Complete Report - University of New South Wales

Progress towards the second milestone has been good with 96% effi ciency for GaAs devices
demonstrated in the Centre’s fi rst year of operation (Section 5.5 of 2003 report). PL EQE
measurements were carried out on an 800 nm sample of GaAs, sandwiched in a double GaInP
heterojunction structure so that carriers are confi ned in the GaAs. Two measurements were
used: a calibrated PL technique which gave 90% EQE for planar samples and a combined
thermal/PL measurement that gave 92% ± 3% EQE (a reasonable agreement). Further
calibrated PL measurements with the ZnSe dome to couple light out of the sample gave an
increase from 90% to 96% EQE.
Further progress predicted to increase the EQE towards the 98% for GaAs was achieved in
2004, with development of the texturing technique required for the back surface. Modelling in
2004 showed that this would bring the 96% with a ZnSe dome to a 97.5% external quantum
effi ciency – remarkably close to the 98% fi gure calculated to be required for PL cooling.
As the primary aim of this task was to improve insight into the physics and technology of
improved radiative emission, rather than to achieve radiative cooling itself, no further work
has been carried out or is planned in this milestone area.
A.9
Efficiency improvement for a silicon based tandem cell over a single cell baseline.
Signifi cant progress has been made in demonstrating an enhanced band gap material for
the top cell for a Si based tandem (see Section 4.5.2). Both Si/SiO2 quantum-well (QW)
and quantum-dot (QD) structures have been fabricated. These have also been grown in the
multilayer structures that will be required for the superlattice structures of an enhanced
band-gap material.
Photoluminescence (PL) at enhanced energies has been demonstrated. The enhancement
in PL energy is in general agreement with quantum confi nement calculations based on the
dimensions for QW and QD structures observed in TEM (within the approximately 20%
errors in the size measurements). X-ray diffraction and work on the artefacts present in TEM
of such small structures is improving the quantitative validity of these calculations.
A PL energy at 1.7 eV has been observed for 1-2 nm QDs; this being the ideal band gap for
an upper cell material on Si. Further evidence for quantum confi nement in the Si QDs comes
from the greatly enhanced PL intensity indicative of localisation in the QDs.
This work includes initial measurement of the electrical properties, with promising, although
still high, resistivities. It has been demonstrated that these can be reduced to about 103
Ω.cm with hydrogen passivation. Further information on resistivity variation with temperature
has been obtained in 2005 which is giving an indication of the activation mechanisms.
Furthermore, there has been success with the fabrication of the analogous structure of
Si QDs in Si3N 4 , with TEM evidence of nano-crystals by both sputtering and PECVD. The
advantage of the nitride matrix is that the lower barrier height should improve conductivity
for a given QD density, although optical and electrical properties are still being tested. PL
evidence for enhanced energy levels in these Si QDs in Si3N4 matrix has been achieved with
preliminary evidence for higher energies than for similar sized dots in oxide – although this
remains to be confi rmed.
This technique is now starting to be applied to Si QDs in silicon carbide (SiC). SiC having an
even lower barrier height and hence expected higher tunnelling probability between QDs and
hence higher conductivity.
150