1. Field of the Invention
The present invention relates to the deposition of thin films of material on a substrate, and more specifically to the high-rate deposition of device quality hydrogenated amorphous silicon (a-Si:H) by the hot wire technique for use as photovoltaic and other semiconducting devices.
2. Description of the Prior Art
In the manufacture and construction of microelectronic semiconductor devices and photovoltaic solar cells, amorphous silicon is a feasible alternative to the use of silicon crystals for layers of a device, due to economics, flexibility in manufacture, and higher through-put. However, unalloyed amorphous silicon has a very high density of midgap (defect) states, and, as a consequence, has very poor electrical properties. Most of these midgap defect states can be passivated by the incorporation of hydrogen into the amorphous silicon layer, which is usually accomplished during the deposition process, and it considerably improves the electrical properties of the individual layers and the device.
Two measures of these electrical properties of hydrogenated amorphous silicon layers are the Urbach tail width and the density of midgap states, both of which should be minimized to achieve device quality semiconductor films. Although exact mechanisms are not known, there is a relationship in glow discharge (GD) deposited films between the amount of hydrogen incorporated and both the Urbach tail width and density of midgap states. At hydrogen concentrations too low, the amorphous silicon film exhibits very poor electrical properties due to the high density of midgap states and is thus not suitable for use in practical devices. At hydrogen concentrations too high, these films show an increased density of microvoids and once again inferior electrical properties.
A pervasive problem when hydrogenated amorphous silicon is used in photovoltaic solar cells has been that these solar cells tend to degrade electrically over time upon exposure to sunlight. This degradation, which is referred to as the Staebler-Wronski effect, has been linked to the concentration of hydrogen within the amorphous silicon film. The prevalent model for the Staebler-Wronski effect has suggested that the degradation is due to movement of hydrogen within the film. Therefore, until recently, hydrogen content in the range of 10-15 at. % was considered to be an optimum balance, i.e., not so low as to cause inferior electrical device qualities and not so high as to be subject to excessive Staebler-Wronski degradation upon exposure to sunlight.
In the last decade or so, since the development of the glow discharge (GD) technique as the standard means for producing device quality hydrogenated amorphous silicon (a-Si:H) films for solar cells and other applications, there has been considerable progress made in increasing the efficiencies of these solar cells. However, most of this progress has been in improved techniques in manufacturing and design of these multi-layer solar cells, and not improvements in the electrical quality of the a-Si:H layer. Examples of such improvements include better uniformity of deposition, better light capture, and better doping of layers. The U.S. Pat. No. 4,237,150, issued to H. Weismann, and the U.S. Pat. No. 4,237,151, issued to Strongin et al. illustrate attempts to improve amorphous silicon as a photovoltaic material by using silane as a silicon source gas in a hot wire deposition technique to eliminate impurities, non-uniformities, and clusters of silicon that they thought limited the utility of the material. J. Doyle et al., in their article, Production of High Quality Amorphous Silicon Films by Evaporative Silane Surface Decomposition, published in the Journal of Applied Physics, Vol. 64, p. 3215-3223, 1988, gave credit to H. Weismann and carried the improvements to better temperature and vacuum ranges, but they apparently were not able to control the degradation from the Staebler-Wronski effect. No improvements beyond that available using glow discharge (GD), either in the material quality or in the Staebler-Wronski effect, were achieved by using the deposition methods that were reported in those publications.
The U.S. Pat. No. 5,397,737 issued to Mahan et al., which is incorporated herein by reference, shows that by keeping four deposition parameters in the hot wire technique--filament temperature, silane pressure, distance between filament and substrate, and substrate temperature--all within certain specified ranges, hydrogenated amorphous silicon (a-Si:H) films with hydrogen content as low as 1 at. % can be produced that still have device quality electrical properties. Such low hydrogen content, device quality films were also shown to have substantially less Staebler-Wronski degradation when exposed to sun light than the previous state-of-the-art, device quality hydrogenated amorphous silicon (a-Si:H) films with 10-15 at. % hydrogen content.
The improvement described above, as well as many other improvements made in device quality hydrogenated amorphous silicon (a-Si:H) films, have advanced the state-of-the-art in electrical quality. However, to maintain such electrical device quality, the deposition rates of the hydrogenated amorphous silicon films have had to remain quite low. For example, the deposition rate for the low hydrogen content, device quality hydrogenated amorphous silicon (a-Si:H) film produced according to the U.S. Pat. No. 5,397,737 discussed above, is in the range of about 5-10 .ANG./sec. In general, past experiences in this field have indicated that as deposition rates are increased, the electrical qualities of the resulting hydrogenated amorphous silicon film devices decrease. To maintain device quality, current industrial GD production of hydrogenated amorphous silicon cells is done at deposition rates of 1-2 .ANG./sec. One reason for using such low deposition rate is believed to be that, as the deposition rate increases, the rate of arrival and incorporation of hydrogen atoms into the a-Si:H compound is faster than the out-diffusion of hydrogen atoms that accompanies Si--Si bonding. In addition, beyond a certain hydrogen content in terms of atomic percent (at. %), this excess hydrogen bonds differently. Instead of monohydride silicon bonds (SiH), which provide effective defect passivation and have good electrical qualities and stability, polyhydride bonds (SiH.sub.2).sub.n begin to form, which are less desirable, remain in the film. Also, at higher deposition rates, hydrogen that does not out-diffuse does not have time to diffuse to preferred sites, thereby forming a dense silicon network. Therefore, the presence of polyhydrides (SiH.sub.2).sub.n result in less dense films with more microvoids, inferior electrical properties, poorer solar cell performance, and greater Staebler-Wronski effect degradation.
There would be significant advantages for higher deposition rates of hydrogenated amorphous silicon (a-Si:H) films, provided that the resulting films have device quality electrical characteristics and minimal Staebler-Wronski effect degradation upon exposure to sun light. In particular, deposition of the intrinsic (i) layer of device, which is commonly ten to twenty times thicker than the p-layer and the n-layer, could be accomplished with much less dwell time in the deposition chamber than is now required at current low deposition rates, thus allowing for higher through-put in manufacturing lines.
There are reports of attempts to achieve higher rate depositions of hydrogenated amorphous silicon films. For example: The Neuchatel Laboratory reported a VHF (very high frequency) glow discharge plasma deposition process in which frequencies varied from 13.56 MHz to 100 MHz to achieve deposition rates up to 20 .ANG./sec, F. Finger et al., MRS Symp. Pro., vol. 192, page 583 (1990); The Electrotechnical Laboratory has used an rf (radio frequency) deposition system in a chemical vapor deposition (with higher discharge powers and higher substrate temperatures) that deposited films at about 10 .ANG./sec, G. Ganguly et al., Phys. Rev. B., vol. 47, page 3661 (1993); and United Solar Systems Corp. has used a microwave deposition and higher substrate temperature to achieve up to 100 .ANG./sec, S. Guha et al., APL, vol. 66, page 595 (1995). However, hydrogenated amorphous silicon films produced by such high deposition rate techniques have not shown sufficient promise with regard to a combination of improved material properties and a reduced Staebler-Wronski degradation to warrant serious consideration for incorporation into industrial manufacturing processes.