Electron Mobilities Approaching Bulk Limits in “Surface-Free” GaAs Nanowires

Abstract

Achieving bulk-like charge carrier mobilities in semiconductor nanowires is a major challenge facing the development of nanowire-based electronic devices. Here we demonstrate that engineering the GaAs nanowire surface by overcoating with optimized AlGaAs shells is an effective means of obtaining exceptionally high carrier mobilities and lifetimes. We performed measurements of GaAs/AlGaAs core-shell nanowires using optical pump-terahertz probe spectroscopy: a noncontact and accurate probe of carrier transport on ultrafast time scales. The carrier lifetimes and mobilities both improved significantly with increasing AlGaAs shell thickness. Remarkably, optimized GaAs/AlGaAs core-shell nanowires exhibited electron mobilities up to 3000 cm(2) V(-1) s(-1), reaching over 65% of the electron mobility typical of high quality undoped bulk GaAs at equivalent photoexcited carrier densities. This points to the high interface quality and the very low levels of ionized impurities and lattice defects in these nanowires. The improvements in mobility were concomitant with drastic improvements in photoconductivity lifetime, reaching 1.6 ns. Comparison of photoconductivity and photoluminescence dynamics indicates that midgap GaAs surface states, and consequently surface band-bending and depletion, are effectively eliminated in these high quality heterostructures.

Distinguishing cap and core contributions to the photoconductive terahertz response of single GaAs based core–shell–cap nanowire detectors

“…This result suggests that the GaAs cap has a significant influence on the photoconductivity properties of the nanowires, rather than solely preventing the AlGaAs shell from oxidation [23]. After a further study [25], we confirmed that the early rapid decay signal is related to the fast electron trapping at the GaAs cap surface for the carriers photogenerated in the GaAs cap. Furthermore, we found that the GaAs/AlGaAs/GaAs core-shellcap nanowires exhibit higher electron mobilities than the core-shell nanowires because the GaAs cap layer also plays a role in spatially separating the carriers in the core from the nanowire surface leading to improved mobility (in the same way as the AlGaAs shell does).…”

“…We also noted an additional signal at the earliest time (a very fast decay at the first 25 ps after photoexcitation) when measuring photoconductivity lifetimes for these GaAs/AlGaAs/GaAs core-shell-cap nanowires [24,25], which cannot be observed for bare GaAs nanowires [16] or GaAs/AlGaAs core-shell nanowires [25]. This result suggests that the GaAs cap has a significant influence on the photoconductivity properties of the nanowires, rather than solely preventing the AlGaAs shell from oxidation [23].…”

“…Ultrafast photoexcitation at 800 nm (1.55 eV) was employed to selectively excite only the GaAs in the nanowires. It was shown that the carrier decay was not single-exponential for the GaAs/AlGaAs/GaAs nanowires, including an initial rapid decay (τ f ~ a few ps) due to fast electron trapping at the un-passivated GaAs cap layer surface, and then slower decay (τ s = 4600 ps) due to recombination in the GaAs core [25]. It is worth noting that the fast decay is only present in the GaAs nanowire sample with the GaAs cap [17, To fabricate the single nanowire detectors, the as-grown nanowire substrate was cleaved into small pieces and placed in an isopropyl alcohol solution for a 30-second ultra-sonication to transfer the nanowires into the solution.…”

Diffusion-driven growth of nanowires by low-temperature molecular beam epitaxy

“…In the following we will (1) use CdTe markers to reveal the presence of an incubation time, (2) analyse the distribution obtained when growing at higher temperature to reveal the role of the shape and size of the gold nanoparticle, and (3) use a model adapted to the present growth conditions, which is amenable to analytical expressions, to examine the role of adatom diffusion.…”

Population dynamics and dephasing of excitons and electron-hole pairs in polytype wurtzite/zinc-blende InP nanowires

“…TRPL measurements showed exciton recombination times faster than 80 ps 7 for zincblende GaAs NWs and of ~1ns for GaAs/AlGaAs core-shell NWs at 20K 7,33,34 , revealing the influence of carrier trapping at non-radiative surface states which is reduced by passivation of the NW surface with a material of higher band-gap. InP NWs of both zincblende (ZB) 35,36 and wurtzite (WZ) 36,37 type instead show exciton decay times in the order of 1ns even without a core-shell structure due to a three orders of magnitude slower surface recombination as compared to GaAs.…”