Elastic properties, intrinsic and photoinduced stress in hydrogenated amorphous-silicon thin films with different hydrogen content

All device-quality hydrogenated amorphous-silicon (a-Si:H) thin films on a substrate exhibit intrinsic compressive stress, induced primarily during growth. Furthermore, it has been established by several previous studies that exposure to intense light at room temperature leads to increase of the compressive stress in addition to the creation of dangling-bond defects, well known as the Staebler–Wronski effect [D. L. Staebler and C. R. Wronski, Appl. Phys. Lett. 31, 292 (1977)]. We present here the results of our investigation of the intrinsic and photoinduced stress as well as the Young’s modulus of a series of samples with different hydrogen content prepared by hot-wire (HW) chemical-vapor deposition. These film properties have been measured with high sensitivity through the bending and flexural oscillation of crystalline silicon microcantilevers upon which we deposited the a-Si:H films. The intrinsic stress decreases with H content in the series. This result complements other investigations linking the saturated defect density to H content in HW samples and validates the generally observed correlation between stress and device performance. The photoinduced stress shows almost an identical initial increase with illumination time in all samples but tends to saturate to a lower value for the lower H concentration. We discuss our results on the variation of the Young’s modulus with H content together with those of a previous study by a different measurement technique and sample growth method and compare both to theoretical calculations. A unified picture emerges clearly suggesting that the microstructure of device quality material is not homogeneous.