Papers by Author: H. Riemann

Abstract: The effect of tin on the formation and temperature transformation of VO centers in Ge upon annealing has been investigated. It was found that the doping of Ge with tin leads to a change of reactions involving oxygen and vacancies and the new defect SnVO appears upon VO annealing. Doping Ge with tin gives rise to a considerable decrease in the formation efficiency of divacancies and VO centers and the latter exist in a very narrow temperatures range. VO2 complexes appear only upon annealing of SnVO centers. The assumption is made that the absorption bands situated at 718.9 and 733.6 cm^-1 belong to the less stable configuration VO₂* and the bands at 731.5 and 771.7 cm^-1 correspond to stable configuration of VO₂ centers.

Abstract: The measurements of stress induced dichroism on oxygen absorption band near 1107 cm-1 in Si1-xGex compounds and subsequent kinetics of the dichroism recovery upon isothermal annealing have been carried out. It has been found that the magnitude of introduced by uniaxial stress dichroism decreases with increasing Ge content. Two components in the dichroism annealing kinetics have been found. On the basis of studying absorption spectra of samples under investigations it was assumed that two components in relaxation correspond to the diffusion of oxygen being in a different nearest environment: the one component corresponds to oxygen surrounded by silicon atoms and the second one to the oxygen the neighbour of which is Ge atom. Diffusivity for each of the components has been determined. It has been shown that the diffusivity of oxygen that is in both of these configurations decreases with increasing Ge content.

Abstract: Terahertz-range photoluminescence from silicon-germanium crystals and superlattices doped by phosphor has been studied under optical excitation by radiation from a mid-infrared CO2 laser at low temperature. SiGe crystals with a Ge content between 0.9 and 6.5 %, doped by phosphor with a concentration optimal for silicon laser operation, do not exhibit terahertz gain. On the contrary, terahertz-range gain of ~ 2.3 - 3.2 cm-1 has been observed for donor-related optical transitions in Si/SiGe strained superlattices at pump intensities above 100 kW/cm2.

Abstract: Silicon crystals, doped with moderate concentration of magnesium or lithium, have been grown for application as optically pumped donor silicon lasers for the terahertz spectral region. The pedestal growth technique accompanied with axial-loaded dopant pills enabled manufacturing of large silicon crystals with a homogeneous donor distribution in the range from 1014 to 1016 cm-3, as required for intracenter silicon lasers. Terahertz-range photoluminescence from the grown crystals has been observed.

Abstract: The performance of optically pumped terahertz silicon lasers with active media made from mono- and polycrystalline silicon doped by phosphorus has been investigated. The polycrystalline silicon samples consist of grains with a characteristic size distribution in the range from 50 to 500 m. Despite of significant changes of the principal phonon spectrum and increased scattering of phonons at grain boundaries, the silicon laser made from polycrystalline material has a laser threshold and an operation temperature only slightly worse than that of monocrystalline silicon lasers.

Abstract: The interstitial carbon impurity (CI) vibrational modes in monocrystalline Si-rich SiGe were investigated by Fourier Transform Infra Red spectroscopy and density functional modelling. The two absorption bands of CI are found to be close to those in silicon, but show shifts in opposite directions with increasing Ge content. The transversal mode band at 932 cm-1 shifts slightly to the high frequency side, while the longitudinal mode at 922 cm-1 suffers a pronounced red-shift. Each Ci-related band is found to consist of two components. An annealing of CI in Si1-xGex occures in two stage. During the first stage (210-250 K) the main components of bands anneals and revealed components grow in intensity. At T>250 K all components disappear. Two component structure of bands is suppose most likely correspond to different combinations of Si and Ge atoms in the neighbourhood of the carbon atom. The interstitial carbon defect was modelled by a supercell density-functional pseudopotential method (AIMPRO) for alloys with 4.69% Ge concentration. From energetics, it has been found that each Ge-C bond costs at least 0.4 eV in excess of a Si-C bond. However, structures where Ge atoms are second neighbors to the C atom are marginally bound, and may explain the two-component band structure in the absorption measurements. The vibrational mode frequencies taken from several randomly generated SiGe cells produce the observed opposite shifts for the transverse and longitudinal modes.

Abstract: The influence of Ge content on the local vibrational mode of substitutional carbon in Si-rich Si1-xGex single crystals has been investigated by infrared Fourier-transform spectroscopy and ab initio modeling methods. Czochralski-grown Si1-xGex samples doped with boron and carbon have been studied. The Ge fractional content was varied from x=0.004 to x=0.044. To reveal the CS-related absorption band in the Si1-xGex the difference spectra between carbon-lean and carbon-reach Si1-xGex samples with the same Ge content were studied. We have found that the CS-related absorption band in the Si1-xGex alloys red-shifts and broadens with increasing Ge content. It has been found that at x0.015 the CS absorption band consists of two overlapping lines corresponding to different combinations of Si and Ge atoms neighboring the CS atom. The calculations show that substitutional carbon atoms avoid Ge ligand atoms, and should be found in Si-rich regions. These results also reveal that the softening of the CS mode frequency arises from the SiGe volumic expansion.