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Electrical resistivity measurements have been used to define the Néel temperature in tensile strained chromium. The Néel temperature was observed to increase linearly with tensile stress at a rate dTN/dS = 2·0 × 10-8 K m2 N-1. A model of the strain distribution in chromium was used to estimate the Néel temperature shift resulting from a tensile strain in the Q direction. The shift was found to be approximately equal to the shift observed previously in hydrostatically compressed chromium.

In a-Si: H films with columnar growth morphology, the material between the columns reacts with air, leaving an Si-O suboxide coating on the columns of estimated thickness 5-10 Å. At the a-Si: H/oxide interface a dangling-bond density of ∼3 × 1011 cm-2 develops with time. We suggest that the oxide layer and the surface defects are characteristic of exposed surfaces of any a-Si: H film. It is emphasized that the surface defect states can have a strong influence on the electronic properties of the material.

Physical properties of variously strained chromium samples have been investigated between 4·2 and 500 K. The techniques employed included neutron and X-ray diffraction, specific heat, electrical resistivity and elasticity measurements. From this data, magnetic phase diagrams for strained chromium have been constructed. These have been interpreted with the aid of a simple dislocation-strain model.

Luminescence properties of hydrogenated amorphous Si-O alloys made by plasma decomposition are described. Alloying with oxygen results in a systematic increase of the luminescence peak position and linewidth, a reduction in the thermal quenching, and a modification of the decay properties. From the data we infer an increase in the width of the band tails, and a larger electron-phonon interaction compared with a-Si: H.

The density of band tail electrons in n-type a-Si: H, at doping levels between 10-3 and 10-6 PH3, has been measured using a transient-voltage technique on layered samples. At a doping level of 10-3 PH3 a density of 8 × 1016 cm-3 is found which agrees well with ESR data. The results confirm that occupied band-tail states account for only a small fraction of the dopant electrons.

Studies of spin-dependent photoconductivity in a-Si: H are reported. It is shown that, with increasing E.S.R. spin density, the photoconductivity decreases, but its spin dependence increases. The results demonstrate that dangling bonds are important trapping or recombination centres at room temperature. Prolonged exposure to above-gap illumination increases the spin dependence of photoconductivity and provides further evidence that light induces dangling bonds. The spin dependence of photoconductivity is compared with that of luminescence.

Experimental data and a new model of luminescence in glow-discharge-deposited amorphous silicon are described. The shape of the luminescence excitation spectrum rules out the previous model based on zero-phonon transitions. Instead, phonon interactions give a Stokes shift of 0.4-0.5 eV and the recombination is due to self-trapping of excitons bound to band tail localized states. The model explains the luminescence peak position and line width, the Stokes shift, and the thermal quenching of the luminescence intensity.

Luminescence and electron spin resonance are investigated in plasma deposited a-Si: H samples after bombardment with electrons and He+ ions. The defects introduced are predominantly singly occupied dangling bonds and act as non-radiative recombination centres. Annealing between room temperature and 300°C decreases the density of these defects, but in some samples annealing apparently leads to the formation of spinless centres. Defect-related luminescence near 0·9 eV is interpreted as a transition between an electron trap and a self-trapped band-tail hole. Disorder induced by the damage broadens the band tails and is observed as a shift of the band-edge luminescence to lower energy. He+ ions cause greater damage of this kind than electrons, in accordance with expectations. The annealing of the E.S.R. spin density is sample-dependent and is explained by the differing amounts of hydrogen in the films.

The model of charged defects is used to interpret transport experiments in As2Se3. An explanation is given for the observation of two activation energies in the hole mobility at different experimental time scales, and for the absence of electron transport.

The photoluminescence decay of glow-discharge-deposited amorphous silicon is reported. The low temperature radiative lifetime of the broad luminescence band near 1·3 eV is found to be about 30 μs, and is independent of luminescence energy. The long lifetime is possibly due to weak electron-hole overlap. A competing non-radiative process dominates in weakly luminescent doped and undoped samples, and is thought to be due to tunnelling of an exciton to a nearby defect.

