The paper summarizes a few basic properties of SiGe showing that SiGe is an interesting material for high speed electronics. The advantage of using heterostructures in silicon-based technologies is demonstrated by taking SiGe heterobipolar transistors as an example. First results obtained with very fast and low-noise heterobipolar transistors are briefly mentioned. The paper is concluded by a short discussion of a few optoelectronic properties observed in various Si/Ge and Si/Si_{1-x}Ge_{x} strained-layer superlattices and quantum wells with particular emphasis on electroluminescence properties.
We used the density functional theory and ab initio pseudopotentials to investigate He-H interaction in crystalline silicon. It was shown that both hydrogen and helium stimulate the formation of vacancy complexes. The presence of hydrogen decreases the vacancy and divacancy formation energies by about 2 eV. The presence of one or two helium atoms reduces the divacancy formation energy by 0.3 and 0.4 eV, respectively. The influence of helium presence on hydrogen diffusion from silicon vacancies under high pressure depends on a helium concentration. Thus, according to our calculation, low concentrations of He increase the hydrogen out-diffusion.
Doppler broadening measurements performed by a slow positron beam on p-type Si samples implanted with He at 20 keV and at a fluence of 5×10^{15} and 2×10^{16} cm^{-2} are reviewed and discussed. The evolution of the open volume defects distribution was studied as a function of isochronal and isothermal annealing of the samples. In the as implanted samples the majority of the open volume defects produced by implantation was passivated by He. The open volume defects density decreases, reaching a minimum at 250°C. In the 250-650°C temperature range there is an increase in defects due to the appearance of vacancy clusters. At the higher annealing temperatures (700-900°C) the vacancy clusters disappear only in the samples implanted at 5×10^{15} cm^{-2}.
High-resolution Laplace-transform deep level transient spectroscopy technique has been used to study a fine structure in the carrier emission process for transition metal- and thermal donors-related defects in silicon. For the case of the transition metal centres the method revealed the fine structure when the defect has a similar emission characteristics to other defects in the crystal. The method also demonstrated the complex emission process for the thermal donors.
Two positron techniques have been applied to study dynamics of oxygen precipitation in Czochralski-grown silicon, annealed under high (up to 1.4 GPa) pressure. Lifetime measurements were performed with 180 ps resolution; Doppler broadening with a variable-energy slow-positron beam. Different thermal treatings rise the mean lifetime of positrons from 222 ps in as-grown samples up to 227 ps. In samples with a high (up to 85%) amount of oxygen precipitated, an intermediate (550-800 ps) lifetime is observed.
For the iron-boron pair in the p-type silicon two different configurations of the defect are observed: stable and metastable. The reported metastable configuration is the first step in a dissociation process of the stable, i.e. of trigonal symmetry, configuration of the pair. Rate equations for the two-step iron-boron pair dissociation allowed us to evaluate the dissociation rates for both configurations of the pair. The driving force for the creation and, then, dissociation of the metastable pair is the minority carrier injection followed by the electron-hole recombination process in the space charge region. A use of the high-resolution Laplace-transform deep level transient spectroscopy allowed us to demonstrate for both of the configurations the influence of the magnetic field on the hole emission.
Nanostructured oxides, like ZrO_2, on ZnO and Bi_2O_3 show interesting electronic and photovoltaic properties. Pressed and annealed samples were obtained from nanopowders grown in hydrothermal or plasma processes. Positron annihilation (the Doppler broadening depth-resolved, positron lifetime) techniques were used to trace structural changes in samples after annealing. Photoluminescence spectra of all investigated samples show broad-band emission in the visible, with intensity depending on annealing temperature. The change in nanoporosity and positron lifetimes correlate well with changes in photoluminescence properties. The nature of broad photoluminescence bands is to be understood; further positron studies via the Doppler coincidence method would help in identifying the nature of defects in these samples.
In this study the technique of Laplace transform (high resolution) deep level transient spectroscopy combined with the uniaxial stress method has been used to study a symmetry and the defect reconfiguration kinetics (the stress induced alignment) of some forms of hydrogen-related centres. We have confirmed the trigonal symmetry of the defect related to the isolated bond centred hydrogen. When hydrogen decorates the vacancy-oxygen pair (the A centre) the apparent defect orthorhombic symmetry is not lowered as a result of a very high hydrogen jumping rate between two unsaturated broken bonds of the vacancy. We also show that the stress-induced defect alignment in some cases can be related to the same microscopic mechanism of the hydrogen motion as it is for the diffusion process.
Many point and extended defects in silicon, and other semiconducting materials, have been relatively well characterised by the standard DLTS technique. In this method the activation energy of carrier emission from the defect is calculated after multiple temperature scans. In this paper we demonstrate a new approach to the technique, in which after a single temperature scan the complete Arrhenius plot can be constructed for defects present in the sample with considerable concentrations. This method is much faster, accurate, and offers a much higher resolution in comparison with the standard DLTS method.
We present a detailed investigation of the growth kinetics of aluminium-related shallow thermal donors: the K-donors. Constraints for the diffusion mechanism of oxygen in silicon at temperatures ≈ 470°C are found. A large entropy of the K-donors is considered as a possible explanation of high diffusivities and interaction radii found for the generation of the K-donors.
By combination of polarization-resolved photoluminescence, transport, and realistic numerics we study energy and recombination spectra of free and acceptor-bound positive trions in a quasi-two-dimensional hole gas. The singlet-triplet crossing in the trion ground state is found at B≈12 T, and a slight reduction of all trion binding energies coincident with the formation of a Laughlin hole fluid is observed at B≈14.2 T.
Recently, genetic algorithms have been applied for ultrafast optical spectrometry in systems with several convoluted lifetimes. We apply these algorithms and compare the results with POSFIT (by Kirkegaard and Eldrup) and LT programme (by Kansy). The analysis was applied to three types of samples: molybdenum monocrystals, Czochralski-grown silicon with oxygen precipitates, Si with under-surface cavities obtained by He + H ion co- implantation. In all three tests, the genetic algorithm performs very well, in particular for short lifetimes. Further developments to model the resolution function in genetic algorithms are needed.
New shallow acceptor magnetoabsorption lines in THz range have been discovered under bandgap photoexcitation in strained Ge/GeSi multiple-quantum-well heterostructures. It is shown, both theoretically and experimentally, that the resonant absorption results from the photoionization of A^+-centers and from 1s → 2p_+-type transitions from the ground state of the barrier-situated A^0-centers into excited states in the 1st and 2nd electronic subbands. The shallowest discovered ground acceptor states (E_B≤0.5 meV) are attributed to the "barrier-spaced" acceptors (a hole bound with an acceptor ion in the neighboring Ge quantum well).
In this report we present high-resolution spectra obtained with a use of the Laplace transform deep level transient spectroscopy for platinum diffused into dilute (0-5% of Ge) SiGe alloys. Very significant changes are observed in the spectra associated with the transition metals as the germanium content is altered. We interpret these spectra in terms of the configurations of silicon and germanium atoms surrounding the transition metal. In order to explain the observed behaviour both the first and second nearest neighbour shells are considered.
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