Terahertz spectroscopy of semiconductor nanowires for device applications

S
emiconductor nanowires are of great interest as active components in numerous optoelectronic
devices. Therefore, accurate characterisation and control of the nanowire transport
properties is of paramount importance for the realisation of nanowire-based devices. With
this aim in mind, this thesis presents THz spectroscopy as an ideal, non-contact technique for
probing the nanowire electrical conductivity and carrier dynamics, with particular focus on the
effect of doping and crystal structure on key device parameters, such as carrier mobilities and
lifetimes.
Firstly, the effect of ‘bulk’ n-type and p-type shell doping is investigated in GaAs nanowires.
For the first time using an optical pump terahertz probe technique, high extrinsic carrier concentrations
on the order of 1018 cm−3 are extracted for these doped nanowires. An increase in carrier
lifetime is demonstrated as a direct result of doping-induced bandbending, highlighting controlled
doping as a method for reducing parasitic surface recombination in optoelectronic nanowire-based
devices. This result is particularly promising for the development of nanowire solar cells and
nanowire lasers, where long carrier lifetimes are required. However, this ‘bulk’ shell doping
technique is synonymous with a reduction in the carrier mobility within the nanowire by over
an order of magnitude in comparison to an undoped reference, as a direct result of increased
impurity scattering due to doping.
As a solution to this inherent reduction in electron mobility associated with ‘bulk’ doping,
modulation doping in GaAs/AlGaAs core-shell nanowires is presented. Enhanced carrier lifetimes
are again observed, as dopant electrons passivate trap states at the core-shell interface. Yet, for
this doping technique, a lower extrinsic carrier concentration of 1016 cm−3
is extracted. More
importantly, a minimal reduction in the electron mobility is observed compared to an undoped reference
sample. By physically separating the donor ions from the photoexcited electrons, impurity
scattering is reduced and a high electron mobility maintained. Temperature-dependent terahertz
and photoluminescence measurements confirm that the dominant scattering mechanism affecting
the electron mobility in these modulation doped nanowires is longitudinal optical phonon
scattering, with impurity scattering reduced in comparison to an undoped reference. From these
measurements, the dopant activation energy in these nanowires is extracted for the first time via
the terahertz spectroscopy, coinciding with literature values for the donors in bulk AlGaAs. An
increase in carrier lifetime and radiative efficiency was observed with increasing temperature
above the dopant ionisation temperature. This demonstrates the suppression of non-radiative
recombination routes in these nanowires, as dopants act to passivate trap states at the core-shell
interface, making modulation doped nanowires promising candidates for use in nanowire-based
optoelectronic devices.
Secondly, the effect of crystal structure in InAsSb nanowires is investigated. Antimony
incorporation in InAs nanowires is presented as a method for achieving catalyst-free growth of
quasi-pure phase nanowires, where the transport properties of the nanowire are unaffected by
i
defects in the nanowire crystal structure. Utilising an optical pump terahertz-probe technique,
an increase in carrier lifetime with increasing antimony content is demonstrated for the first
time, which directly correlates with a reduction in defect density due to antimony incorporation.
The electron mobilities are also extracted and an increase in mobility with increasing antimony
content is observed. This is a direct result of the reduced electron effective mass at higher
antimony concentrations, as well as the reduction of interface and defect scattering, associated
with decreased defect density at high antimony concentrations. As interface and defect scattering
dominates at low temperatures, further enhancement of the electron mobility is expected at low
temperatures.
Finally, from the knowledge gained from these studies of the nanowire carrier dynamics,
two applications of III-V nanowires in terahertz devices are explored. Single-nanowire terahertz
detectors based on InP nanowires are demonstrated, with a broad detection bandwidth of up to
2 THz and signal to noise ratio of 40, comparable to bulk InP terahertz receivers. An ultrafast
terahertz polarisation modulator based on GaAs nanowires is also demonstrated for the first time
with picosecond optical switching speeds, a high extinction ratio of 18%, modulation depth of
-8 dB and dynamic range of -9 dB. The performance of these nanowire-based terahertz modulators
are comparable to graphene-based terahertz modulators and far surpasses those based on carbonnanotubes,
providing a nanoscale platform for ultrafast THz wireless communication.