Grain-Size Control of Nanocrystalline Silicon by Pulsed Gas Plasma Process
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Tokyo Institute of Technology, Meguro-ku, Tokyo 152, Japan. ABSTRACT A new method for the formation of nanocrystalline Si (nc-Si) in the SiH4 plasma using pulsed-H2 supply with very-high-frequency (VHF;144 MHz) excitation is proposed to control the size of nc-Si. Nanocrystalline Si is formed in the gas phase of SiH 4 plasma cell by coalescence of radicals. The principle of size control is based on the separation of nucleation and growth process. Supplying H2 into a Sill4 plasma enhances nucleation of nc-Si and suppresses growth rate of nc-Si. The nucleated nc-Si grows larger in a Sill4 plasma during the off state of the H2 supply. As the newly supplied H2 forces nc-Si grown in the previous cycle out of the plasma cell into the deposition chamber, the next nucleation of nc-Si is enhanced simultaneously. Using this method, we fabricated 8 nm-diameter nc-Si with small dispersion (±-1 ran) successfully. INTRODUCTION Nanocrystalline silicon (nc-Si) has received a great deal of attention for application to quantumeffect optoelectronic devices and single-electron tunneling (SET) transistors for the next generation ultra large scale integrated circuits (ULSI's). Recently, SET devices have been proposed and fabricated using compound semiconductors, metal/insulator, and Si/Si0 2 nanostructure systems [1-6]. Although room-temperature operation has been demonstrated [5,6], realization of stable characteristics has been still difficult at high temperature because of the difficulty in fabrication of nanometer-scale structures. For realization of SET devices operating at room temperature, reproducible fine structures less than 10 nm-scale are strongly required. We have fabricated nc-Si particles with 3-30nm diameter by a Sill4 plasma process using very-high-frequency (VHF;144-MHz) excitation [7,81. The merits of VHF plasma are higher efficiency of radical formation [9] and lower self-bias of plasma [10,11] compared with radio-frequency
(RF;13.56 MHz) plasma. However, it has been difficult to control the size uniformity of nc-Si, causing a problem of reproducibility in current-voltage characteristics of SET devices. In order to apply nc-Si to future electron devices, it is crucially required to fabricate as small particles as possible (less than 10 nm) and to control the size distribution precisely. In this paper, the effects of H2 supply into a Sill4 plasma on the fabrication of nc-Si is described. Based on the effects, a method of pulsed-H 2 supply into a Sill4 plasma is proposed to control the size of nc-Si. Size and distribution of nc-Si were evaluated by transmission electron microscopy (TEM) observation. Successful fabrication of nc-Si less than 10 nm with a small spread of size was accomplished by means of the pulsed-H 2 supply method.
EXPERIMENTAL A schematic diagram of the nc-Si deposition system, which is a modified Si molecular beam epitaxy equipment, is shown in Fig. 1. A plasma cell is attached instead of Knudsen cells. The electrodes of the plasma cell are capacitively coupled. The stainless steel plate with an ori
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