A. Tříska

Density of the gap states in undoped and doped glow discharge a-Si:H

Abstract The density of the gap states in undoped and phosphorus and antimony doped a-Si:H has been determined from the absorption coefficient measurement. A constant photocurrent method was used to determine the value of optical absorption in the low absorption region. A quantitative model has been suggested to explain all experimental data. The model is based on a Gaussian shaped maximum, connected with the dangling bond, within the exponential band tails just below the middle of the gap. Moderate and strong coping rises the concentration of dangling bonds.

Direct measurement of the gap states and band tail absorption by constant photocurrent method in amorphous silicon

A direct way to obtain the spectra dependence of the optical-absorption coefficient in the low-absorption region (10−1−103cm−1) on thin amorphous silicon films is presented. The constant photocurrent method (CPM) is described compared with normalized photoconductivity spectra measurement and the results on a-Si:H are presented. Implications of the CPM for the gap states spectroscopy are suggested.

Electrical properties of glasses in the GeBiSbSe and GeBiS systems

Abstract Measurements of optical absorption, dc conductivity and thermopower were carried out on glasses of the types (GeSe3.5)88sbPin12−xBix. The main results are: 1) observation of n-type conduction in the Ge20BixS80−x glasses containing high concentrations of Bi; 2) in the mixed doped glasses, transition from p-type for a Sb-doped glass (x = 0) to n-type on addition of Bi at nearly equal concentrations of Sb and Bi. The possible types of bonding of the Bi atoms in the structure and their influence on the electrical behaviour are examined.

Is the n-type conductivity in some Bi-doped chalcogenide glasses controlled by percolation?

Abstract The compositional dependences of the resistivity (ϱ) of Ge 20 Bi x Se 80− x and Ge 20 Bi x S 80− x glasses (0 ⩽ x ⩽ 15) have been measured and interpreted from the point of view of percolation-controlled conductivity in these materials. The results of the simulation of the experimental compositional dependences of ϱ show that a transition from p to n -type conductivity most probably occurs in the vicinity of a percolation threshold. In homogeneous transport therefore seems to be typical of these glasses, a feature supported, at least for Ge 20 Bi x S 80− x , by the observation of two softening temperatures.

On the determination of the CuO-BaCuO2 and CuO-YCuO2.5 binary phase diagrams

Abstract The phase relations in the CuO-BaCuO 2 and CuO-YCuO 2.5 systems were investigated by means of DTA and X-ray diffraction analysis. Decomposition of CuO into Cu 2 O and mutual reaction of the melt containing BaO complicated the phase diagrams determination.

On the nature of bleaching of amorphous Ge30S70 films

Abstract The irreversible photobleaching of amorphous Ge30S70 films is found to depend on the air pressure during exposure. This is most probably a result of photo-oxidation and/or hydrolysis of the layers. The appearance of a Ge-O-Ge vibrational band in the infrared transmission spectra of exposed layers clearly indicates the extrinsic origin of photobleaching. Annealing of the layers is also accompanied by considerable bleaching. In this case the role of oxygen seems to be of less importance and an increase in the density of Ge-S bonds is most probably responsible for the increase of the optical gap.

Some physical properties of the glassy (GeS2)x(Sb2S3)1−x system

Some physical properties of a (GeS2)x(Sb2S3)1−x noncrystalline system are described. In contrast to the simple compositional dependences of e.g. the glass transition temperature (Tg), optical gap (Egopt) and high-frequency dielectric constants(ϵ∞), the compositional dependences of both the DC electrical activation energy (ΔEσ) and preexponential factor (σom), and also those of the reciprocal slope of the exponential absorption edge, show abrupt changes in the vicinity of 0.6 < Y < 0.7 (Y is volume fraction of GeS2). From the results of the application of the effective medium theory to the calculation of σom(Y) dependence it can be supposed that there exists an inhomogeneous transport regime.

The composition dependence of the gap in amorphous films of SixGe1−x, SbxSe1−x and AsxTe1−x systems

Abstract The gap energies of the amorphous thin films of Si x Ge 1− x , Sb x Se 1− x and As x Te 1− x systems were correlated with the “mean” bond energy using Mancas relation. The best fit between calculated and experimental values of the gap obtained using random covalent network model (RCNM) is valid. In accordance with Shimakawa the RCNM idea is applicable for chemical bonds network in these thin films.

Optical gaps from ‘mean’ bond energy in Ge1−x S x and Ge k Sb m S n non-crystalline solids

Abstract An attempt has been made to evaluate the optical gaps in Ge1-x S x and Ge k Sb m S n glassy systems using Mancas relation and the bond statistics of various models. The best agreement between calculated and experimental data is obtained for a chain-crossing model in the S-rich region and for a model due to Philipp in the metal-rich region. The change of the optical gap of Ge1-x S x thin films associated with the transition from as-deposited to annealed states is also investigated.

Determination of the mobility gap in amorphous silicon from a low temperature photoconductivity measurement

Abstract Measurement of the spectral dependence of photoconductivity and quantum efficiency at liquid helium temperature is presented. The mobility gap value was found to be quite close to the Tauc optical gap value for undoped good quality material.