R.A.G. Gibson

Majority and minority carrier lifetimes in doped a-Si junctions and the energy of the dangling-bond state

Abstract Electron and hole lifetimes have been measured by transient-delayed-field techniques on a series of a-Si junction specimens deposited with lightly doped n- or p-type central regions. The results, plotted as a function of Fermi level position, show a pronounced sensitization of the majority carriers and a desensitization of the minority carriers. The data are readily explained by electron and hole capture at the dangling-bond centre. An analysis of this centre in terms of the occupation statistics places it at an energy between 0·95 and 1·0 eV below ∊c. The assignment is supported by independent results, but disagrees with recent spectroscopic work.

Photoluminescence properties of a-SiNx: H alloys

Abstract Photoluminescence (PL) studies are reported on a series of a-SiNx:H alloys prepared by glow discharge, with x up to around 1·1. As x increases, the main PL band broadens and moves to higher energies, and there is a marked reduction in the temperature dependence of the PL efficiency, linewidth and peak energy. The 0·9 eV defect band increases by a small factor and there is only a small reduction in the efficiency of the main band, strongly suggesting that nitrogen introduces few extra dangling-bond defects. Excitation studies show a strikingly large red-shift at low excitation energies up to 0·5eV or more in SiN1·1. This suggests a large increase in tail-state widths, probably due to compositional fluctuations, and this is consistent with most of the other changes in PL behaviour with alloying. We also compare our data briefly with PL from a-Si/SiNx multilayers.

Electronic and optical properties of a-Si1-xCx films prepared from a H2-diluted mixture of SiH4 and CH4

Abstract This paper deals with the electronic and optical properties of a-Si1-xCx films deposited by glow-discharge decomposition of SiH4 and CH4 mixtures, in most cases with H2 dilution. These results complement the structural and compositional data on the same specimens reported in the preceding paper. A systematic investigation of optical absorption, dark conductivity, photoconductivity and H evolution has been carried out as a function of the carbon content of specimens prepared with and without H2 dilution. It is shown that the former leads to a significant increase in the photosensitivity of specimens having optical gaps of up to 2.1 eV, in agreement with the work of Matsuda and co-workers. Further support for the improvement in electronic properties produced by H, dilution is obtained from density-of-states measurements, using the space-charge-limited current technique, and from photothermal deflection spectroscopy results, both of which show a significant decrease in the density of deep defect sta...

Analogue Memory Effects in Metal/ a-Si:H /Metal Thin Film Structures

The ac conductivities of non-volatile analogue memory states are measured in electro-formed Cr/p + /V amorphous silicon structures for a broad frequency range (from 0.1 Hz to 32 MHz). The results suggest that the memory action is associated with electronic processes.

The characteristics of high current amorphous silicon diodes

Amorphous siliconp−n junctions with various doping profiles have been prepared by the glow discharge technique and the effect of the barrier profile on electrical properties investigated. Current densities of up to 40 A cm−2 with rectification ratios of 104–105 were obtained withn+−ν−p+ structures. The diode quality factor has also been investigated, both in the dark and under illumination.

Hydrogen profiling in amorphous silicon films and p-n junctions

Abstract Hydrogen depth profiles, obtained by a nuclear reaction technique, have been investigated in a series of glow-discharge deposited Si specimens. These include undoped films, homogeneously doped n- and p-type specimens, junctions and continuously graded devices. It is shown that the hydrogen concentration CH in the bulk of the specimens depends systematically on the doping level and thus on the position of the Fermi energy ∊F in the mobility gap. In junctions there exists an almost linear dependence of Ch on (∊c-∊f)o, extending over a range of about 6 at.%. The lowest values of CH (1-2 at.%) occur in p+ regions. The results are discussed and it is suggested that, with the good depth resolution obtainable, hydrogen profiling may be a useful approach for the study of barrier profiles in a-Si devices. In addition to the bulk effects, the doping dependence of CH at the surface and the specimen-substrate interface has been investigated.

Electroluminescence in amorphous silicon p-i-n junctions

Abstract We present a detailed study of EL (electroluminescence) spectra and EL quantum efficiency (ηEL) for well-characterized a-Si p+-i-n+ junctions deposited on stainless-steel substrates. EL spectra were measured under forward bias and the PL (photoluminescence) characteristics of the i region were also probed using laser excitation. The PL spectra are much more strongly modulated by interference fringes, owing to reflection at the steel substrate, and are shifted to higher energies by ∼0·17 eV as compared to the EL spectra. There is good evidence that EL is generated near the p+ contact, at a distance ∼ 100 to 200 nm from the reflecting substrate. We propose that the relative shift in the EL spectra is also due to optical interference effects, which depend on this distance. We conclude that optically and electrically excited pairs have the same luminescence spectrum. Further evidence for this view is provided by the close similarity in the temperature dependence of ηEL and ηPL· ηEL is ∼ 0·03 times sm...

Recent developments in amorphous silicon p-n junction devices

Abstract Amorphous Si p-n junctions with various doping profiles have been prepared by the glow discharge process to investigate the effect of the barrier profile on the electrical properties of the diodes. The highest current densities, up to 40A/cm 2 , are obtained with n + -ν-p + structures. Under AM-I illumination photovoltaic p + -i-n + cells generate open circuit voltages of 0.7V and short-circuit currents up to 10mA/cm 2 , corresponding to efficiencies between 3 and 4%. The diode quality factors have also been investigated.

The effect of hydrogen dilution on the aminosilane plasma regime used to deposit nitrogen-rich amorphous silicon nitride

Abstract The first hydrogen dilution study of ammonia/silane plasmas tuned to an aminosilane plasma regime is reported. The use of formal statistical experimental design techniques have enabled us to determine the effects that hydrogen dilution level, rf power density and substrate growth temperature have on various film properties. Hydrogen dilution of the ammonia/silane plasma reduced the amino (NH 2 ) content of the deposited nitride films while also driving the intrinsic film stress to compressive values. Increasing the substrate temperature also reduced the amino concentration, but drove the film stress tensile. It was therefore possible to control the film stress and produce nitride films that contained only NH and SiN bonds (with no detectable NH 2 or SiH bonds) by choosing a suitable combination of H 2 dilution level and growth temperature. Exodiffusion experiments on such ‘bond optimised’ films revealed only one hydrogen evolution peak at temperatures in excess of 900°C, with no ammonia exodiffusion detected.

Photoluminescence excitation studies of a-SiNx:H alloys

Abstract Below gap excitation is shown to red shift the PL peak by >14% of the gap in SiNx alloys and produce a marked weakening of its temperature dependence.