Long-lived Hot Carriers in III-V Nanowires

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 photo-generated 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, T, increases exponentially with
lattice temperature and depends inversely on the NW diameter. T is instead independent of other2
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 non-equilibrium 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 photo-excitation 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.