Plasma deposition of thin film silicon at low substrate temperature and at high growth rate

Abstract

To expand the range of applications for thin film solar cells incorporating hydrogenated amorphous silicon (a-Si:H) and hydrogenated nanocrystalline silicon (nc-Si:H), the growth rate has to be increased 0.5 or less to several nm/s and the substrate temperature should be lowered to around 100 C. In this thesis, key issues towards this goal are addressed, for films deposited using Very High Frequency Plasma Enhanced Chemical Vapour Deposition (VHF PEVCD). The commonly encountered decrease of the performance of a-Si:H solar cells with decreasing substrate temperature is often attributed to surface chemistry changes during growth. However, in this thesis it is demonstrated that the gas phase chemistry depends on the substrate temperature as well. It was experimentally found that the ion energy flux towards the substrate decreased when the substrate temperature was reduced. The ion energy flux corresponding to optimal substrate temperatures can be restored during low substrate temperature deposition by increasing the hydrogen dilution. Using plasma impedance measurements, it was confirmed that powder formation is increased at lower substrate temperature. The transition from a-Si:H growth to nc-Si:H growth, which is determined by the relative fluxes of atomic hydrogen and silicon species, can be predicted using the optical emission associated with the origin of these fluxes. At higher power and pressure, the fluxes are also influenced by gas phase reactions. Moreover, the emission rate constants depend on the electron temperature. Using data from a 1 D computer simulation, a correction factor is derived that is purely based on measurable quantities, and that considerably extends the range of deposition conditions for which the phase transition can be monitored using optical emission from the plasma. In a high pressure, high power regime, where source gas usage is high, the phase composition has to be manipulated via the degree of depletion rather than via the dilution with hydrogen. Because the growth rate is very high, the stabilisation time associated with back diffusion of silane affects a large section of the growing film, causing a thick incubation layer. This problem is solved using an initially hydrogen filled reactor in combination with a few seconds of hydrogen plasma prior to the actual deposition. Both a-Si:H and nc-Si:H films show device-quality photosensitivity and activation energy values. The as-deposited conversion efficiencies of solar cells deposited at high deposition rate with a-Si:H (3.2 nm/s) and nc-Si:H (4.5 nm/s) absorber layers were both 6.4%. The conversion efficiency of the a-Si:H solar cell showed a light-induced degradation of 20%, whereas the nc-Si:H cell was stable. The defect density in the high deposition rate nc-Si:H cell was an order of magnitude larger compared to performance-optimized nc-Si:H material. Proton irradiation of the nc-Si:H solar cell revealed that radiation resistance was comparable to that of a-Si:H cells from literature with comparable thickness. The results presented in this thesis can be applied in industrial, in-line systems for the production of high deposition rate nanocrystalline silicon solar cells with enhanced homogeneity and for the quality improvement of solar cells deposited at low substrate temperatures on less expensive substrates, thus reducing production costs.