This invention relates to semiconductor materials. In particular, this invention relates to a method for doping amorphous semiconductors to produce a doped material having higher conductivity with fewer defects.
Control of the conductivity type (n-type or p-type) of a semiconductor material is found to be necessary in many device applications, such as diodes solar cells and transistors. Normally the conductivity type is controlled by the addition of trace amounts of substitutional impurities with one extra or one less valence electron, for n-type and p-type material, respectively.
The problem with this doping mechanism, in both crystalline and amorphous semiconductors, is that some fraction of the dopant impurities inevitably form complexes with each other or other impurities or defects and generate states near the middle of the semiconductor's bandgap. Such states, according to the Shockley-Read-Hall theory of recombination, reduce the electron-hole pair recombination lifetime and degrade the performance of devices such as solar cells that require long recombination lifetimes; they also take up electrons (or holes) from the dopant atoms and thus degrade the substitutional doping efficiency.
Midgap states associated with impurity dopants are a particularly severe problem for amorphous semiconductors where the doping efficiency is normally low in the first place because of the ability of the amorphous network to accommodate impurity atoms in their preferred coordination. In a-Si:H for example, the doping efficiency is of order 1% with phosphorous or boron. An additional problem with substitutional doping, of particular importance for crystalline semiconductors, is ionized impurity scattering from the dopant atoms themselves. This ionized impurity scattering associated with substitutional dopants reduces the electron and hole mobility, particularly at low temperatures. This effect degrades the switching speed of transistors made from doped material and reduces the diffusion length in solar cells.
In substitutionally doped amorphous silicon the large density of gap states associated with the dopants means that the depletion or accumulation layers associated with Schottky contacts or externally applied gate voltages are thin in doped material, being of order 500A thick, which is not as thick as would be desirable in field effect transistors.
These deficiencies, and others, are avoided by the method of the present invention wherein the conductivity type of amorphous semiconductors is controlled by fabricating the semiconductor in the form of a plurality of relatively narrow bandgap layers and proximity doping these layers from a second plurality of wider bandgap semiconductor layers which are interleaved with the first plurality. The multilayered structures are commonly known as superlattices.