Principles for controlling the optical and electrical properties of hydrogenated amorphous silicon deposited from a silane plasma

Abstract

The optical, electrical, and structural properties of hydrogenated amorphous silicon (a-Si:H) films are systematically investigated as functions of the substrate temperature (Ts) and plasma parameters, such as the rf power, gas pressure, and electrode dimensions. The films are deposited by the plasma chemical vapor deposition method. The properties of a-Si:H can be controlled over a wide range by varying the plasma parameters at fixed Ts. Reducing the film deposition rate and raising Ts have the same effect on the properties of a-Si:H. A unified relationship is found to exist among those properties of a-Si:H in the range of deposition conditions in this study, which includes ‘‘device-quality’’ conditions. No apparent effects of gas-phase polymerization or ion bombardment are observed. The experimental results suggest that during device-quality a-Si:H film deposition under conventional plasma conditions, the film properties are governed by a competition between the rate of film growth and the rate of thermally activated surface reactions at or near the film-growing surface. The limitations on the controllability of plasma-deposited a-Si:H, especially at low Ts, can be surmounted by adding hydrogen or helium to the plasma, and by treating a-Si:H with the hydrogen plasma.