GaAs thin films for solar cells by ion-assisted deposition

A feasibility study of producing thin polycrystalline GaAs-films on neutral substrates in a cost-efficient medium-quality vacuum of around 10^-7 mbar is presented. To improve morphology and crystal structure of these films at the lowest possible temperature, the effectiveness of ion assistance during the process is examined. By adjusting the energy and intensity of the bombardment, well-defined energy can be brought to impact, which will be equivalent to an increased substrate temperature, levitating the actual thermal stress imposed on the substrate. This temperature is referred to as virtual temperature. If successful, this process will allow commercially attractive substrates to be used under deposition conditions corresponding to significantly higher temperatures. The ion-bombardment will continuously sputter-away loosely bound impurity atoms, so that a less ideal and therefore more cost-efficient vacuum in the 10^-7 mbar range might be used. Experimental results demonstrate that GaAs epitaxial and polycrystalline films with good photovoltaic properties can be deposited in a common high vacuum using ion assistance