Alloy Fluctuations Act as Quantum Dot-like Emitters in GaAs-AlGaAs Core–Shell Nanowires

Abstract

GaAs-AlxGa1-xAs (AlGaAs) core-shell nanowires show great promise for nanoscale electronic and optoelectronic devices, but the application of these nonplanar heterostructures in devices requires improved understanding and control of nanoscale alloy composition and interfaces. Multiple researchers have observed sharp emission lines of unknown origin below the AlGaAs band edge in photoluminescence (PL) spectra of core-shell nanowires; point defects, alloy composition fluctuations, and self-assembled quantum dots have been put forward as candidate structures. Here we employ laser-assisted atom probe tomography to reveal structural and compositional features that give rise to the sharp PL emission spectra. Nanoscale ellipsoidal Ga-enriched clusters resulting from random composition fluctuations are identified in the AlGaAs shell, and their compositions, size distributions, and interface characteristics are analyzed. Simulations of exciton transition energies in ellipsoidal quantum dots are used to relate the Ga nanocluster distribution with the distribution of sharp PL emission lines. We conclude that the Ga rich clusters can act as discrete emitters provided that the major diameter is ≥4 nm. Smaller clusters are under-represented in the PL spectrum, and spectral lines of larger clusters are broadened, due to quantum tunneling between clusters.

Tuning adatom mobility and nanoscale segregation by twin formation and polytypism

“…The QD potential well is simplified as pure GaAs in order to use the experimental values in [13] and because the exact band structure of WZ AlGaAs is still a matter of debate [38][39][40]. The potential barrier is set by Al 0.2 Ga 0.8 As, in agreement with the experimental Al fluctuations in the shell away from the Al-rich ridges [27,33]. The QD width is chosen in agreement with previous studies [26,27,41].…”

“…On the right: PL spectra (not normalized) from the positions indicated by the dashed vertical lines in the contour plot. extensively reported to be spatially non-uniform, with Al-rich planes at the ridge between two NW facets [26][27][28]33]. By changing the deposition time of the AlGaAs shell we grew three different samples with nominal shell thickness of 30 , 50 , and 100 nm.…”

N -induced Quantum Dots in GaAs/Ga(N,Jansson1, Ishikawa2, Chen3 et al. 2018Phys. Rev. Applied62130View full textAdd to dashboardCite

“…The latter was demonstrated in, e.g., GaP/Ga(As x P 1-x ) [1], In(Ga)As/GaAs [10], In(As x P 1-x )/InP [3,7], and GaAs/(Al x Ga 1-x )As [8] material systems. In addition, it was found that in GaAs/(Al x Ga 1-x )As NWs the QDs can be formed in the (Al x Ga 1-x )As region due to variations in the Al content [6,15]. Independent of the specific mechanism for the QD formation, the majority of such hybrid NW/QD systems utilize heterostructure design and, therefore, often experience strain caused by lattice mismatch between the involved materials.…”

Exciton dynamics in GaAs/(Al,Ga)As core-shell nanowires with shell quantum dots

“…The numbers on the right indicate the excitation power in mW. The solid and dotted lines show the results of a fit using Eq (14). in Ref 29.…”