Contactless photoconductivity measurements with the time-resolved microwave conductivity technique have been performed during the growth of hydrogenated amorphous silicon films. it has been shown that low substrate temperature and thin films lead to a larger electron decay rate and to an increased infrared absorption compared with high quality films. Addition of H2S to SiH4 during the glow discharge process leads to a worse fi1m quality which can be detected in-situ.

A theoretical model has been developed for recombination at dangling bonds which explains the γ variations between 0.5 and 1 depending on the Fermi level position. The occupation probabilities of the T3+, T3° and T3- states under illumination have been calculated using the statistics of correlated levels. The γ exponent is derived through a parametric representation of the equations of detailed balance and charge conservation. A good agreement with experiment is obtained with a dangling bond density of 5×1015 cm-3, a placing of the T3° level at 0.95 eV below Ec, an effective correlation energy of 0.4 eV and a charged to neutral capture cross section ratio of 50.

A general expression for generation-recombination rate in a-Si based on classical SRH theory including different electron and hole capture cross-sections for donor-like and acceptor-like centers inside the mobility gap is derived. Applying appropriate approximations and two-exponential model for localized states distribution two methods of analytical solution are presented and discussed.

The effects of illumination upon the absorption electron spin resonance spectrum of the dangling bond defect system have been studied in undoped amorphous hydrogenated silicon (a-Si:H). A small shift of the inhomogeneous envelope of the system towards higher g-value is observed at roomtemperature. The shift is not accompanied by a significant change in the signal. Results are reported which indicate that this shift is not due to illumination induced heating of the specimen or calibration changes of the spectrometer. The results may be related to previously reported optical bias effects upon transient photocurrent and photoinduced absorption studies.

The isothermal capacitance transient spectroscopy (ICTS) data of a- Si:H is consistently analyzed in terms of a relaxation function with a fractional exponential time decay of the form φ(t)=exp[-(t/τ*)λ] with λ≅0.8. The anomalous variation of the effective relaxation time τ* (and hence that of the attempt-to-escape frequency ν* and the capture cross section σ* observed by Okushi and coworkers) with energy and temperature is shown to follow an additional prediction of the time dependent relaxation rate coupling model developed by Ngai and coworkers for relaxations in many complex systems. A modified energy scale is extended from the ICTS analysis which brings its electronic density of states structure in closer agreement with results obtained by DLTS experiments.

The density of states (DOS) has for the first time been calculated throughout the entire bandgap region of undoped amorphous silicon from quasi-static capacitance-voltage (QSCV) measurements using MOS structures. The QSCV DOS is compared with the DOS obtained by the field-effect method. It is shown, that the coexistence of states of a different nature at the same bandgap level can be revealed by the temperature dependence of low frequency MOS CV measurements.

The dependence of the conductance prefactor on the activation energy in accordance with the Meyer-Neldel relation has been observed in a-Si:H, by measuring the temperature dependence of the field-effect in a-Si:H thin-film transistors. The Meyer-Neldel rule is for the first time properly taken into account in the analysis of the field-effect, thereby considering the non-uniform shift of the Fermi-level as induced by the gate bias. The analysis also yields the flat-band voltage, which is an important parameter in the density of states evaluation. The density of states is shown to be considerably overestimated in conventional analysis.

Measurements of high temperature conductivity on a-Si:H obtained by LPCVD from Si2H6 at temperatures between 450 and 500 °C indicate clearly a change on the activation energy from 0.9 - 1.0 eV to 0.55 - 0.6 eV around 600 °K.

The results are not strongly different from those obtained in GD a- SiH4:however, the coefficent of the energy shift of the mobility edge with temperature is greater by a factor of 2, while the average extension of the tail state distribution at T= 0 °K is roughly a factor 1.5 larger. The main discrepancy concerns the pre-exponential factor, which is one or two orders of magnitude larger, giving a product (NcukT)of the order of 1021 cm-1 s-1. By assuming a conduction mobility two orders of magnitude larger than the maesured Hall mobility one obtains a value for N which is only a factor four times larger than what currently assumed for a-Si:H.

I-V characteristics have been measured in a-Si:H films deposited from Si2 H6 by LPCVD and have been analyzed according to a direct method. DOS distribution is thickness-dependent, similarly to what has been found on SiH deposited films, with values at Fermi level ranging from below 1016 cm-3 eV-1 up to 1018 cm-3 eV-1. Deposition temperature, at least in the interval 450 - 500°C, does not seem to affect DOS distribution. Hydrogen content is pratically constant in all the films. The only possible conclusion is that only thickness influences DOS distribution and that only for thickness larger than 0.8 μm it is possible to get densities at Fermi level below 1016 cm-3 eV-1.

Optical and electrical properties are investigated as a function of thickness for two sets of samples of a-Si:H films deposited respectively by RF magnetron sputtering and by RF glow discharge under the same conditions. The hydrogen concentration, the transmittance and reflectance spectra in the UV-visible-NIR and the resistivity were measured for both sets of samples. Relationships between index of refraction, absorption coefficient, imaginary part of the dielectric constant, optical band gap, resistivity and the thickness are found and interpreted. The thickness dependence of these parameters is related to the larger amount of inhomogeneities present in sputtered samples with respect to glow discharge samples.

Flicker noise, due to trapping and emission from states in the mobility gap, leads to a spectrum in which noise power is proportional to 1/fn. Experimentally we find 0.6 <n<1.1 for an Si:H film in an n+-i-n4 structure. Theoretical development allows the extended state electron mobility, and the density of states at the conduction band edge to be obtained. Similar measurements on Schottky diodes provide the distribution of the localized state density in the mobility gap.

