First principles calculations of indium impurity-cadmium vacancy complex in CdTe

First principles calculations are used to study stability of the complex formed by indium impurity and cadmium vacancy in CdTe. Formation energies and transition energy levels of the cadmium vacancy, indium impurity and their complex in different charge states are calculated using supercell method within density functional theory in the local density approximation. From the analysis of binding energy of the complex it is found that formation of the complex is favorable and the neutral and single charged states of the complex are stable. The studies of the formation energy as a function of the Fermi level show that interaction between the shallow indium impurity and cadmium vacancy results in the Fermi level pinning near the middle of the semiconductor band gap and leads to the formation of semi-insulating material

Electrical Instability of CdTe:Si Crystals

Results of Hall effect measurements of cadmium telluride crystals, doped by silicon (dopant concentration in the melt was 1018 - 1019 cm-3), allowed to classify the studied samples and the conditions under which probably the definite crystal and impurity states are realized. We have found the distinction between 3 type of CdTe:Si crystals: (1) low-resistance p-type crystals with shallow acceptors, in which Si impurity is localized mainly in the large inclusions; (2) semi-insulating crystal with deep acceptors and submicron size dopant precipitates that are source/drain for interstitials Sii - shallow donors; and (3) low-resistance crystals in which the n-type conductivity is provided by shallow donors: Sii (and/or SiCd). Therefore the silicon is responsible for n-type conductivity of doped samples, introducing as a donor Siі and provides semi-insulating state by forming deep acceptor complexes (SiCd-VCd2-)- with (Еv + 0.65 eV). Besides, the submicron silica precipitates, that have a tend to "dissolution" at relatively low temperatures, can act as electrically active centers. Keywords: cadmium telluride, silicon, doping, electrical properties, impurity, precipitates.</span

Peculiarities of Electrical Characteristics of Semi-Insulating CdTe-Cl crystals

Electrical properties of semi-insulating CdTe-Cl crystals, grown by the vertical Bridgman and the travelling heater method, have been studied. It is found that the travelling heater method provides electron conductivity of the crystals, and the vertical Bridgman method - hole conductivity. Specific resistance of the samples is of (108-109) Ohm*cm at 300 K, and Hall mobility of the holes and electrons is of (45 - 55) cm2/V*sec. and (10 - 20) cm2/V*sec., respectively. Very low values of electron mobility and an exponential temperature dependence of µn are due to drift barriers with a height of εb ≈ 0,20 eV. Formation of the barriers is caused by the fluctuations of the potential relief resulting from the microheterogeneity of the defect-impurity system. Quasi-photochemical reactions that reduce electron mobility after photo-excitation have been observed in n-CdTe-Cl samples. In p-CdTe-Cl samples, neither drift barriers, nor quasi-photochemical reactions were detected. Key words: transport phenomena, scattering of charge carriers, cadmium telluride.</span

Transferrin-Associated Lipoplexes as Gene Delivery Systems: Relevance of Mode of Preparation and Biophysical Properties

Abstract The successful application of gene therapy depends highly on understanding the properties of gene carriers and their correlation with the ability to mediate transfection. An important parameter that has been described to improve transfection mediated by cationic liposomes involves association of ligands to cationic liposome–DNA complexes (lipoplexes). In this study, ternary complexes composed of 1,2-dioleoyl-3-(trimethylammonium) propane:cholesterol, plasmid DNA and transferrin (Tf, selected as a paradigm of a ligand) were prepared under various conditions, namely, in medium with different ionic strengths (HEPES-buffered saline [HBS] or dextrose), at different lipid/DNA (+/–) charge ratios and using different modes for component addition. We investigated the effect of these formulation parameters on transfection (in the absence and presence of serum), size of the complexes, degree of DNA protection and extent of their association with cells (in terms of both lipid and DNA). Our results show that all the tested parameters influenced to some extent the size of the complexes and their capacity to protect the carried genetic material, as well as the levels of cell association and transfection. The best transfection profile was observed for ternary complexes (Tf-complexes) prepared in high ionic strength solution (HBS), at charge ratios close to neutrality and according to the following order of component addition: cationic liposomes–Tf–DNA. Interestingly, in contrast to what was found for dextrose–Tf-complexes, transfection mediated by HBS-Tf-complexes in the presence of serum was highly enhanced