Early Research On Amorphous Silicon: Errors and Missed Opportunities
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EARLY RESEARCH ON AMORPHOUS SILICON: ERRORS AND MISSED OPPORTUNITIES Hellmut Fritzsche Energy Conversion Devices, Inc., Troy, MI ABSTRACT It is instructive to reflect on the past research of an entirely new material such as hydrogenated amorphous silicon in order to analyze which impediments lay in the way of reaching our present understanding. What in retrospect appears to be clear and important evidence was often not recognized as such and, therefore, not incorporated into a coherent picture for a surprisingly long time. One reason for the inability to see and understand is one’s mode of thinking, which is conditioned by training and prior experience. Other impediments to progress are some persuasive, but erroneous, views of respected members of the scientific community. A third factor that tends to slow progress is a false sense of competition which, for example, prevents research laboratories from a fruitful exchange of materials and samples. It appears that human faults run neck to neck with nature’s intricacies in slowing the pace of our understanding. INTRODUCTION From the perspective of hindsight, progress in science does not appear as a smooth sequence of ingenious experiments, great insights, and triumphs of understanding. One sees fumbling and blind stumbling. Our thinking is hampered by incorrect concepts and influenced by views of a few authoritative scientists. Moreover, projects assigned to students often depend more on the available equipment and the expertise of the professor’s laboratory than on intrinsic scientific values. All this activity feeds the large stream of publications from which we distill at any one time our current understanding. It is interesting and instructive to pause and reflect on the reasons for our errors in judgment and the blind spots in our thinking. The history of a-Si:H research offers us a good opportunity to do this because this material is quite unlike any other studied before, and early on showed promise for important applications. Its defect and dopant equilibration process fostered by hydrogen motion is unique and required a change in paradigm for its understanding. Moreover, the details of the plasma-enhanced deposition mechanism were new to all of us, and the microstructure of the material is very complicated. No other material shows such interesting metastabilities caused by nonequilibrium carrier distributions and their recombinations. These still pose challenging scientific problems. I select only one topic for discussion today, the surprisingly slow manner by which we came to realize the crucial role played by hydrogen in our material.
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BACKGROUND In the last 1960s research on a-Ge and a-Si was very frustrating compared to that of chalcogenide glasses. Only thin films could be produced by vacuum evaporation or by sputtering. Their electrical and optical properties were very unreproducible. This was surprising because we expected elemental semiconductors to be simpler and better behaved than the multicomponent chalcogenide glasses with their concomitan
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