acta physica slovaca

Abstract: The acoustoelectric interaction between a longitudinal acoustic wave and heterojunctions is used to investigate the deep centers in GaAs/AlGaAs heterostructures. The acoustoelectric response generated by the interface when acoustic wave propagates through the heterostructure reflects any changes in the charge distribution, connected also with the charged traps. The time developments of the acoustoelectric response after an optical excitation pulse reflect then relaxation processes associated with the thermal recombination of excited carriers moving towards their equilibrium state. The measured signal that is proportional to the nonequilibrium carrier density in the interface regions and can be used to the determination of activation energy and corresponding cross-section of deep centers. Planar GaAs/AlGaAs heterostructures with both two dimensional electron system (2 DES) and two dimensional hole system (2 DHS) were investigated by optically induced acoustic deep-level transient spectroscopy (OI A-DLTS). The method of computer evaluation of isothermal acoustoelectric transients was used in the A-DLTS technique. Several deep centers were found and their parameters were determined.

Abstract: Defect states in Si can be passivated by cyanide treatment which simply involves immersion of Si materials in KCN solutions, followed by rinse. When the cyanide treatment is applied to pin-junction amorphous Si (a-Si) solar cells, the initial conversion efficiency increases. When the crown-ether cyanide treatment using a KCN solution of xylene containing 18-crown-6 is performed on i-a-Si films, decreases in the photo- and dark current densities with the irradiation time are prevented. The cyanide treatment can also passivate interface states present at $\rm Si/SiO_{2}$ interfaces, leading to an increase in the conversion efficiency of $\rm \langle ITO/SiO_{2}/Si(100)\rangle $ solar cells. Si-CN bonds formed by the reaction of defect states with cyanide ions have a high bond energy of about 4.5 eV and hence heat treatment at $\rm 800^{\circ}C$ does not rupture the bonds, making thermal stability of the cyanide treatment. When the cyanide treatment is applied to ultrathin $\rm SiO_{2}/Si$ structure, the leakage current density is markedly decreased.

Abstract: In this work an experimental study on the etching reaction of low-doped silicon was carried out in HF/K$_{2}$Cr$_{2}$O$_{7}$/H$_{2}$O mixed solutions. The morphology of the etched surface was examined with varying HF and K$_{2}$Cr$_{2}$O$_{7}$ concentration, reaction time and agitation speed. The surface of etched wafers were analysed using SEM (scanning electron microscope) and EDX (Energy-dispersive X-ray). The structure of porous silicon films created by etching in HF/K$_{2}$Cr$_{2}$O$_{7}$ is shown to depend on the solution composition and the resistivity of the silicon substrate. The investigation of the HF/K$_{2}$Cr$_{2}$O$_{7}$ system on polycrystalline silicon surface shows that in the concentration range 1 $\div $ 2 M of potassium dichromate (K$_{2}$Cr$_{2}$O$_{7})$, the surface is microporous. The texture developed is a network of hemispherical tubs. It is seen that reflection losses in micropores (tubs) decreased greatly, to be comparable with the defect etchants method and much lower that obtained by Stocks et al. Simultaneous porous silicon growth and deposition of hexafluorosilicates has been observed. Taking into account this observation a dissolution mechanism of the silicon is proposed.

Abstract: Optical parameters of thin films are important for several optical and optoelectronic applications. In this work the genetic algorithm method is proposed to solve optical parameters of thin film values. The experimental reflectance is modelled by the Forouhi -- Bloomer dispersion relations. The refractive index, the extinction coefficient and the film thickness are the unknown parameters in this model. Genetic algorithm use probabilistic examination of promissing areas of the parameter space. It creates a~population of solutions based on the reflectance model and then operates on the population to evolve the best solution by using selection, crossover and mutation operators on the population individuals. The implementation of genetic algorithm method and the experimental results are described too.

Abstract: In this paper results of a characterization of the surfaces of ZnTe epitaxial thin films exhibiting the different thicknesses are presented. The results mentioned are obtained using the procedures enabling us to determine the values of the following quantities: mean grain size, grain size distribution, root--mean square values of the heights of the irregularities and the diagram describing the distribution of the directions of the normals. For the analysis of the grain structure a watershed algorithm is used. It is shown that the values of these quantities can describe the morphology of the ZnTe film surfaces in a sufficient way. Further, it is shown that the structure of the surfaces of the ZnTe films exhibit facets forming a grain structure. Moreover, it is presented that the ZnTe film surfaces exhibit a strong slope anisotropy and that the linear dimensions of the grains increase with increasing values of the thicknesses of the ZnTe films.

Abstract: Photovoltaic characteristics of a heterojunction solar cell composed of reactively evaporated indium oxide ($\rm{In_2O_3}$) film and single crystalline p-type indium phosphide (InP) was found to depend on acceptor concentration at the interface. The value of acceptor concentration was preferable to be high to obtain a high performance cell because larger open-circuit voltage can be obtained due to decrease of diode saturation current of the cell with the increase of the acceptor concentration. The acceptor concentration of the cell was increased by annealing during forming an ohmic contact. The increase of acceptor concentration by annealing thought to be able to explain in terms of outdiffusion of the interstitial zinc atoms in InP bulk. Further, the value of acceptor concentration is modified by substrate heating during deposition of transparent and conductive $\rm{In_2O_3}$ film. In order to produce a high performance cell, low substrate temperature ($200^{\circ}$C) was preferable during deposition of $\rm{In_2O_3}$.

Abstract: In the present article, after defining biquaternions, the general properties of biquaternion's algebra are introduced. The matrix representations of biquaternions are presented, as well. Then, the biquaternionic angular momentum is reformulated in terms of biquaternionic product. A new biquaternionic definition of the Dirac equation and its solution are given by the use of biquaternion's basis.

Abstract: After introducing the quaternions and its algebra, the equations of the linear acoustics are defined. The local conservation equation for energy of the linear acoustics using quaternions and a quaternionic first-order Lagrangian description is then formulated. Using the variational principle, the local conservation equation for the energy is derived from the quaternionic gauge transformation. The purpose is to provide an alternative for the usual derivations.

Abstract: The hybrid electron pencil beam model (HPBM) can usually be applied to middle or large fields and get good results, whereas it is sometimes not so successful to apply HPBM to small fields. In this note we report some results obtained by using an extended hybrid electron pencil beam model for small fields. Considering the condition of small fields, we redefine the central-axis depth dose for mono-energetic electrons in small fields and calculate the fitted spectrum of incident electron beam for these small fields. Through these improvements, we can make the extended hybrid electron pencil beam model applicable to small fields and obtain good results. And at the same time, the extended HPBM retains the calculation accuracy and efficiency for middle and large fields.