Liquid Vapor Deposition Using Liquid Silicon (LVD)

Abstract

In Chap. 4, a liquid process using silicon ink was described along to the centered production steps shown in Fig. 2.2. In the figure, the other path which enables the fabrication of amorphous silicon films from liquid silicon is also shown at the left side of it. This path is named a liquid-source vapor deposition (LVD) method, which is a kind of thermal CVD in inert gas at atmospheric pressure. In particular, the LVD uses CPS, which is a liquid state, as a gas source. The unique feature of CPS makes the deposition of amorphous silicon possible. Although the LVD is not a liquid process, it utilizes a unique feature of liquid silicon. So it is worth introducing as a variation of liquid process.

In Sect. 5.1, the possibility of the LVD method to make a good amorphous Si films was demonstrated with using a very simple experimental set. The deposition chamber was a petri dish! It was put on a substrate which was heated on a hot plate. Small amount of CPS was attached to the corners of the petri dish and vaporized within the petri dish. Vaporized CPS decomposed and transformed immediately into a-Si:H on the substrate surface. We deposited both intrinsic and doped a-Si:H films at the temperature of 370 °C in nitrogen gas at atmospheric pressure. Deposition process of the LVD was studied, and the properties of the resultant films were investigated.

In Sect. 5.2, a new sophisticated deposition system for LVD, which has better controllability by introducing a unique structure, was developed. In the experimental set used in Sect. 5.1, there was no way to control parameters of evaporation and deposition. As a result, the film quality being represented by a microstructure factor (MSF) was inferior to those obtained by conventional CVD methods. The developed new deposition system enables controlling two process parameters, i.e., processing temperature and CPS supply speed, separately to optimize the process condition. The MSF was improved from 61.2% to 21.4%, while the bandgap was kept constant. The a-Si:H film prepared at 360 °C showed the best performance with the highest photoconductivity of 1.09 × 10⁵ S cm⁻¹ and the lowest dark conductivity of 1.71 × 10¹¹ S cm⁻¹.

This system offers a simple instrument for depositing an a-Si:H film with semiconductor device compatibility, obtaining a high raw material conversion rate and reducing precursor loss. This system doesn’t require storage of large amounts of hazardous gases, since CPS, white phosphorus, and decaborane are in liquid or solid state.