Decai Yu

On the origin of Si nanocrystal formation in a Si suboxide matrix

We examined mechanisms underlying Si nanocrystal formation in Si-rich SiO2 using a combination of quantum mechanical and Monte Carlo (MC) simulations. We find that this process is mainly driven by suboxide penalty arising from incomplete O coordination, with a minor contribution of strain, and it is primarily controlled by O diffusion rather than excess Si diffusion and agglomeration. The overall behavior of Si cluster growth from our MC simulations based on these fundamental findings agrees well with experiments.

Structure and Dynamics of Ge in the Si – SiO2 System: Implications for Oxide-Embedded Ge Nanoparticle Formation

Using gradient corrected periodic density functional theory calculations, we have investigated the structure, energetics, bonding, and diffusion of Ge in bulk α-quartz and amorphous a-SiO 2 matrices as well as at the Si(001)/a-SiO 2 interface. Our calculations show that Ge atoms undergo migration in a-SiO 2 with a moderate barrier (<2.5 eV) and prefer to remain in the Si part near the Si(001)/a-SiO 2 interface via site exchange reaction with Si lattice atoms, while the kicked-out Si atoms are preferentially incorporated into the a-SiO 2 matrix. We also discuss implications of the Ge-Si exchange process for Ge nanoparticle formation in an oxide matrix.

Origin of vacancy and interstitial stabilization at the amorphous-crystalline Si interface

Using plane-wave pseudopotential density functional theory calculations, we have investigated the behaviors of neutral interstitials and vacancies at the amorphous-crystalline (a–c)Si interface. A continuous random network model is employed in the construction of defect-free a-c interface structure. We find that both vacancies and interstitials prefer to reside on the amorphous side of the interface. In both cases, the most stable defects occur 3–4A from the a-c interface. Vacancy stabilization is found to be due to strain relief provided to the substrate lattice while interstitial stabilization is due largely to bond rearrangement arising from interstitial integration into the substrate lattice. We also discuss the effect of the “spongelike” behavior of the amorphous phase toward native defects on ultrashallow junction formation in the fabrication of ever-shrinking electronic devices.

Vacancy At Si-SiO2 Interface: Ab-Initio Study

Using density functional theory calculations within the generalized gradient approximation we have examined structure and dynamics of neutral Si vacancies at Si/SiO2 interface. We show that Si/SiO2 interface may serve as a limited sink for Si vacancies. Single vacancy and vacancy cluster defects are substantially more stable at c-Si/a-SiO2 interface compared to the bulk c-Si layers away from interface, mainly due to termination of dangling bonds with bridging O atoms and reduction of interface strain

Understanding of the Synthesis and Structure of Si Nanocrystals in an Oxide Matrix from First Principles Based Atomistic Modeling

Kinetic Monte Carlo simulations were performed to examine mechanisms underlying the formation of Si nanoparticles in Si-rich SiO 2 . We have determined two important features of the embedded Si nanoparticle growth: “coalescence-like” and “pseudo Ostwald ripening”. The former is mainly responsible for fast Si particle growth at the early stage of annealing where the particles are close to each other, while the latter becomes important when the density of particles is low such that they are separated by large distances. The pseudo ripening process takes place several orders of magnitude slower than the “coalescence-like” growth. The predominance of “coalescence-like” behavior in the growth of Si nanoparticles results in a big variation in the particle size in terms of the Si:O ratio. Overall the predicted growth behavior based on our Monte Carlo simulations agrees well with experiments.

Multiscale Modeling of Growth and Structure of Silicon Nanoparticles in an Oxide Matrix

A first principles-based multiscale model is developed to examine mechanisms underlying Si nanocrystal formation in Si-rich SiO 2 . Using the multiscale approach, we have found that the embedded nanocrystal formation is mainly driven by suboxide penalty arising from incomplete O coordination, with a minor contribution of strain, and it is primarily controlled by O diffusion rather than excess Si diffusion and agglomeration. The overall behavior of Si cluster growth from our Monte Carlo simulations based on these fundamental findings agrees well with experiments.

Tunnel oxide thickness dependence of activation energy for SiGe quantum dot flash memory

For nonvolatile memory devices, a long retention time is very important. Nanocrystal floating gate has been demonstrated to lead to an improvement for retention time compare to conventional continuous floating gate. In this paper, the authors present our studies of activation energy for SiGe nanocrystal flash memory devices as a function of tunnel oxide thickness to try to clarify this issue