Electronic properties of GaAs, InAs and InP nanowires studied by terahertz spectroscopy

Abstract

We have performed a comparative study of ultrafast charge carrier dynamics in a range of III-V nanowires using optical pump-terahertz probe spectroscopy. This versatile technique allows measurement of important parameters for device applications, including carrier lifetimes, surface recombination velocities, carrier mobilities and donor doping levels. GaAs, InAs and InP nanowires of varying diameters were measured. For all samples, the electronic response was dominated by a pronounced surface plasmon mode. Of the three nanowire materials, InAs nanowires exhibited the highest electron mobilities of 6000 cm² V⁻¹ s⁻¹, which highlights their potential for high mobility applications, such as field effect transistors. InP nanowires exhibited the longest carrier lifetimes and the lowest surface recombination velocity of 170 cm s⁻¹. This very low surface recombination velocity makes InP nanowires suitable for applications where carrier lifetime is crucial, such as in photovoltaics. In contrast, the carrier lifetimes in GaAs nanowires were extremely short, of the order of picoseconds, due to the high surface recombination velocity, which was measured as 5.4 × 10⁵ cm s⁻¹. These findings will assist in the choice of nanowires for different applications, and identify the challenges in producing nanowires suitable for future electronic and optoelectronic devices.

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

“…The presence of a wide distribution of time constants for this process requires a spatial separation of the minority carriers, the holes, from the electrons. This can be provided by an upward band bending close to the nanowire surface due to negative surface charges 49 , which leads to a spatial separation between holes at the NW surface and electrons in the NW center, with small wavefunction overlap and consequently small recombination rates, which is consistent with the low surface recombination velocity found in InP NWs 61 . The relaxation time T5 decreases with increasing excitation fluence, which could be due to enhanced thermal activation of the surface trapped holes resulting from the screening of the band bending by the trapped hole density.…”

Strong surface passivation of GaAs nanowires with ultrathin InP and GaP capping layers

“…Moreover, the surface states can further reduce conductivity by scattering the charge carriers. 6 These effects are pronounced in GaAs due to its high surface recombination rate. GaAs is otherwise a widely used material with its several desired properties, such as high electron mobility and a direct band gap with suitable energy to single-junction solar cells.…”

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

“…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].…”

“…A time-domain optical-pump THz-probe (OPTP) spectroscopy system [16,20,22] was used to measure the photoconductivity lifetime of an ensemble of GaAs/AlGaAs/GaAs nanowires. This photoconductivity lifetime determines the detector operation type (direct or integrating sampling) [26] and the signal processing technique required to recover the THz electric field from the current measured in the photoconductive detector.…”

“…It is well known that the carrier lifetimes in core-only GaAs nanowires are extremely short [15], of the order of picoseconds, owing to their high surface recombination velocity [16,17]. These short lifetimes lead to large surface scattering and poor charge collection, resulting in poor performance for optoelectronic applications.…”