Evolution of InGaAs quantum dot molecules

Abstract

The formation and evolution process of self-assembled InGaAs quantum dot molecules (QDMs) are studied in terms of configuration, volume, and types of QDMs. QDMs are formed around self-assembled GaAs nanoscale island induced by adapting a hybrid growth approach combining droplet homoepitaxy and Stranski–Krastanov mode. In distinction from our previous results [Lee et al., Appl. Phys. Lett. 89, 202101 (2006)], hexa-QDMs are fabricated without the formation of background QDs, which can be due to a combinational effects of enhanced intermixing of Ga and In atoms, enhanced surface diffusion (high mobility) of adatoms, and higher In desorption rate due to the higher thermal energy provided during the fabrication of QDMs. In addition, a detailed evolution mechanism from bi-QDMs (two QDs per each GaAs island) to hexa-QDMs (six QDs per island) is proposed based on atom diffusion, material transfer, and equilibrium dimension (saturation) of QDs. Under a fixed InAs coverage, depending on postannealing process after liquid Ga droplet formation, highly uniform as well as various types of QDMs can be fabricated and the resulting configurations show a very strong correlation with the size of initial GaAs islands. With relatively smaller GaAs islands, quad-QDMs (four QDs per island) with a squarelike configuration were formed and also, quad-QDMs with a rectangularlike positioning were fabricated with relatively larger size of islands, while hexa-QDMs were formed with middle sized ones. Relatively, broader size distribution of GaAs nanoisland can be a direct result of Ostwald ripening, which can be well controlled by adjusting postgrowth interruption of liquid Ga droplets.