J. Allison

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.

The 0·9 eV defect luminescence band in sputtered and other forms of plasma-deposited a-Si: H

Abstract The photoluminescence properties of a low-energy (0·9 eV) emission band are examined for a wide variety of samples of a-Si: H prepared in both glow-discharge and sputtering reactors, and are compared with those of similar bands seen in microcrystalline and crystalline silicon. We also report the first photoluminescence measurements on magnetron-sputtered films, which suggest that this material has defect densities as low as those found in good quality glow-discharge a-Si: H.

Luminescence and non-radiative decay in sputtered amorphous Si:H

Abstract Photoluminescence (PL) decay has been studied (between 5ns and ∼ lms) on a range of sputtered a-Si:H samples, prepared at different hydrogen partial pressures. The hydrogen content has been studied by infrared spectroscopy. Some PL intensity and temperature effects are reported and discussed.

Switching behaviour of amorphous threshold devices in the gap configuration

Abstract An investigation is reported on the switching mechanism in threshold gap-cell switches which use SiTeAsGe glasses as the active material. Measurements made of the I – V characteristics in the off state, “forming” conditions and double-pulse performance are described. The results obtained indicate that switching is probably largely thermally dominated. A thermal time constant associated with these devices is shown to be strongly influenced by the nature of the substrate. A lower limit for the temperature attained within the switching filament is estimated. “Formed” threshold voltages are calculated using the impulse thermal breakdown approximation and a reasonable agreement with the measured values is obtained.

The growth and properties of bias‐sputtered a‐Si‐H

a‐Si‐H films are sputtered from a Si target in an argon‐hydrogen atmosphere. It has been found that the bias voltage induced on the heated substrate by the r.f. plasma influences film growth and has a profound effect on the properties of the a‐Si‐H. The photoconductivity of the films increase to a maximum and then decrease with increasing bias voltage applied to the substrate during growth. Although the hydrogen content of the films does not change significantly with bias, the variation in the relative magnitude of the vibrational stretching bands at 2000 and 2100 cm−1 appears to correlate with the photoconductivity bias dependence. The deposition rate and optical absorption edge decreases monotonically with increasing bias.

High-temperature resistivity measurements of YBa2Cu3O7−δ doped with sulphur

Abstract The diffusion of sulphur into YBa 2 Cu 3 O 7− δ using an open tube technique operating between 450 and 650°C is demonstrated. The resulting material has unchanged low-temperature characteristics but high-temperature resistivity measurements indicate that the sulphur plays a role in suppressing the out-diffusion of oxygen prior to the transition between the orthorhombic and tetragonal phases. This diffusion technique and the inclusion of sulphur generally may have a use in widening the superconductor processing temperature range without a phase change and in reducing the out-diffusion of oxygen which can be a problem in device structure fabrication.

Luminescence recombination in doped magnetron sputtered a-Si:H

Abstract New photoluminescence (PL) results on phosphorus and oxygen doped a-Si :H produced by magnetron sputtering are reported. These dopants produce PL peaks at 0.9 eV and 1.1 eV respectively. Both PL peaks can be excited selectively with sub-gap energies. The temperature dependence and kinetics of these peaks are compared.

Relaxation processes in memory and threshold chalcogenide switches

Abstract This paper concerns the relaxation processes that occur during and following switching in amorphous threshold and commercial memory devices. Relaxation with time of the conduction activation energy in the low-field OFF state following switching events, has been measured for a large number of materials. The results can be explained by assuming that the switching operation creates a severe thermal cycling process which influences the localized structure and thus the electronic properties of the glass.