Interaction of aluminum with hydrogenated amorphous silicon at low temperatures

Abstract

Annealing effects on aluminum/hydrogenated amorphous silicon (a-Si:H) contacts in the temperature range from 100 to 300 °C were studied. Al was evaporated on device-quality, phosphorus-doped (n+) a-Si:H films deposited in a UHV plasma-enhanced chemical- vapor-deposition system. Both electrical measurements and surface morphological analyses were performed to characterize the interaction. The transmission line model technique was used to measure sheet resistance and contact resistivity. For samples where Al covered the entire a-Si:H surface during annealing, sheet resistance and contact resistivity were found to decrease monotonically with annealing temperature; whereas, samples annealed after patterning of the Al pads exhibited a minimum in sheet resistance and contact resistivity at temperatures between 150 and 200 °C. Optical and scanning electron microscopy, surface profilometry, and Raman spectroscopy were used to study the surface morphology. Interaction of Al with a-Si:H was observed to initiate at a temperature of 150 °C. Crystallization of the interacted Al/a-Si:H film starts at a temperature as low as 180 °C. A model, involving Al counterdoping of the n+ a-Si:H film at the surface, is proposed to explain the electrical behavior of the contacts and films following different annealing cycles.