Studies of Ga diffusion and the elimination of pinholes in Ga-doped β-FeSi2 films prepared by MBE

Our previous studies on the Ga doping of β-FeSi2 films proved that Ga atoms could be an effective p-type dopant by using the molecular beam epitaxy (MBE) method. However, a crucial problem was the generation of a great number of pinholes when a high concentration of Ga atoms was introduced into the β-FeSi2 films. In this report, we perform a qualitative investigation into Ga diffusion in β-FeSi2 layers with secondary ion mass spectroscopy (SIMS) and Auger electron spectroscopy (AES). We found that Ga atoms diffuse much faster in the β-FeSi2 layers than in Si, and they tend to congregate on the front surface of β-FeSi2 films during the post thermal annealing at 800 °C. However, the Ga is supposed to evaporate from the surface. Based on these results, we assumed that the formation of pinholes may be closely related to the Ga diffusion and evaporation in β-FeSi2 layers during the annealing process. In order to suppress the evaporation of Ga, we deposited an SiO2 capping layer (∼500 nm in thickness) terminating on Ga-doped Fe/Si multilayers prior to the annealing. Preliminary results demonstrated that pinhole density was reduced by the SiO2 capping layer.