Designable Ge Quantum Dot Formation with Molecular Dynamics Study

Abstract

As the size of the device is approaching nanoscale dimensions, it is necessary to understand the new current mechanisms proceeding at the nanoscale [1]. The semiconductor quantum dots (QD) have gained attention due to their property to control the quantum confinement effect, enabling fine current control at the nanoscale device. As shown in Figure 1, this study proposes a method of self-assembly Ge QD fabrication for room temperature operation. The SiGeO mixed layer is created on the Si1-xGex layer through an oxidation process via annealing in the Si1-xGex layer. After the three-layered structure of Si1- xGex/SiGeO/Si1-xGex is patterned into a pillar limited to the nanoscale, and Ge QD is automatically formed in the middle of SiGeO mixed layer through an additional annealing process. We conducted the Ge QD formation study with molecular dynamics calculation using Tersoff potential, which is suitable for describing the interaction of covalently bonded materials, such as semiconductor materials [2]. We created five models of SiGeO alloy to investigate the environment of Ge QD formation. In our simulation, through high-temperature annealing at 3000 K, where the five models reach a near-molten state, Ge atoms tended to dissociate from oxygen and bond with other Ge atoms. This is a corresponding result of the Gibbs triangle at 1000 K, suggesting that Ge nanoparticles are inevitably formed at sufficiently high temperatures in SiGeO ternary system [3]. Nearly spherical nanoparticles were observed in models with concentrations of Ge at 10 %, 20 %, and 30 %. We also conducted a simulation of Ge nanoparticle formation in the Si1-xGex/SiGeO/Si1-xGex layers. After annealing process at 1600 K, amorphous 2 nm Ge nanoparticle were generated in SiGeO mixed alloy. The crystalline 2 nm Ge QD’s energy gap was also investigated to be approximately 2.12 eV through density functional theory (DFT) simulation. Although the size of the nanoparticle from our simulation work are inherently limited due to computational cost, our proposed process methodology suggests highly applicable self-assembly approach for Ge QD formation with diverse sizes in real Si-based fabrication processes.