Papers by Author: John W. Steeds

Abstract: In the course of studying by low temperature photoluminescence spectroscopy a wide range of electron-irradiated samples of p(Al)-type epitaxial layers of 4H SiC, from a variety of different sources of supply, the results were found to fit into two very different categories. The origin of these differences has been explored using a wide range of experimental techniques and found to result from the degree of compensation of the aluminium by nitrogen in the layers. Nitrogen concentrations deduced by SIMS experiments on these materials were found to be unreliable. The two different categories of material, called V and AB here, showed marked differences in their subsequent annealing behaviour and the implications of this distinction are discussed.

Abstract: Low voltage electron irradiations with electron energies down to the C-displacement threshold have been performed and the irradiated samples studied subsequently by low temperature photoluminescence microscopy. The results were found to depend on the electron energy, the n(N)- or p(Al)-doping and the C- or Si-face irradiated. The implications of these results for the current understanding of the atomic origins of these lines are discussed.

Abstract: An attempt is made, in the light of recent developments in the identification of intrinsic defects in 4H SiC, to account for differences that have been reported after electron-irradiation of different samples and to discuss the progression of defects that is observed on annealing. The emphasis is placed on internal stress in the material and on defects involving carbon anti-sites and silicon vacancies because they are readily detected by photoluminescence.

Abstract: 1 MeV ion implantations of 4H SiC have been performed to various doses with ion probes of 5 µm diameter. Defect introduction has been studied by microscopic photoluminescence.

Abstract: A common set of optical centres found in photoluminescence spectra of electron irradiated 4H SiC is interpreted as originating in neutral carbon-vacancy carbon anti-site pairs.

Abstract: Local vibrational mode energies of optical centres, obtained by low-temperature photoluminescence experiments, are compared with the results of published local density approximation calculations to arrive at atomic models for the carbon interstitial-related defects responsible for the light emission.

Abstract: The high-temperature persistent PL defect known as DII is commented on within this study, seen for the first time in low-energy electron irradiated 4H SiC. The local vibrational modes associated with the defect have been identified and the temperature dependence, spatial variation and electron-energy/electron-dose variation of this defect have all been investigated.

Abstract: Use of a transmission electron microscope to irradiate silicon carbide samples has been demonstrated as a useful additional characterisation technique. The photoluminescence spectra of crystal defects introduced in this way have been found to be extremely rich in detail, involving more than 50 zero phonon lines. It is perhaps disappointing that relatively few of these optical centres have been identified conclusively. Indeed, controversy exists over most of the interpretations that have been advanced. As a step towards clarifying this situation we have been studying many of the more important photoluminescent systems by investigating the dependence of the results on the sample n- and p-doping levels, their stoichiometry, the source of supply, the electron dose, the subsequent annealing history, and by exploiting two new aspects of the technique that will be introduced here. A brief review will be given of new results obtained for some of the major optical centres. Most of the irradiations have been performed at room temperature using 300 kV electrons but some were carried out at 750°C.

Abstract: We have carried out the growth and basic characterization of isotopically enriched 4HSi 13C crystals. In recent years the growth of 13C enriched 6H-SiC has been performed in order to carry out fundamental materials studies (e.g. determination of phonon energies, fundamental bandgap shift, carbon interstitial defect study, analysis of the physical vapor transport (PVT) growth process). For electronic device applications, however, the 4H-SiC polytype is the favored material, because it offers greater electron mobility. In this paper we present the growth of 4H-Si13C single crystals with up to 60% of 13C concentration. From a physical point of view we present first results on phonons as well as the fundamental bandgap energy shift due to 13C incorporation into the SiC lattice.

Abstract: The migration of carbon interstitials in n-type 4H-SiC has been revealed with optical and electrical measurements. Furthermore, clear evidence is found that carbon interstitials are involved in the formation of the Z- and S-centers detected by DLTS within the electronic band gap of n-type 4H-SiC.