GaAs solar cells on a silicon substrate have advantages over those grown on a germanium substrate. Si has lower mass, higher mechanical strength and better thermal properties than Ge. Good thermal conductivity is required to dissipate heat in concentrator solar cells, while weight is of prime concern for satellites. State-of-the-art GaAs space solar cells have efficiencies of 19-27% while silicon space solar cells have an efficiency of 17%. For terrestrial applications, state-of-the-art commercial solar cells from crystalline Si have efficiencies of 12-15% and Si research cells attain efficiencies of 20%. GaAs cells are not used in terrestrial applications due to their high cost. Reducing this cost by employing Si-based virtual substrates (VSO) makes GaAs solar cells cost effective for terrestrial applications. Reduced weight, good thermal properties and radiation tolerance make them applicable for satellite power sourcing. In the report a virtual substrate consisting of a Ge layer grown directly on Si without an intervening SiGe graded layer is characterized. The nominally 100% Ge overlayer is fully relaxed and contains a small amount of unintentional Si. A dislocation density of 10exp 8/cm exp2 is estimated for the virtual substrate prior to GaAs epitaxial growth, which is reduced by a factor of 100 after the growth of GaAs. On this novel virtual substrate 1 cm exp2 single-junction GaAs photovoltaic cells were realized with an efficiency of 11.7% under AMO compared with 20.2% for cells grown on a crystalline Ge substrate. Due to the high dislocation density a 50-fold higher dark current is measured in the virtual substrate cells compared to the crystalline Ge cells, leading to a lower short circuit current and open-circuit voltage of the cells fabricated on the virtual substrates. The post-GaAs growth dislocation density is estimated as 1 x 10 exp 7/cm exp2 in the base region and 4 x 10exp5/cm exp2 in the emitter region based on modelling and measurements.


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    Title :

    Characterization of Ge-on-Si virtual substrates and single junction GaAs solar cells


    Contributors:
    Ginige, R. (author) / Corbett, B. (author) / Modreanu, M. (author) / Barrett, C. (author) / Hilgarth, J. (author) / Isella, G. (author) / Chrastina, D. (author) / Känel, H. von (author)

    Published in:

    Publication date :

    2006


    Size :

    6 Seiten, 6 Bilder, 1 Tabelle, 27 Quellen




    Type of media :

    Article (Journal)


    Type of material :

    Print


    Language :

    English