Crystalline silicon is the most important semiconductor material in the electronics industry. However, silicon has poor optical properties because of its indirect bandgap, which prevents the efficient emission and absorption of light. The energy structure of silicon can be manipulated through quantum confinement effects, and the excitonic emission from silicon nanocrystals increases in intensity and shifts to shorter wavelengths (a blueshift) as the size of the nanocrystals is reduced. Here we report experimental evidence for a short-lived visible band in the photoluminescence spectrum of silicon nanocrystals that increases in intensity and shifts to longer wavelengths (a redshift) with smaller nanocrystal sizes. This higher intensity indicates an increased quantum efficiency, which for 2.5-nm-diameter nanocrystals is enhanced by three orders of magnitude compared to bulk silicon. We assign this band to the radiative recombination of non-equilibrium electron-hole pairs in a process that does not involve phonons.

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The problem of electron tunnelling through a symmetric semiconductor barrier based on zincblende-structure material is studied. The k 3 Dresselhaus terms in the effective Hamiltonian of bulk semiconductor of the barrier are shown to result in a dependence of the tunnelling transmission on the spin orientation. The difference of the transmission probabilities for opposite spin orientations can achieve several percents for the reasonable width of the barriers. Lately spin polarized electron transport in semiconductors attracts a great attention.1 One of the major problems of general interest is a possibility and methods of spin injection into semiconductors. A natural way to achieve spin orientation in experiment is the injection of spin polarized carriers from magnetic materials. Although significant progress has been made recently, 2,3,4,5 reliable spin-injection into low-dimensional electrons systems is still a challenge. Schmidt et al. pointed out that a fundamental obstacle for electrical injection from ferromagnetic into semiconductor was the conductivity mismatch of the metal and the semiconductor structure.6 However, Rashba showed that this problem could be resolved by using tunnelling contact at the metal-semiconductor interface.7 On the other hand Voskoboynikov et al.8 proposed that asymmetric nonmagnetic semiconductor barrier itself could serve as a spin filter. It was demonstrated that spin-dependent electron reflection by inequivalent interfaces resulted in the dependence of the tunnelling transmission probability on the orientation of electron spin. This effect is caused by interface-induced Rashba spin-orbit coupling 9 and can be substantial for resonant tunnelling through asymmetric double-barrier 10,11 or triple-barrier 12 heterostructures. However, in the case of symmetric potential barriers, the interface spin-orbit coupling does not lead to a dependence of tunnelling on the spin orientation.In this communication we will show that the process of tunnelling is spin dependent itself. We demonstrate that a considerable spin polarization can be expected at tunnelling of electrons even through a single symmetric barrier if only the barrier material lacks a center of inversion like zinc-blende structure semiconductors. The microscopic origin of the effect is the Dresselhaus k 3 terms 13 in the effective Hamiltonian of the bulk semiconductor of the barrier.We consider the transmission of electrons with the initial wave vector k = (k , k z ) through a flat potential barrier of height V grown along z [001] direction (see

The enhancement of the emission rate of charge carriers from deep-level defects in electric field is routinely used to determine the charge state of the defects. However, only a limited number of defects can be satisfactorily described by the Poole-Frenkel theory. An electric field dependence different from that expected from the Poole-Frenkel theory has been repeatedly reported in the literature, and no unambiguous identification of the charge state of the defect could be made. In this article, the electric field dependencies of emission of carriers from DX centers in Al x Ga 1Ϫx As:Te, Cu pairs in silicon, and Ge:Hg have been studied applying static and terahertz electric fields, and analyzed by using the models of Poole-Frenkel and phonon assisted tunneling. It is shown that phonon assisted tunneling and Poole-Frenkel emission are two competitive mechanisms of enhancement of emission of carriers, and their relative contribution is determined by the charge state of the defect and by the electric-field strength. At high-electric field strengths carrier emission is dominated by tunneling independently of the charge state of the impurity. For neutral impurities, where Poole-Frenkel lowering of the emission barrier does not occur, the phonon assisted tunneling model describes well the experimental data also in the low-field region. For charged impurities the transition from phonon assisted tunneling at high fields to Poole-Frenkel effect at low fields can be traced back. It is suggested that the Poole-Frenkel and tunneling models can be distinguished by plotting logarithm of the emission rate against the square root or against the square of the electric field, respectively. This analysis enables one to unambiguously determine the charge state of a deep-level defect.