Spontaneous Alloy Composition Ordering in GaAs-AlGaAs Core–Shell Nanowires

Abstract

By employing various high-resolution metrology techniques we directly probe the material composition profile within GaAs-Al0.3Ga0.7As core-shell nanowires grown by molecular beam epitaxy on silicon. Micro Raman measurements performed along the entire (>10 μm) length of the [111]-oriented nanowires reveal excellent average compositional homogeneity of the nominally Al0.3Ga0.7As shell. In strong contrast, along the radial direction cross-sectional scanning transmission electron microscopy and associated chemical analysis reveal rich structure in the AlGaAs alloy composition due to interface segregation, nanofaceting, and local alloy fluctuations. Most strikingly, we observe a 6-fold Al-rich substructure along the corners of the hexagonal AlGaAs shell where the Al-content is up to x ~ 0.6, a factor of 2 larger than the body of the AlGaAs shell. This is associated with facet-dependent capillarity diffusion due to the nonplanarity of shell growth. A modulation of the Al-content is also found along the radial [110] growth directions of the AlGaAs shell. Besides the ~10(3)-fold enhancement of the photoluminescence yield due to inhibition of nonradiative surface recombination, the AlGaAs shell gives rise to a broadened band of sharp-line luminescence features extending ~150-30 meV below the band gap of Al0.3Ga0.7As. These features are attributed to deep level defects under influence of the observed local alloy fluctuations in the shell.

Lasing from individual GaAs-AlGaAs core-shell nanowires up to room temperature

“…S2 showing the relative PL yield from a series of NWs grown under nominally identical growth conditions as a function of the shell thickness in the range of 0-100 nm. These data clearly show that a shell with thickness even as low as 8 nm results in a B10 3 -fold enhancement in the luminescence efficiency, which dramatically reduces the intrinsic losses of the NW core and makes lasing even up to room temperature possible 25 .…”

“…To strongly enhance the optical efficiency of the GaAs NWs, the core was overgrown with a 5-nm-thick AlGaAs passivation layer and finally capped with a thin layer of GaAs to prevent oxidation ( Supplementary Fig. S2) 25 . Figure 1a shows a typical scanning electron microscopic image of the as-grown NW-laser structures, with lengths varying from 11 to 16 mm.…”

“…Furthermore, gainpinning effects are not evident in our experiments for any of the excitation conditions used. The very clear threshold and strongly nonlinear input-output characteristic is attributed to the high quality of the lattice-matched GaAs/AlGaAs material system 25 in combination with the homogeneous optical excitation of the entire NW. As such, the results presented in Fig.…”

Determination of the Optimal Shell Thickness for Self-Catalyzed GaAs/AlGaAs Core–Shell Nanowires on Silicon

“…Therefore, a strain relaxation and defect formation in such a small strain system must be considered. So far, the majority of the reports concerning the GaAs/AlGaAs core−shell NWs have focused on a feasibility of the growth 23,24,25,26 , elaborating the detailed structural characteristics 27,28,29,30,31 , and/or showing the positive effects of the AlGaAs shell on the optical properties of the GaAs NW core 32,33,34 . On the other hand, fewer studies were dedicated to systematically explore relative contributions of various surface effects on the electronic properties of the core-shell NWs, which could have large implications on the optimum design for ultimate performance of the devices based on such complex structures.…”

Quantum dots in the GaAs/AlxGa1−xAs core-shell nanowires: Statistical occurrence as a function of the shell thickness

“…[20][21][22] Another explanation for the peculiar peaks in the emission spectrum from the Al x Ga 1Àx As shell has been proposed, where the luminescence could originate from recombination at lattice point defects. 23 If this would be the case, one would expect that the QD emission would be proportional to the Al x Ga 1Àx As shell volume and mainly independent of the NW crosssection morphology.…”