Long-Lived Hot Carriers in III–V Nanowires

Abstract

Heat management mechanisms play a pivotal role in driving the design of nanowire (NW)-based devices. In particular, the rate at which charge carriers cool down after an external excitation is crucial for the efficiency of solar cells, lasers, and high-speed transistors. Here, we investigate the thermalization properties of photogenerated carriers by continuous-wave (cw) photoluminescence (PL) in InP and GaAs NWs. A quantitative analysis of the PL spectra recorded up to 310 K shows that carriers can thermalize at a temperature much higher than that of the lattice. We find that the mismatch between carrier and lattice temperature, ΔT, increases exponentially with lattice temperature and depends inversely on the NW diameter. ΔT is instead independent of other NW characteristics, such as crystal structure (wurtzite vs zincblende), chemical composition (InP vs GaAs), shape (tapered vs columnar NWs), and growth method (vapor-liquid-solid vs selective-area growth). Remarkably, carrier temperatures as high as 500 K are reached at the lattice temperature of 310 K in NWs with ∼70 nm diameter. While a population of nonequilibrium carriers, usually referred to as "hot carriers", is routinely generated by high-power laser pulses and detected by ultrafast spectroscopy, it is quite remarkable that it can be observed in cw PL measurements, when a steady-state population of carriers is established. Time-resolved PL measurements show that even in the thinnest NWs carriers have enough time (∼1 ns) after photoexcitation to interact with phonons and thus to release their excess energy. Nevertheless, the inability of carriers to reach a full thermal equilibrium with the lattice points to inhibited phonon emission primarily caused by the large surface-to-volume ratio of small diameter NWs.

Impact of Outer Shell Structure and Localization Effects on Charge Carrier Dynamics in GaAs/(In,Ga)As Nanowire Core–Shell Quantum Wells

“…The extracted radiative lifetime of free excitons is 1.3 ns, noticeably longer than typical values of 400 ps reported for planar (In,Ga)As/GaAs(001) QWs . This result indicates an increased broadening of excitons in k ‐space possibly due to a high carrier temperature that was observed previously for NWs . The radiative lifetime of the localized exciton is 3.5 ns, longer than lifetimes reported for GaAs/(Al,Ga)As shell quantum dots ranging from 450 ps to 1.7 ns .…”

“…The capture time of 0.5 ns is longer than expected, since capture times in GaAs QWs have been found to be around 250 ps for samples of the highest quality . This finding might be related to the slow relaxation dynamics reported for NWs …”

Exciton dynamics in GaAs/(Al,Ga)As core-shell nanowires with shell quantum dots

“…The origin of such the slow cooling rate of carriers in GaAs nanowires, also reported recently in Ref. 48 for GaAs and InP nanowires, remains to be clarified. Radiative recombination (with a lifetime τ r ), phonon-assisted QD-to-QD tunneling of excitons, and thermally activated transfer of carriers from the shell to the core of the nanowire (with characteristic time τ t and activation energy E A2 ) are shown by arrows (1), (2) and (3), respectively.…”

Radiative Channeling of Nanowire Frenkel Exciton Polaritons through Surface Plasmons

“…[36] A variety of schemes have been explored to interface excitonic systems with surface plasmons such as coupling of quantum dots with plasmonic substrate, [37] layered materials with plasmonic lattice, [38] core-shell nanoparticles, [33] molecular J-aggregates layer with plasmonic nanoparticles, [39,40] and photonic nanowire with gold nanoparticles. [36] These investigations are predominantly confined to probe energy transfer mechanism, recombination processes, [41] photoluminescence enhancement, [42] Rabi splitting, [43] generation of hybrid states, [44] biosensing applications, [45,46] and low-threshold nonlinear effects. [21] In many of these approaches, the dimensionality of the molecular material is either quasi 0D or quasi 2D in nature.…”

“…[49] Such a single, vertical nanowire provides a unique opportunity to study coupling of 1D propagating exciton polaritons with 2D propagating plasmon polaritons at nanoscale at a single point of contact. Such directional outcoupling of emitted light has consequence on designing photovoltaic devices such as single-nanowire solar cells, [41,50,51] organic light emitting devices, [3,23] exciton-polariton lasers, [26] and nanooptical biosensors. Given that spontaneous emission is isotropic, there is an imperative to channel the nanowire emission, such that a majority of the light is collected and harnessed.…”