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THE EFFECT OF DEPOSITION PROCEDURE ON THE CONDUCTIVITY OF HYDROGENATED AMORPHOUS SILICON MULTILAYER FILMS. 3

2 F.-C. SU AND P.E. VANIER Department of Electrical and Computer Engineering, University of Colorado. Boulder, CO 80309-0425, 2 Department of Materials 3Science and Engineering, State University of New York, Stony Brook, NY 11794, Metallurgy and Materials Science Division, Brookhaven National Laboratory, Upton, NY 11973.

" MODDEL, 1

ABSTRACT The conductivity of multilayer P-doped amorphous hydrogenated sili'on (aSi:lI) thin films is measured for films prepared with different, deposition procedures. Multilayer films are deposited by plasma enhanced CVD following a procedure in which the plasma is extinguished and the deposition chamber is filled with air or argon after the deposition of each layer. These films are compared to films grown in continuous deposition runs. The technique provides a direct means to determine the effects of continuous versus interrupted deposition and to analyze oxide interface and bulk gap state densities. Exposing the layers to air between depositions produces deleterious effects whereas the effect of argon exposure are slight. Literature values for the density of states in oxidized a-Si:lI are used to provide evidence for a defective layer in very thin P-doped a-Si:H having a defect density of over 1013 cm- 2 eV-I approximately 0.3 eV below the transport level. INTRODUCTION In the initial studies of amorphous semicondluctor superla.ttices emphasis was placed on the prol)erties of the structures, per se rather than on the properties of the materials. Light-induced excess conductivity was observed in dopingmod ulated hydrogenated silicon amorphous (a-Si:HI) multilayer structures.' This led to discussion of whether the sl)ecial effects are a consequence of the multilayer structure of materials properties which the structure reveals. 2 It is becoming increasingly apparent that. multilayer structures may be used as a vehicle to probe basic properties of the material. Recentlv a method was proposed to determine the density of states in the gap of amorphous semiconductors using doping superstructures . The present study makes use of multilayer structures to address some basic properties of phosphorus-doped a-Si:H thin films. In fabricating a-Si:H solar cells and forming ohmic contacts to other a-Si:II devices, a P-doped layer is often i[tcorporated into the structure. We examine whether the electronic properties of this P-doped layer are compromised by interrupting the deposition between this layer and other layers. We take advantage of the enhanced interface-to-bulk-thickness ratio of multilayer films to study the interface. Multilayer interrupted deposition P-doped films (inn) which have been deposited under various conditions are compared to continuously deposited n-type films. A relation is developed between the density of states in the gap and the denisty of surface states at oxide interfaces.

Mat. Res. Soc. Symp. Proc. Voe. 70.

1986 Materials Research Society

424

EXPERIMENT