Mu centers in crystalline Ge under illumination

A slow conversion to a diamagnetic state has been observed for muonium centers at the tetrahedral interstitial site (Mu⁰ _T) in dark Ge at low temperatures. While the conversion process is affected by illumination, no effect of illumination upon the initial (Mu⁰ _T) centers themselves was observed at 10 K. This is in marked contrast with the case of (Mu⁰ _T) centers in Si where strong interaction with photo‐induced carriers is observed, suggesting that the electronic level associated with (Mu⁰ _T) state in Ge is not located in the energy gap. This revised version was published online in August 2006 with corrections to the Cover Date.     

Muonium centers in heavily n‐doped Si under illumination: Evidence for Mu–hole interaction

A small fraction of implanted muons exists as a paramagnetic state (presumably Mu_BC ⁰, muonium at the Si—Si bond center) in heavily Sb‐doped Si (n-type, [Sb]\ \simeq 10¹⁸\ cm^–3). The paramagnetic state is susceptible to illumination both at 10–20 K and 290 K, providing evidence that holes (minority carriers) play an important role in determining the dynamical properties of muonium centers, where change may occur via a process Mu_BC ⁰+ h⁺\to Mu_BC ⁺ followed by charge exchange reaction (or transition Mu⁺ _BC+ e⁻→ Mu⁰ _T). This revised version was published online in August 2006 with corrections to the Cover Date.     

Muonium defect centers in semiconductors

After a brief summary of the properties of the muonium defect centers observed in the elemental group IV semiconductors, the status of studies of muonium centers in semiconductors at the time of the last μSR conference in 1983 will be compared with what is currently known. With the introduction of new experimental techniques, such as high-transverse-field μSR and level-crossing spectroscopy, many new results are or soon will be available on muonium centers. These, combined with new theoretical studies, should lead to rapidly increased insight into a subject which has been both puzzling and resistant to clarification.     

Muonium studies of p-type semi-conducting diamond under conditions of UV-visible illumination

This work reports on the promptly forming fraction and the spin relaxation rate of the isotropic muonium (Mu_T) component in p-type semi-conducting diamond, measured under the condition of illumination. The data are the first such investigations for diamond. A broad band illumination with wavelengths ranging from 0.5 μm to 3 μm was obtained from a Xenon lamp. The energy of the photons was sufficient to excite electrons from the valence band to the 0.28 ppm boron impurity band (0.37 eV). The Transverse Field Muon Spin Rotation (TF-μSR) measurements were conducted as a function of temperature, ranging from 5 K to 300 K. An illumination effect at temperatures below 100 K is observed. It is not yet clear from these data whether the effect is due to Mu_t scattering off delocalized holes, which are removed by illumination or whether there is prompt trapping of Mu_t at boron impurities (passivation) which is affected by illumination. This revised version was published online in August 2006 with corrections to the Cover Date.     

Muonium centers in the alkali halides

Muonium centers (Mu) in single crystals and powdered alkali halides have been studied using the high-timing-resolution transverse field μSR technique. Mu has been observed and its hyperfine parameter (HF) determined inevery alkali halide. For the rocksalt alkali halides, the HF parameter A_μ shows a systematic dependence on the host lattice constant. A comparison of the Mu HF parameter with hydrogen ESR data suggests that the Mu center is the muonic analogue of the interstitial hydrogenH _i ⁰ -center. The rate of Mu diffusion can be deduced from the motional narrowing of the nuclear hyperfine interaction. KBr shows two different Mu states, a low-temperatureMu ^I -state and a high-temperatureMu ^II -state.     

Electron states in Ge and Si under uniaxial stress

The electron states in Ge and Si under uniaxial stress in the (100) direction are computed with the pseudopotential method. The theoretical results are compared with optical measurements of the indirect absorption edge in Si and Ge under uniaxial stress and with the polarization dependence and the splittings of donor levels.     

Photo-induced creation of nitrogen-related color centers in diamond nanocrystals under femtosecond illumination

Diamond nanocrystals deposited on a dielectric mirror at the focus of a microscope objective have been illuminated by femtosecond laser pulses. We have observed the photo-creation of color centers, under peak power corresponding to an intensity of about . In a nanocrystal initially containing a single Nitrogen Vacancy (NV) center, femtosecond illumination resulted in the transformation of this center into another one with different spectral features. These features are tentatively attributed to the neutral form NV⁰. This irreversible transformation goes together with the photocreation of other unstable color centers at the laser focus. Such behavior under femtosecond laser illumination place some limitations on the use of sub-picosecond pulses to trigger single photon emission from a single NV center in diamond nanocrystal.     

Phonons in Si3Ge1 and Si1Ge3 short-period superlattices

Oscillatory spectra and single-phonon state densities are calculated in the Keating atomic interaction model for the ultrathin superlattices Si₃Ge₁ and Si₁Ge₃. Fundamental features of the superlattice phonon spectra and their relationship to the spectra of Si and Ge crystals are analyzed. Conditions required for oscillation localization in the interface region are studied and the role of the Si-Ge bond in formation of interlayer oscillations is considered.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii. Fizika, No. 8, pp. 40–44, August, 1993.     

