Papers by Author: Eugene B. Yakimov

Abstract: Commercial 4H-SiC p⁺n structures with an uncompensated donor concentration (N_d-N_a) of ~1.5∙10¹⁵ cm^-3 in the n-type epitaxial layer are studied. The measurement of the photocurrent, electron beam induced current and transient switching characteristics (from forward to reverse voltage), altogether showed that the value of the hole diffusion length, about 2 μm at room temperature, increases to at least 7 μm at 620 K.

Abstract: The approach for imitation of beta radiation using the e-beam of scanning electron microscope (SEM) for semiconductor energy converter testing is proposed. It is based on the Monte-Carlo simulation of depth-dose dependence for beta-particles and a determination of collection probability from the EBIC measurements of collection efficiency dependence on beam energy. Experiments with the ⁶³Ni radiation source confirm that such approach allows to predict the efficiency of semiconductor structures for radiation energy conversion to electric power.

Abstract: 4H-SiC p⁺nn⁺ structures fabricated by implantation of Al into a commercial n-type 4H-SiC epitaxial layer doped to (3-5)Ÿ10¹⁵cm^-3 have been studied. Structures with unstable excess forward current were characterized by electron beam induced current (EBIC) and secondary electron (SE) methods and by Auger-electron spectroscopy (AES). Numerous defects were found with a depth which exceed the thickness of the p⁺-layer. Also, it was demonstrated that the concentration of carbon on the SiC surface always exceeds that of silicon, which may be the reason for the initially unstable conductivity via the defects.

Abstract: A comparative study of multicrystalline Si based solar cells and plastically deformed single crystalline Si by the EBIC, LBIC and XBIC methods as well as a computer simulation were carried out. The XBIC measurements were realized on a laboratory X-ray source. Simulations of LBIC and XBIC contrast values for grain boundaries, dislocations and spherical precipitates were carried out for different diffusion length and beam diameter values. It is shown by a computer simulation that the LBIC and XBIC contrast of two-dimensional defects in the crystals with a large enough diffusion length can be a few times higher than that in the EBIC mode, i.e. these methods in recent multicrystalline Si structures allow to reveal grain boundaries with the lower recombination strength. The contrast of dislocations perpendicular to the surface can be comparable in all three methods. The XBIC and LBIC contrast of precipitates usually is essentially smaller than that in the EBIC mode and could approach it in the structures with the small diffusion length only. Experimental data confirming the results of simulations are presented.

Abstract: Investigations of silicon layers grown on carbon foil were carried out using the Electron Beam Induced Current (EBIC) methods. The most of grain boundaries in these ribbons are (111) twin boundaries elongated along the direction. The EBIC measurements showed that the recombination contrast of dislocations and of the most part of twin boundaries at room temperature is practically absent and only random grain boundaries and very small part of twin boundaries produce a noticeable contrast. At lower temperatures a number of electrically active twin boundaries increases but the most part of them remains inactive. A contamination with iron increases the recombination activity of random boundaries but not the activity of twin boundaries.

Abstract: It is shown that the X-ray beam induced current method (XBIC) can be realized at the laboratory X-ray source using the polycapillary x-ray optics. The images of iron contaminated grain boundaries in multicrystalline Si are obtained. It is shown that the grain boundary XBIC contrast is 2-3 times smaller than the EBIC one. A simulation of XBIC and EBIC contrast values for two-dimensional defects is carried out and a good correlation between the experimental and calculated values is obtained. The dependence of grain boundary XBIC contrast on the X-ray beam width is calculated.

Abstract: Characterization of defect structure in silicon ribbon grown on carbon foil has been carried out. The structure of grown Si layers and a dislocation density in these layers have been studied using selective chemical etching and the Electron Backscattering Diffraction. It is observed that the layers consist of rather large grains, the majority of which is elongated along the growth direction with a similar surface orientation and with a misorientation angle between neighboring grains of 60º. This means that such grains are separated by the (111) twin boundaries. The dislocation density in different grains is found to vary from 102 to 107cm-2. The energy dispersive X-Ray microanalysis has shown that some twin boundaries are enriched with carbon.

Abstract: Simulation of contrast for small spherical defects in the X-ray beam-induced current (XBIC) mode has been carried out. Under simulations the excess carrier generation function is described by the rigid cylinder with the constant generation rate inside it. The dependence of maximum contrast value on the precipitate depth, diffusion length and effective beam radius is calculated. The XBIC contrast profile as a function of diffusion length, of beam radius and of precipitate depth has been calculated that allows to evaluate the spatial resolution of the technique. The results obtained are compared with those calculated for the EBIC contrast of the same defect. It is shown that in the semiconductor materials with the small diffusion length the XBIC contrast could be comparable with the EBIC one.

Abstract: Calculation of relation between the EBIC contrast and the recombination strength for dislocations and quasi-two-dimensional dislocation trails has been carried out taking into account the real values of depletion region width. Using the relations obtained the linear defect density along dislocations and sheet density in dislocation trails are estimated. The results of EBIC investigations of dislocations and dislocation trails in plastically deformed n- and p-Si are analyzed.

Abstract: This paper presents results of investigations about the influence of Hydrogen (introduced by annealing or plasma implantation), and Helium (ion implantation followed by a proper annealing for creating nanocavities) on the electrical properties of 4H-SiC n-type epitaxial samples. First, 4HSiC epitaxial layers were hydrogenated either by annealing under H2 ambient or by a RF plasma treatment. This last process took place before or after the deposition of Schottky contacts. Two different annealing temperatures were imposed (300°C and 400°C), as well as two plasma hydrogen doses for the same low energy. An improvement of electrical characteristics (25 % increasing of the minority carrier diffusion length, lowering of ideality factor, better switching characteristic) is detected for samples annealed at 400°C. The treatment of 4H-SiC surface in hydrogen plasma through Ni metal also increases the diffusion length, but not sufficiently to have an effect on I-V characteristics. A second set of 4H-SiC epitaxial layers were secondly implanted with He+ ions at two distinct temperatures. An annealing at 1700°C during 30 minutes under argon atmosphere was then carried out. C-V measurements revealed the presence of a high charge density zone around the nanocavities, containing fixed negative charges, opposite in sign to the donor atoms.