Activity Report 2010 - CNRS

SCIENTIFIC REPORT
Exploring new concepts for
photovoltaics with II-VI
heterostructures
Chair of Excellence 2009: Yong ZHANG
Coordinator: Henri MARIETTE (Institut
Néel)
Thanks to their appropriate range of
energy gaps, II-VI materials are already
intensively used in solar cells.
This novel project explores new concepts
which are likely to improve the efficiency
of II-VI solar cells and/or reduce their
cost. Among the most promising ones
figure type II band configurations, which
favour the separation of photogenerated
electron – hole pairs, and the nanowire
geometry, which can be used to
enhance light absorption and improve
current collection.
Promising first results have already been
obtained along both lines. Noticeably,
test planar CdSe/ZnTe heterostructures,
grown by molecular beam epitaxy,
display a three orders of magnitude
enhancement of the lifetime of
photogenerated carriers thanks to their
type II band configuration.
Novel type II core-shell nanowire
heterostructures have been synthesized
using either molecular beam epitaxy or
“low cost” techniques to coat ZnO
nanowires by a thin layer of CdSe or
CdTe (see Figure 2).
OPTICAL
MICROCAVITIES
High Q silica microtoroids
and microspheres
“New comers” Project 2007: Julien
CLAUDON (INAC/SP2M)
PhD student: Nitin Singh MALIK
Among dielectric microcavities, silica
microspheres and microtoroids display
remarkably high quality factors. This
project aims at exploring the physics of
hybrid systems formed by a microtoroid
coupled to one or few self-assembled
QDs.
Within this project, high Q (Q>10 7 )
microtoroids and microspheres have been
fabricated on a Si chip, through a CO 2
laser-assisted melting of the silica. The
assembly of microtoroids to tiny GaAs
mesa structures containing InAs QDs has
been realized for the first time in the
RTRA dual beam FIB system, following a
cut, pick and place procedure (see Figure
3).
µ-toroid
Mesa
Fig. 3: Scanning electron micrograph of a silica
microtoroid coupled to InAs self-assembled
QDs in a GaAs mesa structure.
Fig. 2: Scanning electron micrograph of an
array of ZnO nanowires after conformal
coating by CdTe (typical diameter: 200 nm)
The modes of the coupled system show
up on the emission spectra, with Q’s in
the few 10 3 range, in agreement with
calculated values for a mesa-toroid
system in contact. Modelling shows that
the introduction of a small air gap should
restore a higher quality factor (Q>10 5 ),
enabling a laser operation and possibly
the vacuum Rabi-flopping of a single QD.
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