UBC Theses and Dissertations
Metal-insulator-semiconductor tunnel junctions and their application to photovoltaic energy conversion Tarr, Nicholas Garry
This thesis is concerned primarily with an experimental and theoretical investigation of the properties of the metal-insulator-semiconductor (MIS) tunnel junction. Particular emphasis is placed on those properties which might be of use in the production of photovoltaic cells. The MIS junctions studied are divided into two basic classes: those in which the semiconductor surface is depleted or strongly inverted at equilibrium, and those in which the surface is accumulated at equilibrium. Junctions falling in the former category are termed positive barriers, while those in the latter group are termed negative barriers. Recent theoretical studies have predicted that it should be possible to form positive barrier MIS junctions in which the dark current flow at moderate forward bias is dominated by the injection of minority carriers into the bulk semiconductor. This prediction is quite remarkable, in that it appears to contradict the abundant experimental evidence indicating that the dark current in non-ideal Schottky diodes is dominated by majority carrier thermionic emission. In this thesis two independent experiments providing the first incontrovertible evidence for the existence of minority carrier MIS diodes are reported. The first of these experiments involved the measurement of the current-voltage characteristics of Al-SiO[sub x]-pSi diodes at various temperatures. From these measurements, an activation energy describing the temperature dependence of the dark current was extracted. This activation energy was found to agree exactly with that expected for a minority carrier injection-diffusion current, and to be significantly larger than that possible for a majority carrier thermionic emission current. In the second experiment, it was shown that an alloyed aluminum back surface field region could be used to enhance the open-circuit voltages of Al-SiO[sub x]-pSi solar cells. This demonstration that a modification to the rear surface of an MIS solar cell could alter the cell open-circuit voltage provided further irrefutable evidence for the existence of minority carrier MIS diodes. Negative barrier MIS junctions do not function as rectifiers. Instead, these junctions are of use in forming low-resistance contacts to semiconductors. A simple analytic model of current flow in the negative barrier MIS junction is developed here. This model predicts that with a suitable choice of insulator thickness and barrier metal work function, it should be possible to form a negative barrier MIS contact which presents a very low effective surface recombination velocity to minority carriers, yet which offers negligible impedance to the flow of majority carriers. The minority carrier reflecting properties of the negative barrier MIS junction were demonstrated experimentally by incorporating this structure as the back contact in induced back surface field solar cells. Induced back surface field cells were successfully fabricated on both n- and p-type silicon. For both types of substrate, it was found that the minority carrier reflecting negative barrier MIS back contact could provide an enhancement in cell open-circuit voltage comparable to that obtained with a conventional back surface field formed by diffusion or alloying. In addition to the studies of the MIS tunnel junction outlined above, this thesis includes a comprehensive investigation of the conditions under which the principle of dark current and photocurrent superposition provides an accurate description of the characteristics of homojunction solar cells. In particular, it is shown that the superposition principle should apply even if a significant fraction of both recombination and photogeneration occur in the depletion region. This contradicts the conclusions drawn recently by other investigators. It is also found that the superposition principle may seriously overestimate the efficiency of cells fabricated on substrates with very poor lifetimes and low mobilities, a point which had not been appreciated previously.
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