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Abstract
Chalcogenide glasses are one of the most widely known families of amorphous materials and have been extensively studied for several decades. Our present work is devoted to investigate the structural, optical and electrical transport properties of Ge¬1-x Se2 Pbx (x = 0, 0.2, 0.4, 0.6 and 0.8 at %). The bulk of Ge1-xSe2Pbx semiconducting materials (with x = 0, 0.2, 0.4, 0.6 and 0.8 at%) were prepared by the standard quenching technique. Materials (99.999 % Pure) were weighed according to their atomic percentage and sealed in evacuated (~10-4 Pa) silica ampoules tubes. The sealed ampoules were heated up to 1273 K and were held at this temperature for 12 hrs inside a furnace.
The temperature of the furnace was raised slowly at a rate of 3 - 4 K/min. during heating; all the ampoules were constantly rocked, by rotating a ceramic rod in which the ampoules were tucked away in the furnace. After rocking for 12 hrs, the obtained melts were quenched into ice cooled water.
In order to produce high quality Ge1-xSe2Pbx (with x = 0, 0.2, 0.4, 0.6 and 0.8 at%) films by thermal evaporation in vacuum having reproducible structure. Carefully cleaned glass substrates were used for both structural and transport properties studies, while for the optical constants determination, optical flat quartz substrates have been used.
The stoichimetry of these compositions was tested by energy dispersive analysis of X-ray (EDAX), the results indicated that the obtained films are nearly stiochiometric for all compositions under investigation.
The differential thermal analysis technique (DTA), determined the glass transition, Tg crystallization temperature, Tc and melting point Tm for all compositions under investigation.
X-ray diffraction (XRD) results indicated that as deposited films are amorphous materials, annealing at 537 K for 2 hrs transforms amorphous structure into nano-crystallite one and the formed preferred orientation in Ge1-xSe2Pbx films is different from that of Ge Se2 films.
The optical properties of thin chalcogenide films of Ge1-xSe2Pbx with (x =0, 0.2, 0.4, 0.6 and 0.8 at%) have been determined from their corresponding transmittance and reflectance. The results clearly show that the chalcogenide amorphous materials alloyed with lead change in a remarkable fashion all their optical properties. The optical constants for as deposited and annealed Ge1-xSe2Pbx thin films had not been influenced by film thickness in the utilized film thickness range (75-350 nm). It has been found that the value of the optical band gap decreases clearly with increasing Pb content, whereas the value of the refractive index at long wavelength (h→0) increases from 2.50 to 3.17 and 2.30 to 2.93 for as deposited and annealed Ge1-xSe2Pbx thin films respectively. A detailed analysis of the dispersion curve based on the effective single oscillator frame work allows us to obtain two physical parameters known as dispersion energy, Ed, and oscillator energy, Eo. These parameters have been related to the optical gap and structural properties of the deposited films. The average gap, Eo, gives quantitative information on the overall band structure of
the material. This is quite different from the information coming from the value of the optical gap, Eg, which probes the optical properties near the band edges of the material. In particular, localized states near the conduction or the valence band “tail states” might have a strong effect on the optical absorption and thus decrease the optical gap, whereas if they have a small polrizability, they will result in a small effect on the refractive index: such tail states increase the “Urbach tail”, but have little effect on the average gap, Eo
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The DC electrical conductivity of amorphous Ge1-xSe2Pbx films prepared by thermal evaporation technique measured in the temperature range of 300 450 K and analyzed to exhibit two different transport mechanisms. In the high temperature range T > 385 K, the conduction was found to be of activated type and the Meyer-Neldel Rule (MNR) has been found to be satisfied for the thermally activated conductivity. It is observed that there exist a strong correlation between (MNR) conductivity prefactor and activation energy E. And we found that
the activation energy E in this region decreases from 1.11 to
0.37 eV by increasing pb content. In the low temperature range
T < 385 K, the conduction mechanism described by Mott’s law due to variable range hopping of electrons between localized states is the operating mechanism. The values of To and N(EF) have been calculated. Two other Mott’s parameters are hopping distance, R, and the average hopping energy, W, can also be evaluated.
The AC conductivity , dielectric constant , and dielectric loss of the studied Ge1-x Se2 Pbx films, seem likely to be both frequency and temperature dependent regarding the small values of AC activation energy , the discrepancy between AC and DC activation energies and the frequency dependence of the AC conductivity. The conduction mechanism was suggested to be hopping conduction.
In addition, the value of the frequency exponent s and its temperature dependence confirmed the applicability of the correlated barrier hopping, (CBH) model to the investigated system.
Both frequency and temperature dependence of the dielectric constant were attributed to interfacial and orientational polarization, respectively. whereas, the temperature dependence of dielectric loss is associated with the conduction loss