Time resolved photoluminescence (PL) decays have been measured for Si nanocrystals embedded in silicon dioxide. The nanocrystals were formed by implanting 40 keV Si ions into a 1000 Å thick film of thermally grown SiO2, followed by thermal annealing at 1000–1200 °C. The observed luminescence, peaking at 700–850 nm, is compared to similar measurements performed on porous Si emitting in the same wavelength range. The results show that the PL from the nanocrystals exhibits a stretched exponential decay with characteristic decay time τ in the range 10–150 μs and dispersion factor β in the range 0.7–0.8. Both parameters are, however, higher for the nanocrystals compared to those of porous Si indicating superior passivation of the nanocrystals in the SiO2 matrix. Evidence is also presented for a single exponential behavior at the decay end suggesting a remaining fraction of excitons in isolated nanocrystals. We attribute the highly nonlinear dose dependence of the PL yield to a nucleation process for the nanocrystals and a more curved decay line shape for higher ion doses to a higher crystal density, promoting excitonic migration to nearby nanocrystals. These observations provide strong evidence that the origin of the stretched exponential line shape of the PL decay results from migration and trapping of excitons in a system of randomly distributed and interconnected nanocrystals with a dispersion in size.

The band-to-band Auger recombination in 4H-SiC material is studied using a time-resolved photoinduced absorption technique. The Auger recombination coefficient is derived from the kinetics of electron-hole plasma in heavily doped n-type 4H-SiC and in low-doped 4H-SiC epitaxial layers in the temperature interval 300–565 K. Within this range, its value decreases from γ3=(7±1)×10−31 cm6 s−1 to γ3=(4±1)×10−32 cm6 s−1. The observed pronounced reduction of Auger recombination rate with temperature is correlated to temperature dependent threshold energy of Auger process.

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Ternary TlGaSe 2 compounds are ferroelectric semiconductors possessing a complex layered structure. Each layer of a single TlGaSe 2 crystal comprises strongly covalently bonded GaSe 4 tetrahedral units and exhibits overall anionic character. Weak interlayer bonds in the crystal are ensured by the Tl + cations stacked in the trigonal cavities between the layers. The elementary 32 atoms cell of TlGaSe 2 has monoclinic symmetry and belongs to the C 2/c (C 6 2h ) space group [1]. It is known that the compound undergoes two-phase transformations at temperatures between 105 and 120 K [2,3]. Unique optical properties of the semiconductor dictated by its structural complexity received a great deal of attention for applications in photoelectronics [4].In the past, several works were dedicated to the fundamental optical absorption of TlGaSe 2 [5-10]. Parameters extracted in these studies primarily concern the direct band gap character of the compound. Information obtained about the simultaneous indirect optical transitions, on the other hand, remains highly inconsistent and controversial due to differences in sample quality and measurement arrangements. In addition, most of the detected spectra exhibit poorly resolved absorption features [8,9]. To the best of our knowledge, the fine absorption structure involving assisting phonons has been only detected in narrow 2.05-2.08 eV spectral range [6]. In this paper we present highresolution absorption measurements at different temperatures performed in the high-quality TlGaSe 2 crystals. The analysis of low temperature spectrum provides us a clue to insight into the role of indirect optical transitions.The data reported here refer to a set of crystals synthesized by the Bridgman method from high-purity elements taken in stoichiometric proportions. Samples of a few millimetres thickness and mirror-like surfaces were obtained by cleaving the crystals along the layers. For absorption measurements along these layers (see text below) side surfaces of specimens were polished to optical quality. The examined samples showed weak p-type conduction at room temperature while their optical trapping concentration was less than 10 16 cm -3 . More detailed results on sample structure and free carrier dynamics are presented elsewhere [11].Optical transmission of samples was measured using a dual beam spectrometer with a resolution not less than 1 nm equipped with a xenon flash tube. Such apparatus simultaneously acquires data from the sample and reference detectors avoiding fluctuations of the lamp brightness. Non-polarized light was used in the experiments. During measurements the samples were cooled down to helium temperatures using a close cycle cryostat. The absorption coefficient, α, has been calculated taking into account multiple reflections from parallel surfaces. We also accounted for a small increase of reflectivity versus photon energy R(E) recorded for E⊥c polarization at room temperature [8]. For low temperatures this dependence was uniformly shifted in energy according to the blue...

Fundamental band edge absorption is investigated in nominally undoped (n<1014cm−3) and heavily doped (n∼8×1018cm−3) 4H-SiC by a spectroscopy technique based on spatially and time-resolved free-carrier absorption. The spectra are extracted over a wide absorption range (0.02–500cm−1) at temperatures from 75to450K. The experimental results are supported by an indirect transition theory with a unique set of dominating momentum-conserving phonons, showing good correlation with earlier findings of differential absorption measurements at 2K. Exciton binding energy of 30±10meV is derived from fitting the data at 75K. The detected polarization anisotropy of absorption with respect to c axis is shown to be consistent with the selection rules for the corresponding phonon branches. An analytical model related to constant degree of involved phonons describes well the obtained energy gap variation with temperature. Finally, doping induced band gap narrowing is characterized above the impurity-Mott transition and compared with theoretical calculations in the random phase approximation. The shape of the fundamental absorption edge at high carrier concentrations is discussed in terms of excitonic enhancement above the Mott transition, as recently detected in Si.