A calibration method for accurate prediction of amorphous to nanocrystalline transition from line intensities of optical emission spectrum

To be able to use the simple technique of optical emission spectroscopy (OES) for the prediction of the transition of growth from a-Si to nc-Si via the Hα/Si⁎ emission ratio, a regime-dependent correction factor is required to relate the measured Hα/Si⁎ emission ratio to the true flux (to the substrate) ratio of atomic hydrogen to deposited silicon radicals. Through an in-depth study in a very high frequency plasma enhanced chemical vapor deposition process, we obtained that the flux ratio of atomic hydrogen and deposited silicon radicals to the growing surface,ΓH/ΓSi, is related to the emission ratio of Hα and Si⁎, IradHα/IradSi*, by the relation, R rad I rad H α I rad Si * / Γ H Γ Si = a ( p d ) 2 / k T gas + b , where the parameters p (pressure), d (inter-electrode distance) and Tgas (gas temperature) are experimentally obtained quantities and Rrad is the ratio of the rate coefficients for radiation of Si⁎ and Hα. We obtained the calibration parameters a and b to be 1.9·10− 21 ± 2·10− 22 Pa m− 1 and 5.5 ± 1.9 respectively which is valid in a broad range of power and pressure settings. With these parameters, it is easy to estimate the flux ratio of atomic hydrogen and silicon species at any deposition condition using the OES data and this will allow accurate prediction of the phase transition. According to simulations in the linear low-pressure regime, the amorphous to nanocrystalline phase transformation occurs at the flux ratio ΓH/ΓSi = 12, which translates, using the factors a and b, to the required emission ratio

Ultrafast spectroscopy of localised vibrational modes in amorphous silicon using a free electron laser

Ultrafast vibrational relaxation of localised modes associated with hydrogen, deuterium and oxygen in amorphous silicon based alloys are discussed. The availability of internal defect vibrations to act as accepting modes is discovered to be a crucial factor in the population decay dynamics. Phase relaxation is shown to have both a temperature dependent component due to elastic phonon scattering as well as an excitation dependence contributed by non-equilibrium phonons. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim