Paper NM+NS+MS+EM-MoA9
An Industrial Solution for Surface Passivation of c-Si using AlO_x Film Deposited by In-line Atmosphere Chemical Vapor Deposition

Monday, October 29, 2012, 4:40 pm, Room 16

Among the different dielectric passivation layers for crystalline silicon (c-Si) solar cells, AlO_x has recently received a great attention due to its excellent chemical and field effect passivation performance for p-type c-Si surface. It offers great promise as a rearside passivation material for passivated emitter and rear cell (PERC) designs. However, up to this point in time, most of the development has been based on laboratory scale deposition systems and methods. Common approaches for synthesizing these passivation layers are thermal or plasma-assisted atomic layer deposition (ALD), whose deposition rates are typically too low (< 10 nm/min) to be compatible with high-volume manufacturing. Other deposition methods like PECVD or spatial separated ALD enable an increase in deposition rate by one order of magnitude (i.e. 100 nm/min). An industrially-compatible deposition technique with low processing cost, easy-handling, compact size, and high throughput that still retains comparable passivation performance to ALD films remains a challenging task.

Using an in-line atmosphere chemical vapor deposition (APCVD) tool, we have synthesized amorphous AlO_x films from precursors of trimethylaluminum and O₂, yielding a maximal deposition rate of up to 150 nm/min per wafer. Deposition rate is determined by the film thickness divided by wafer transportation time through the CVD injector. Both top view and the cross-sectional SEM images present an intact AlO_x/Si interface. A smooth surface is shown without any outgassing (blistering) after deposition and a subsequent firing step. The as-deposited layers exhibit an over stoichiometric O/Al ratio of 1.65~1.75 due to the incorporation of an OH group inside the layer. For both high and low doped p-type c-Si wafers deposited with APCVD AlO_x, excellent surface passivation is achieved with a maximum effective surface recombination velocities (S_eff.max) of 8 cm/s following by a firing step. These findings are attributed to the buildup of a large negative charge (Q_f ≈ -3 × 10¹² cm^-2) and low interface defect density (D_it ≈ 4 x 10¹¹ eV^-1cm^-2) following the firing process. It is believed that the incorporated OH group plays an essential role during the firing step. During the annealing/firing step, a certain degree of dehydration takes place (i.e. Al sites bonded OH termination start to bond via an O bridge), which may involve an octahedral to tetrahedral coordination change. This could facilitate the negative charge formation and release of atomic H for passivating the Si dangling bonds at the AlO_x/Si interface.

This data implies a high application potential of APCVD AlO_x for low cost industrial solar cell applications.