Assembly of Nanostructures in III-V Semiconductor Films.

Abstract

In film growth, assembly of nanostructures allows precise placement and reliable dimensions for higher efficiency in devices. This work looks at two extremes of assembly: spontaneous assembly manipulated by experimental parameters and directed assembly by altering surface patterns. First we vary experimental procedures to change feature sizes, and then we directly assemble dots on patterned surfaces. The morphologies in these films are characterized and then reproduced. We examine two different strained material systems: mesa formation in 2 monolayer GaAs films on In0.53Ga0.47As/InP and quantum dots in 2 monolayer InAs films on GaAs. We employ focused ion beam (FIB) patterning on the latter system to direct the formation of quantum dots. When varying growth parameters in the GaAs films, the mesa-trench morphology shown by scanning tunneling microscopy images changes significantly. There is roughening and mesa narrowing at higher temperatures, and intermixing is confirmed by X-ray coherent Bragg rod analysis. We use a Ga adatom density model to correspond to step edge density to predict morphological trends. This shows the commonly used metric of V:III growth ratio is not applicable at low As growth rates because of roughening. In the second material system we grow InAs quantum dots on GaAs. Explorations of ex situ FIB patterning show the technique is not successful due to oxide desorption roughness. We instead use in vacuo FIB to successfully assemble quantum dots on FIB-irradiated holes. We vary growth conditions, irradiation dose, and periodicity to yield single or multiple quantum dots. Elastic kinetic Monte Carlo simulations help predict the number of dots at sites and show that dot nucleation begins within the hole walls. The simulations show agreement with multiple dots, but discrepancies arise because of the limited amount of intermixing and initial hole shape. We characterize the quantum dot shape and holes and attempt to reconcile the large range of sizes with our experiments. Photoluminescent structures grown from the FIB-patterned quantum dots are measured. Spatial mapping shows that the FIB decreases InAs quantum dot peaks. Transmission electron microscopy images indicate that the lowered emission is due to the presence of defects caused by FIB.