Practical Nanoscale Silicon Light Emitters
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ABSTRACT Impressive advances have been made over the last few years in teaching silicon how to emit light. Recently, light-emitting devices made of porous silicon and other forms of nanoscale silicon have been demonstrated with specifications that start to make them attractive for commercial applications. This paper reviews the state-of-the-art in the materials science and device properties of nanoscale silicon-based LEDs, including their integration with microelectronic circuits.
INTRODUCTION
Since 1990, there has been a renewed interest in the preparation, properties, and device applications of porous silicon (PSi) and other silicon nanostructures containing crystallites in the submicron to nanometer size range. The intense visible luminescence from nanoscale silicon crystallites has been a source of numerous investigations and considerable debate, which are reviewed in Refs. 1-3. This report on the status of nanoscale silicon light emitters starts by a survey of the methods used to manufacture stable PSi, and then discusses other nanoscale silicon structures. Next, the physical, optical, electrical and structural properties of nanoscale Si are examined. The fabrication of electroluminescent devices (LEDs) is then discussed, and the stability, efficiency, speed, and emission wavelengths of nanoscale Si light emitters are reported. Finally, the integration of PSi LEDs with microelectronic circuitry is reported and the prospects for practical devices are briefly examined. THE MATERIALS SCIENCE OF NANOSCALE SILICON
The manufacture of porous silicon In 1956, Uhlir [4] made PSi for the first time, by anodically etching a p-type silicon wafer in a solution containing HF. PSi is formed below a critical current density Jcrit in forward bias, whereas above Jcrit electropolishing takes place [5]. It is also possible to produce PSi starting from an n-type Si wafer by photogenerating the holes that are necessary for the dissolution of the Si wafer. PSi is classified as macroporous, mesoporous, or microporous, in order of decreasing pore sizes [6]. The term nanoporous silicon is also used when the size of the Si crystallites is in the nanometer regime. The nanocrystallite size is measured by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. Strongly luminescent samples are found to contain nanocrystallites of sizes below 4 nm (nanoporous Si) [7]. The variation of size with porosity (P) for PSi films made of p+ and p-type wafers is plotted in Fig. I. The stabilization of porous silicon The open structure of very high porosity films [8], which produce the brightest PL [7], makes them easily destroyed by the capillary stresses present during drying. After supercritical drying [9,10], a technique well known from silica aerogels, PSi films survive intact even when the
21 Mat. Res. Soc. Symp. Proc. Vol. 486 01998 Materials Research Society
porosity exceeds 95%. For porosities below 85%, normal drying is usually sufficient. The surface of PSi is naturally passivated by silicon hydride bond
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