Radiative recombination in hydrogenated amorphous silicon

Hydrogenated amorphous silicon exhibits efficient optical transitions across a gap larger than that of crystalline Si. Hydrogen passivates the dangling bonds and endows the material with a reduced number of non-radiative recombination centers. A gap widening has been observed in other hydrogenated semiconductors.Research reported herein was supported by the Department of Energy, Division of Solar Technology, under Contract No. EY-76-C-03-1286 and by RCA Laboratories, Princeton, NJ 08540.     

Time-resolved optically detected magnetic resonance of conduction band tail electrons and A-centres in hydrogenated amorphous silicon

The conduction band tail electron resonance was observed at 2K in a-Si:H by time-resolved optically detected magnetic resonance experiments. The g - value of the A centre (the trapped hole centre) shifted towards the small value with decreasing the delay time of a microwave pulse relative to a laser pulse. This result is accounted for in terms of exchange interaction between the tail electron and the A centre.     

Level of dangling bond centres and its broadening due to disorder in amorphous silicon as elucidated by optically detected magnetic resonance measurements

The position of the energy level of dangling bond centres and its broadening have been investigated by optically detected magnetic resonance measurements in aSi, aSi:H and aSi:H, F. Correlation between level broadening and increase in the bandgap energy is discussed.     

Effect of substrate temperatures and annealing on optically detected magnetic resonance in glow-discharge amorphous silicon

Optically detected magnetic resonance experiments have been performed at 2 K in glow-discharge amorphous silicon with different substrate temperatures. Effects of annealing and illumination on ODMR have been examined. A broad line is interpreted in terms of a three-centre bond model.     

Light-induced metastable defects in a-Si:H as elucidated by optically detected magnetic resonance measurements at 2K

We observed a gradual increase of a nonradiative recombination channel in a-Si:H by strong light exposure at 2K, i.e., an increase in the intensity of the quenching optically detected magnetic resonance (ODMR) line at g=2.005 and no change of the other quenching lines at g=2.004 and g=2.013. The sample with previous light soaking at 300K still showed a degradation of photoluminescence (PL) efficiency as much as that for the sample without previous light soaking, but no obvious change of the quelching ODMR lines. An increase of silicon dangling bond (db) density and the other nonradiative recombination process are suggested to explain the experimental results.     

Luminescence fatigue and optically detected magnetic resonance in amorphous silicon-hydrogen alloys

Fatigue of the luminescence in amorphous SiH alloys has been investigated as a function of dangling bond density and of hydrogen content at 2 K. Optically detected magnetic resonance measurements have also been carried out to elucidate the origin of the fatigue effect for those samples.     

Radiative and nonradiative recombination of self-trapped exciton on silicon nanocrystal interface

The theory of the multiphonon and radiative recombination of a self-trapped exciton on the interface of a silicon nanocrystal in a SiO₂ matrix is developed. Self-trapped excitons play a key role in the hot carrier dynamics in nanocrystals under photoexcitation. The ratio of the probabilities of the multiphonon and radiative recombination of the self-trapped exciton is estimated. The probabilities of exciton tunnel transition from the self-trapped state to a nanocrystal are calculated for nanocrystals of various sizes. The infrared range spectrum of the luminescence of the self-trapped exciton is obtained.     

Optically detected nuclear magnetic resonance in amorphous silicon related materials

Optically detected nuclear magnetic resonance (ODNMR) measurements have been carried out for a-Si:H samples with different hydrogen content and for a-Si_xN_1−x:H (x = 0.01 and 0.05). We have observed for the first time the ODNMR signal of hydrogen. The ODNMR spectrum is decomposed into two Lorentzians with different widths (FWHM) which arise from the dipole-dipole interactions of a trapped hole with its nearby and distant H¹ nuclei, respectively. The trapping site of the hole is discussed based on the experimental results.     

Radiative and nonradiative recombination times in semiconducting films

The radiation emitted spontaneously by a semiconductor which has been excited for a very short time decays exponentially with a time constant that depends on the recombination rate of electrons and holes. This recombination rate is the combination of radiative and nonradiative transition rates between conduction and valence bands of the semiconductor. The radiative recombination rate depends on the density of states of the electromagnetic field, which can be made to be dependent on the geometry. In this paper, we report on the dependence of the fluorescence lifetime upon the thickness of active thin films. For systems in which the radiative recombination rate is dominant over the nonradiative ones, the total recombination time can be changed by suitable modifications of the thickness of the film. In this situation, the nonradiative rate can be evaluated. We present experimental results for the case of cadmium sulphide (CdS) thin films.     

Dispersive processes of light-induced defect creation in hydrogenated amorphous silicon

The growing curve of light-induced dangling bonds under illumination has been observed for various intensities of illumination in a-Si:H. It is fitted to a stretched exponential function and then two parameters β and τ involved in the function are estimated as a function of saturated dangling bond density . The experimental values of β, τ, and are compared with those calculated based on our model of light-induced defect creation in a-Si:H.     

Microscopic identification of the origin of generation-recombination noise in hydrogenated amorphous silicon with noise-detected magnetic resonance

Spin-dependent changes in the noise power of undoped amorphous hydrogenated silicon ( a-Si:H) are observed under electron spin resonance conditions. The noise-detected magnetic resonance (NDMR) signal has the g value of holes in the valence band tail of a-Si:H. Both the sign of the NDMR signal and the frequency dependence of its intensity can be quantitatively accounted for by a resonant reduction of the generation-recombination noise time constant tau. This identifies hopping in the valence-band tail as the dominant spin-dependent step governing noise in this material.     

Characteristics of recombination processes in doped hydrated amorphous silicon films

The results of measurements of the temperature dependences of the dark conductivity and photoconductivity ofa-Si:H films, doped from the gas phase or by implantation of phosphorus or boron ions, as well as the effect of preillumination with white light with different duration on the photoconductivity are presented. A model is proposed for carrier recombination in doped films, taking into account the broadening of the levels of dangling bonds and the difference in the coefficients trapping of electrons and holes on neutral and charged dangling bonds. The dependence of the stationary interband photoconductivity on the equilibrium Fermi level and the appearance of the temperature-induced quenching of the photoconductivity in doped films after preillumination are studied on the basis of this model.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 7–17, June, 1987.     

Nitrogen doping in hydrogenated amorphous silicon

A direct evidence of substitutional doping in ion beam deposited amorphous hydrogenated silicon by nitrogen is presented. From the analysis of infrared (IR) absorption spectra and Si-2p core level shape, measured with X-ray photoelectron spectroscopy (XPS), the preferential tendency of nitrogen to go in for three-fold coordination at higher concentration and tetrahedral bonding at lower concentration (⩽4 at %) is established. XPS technique has been used for the first time to deduce the upper limit for substitutional solid solubility of the impurity.     

Persistent photoconductivity in hydrogenated amorphous silicon

The persistent photoconductivity (PPC) has been observed in undoped, phosphorus-doped, and boron-doped hydrogenated amorphous silicon (a-Si: H) films. As the annealing temperature is raised for these films the decay of residual conductivity is accelerated and the PPC disappears almost completely after annealing at 500°C. These experimental results rule out the mechanism requiring the phosphorus-boron complexes or the deep defects. The PPC is found to be related with the sample inhomogeneity from the experimental observation that the decay of residual conductivity is closely correlated with the microstructure. A model is proposed to explain the PPC in a-Si: H films.