Light Emission from Porous Silicon Subjected to Rapid Thermal Oxidation
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LIGHT EMISSION FROM POROUS SILICON SUBJECTED TO RAPID THERMAL OXIDATION A.G. CULLIS, L.T. CANHAM, G.M. WILLIAMS, P.W. SMITH and O.D. DOSSER DRA Malvern, St Andrews Road, Malvern, Worcs WRI4 3PS, UK ABSTRACT Luminescent oxidised porous Si is produced by rapid thermal annealing of the anodised material in a dry oxygen ambient. Its light-emitting properties are studied by both photoluminescence and cathodoluminescence methods. The structure of the oxidised material is examined by transmission electron microscopy, while its oxygen content is determined by X-ray microanalysis. These investigations show that crystalline Si nanostructures remain in the oxidised porous material and account for its luminescence properties. The work demonstrates that the speculated importance of either Si-based amorphous phases or the interesting material, siloxene, in this regard is unrealistic. INTRODUCTION Under photostimulation, as-anodised hydrogen-passivated porous Si emits light very efficiently in a manner which is consistent with the recombination of quantum-confined carriers in nanometre-scale, crystalline Si structures [1-4]. Furthermore, it has been shown that porous Si subjected to limited, high temperature oxidation can also exhibit efficient luminescence properties [5-7]. In the present paper, we examine the light-emitting behaviour of this latter material using a number of methods of physical investigation which allow a direct correlation between structural and optical properties to be made. It is demonstrated that the properties of the oxidised porous Si are fully consistent with light emission resulting from carrier recombination within quantum-domain Si crystallites, since the latter are shown to be present at the nanometre scale by direct imaging in the transmission electron microscope (TEM). In addition, the present observations militate strongly against the suggested [8-11] possibility that Si/H-based amorphous phases and crystalline compounds are relevant to the light emission process in the porous Si system. Indeed, the heat treatment associated with the oxidation in these experiments would result in the restructuring/decomposition of Si/H-based materials, which eliminates them from consideration in these circumstances. EXPERIMENTAL PROCEDURES Mesoporous Si layers - 10um in thickness and - 77% porosity were produced by the anodization of n' Si wafers in 20% ethanoic HF at 1.5mA/cm2 for a time of 2hr. Samples were then either subjected to a low temperature (300'C), lhr anneal in an oxygen atmosphere or allowed to age in ambient air for a period of 1 year before additional treatment. All samples were ultimately subjected to rapid thermal oxidation (RTO) for times of 30 sec to 5 min in pure oxygen at temperatures ranging up to 1150'C. The photoluminescence spectra of the oxidised porous layers were obtained using either 488 or 325nm excitation at 0.1W/cm2 with the sample at room temperature. Cathodoluminescence (CL) emission from the layers was measured using a Cambridge 150 scanning electron microscope (SEM) equipped wi
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