Electron and hole gas in modulation doped GaAs/AlGaAs radial heterojunctions

TL;DR

This study uses self-consistent calculations to explore how electron and hole gases localize in modulation-doped GaAs/AlGaAs nanowires, revealing tunable distributions and symmetry-breaking effects in a field-effect transistor setup.

Contribution

It provides a detailed analysis of electron and hole gas distributions in radial heterojunction nanowires, highlighting their dependence on doping levels and external electric fields, which was not previously characterized.

Findings

Electrons transition from cylindrical to quasi-1D channels with doping.

Holes form various configurations, including separated 2DEGs and bent channels.

Gate voltage can deform and control the electron/hole gas distributions.

Abstract

We perform self-consistent Schr\"odinger-Poisson calculations with exchange and correlation corrections to determine the electron/hole gas in a radial hetero-junction formed in a modulation doped GaAs/AlGaAs core-multi-shell nanowire (CSNW) which is n-/p-doped. Realistic composition and geometry are mapped on an symmetry compliant two-dimensional grid, and the inversion/accumulation layers are obtained assuming mid-gap Fermi energy pinning at the surface. We show that the electron and hole gases can be tuned to different localizations and symmetries inside the core as a function of the doping level. Contrary to planar hetero-junctions, conduction electrons do not form a uniform 2D electron gas (2DEG) localized at the GaAs/AlGaAs interface, but rather show a transition between i) an isotropic, cylindrical distribution deep in the GaAs core (low doping), and ii) a set of six tunnel-coupled quasi-1D channels at the edges of the interface (high doping). Holes, on the other hand, are much more localized at the GaAs/AlGaAs interface and form either i) six separated planar 2DEGs at the GaAs/AlGaAs interfaces (low doping), ii) a quasi uniform six-fold bent 2DEG (intermediate doping), or iii) six tunnel-coupled quasi-1D channels at the edges (high doping). We also simulate the electron/hole gas in a CSNW-based field effect transistor. The field generated by a back-gate may easily deform the electron or hole gas, breaking the six-fold symmetry. Single 2DEGs at one interface or multiple quasi-1D channels are shown to form as a function of voltage intensity, polarity, and carrier type.