Effect of high oxygen doping on the minority carrier lifetime in heat-treated silicon crystals

Effect of high oxygen doping on thermally induced changes in the minority carrier lifetime τ in 800 °C heat-treated silicon crystals is studied and analyzed. Deteriorated τ values in prolongly annealed highly oxygen-enriched Si crystals compared with those found in annealed oxygen-poor crystals, are observed. This is explained by an appearance in prolongly annealed highly oxygen-enriched silicon crystals of oxygen-induced microdefects which are effective in the minority carrier recombination.     

Recombination and transport through localized states in hydrogenated amorphous and microcrystalline silicon

Electrical transport and recombination mechanisms in hydrogenated amorphous silicon, a-Si:H, are determined by localized band-tail states and deep defects. At low temperatures (T < 100 K) the photoluminescence originates from tunneling recombination between localized band-tail states and the photoconductivity arises from hopping in the band tail. This review describes the present understanding of transport and recombination mechanisms in this low-temperature regime with a focus on two aspects: (i) the kinetics of carrier recombination and the competition between geminate and non-geminate recombination, and (ii) the microscopic identification of recombination paths by magnetic resonance techniques and the proof of excitonic recombination. Inspite of its complex nanocrystalline morphology, hydrogenated microcrystalline silicon, μc-Si:H, behaves in many respects similarly to a-Si:H in that the low-temperature properties are also determined by disorder-induced localized band-tail states.     

Modulated photoconductivity method for investigation of band gap states distribution in silicon-based thin films

It has been demonstrated that the well known modulated photocurrent technique can be modified to escape imperfect data in intrinsic parameters of amorphous and microcrystalline silicon films and to simplify measurements of the density of localized states distribution in the band gaps of these semiconductors. The information on the density-of-states distribution can be extracted from temperature dependence measurements of the constant and modulated components of the photoconductivity in the film illuminated by the light modulated with some selected frequencies. The modified method has been applied to microcrystalline hydrogenated silicon films with n- and p-type conduction. The study has demonstrated that the tail of the density-of-states distribution near the valence band of microcrystalline hydrogenated silicon is less steep than that near the conduction band.     

Luminescence decay in hydrogenated amorphous silicon and silicon nanostructures

The stretched exponential luminescence decay observed at temperatures lower than 20 K transits to the power law decay due to the electron-hopping at localized band tail states near 60 K in the hydrogenated amorphous silicon (a-Si:H). The luminescence decay at 4.2 K in a-Si:H is quite similar to that of Si-nanoparticles in the porous Si (p-Si). It is explained from the comparison with p-Si that the slow luminescence of the life time of ~ 1 ms is due to the recombination of excitonic electron–hole pairs at the spin triplet state quantum-confined in the hydrogen-free Si nanostructure in a-Si:H. The fast luminescence of the life time of ~ 1 μs is due to the recombination of the pairs at the spin-singlet state and the life time is explained as due to the indirect optical transition.     

Photoconductivity of amorphous CdS films

The photoconductivity of amorphous CdS films, vacuum evaporated onto cooled substrates, has been investigated. The conditions and possible reasons for the appearance of a low temperature maximum in the photoconductivity temperature dependence of chalcogen rich samples have been found. The presence of a slow recombination centre related to the lone pair level of the one-fold coordinated negative chalcogen has been assumed. The energy necessary for both the electron (2 eV) and hole (0.4 eV) escape from this centre has been determined. Photoinduced changes in photoconductivity have been examined as well.     

Electrical and Photoelectrical Properties of n-CuInS2 Single Crystals

The temperature dependences of free electron density and mobility in n-CuInS₂ single crystals have been determined from the Hall effect measurements. The ionization energy of donors and their densities have been estimated. The results of measurements show that the crystals are strongly compensated. The spectral distribution of photoconductivity, the dependence of photoconductivity on excitation intensity, and the photoconductivity decay with time have been measured in the n-CuInS₂ crystals at different temperatures. The results of photo-conductivity measurements suggest linear recombination of photo-electrons at weak excitation and quadratic recombination at high excitation intensity. The compensated deep acceptor probably acts as the recombination centre.     

