Four-Junction Solar Cell with 40% Target Efficiency Fabricated by Wafer Bonding and Layer Transfer: Final Technical Report, 1 January 2005 - 31 December 2007 Page: 4 of 17
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Summary:
During the grant period 2005-2008, we accomplished the key milestones of the project,
which were to realize (i) high quality InGaP/GaAs two junction 'top cells' on Ge/Si
templates, (ii) InGaAs/InP 'bottom cells', (iii) direct bond series interconnection of
tandem junction solar cells and (iv) modeling of bonded three and four junction solar cell
device performance. During the grant period, two graduate students (Katsuaki Tanabe
and Melissa Archer) progressed toward their PhD. theses, and both graduated with Ph.D.
degrees in 2008. Results from the project were presented at technical meetings (IEEE
PVSC, WCPEC, MRS) and disseminated in technical publications. Significant aspects of
the project included close industrial collaboration with Spectrolab, Aonex Corporation
and Emcore Photovoltaics.
Technical Accomplishments:
GalnP/GaAs Dual Junction Solar Cells on Ge/Si Epitaxial Templates
The Caltech/Spectrolab team achieved a major milestone by demonstrating large area,
crack-free GaInP/GaAs double junction solar cells grown by metal organic chemical
vapor deposition on Ge/Si templates fabricated using wafer bonding and ion implantation
induced layer transfer. The photovoltaic performance of these devices was comparable to
those grown on bulk epi-ready Ge, demonstrating the feasibility of alternative substrates
fabricated via wafer bonding and layer transfer for growth of active devices on lattice-
mismatched substrates.
One of the key milestones of our wafer bonded 4-junction solar cell
(GaInP/GaAs/InGaAsP/InGaAs with 1.9eV/i.42eV/i.05eV/0.72eV bandgaps) was the
demonstration of layer transfer and wafer bonding to realize GaInP/GaAs dual junction
grown on a GaAs or Ge template suitable for integration InGaAsP/InGaAs grown on an
InP/Si template. For this structure to be viable, we must have ohmic contacts at the
bonded interfaces and good quality epitaxial growth on the bonded templates.
The first step in
fabrication of these
epitaxial templates was
to implant a Ge wafer
with H+ at 180keV and a
dose of 1x101 cm-2
Next, wet chemical
cleaning removed
organic and particulate
contaminants from both 5m
the oxidized Si and Ge
wafers. We employed a Figure 1. Optical micrographs of a full 50mm Ge/Si
Si02 bonding layer for template made with layer transfer and wafer bonding (left),
thermal stability of the and GalnP/GaAs solar cells grown on a Ge/Si template
transferred film. Just (right).
before initiating the1
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Atwater, H. A. Four-Junction Solar Cell with 40% Target Efficiency Fabricated by Wafer Bonding and Layer Transfer: Final Technical Report, 1 January 2005 - 31 December 2007, report, November 1, 2008; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc898332/m1/4/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.