الفهرس 
coverOnly 14 pages are availabe for public view
 |  | from | 126 |  |  |
| | | from | 126 | | |
Abstract
In the present work, we present the experimental results and discuss the results of structural, compositional, optical and electrical properties of thermally evaporated ZnTe thin films treated with AgNO3 solution for different periods of time The structural characterization of thermally evaporated ZnTe thin films as well as those treated with AgNO3 solution has been investigated using X-ray diffraction (XRD), transmission electron microscope (TEM) and electron diffraction (ED). The surface morphology and chemical composition of the investigated films have been studied using scanning electron microscope (SEM) attached to energy dispersive X-ray spectrometry unit (EDX).
To identify the crystal structure of the deposited pure and Ag-doped ZnTe films, the X-ray diffraction method was employed. The presence of a number of peaks in such diffraction pattern of the investigated samples is the indication of polycrystalline nature of the deposited film. The main diffraction peaks at 2= 25.65°, 42.02°, and 49.530 are attributed to the (111), (220), and (311) planes, respectively. These positions compared well with data reported in JCPDS cardNo.75-2085.The deposits are cubic with a preferential orientation along the (111) plane. The deposits have a close stoichiometric composition. There is no peaks related to zinc or tellurium was observed. It was also observed the X-ray diffraction patterns of ZnTe films immersed in AgNO3 solution for different periods of time followed by annealing at temperature 473 K for 1h have the same feature.
The transmission electron micrographs of the as-deposited ZnTe film consist of small particles uniformly distributed through the investigated region. It is clearly show a significant difference between Ag/ZnTe films before and after annealing process. whereas, the transmission electron micrograph for the ZnTe film immersed in AgNO3 solution (before annealing process) showed agglomerate randomly distributed regions of Ag particles on the surface of the deposited ZnTe films. When these films being annealed at 473 K for 1h, the randomly distributed regions of the Ag particles which were observed onto the surface of the ZnTe films disappeared and diffused into the ZnTe layer. The selected area electron diffraction patterns for pure as well as Ag-doped films shows continuous rings of different intensities, confirming the polycrystalline structure. This result confirmed that Ag diffused into the ZnTe layer at an annealing temperature of 473 K.
It was seen from the SEM micrograph that there are white like needle regions observed onto the surface of the Ag-doped ZnTe layer in comparison with the pure one. Those regions correspond to agglomerate of Ag particles, which formed due to immersed the ZnTe films into the AgNO3 solution. The area of those regions was found to be increases as the periods of the immersion time increases.
The chemical composition of pure and Ag-doped ZnTe films was investigated by means of energy dispersive X-ray spectrometry using energy dispersive. Through which it can be seen the presence of elemental Zn, Te and Ag in the investigated films. Also, it is seen from the data that zinc and telluride for the as deposited ZnTe film are present almost in stoichiometric ratio, whereas, the Ag at % for ZnTe films immersed in AgNO3 solution increases as the periods of the immersion time increases.
The optical transmission T and reflection, R spectra of pure and Ag-doped ZnTe films exhibit an oscillatory behavior, which the envelope method can be applied to calculate the refractive index, n and film thickness, t. The behavior of refractive index of all the investigated samples is similar, which is due to the normal dispersion. The refractive index, n has a higher values at low wavelengths spectral region < 750 nm (strong absorption region), thereafter, the refractive index was found to decreases with increasing wavelength and becomes fairly flat above 1250 nm. In addition, the effect of increasing immersion time of ZnTe films in AgNO3 solution reflects a gradually increasing in the refractive index at any wavelength value. The absorption coefficient value markedly increases as photon energy increases in the range from 1.6 to 3.3 eV. In addition, the value of the absorption coefficient in the main fundamental absorption edge (2-3.1 eV) was found to increases as the periods of the immersed ZnTe films in AgNO3 solution increases.
The spectral dependence of absorption coefficient could be described by the following equation:
where is the photo energy, A is a parameter that depends on the transition probability, Eg is the optical band gap and the exponent p depends on the type of the optical transition between the valence and conduction band, direct or indirect transition We plotted versus for investigated samples, and found the best fit for these samples was obtained for p =1/2 indicating a direct allowed optical transition.
The electrical resistance of the pure and Ag doped ZnTe films were carried out in the temperature range 300-575K. It is observed that electrical resistance of the films decreases with increasing the periods of the immersion time. The resistance decrease from for as-deposited film to for ZnTe film immersed in AgNO3 solution for 90 sec. The variation of the conductivity versus temperature exhibits two conduction mechanisms with two linear parts at two different temperature ranges, the first extends between 303 and 388 K and the second between 388 and 575 K, indicating two types of conduction mechanisms through thermally activated process. The values of the activation energy calculated for such conduction mechanisms decreases with increasing the periods of the immersed time.