Determination of the density of states of semiconductors from steady-state photoconductivity measurements

We discuss a method to obtain the density of states of photoconductive semiconductors from the light-intensity-dependence of the steady-state photoconductivity. Considering a material having different species of gap states – i.e., with different capture coefficients – we deduce a simple expression relating the defect density to measurable quantities. We show that the relevant capture coefficient appearing into the formula is that of the states that control the recombination. We check the validity of the approximations and the applicability of the final expression from numerical calculations. We demonstrate the usefulness of the method by performing measurements on a standard hydrogenated amorphous silicon sample.     

a-Si:H transport parameters from experiments based on photoconductivity

In this paper we review some of the techniques based on the photoconductivity property of hydrogenated amorphous silicon (a-Si:H) from which it is possible to extract transport parameters as well as density of states (DOS) spectroscopies. We also present a new experiment based on the steady state photocarrier grating technique. We show that combined with simple steady state photoconductivity it gives information on the DOS. The comparison of these results with those of other techniques used for DOS measurements theoretically allows determination of transport parameters in a-Si:H.     

Structure and photoluminescence of the a-GexSe1−x system

GeGe like-bonds are formed in as-deposited films at x < 0.33 i.e. they are both chemically and structurally disordered. Photoluminescence (PL) intensity along with the fatigue of PL is presumably controlled by the positions of the demarcation levels which are displaced by changing the concentrations and cross sections of the recombination centres. The increase of Se content in these binary alloys leads to an increase of the concentration of non-radiative recombination centres. During fatigue both new centres are created and the cross sections of the pre-existing ones are changed.     

Improvement of photoconductivity in Silicon Tin (SiSn) thin films

Hydrogenated-amorphous silicon tin alloy (a-SiSn (C:H)) films have been prepared by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) technique. We present investigations of the absorption and photoconductivity characteristics of a-SiSn(C:H) thin films as a function of the tin concentration, which could be a pivotal feature for the bottom solar cell development. We demonstrate that the photoconductivity is significantly improved by the film growth under enhanced hydrogen dilution. The results suggest that the hydrogen dilution plays an important role in development of high quality a-SiSn(C:H) photosensitive thin films.     

Improvement of the electrical properties of Se3Te1 thin films by In additions

The electronic and photoconductivity properties of semiconducting chalcogenide glasses have been largely stimulated by attractive micro-electronic device applications. The present paper aims to study the effect of In additions on the steady state and transient photoconductivity of amorphous In_x(Se₃Te₁)_100 ? x (0 ≤ x ≤ 10 at.%) chalcogenide films. It was found that, the Indium additions lead to the decrease of both the activation energies (ΔE_dc in the dark and ΔE_ph for the photoelectrical conduction) and the optical band gap E_g that improved the electrical properties of these films. The photoconductivity increases while photosensitivity changes from 8.73 to 7.18 with the increase of In content. The exponential dependence of photocurrent on the light intensity suggests that, the recombination mechanism in these films is due to bimolecular recombination. The transient photoconductivity measurements stated that, the carrier lifetime decreased by the increase of the light intensity and In content. The obtained results were discussed in terms of the width of localized states (Mott and Davis model) and the chemical-bond approach.     

Density-of-states in microcrystalline silicon from thermally-stimulated conductivity

The technique of thermally-stimulated currents has been applied to extract the density-of-states profile in microcrystalline silicon. Exploiting the experimental parameter space a consistent density-of-states profile emerges with an exponential conduction band tail and a broader deeper distribution. Calibrating the absolute density-of-states profile from other techniques like modulated photoconductivity, steady-state photocarrier grating technique and intensity-dependent photoconductivity allows a determination of the capture coefficient of the probed localized states.     

Frequency resolved photoconductivity of bulk CdS single crystals

Photoconductivity measurements in CdS single crystal showed that a.c. photoconductivity decays as ω^−s, with s value - {1 + α} for ω > 1/τ and - {1–α} for ω < 1/τ. At low frequency values, recombination might be controlled by monomolecular behaviour. As the frequency increases bimolecular behaviour, in shallow states, dominates. Direct recombination is found to be weak dependent on both the intensity and applied voltage. However when band tail recombination is dominant (at higher frequencies), strong dependence is observed.     

Global simulation of a Czochralski furnace for silicon crystal growth against the assumed thermophysical properties

In order to understand the effects of the thermophysical properties of the melt on the transport phenomena in the Czochralski (Cz) furnace for the single crystal growth of silicon, a set of global analyses of momentum, heat and mass transfer in small Cz furnace (crucible diameter: 7.2 cm, crystal diameter: 3.5 cm, operated in a 10 Torr argon flow environment) was carried out using the finite‐element method. The global analysis assumed a pseudosteady axisymmetric state with laminar flow. The results show that different thermophysical properties will bring different variations of the heater power, the deflection of the melt/crystal interface, the axial temperature gradient in the crystal on the center of the melt/crystal interface and the average oxygen concentration along the melt/crystal interface. The application of the axial magnetic field is insensitive to this effect. This analysis reveals the importance of the determination of the thermophysical property in numerical simulation. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of lead additive on photoconductive properties of Se–Te chalcogenide films

