Study the effect of Cd addition on some physical properties of Se–Ge thin films

Abstract

The structural, optical properties, dc electrical properties switching phenomenon, ac conductivity and dielectric properties of Se80Ge20-xCdx (0≤ x ≤ 12 at%) film compositions were studied. For this purpose amorphous Se80Ge20-xCdx (0≤ x ≤ 12 at%) composition were prepared in bulk form by quenching the molten materials in an ice-water after synthesis according to a special regime in an evacuated silica tube and then thin film form were prepared from bulk compositions by thermal evaporation technique. X-ray diffraction analysis (XRD) showed that the prepared samples in powder and thin films forms were in amorphous state. Differential thermal analysis DTA for the investigated compositions in powder form revealed that the glass transition temperature Tg are 435, 410,399K respectively. Energy dispersive X-ray analysis (EDX) for the investigated compositions indicates that the real percentage of the constituent elements in the investigated compositions Se80Ge20-xCdx (0≤ x ≤ 12 at%), are close to that prepared. The optical properties of Se80Ge20-xCdx (0≤ x ≤ 12 at%) thin film samples are studied as a function of the Cd content. Transmittance and reflectance were measured at room temperature for samples of different thicknesses in the range (200 - 620 nm) in the wavelength range (350-2500 nm) to calculate the optical constants (refractive index n, absorption index k and absorption coefficient ) using Swanepoel method. Analysis of absorption index k indicates that the allowed optical transitions were found to be indirect transitions and the optical band gap Egopt decreases with the increase of Cd but the width of the tail of localized states Ee the increases with the increase of Cd. The temperature dependence of the dc electrical conductivity σdc , studied in the temperature range (293-333 K) for the Se80Ge20-xCdx (0≤ x ≤ 12 at%) film compositions, showed that σdc increases linearly with the increase of temperature. The obtained results showed that each composition has two values of activation energy and through the considering range of temperature, which can be interpreted according to Mott and Davis model. The dc electrical conduction activation energy thickness independent in the investigated range (200-620nm), it decreases with Cd addition. Room temperature electrical conductivity RT increases and the activation energy (E) and the pre-exponential factor  decrease with the increase in Cd in the investigated compositions. The obtained I-V characteristic curves for Se80Ge20-xCdx (0≤ x ≤ 12 at%) films revealed that they are typical for a memory switch. The mean value of the threshold voltage Vth, measured at room temperature increases linearly with increasing film thickness (t) in the investigated range (200-620nm) and decreases exponentially with increasing temperature in the range (293-393K) for the studied films. The obtained values of the threshold voltage activation energy εth are independent of film thickness in the investigated range, its mean value εth decreases with Cd content. The obtained values of εth/ΔEσ 0.495 agree with that obtained theoretically on the basis of an electrothermal model 0.5 for switching process. Moreover values of the temperature difference between the inside of the film and that of its surface (Tbreakdown), calculated on the basis of the electrothermal model are in the same order with those obtained previously for other chalcogenide glasses. Therefore the switching process in the investigated compositions can be explained satisfactorily according to the electrothermal model. Ac conductivity , of Se80Ge20-xCdx (0≤ x ≤ 12 at%) chalcogenide glass films were studied as a function of temperature in the range (293 – 393 K), and frequency in the range (102 Hz – 105 Hz) and thickness in the range (200 – 620 nm). The obtained results of the frequency dependence of the ac conductivity of the studied films satisfies the power low , where S is the frequency exponent. The frequency exponent S decreases slightly with increasing temperature, in accordance with the correlated barrier hopping (CBH) model between centers forming intimate valence alternation pairs (IVAP's). The temperature dependence of is linear with a single activation energy . The activation energy for the studied film composition decreases with increasing frequency. The dielectric constant and the dielectric loss are studied for Se80Ge20-xCdx (0≤ x ≤ 12 at%) films in the temperature range (293 – 333 K), and frequency range (102 – 105 Hz). It is found that both dielectric constant and dielectric loss decreased with frequency while increased with temperature and Cd addition due to the increase concentration of localized states. Maximum barrier height Wm values, were calculated according to Guintinni equation (2 =Bm) for the investigated compositions. The obtained values of Wm are 0.3, 0.26, 0.2 eV for Se80Ge20-xCdx (0≤ x ≤ 12 at%) films respectively, which agree with the theory of hopping of charge carriers over a potential barrier as suggested by Elliott in the case of chalcogenide glasses.