Surface lifetime degradation in Si and Ge under low energy electron irradiation

The bombardment of n-type Ge and Si by 10–30 keV electron beams is shown to reduce the surface lifetime τ_P of minority carriers but not to affect mobility μ_P or diffusion constant D_P. For experiments, the electron beam in a scanning electron microscope is replaced a chopped light source to perform modified Haynes-Shockley experiment, and the measurements is used to calculate μ, D and τ. The reduction in τ is interpreted in terms of an increase in surface recombination velocity s. At a dose of approximately 10¹⁷ electrons cm^?2 for Ge and 10¹⁵ electrons cm^?2 for Si, a saturation value for s appears to be reached after which further bombardment has no effect. The values of τ obtained from both light and SEM Haynes-Shockley experiments performed in vacuum agree within approximately 5% both prior to and after bombardment. The initial values of lifetime can be restored by etching the samples.     

Visible photoluminescence from Ge+-implanted SiO2 films thermally grown on crystalline Si

+ -implanted SiO₂ films is studied as a function of different fabricating conditions (implantation dose, annealing temperature and time). The SiO₂ films containing Ge nanocrystals exhibit two photoluminescence (PL) bands peaked at 600 nm and 780 nm. There are two excitation bands in the PL excitation (PLE) spectra. With variation in Ge nanocrystal size, the PL and PLE peak energies show no appreciable shift. The PL and PLE spectral analyses suggest that during the PL process, electron–hole pairs are generated by the E(l) and E(2) direct transitions inside Ge nanocrystals, which then radiatively recombine via luminescent centers in the matrix or at the interface between the nanocrystal/matrix. Received: 27 January 1998/Accepted: 18 March 1998     

Impurity-Vacancy Complexes in Si and Ge

We examine the electronic and geometrical structure of impurity-vacancy complexes for 11 sp-impurities in Si and Ge, using the pseudopotential plane wave (PPW) and the all-electron Kohn–Korringa–Rostoker (KKR) methods. We find that all impurities of the 5sp and 6sp series prefer the split-vacancy configuration. For Cd and Sn we obtain good agreement of the calculated hyperfine parameters with experimental PAC and EPR data. Impurities of the 3sp and 4sp series form distorted substitutional complexes (except Al, which forms a split complex in Si). This trend strongly correlates with the lattice relaxations of nearest neighbors around the isolated (without vacancy) substitutional impurities.     

Structural phase transitions in Si and Ge under pressures up to 50 GPa

The crystal structures of Si and Ge were studied by energy dispersive X-ray diffraction at room temperature and pressures up to 50 GPa. Si transforms to a primitive hexagonal (Si-V) structure around 16 GPa, to an intermediate phase Si-VI between 35 and 40 GPa, and to hcp (Si-VII) around 40 GPa. In contrast, Ge remains in the β-tin structure up to at least 51 GPa. The pseudopotential method reproduces these differences in the high-pressure behavior of Si and Ge.     

Thermoelectric transport in strained Si and Si/Ge heterostructures

The anisotropic thermoelectric transport properties of bulk silicon strained in the [111]-direction were studied by detailed first-principles calculations focusing on a possible enhancement of the power factor. Electron and hole doping were examined in a broad doping and temperature range. At low temperature and low doping an enhancement of the power factor was obtained for compressive and tensile strain in the electron-doped case and for compressive strain in the hole-doped case. For the thermoelectrically more important high-temperature and high-doping regime a slight enhancement of the power factor was only found under small compressive strain with the power factor overall being robust against applied strain. To extend our findings the anisotropic thermoelectric transport of a [111]-oriented Si/Ge superlattice was investigated. Here, the cross-plane power factor under hole doping was drastically suppressed due to quantum-well effects, while under electron doping an enhanced power factor was found. For this, we state figures of merit of ZT?=?0.2 and 1.4 at T?=?300?and 900?K for the electron-doped [111]-oriented Si/Ge superlattice. All results are discussed in terms of band structure features.     

Certain UV-luminescence centers in crystalline quartzes

Models of luminescence centers are proposed in a number of natural quartz crystals on the basis of ESR, optical absorption, thermoluminescence, and luminescence data; these centers are responsible for their radiation in the 335–380-nm band and hole trapping centers. Corresponding mechanisms for x-ray and thermally stimulated luminescence are also suggested.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 5, pp. 23–26, May, 1984.     

Energy and momentum relaxation of charge carriers in Ge and Si under uniaxial stress

Measurements of the piezoresistance and the energy relaxation time T_?, in Ge and Si under uniaxial stress up to 4 kb are reported in the temperature range 30 K < T < 300 K. The measurements of t_? have been performed in the warm carrier range using the harmonic mixing technique. The experimental results for the piezoresistance and energy relaxation time in n-type material can roughly be understood in terms of carrier redistribution in the conduction band valleys whose degeneracy is lifted by the stress. Information is obtained from these measurements about the relative strength of ? and g type intervalley scattering in n-Si; we find nearly equal coupling strength for both scattering types. For the p-type material the experiments show convincingly that the main effect of stress on transport quantities is also caused by the lifted degeneracy of the heavy and light hole bands, as predicted already by Adams, and by Pikus and Bir. The nonlinear dependence of the piezoresistance and τ_? on the stress can very well be explained by approximating the heavy and light hole bands as spherical and parabolical and using the deformation potential constant b as a parameter. The value of ≈2eV obtained for |b| is in good agreement with earlier results for p-Ge.