Roughening transition in nanoporous hydrogenated amorphous germanium: Roughness correlation to film stress

Hydrogenated amorphous germanium ( a - Ge : H ) is a material of interest for optoelectronic applications such as solar cells and radiation detectors because of the material's potential to extend the wavelength sensitivity of hydrogenated amorphous silicon ( a - Si : H ) . An increase in porosity is observed in amorphous germanium compared to a - Si : H , and this increase in porosity has been correlated with a degradation of the electrical performance. Improved understanding of the mechanisms of porous formation in a - Ge : H films is therefore desirable in order to better control it. In this paper we describe a correlation between film stress and surface roughness, which evolves with increasing thickness of a - Ge : H . A roughening transition from planar two-dimensional growth to three-dimensional growth at a critical thickness less than 800 Å results in a network of needlelike nanotrench cavities which stretch from the transition thickness to the surface in films up to 4000 Å thick. Surface roughness measurements by atomic force microscope and transmission electron microscopy indicate that the transition is abrupt and that the roughness increases linearly after the transition thickness. The roughening transition thickness is, furthermore, found to correlate with the maxima of the integrated compressive stress. The compressive stress is reduced after this transition thickness due to the incorporation of nanovoids into the film that introduce tensile stress as the islands coalesce together. The roughening transition behavior is similar to that found in a general class of Volmer-Weber mode thin film deposition (e.g., Cu, Ag, and nonhydrogenated amorphous silicon), which offers additional insight into the underlying mechanisms of the stress and roughening in these a - Ge : H films. The suppression of the roughening transition by changing the kinetics of the deposition rates (e.g., slowing the deposition rate with a weak sputtering bias) is also observed and discussed.