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Abstract
Summery and conclusions
Tin telluride is a IV-VI compound. It has a very interesting properties as narrow gap, high dielectric constant, high mobility, high density of charge carriers and has two maxima valance band. In addition, SnTe has been launched in many application such as, low band gap infrared detectors, injection lasers, luminescent injection diodes and radiation receivers.
In this study SnTe was synthesized and deposited in thin film form utilizing thermal vacuum deposition technique. For best quality films, flash evaporation was applied in order to obtain high quality thin films of SnTe. Prior to the study of the physical properties of SnTe thin film and to have better understanding to these properties, a structural investigation has been established. Such a structural study includes X-ray diffraction, energy dispersive X-ray analysis, EDX, electron diffraction and differential thermal analysis, DTA, techniques. According to the X-ray examination of SnTe thin film the crystal structure has been revealed and found to be polycrystalline and have fcc structure. The lattice constants have also been extracted from the diffraction pattern of SnTe in a bulk and thin film form. Such results were confirmed by electron diffraction utilizing transmission electron microscope. Since the stoichiometric state of the film has a drastic effect on the physical properties of the film, EDX has been performed. According to the EDX data analysis the stoichiometric state has been sustained. Regarding the X-ray diffraction pattern and DTA, the investigated SnTe thin film have the polycrystalline structure for the as-deposited thin films and no phase changes were detected in the temperature range between 300K up to 450K. After heat treating of SnTe thin film samples, no significant changes were detected when structurally reexamined.
In order to investigate the electrical properties of SnTe thin films, the DC resistivity, thermoelectric power, AC conductivity and the dielectric constants were considered. According to the DC resistivity measurements of thin films of SnTe, it has been found that SnTe thin films behave as semimetal in the temperature range between room temperature and 450K. However the thermoelectric power and Seeback coefficient in the above temperature range indicate that SnTe thin films behave as a semiconductor. Moreover the type of conduction has been identified to be p-type with density of charge carriers, p, in the order of 1-5x1019cm-3. Throughout this study, the temperature and thickness dependence of the Fermi energy, Ef, and the carriers mobility, ?, of SnTe thin films were tentatively determined. Form the analysis of the obtained results, the temperature dependence of p, Ef and ? can be attributed to the transition from extrinsic conduction to the intrinsic conduction. On the other hand, the thickness dependence of DC resistivity and the semiconducting parameters are associated with the lattice defects introduced to the films during the deposited stage. Such defects behave as an acceptor impurities. From the thickness dependence of DC resistivity, the size effect parameters for the specular and non specular cases were revealed as a new data of SnTe thin film.
Considering the dielectric properties for their importance particularly in the technical application, the AC conductivity, the dielectric constant, the dielectric loss and the loss tangent were investigated as a function of field frequency, temperature and film thickness. Such properties were analyzed in terms of the small polaron hopping mechanism. According to this model the hoping energy, relaxation time and frequency exponent factor were evaluated and reported for SnTe thin films.
For the significant informations which can be extracted from the optical properties as the optical constants, dielectric constants, absorption coefficient plasma frequency, energy gap and type of transition, thin films of SnTe were optically investigated in a wide range of photon energy. Based on the normal incidence reflectivity and transmissivity measurements, the optical properties of SnTe thin films were exploited.
Following are the overall conclusions:-
1- SnTe thin films have been synthesized and thermally vacuum evaporated using flash evaporation technique.
2- From the X-ray study, and the TEM investigation the studied thin films were polycrystalline and have fcc structure with lattice constant a ? 6.3075?.
3- According to the EDX analysis, the deposited thin films were stoichometric and no phase changes were detected after heat treatments of the films.
4- Regarding the DC electrical resistivity of SnTe thin films, the films behave as a semimetal and metals.
5- In respect of the thermoelectric power measurements the SnTe thin films have a semiconducting properties with type p of charge carriers.
6- A transition from the extrinsic conduction at low temperature to an intrinsic conduction at high temperature was detected in SnTe thin films characteristics.
7- SnTe thin films are characterized with a relatively high density of charge carriers in order of 1-5x1019cm-3 and high drift mobility ? 400 cm2 /V.sec.
8- It is concluded that the AC conduction is governed by the small polaron hopping mechanism with hopping energy in the order of 0.24eV and relaxation jump time in order of 1-4x10-6sec.
9- Form the optical investigation it has been concluded that the SnTe thin films are characterized by allowed direct transition with two values of optical energy gaps.
10- In agreement with the band structure and the electrical properties the values of the optical energy gapes were found to be 0.18eV and 0.52eV.
11- A quite good agreement between the obtained results and the reported results concerning the energies of the critical points using different techniques was found.