Vegard's law relationship between the activation energy for blister formation and the Si concentration in RF co-sputtered a-SixGe1-x layers

The quality of the surface is a key issue of semiconductor structures for device applications like solar cells and infrared detectors. Typical surface defects are blisters [1]. They often form in hydrogenated semiconductors, especially when they undergo some annealing either during growth or after-growth processing or device operation. Blisters are bubbles of H2 inside the semiconductors that have reached so big a size to deform the top surface. In this work we have determined the activation energy for blistering in hydrogenated a-SixGe1-x, 0 <= x <=1. Its knowledge allows to reliably establish the atomic mechanisms at the origin of the blistering. The amorphous hydrogenated thin SixGe1-x layers were deposited on polished silicon by RF co-sputtering. Each sample was submitted to heat treatment up to the temperature where the onset of blistering was observed by change of the surface reflectivity. The structural changes of the surface of the a-SixGe1-x films have been explored systematically by light beam reflection, scanning electron microscopy (SEM) while the H depth distribution was measured by secondary neutral mass spectrometry. Fig. 1 shows typical blisters in the sample with x=0, i.e. a-Ge, annealed at 179 °C. By comparing the blistering activation energy Ea (SixGe1-x) determined by means of Arrhenius plots in several samples with different compositions from x=0 to x=1, it was found that it follows a Vegard's law as a function of the silicon x composition: Ea (SixGe1-x) = xoE2a + (1-x)oE3a (Fig. 2). Here E2a and E3a are the activation energy of Si and Ge, experimentally determined to be 2.42 and 1.68 eV, respectively. Modelling of the reaction kinetics governing the evolution in time of the concentration of H2, d[H2]/dt, by assuming that the formation of H2 is due to the reaction between two H atoms produced by the scission of the SiH and GeH monohydrides, confirmed the validity of the Vegard's-law-like dependence of the activation energy on the x composition. This agreement suggests that the key phenomenon determining the conditions for blistering is the rupture of the monohydrides bonds rather than of the dihydrides ones or of the SiSi and GeGe ones. The energetic reaction leading to the formation of the H2 molecules follows a linear law as a function of the x composition similarly to the activation energy. It should be noticed that the temperature range at which blistering onset was detected, i.e. from 179 °C (for pure a-Ge) to 271 °C (for pure a-Si), concerns temperatures often applied for the a-SixGe1-x growth, e.g. by CVD [2], or during device manufacturing processes and close to the operating temperature of solar cells. Acknowledgements Work supported by the Scientific Cooperation Agreement between CNR (Italy) and MTA (Hungary). References [1] B. Terreault, phys. stat. sol. (a) 201 (2007) 2129-2184 [2] V. Steenhoff, A. Neumüller, O. Sergeev, M. Vehse and C. Agert, Sol. Energy Mater. Sol. Cells 95 (2016) 148-153. Fig. 1 - SEM image of surface blisters taken soon after their formation (at 179 °C) in the hydrogenated a-Ge sample (x=0). One blister is arrowed. Fig. 2 - Blistering activation energy Ea(SixGe1-x) in SixGe1-x as a function of the silicon composition x. The equation of the fitting curve is Ea(SixGe1-x) = 1.663 + 0.733ox