Demonstration and study of Ga-droplet migrating propelled by <italic>in-situ</italic> laser based on MBE

Abstract

In droplet epitaxy, as is well known that the metallic droplets would closely determine the density, size and site of the following quantum structures such as quantum dot/ring, nano wire and son on because they usually act as the precursors. Recently, to search for the method of effectively manipulating the droplets shows great significance. In this paper, we have conducted a pioneering study of in-situ laser manipulating droplets in the droplet epitaxy basing MBE. A layer of Ga-droplets (density: 5.7×1010 cm−2) on GaAs (001) was first prepared by MBE deposition of 6ML Ga at the temperature of 150°C with a growth rate of 0.18 ML/s and an interrupt of as flux. Then a single pulse of mono-beamed laser (355 nm/10 ns) was in-situ introduced to shoot on it and the morphology evolution was carefully studied by AFM and the mechanism was also discussed simply. It is observed: As (laser) energy density reached above 10 mJ cm−2, the Ga droplet was able to escape from the surface potential trap in all-direction and began to migrate randomly and even in a very small area of several hundred nanometers various migrations (along different surface orientations) could been seen. This was explained as: since the Ga droplets are disorderly locating on the GaAs surface, even for two closest neighbor droplets within a typical distance of hundred nanometers, the surrounding situations for them might be distinctly non-equivalent in the terms of the droplet size (height/diameter), density and the inter-droplet relative position. So when the laser pulse heated the surface, such disorder will cause a chaotic fluctuation in surface local thermal field. Then as a consequence, the droplets will move around with the diffusion force under the gradient of temperature. Accompanying with the migration, coalescence (among different droplets) might take places during the collision which could remarkably modify the Ga-droplet morphology. And this kind of modifying strongly depended on laser energy density. Statistically, as the energy density increased from 10 mJ cm−2 to 30 mJ cm−2, the density of droplet would reduce 1/3 from 5.22×1010 cm−2 to 2.23×1010 cm−2 and rapidly the dominant size was enlarged with corresponding increasement of diameter from (35–65 nm) to (65–95 nm) and height from (1–7 nm) to (7–16 nm) respectively. And meanwhile, the overall size linewidth broadened a lot resulting in an extreme “wide-band” distribution and it was hardly observed in the normal droplet growth. Such wide character of droplet size distribution would find huge use in the photoelectric detection or conversion with the special need of “broad-spectrum absorption” in many actual occasions. Herein, we have demonstrated a novel finding of “photo-induced diffusion” on droplets and moreover it seems that the diffusion has less relation with the surface orientation and the size of droplets but the energy density of laser straightly. So, it is promising to realize a highly controlled-droplet epitaxy by taking use of laser interference patterning in the future which will must make significant developments for the whole fabrication area of quantum semiconductor.