VHF plasma deposition for thin‐film solar cells

Abstract

AbstractFast deposition rates, typical for very high frequency (VHF) plasma deposition, up to now have mainly been attributed to a more efficient silane decomposition as the results of an enhanced density of high‐energy electrons in the plasma.In this work a‐Si:H was prepared at excitation frequencies in a wide frequency range between 25 and 250 MHz and at otherwise constant conditions. In order to understand the processes leading to the observed increase in deposition rate with frequency, the plasma was investigated by optical emission spectroscopy and mass spectrometry. Plasma‐substrate interactions were studied by impedance analysis and ion flux measurements. the results show that the high deposition rates are mainly the result of an increased surface reactivity of film precursors resulting from the ions impinging on the growth surface. It is shown that variation of the excitation frequency allows a flexible control of ion flux and energies. Conditions were optimized for the preparation of a‐Ge:H films, which require a flux of high‐energy ions to the substrate. Material properties were obtained that were comparable to results from deposition on the cathode of a radio frequency (RF) discharge.