Preparation of Nanocrystalline Silicon by Pulsed Plasma Processing
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S. ODA* AND M. OTOBE Department of Physical Electronics, Tokyo Institute of Technology O-okayama, Meguro-ku, Tokyo 152, Japan *also PRESTO, Research Development Corporation of Japan
ABSTRACT We have proposed digital plasma processing for the fabrication of silicon quantum dots with grain size less than 10nm. By using the pulsed gas supply of SiH4 and H2 in the very-highfrequency (VHF) plasma, we have clarified the role of atomic hydrogen in the nucleation, crystallization of nanocrystalline Si (nc-Si) as well as in the selective etching of amorphous Si to nc-Si. Recently, we have prepared nc-Si by employing an ultra-high-vacuum (UHV) chamber equipped with VHF plasma cells of SiH4 and H2. Flux rate of Si cluster depends significantly on the pressure of the plasma cell and VHF power. Spherical shaped nc-Si clusters less than 6nm in diameter have been observed by transmission electron microscopy (TEM). Infrared absorption measurements have clarified that the surface of nc-Si is covered by hydrogen. In an attempt to control the position of nuclei, we have prepared nc-Si on Si02 with micro trenches, 40nm wide and 20nm deep, fabricated by electron beam exposure and electron cyclotron resonance (ECR) etching. It has been revealed by TEM observation that nc-Si are formed preferentially along micro trenches.
INTRODUCTION Recently, increasing interest has been shown towards fabrication and characterization of nanocrystalline silicon (nc-Si), fine particles of silicon with less than 10nm in size. The expected properties of nc-Si such as quantum effects and single electron effects can be useful for the application of future ultra large scale integrated devices and optoelectronics devices overcoming the limitation of miniaturization of present transistor-based devices. Silicon-based quantum devices are more promising than its compound semiconductor counterpart since it can make use of processing technology already developed for Si VLSI. The study of nc-Si is also interesting from the view point of basic science. It may answer the question of how small crystalline Si can be formed and what the property is like. It may answer the question what the structure of amorphous Si (a-Si) is. It may also render the idea of the mechanism of powder formation in the glowdischarge plasma of SiH4 for the fabrication of a-Si devices and suggest a method to control it. In order to apply nc-Si to future electron devices, precise control of grain size, grain position and interface states are required. Conventional methods, e.g., gas evaporation, sputtering and chemical vapor deposition (CVD), provide Si fine particles with scattered grain size without position control. The idea for the fabrication of monodispersed Si fine particles is as follows. The first principle is the separation of nucleation and growth processes.' The choice of the diffusionlimited condition, 2 where the growth rate decreases with increasing grain size, is also helpful by taking advantage of a negative feedback system. Finally, the natural self-limiting mechanism can be em
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