Electronic and Optical Properties of Si-Al Contacts Produced by Optical Excitation

In many silicon opto-electronic devices, it is desirable to control both the optical and electrical characteristics of back metal contact. Typically, one desires a contact with low resistance and high reflectance. This is especially important for solar cells in which a high absorption of near bandgap photons can be achieved through multiple reflections. However, conventional processing for lowresistance contacts results in low reflectance. Although there have been several explanations given for the loss of reflectance, we believe it is due to the creation of a thick silicon/metal alloy layer at the interface. The alloy layer acts to couple light from the silicon into the metal (where it is absorbed), resulting in a loss of reflectance. In order to limit this loss, one needs tomaintain an abrupt interface. By using a rapid, low temperature process, one can control the thickness of the alloy layer and, hence, the optical properties of the contact. This paper discusses the influence of the alloy layer thickness on reflectance and describes processes for creating low resistance aluminum contacts to silicon with controlled optical characteristics.