Observation of gallium droplets directly formed on GaAs substrate by in-situ laser irradiation

In this study, following the deposition of a 500nm GaAs buffer layer, the GaAs substrate was irradiated in MBE with an in situ laser shooting at a very low temperature of around 12°C. We carefully observed the morphology evolution of the irradiated surface with different pulse energy varying from 20mJ to 50mJ. After being pulsed-irradiated with 20mJ, a density of Ga droplets as high as 7.2×10¹⁰/cm² was directly formed on the surface. The droplets have a height range of 1.0nm to 4.1nm (averaging at 2.4nm) and a width range of 15.7nm to 39.2nm (averaging at 28.1nm). With increasing irradiation energy, it is observed the droplet density will gradually decrease from 2.2 × 10¹⁰/cm², 1.2 × 10¹⁰/cm² to 5.2 × 10⁹/cm² while the size (average width/height) will continue to grow up from 64.1nm/8.2nm, 63.3/9.2nm to 76.1nm/12.2nm, respectively corresponding to exposure energy from 30mJ, 40mJ to 50mJ. The results demonstrate the successful provision of a new technology to generate Ga droplets on the surface of GaAs directly through in-situ pulsed laser irradiation without the need of epitaxial growth. Furthermore, both the resulting density and size can be easily and effectively adjusted by varying the laser energy. Additionally, this technique offers significant advantages such as low cost, free of contamination and defects and high controllability. Therefore, we believe this technology may find great application prospects for nano-fabrication of semiconductor quantum structures and devices.