Silicon Transistors and Photodetectors Integrated on a Lithium-Tantalate Substrate

Metal-Oxide-Semiconductor Field-Effect-Transistors (MOSFETs) have been fabricated in laser recrystallized silicon films on a lithium tantalate substrate with an intervening silicon dioxide layer. Deposited polycrystalline silicon films were irradiated with a scanning focussed argon ion laser beam which caused melting and subsequent crystallization of crystallites up to 5 (mu)m long. Conditions were determined for which the process yielded recrystallized silicon films without damaging the lithium tantalate substrate. A theory based on the existence of a maximum allowable thermal gradient in the substrate was developed which explains the observed scan speed dependence of laser induced substrate damage. The theory is also applicable to predicting proper scan conditions for other material combinations. Transistors were fabricated in bulk silicon, in silicon on insulator (SOI) and in silicon on electrooptic substrates (SOES), specifically in silicon on lithium tantalate. The transistors in each material were evaluated and compared. The electron channel mobility in the SOES structure was measured to be 50 cm('2)/V-sec. Other electrical data are presented, including leakage current, threshold and breakdown voltages. The leakage current data were used to determine that lithium migration from the substrate into the silicon film had no measurable effect on transistor performance. A theory of operation for the MOSFETs as integrated photodetectors was derived. The devices were operated as integrated photodetectors and performance was shown to be in accordance with the theory. Optical responsivity of the devices was measured to be approximately 0.5 A/W. The photodetector response was used to estimate the minority carrier lifetime in the silicon to be approximately 0.1 (mu)sec.