Tuning of ion bombardment in plasma processes for the fabrication of crystalline silicon solar cells

Abstract

Crystalline silicon solar cells with the highest energy conversion use the heterojunction concept: very thin amorphous silicon layers on the thick crystalline wafer. The thesis elucidates the role of low energy ion bombardment during low temperature plasma enhanced vapor deposition (PECVD) of an intrinsic hydrogenated amorphous silicon (i-a-Si:H) layer. The i-a-Si:H layer is used to passivate the crystalline silicon (c-Si) surface in the silicon heterojunction (SHJ) solar cell. Excellent i-a-Si:H passivation layers were grown by PECVD with simultaneous ion bombardment at energies of 27 eV and with a Si ion flux that forms between 10 and 20% of the Si atom deposition flux. The ion bombardment during PECVD was measured with an advanced energy resolved mass spectrometer. These measurement results were explained with modeling using a self-consistent fluid model of the discharge and a Monte Carlo code. Si ion bombardment with energies below 36 eV, the energy for bulk Si atom displacement, mainly enhances surface diffusion of growth precursors at a substrate temperature of only 130°C. The best i‑a‑Si:H passivating layers are usually grown with plasma conditions close to conditions for epitaxial growth. Small fluctuations in the plasma parameters can thus lead to epitaxial growth. Local epitaxial growth occurred in our study at very low silane (SiH4) concentration (1.7%) in the SiH4-H2 feedstock gas mixture and a slightly raised discharge power. This crystalline growth in the amorphous matrix obstructs the wafer surface passivation. For these conditions the ion bombardment brings a large amount of energy to the surface per deposited silicon atom. This energy flux probably enables equilibrated growth that allows seeded conical crystalline growth inside the amorphous matrix. Moreover, we found that hydrogen ions with energies above 20 eV etch deposited silicon during PECVD, especially during growth at low silane concentration. Another low temperature method to passivate the c-Si surface has been studied: silicon oxide created with a wet chemical solution. The RCA oxide, which is created when a Si wafer is cleaned in a diluted HCl + H2O2 solution, can be modified by treatment with an Ar-H2 VHF PECVD discharge such that hydrogenation results in a well‑passivated c‑Si/SiOx:H interface. This finding supports the broad based effort to replace the i-a-Si:H layer in the SHJ cell with a thin silicon oxide layer. Our findings can be used both in in‑line and batch‑type reactors for commercial mass production.