Influence of a-Si:H deposition power on surface passivation property and thermal stability of a-Si:H/SiNx:H stacks

摘要

The effectiveness of hydrogenated amorphous silicon (a-Si:H) layers for passivating crystalline siliconsurfaces has been well documented in the literature for well over a decade. One limitation of such layers however has arisen from their inability to withstand temperatures much above their deposition temperature without significant degradation. This limitation is of importance particularly with multicrystalline silicon materials where temperatures of at least 400°C are needed for effective hydrogenation of the crystallographic defects such as grain boundaries. To address this limitation, in this work the surface passivation quality and thermal stability of a stack passivating system, combining a layer of intrinsic a-Si:H and a capping layer of silicon nitride (SiNx:H), on p-type crystalline silicon wafers is studied and optimized. In particular the sensitivity of different microwave (MW) power levels for underlying a-Si:H layer deposition are examined. Both effective minority carrier lifetime (ζeff) measurement and Fourier transform infrared (FTIR) spectrometry were employed to study the bonding configurations, passivating quality and thermal stability of the a-Si:H/SiNx:H stacks. It is established that the higher MW power could result in increased as-deposited ζeff and implied Voc (iVoc) values, indicating likely improved surface passivation quality, but that this combination degrades more quickly when exposed to prolonged thermal treatments. The more dihydride-rich film composition corresponding to the higher MW power appears to be beneficial for bond restructuring by hydrogen interchanges when exposed to short term annealing, however it also appears more susceptible to providing channels for hydrogen out-effusion which is the likely cause of the poorer thermal stability for prolonged high temperature exposure compared with stacks with underlying a-Si:H deposited with lower MW power.