Fabrication of Nanocrystalline Si by SiH 4 Plasma Cell
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Department of Physical Electronics and Research Center for Quantum Effect Electronics, Tokyo Institute of Technology, O-okayama, Meguro-k-u, Tokyo 152, Japan * also with PRESTO, Research Development Corporation of Japan ABSTRACT Nanocrystalline silicon (nc-Si) has been fabricated by a very-high-frequency plasma cell attached to an ultra-high-vacuum chamber using SiH4 gas. Nanocrystalline Si is formed in the gas phase of the plasma cell and is extracted out of plasma cell through the orifice to the ultrahigh-vacuum chamber. The shape of nc-Si is spherical or octahedral with the diameter of 330nm. Giant Si particles about 100nm in diameter are also formed at the lower cell pressure condition. A 1000keV transmission electron microscopy measurement has revealed that the core region of giant Si particle with the diameter about 30nm was crystalline and the shell region is amorphous. We have demonstrated that the spread of particle size can be decreased using pulsed gas supply of H2 into SiH4 plasma. INTRODUCTION Recently, nanocrystalline silicon (nc-Si) with a grain size of less than 10nm attracts considerable attention because of an expectation to manifest a quantum dot structure, which has potential application to silicon-based optoelectronics and the next generation ultra large scale integrated circuits. 1 In order to investigate physics of Si nanostructure and apply nc-Si to future electron devices, it is required to establish the technology to control the size and the surface states
of nc-Si. However, current methods, i. e., plasma enhanced chemical vapor deposition (PECVD) of SiH4 highly diluted by H2, sputtering and gas evaporation, cannot control them precisely. In an attempt to implement these controls, we have designed an apparatus for fabrication of nc-Si which consists of very-high-frequency (VHF) plasma cell and an ultra-high-vacuum (UHV) chamber. The features of VHF plasma are higher efficiency of radical formation 2 and lower self-bias of plasma 3 compared to radio-frequency (RF) plasma. A gas phase impurity which can affect the formation of nc-Si is reduced by UHV system. In this paper, we describe fabrication of nc-Si with this apparatus. Prepared samples were characterized by a transmission electron microscopy (TEM) (accelerating voltage of 200kV) and an ultra-high-voltage TEM (1000kV). An attempt of the fabrication of monodispersed nc-Si with pulsed gas supply method is also described. EXPERIMENTAL A schematic diagram of the experimental apparatus is shown in Fig. 1. This is a modified Si molecular beam epitaxy machine equipped with a plasma cell. The UHV main chamber can be evacuated to the order of 10-I°Torr by using a turbo molecular pump and a Ti getter pump. Samples can be exchanged through the load-lock system. The electrodes of the plasma cell are capacitively coupled. The diameter of the power electrode is 3cm. The stainless plate with an orifice, separating UHV chamber and plasma cell, is used for grounded electrode and 3.6cm apart from the power electrode. The dimension of the orifice is 6mm
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