Tuning the refractive index of transparent conducting oxides via oxide/oxide periodic
heterostructures
David Caffrey, Emma Norton, Cormac O'Coileain, Leo Farrell, Brendan Bulfin, Christopher
M. Smith, Igor V. Shvets and Karsten Fleischer
School of Physics and Centre for Research on Adaptive Nanostructures and Nanodevices
(CRANN), Trinity College, The University of Dublin, Dublin 2, Ireland
Superlattice structures are a novel method of improving upon the optoelectronic properties of
Transparent Conducting Oxide (TCO) structures. The invariability of the refractive index of
TCO materials leads to reflection losses at the interfaces of transparent devices such as solar
cells. The development of a transparent material or structure of tuneable refractive index
would allow for the integration of anti-reflective coatings which would reduce such losses
significantly, thus improving device efficiency. Previous attempts to modify the refractive
index have been marred by the degradation of the electrical or optical properties of the tuned
material. We demonstrate the novel use of a TCO/dielectric superlattice structure to achieve
an effective medium of altered refractive index, while maintaining high values of
transparency, conductivity and mobility. We demonstrate the efficacy of these superlattice
structures on both amorphous InGaZnO
4
and ZnO:Al via TCO/SiO
2
superlattices. The
effective refractive indices of the films were successfully tuned over a range of Δn≈0.2 with a
decrease in conductivity of less than an order of magnitude. Mobility of the films was also
well conserved a change of n≈0.2 resulting in a variation from 16.5cm
2
/Vs to 6cm
2
/Vs for
the amorphous InGaZnO
4
/SiO
2
superlattices,
while the ZnO:Al/SiO
2
superlattices varied from
7.3cm
2
/Vs to 1cm
2
/Vs. We will discuss the flexibility of our approach, allowing not only for
a
reduction
of refractive index by using low refractive index interlayers (SiO
2
, MgO) but also
an increase in n by using high refractive index interlayers.
Cross-sectional SEM of of TCO/SiO
2
superlattices Cross sectional image of (a) 10/7.5nm
a-IGZO/SiO2 (b) 10/5nm ZnO:Al/SiO
2
superlattices on glass substrate. (a) was obtained
using a backscatter electron detector and (b) with a secondary electron detector. Inset of (b)
has been post processed to maximise contrast of the layered structure.
PS1 28
-184-
