 1 for direct 2
semiconductors. In order to investigate the optical properties of CdSe thin films we used UV–vis spectrometry (Perkin-Elmer, UV/ VIS Spectrometer Lambda 25-USA) in the wavelength region between 300 and 1100 nm. Through this measurement, the absorption spectra of the CdSe thin films with different deposition temperatures are presented in Fig. 3. The absorption spectra revealed a blueshift with decreasing deposition temperature from 50 °C to room temperature. Based on the fact that the absorption edge of small nanoparticles size are in low wavelengths, the observed blueshifts are most possibly due to decreases in nanoparticles size. Dependence of nanoparticle size on the temperature of the chemical bath is governed by the reaction in solution and on the substrate as a heterogeneous site for nucleation. In other words, the growth of crystals through precipitation via primary and secondary nucleation in a supersaturated solution will be different in the solution and on the substrate.
Fig. 4. The optical gap for deposited thin films at room temperature, 40 °C and 50 °C.
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Fig. 5. Experimental RBS spectra (black color) and simulations by SIMNRA (red color) for the thin film of CdSe on a SiO2 substrate. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
prepared CdSe thin films exhibit size-dependent electronic band gap energies. 3.4. Rutherford backscattering study Rutherford Backscattering Spectrometry (RBS) is an ion scattering technique used for compositional thin film analysis. RBS is unique in that it allows quantification without the use of reference standards. During an RBS analysis, high-energy (MeV) He2 þ ions (i.e. alpha particles) are directed onto the sample and yield of the backscattered He2 þ ions for each channel is measured at a given angle. Since the backscattering cross-section for each element is known, it is possible to obtain a quantitative compositional depth profile from the RBS spectrum for films that are less than 1 μm thick. C. Jeynes et al. have demonstrated %1 of accuracy in RBS measurements [27]. In RBS spectrum the energy (E) of the detected particle is linearly proportional to the channel number (ch). Two channel positions of known energy values are sufficient to calibrate the analyzer, but it is better to have more and calculate the two coefficients by linear regression. From the position of the surface peaks and the corresponding energy values the calibration coefficient are evaluated through linear regression. Individual peaks were assigned to specific elements by fitting the channel numbers to a line equation using tabulated kinematics factor. The peaks appear in order of atomic number and lighter element appears at lower channel number. With the use of a standard, such as bismuth implanted, one can determine the areal density Nx (atoms/cm2) which is multiples of mass density ρ (g/cm3), Avogadro’s Number (atoms) and average thickness t (cm) divided by molecular mass M (g), Nx ¼(ρNA/M)t, of the sample through this ratio,
σ YQ Nx = Bi x Bi σxYBiQ x NBi
(5)
In above equation Yx is the peak area of detected element x, Qx the amount of collected current during sample acquisition. sx is the non-Rutherford cross-section and is determined by following equation,
σx = 1 − ((0.049)Z1(Z2)4/3/Elab)
(6)
where Z1 is the atomic number of detected element, Z2 the atomic number of the gas used to create ion beam and Elab the beam energy in keV.
Fig. 6. The elemental depth profile of the Cd and Se concentration in percentage with respect to the thin film thickness.
The Van de Graff accelerator was set-up to 2 MeV with He ion flux of 2.1 1010 particles/Sr. The surface barrier detector is positioned at 165° with respect to the beam direction and current of around 10 nA on the Faraday cap is adequate for characterization of thin films. Fig. 5 exhibits the RBS spectrum of CdSe nanocrystal. An acceptable fit between experimental and simulated RBS data using SIMNRA software was obtained by considering 5 layer of homogeneous CdSe deposited on SiO2 as the substrate. The lighter Se appears at lower energies than Cd because of lower kinematic factor and the substrate appears as a step edge followed by a sloping continuum of counts. This is because of the loss of scattering energy and increasing in cross-section as the beam penetrates deeper into the substrate. The fitted elemental profile data for Cd and Se concentration in depth is plotted in Fig. 6. The concentration scale assumes that no light element except than O and Si exist in the film. Also, in order to convert the areal density (atoms/cm2) of layer to the thickness (nm) a weighted average of bulk elemental densities (g/cm3) for Cd and Se was considered. This is because of that the density of atoms in thin film is less or equal than bulk density, as the atoms are generally less dense packed in thin films. The reason is surface phenomena as well as specifies in film growth. The presence of excess Cd is not due to contamination from unreacted starting material as RBS analysis of the supernatants used in the washing procedure verified that unreacted Cd and Se are completely removed with the third methanol wash. It is therefore consistent here that excess Cd would passivate surface Se atoms.
4. Conclusions In this study, the CdSe nanocrystalline thin films were prepared using the chemical bath deposition technique onto glass substrate at different deposition temperature. Using the various investigation techniques it was concluded that the deposition temperature has a characteristic role for fabrication of the thin layers with high quality and desired features. The optical gap studies for deposited thin films after 18 h indicated that the optical gap decreases by increasing the deposition temperature from 3.52 eV to 1.86 eV ones temperature changes from room temperature to 50 °C. The XRD results showed that CdSe are well crystallized with cubic structure. It was also found that the crystallite size diminish when deposition temperature decreases from 50 °C to room temperature. The RBS analysis revealed intriguing insight in to the structure of these nanocrystals by depth profile viewing the concentration of thin films. The presence of excess Cd is independent from initial amount of Cd in reaction mixture.
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