1. Field of the Invention
This invention relates to hydrogenated amorphous silicon materials, and, in particular, it relates to a method of making hydrogenated amorphous silicon films which are characterized by an improved stability to metastable degradation and useful in amorphous silicon devices.
2. Description of the Related Art
An amorphous silicon device, such as a silicon solar cell, is comprised of a body of hydrogenated amorphous silicon (a-Si:H) material, which can be formed in a glow discharge of silane or other chemical vapor deposition techniques. Such cells can be of the type described in U.S. Pat. No. 4,064,521 entitled: Semiconductor Device Having a Body of Amorphous Silicon, issued to D. E. Carlson on Dec. 20, 1977. Amorphous hydrogenated silicon based device technology is currently the leading candidate for large area, low-cost photovoltaic applications.
For solar cells, the basic device structure is a single p-i-n junction or an n-i-p junction in which all layers are traditionally amorphous and are made in a continuous plasma deposition process. The substrate of the solar cell can be made of glass or a metal, such as aluminum, niobium, titanium, chromium, iron, bismuth, antimony or steel. A metallic contact can be formed on the back of the substrate.
However, since its discovery in 1977, a distinct disadvantage in application of these materials in devices has heretofore been the problem of light-induced metastability of the a-Si:H films themselves. See, Staebler, D. L. and Wronski, C. R., Appl. Phys,. Lett., 31, 1977, 292. Briefly, the exposure of device-quality a-Si:H films to light or excess carriers results in an increase in the density of neutral threefold-coordinated dangling-bond (DB) defects by one to two orders of magnitude. The excess in defects reduces carrier lifetimes and photoconductivity in the films which sharply limits the usefulness of a-Si:H as an inexpensive semiconductor material.
A new model of light-induced metastability (Staebler-Wronski effect) in a-Si:H has more recently been disclosed. There, it is postulated that when two mobile H atoms, generated by photo-induced carriers, collide they form a metastable-immobile-complex which contains two Sixe2x80x94H bonds. Excess metastable dangling bonds remain at the uncorrelated sites, from which the colliding hydrogen molecules were excited. This quantitative model accounts for many of the experimental observations which relate to the microscopic nature of the degradation problem and the associated kinetics of light-induced-defect-creation under various conditions. See Branz, H., Solid State Communications, Vol. 105, No. 6, pp. 387-391, 1998.
It is well known that the light-induced DB defects are metastable because they can be reversed. In the prior art, one method of reversing metastability includes annealing the films for 2 hours at temperatures greater than 150xc2x0 C. Another way of annealing light-induced changes in the dark conductivity and photoconductivity of a-Si:H thin films involves the ultraviolet (UV) irradiation (wavelength≅254 nm) of the films at room temperature. With this annealing process, a problem exists in that although the bulk photoconductivity of the film is improved, the UV irradiation is mostly absorbed near the top surface of the films and causes considerable surface damage. G. Ganguly, et al., Appl. Phys. Lett. 55, 1975 (1989). Further, illumination will cause Staebler-Wronski degradation of all amorphous silicon after such reversal treatments. Thus, what is needed is a process which, unlike the foregoing reversal methods, produces device-quality a-Si:H films which are highly resistant to metastable degradation without deleterious surface damage and thereby demonstrate an improvement in stability under light exposure or excess carrier conditions when used in amorphous silicon devices.
It is accordingly an object of the invention to provide novel hydrogenated amorphous silicon films which are characterized by an improved resistance to metastable degradation.
It is another object of the invention to provide a novel method for producing device-quality a-Si:H films which are highly resistant to metastable degradation and thereby demonstrate an improved stability when exposed to light or excess carriers.
It is yet another object of the invention to provide amorphous silicon devices which, through use of the novel a-Si:H films made according the method herein, are characterized by an improvement in stability when used under light or excess carrier conditions.
Briefly, to overcome the problems associated with the prior art methods and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention is intended to provide a method of producing amorphous hydrogenated silicon films which are resistant to metastable degradation, the method comprising the steps of growing a hydrogenated amorphous silicon film, the film having an exposed surface, illuminating the surface using an essentially blue or ultraviolet light to form high densities of a light induced defect near the surface, and etching the surface to remove the defect.
Additional advantages of the present invention will be set forth in part in the Id description that follows and in part will be obvious from that description or can be learned from practice of the invention. The advantages of the invention can be realized and obtained by the method particularly pointed out in the appended claims.