The epitaxial lift-off (ELO) technique can be used to separate a III-V solar cell structure from its underlying GaAs or Ge substrate. ELO from 4-inch Ge wafers is shown and 2-inch GaAs wafer reuse after lift-off is demonstrated without degradation in performance of the subsequent thin-film GaAs solar cells that were retrieved from it. Since a basic wet chemical smoothing etch procedure appeared insufficient to remove all the surface contamination, wafer re-preparation is done by a chemo-mechanical polishing procedure.
The present work describes the study and improvement of the Epitaxial Lift-Off (ELO) technique, which is used to separate III/V device structures from their GaAs substrates. As a result the ELO method, initially able to separate millimetre sized GaAs layers with a lateral etch rate of about 0.3 mm/h, has been developed to a process capable to free entire 2″ epitaxial structures from their substrates with etch rates up to 30 mm/h. It is shown that with the right deposition and ELO strategy, the thin-film III/V structures can be adequately processed on both sides. In this way semi-transparent, bifacial solar cells on glass were produced with a total area efficiency in excess of 20% upon front side illumination and more than 15% upon back side illumination. The cell characteristics indicate that, once the thin film processing has been optimized, ELO cells require a significantly thinner base layer than regular III/V cells on a GaAs substrate and at the same time have the potential to reach a higher efficiency.
Centimeter sized, crack-free single crystal InGaP films of 1 μm thickness were released from GaAs substrates by a weight-induced epitaxial lift-off process. At room temperature, the lateral etch rate of the process as a function of the applied Al0.85Ga0.15As release layer thickness was found to have a maximum of 3 mm/h at 3 nm. Using 5-nm-thick AlAs release layers, the etch rate increased exponentially with temperature up to 11.2 mm/h at 80 °C. Correlation of the experimental data with the established theoretical description of the process indicate that the model is qualitatively correct but fails to predict the etch rates quantitatively by orders of magnitude.
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