T.M. Searle

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.

Redox and clustering of iron in silicate glasses

Soda–lime–silica glasses (70 mol% SiO2:15 mol% Na2O:15 mol% CaO) containing 0.1–5 mol% Fe2O3 have been investigated using optical absorption spectroscopy, Mossbauer spectroscopy, and wet chemical analysis. Iron redox ratios measured by these different techniques are in agreement. Samples were annealed at 550°C for 1 h in air. As the iron content increases more of the iron ions are in adjacent sites and this manifests itself as changes in the optical and Mossbauer spectra. For example an absorption peak in the visible region which appears with increasing iron content can be assigned to Fe2+–O2−–Fe3+ interactions. Mossbauer parameters indicate that, as Fe2+ and Fe3+ ions cluster, their co-ordination becomes more tetrahedral although the iron concentration has no effect on the Fe2+/ΣFe ratio in the glasses studied. Estimates of the proportion of Fe3+ ions in adjacent sites have been made using Mossbauer data.

Upconversion in neodymium doped fluoride glasses

Abstract Room temperature upconversion in Nd doped fluoride glasses excited with a cw diode laser is reported for the first time. With 801 nm diodes, efficiencies are high enough for the protected eye to see white light with pump powers as low as 10 mW. In addition, near-UV emission is produced. Upconversion spectra are presented for a range of fluoride glasses between 800 and 350 nm, together with excitation spectra and efficiency estimates. Possible mechanisms for two and three photon excitation processes are suggested.

Novel structural behaviour of iron in alkali-alkaline earth-silica glasses

The combined effect of alkali and alkaline-earth ions on the redox, distribution, co-ordination and environment of Fe ions in alkali–alkaline-earth–silica glasses has been studied using a multi-technique approach. Wet chemical analysis and Mossbauer, electron spin resonance (ESR), optical absorption and photoluminescence spectroscopies were utilised. Behaviour generally falls into two categories, which we have termed ‘collective’ and ‘selective’. Collective behaviour occurs when alkali and alkaline-earth ions have similar effects on a property and the overall effect is cumulative. This is characterised by a linear relationship with optical basicity of the glass. Some parameters associated with the environment of Fe2+ ions fall into this category. Selective behaviour occurs when alkali and alkaline-earth ions have opposing effects on a property, suggesting competition or selectivity. This is characterised by a linear relationship with the alkali/alkaline-earth ionic radius ratio, cation field-strength ratio or oxide-basicity ratio. The Fe2+/ΣFe ratio and several parameters associated with the distribution, coordination and environment of Fe3+ ions fall into this category. These results have implications for the local structure surrounding Fe species. A relationship has been suggested linking coordination and distribution of Fe3+ ions.

Static versus electron-phonon disorder in amorphous Si: H and its alloys

Abstract We present new results from photoluminescence studies of a-Si: N: H alloys which enable us to distinguish between the effects of rigid-band (i.e. static) and electron-phonon disorder. Using predictions of correlations between the M-band width, the Urbach slope and the Tauc gap, we show that static structural disorder determines the photoluminescence band width. Analysis of earlier work on the a-Si: C: H and a-Si: Ge: H systems supports these conclusions. By using subgap excitation of luminescence, we provide further evidence that in the nitrogen-rich alloys this disorder is predominantly due to long-range alloy compositional fluctuations.

Radiative and non-radiative tunnelling in glow-discharge and sputtered amorphous silicon

Abstract Photoluminescence decay measurements are reported for doped and undoped glow-discharge a-Si, and for a-Si prepared by sputtering in a hydrogenated atmo-sphere. It is concluded that a distribution of radiative lifetimes is present, due either to non-crystalline disorder or to a donor-acceptor pair mechanism. The main competitive mechanism is tunnelling to defects.

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.

Low energy excitation of photoluminescence in a-Si:H : Temperature and intensity effects

Abstract Photoluminescence (PL) spectra from both glow discharge and sputtered a-Si:H are reported, excited with photons of energies between 2.4eV and 1.55eV. Low energy excitation produces a red shifted PL spectrum, which responds quite differently to changes of temperature compared with PL excited by above-gap photons. With rising temperature, the peak moves more slowly to lower energies, the efficiency falls less rapidly, and the kinetics are bimolecular even at the lowest excitation intensities. We compare two possible models to explain these effects, one based on a thermalisation gap and the other on two centre emission.

Luminescence phenomena in a-Si:H p-i-n junctions

Abstract Injection electro-luminescence (EL) and photoluminescence (PL) have been studied in a range of a-Si p-i-n junctions on stainless steel and ITO glass substrates, in which the i layer thickness is varied. New data are presented on photoenhanced EL (or PEL) and emission and excitation spectra for this process.

A comparison of photo- and electroluminescence in a-Si p-i-n junctions

Abstract New measurements over an extended temperature range confirm the similarity between electro- and photoluminescence from a-Si:H junctions. The first lifetime studies show significant differences in the kinetics.