Steady state and transient characteristics of photoconductivity have been studied in amorphous thin films of Se₈₅Te_15−xPb_x (x = 0, 2, 4 and 6). The studies of temperature dependence of photoconductivity show that photoconduction is a thermally activated process. The value of activation energy of photoconduction is, however, smaller as compared to activation energy in dark. The results of intensity dependence of steady state photoconductivity indicate that bimolecular recombination is predominant in these materials. The Se₈₅Te_15−xPb_x is found highly composition dependent as lead impurity drastically changes the conduction parameters. The incorporation of Pb impurity is found to affect the transient photoconductivity properties drastically. A spike is observed in the rise curve of photocurrent in undoped a-Se₈₅Te₁₅, which disappears in Pb doped samples. The lead incorporation is found to delay the onset time of recombination in the rise and decay of photocurrent which is attributed to the trapping of charge carriers in deep localized states produced by lead impurity. The photocurrent rise and decay is explained by the trap-controlled recombination model proposed by Iovu et al. and the dispersion parameter α of localized state energy distribution is determined from the experimental results.     

Metastable dark and photoconductive properties of microcrystalline silicon

Metastable changes in the dark conductivity of microcrystalline silicon upon heat treatment at different temperatures obey the Meyer–Neldel rule. Dark conductivity variations are accompanied by changes in the photoconductivity or the majority-carrier mobility-lifetime product. The minority-carrier mobility-lifetime product is not affected. The observations can be related to Fermi-level induced changes of the excess carrier lifetimes.     

The impact of the nitridation process on the properties of the Si–SiO2 interface

A newly developed technique for the simultaneous measurement of the oxide–silicon interface properties and of minority carrier lifetime in the silicon volume was used for a systematic study of the nitridation process of oxide films.This technique is based on the surface recombination velocity measurements, and does not require the formation of a capacitor structure, so it is suitable for the measurement of as-grown interface properties. Oxides grown both in dry and in wet environments were prepared, and nitridation processes in N₂O and in NO were compared to N₂ annealing processes. The effect of nitridation temperature and duration were also studied, and processes of rapid thermal oxidation (RTO) and nitridation (RTN) were compared to conventional furnace nitridation processes. Surface recombination velocity was correlated with nitrogen concentration at the oxide–silicon interface obtained by secondary ion mass spectroscopy (SIMS) measurements. Surface recombination velocity (hence surface state density) decreases with increasing nitrogen pile-up at the oxide–silicon interface, indicating that in nitrided interfaces surface state density is limited by nitridation. NO treatments are much more effective than N₂O treatments in the formation of a nitrogen–rich interface layer and, as a consequence, in interface state reduction. X-ray photoelectron spectrometry (XPS) analyses were used to extend our correlation to very thin oxides (3 nm).     

A systematic investigation of the role of material parameters in metastability of hydrogenated amorphous silicon

In this work, the role of structural, electronic and optical parameters of as-deposited amorphous silicon films in photoconductivity decay during light soaking was systematically investigated. Deposition temperature was varied in the range 130–270°C, in order to obtain samples with different structural, optical and electronic properties. As a result, two degradation regimes were identified. At short illumination times (within a few days in typical samples, and within a few hours in the low quality samples), the material showed different tendency to degradation depending on the content of the SiH bond clusters. At long illumination times, in all the light soaked samples the photoconductivity decay followed the t^−1/3 law. The measured photoconductivity degradation was simulated starting from the bond-breaking model. The observed correlation between the material structural parameters and the different tendencies to degradation is explained in terms of variations of the Staebler-Wronski susceptibility.     

Photoconduction of glasses in the TeSeSb system

The photoconductivity of bulk glasses of the TeSeSb system is measured as a function of light intensity and photon energy. The relative sensitivity (ΔI/I_d) has linear and square-root dependences on light intensity in low and high illumination intensities, respectively, and is nearly proportional to the square-root of the resistivity at room temperature. The spectral response of photoconductivity, which is calculated by taking into account the effect of surface recombination of carriers, agrees qualitatively with the experimental results. The experimentally determined broad spectral response suggests the presence of band tails below the conduction band and above the valence band. The large residual dark conductivity in the decay response is associated with the presence of many deep trapping centers.     

Study of recombination of photocarriers in a-As2Se3 and As2Te3 by photoconductivity and photo-absorption measurements

Time-dependent photoconductivity and photo-absorption measurements, the latter using probe beams of 0.1 eV and 1.4 eV, were analyzed on the basis of a saturated band tail model. By a best fit procedure of theory to experiment band structure and recombination parameters for a-As₂Se₃ were obtained.