Thermal effusion measurements: Probing hydrogen in surface passivation schemes

Hydrogen is a vital component of surface passivation materials because it can diffuse towards the silicon interface and passivate surface and bulk defects during thermal treatment. To gain more insight into these complex processes, thermal effusion measurements are an innovative and technologically-relevant approach. During these experiments, thin films are heated under high vacuum conditions while the gaseous species released from bulk and surface are detected by a mass spectrometer. The temperature range (T= 200-1000°C) includes the range for activation of the surface passivation (typically, T= 350-500°C), but also the higher temperature range (T >; 700°C) typically used during the metallization processes of solar cells. As case studies, we consider Al₂O₃ and Al₂O₃:Er films. In addition, the effusion of inert gas atoms-implanted in the Al₂O₃ films- is discussed as a sensitive, complementary approach to study gas diffusion during annealing. We demonstrate that effusion measurements reveal significant differences in hydrogen diffusion and hydrogen loss for films with a range of structural properties. These results were consistently correlated with the thermal stability of the surface passivation.