Laser-Induced Current Transients in Strained-Si Diodes

Laser-induced current transients are measured on uniaxially stressed silicon (Si) N+/P diodes using a high speed measurement system. Controlled external mechanical stress along the (110) direction is applied via a four-point bending jig while the samples are irradiated using a cavity-dumped dye laser with a wavelength of 590 nm. A decrease in the peak current is observed for increasing tensile stress applied to the diode. Unlike tensile stress, compressive stress increases the peak current. Charge collection is observed to decrease with tensile and increase with compressive stress. These results suggest that uniaxial mechanical stress alters the current transients due to strain-induced changes in electron mobility along the (001) direction. The average effective electron mass along the (001) direction increases with tensile and decreases with compressive stress resulting from the splitting of degenerate conduction band valleys and the repopulation of electrons from higher to lower valleys. The Florida object oriented device simulator (FLOODS) is used to explain the mechanism of current transients in unstressed and stressed diodes. FLOODS is also used to predict results for values of applied stress ( ~ 1 GPa) beyond those that can be obtained using the bending jig (~ 240 MPa).