Value and Anisotropy of the Electron and Hole Mass in Pure Wurtzite InP Nanowires

Hole and Electron Effective Masses in Single InP Nanowires with a Wurtzite-Zincblende Homojunction

The formation of wurtzite (WZ) phase in III-V nanowires (NWs) such as GaAs and InP is a complication hindering the growth of pure-phase NWs, but it can also be exploited to form NW homostructures consisting of alternate zincblende (ZB) and WZ segments. This leads to different forms of nanostructures, such as crystal-phase superlattices and quantum dots. Here, we investigate the electronic properties of the simplest, yet challenging, of such homostructures: InP NWs with a single homojunction between pure ZB and WZ segments. Polarization-resolved microphotoluminescence (μ-PL) measurements on single NWs provide a tool to gain insights into the interplay between NW geometry and crystal phase. We also exploit this homostructure to simultaneously measure effective masses of charge carriers and excitons in ZB and WZ InP NWs, reliably. Magneto-μ-PL measurements carried out on individual NWs up to 29 T at 77 K allow us to determine the free exciton reduced masses of the ZB and WZ crystal phases, showing the heavier character of the WZ phase, and to deduce the effective mass of electrons in ZB InP NWs (m_e= 0.080 m₀). Finally, we obtain the reduced mass of light-hole excitons in WZ InP by probing the second optically permitted transition Γ₇^C ↔ Γ_7u^V with magneto-μ-PL measurements carried out at room temperature. This information is used to extract the experimental light-hole effective mass in WZ InP, which is found to be m_lh = 0.26 m₀, a value much smaller than the one of the heavy hole mass. Besides being a valuable test for band structure calculations, the knowledge of carrier masses in WZ and ZB InP is important in view of the optimization of the efficiency of solar cells, which is one of the main applications of InP NWs.

Nonpolar-Oriented Wurtzite InP Nanowires with Electron Mobility Approaching the Theoretical Limit

As an important semiconductor nanomaterial, InP nanowires (NWs) grown with a typical vapor-liquid-solid mechanism are still restricted from their low electron mobility for practical applications. Here, nonpolar-oriented defect-free wurtzite InP NWs with electron mobility of as high as 2000 cm² V^-1 s^-1 can be successfully synthesized via Pd-catalyzed vapor-solid-solid growth. Specifically, PdIn catalyst particles are involved and found to expose their PdIn{210} planes at the InP nucleation frontier due to their minimal lattice mismatch with nonpolar InP{2̅110} and {1̅100} planes. This appropriate lattice registration would then minimize the overall free energy and enable the highly crystalline InP NW growth epitaxially along the nonpolar directions. Because of the minimized crystal defects, the record-high electron mobility of InP NWs ( i.e., 2000 cm² V^-1 s^-1 at an electron concentration of 10¹⁷ cm^-3) results, being close to the theoretical limit of their bulk counterparts. Furthermore, once the top-gated device geometry is employed, the device subthreshold slopes can be impressively reduced down to 91 mV dec^-1 at room temperature. In addition, these NWs exhibit a high photoresponsivity of 10⁴ A W^-1 with fast rise and decay times of 0.89 and 0.82 s, respectively, in photodetection. All these results evidently demonstrate the promise of nonpolar-oriented InP NWs for next-generation electronics and optoelectronics.

Addressing the Fundamental Electronic Properties of Wurtzite GaAs Nanowires by High-Field Magneto-Photoluminescence Spectroscopy

At ambient conditions, GaAs forms in the zincblende (ZB) phase with the notable exception of nanowires (NWs) where the wurtzite (WZ) lattice is also found. The WZ formation is both a complication to be dealt with and a potential feature to be exploited, for example, in NW homostructures wherein ZB and WZ phases alternate controllably and thus band gap engineering is achieved. Despite intense studies, some of the fundamental electronic properties of WZ GaAs NWs are not fully assessed yet. In this work, by using photoluminescence (PL) under high magnetic fields (B = 0-28 T), we measure the diamagnetic shift, ΔE_d, and the Zeeman splitting of the band gap free exciton in WZ GaAs formed in core-shell InGaAs-GaAs NWs. The quantitative analysis of ΔE_d at different temperatures (T = 4.2 and 77 K) and for different directions of B⃗ allows the determination of the exciton reduced mass, μ_exc, in planes perpendicular (μ_exc = 0.052 m₀, where m₀ is the electron mass in vacuum) and parallel (μ_exc = 0.057 m₀) to the ĉ axis of the WZ lattice. The value and anisotropy of the exciton reduced mass are compatible with the electron lowest-energy state having Γ_7C instead of Γ_8C symmetry. This finding answers a long discussed issue about the correct ordering of the conduction band states in WZ GaAs. As for the Zeeman splitting, it varies considerably with the field direction, resulting in an exciton gyromagnetic factor equal to 5.4 and ∼0 for B⃗//ĉ and B⃗⊥ĉ, respectively. This latter result provides fundamental insight into the band structure of wurtzite GaAs.