The Role of the Species Formed in Pecvd Systems on the Density of States of a-Si:H Films

Abstract

Here the species detected by a quadropole mass analyser and resulting from the decomposition of SiH₄ induced by a plasma [using a diode-like PECVD (Plasma Enhanced Chemical Vapour Deposition) system, in which has been varied the following experimental conditions: temperature of the substrate (T_s); deposition pressure (pd); power density (dp) and electrode geometries], have been correlated with the electro-optical properties and Density of States (DOS), inferred from Constant Photocurrent Measurements (CPM). The mass spectra analysis reveal the existence of atomic and molecular hydrogen as well as on the SiH₃ and SiH₂ species whose ratios are strongly dependent on electrode geometries used, that also roles the adequated deposition conditions for producing good quality materials. Thus, for the recessed electrode we observe an enhancement on plasma density at low self bias voltage (V_dc), leading to the production of high quality films at Ts = 180 °C, using moderate dp and pd in the range of 75–80 mtorr, with V_dc ≃ 8 V. On the other hand, for the grid electrode we observe an enhancement in molecular hydrogen at growth surface for high T_s, leading to the production of the best films for Ts in the range of 280–300 °C, using dp ≃ 25 mWcm⁻³, pd ≃ 100 mtorr and V_dc ≃ 30 V. For both electrodes we observe that high bombardment is important for getting microstructures, defect free and dense. The CPM data show that DOS increases with pd indicating that the enhancement on the number of collisions in the gas phase lead to the increase on chain polymerization formation. We also observe that the hydrogen coverage onto the growth surface is important for activating the growth mechanism, producing homogeneous material with a low DOS.