Light induced stress in a-Si1−xGex:H alloys and its correlation with the Staebler–Wronski effect

Photoinduced compressional stress ΔS has been studied in hydrogenated amorphous silicon and silicon–germanium alloys, a-Si1−xGex:H with x=0, 0.4 and 0.67. The films were deposited by plasma enhanced chemical vapor deposition onto 4 μm thick crystalline silicon microcantilevers commonly used in scanning probe microscopy. The initial stress S0 of the films was obtained from the initial bending and the Youngʹs modulus from the cantilever resonance frequency. The kinetics of ΔS(t) follow a stretched exponential. ΔS(t) cannot be a consequence of photoinduced defect creation because ΔS continues to rise when defect creation has saturated and the largest ΔS corresponds to a relative volume change ΔV/V=1×10−3, too large for 4×1017 cm−3 defects. We observe a significant decrease in ΔS between x=0.4 and x=0.67 alloy composition just where defect creation is greatly diminished. We suggest that defect creation is associated with the time dependent large local strains in and around the volume elements of electron–hole recombination. ΔS is the time and spatial average of the local configuration changes.