Electronic Properties of Amorphous Semiconductors

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w:\awards\awards96\*.doc 9624002 Cohen This project involves research into the fundamental properties of defects in amorphous semiconductors. First, studies will be carried out to further verify and understand the novel type of deep defect relaxation in hydrogenated amorphous silicon (a-Si:H) that has been suggested by capacitance transient measurements in the previous phase of this research. These experimental studies will focus on (1) observing relaxation through its effect on the optical spectra associated with deep defects in a-Si:H, and (2) comparing modulated photocurrent spectra (which also indicate a defect relaxation effect) with electron spin resonance data to identify the deep defect sub-bands responsible. Second, the project will investigate the deep defect properties of a novel form of carbon containing a predominance of tetrahedral bonding (ta-C). This material has recently been made with sufficiently good semiconducting properties to be amenable to our junction capacitance methods. The disclosed deep defect properties are likely to be fundamentally different than those of the tetrahedral amorphous semiconductors based on Si and Ge. %%% Amorphous or "glassy" materials represent a true frontier area to test our understanding of solid state physics. One increasingly important class of such materials are amorphous semiconductors such as hydrogenated amorphous silicon (a-Si:H). This latter material is finding important technological applications in photovoltaics and in thin film transistor arrays used to drive liquid crystal displays (flat panel TV). However, the theoretical understanding of a-Si:H is still in its infancy. This experimental research is directed at understanding the basic electronic properties of defects in such materials. These defects currently limit their future technological exploitation. Of partic ular interest are newly discovered aspects of the defect behavior in a-Si:H which suggests a glassy aspect to the electronic structure itself. In addition, preliminary studies of the electronic properties of a relatively new type of amorphous semiconductor, tetrahedral amorphous carbon (the amorphous counterpart of crystalline diamond) will be initiated. ***