Istituto dei materiali per l'elettronica ed il magnetismo     
Frigeri C., Serényi M., Csik A., Erdélyi Z., Beke D. L., Nasi L. TEM, SEM and AFM study of hydrogenated sputtered Si/Ge multilayers. In: BIAMS 2008 - BIAMS 2008, 9th International Workshop on Beam Injection Assessment of Microstructures in Semiconductors (Toledo (E), 29/06 - 03/07 2008). Abstract, pp. 112 - 112. J. Piqueras, Ed (eds.). Universidad Complutense, Madrid (E), 2008.
The electro-optical properties of amorphous Si/Ge multilayers (MLs) can be much impaired by the presence of dangling bonds which can act as effective deep electronic levels. A way to improve those properties is to hydrogenate the MLs as H passivates the dangling bonds. As the amorphous MLs need to be submitted to heat treatments to form solid state solutions by interdiffusion to prepare devices, it is mandatory to know the stability of H and the MLs behaviour upon annealing. The amorphous Si/Ge MLs studied here were hydrogenated by introducing H at different concentrations into a radio frequency sputtering chamber during the deposition of the MLs. Ar was the sputtering gas. Si and Ge were sputtered from Si and Ge targets so as to obtain MLs of typically 50 couples of Si and Ge layers, each one 3 nm thick. Various heat treatments were applied with temperature varying between 150 and 450 °C. By TEM, SEM and AFM it was seen that heat treatments caused a structural degradation of the MLs consisting in formation of surface bumps and craters originated from zones of layers intermixing where the original layer sequence fully disappeared for annealing temperature and time >150 °C and 22 hours, respectively. The size and density of the intermixing zones and of the bumps and craters increased with increasing annealing time and temperature and, for a given annealing condition, with H concentration. Not-hydrogenated samples did not show any degradation at all. The above results suggest a close correlation between the presence of H in the Si/Ge MLs and their structural degradation upon annealing according to the following mechanism. First, the H is released from the Si and Ge atoms because of the energy supplied by the heat treatment. Very likely H is mostly liberated from the Ge layers due to the lower binding energy of the Ge-H bond with respect to the Si-H one. Next, the H atoms gather together forming bubbles in the MLs that eventually give rise to surface bumps or even blow up with formation of craters with increasing inside pressure.
Subject Amorphous multilayers

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