Large spin-dependent changes in the luminescence of undoped and doped glow-discharge-deposited amorphous silicon have been observed. A pair of luminescence quenching lines was found for all doping levels, the fractional change in luminescence intensity ΔL/L being independent of temperature. A pair of enhancing lines with ΔL/L falling off as T-2 was observed in undoped and lightly doped samples. The resonant lines are very similar to those found in light-induced electron spin resonance under the same conditions. Non-resonant field-dependent luminescence is also reported. Models related to spin excitation of thermaiized and non-thermaiized electron-hole pairs are presented.

A detailed study has been performed concerning the temperature and electric-field dependence of hole carrier time-of-flight pulses in amorphous silicon. Various aspects of the experimental behaviour are employed in the estimation of the energy distribution and other characteristics of localized states in the films. Over the depth range 0·2 to 0·45eV, the trap concentration is found to vary more rapidly than the exponential form assumed in earlier studies, and a Gaussian tail is in better agreement with the data. Localized-state capture cross-sections are calculated as 1-3 × 10-16 cm2, and are essentially independent of depth over the range studied.Taking the present data in conjunction with the energy distribution of states as determined by other techniques allows an estimation of the mobility of free holes. The value of about 10cm2V-1s-1 thus obtained is an order of magnitude larger than the previous estimate from time-of-flight measurements, but is comparable to the figures suggested by other transport data.

The present work shows how transient photoconductivity can be used to characterize surfaces, interfaces, and contacts to hydrogenated amorphous silicon (a-Si:H). At a silicon nitride/a-Si:H interface an electron accumulation layer is found, whereas the native oxide induces a depletion layer. The accumulation is much greater when the nitride is deposited on top of the a-Si:H, rather than vice versa. The injection properties of n+ contacts are also investigated, and a study is made of the conditions under which these are partially or completely depleted, thereby affecting the carrier injection.

Disorder and band-tail localization are shown to influence the deep trapping processes in an amorphous semiconductor. The capture cross-section includes a term involving tunnelling from a cluster of tail states near a deep centre. Data for trapping in hydrogenated amorphous silicon is analysed to obtain the capture cross-sections and to identify the tail-state contribution.

The changes in the density-of-states distribution upon doping of hydrogenated amorphous silicon (a-Si:H) are discussed. A previous doping model is extended to explain the presence of occupied band-tail states, and their predicted doping dependence is shown to be consistent with experimental observations. The energy levels of dangling bonds are discussed, and we describe a complete set of optical and thermal emission data which place the upper dangling bond level at around 0·8 eV below the conduction band. The results also indicate that the defect has little atomic relaxation. It is argued that the defect-dopant pair does not have a negative correlation energy, contrary to a recent suggestion.

The sweep-out experiment, used to measure the density of band-tail carriers in hydrogenated amorphous silicon (a-Si:H) is described. The experimental configuration, sample structure and analysis are discussed in some detail. The band-tail carrier density nBT is obtained with good accuracy from the charge collection, but their energy distribution is not easily deduced from the time dependence of the current. The sweep-out technique is applied to doped material and the results are related to the transport, the density-of-states distribution and the thermal equilibration of the localized states. In the equilibrium regime, nBT is activated with a doping-dependent energy, which is explained by the thermodynamics of the defect and dopant states. The measurements of nBT are combined with conductivity data and the density of states, to give the drift mobility of doped material. Sweep-out measurements also prove that the doping efficiency of a-Si: H depends on the position of the Fermi energy, as predicted by the thermodynamic models. Lastly, it is shown that the technique can be used to obtain the photoconductivity and response time of undoped a-Si: H.

