Parameters Affecting Light-Induced Excess Conductivity in Amorphous Silicon Doping-Modulated Multilayers
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PARAMETERS AFFECTING LIGHT-INDUCED EXCESS CONDUCTIVITY AMORPHOUS SILICON DOPING-MODULATED MULTILAYERS
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F.-C. Su , S. Levine and P. E. Vanier * Materials Science Engineering Department, State University of New York at Stony Brook, Stony Brook, NY 11794 ** Division of Metallurgy and Materials Science, Brookhaven National Laboratory, Upton, NY 11973
ABSTRACT The phenomenon of light-induced excess conductivity (LEC) which occurs in a-Si:H npnp doping-modulated multilayers is found experimentally to be dependent on several different factors. The concentrations of the dopants in n-type and p-type layers affect the Fermi level position, the height of the barriers, and also the density of defects. These parameters are altered by different choices of inert gas diluent (Ar or He) and substrate temperature T . For a given set of deposition conditions, the LEC effect can be maximized by varying the layer thickness. With undiluted silane at 0 T = 250 C, the effect was relatively small, reaching a maximum in relatively t~ick layers (540 A). The largest effects were obtained for films deposited from silane diluted in helium, using thinner (330 A) layers. However, for films deposited from silane diluted in argon, thS magnitude of the effect and optimum layer thickness was intermediate (440 A). When T was varied, a 0 minimum in LEC was found near 200-250 C. The influence of Sinternal field was examined by using nini, pipi and npnp multilayers. The internal field is necessary factor to observe a large LEC effect. A compensated film shows small LEC effect.
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INTRODUCTION
Amorphous doping-modulated semiconductor multilayer structures consisting of alternating n-type and p-type doped layers of hydrogenated amorphous silicon (a-Si:H) exhibit interesting electrical and optical properties. These multilayered films show the novel phenomenon of light-induced excess conductivity (LEC) associated with a metastable state having a lifetime of the order of days. Suggested mechanisms for LEC in such a-Si:H structures have included trapping in phosphorus-boron complexes, AX centers and E centers (1-3]. However, it is well documented that two main types of traps exist in a-Si:H. One type is the shallow band tail states that have an intrinsic character because they are due to the disorder itself, i.e. to the potential fluctuations related to bond angle, dihedral angle and bond length fluctuations; the other type is the deep defect states caused by dangling bonds in the disordered network. The dangling bond can be occupied by zero, one and two electrons leading to three different 0 charged states labeled as D+, D and D-. It may be possible to explain the observations without invoking more exotic defects than these. In this study,we- have varied a number of deposition parameters such as process gas composition, layer thickness, illumination time, doping concentration, and substrate temperature, and measured the magnitude of the LEC. We then examine whether the results can be explained consistently by a gap state model containing only band-
