Roughness, impurities and strain in low-temperature epitaxial silicon films grown by tantalum filament hot-wire chemical
- PDF / 1,423,690 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 65 Downloads / 142 Views
0910-A15-01
Roughness, impurities and strain in low-temperature epitaxial silicon films grown by tantalum filament hot-wire chemical vapor deposition Charles W. Teplin, Matthew W. Page, Eugene W. Iwaniczko, Kim M. Jones, Robert M. Ready, Bobby M. To, Helio M. Moutinho, Qi M. Wang, and Howard M. Branz National Renewable Energy Lab, Golden, CO, 80401 ABSTRACT We grow epitaxial silicon films onto (100) silicon wafers from pure silane by hotwire chemical vapor deposition (HWCVD). The films grow epitaxially for a thickness hepi before a-Si:H cones nucleate and expand. We study the dependence of hepi on growth rate and the differences between Ta and W filaments. The surface morphology of thin but completely epitaxial films are studied in order to correlate the surface roughness during growth with the eventual epitaxial breakdown thickness. Surface roughness, strain and H at the wafer/film interface are not likely to cause the observed breakdown. INTRODUCTION Thin crystal silicon (c-Si) films have many potential applications in large-area semiconductor devices such as display driver circuitry and photovoltaics. This has motivated considerable recent research into the epitaxial growth of silicon on silicon by scalable, inexpensive growth techniques. If c-Si films are to be used in large-area photovoltaic applications, it will be necessary to quickly thicken seed layers at temperatures compatible with inexpensive substrates (e.g., below about 600°C for float glass) [1-4]. In microelectronics applications, the ability to deposit epitaxial Si layers at low temperature on wafers, without smearing doping profiles, could be a key to fabricating novel 3-dimensional c-Si structures. Low temperature silicon homoepitaxy was first demonstrated by molecular beam epitaxy (MBE) [5-7]. Using MBE, it was found that epitaxial films thicker than 3 µm could be grown at temperatures above 480°C on (100)-oriented wafers. Below 480°C, however, the films were found to break down into amorphous material after a critical film thickness, designated hepi. Generally, epitaxial thicknesses were found to be thinner on crystalline orientations other than (100) [8]. Research with other deposition techniques has shown that a breakdown thickness is a universal feature of low-temperature epitaxy. Typically, hepi increases with higher substrate temperatures (Tsub) and/or lower deposition rates (Rg). Figure 1 summarizes the best epitaxial thickness obtained on (100)-oriented wafers using MBE [5], ion-assisted deposition (IAD) [9, 10], electron-cyclotron resonance chemical vapor deposition (ECR-CVD) [11], and hot-wire chemical vapor deposition (HWCVD) [4, 12-14]. Above 420°C, films thicker than 2 µm have been demonstrated using all of these techniques except HWCVD. The best epitaxial thicknesses were achieved by ultra-high vacuum (UHV) IAD where >8 µm thick epitaxial films can be grown at above 420°C at a high growth rate, Rg = 300 nm/min [9]. More recently, IAD with a careful surface preparation [10] has also been used to grow thick epitaxial films at higher
Data Loading...
