Modeling Carrier Transport in Oxide-Passivated Nanocrystalline Silicon Leds
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MODELING CARRIER TRANSPORT IN OXIDE-PASSIVATED NANOCRYSTALLINE SILICON LEDS KARL D. HIRSCHMAN Department of Microelectronic Engineering, Rochester Institute of Technology, Rochester, NY 14623
PHILIPPE M. FAUCHET Department of Electrical and Computer Engineering, University of Rochester, Rochester, NY 14627
ABSTRACT This report presents an investigation on carrier transport in LED structures based on oxide passivated nanocrystalline silicon (OPNSi), formed by oxidation of porous silicon. This material, like its precursor, can luminesce quite efficiently while demonstrating several advantages in stability (i.e. chemical, thermal, electrical and electroluminescence). OPNSi can be best described as a porous glass structure with defects that facilitate transport, and remaining embedded nanocrystals of silicon that support light emission. Although this study does not provide a direct measurement of the density of states in OPNSi, the following transport study suggests a high density of states having a broad energy distribution that readily exchange charge with the silicon electrodes. Experimental data also suggests the existence of deeper trap centers that do not facilitate transport, yet influence transport behavior significantly. The device operation is explained by bipolar injection from an electron-injection cathode and a hole-injection anode into the semi-insulating OPNSi layer. The device is modeled as a " field effect diode", where untraditional concepts are applied in the interpretation of experimental observations. Extensive electrical characterization of OPNSi LEDs has lead to the development of a comprehensive transport model that is self-consistent with all experimental observations. INTRODUCTION Electroluminescence (EL) from porous silicon (PSi) was observed [1] shortly after photoluminescence (PL) was discovered by Canham in 1990. A typical PSi-based LED consists of a transparent or semitransparent contact (Au, ITO or conducting polymers) and a PSi layer (thickness between 1-10µm) fabricated on an n-type or p-type crystalline silicon substrate. These devices have typically been modeled as a type of Schottky junction device. The use of pn-junctions has also been implemented, with some results claiming improved electrical characteristics and EL efficiency [2,3]. A variety of energy-band diagrams have been developed to model carrier injection into the active PSi light emitting region. In most device models, the PSi layer is taken to be an effective medium material with bulk-like properties. There is an effective bandgap assumed for the high resistivity PSi material. There are some assumed potential barriers to the contact material and the crystalline silicon substrate. Several empirical models have been developed from carrier transport investigations. The OPNSi LED is quite different from other porous-silicon based LEDs. OPNSi is formed by partial oxidation of PSi, and has been characterized as a porous glass (~15% void) with remaining embedded nanocrystals of silicon. The OPNSi composite is highly interconne
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