Increased Conductivity in P-Type Hydrogenated Amorphous Silicon
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INCREASED CONDUCTIVITY IN P-TYPE HYDROGENATED AMORPHOUS SILICON JACQUES I. PANKOVE* AND JOSEPH DRESNER * Firstauthor now at SERI, 1617 Cole Blvd., Golden, CO, Boulder, CO 80309 Work done at RCA Laboratories, Princeton, NJ 08540
CO 80401 and Univ.
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ABSTRACT The low conductivity of B-doped a-Si:H is usually attributed to the fact that only a small fraction of the boron is tetrahedrally coordinated. In the presence of hydrogen, that small fraction can be inactivated via the acceptor-neutralization process that was described for the case of B-doped crystalline Si. When a B-doped sample of a-Si:H was annealed to drive away hydrogen near the boron atoms, the conductivity increases by a factor of 600. Although a-Si:H can be doped either n-type or p-type, the doping efficiency is orders of magnitude poorer than in crystalline Si. In fact the doping efficiency of boron is one order of magnitude lower than that of phosphorus. Several models account for the low doping efficiency of aSi:H, the most plausible being the location of B in a trigonal site, i.e. surrounded by three Si-atoms as shown in Fig. 1. Such a center is neutral and cannot act as an acceptor. The present work is an offshoot of our study of the hydrogenation of dangling bonds in crystalline Si (1). The awareness that H ties to a Si dangling bond more strongly than another Si-atom led us to passivate the numerous dangling bonds on the surface a Si-crystal. Then, we passivated dangling bonds in grain boundaries and in dislocations and we showed that ion implantation damage also could be neutralized by atomic hydrogen thus removing non-radiative recombination centers and allowing the luminescent transitions to become more efficient. Finally, realizing that an acceptor is the site of a Si dangling bond (Fig. 2), we showed that it is possible to neutralize the acceptor by inserting H between B and a neighboring Si (Fig. 3). The success of this
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treatment could be measured as a several orders of magnitude increase in This change could be reversed by resistivity of the p-type crystal. This thermal anneal that dissociates the SiH complex near the boron atom. dissociation occurs above 130*C. It is on the basis of this experience that we turned to p-type a-Si:H. We assumed that the reason for a-Si:H,B to be so resistive was the (1) the location of B in a trigonal site combination of the two effects: (Fig. 1) and (2) the H neutralization of B in a four-fold coordinate site Hence, we deposited a-Si:H,B on a glass substrate kept at 100%C (Fig. 3). to form a H-rich semiconductor using a DC glow discharge in a mixture of Then one piece of the sample was annealed at 180%C silane and diborane. for one half hour to dissociate the Si-H bond near the B-atom without disturbing the remaining hydrogen (Si-H dissociates at -550*C while SiH2 Aluminum electrodes were evaporated onto the two dissociates at 350OC). We found ohmic pieces and the I(V) characteristi
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