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 gr owth. Furthermore, it has been established by several previous studies that exposure to intense light at room temperature leads to increase of the com pressive stress in addition to the creation of dangling-bond defects, well known as the Staebler-Wronski effect [D. L. Staebler and C. R. Wronski, App l. Phys. Lett. 31, 292 (1977)]. We present here the results of our investig ation of the intrinsic and photoinduced stress as well as the Young's modul us of a series of samples with different hydrogen content prepared by hot-w ire (HW) chemical-vapor deposition. These film properties have been measure d with high sensitivity through the bending and flexural oscillation of cry stalline silicon microcantilevers upon which we deposited the a-Si:H films. The intrinsic stress decreases with H content in the series. This result c omplements other investigations linking the saturated defect density to H c ontent in HW samples and validates the generally observed correlation betwe en stress and device performance. The photoinduced stress shows almost an i dentical initial increase with illumination time in all samples but tends t o saturate to a lower value for the lower H concentration. We discuss our r esults 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 pictu re emerges clearly suggesting that the microstructure of device quality mat erial is not homogeneous. (C) 2001 American Institute of Physics.