Preparation of Nanocrystalline Silicon Quantum Dots by Pulsed Plasma Processes with High Deposition Rates

A new method for the fabrication of nanocrystalline silicon (nc-Si) in SiH4 plasma with very-high-frequency (VHF; 144MHz) excitation is proposed to increase the deposition rate, to control the size, and to minimize size dispersion of nc-Si. Nanocrystalline silicon is formed in the gas phase of the SiH4 plasma cell by coalescence of radicals. Supplying Ar enhances the nucleation of nc-Si because of high efficiency of SiH4 excitation into SiH2 radicals resulting in the nucleation. The deposition rate is thus increased by a factor of 100 to 10/cm・h. At the low flow rate of SiH4, smaller nc-Si with small dispersion is obtained. Moreover, when pulsed-SiH4 is supplied into Ar plasma, the growth of nuclei is limited by the time when SiH4 flows. The size of nc-Si and its dispersion are adjusted by the duration of SiH4 gas pulse. INTRODUCTION Recently nanocrystalline silicon has received a great deal of attention for application to quantum-effect optoelectronic devices and single-electron tunneling (SET) transistors for the next generation ultra large scale integrated circuits (ULSI). SET devices have been proposed and fabricated using compound semiconductors, metal/insulator, and Si/SiO2 nanostructure systems [1-6]. Although room temperature operation has been demonstrated, realization of stable characteristics is still difficult at room temperature because of the difficulty in fabrication of nanometer-scale structures. For realization of SET devices operating at room temperature, uniform fine structure smaller than 10nm-scale is required. Moreover, by decreasing the size of nc-Si, one dimensional confinement increases the band gap of Si to visible energies. We have investigated the formation of nc-Si in SiH4 plasma using pulsed-H2 gas supply by very-high-frequency (VHF; 144MHz) excitation [7-9]. Nanocrystalline silicon is formed in the gas phase of a SiH4 plasma cell by coalescence of radicals. By inserting a H2 gas pulse into the SiH4 gas flow, the period of nucleation and growth of nc-Si are separated, thus the fabrication of nc-Si with 8nm diameter and narrow dispersion (±1nm) of particle size is realized. But a problem of the method is that the deposition rate of nc-Si is very low (10/cm・h), thus it is difficult to utilize nc-Si in future devices, e.g. SET devices and optoelectronic devices. In this investigation the deposition rate of nc-Si is remarkably increased by adding Ar gas to SiH4. Moreover, by inserting pulsed-SiH4 gas into Ar plasma, size control of nc-Si, within 6±2nm, is also achieved with high deposition rate (10/cm・h) compared to pulsed-H2 supply. EXPERIMENTAL A schematic diagram of nc-Si deposition system, which is a modified silicon molecular beam epitaxy reactor, is shown in Fig. 1. A plasma cell is attached in place of Knudsen cells. The electrodes of the plasma cell are capacitively coupled. The stainless steel plate with an orifice of 6mm diameter and 2mm length, separating the UHV chamber (10 Torr) from the plasma cell, is used as the grounded electrode. The volume of the plasma cell is 230cm. The deposition rate is monitored by a quartz crystal sensor during deposition. The pressure in the cell is monitored by a capacitance manometer and is controlled by the flow rates of gases. The flow rate of gas is controlled by a mass flow controller and computer controlled air valves. Nanocrystalline silicon is formed by coalescence of radicals produced from SiH4 plasma in the plasma cell and is extracted out of the plasma cell through the orifice to the UHV chamber. Nanocrystalline Si is deposited on a silicon substrate. Figure 2 shows a high-resolution TEM image of nc-Si. The Si(111) lattice image indicates that nc-Si grown by the process is single domain crystal. The amorphous layer covering crystal Si is natural oxide. RESULTS AND DISCUSSION The first experiment is dilution of SiH4 with Ar in the plasma cell that will enhance SiH2 radical formation. Plasma power is 20 W, SiH4 partial pressure in the plasma cell is 0.42 torr with 8.6 sccm flow rate and the flow rate of Ar is varied from 0 to 30 sccm. Figure 3 shows the deposition rate of nc-Si as a function of Ar flow rate. The deposition rate increases by two orders of magnitude compared to the case of no Ar dilution. Before discussing the effect of Ar supply, some background information of nc-Si growth [10-11] is useful. SiH4 gas in the plasma cell is excited into Si, SiH, SiH2, and SiH3 radicals. The densities of Si and SiH radicals are low enough to be ignored compared to SiH2 and SiH3 radicals. The SiH2 radical has a short lifetime and reacts with a source gas (SiH4) to nucleate nc-Si. Nucleus growth results from high-density and long-lifetime SiH3 radicals and SiHn + (n=0-3) ions reaching surfaces of particles. Addition of Ar gas increases a number of SiH2 radicals resulting in the increased nucleation of nc-Si [12]. At the proper plasma condition, SiH2 density in the SiH4(1%)/Ar plasma increases by one order of magnitude compared to the case of pure SiH4. It will be explained as follows. One explanation is that electron temperature in Ar dilution is higher than that in the absence of Ar because of higher ionization Plasma power : 20 W SiH4 flow rate: 8.6 sccm (partial pressure : 0.42 torr) Fig. 3: Deposition rate of nc-Si as a function of Ar flow rate. Ar flow rate (sccm) (n m /m in ) 0 10 20 30 10 -1 10 10 10 D ep os iti on r at e of n cS i 10nm Fig. 2: High-resolution TEM image of nc-Si. Fig. 1: Schematic diagram of nc-Si deposition apparatus. SUBSTRATE