Abstracts Book - IMRC 2018

• SA4-O008 Invited Talk
IS HEXAGONAL Si THE NEXT BIG THING IN NANOWIRES?
Riccardo Rurali 1
1 Institut de Ciència de Materials de Barcelona, Theory and Simulation of Materials, Spain.
Crystal structure and interface engineering are acquiring an increasing
importance in nanoscience because of their enormous potential to conceive
new properties and functionalities. In the case of nanowires (NWs)1, the
emergence of new stable polytypes of common semiconductors promises to
have an important impact in materials design. Driven by this promising
evidence, we use first-principles methods to investigate theoretically the
electronic and optical properties of hexagonal–diamond and cubic–diamond Si
NWs as well as their homojunctions 2 . We show that hexagonal–diamond NWs
are characterized by a more pronounced quantum confinement effect than
cubic–diamond NWs. Furthermore, they absorb more light in the visible region
with respect to cubic–diamond ones and, for most of the studied diameters, they
are direct band gap materials. The study of the homojunctions reveals that the
diameter has a crucial effect on the band alignment at the interface. In
particular, at small diameters the band-offset is type-I whereas at
experimentally relevant sizes the offset turns up to be of type-II. These findings
highlight intriguing possibilities to modulate electron and hole separations as
well as electronic and optical properties by simply modifying the crystal phase
and the size of the junction. We also show that a suitable mixing of hexagonal Si
and hexagonal Ge yields a direct bandgap with an optically permitted transition.
Comparison of the calculated radiative lifetimes with typical values of
nonradiative recombination mechanisms indicates that optical emission will be
the dominant recombination mechanism. These findings pave the way to the
development of silicon-based optoelectronic devices, thus far hindered by the
poor light emission efficiency of cubic Si 3 . Additionally, we present results of the
thermal conductivity of hexagonal Si NWs calculated first-principles. We
highlight the differences with the conventional cubic polytype and discussing
possible applications in the field of thermoelectricity 4 .
References
1
R. Rurali, Re. Mod. Phys. 82, 427 (2010) 2 M. Amato, T. Kaewmaraya, A. Zobelli,
M. Palummo, and R. Rurali, Nano Lett. 16 (2016), 5694 3 X. Cartoixà, M. Palummo,
H. I. T. Hauge, E. P. A. M. Bakkers, and R. Rurali, Nano Lett. 17, 4753 (2017) 4 M.