Production of high-quality amorphous silicon films by evaporative silane surface decomposition

Abstract

High-quality hydrogenated amorphous silicon films (a-Si:H) have been produced by decomposition of low-pressure silane gas on a very hot surface with deposition on a nearby, typically 210 °C substrate. A high-temperature tungsten filament provides the surface for heterogeneous thermal decomposition of the low-pressure silane and subsequent evaporation of atomic silicon and hydrogen. These evaporated species (primarily) induce a-Si:H growth on nearby substrates which are temperature controlled using a novel substrate holder. The light and dark conductivities, optical band gap, deposition rates, and light-soaking effects of preliminary films are reported. The decomposition-evaporation process has been examined using a mass spectrometer to directly detect the decomposition rate and the evaporated radical species. Based on this data and other information, a simplified model for the deposition process is suggested. The excellent film quality and the attributes of the deposition process make this technique, which was originally suggested by Wiessman, viable for the fast rate, large-area deposition of a-Si:H for solar cells and other applications.

Controlling the quality of nanocrystalline silicon made by hot-wire chemical vapor deposition by using a reverse H2 profiling technique

“…The accumulated silicon atoms can react with or diffuse into the metal wires, forming metal silicon alloy or metal silicide. At an even lower temperature or at a very high silane partial pressure, depending on the type of filament, liquid silicon may appear on the filament surface, which further reduces the filament's catalytic ability [10,13].…”

“…For a fresh filament at a high enough filament temperature, all silane molecules are decomposed within a few collisions with the hot-wire surface, resulting in a significant amount of atomic Si and H evaporating to the gas phase [10]. At a moderate filament temperature, most of the SiH 4 molecules are also dissociated upon collision with the filament surface, but the evaporation of silicon atoms becomes slower, as insufficient energy is available for silicon to immediately desorb from the filament surface.…”

“…The most important reaction taking place on the filament surface is the silane dissociation reaction, namely SiH 4 = -Si + 4H. While this reaction is mainly catalytic, an increasing fraction of the reactions may be due to pyrolytic dissociation at the filament when it is ageing [10][11][12].…”

The aging of tungsten filaments and its effect on wire surface kinetics in hot-wire chemical vapor deposition

“…There have been a number of recent reports [3][4][5][6] of the distribution of wire-desorbed radicals, following the early report by Doyle et al 7 Experiments conducted using similar detection schemes, such as vacuum ultraviolet photoionization mass spectrometry, have shown quite different results in some cases. 4,5 Whether these differences are due to the different histories of the wire used or differences in the reactor condition ͑e.g., amorphous silicon-coated walls͒ is unclear.…”

Thermodynamic analysis of gas phase chemistry in hot wire chemical vapor deposition of a-Si:H and μc-Si:H

“…Doyle et al [1] have observed that SiH 4 dissociation is high than at lower pressure than at higher pressure. This, they explain in terms of the availability of reaction sites on the filament surface.…”