Istituto dei materiali per l'elettronica ed il magnetismo     
Frigeri C., Serényi M., Khánh N. Q., Csik A., Erdélyi Z., Nasi L., Beke D. L., Boyen H. Relationship between structural changes, hydrogen content and annealing in stacks of ultrathin Si/Ge amorphous layers. In: E-MRS Fall Meeting 2010 - E-MRS Fall Meeting 2010, Symposium E (Nanoscaled Si, Ge based materials) (Varsavia (PL), 13-17 Sept. 2010). Abstract, p. 13/9. E.MRS, 2010.
Amorphous Si, Ge and SiGe alloys are often doped with H in order to passivate the dangling bonds. However, H is not stable against light soaking and heat treatments yielding degradation of the electrical-optical properties. We present results on the structural instability, as a function of annealing, caused by H in multilayers (MLs) of alternating 3 nm thick a-Si and a-Ge layers deposited by sputtering. H was added at flow rates of 0.4, 0.8, 1.5, 3 and 6 ml/min. By ERDA it was seen that for flow rates ≥1.5 ml/min the effective H content incorporated in the samples saturates at ∼16 at. %. IR optical absorbance shows that mostly Si and Ge monohydrides form. Annealing was done at 673 K for times of 1 to 10 h. The evolution of the properties of the MLs as a function of annealing and H content was followed by IR optical absorbance, TEM, AFM, ERDA. With increasing annealing time/temperature and H content the surface morphology degrades with formation of bubbles and craters whose size and density increase up to 9 μm and 6.7x105 cm-2 for a H flow rate of 6 ml/min. The signal of Ge-H and Si-H complexes almost completely vanish in the IR absorbance spectra upon annealing indicating that H is released to the lattice. This supports the conclusion that it is the released H that produces the bubbles and the craters when the H bubbles blow up because of a too high internal pressure. ERDA experiments performed on single layers of a-Si and a-Ge, showing a faster H released from a-Si than from a-Ge, and energy filtered TEM (EFTEM) maps, showing larger broadening of the a-Si layers in the ML structure, suggest that upon annealing H is first released from a-Si layers. This is in agreement with published data reporting on the lower binding energy of Si-H with respect to Ge-H in amorphous materials.
Subject amorphous multilayers
Amorphous silicon
Amorphous Ge

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