Methods
In this work, the fabrication of the self-assembled Au droplets was investigated on various GaAs type-B (n11) substrates, where n is 9, 8, 7, 5, 4, and 2 in a pulsed laser deposition (PLD) system. The GaAs wafers utilized in this work were semi-insulating or undoped with an off-axis of ±0.1° from VX-809 cost the Wafer Technology Ltd. (Milton Keynes, UK). To start with, a batch of samples including the various type-B GaAs substrates was indium soldered on an Inconel sample holder side by side to maintain the uniformity among the samples and then was treated with a 30-min degas process at 350°C under 1 × 10-4 Torr to remove the contaminants. Subsequently, Au deposition was equally performed on the various type-B GaAs substrates
at a growth rate of 0.05 nm/s with an ionization current of 3 mA under 1 × 10-1 Torr in XL184 in vivo a plasma ion-coater chamber. Au deposition of 2, 3, 4, 6, 9, and 12 nm was systematically performed, and regardless of the deposition amount, the surface showed a quite smooth morphology as shown in Figure 1b,b-1. As an example, Table 1 shows the root-mean-square (RMS) roughness (R q) of the various GaAs surfaces after the 3-nm Au deposition as compared to the Figure 1b. Next, annealing process was implemented by a programmed recipe, and the substrate temperature (T sub) was gradually increased to 550°C from the ambient temperature (approximately 25°C) at a fixed rate of 1.83°C/s under a chamber pressure of 1 × 10-4 Torr. After reaching the target T sub (550°C) [35], the samples were Sulfite dehydrogenase dwelt for 150 s to ensure the maturation of the droplets. Immediately after the dwell process, the samples were quenched down to the ambient temperature to minimize the ripening effect [36, 37]. An atomic force microscope (AFM) under atmospheric pressure was employed to characterize the surface morphology
with non-contact tapping mode. The tips used in this work were NSC16/AIBS (μmasch, Lady’s Island, SC, USA) with a curvature radius less than 10 nm. The RG7420 concentration spring constant was approximately 40 N/m, and the resonation frequency was approximately 170 kHz. A scanning electron microscope (SEM) under vacuum was utilized for the characterizations of the resulting samples, and energy-dispersive X-ray spectrometry (EDS) was utilized (Thermo Fisher Noran System 7, Thermo Fisher Scientific, Waltham, MA, USA) for the elemental analysis. Table 1 Root-mean-square (RMS) roughness ( R q ) of various GaAs surfaces after 3-nm Au deposition Surface (211)B (411)B (511)B (711)B (811)B (911)B R q [nm] 0.361 0.264 0.232 0.351 0.347 0.269 Results and discussion Figure 2 shows the self-assembled Au droplets on GaAs (211)B by the systematic variation of the Au DA from 2 to 12 nm with subsequent annealing at 550°C. Figure labels indicate the related DAs. AFM top views (3 × 3 μm2) of the corresponding samples are shown in Figure 2a,b,c,d,e,f along with enlarged 1 × 1 μm2 images below.