The dependence of the hydrogen diffusion coefficient on the material growth conditions in doped hydrogenated amorphous silicon has been measured, using secondary-ion mass spectrometry. A greatly enhanced diffusion was found in material with a columnar microstructure because of the preferential motion of hydrogen along the columns. Increasing the deposition temperature in non-columnar material results in a higher diffusion coefficient and a lower hydrogen concentration. No significant change in the diffusion was found with different n-type dopants.

Hydrogen diffusion in doped and compensated a-Si:H has been measured by secondary-ion mass spectrometry profiling in the temperature range 155-300°C. Doping reduces the activation energy and enhances the diffusion coefficient by up to three orders of magnitude at 200°C, and a correlation between the diffusion coefficient and the dangling-bond density is found. An analysis of three different diffusion models indicates that the breaking of weak Si - Si bonds by hydrogen may be an important process. The relation between the diffusion results and the thermal equilibration of the electronic structure is discussed.The hydrogen diffusion coefficient in a-Si:H has been measured over the temperature range 155-300°C, with particular emphasis on the effects of doping and compensation. In all cases DH is thermally activated with an energy 1⋅2-1⋅5 eV. The diffusion coefficient decreases slowly with time which is attributed to the disorder-induced variation in site energies. We find that DH is greatly enhanced by doping, and its magnitude correlates with the dangling-bond density in the samples. At a doping level of 10-2 B2H6 in SiH4, DH is about 103 times larger than in undoped a-Si:H. The diffusion coefficient is very low in compensated material, again showing the correlation with the dangling-bond density. We have also measured the boron diffusion and found that it does not follow the same trends as the hydrogen.In order to explore the diffusion mechanisms, we have analysed three models. A model in which hydrogen can only move if there is a neighbouring dangling bond to receive it is able to explain the data but is doubtful because of the large distance over which the Si-H interaction must occur. A second model in which hydrogen is released into an interstitial site where it diffuses until it is captured by a dangling bond seems to be unable to account for the data. However, a third model in which interstitial hydrogen breaks weak Si-Si bonds results in agreement with experiment. This latter model is attractive because it can be related both to the process of thermal equilibration and to the Staebler-Wronski effect. However, a much more detailed understanding of the atomic bonding is needed before the diffusion mechanism can be conclusively determined. Irrespective of the details of the mechanism, we argue that the observed activation energies are consistent with the breaking of Si-H bonds. Furthermore, the energy will be influenced by the presence of band-tail electrons or holes in doped a-Si:H. Thus we anticipate that a full analysis of diffusion will include electronic transitions.

Measurements are reported of the luminescence, time-of-flight photoconductivity and electron spin resonance (ESR) in a-Si: Ge: H alloys. In the alloys it is found that the luminescence intensity varies with defect density according to a tunnelling model similar to that in a-Si: H. However, the tunnelling distance decreases with Ge concentration up to about 30% and then increases. This nonmonotonic behaviour is attributed to the additional chemical disorder of the alloys. Time-of-flight experiments confirm that the dangling bonds are the dominant deep trap in the alloys. We find that the capture cross-sections for electrons and holes are very similar to those measured in a-Si: H, and are independent of whether the trap is a Ge or Si dangling bond.

The 'time of flight' technique has been employed to study the transient photoconductivity due to excess electron carriers in amorphous Si:H, over a wide range of temperature and electric field. The data are analysed in terms of the temperature/field dependence of the carrier transit time, and also in terms of the variation in pulse shape with temperature. In both cases, the behaviour suggests interactions with a tail of states extending about 0·15 eV from the band edge and showing an almost linear variation of trap density with energy over at least the range 0·085 to 0·145 eV. The capture cross-section of the centres is estimated to be of order 10-17-10-l6cm2. Interaction with states beyond the tail is comparatively weak, and is dominated by deep trapping into centres believed to be dangling bonds.

Experiments are described which explore the changes in the densities of electrons or holes in doped amorphous silicon when it is annealed with an applied bias. Such bias annealing produces a metastable enhancement of the doping efficiency by altering the distribution of localized states. A model is proposed in which the effects are directly related to the doping mechanism of amorphous silicon.