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STUDY OF PHOTOLUMINESCENT THIN FILM POROUS SILICON ON SAPPHIRE W. B. DUBBELDAY, D. M. SZAFLARSKI', R. L. SHIMABUKURO, and S. D. RUSSELL Naval Command Control and Ocean Surveillance Center, RDT&E Division (NRaD),

Solid State Electronics, Code 553, San Diego, CA 92152-5000 * Postdoctoral Fellow from Office of Naval Technology, administered by the American Society for Engineering Education. Present address: Molecular Biosystems Inc., San Diego, CA 92121. ABSTRACT

Photoluminescence in porous silicon on sapphire (SOS) prepared through chemical staining and photo-initiated chemical etch is demonstrated. SOS consists of single crystal quality material on a transparent substrate through which photoluminescence can be excited and observed. Porous silicon films as thin as 270 nm are shown to photoluminesce. This study explores the effect of varying film thickness and crystal quality on the photoluminescent emission from porous silicon. Introduction of crystal defects by ion implantation is used to create controlled damaged layers prior to fabricating porous silicon. No photoluminescence is observed from chemically stained or electrochemically etched amorphized layers. Corroborative results are obtained using LPCVD deposited amorphous silicon layers. BACKGROUND

The majority of the literature on photoluminescent porous silicon (PS) has reported electrochemical etching of bulk silicon material.' 2 However, there are reports of photoluminescence from samples prepared by commonly used stain etches. 3"4 This study focusses on chemical and photochemical induced etching of silicon so that thin (270nm) films of PS on insulating sapphire could be fabricated and studied without the requirement for electrical contact for anodization. Silicon on sapphire (SOS) is a proven technology for large scale integrated circuitry. It has several advantages over bulk silicon, including lower parasitic capacitance and better device isolation. The transparent substrate provides a unique opportunity for electro-optic integration and flat panel display technology. Therefore, luminescent silicon on sapphire may have various applications not possible with bulk material. In addition, thin film silicon on sapphire enables the separation of diagnostic signals from the underlying bulk material and allows for focus on the luminescing layer. It also permits excitation and detection of photoluminescence through the transparent sapphire substrate. The controversy concerning the light emission mechanism in porous silicon continues between surface and quantum confinement effects. An investigation into the dependence of photoluminescence on damage in the starting silicon layers may help to elucidate the mechanism by decoupling the surface species formed by the chemistry from the crystalline state of the nanostructures. This study applied identical etch procedures to damaged silicon layers to examine the emission mechanism. EXPERIMENTAL PROCEDURES AND RESULTS

Sample Preparation and Data Collection The chemically etched samples were prepared as described in the li