Carrier Separation Effects in Doping Modulated Amorphous Silicon
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Carrier Separation Effects in Doping Modulated Amorphous Silicon J. Kakalios, C. C. Tsai and R. A. Street Xerox Palo Alto Research Center, Palo Alto CA 94304 ABSTRACT Doping modulated amorphous silicon films have been synthesized which exhibit photoconductivity decay times in excess of 1000 seconds at 78 K. The time and intensity dependence of the photoconductivity is described by a model in which photo-excited charge carriers are spatially separated to separate layers by the built-in pn junction fields and reside in band tail states. INTRODUCTION Recently, there has been great interest in the optical and electrical properties of doping modulated amorphous silicon consisting of hydrogenated amorphous silicon (a-Si:H) which has been alternatively doped n-type and ptype 1 -4. These amorphous multilayer structures exhibit many novel properties, such as a room temperature persistent photoconductivity' and a blue shift of the low temperature photoluminescence spectrum 2. The initial interest in doping modulated a-Si:H was based on the expectation that field separation of excess charge carriers by the built-in pn junction fields should yield extremely long recombination lifetimes, as observed in crystalline nipi doping superlattices 5. Recombination lifetime enhancements in doping modulated aSi:H have been observed by Hundhausen et al using transient infrared photoconductivity measurements below 30 K 3. However, the conductivity of the a-Si:H multilayers is so small at these low temperatures that the effects of the spatially separated charge carriers can only be seen via pump and probe techniques. In this paper we report for the first time measurements of carrier separation effects on the dc dark conductivity at 80 K in doping modulated aSi:H. EXPERIMENTAL TECHNIQUE The doping modulated amorphous silicon multilayers were made in an ultrahigh vacuum r.f. glow discharge deposition system, described earlier 6. Alternating layers of a-Si:H were grown by switching the reactive gases from silane premixed with 100 ppm phosphine for n-type doping to silane premixed with 100 ppm diborane to deposit p-type a-Si:H while maintaining a continous r.f. plasma. To minimize the likelihood of dopant compensation at the p-n junction interfaces, a 5 A thick intrinsic a-Si:H layer was grown between each n- and p-layer. The multilayer samples were deposited on Corning 7059 glass substrates held at a temperature of 200 C and deposited at a rate of 1 A/ sec. The individual layers were of equal thickness (dn = dp). We report here measurements on a sample having six n-layers alternated with five p-layers, each 500 A thick, for a total sample thickness of 0.55 tim. For co-planar conductance measurements the multilayer sample is scratched with a diamond scribe before depositing electrodes (length 0.4 cm, separation 0.2 cm) using carbon paint, to insure that electrical contact is made to all layers. The conductivity data presented here were obtained using voltages less than 10 V, which yielded linear I-V characteristics. To calculate the
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