Influence of a Thermally Grown SiO2 Interface on the Passivation Quality of PECVD AlOx-SiNx Passivation Layers for PERC Solar Cells

Aluminum oxide (AlOX) serves as high quality passivation layer for p-doped Silicon surfaces and is in combination with silicon nitride (SiNX) widely introduced for rear passivated cells (e.g. PERC). During typical processing sequences as annealing or firing, between the deposited AlOX and the silicon surface a thin intermediate silicon oxide (SiOX) is intrinsically growing. In this paper we analyzed the passivation mechanism of inline PECVD AlOX/SiNX stacks by independently varying the thickness of an intermediate SiO2. The changing of passivation mechanism was investigated using techniques as QSSPC lifetime, Fourier transform infrared spectroscopy (FTIR) and capacitance-voltage measurements (CV). It was found that the best passivation quality can be achieved, either using the native oxide grown in air or very thin oxides less than 4 nm thick. Also, a very high surface passivation quality is achieved using a thin (~ 1.6 nm) high-quality chemical oxide. In most cases an annealing or firing step helps to activate the passivation stack by forming negative charges and accordingly leading to field effect passivation. Additionally, hydrogen passivation induced by the PECVD deposited -rich AlOX and SiNX layers leads to an effective defect passivation of interface defects as dangling bonds.