Growth studies of Ge-islands for enhanced performance of thin film solar cells

We have studied the self organized growth of Ge-islands on Si-substrates in the Stranski–Krastanow growth mode with Si MBE for use in thin film solar cells. For samples with high Ge concentrations and high crystal quality, the increased infrared absorption of the SiGe-islands incorporated in the base material of a Si solar cell should overcome the reduced open circuit voltage due to the lower band gap. Series with growth temperatures from 500–700 °C and different Ge-coverage have been grown to optimize Ge island/dome formation and crystal quality of single and multiple island layers. Photoluminescence measurements, atomic force and transmission electron microscopy exhibit optimized three dimensional growth at temperatures ≈650 °C and eight mono layers Ge coverage. In addition, the use of Sb as surfactant during Ge-island growth was studied intensively. Highly ordered Ge-islands with lateral densities up to 1·1011 cm−2 were grown at temperatures ≈700 °C.