Three-Dimensional Magneto-Photoluminescence as a Probe of the Electronic Properties of Crystal-Phase Quantum Disks in GaAs Nanowires

Abstract

Crystal-phase engineering has emerged as a novel method of bandgap engineering, made feasible by the high surface-to-volume ratio of nanowires. There remains intense debate about the exact characteristics of the band structure of the novel crystal phases, such as wurtzite GaAs, obtained by this approach. We attack this problem via a low-temperature angle-dependent magneto-photoluminescence study of wurtzite/zinc-blende quantum disks in single GaAs nanowires. The exciton diamagnetic coefficient is proportional to the electron-hole correlation length, enabling a determination of the spatial extent of the exciton wave function in the plane and along the confinement axis of the crystal-phase quantum disks. Depending on the disk nature, the diamagnetic coefficient measured in Faraday geometry ranges between 25 and 75 μeV/T(2). For a given disk, the diamagnetic coefficient remains constant upon rotation of the magnetic field. Along with our envelope function calculation accounting for excitonic effects, we demonstrate that the electron effective mass in wurtzite GaAs quantum disks is heavy, mostly isotropic and results from mixing of the two lower-energy conduction bands with Γ7 and Γ8 symmetries. Finally, we discuss the implications of the results of the angle dependent magneto-luminescence for the likely symmetry of the exciton states. This work provides important insight in the band structure of wurtzite GaAs for future nanowire-based polytypic bandgap engineering.

Electrons, holes, and excitons in GaAs polytype quantum dots

“…17,18 Lacking effective mass parameters describing such a coupling, we model WZ electrons with a single-band Hamiltonian of hybrid character: Γ 8c masses but optically bright, like the Γ 7c band. This picture is consistent with the observations of different recent experiments 11,19,20 and suffices to assess the role of the physical factors we investigate. The polytype Hamiltonian then reads:…”

“…11 As Fig. 5 shows, due to the VB mixing, the hole g-factor value can fluctuate substantially for different QD dimensions.…”

“…In particular, we assess the validity of the usual assumption that the ground state has a well defined, single-band, HH character. [9][10][11] The eigenfunctions of Hamiltonian H 6B h are sixcomponent spinors of the form: Ψ 6B h = 6 i=1 f i (r)|u i , where f i (r) is the envelope function associated with the |u i Bloch function. The weight of an individual component is computed as |f i | 2 .…”

Charge carrier generation, relaxation, and recombination in polytypic GaAs nanowires studied by photoluminescence excitation spectroscopy

“…[14][15][16] Despite this technical progress, the band structure of WZ GaAs remains unclear. There is still controversy surrounding the value of the bandgap 15,[17][18][19][20] and of the electron effective mass, 11,[21][22][23] and the symmetry of the lowest energy conduction band has yet to be clarified. 15,16,24 Nonresonant photoluminescence (PL) has been useful in elucidating the interplay between charge carrier relaxation and recombination mechanisms, but it cannot alone answer the questions regarding the band structure as the nanowire emission near the band-edge at low-temperature is dominated by the recombination of excitons bound to point [25][26][27] and extended defects.…”

Electronic structure of quantum dots: response to the environment and externally applied fields

“…Instead, we use a single-band Hamiltonian of hybrid character: Γ 8c masses but optically bright as Γ 7c band. [171,172] The polytype CB Hamiltonian reads…”