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Chalcogenide glasses have a number of individual properties such as photosensitivity, low phonon energy, high linear and non-linear refractive index, high excitation times and wide range of transparency in the infrared region. Thin films of these materials have many applications in a field of optical technologies (optical fiber connections, optical switches, media for optical recording of information and optical sensors) [1-7]. These materials with appropriate properties have a potential for thermoelectric and photovoltaic device applications. Chalcogenide is p-type semiconductor due to pinning of the Fermi energy level near the middle of band gap [8- 10].
Tohge et al. [11, 12] reported that n-type glasses could be obtained by adding suitable amounts of Bi or Pb to Ge-Se glasses. On the other hand, selenium has unique property of reversible phase transformation and also applications like photocells, xerography, memory switching etc., but pure selenium has disadvantage as short life time and low photo sensitivity. To get over this problem, we can use some impurities such as Ge, In, Bi, Te, Sb and Ag to make alloys with Se which may improve sensitivity, crystallization temperature and reduce ageing effects .
To understand mechanisms of conduction processes and type of polarization in amorphous chalcogenide semiconductors, AC conductivity and dielectric measurements are necessary [14-18]. This work aims to investigate XRD, optical properties, electrical conductivity and Hall effect of PbxGe42-xSe48Te10 at room temperature.
Samples were prepared from high purity Pb, Ge, Se, and Te (99.999% Mathy Chem. Ltd) by melt quenching method. Calculated ratios of alloying elements were weighted according to their atomic weights percentage and then placed in a cleaned graphitized silica tube.
Ampoules were evacuated and sealed under a vacuum of 10-5 m.bar. Silica tubes were heated gradually to 870K for about 24h and then raised to 1273K and kept at this temperature for 36h. Thin films were prepared by thermal evaporation in vacuum 10-6 m.bar. Powder and thin films were analyzed by XRD and the composition was determined by (EDAX) Philips (XL 30 attached with EDAX unit). Transmission (T) is measured by using double beam spectrophotometer (JASCO V-570 model).
Elements of all compositions are examined by energy dispersive X- ray analysis (EDAX). Fig. shows (EDAX) of Pb18Ge30Se48Te10 as example curve. It is seen that a compound is nearly stoichiometric as obvious from data.
Transmittance spectra of thin PbxGe42- films are measured at wavelength (200 ? ?? 2500 nm) as seen in figure (). Curves show fringes due to interference at different wavelengths which predict that thin film is homogeneous. To calculate optical constants the maximum (TM) and a minimum (Tm) of these fringes are used shows the absorption coefficient, It is obvious that decreases with increasing Pb content and increases with increasing of photon energy. To determine a nature and value of optical energy gap (Eg), absorption coefficient was used. A dependence of ? on h? is given by Tauc relation [19-21]:
Where B, Eg and m are constant, energy band gap and exponent depends on a type of transition respectively. Indirect Eg is calculated from the absorption coefficient by plotting a relation between (?h?) 0.5 and h? (Fig. ). Values of indirect energy band gap have been estimated from the intercept on the h? -axis. We noticed that there are two energy band gaps Eg1 and Eg2 which referred to fundamental edge and band splitting by crystal field. Calculated values of Eg1 and Eg2 for all samples of a PbxGe42-xSe48Te10 are tabulated in Table1. It is clear that from Table1, both Eg1 and Eg2 increased with Pb increment. An increment in energy band gap attributed to, in chalcogenide glasses the valence band (?-bonding) arises from lone-pair (LP) electron states while the conduction band arises from anti-bonding (?*) states . Variation of Eg with Pb concentration can be clarified on the basis of the change in average bond, bond strength, network connectedness and density of amorphous solids as a function of composition .On the other hand from a chemical approach, an increase in Eg may be attributed to the increase in bond strengths which leads to a larger splitting between ? and ?* bands . Energy band tail (Eu) can be obtained according to Urbach equation :
ln = ln D +(hEu)
where D and Eu are constant and band tail width shows relation between ln and photon energy (h?), Eu can be estimated from the inverse slope of straight line and recorded in table 1. It is seen that Eu decrease with increasing Pb content. This may be referred to the density of localized states decrease with increasing Pb content in Ge-Se-Te system. Fig.() shows a dependence of refractive index (n) and extinction coefficient as representative curve. From this figure we notice that k decreases linearly with increasing . This behavior may be attributed to decrease in absorption coefficient with increasing wavelength, while n increases at low then becomes constant at high this increasing means that the film is more optically dense. Moreover dielectric function is complex quantity and contains both real (?r) and imaginary parts. shows how a dielectric material with dielectric constant absorbs energy from electric field due to dipole motion while ?r points to how the speed of light in material can be slowed. Dielectric constant of solid material is significant for optoelectronics applications due to a change in optical energy band gap causes a variation in ?, which indirectly changes ionization energies of impurity atoms and binding energy of the excition.
