(Color online) Current as a function of time during the second step anodization step. The rapid initial decay indicates oxide formation (period A) followed by pore nucleation and growth (period B). After complete conversion of the Al to alumina (point C), GaAs substrate etching occurs indicated by the subsequent current increase (period D).
SEM images of the AAM surface formed on Al thin films deposited on GaAs substrates after the completion of the outlined electrochemical anodization procedure. For the (a) 30 V anodizations, the hole size and interpore spacing appear to be smaller than that achieved using (b) 40 V or (c) 50 V. Using 50 V, the arrangement of the holes was irregular over the sample. In the case of (d) single step 40 V anodization, irregularity was also observed.
SEM images of the GaAs(111)B surfaces after the electrochemical anodizations and removal of the AAM layer. [(a) and (b)] The surfaces consist of numerous inverted-dome structures for the anodization terminated 6 min after the complete consumption of Al. [(c) and (d)] A reduced surface density of the domes is present when the anodization was stopped 2 min after the complete consumption of the Al film. Higher magnification images of the domes in (a) and (c) are shown in (b) and (d), respectively.
SEM images of the annealed Au nanoparticles for samples (a) (40-2), (b) (control-2), (c) (30-2), (d) (50-2), (e) (40-1), (f) (40-3), (g) (control-1), and (h) (control-3).
SEM images of single GaAs nanowires grown on (40-2) samples shown in (a) plan-view and (c) tilt-view . The corresponding controls of single GaAs nanowires grown on (control-2) are also shown in (b) plan-view and (d) tilt-view.
(a) SEM images of QW nanowires grown on (40-2) samples with GaAs shell, grown to surface passivate the nanowire. Inset: low magnification TEM image of , which shows that the nanowires are single crystals. The contrast along the nanowire axis is due to thickness variation, not stacking faults. (b) HRTEM image of a section of QW nanowires without GaAs shell. Oxide layers along the nanowire walls, due to air exposure, can easily be seen. The inset shows selected-area diffraction along zone axis from the nanowires confirming the single crystal structure. (c) -contrast HAADF-STEM image of the QW nanowires. The brighter section along the nanowire axis (inset: intensity line scan along the InGaAs layer normalized to the intensity from the GaAs regions) of about 70 nm indicates the presence of InGaAs well layer. This was further confirmed by (d) EDX data on this bright section. Quantification of this spectrum results in 5% In.
(Color online) (a) PL data obtained at 30 K for nanowires (spectrum A), films on GaAs(100) (spectrum B), and GaAs nanowires from Fig. 5(a) (spectrum C). The laser power was adjusted to obtain relatively similar magnitude of PL intensity from the film and the nanowires. Spectra A and C were obtained when the nanowires were dispersed on sapphire substrates. (b) PL spectra from nanowires obtained from different regions on the sapphire substrates showing the consistency of the InGaAs peaks (1.375 eV). This implies the low variation of In concentration among nanowires of slightly different diameters (±4.7 nm).
Calculated PL energy based on square well potential under the influence of piezoelectric field, electron-hole separation induced field, and band filling effects for (a) and (b) . The choice of was based on EDX result, while was the result of trial and error method to match the measured PL energy at 1.375 eV in Fig. 7.
Summary of annealing statistics for the samples in Fig. 4 showing the effects of anodizing voltage and thickness of evaporated Au films on the average diameter , the size dispersion , and the density of Au nanodots after annealing.
Summary of MOVPE operating conditions for InGaAs layer in QW nanowires and films in comparison with literature values (Ref. 37). The growth time and growth temperature for the InGaAs layer in this study were shorther than those of the literature values. indicates the mole fraction of the corresponding precursor . The In content is denoted by , which was calculated from the measured PL energy using square well approximation under piezoelectric fields, electron-hole separation induced fields, and band filling effects (Ref. 38).
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