Doping of hydrogenated amorphous silicon and germanium with boron, phosphorus, and arsenic is investigated. The incorporation of the dopants from the gas phase into the solid film is found to differ strongly for the various dopant-host systems. The doping efficiency is calculated from measurements of the density of dangling bond defects and of shallow band-tail states as a function of the doping level. A common square root dependence of the efficiency on dopant gas concentration is obtained.

The low conductivity of B-doped a-Si:H is usually attributed to the fact that only a small fraction of the boron is tetrahedrally coordinated. In the presence of hydrogen, that small fraction can be inactivated via the acceptor-neutralization process that was described for the case of B-doped crystalline Si. When a B-doped sample of a-Si:H was annealed to drive away hydrogen near the boron atoms, the conductivity increases by a factor of 600. Although a-Si:H can be doped either n-type or p-type, the doping efficiency is orders of magnitude poorer than in crystalline Si. In fact the doping efficiency of boron is one order of magnitude lower than that of phosphorus. Several models account for the low doping efficiency of a-Si:H, the most plausible being the location of B in a trigonal site, i.e. surrounded by three Si-atoms as shown in Fig. 1. Such a center is neutral and cannot act as an acceptor. The present work is an offshoot of our study of the hydrogenation of dangling bonds in crystalline Si (1). The awareness that H ties to a Si dangling bond more strongly than another Si-atom led us to passivate the numerous dangling bonds on the surface a Si-crystal. Then, we passivated dangling bonds in grain boundaries and in dislocations and we showed that ion implantation damage also could be neutralized by atomic hydrogen thus removing non-radiative recombination centers and allowing the luminescent transitions to become more efficient.

We have studied the light induced instability problem in hydrogenated amorphous silicon using junction capacitance techniques. These techniques are used to examine specific changes in the density of gap states, and occupation of gap states, for undoped a-Si:H samples after light saturation and for a series of partial anneal “states” which culminate in the original dark annealed state (state A). We find that the observed changes in the metastable occupied and unoccupied defects contradict the Si-Si bond breaking model and indicate at least two defect creation processes. In several samples we also find clear evidence that the metastable defect distribution near midgap has a slightly different energy distribution than the stable deep state (dangling bond) distribution. At the same time, these results seem to be qualitatively consistent with many aspects of recent ESR and optical absorption studies of metastable defect creation. We discuss these findings in terms of alternative possible microscopic models for metastable effects in a-Si:H.

The occurrence of optically-induced, metastable changes in the paramagnetism in films of a-Si:H is well known. The effect was first observed with white light in powdered samples, but recent experiments with both white and monochromatic light incident on films on substrates have observed a similar effect. This optically-induced ESR appears to be stable at temperatures < 400 K. Typical inducing curves for samples with initial dark spin densities ns > 1016 spins/cm3 approach a power law behavior (ns ∼ t1 / 3) at long times. However, when the dark spin density is less than 1016 spins/cm3, the samples exhibit an inducing curve (on a log-log scale) with a continuously decreasing slope. The curve does not exhibit a unique power law behavior over an extended range of time and is at all times < t1 / 3.

The temperature dependences of response time tr and steady state photoconductivity (PC) were used to deduce the DOS at energies above the dark Fermi level. The tr and PC of annealled state A and light soaked state B were measured from 115K to 300K. Light soaking causes degradation of PC. Compared with state A, the PC response of state B is faster at low temperatures, but is slower at high temperatures. The difference between state A and state B was interpreted by a decrease of shallow traps and an increase of deep traps.

The effects of illumination on the optical and electronic properties of narrow gap hydrogenated and fluorinated amorphous Si-Ge (a-Si1-xGex:H, F) alloys have been evaluated. A series of alloys with optical gaps ranging from 1.30 eV to 1.64 eV has been light soaked at ∼1 sun intensity for 354 hours. Measurements of sub-gap absorption, photo- and dark conductivities and dark conductivity activation energy were made on alloys in the annealed and the light-soaked states. The results indicate that samples with optical gaps ≳ 1.4 eV degrade significantly. The 1.3 eV sample shows no degradation in its optical or electronic properties except for a factor of 5 increase in the dark conductivity.

Firstly, on the basis of the classical model on the surface process of adsorbed species, a guiding principle is proposed for the preparation of high-quality a-SiGe:H alloys via plasma-CVD technique. Secondly, a- SiGe:H alloys were prepared by the rf-discharge technique with three intentional modifications; (A) from SiH4/GeH4 using a diode reactor (conventional method), (B) from SiH4/GeH4 using a triode reactor (triode method), (C) from SiH4/GeH4/H2 mixture using the diode reactor (H2-dilution method). Finally, growth mechanism is discussed in terms of the selection of long-lifetime radicals in a plasma and the H-coverage factor of the growing surface, providing us a clue to materials design.

A review is given of the current status of amorphous Si(a-Si) and related alloys prepared by photo-CVD. Preparation techniques and film quality of the following materials will be reviewed;

• • undoped a-Si films,

• • highly conductive n-type and p-type μc-Si films, a-SiGe films.

• • a-SiC films, superlattice structures.

Finally, the present status of a-Si solar cell performance prepared by photo-CVD will be introduced and the merit of photo-CVD will be discussed.

The electronic and optical properties of amorphous silicon-germanium alloys produced by d.c. and r.f. glow discharge are reported. Data on the sub-gap absorption, dark and photo conductivities, drift mobilities and drift mobility-lifetime products are used to propose a density of states model.