tan = i/ r
Variation of tan vs. frequency for Pb10Ge32Se48Te10 is shown in Fig. as a representative curve. It is obvious that tan? increases with increasing frequency. On the other hand, Wemple- DiDomenico (W-DD) model of single oscillator is used to calculate the refractive index in the normal dispersion region:
n2(h?) = 1 + Ed E° [E°2- (h?)2] (6)
Ed and E° values are determined by using equation 3 (W-DD) and denoted as dispersion energy and energy of effective dispersion oscillator. A dependence of (n2-1)-1 on (h?)2 is shown in Fig.(). From this figure, Ed, E° and ?? can be computed from both slope and an intercept of linear relation. From the relations, n?2=1+EdE° and then n?2=?? is dielectric constant at infinite wavelength can be obtained. Values of Ed, E° and ?? are listed in table 1; It is noticed decreasing with increasing Pb content. The decrement of Ed may be attributed to the dependence upon the distribution charge carriers in each unit cell and this is related to chemical bond . Moreover, first and third order of moments (M?1) and (M?3) can be derived from equations:
E°2=M-1M-3 ,Ed2=M-13M-3 (7)
Obtained values are recorded in table 2. It is obvious that both moments varied with Pb content and M?1 decreases with increasing Pb. On the other hand, both optical moments related to effective dielectric constant, effective number of valence electrons in investigated thin films . A relation between n2 and ?2 is seen in figure as a representative curve. Lattice dielectric constant (?L) can be determined from an extrapolation the straight line to n2. (N/m*) can be estimated by using the following equation:
n2= ?L-(e2N?c2m*)?2 (8)
Where c, e and m* are velocity of light, electronic charge and electron effective mass respectively. N/m* values increase with the increase of Pb concentration. This increment may be due to the saturation of dangling bonds increases which indicates that this parameter is related to internal microstructure of the films. From a relation n°=?? , static refractive index can be calculated. Values of ?L, N/m* and n° are tabulated in table 2.
Optical conductivity (?opt) is a tool for studying the electronic states in materials. ?opt is determined by using the following equation :
?opt = ? nc4? (9)
Where ?, n and c are absorption coefficient, refractive index and velocity of light respectively. Variation of ?opt vs. h? is shown in Fig.. It is seen an increase in ?opt at high photon energy may be due to electrons excited by photon energy and high absorbance of thin film . Also, when absorption of photon by compound increases, the compound has high ?opt; it suggests that a material suitable for optoelectronic applications. Electrical conductivity (?ele) is derived from this relation:
?ele = 2??opt? (10)
Dependence of ?ele on h? is shown in Fig. as example curve. It is found that a decrease of ?ele with increasing h?, values of electrical conductivity indicate semiconducting nature of thin films.
Surface energy loss (SEL) and volume energy loss (VEL) are important parameters which describe the optical electron transitions in an examined material, (SEL) can describe electron transition in thin-film while (VEL) describes a transition in bulk material.
It is clear that from figure an increase in (SEL/VEL) and attains to a maximum then decrease with increasing h?.
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