(a-e) 500 × 500 nm2 AFM images of different stages of the nanodrilling process during the Ga droplet consumption. (f) Profiles along the direction [dashed line marked in (e)], normalized to the smallest ring diameter, showing the progressive droplet reduction, the local etching

of the GaAs substrate, buy Stattic and the progressive filling of the part of the hole free of Ga droplet. These results show that the nanohole formation process is activated when Ga droplets are exposed to arsenic, while in the absence of arsenic, only flat depressions beneath the Ga droplets are observed. Arsenic exposure also leads to the consumption of the Ga droplets. It is well known that As supply to Ga droplets triggers the onset of different processes [4, 21–23], in particular

a change in Ga droplet composition due to the incoming arsenic diffusion through metallic Ga, driving the Ga droplet arsenic content out of the equilibrium value at the corresponding temperature. In order to restore the arsenic equilibrium composition, Ga atoms belonging to the substrate would migrate towards the Ga droplet, if kinetics is not inhibited, with the subsequent enhancing of local substrate dissolution and the onset of the nanohole formation process. TPCA-1 manufacturer This explains why nanoholes penetrating in the substrate only appear in the presence of arsenic at high enough substrate temperatures. Simultaneously to the nanodrilling effect, GaAs is forming around and at the edge of the PRKACG Ga droplet as has been

previously reported [6, 23], Sapanisertib chemical structure leading to its consumption at a rate that will depend on T S and As flux. In this way, there is a competition between Ga coming from the substrate that incorporates at the Ga droplet and droplet consumption by forming GaAs. Altogether, a Ga droplet under As gives rise to ringlike nanostructures surrounding a deep and narrow hole that can penetrate up to tens of nanometers into the substrate. These processes are closely related to the Ga-assisted vapor-liquid-solid growth of nanowires, where the incorporation of Ga and As and the GaAs crystallization take place below and around the Ga droplet [35], being in our case the source of Ga is the GaAs substrate instead of an incoming Ga flux. According to the critical role of arsenic in nanohole formation, arsenic flux and time to arsenic exposure of Ga droplets would be key parameters to control the process. In order to have a deeper insight into this process, samples exposed to different As flux intensities during different annealing times, keeping the substrate temperature at T S = 500°C, were grown and characterized.Figure 5 shows the average depth of nanoholes as a function of annealing time for the two different As flux intensities employed. The data points at annealing time 0 s correspond to the depth of the depressions remaining after HCl etching of the Ga droplets annealed in the absence of As.