Electron Transport in a-Si:H at Low Temperatures
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ELECTRON TRANSPORT IN a-Si:H AT LOW TEMPERATURES
C. E. NEBEL* and J. KOCKA** *Xerox Palo Alto Research Center, Palo Alto, California 94304 USA "**Institute of Physics, Czech Academy of Sciences, 16200 Praha 6, CSFR
ABSTRACT Time-of-flight and charge collection experiments performed on a-Si:H p-i-n structures in the temperature range 40-300K reveal electron drift mobilities which continuously decrease with decreasing temperature and electric field. At 40K, fields !- 2 X 105 V/cm are necessary to collect almost all carriers. A discussion of the data on the basis of a nearest neighbour hopping model shows the significant influence of the applied field on the hopping down process of electrons. Fieldinduced re-emission of deep trapped carriers back into shallow tail states establishes a narrow band of energy levels where electrons accumulate and propagate. The band width and energy position of the accumulation levels in the tail depends strongly on the applied field and the distribution of localized states. With increasing electric field, it shifts up in energy closer towards the conduction band edge and changes the hopping mobility for orders of magnitude. The calculated and experimental data are in reasonable agreement with the model of a hybrid distribution of localized states. INTRODUCTION One of the most interesting properties of disordered semiconductors like amorphous hydrogenated silicon (a-Si:H) is the existence of different transport mechanisms. Due to the disorder-induced density of localized states (DOS) in the mobility gap, carriers can propagate via quantum mechanical tunneling transitions from localized state to localized state or by thermal activation to states higher in energy. Which of these mechanisms prevails is dependent on DOS, temperature (T), and electric field (F). At lower temperatures, nearest neighbour- or variable range hopping is very likely [1], whereas at higher T, transport in extended states can dominate. The interpretation of data deduced from experiments like photoconductivity or photoluminescence [2,3] is fundamentally dependent on an exact knowledge of the dominant process. In the past, the focus of investigations has been the temperature range T>100K. However, recently published transport data for T
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