Global Research and Education

Abstract: A unique single-photon detector is reported, which utilizes scaled-down silicon-on- insulator (SOI) metal-oxide-semiconductor field-effect transistor (MOSFET) with single-electron sensitivity, and features low-voltage operation without carrier multiplication and low dark counts. Primary single-photon detection characteristics are presented, and then several issues related to operation speed and quantum efficiency are to be addressed.

Abstract: This paper reviews results of studies on different materials and technologies for polycrystalline solar cells conducted at Tallinn University of Technology. Structural properties and defect structure of kesterite CZTS compounds (Cu2ZnSnSe4, Cu2ZnSn(SSe)4) were studied. Influence of selenization parameters of a Zn-Cu-Sn stacked layer on the CZTS layer growth and on the morphology, distribution of elements was analyzed. All the results obtained have been used to optimize the technology of producing solar cell structures in different designs. Cu2ZnSnSe4 and, Cu2ZnSn(SSe)4 based monograin layer solar cells were developed.

Abstract: A review is given of what kind technological aspects were used for realizing the defect engineering in semiconductor layers or crystals. The possibilities to change the free carrier capture are presented. The effect of Fermi level pinning at the surface levels allow to avoid the influence of barriers on the photoconductivity as well as to increase a role of recombination in the inter-crystalline region. The isovalent doping or the creation of the clusters allows transforming the defect distribution in the crystal bulk. The detector structure using the high electric field can introduce the recombination in at the contacts therefore allow diminishing a role of carrier capture in the bulk of structure. The cluster generation allows to increase the capture rate in the definite volume by a proton irradiation. The experience of different technologies for Si, GaAs, PbS, CdSe are presented.

Abstract: The current state of knowledge about mechanisms of metal nano-particles (NP) formation processes induced by the interaction of high-energy laser beam with surface of the metallic lattices (Au, Ag, Cu) is presented. The review includes an evaluation of the contribution of self-organized effects into the processes of the metal nano-structurization depending on the laser mode, external factors and internal parameters of an active zone. It was noticed that intensive pulsed laser illumination enabled to stimulate nano-fragmentation at the fluencies near and above the melting threshold of the metal in different mediums The laser induced processes of metallic particles formation by ablation of the metal target with consequent NP sizes stabilization by precise temperature tuning in the active zone, local plasmon resonance in liquids, and microablation mechanism in metal films in the conditions of surface plasmon resonance (SPR) due to self-organization effects are considered. Comparative analysis of the laser nano-technologies in air, vacuum, rarefied gas and liquid environments showed the advantages of self-organization in NP generation processes based on the SPR effects and their perspectives.

Abstract: Rutile titanium dioxide nanorods (TNR) were successfully prepared by hydrothermal method. By using hydrochloric acid (HCl) as chelating agent and titanium butoxide (TBOT) as precursor, aligned TNR with titanium nanoflower (TNF) was successfully prepared onto fluorine-doped tin oxide (FTO) at 150°C and 20 h. The influence of TBOT concentration on the surface morphology, structural properties and solar cell efficiency are discussed. The highest light-to-electric energy conversion efficiency, 0.4% is achieved using different concentration under simulated solar light illumination of 100 mWcm-2 (1.5 AM).

Abstract: Optical properties and surface state of semiconductor CdTe crystals subjected to irradiation with nanosecond laser pulses were studied. Ellipsometric parameters Δ and Ψ were measured on two opposite surfaces of (111) oriented CdTe wafers before. The samples were subjected to polishing chemical etching and laser irradiation with energy densities lower and higher than the melting threshold of CdTe. The morphology and structure of CdTe crystals were monitored before and after treatments. Irradiation formed a thin modified surface layer on the both sides of CdTe crystals (Cd- and Te-terminated) with the similar optical constants n and k, respectively. However, the effective thicknesses of such modified layers differed and depended on the chemical and laser treatment conditions and also on the faces of CdTe(111) crystals.

Abstract: Different procedures of laser-induced doping of the surface region of semi-insulating CdTe semiconductor are discussed. CdTe crystals pre-coated with an In dopant film were subjected to irradiation with nanosecond laser pulses in different environments (vacuum, gas or water). The dopant self-compensation phenomenon was overcome under laser action and In impurity with high concentration was introduced in a thin surface layer of CdTe. In the case of a thick (300-400 nm) In dopant film, laser-induced shock wave action has been considered as the mechanism of solid-phase doping. Formed In/CdTe/Au diode structures showed high rectification depending on the fabrication procedure. Diodes with low leakage current were sensitive to high energy radiation.

Abstract: The performance of the electrodynamic human motion energy converter with planar structure is under investigation. The electrodynamic converter consists of a flat, spiral-shaped coil and a rectangular magnet that during human motion moves in respect to one another. During the motion of the rectangular magnet with a remanence 0.1 T over a flat, formed up as an Archimedes spiral coil with 25 turns, an average power of about 10 W is generated. The generator elements can both be deployed on a variety of clothing items which move relative to one another during the human motion, and also realizing the forced oscillation of the magnet during walking.

Abstract: The scanning of human body as a method for gaining human measurements has several preferences. The gathering of data is possible in a very short time. In comparison to manual measuring methods, scanning acquires a larger amount of measurements. There are several modes of gaining human body measurements using the scanning system: laser scanning, light beam scanning, etc. A research on the laser beam reflection capabilities on different textile materials has been performed. The description of laser reflections has been compared to the Lambert’s law’s characteristics. The matrix of material selection is made in the process of planning the experiment and all possible materials are presented in this matrix. Eight textile materials are chosen for the experimental work: six lingerie and two additional materials. A laser beam with an angle of incidence of 0º and 45º is used to make the experiment. The dependence of the results on the wavelength of laser beams has also been compared.