3D Nanostructures on Ge/Si(100) Wetting Layers: Hillocks and Pre-Quantum Dots

TL;DR

This study investigates how the thickness of Ge wetting layers influences the formation and morphology of 3D nanostructures, such as hillocks and pre-quantum dots, on Si(100) surfaces, revealing pathways for controlled nanostructure fabrication.

Contribution

It demonstrates the direct relationship between wetting layer thickness and nanostructure type, providing new insights into strain-driven nanostructure control in Ge/Si systems.

Findings

Wetting layer thickness determines nanostructure type: hillocks or pre-quantum dots.

Crossover thickness between structures is between 1.6 and 2.1 ML.

Pre-quantum dots are smaller, with a minimum width of 10 nm.

Abstract

The annealing of sub-critical Ge wetting layers (WL<3.5 ML) initiates the formation of 3D nanostructures, whose shape and orientation is determined by the WL thickness and thus directly related to the strain energy. The emergence of these nanostructures, hillocks and pre-quantum dots, is studied by scanning tunneling microscopy. A wetting layer deposited at 350 C is initially rough on the nanometer length-scale and undergoes a progressive transformation and smoothening until for T>460 C vacancy lines and the 2xn reconstruction are observed. The metastable Ge wetting layer (WL) then collapses to form 3D nanostructures whose morphology is controlled by the WL thickness: firstly, hillocks, with a wedding cake-type structure where the step edges run parallel to the <110> direction, are formed for thin wetting layers, while {105}-faceted structures, so-called pre-quantum dots (p-QDs), are formed from thicker layers. The wetting layer thickness and thus the misfit strain energy controls the type of structure. The crossover thickness between hillock and p-QDs regime is between 1.6 and 2.1 ML. The hillocks have larger lateral dimensions and volumes than p-QDs, and the p-QDs are exceptionally small quantum dots with a lower limit of 10 nm in width. Our work opens a new pathway to the control of nanostructure morphology and size in the elastically strained Ge/Si system. Keywords: Quantum dots, Stranski-Krastanov, epitaxy, strained layers, scanning tunneling microscopy