Thin-film technology is used in many applications such as microelectronics, optics, magnetic-, hard-, and corrosion-resistant coatings, and micro-mechanics [
Optical measurements on thin films provide a good technique for assessing the properties of semiconductors. Particularly, the measurement of absorption coefficient for various energies gives a lot of information about the band gaps of the material. Knowledge of the band gap and its dependence on the film thickness is extremely important in achieving the required electrical properties of a semiconductor for specific applications. In particular, the measurement of absorption coefficients for various incident energies of photons on Se films throws light on the band gap of materials. Knowledge of optical constants of a material such as optical band gap, refractive index and extinction coefficient is quite essential to examine material’s potential opto-electronic applications [
Electrical and optical properties are very much influenced by various deposition parameters such as film thickness [
Apart from this, selenium is used as a material in electro-photography [
In view of the above, interesting properties and little work have been reported on the optical properties of thin selenium films [
Analysis of optical absorption spectra is one of the most productive tools for understanding and developing the band structure and energy gap. The optical absorption coefficient (α) is related to the transmission (
Where,
Since Se, a non-metal, is a direct band gap material and for direct allowed transitions, the absorption coefficient (α) is related to band gap energy by the relation [
Where,
and
The energy gap is evaluated for different thicknesses (t) and the band gap variation with the film thickness follows the relation [
Where,
(t) is the thickness,
(ћ) is the Planck’s constant,
and ΔE is the kinetic energy contribution due to motion normal to the film plane.
In the present investigation, Se is a semiconducting material and it becomes necessary to estimate the De Broglie wavelength in order to select the thickness range of the film, where the investigation is to be carried out. The De Broglie wavelength (λ') can be estimated using
Where,
h = Planck’s constant,
c = Speed of light,
Ef = Fermi energy = 1.0643 eV = half of the average band gap energy.
Selenium of 99.99 % purity (obtained from Leico Industries, New York, USA) was thermally evaporated by resistive heating at room temperature (22 °C) under a pressure of (2 x 10-6) Torr using Hind High Vacuum coating unit model 12A4D. The rate of deposition was around (1-2) nms-1.
The distance between the substrate and the evaporation source was approximately 20 cm. Before deposition, the substrates were treated with various cleaning techniques including ultrasonically agitation method. Lastly, the substrate was cleaned through the ionic bombardment method in the vacuum chamber before deposition. During deposition, the substrate holder was rotated with a constant speed as to have a smooth surface of the sample as required in optical studies.
The film thickness and deposition rate were controlled by means of an built-in quartz crystal digital thickness monitor (Model DTM-101), which could resolve thickness up to 0.1 % in the vacuum coating unit system. The transmittance spectra (T %) were taken at room temperature by using a spectrophotometer in the wavelength (λ) spectral range of 200 to 800 nm.
Nowadays, there are several thin film coating methods available [
Many experiments have been conducted to study the optical properties of Se thin films.
There were similar results obtained in other studies [
The x-ray diffraction studies of selenium film grown at room temperature reveal that they are fine grained and polycrystalline. X-ray diffraction data of selenium films well agrees with that of trigonal selenium peaks [JCPDS: 0362] [
* Series 1, t1 = 200 nm; Series 2, t2 = 350 nm; Series 3, t3 = 500 nm; Series 4, t4 = 600 nm; Series 5, t5 = 700 nm; Series 6, t6 = 850; and Series 7, t7 = 1000 nm (
It is observed that transmittance increases with a decrease in film thickness. According to
* See
Since absorption is a function of incident photon energy [
* Series 1, t1 = 200 nm; Series 2, t2 = 350 nm; Series 3, t3 = 500 nm; Series 4, t4 = 600 nm; Series 5, t5 = 700 nm; Series 6, t6 = 850; and Series 7, t7 = 1000 nm (
The extrapolation of plots of (αhν)2 versus (hν) onto energy axis will give the energy band gap. The energy band gap of films varied in the range (2.0 to 2.3) eV [
In general, density of localized state in the film increases with the thickness, resulting in the decrease of band gap. Such a variation in energy band gap with the increase in thickness in ZnO [
From the results obtained in our experiment, the band gap mainly depends on the thickness of the film and the shift in the bandgap energy is attributed to the nanocrystalline quantum size effect [
The values of the optical band gap energy (Eg) investigated for Se are given in
The plot of (
Semi-conductors and semi-metal films are expected to exhibit the quantum size effect [
* See
From the slope of the curve of
The optical absorption coefficient (α) has high values for higher thickness films compared to that of lower thickness films. On the other hand, the optical density (OD) or the absorption is proportional to the thickness of the films [
* Series 1, t1 = 200 nm; Series 2, t2 = 350 nm; Series 3, t3 = 500 nm; Series 4, t4 = 600 nm; Series 5, t5 = 700 nm; Series 6, t6 = 850; and Series7, t7 = 1000 nm (
Optical Intensity (OD) measures the amount of attenuation, or intensity lost, when light passes through an optical component. It also tracks attenuation based on the scattering of light, whereas the absorbance considers only the absorbance of light within the optical component. We obtained a minimum OD at the photon energy of 2.25 eV for all selenium films, as it mainly depends on the incident photon energy. Initially as the incident photon energy increases from (1.5-2.25) eV, the OD decreases for all the thicknesses. Afterwards, the OD increases as the film thickness increases and with that, an increase in the photon energy from (2.5-5) eV. The maximum value of OD achieved in the present investigation is around 8.5 x 10 -14 as seen in
It is a customary to estimate electron conduction mean free path or De Broglie wavelength (λ'), while making a study on electrical/optical properties with metals or non-metallic films, respectively.
