Metastable Non-Equilibrium Conductance States in Doping-Modulated a-Si:H Multilayers
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METASTABLE NON-EQUILIBRIUM CONDUCTANCE STATES IN DOPING-MODULATED a-Si:H MULTILAYERS H. FRITZSCHE*, S.-H. YANG** AND J. TAKADAt *James Franck Institute, The University of Chicago, 5640 Ellis Ave., Chicago, IL 60637 "**Departmentof Physics Education, Kyungpook National University, Daegu 635, Korea tCentral Research Lab., Kanegafuchi Chemical Industries Co., Kobe 652, Japan
ABSTRACT In doping-modulated a-Si:H multilayers metastable excess conductance states can be excited by brief illumination. In addition to these so-called persistent photoconductivities (PPC) we observe new metastable conductance states by bias-cooling the dopingmodulated multilayers from a temperature Ta>130°C. These metastable states exhibit greatly different nonlinear current-voltage characteristics both parallel and perpendicular to the layers. Furthermore, the magnitude of the PPC as well as the relaxation of the PPC to equilibrium are strongly affected by applying a bias during illumination and during relaxation, respectively. INTRODUCTION A number of semiconducting materials exhibit a variety of anomalies in their electronic properties [1,2] such as nonohmicity, very long time constants for establishing a constant current after a change of applied voltage, anomalously large current fluctuations, and an excess conductance that persists for hours or days after illumination, called persistent photoconductivity (PPC). Little is known about the origin of these phenomena but features common to many of these materials are a heterogeneous structure and the presence of sizable internal potential barriers against recombination and drift of excess photocarriers. Since these internal inhomogeneities are in general uncontrolled and not quantified it is difficult to relate them with the conductance anomalies. We therefore hope to shed some light on this problem by studying samples that have well controlled and geometrically simple heterogeneities. These are for example semiconducting multilayers, i.e., alternating layers of different materials (compositionally modulated multilayers) or alternating layers of the same but differently doped semiconductor (doping modulated multilayer). These are easily made [3,41 when the layers are amorphous because one need not worry then about lattice matching at the interfaces. Persistent photoconductivity effects have been observed in doping modulated multilayers made of hydrogenated amorphous silicon, a-Si:H, [4-7] in amorphous silicon/silicon nitride compositionally modulated multilayers [8,9], as well as in compensated a-Si:H [10,11]. In this paper we shall report metastable conductance states in doping modulated a-Si:H that are produced not by a brief illumination, which gives rise to the PPC effect, but by cooling the samples from an annealing temperature with different values of bias applied. We shall discuss the similarities and differences between the bias-cooled metastabilities and the PPC effect and the interplay between the two. The PPC effect for example is greatly diminished at large applied biases. Both co
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