Optical properties of silicon layers produced by PVD method as a function of thickness.
The most significant properties of the most widespread layered film structures include the layer thickness and basic optical properties such as refractive index (n) and extinction coefficient (k) . The basic thin film properties are well known to depend mainly on its microstructure [2-10] which is controllable by deposition method and parameters . Refractive index of thin films is thickness dependent . Silicon is non-toxic, relatively inexpensive, easy to process, and has quite good mechanical properties. Silicon is very important material in electronics market, dominating the microelectronics industry with about 90% of all semiconductor devices sold worldwide being silicon based . Currently, the vast majority of flash-memory devices are charge storage based, fabricated in CMOS technology. Because of the increasing demand for information storage, memory device developers and manufacturers are constantly attempting to increase storage capacity for memory devices (e.g., increase storage per die or chip). Silicon-based devices are approaching their fundamental physical size limits . The aim of this work is to produce silicon thin layers of different thicknesses and other same deposition conditions (such as: deposition angle, deposition rate, deposition temperature and vacuum condition), calculate optical constants by using kramers-kronig relations on reflectivity curves and investigated about changes of optical properties as a function of film thickness.
Silicon layers were prepared on glass substrates (1 x 20 x 20 [mm.sup.3]) using an ETS160 system with a pressure of 2.9 x [10.sup.-6] mbar. The layers were deposited in high vacuum condition, using an electron gun evaporation method with the deposition rate of 0.9A [degrees]/s. The purity of Silicon disk was 95%. All substrates were cleaned with an ultrasonic-bath technique before the deposition process. The layers were produced at 300 K (room temperature). The thicknesses of the layers were determined by quartz crystal technique about 10, 70 and 110 nano meters. Optical transmittance and reflectance of the films were measured by using UV-VIS spectrophotometer (Hitachi U-3310) instrument. The spectra of the layers were measured in the visible light wave length range. The optical constants of our samples were derived on the basis of standard Kramers-Kronig relations using computer techniques. Aspens & Thee'ten's  data were added to calculated results for comparison.
RESULTS AND DISCUSSION
In this work Kramers-Kronig relations were used to calculate the phase angle [theta] (E) :
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)
Where E denotes the photon energy, [E.sub.2] the asymptotic limitation of the free-electron energy, and R(E) the reflectance. Hence, if [E.sub.2] is known, the [theta] (E) can be calculated. Then the real and imaginary parts of the refractive index were calculated, from which other parameters were obtained.
Figure 1shows the reflectance curves for the silicon layers deposited on the glass substrates at 300 K. Here different curves correspond to different thicknesses of layers. The results by Aspnes and Thee'ten  for silicon samples are included in Figure 1 and all the further figures for the sake of comparison. By increasing thickness of layers optical reflectance increases. Because the layers get more completed and voids fill up with silicon grains.
In Figure 2, the real part of the refractive index is shown. The results in our data is in good agreement with Aspnes & Thee'ten's . There is a peak at 3.4 eV energy for all layers. This peak for Aspnes & Thee'ten . result is sharper; which for our results is wide and short. It can be seen from figure 2 that by increasing thickness real part of refractive index decreases in general which is because of configuration of homogeneous and completed layers.
Figure 3 shows the imaginary part of refractive index. Extinction coefficient increases with increasing thickness. That is because of filling voids with silicon grains, there for transmittance decreases and absorbance increases. Aspnes & Thee'ten  data are also included for comparison.
Figure 4 shows the real part of the dielectric function for the layers produced in this work. The general trends of our results are same with Aspnes & Thee'ten . There is a peak at 3.4 eV energy for all layers. As it can be seen from figure 4, by increasing thickness real part of dielectric function decreases. That is because of formation more metallic layers by increasing thickness. For thicker layers the effect of substrate is also observed.
Figure 5, shows the imaginary parts of dielectric constant, The general trend of our results is same with Aspnes & Thee'ten . There is a peak at 3.5 eV energy for all layers. As it can be seen from figure 5, by increasing thickness, imaginary part of dielectric constant increases, that depends on more absorbance of the light.
Figure 6 shows, the real part of conductivity index which in general agrees well with the results by Aspnes & Thee'ten's  data. By increasing thickness, real part of conductivity index increases, that is because of formation completed metallic silicon layers.
The dependences of the imaginary part of the conductivity coefficient upon the photon energy is shown in Figure 7. The general trend of our results is the same as Aspnes & Thee'ten  results. By increasing thickness, imaginary part of conductivity index decreases that depend to absorbance as we discussed before.
Figure 8 represents the dependence of the absorption coefficient on the photon energy. Our results are similar to those obtained in the earlier studies (1980). The general trend of all layers is the same. The peak at 3.4 eV is shown already. By increasing thickness absorption coefficient increases. That is because of formation of completed layers and filling up the voids with silicon metallic grains, there for transmittance decreases and absorbance increases.
In this work we have deposited thin silicon layers on the glass substrates under the same deposition conditions with different thicknesses of 100, 140 and 180 nm by electron gun evaporation method. Optical reflectance and transmittance of the layers were measured. The optical constants of our samples have been calculated, using the Kramers-Kronig method. Our results are compared with those obtained earlier by Aspnes and Thee'ten .The general trend of our results are quite similar to those found in the works (1980). By increasing thickness of the layers there is an increasing trend for the reflectance spectra. By increasing thickness real part of refractive index decreases and imaginary part of refractive index increases, that was because of formation more completed layers and filling up the voids. By increasing thickness real part of dielectric constant shows a decreasing trend and imaginary part of dielectric constant increases, for the same reasons there is an increasing and decreasing trend for real and imaginary parts of conductivity constant respectively. By increasing thickness absorption coefficient increases. The band gap energy of the layers at about 3 eV energy that is in agreement with silicon standard band gap.
Received 23 December 2013
Received in revised form 25
Accepted 26 February 2014
Available online 25 March 2014
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Department of physics, faculty of science, Urmia branch, Islamic Azad University, Urmia, Iran
Corresponding Author: Haleh Kangarlou, Department of physics, faculty of science, Urmia branch, Islamic Azad University, Urmia, Iran
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|Title Annotation:||physical vapor deposition|
|Author:||Khezri, Kamal Hasan; Kangarlou, Haleh|
|Publication:||Advances in Environmental Biology|
|Date:||Feb 14, 2014|
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