The De Broglie wavelength of electrons or holes, estimated by Fermi-energy taken to be half of the average band gap energy (1.0643 eV), turns out to be about 1167 nm. Hence, we need to select the thickness range around 1167 nm or less than that. Thus, the quantum size effect is expected to be shown by the films in the thickness range of 200-1000 nm [
In this study, a decrease in band gap energy with increase in film thickness is noticed which has been depicted in the
* Series 1, t1 = 200 nm; Series 2, t2 = 350 nm; Series 3, t3 = 500 nm; Series 4, t4 = 600 nm; Series 5, t5 = 700 nm; Series 6, t6 = 850; and Series7, t7 = 1000 nm (
The band gap in the case of CdS thin films decreases with film thickness. In this study, the optical band gap decreases with the increase in the thickness of the film.
This variation is due to quantum size effect occurring in semi-conducting thin films. This is verified by
The morphology of Se films of thicknesses 157 and 210 nm were observed by SEM and the images are presented in
The scanning electron micrographs are helpful in elucidating the incipient growth of Se film on glass substrate.
Atomic force microscopy (AFM) is a technique for analyzing the surface morphology and texture of a rigid material all the way down to the level of the atom. AFM utilizes a mechanical cantilever probe to magnify surface features up to 100,000,000 times and produces 3-D images of the surface of the sample.
If the film is continuous, homogeneous, and has no discontinuities, the AFM image is a reflection of that area. AFM measurements were performed to obtain the surface roughness of the Se films.
Electron Dispersive Spectroscopy (EDS) analysis yields information on the quantitative analysis of the films, which was carried out using Scanning Electron Microscopy (SEM) measurements in order to study the stoichiometry of the films.
The following conclusions may be drawn from the results obtained.
We have successfully grown the thin films of Se on glass substrates held at room temperature below the De Broglie wave-length limit by thermal evaporation in vacuum. The XRD analysis reveals that structure of Se is amorphous. The direct band gap energy calculated from absorption data are in the range (2.0-2.3) eV, which is in good agreement with the expected value. The results were found to be in consistent with other studies that used different procedures. The direct band gap decreases from 2.3 eV to 2.0 eV as the thickness increases. The optical band gap energy of thin Se films has been found to obey the inverse square law with respect to thickness. The optical density studied is found to be a function of film thickness. The effective electron mass estimated from this investigation (0.3375 x 10-4) times m. is comparable to that obtained by other authors. The absorption coefficients were in the range (0.5-4.0) × 10.m-1. The AFM results confirmed that the Se nanosize increases with the increase in the film thickness. Both the grain boundaries and sub-grain regions are clearly visible in the SEM micrographs. We observe that the grain and sub-grain microstructure is a result of the bombardment of growth surface by energetic ions during deposition. The EDS spectra confirms the material used in the present investigation is pure selenium.
The authors Prof. Dr. L. A. Udachan and Prof. Dr. N. H. Ayachit acknowledge the Vision Group on Science & Technology, Government of Karnataka, India, for providing the necessary research facility for carrying out the research work and the instruments procured under grant sanctioned, bearing GRD No. 185 & GRD No. 539, respectively.
Research article