engleski

Metastable amorphous structure in ion implanted GaAs

Disorder is introduced into GaAs by implantation of 30 Si+ ions and studied by Raman scattering, RS. A very wide range of ion doses (8x10E12 – 3x10E16/cm2), dose rates, and implant temperatures was used. RS spectra were deconvoluted in a novel way so that the apparent "background" signal was also included in the analysis. New arguments are given that: a) this "background" is in fact a low frequency peak and hence a meaningful signal, equivalent to the analogous "boson peak", BP, regularly observed in glassy solids, and b) represents a specific amorphous phase, different than a well known continuous – random – network amorphous phase, a- CRN. A systematic and self-consistent behaviour of the BP signal is observed in all studied implantation conditions. BP is the only component of RS spectra whose relative intensity was always proportional to the total disorder, as evaluated by RBS, regardless as to whether higher disorder is produced by higher dose, or higher rate or lower implant temperature. BP increases monotonically with the total disorder at the expense of fractons of other crystalline and a- CRN components. Particularly, for already completely amorphized layer, after all crystalline-phase-related peaks had disappeared ; further implantation converts the a-CRN into a- BP. Relative fractions of these two amorphous components can be changed in a predictable way by varying implantation parameters: dose, rate and implant temperature. These results are consistent with the physical picture in which the BP signal in a-GaAs has been interpreted within the fractal model. The amorphous phase in GaAs is then envisioned as a two-phase composite of strained nanometer-size regions within relaxed continuous random network. The excess vibrational states in RS and resulting "boson peak" originate from the strained bonds within these "blobs" and at the interface of these two regions. Analysis of temperature dependence revealed that for any selected dose and rate a-BP component in much more sensitive to implantation temperature, Timpl, than a-CRN. For example, for the dose 3x10E16/cm2 the a-BP/a-CRN intensity ratio is 400% lower for Timpl=40 deg C than for Timpl= -2 deg C. Hence, a-BP structure is thermally less stable and can be dramatically decreased by appropriate selection of implantation parameters. On the other hand, a-BP relative fraction can be considerable increased by additional implantation, i.e. by additional impact-energy transfer to the layer. a-BP can thus be envisioned as metastable structure of amorphous phase having higher free energy than a regular a-CRN.

GaAs; implantation; amorphous structures; order/disorder; boson peak

nije evidentirano