Luminescence centers in porous silicon

Photoluminescence studies on porous silicon show that there are luminescence centers present in the surface states. By taking photoluminescence spectra of porous silicon with respect to temperature, a distinct peak can be observed in the temperature range 100–150 K. Both linear and nonlinear relationships were observed between excitation laser power and the photoluminescence intensity within this temperature range. In addition, there was a tendency for the photoluminescence peak to red shift at low temperature as well as at low excitation power. This is interpreted as indicating that the lower energy transition becomes dominant at low temperature and excitation power. The presence of these luminescence centers can be explained in terms of porous silicon as a mixture of silicon clusters and wires in which quantum confinement along with surface passivation would cause a mixing of andX band structure between the surface states and the bulk. This mixing would allow the formation of luminescence centers.     

Positron-trap centers in neutron-irradiated silicon containing hydrogen

The defect evolution as a function of the annealing temperature has been studied in monocrystalline silicon grown in a hydrogen atmosphere and irradiated with 3.6×10¹⁷ neutrons/cm². Positron lifetime spectroscopy has been used and the results compared with infrated absorption measurements. Vacancy-H, vacancy-2H, vacancy-O–H and divacancy complexes withm hydrogen atoms (m<6) have been identified for the first time as possible positron traps.     

EPR of neutral vacancy-helium centers in silicon

In this letter we report the first observation by EPR of helium-associated defects in solid. Two new (S = 1) EPR spectra, labeled Si-AA5 and Si-AA6, arise from the association of helium and intrinsic defects in crystalline silicon. Both are produced by helium ion implantation at room temperature and are stable to 180°C. By implanting ³He isotope, the Si-AA6 ³He hyperfine spectrum has been observed. Both centers are tentatively identified as vacancy-helium centers in neutral charge states.     

Nonequilibrium polarization of triplet centers in silicon

The Liouville equation is considered for the density operator of a triplet center with phenomenological population and decay constants (K₊ and K_-). In the case of anisotropy of K- with respect to the internal magnetic axis of the triplet, the spin polarization and intensities of the EPR lines of the triplet center in a strong magnetic field are calculated. On this basis, an interpretation of experiments on Si-Si centers in irradiated silicon is given. Quantum-chemical calculations of the model of the center in the form of a cluster OSi₄H₁₂ are performed, taking account of the spin-orbital interaction, the anisotropy of which agrees with the EPR spectra of Si-Si centers in silicon.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 97–101, June, 1989.     

EPR spectra of heat-treatment centers in oxygen-rich silicon

After heat-treatment of oxygen-rich silicon at 410–550 °C ten different EPR spectra were observed. Nine of these are new spectra, seven of them reveal 2mm symmetry for the corresponding heat-treatment center, thereby reducing considerably the number of possible atomic configurations. In some cases the number of paramagnetic centers could account for the observed changes in carrier concentration.     

New growth technique for luminescent layers on silicon

We report the growth of siloxene on crystalline silicon (111) surfaces based on the chemical transformation of a calcium disilicide layer. The siloxene films obtained show a prominent luminescence in the green, with intensities comparable to the luminescence observed from porous silicon. The structural properties of the siloxene films are studied with infrared transmission and TEM micrographs.     

Oxygen-induced modification of dislocation luminescence centers in silicon

Prospects for using the long-wavelength dislocation luminescence line D1 in silicon-based light-emitting diodes are considered. The standard spectral position of this line at 807 meV, rather than being canonic, depends on the morphology of the dislocation structure and the impurity environment of an individual dislocation. Data on the spectral distribution of luminescence intensity in the region of the D1 line have been analyzed in terms of the concentration of interstitial oxygen in a sample, plastic deformation parameters, and thermal treatment. The results obtained suggest that oxygen exerts a dominant effect on the spectral position of line D1 and luminescence intensity in its vicinity. It is shown that the probable structure of recombination centers can be described in terms of the donor-acceptor pair model, in which oxygen complexes serve as donors and the acceptors are structural defects in the dislocation core.     

Evidence of localized luminescence centers in porous silicon

The mechanism by which porous silicon luminesces when excited by UV radiation is still unknown. As opposed to de-localized models, proposed by other researchers, in which conduction band carriers are believed to recombine with traps during luminescence, we show evidence for and argue ha favour of a localized structure in which, on excitation, electrons are promoted to higher energy levels within a luminescence center but are not given up to the conduction band. Luminescence occurs when they relax to a lower level in the center.     

Recombination centers in phosphorous doped hydrogenated amorphous silicon

Optical absorption below the mobility gap of a-SiH_x:P films is derived from photoconductivity measurements and interpreted in terms of optical transitions from occupied localized states in the exponential valence band tail and dangling bond states 0.8 eV above the valence band edge to unoccupied free electron conduction band states. Collection efficiency measurements of Schottky barrier structures indicate that P doping introduces centers with large capture cross-section for holes.     

Effect of trapping centers of luminescent properties of bismuth orthogermanate

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 6, pp. 970–975, June, 1991.     

Advanced optical spectroscopy of luminescent color centers in lithium fluoride thin films

Lithium fluoride, LiF, is a well-known dosimeter material. In thin films the direct use of optical absorption spectra to individuate the stable formation of different kinds of point defects is often precluded by the presence of interference fringes due to the refractive index difference between film and substrate. Photoluminescence measurements are more sensitive. Recently a very effective investigation method is developing: it is often called combined excitation–emission spectroscopy (CEES) in the literature. In this work we present the basic characteristics of this technique and the first results of the investigation of polycrystalline LiF films grown by thermal evaporation on fused silica substrates and gamma irradiated at several doses up to 10⁶ Gy in air.     

Laser pyrolysis synthesis and characterization of luminescent silicon nanocrystals

Quantum dots of silicon are very attractive materials due to their photoluminescence (PL) emission that can be very strong at room temperature in the visible range under UV illumination. Weighable batches are demanded for several applications in opto-electronic, photovoltaic, medicine, and so on. Laser pyrolysis of silane in a flow reactor is an efficient method to synthesize silicon nanocrystals, but up to now the production rate for the smallest particles was very low. We present here results of a work aimed to overcome this limitation. Optimization of the laser pyrolysis process has been performed through an elaborate study of the synthesis parameters. Weighable batches of very small silicon particles were obtained in a controlled and reproducible way, with production rate in the 0.1–1 g/h. High-resolution electron microscopy and specific surface measurements show that the particles were true silicon nanocrystals in the 4–9 nm range. We have then studied their PL properties. For this purpose, we have paid a particular attention to the surface passivation, an essential step to obtain efficient PL. Various ways were explored: natural oxidation under air and dispersion in liquids. We show that after natural oxidation, the PL properties are, as expected, in agreement with the quantum confinement model. Strong PL is also obtained by dispersion in solvents, but the interpretation is less straightforward in this case, and as discussed in the paper, needs further investigation.     

Effect of copper on dislocation luminescence centers in silicon

The effect of copper on dislocation luminescence centers in silicon has been investigated using photoluminescence and transmission electron microscopy. It has been demonstrated that there exist two main mechanisms responsible for quenching of dislocation luminescence by the copper impurity. The first mechanism is dominant at high copper concentrations and associated with the decrease in the time of nonradiative recombination of nonequilibrium charge carriers due to the formation of copper precipitates in silicon. This leads to the quenching of the entire dislocation luminescence and the edge exciton luminescence. The second mechanism is associated with the interaction of individual copper atoms with deep dislocation centers D1/D2, which results in the passivation of the recombination activity of these centers. This mechanism takes place even at room temperature and is highly effective at low copper concentrations.