Morphological, optical and photovoltaic characteristics of MoSe2/SiOx/Si heterojunctions.

This work reports the effect of different processing parameters on the structural and morphological characteristics of MoSe2 layers grown by chemical vapour deposition (CVD), using MoO3 and Se powders as solid precursors. It shows the strong dependence of the size, shape and thickness of the MoSe2 layers on the processing parameters. The morphology of the samples was investigated by field emission scanning electron microscopy (FESEM) and the thickness of the deposited layers was determined by atomic force microscopy (AFM). Raman and photoluminescence (PL) spectroscopies were used to confirm the high quality of the MoSe2 layers. Surface composition was examined by photoelectron spectroscopy (XPS). Moreover, the MoSe2/SiOx/Si heterojunctions exhibit diode behaviour, with a rectification ratio of 10, measured at ±2.0 V, which is due to the p-i-n heterojunctions formed at the p-Si/SiOx/MoSe2 interface. A photovoltaic effect was observed with a short circuit current density (Jsc), open circuit voltage (VOC) and efficiency of -0.80 mA/cm2, 1.55 V and 0.5%, respectively. These results provide a guide for the preparation of p-i-n heterojunctions based on few-layer MoSe2 with improved photovoltaic response.

Deep level transient spectroscopy of hole traps related to CdTe self-assembled quantum dots embedded in ZnTe matrix.

The capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements have been made on a Schottky Ti-ZnTe (p-type) diode containing CdTe self-assembled quantum dots (QD) and control diode without dots. The C-V curve of the QD diode exhibits a characteristic step associated with the QD states whereas the reference diode shows ordinary bulk behavior. A quasistatic model based on the self-consistent solution of the Poisson's equation is used to simulate the capacitance. By comparison of the calculated C-V curve with the experimental one, hole binding energy at the QD states is found to be equal about 0.12 eV. The results of DLTS measurements for the sample containing QDs reveal the presence of a low-temperature peak which is not observed for the control diode. Analysis of its behavior at different bias conditions leads to the conclusion that this peak may be related to the hole emission from the QD states to the ZnTe valence band. Its thermal activation energy obtained from related Arrhenius plot equals to 0.12 eV in accordance with the energy obtained from the Poisson's equation. Thus based on the C-V and DLTS studies it may be concluded that the thermal activation energy of holes from the QD states to the ZnTe valence band in the CdTe/ZnTe QD system is equal about 0.12 eV.

On the relaxation rate distribution of the photoionized DX centers in indium doped Cd(1-x)Mn(x)Te.

It was recently shown that the kinetics of persistent photoconductivity (PPC) build-up in indium doped Cd(1-x)Mn(x)Te are non-exponential and can be described solely by the stretched-exponential function. The non-exponentiality is attributed to the indium related DX centers present in the materials. In order to explain this observation, low temperature photoconductivity build-up was studied for Cd(1-x)Mn(x)Te:In of two different manganese contents. It was found that this type of response has its origin in the heavy-tailed distribution of the DX centers. The distribution was analyzed in terms of photon flux. Increasing photon flux leads to the more dispersive behavior. It was also confirmed that the heavy-tailed distribution is due to the different local configuration of atoms surrounding DX centers in the alloy.

Properties of the relaxation time distribution underlying the Kohlrausch-Williams-Watts photoionization of the DX centers in Cd(1-x)Mn(x)Te mixed crystals.

In this paper we clarify the relationship between the relaxation rate and relaxation time distributions underlying the Kohlrausch-Williams-Watts (KWW) photoconductivity build-ups in indium- and gallium-doped Cd(1-x)Mn(x)Te mixed crystals. We discuss the role of asymptotic properties of the corresponding probability density functions. We show that the relaxation rate distribution, as a completely asymmetric α-stable distribution, leads to an infinite mean value of the effective relaxation rate. In contrast, the relaxation time distribution related to it leads to a finite mean value of the effective relaxation time. It follows from the experimental data analysis that for all the investigated samples the KWW exponent α decreases linearly with increasing photon flux in the range of (0.6-0.99) and its values are more spread in the case of gallium-doped material. We also observe a linear dependence of the mean relaxation time on the characteristic material time constant, which is consistent with the theoretical model.

[The study of the chemical composition of nitrogenase Fe-Mo-cofactor by a new fluorimetric method of thiocompound analysis]. / Izuchenie khimicheskogo sostava Fe-Mo-Kofaktora nitrogenazy novym fluorimetricheskim metodom analiza tiosoedinenii.

A purification technique for a large scale production of crystalline Mo-Fe protein of nitrogenase from Azotobacter vinelandii and of its fragment, Fe-Mo-cofactor (Fe-Mo-co) in argon atmosphere has been elaborated. The novel fluorimetric method of thiol compounds analysis has been proposed for identification of Fe-Mo-co thiol ligands; this procedure allows the determination of concentration and class of thiol compounds and of the distance between sulphur atoms in the case of dithiols. The use of this method for an analysis of Fe-Mo-co has demonstrated that it contains a thiomolybdate fragment and two atoms of inorganic sulphur. Organic thiol as a Fe ligand has not been revealed.