Mechanical properties of SiC nanowires with polytypes

Abstract

In this report, we model the mechanical properties and fracture behavior of SiC nanowires with different polytypes using Molecular Dynamics (MD) simulations. The mechanical properties investigated are the Young’s modulus, the maximum tensile stress and the fracture strain. The three polytype tested are SiC (3C), (2H) and (4H). Tensile tests are performed on bulk and nanowire samples using three commonly know inter-atomic potentials: the Tersoff, the Vashishta and the MEAM potential. Our report finds large differences in how the potentials predict the mechanical properties and fracture behavior of the SiC structures. Using the MEAM potential, we perform tests on two similar sized nanowires with different side facets: one with {11-2} surfaces and one with {1-10} surfaces. The surface energies of the two surface types are estimated. Our studies find that the type of surfaces will affect the mechanical properties of the nanowire. The mechanical properties of the three SiC polytypes are obtained at four different temperatures. A dependence of the Young’s modulus on the hexagonality of unit cell is found, a dependence also reported for diamond polytypes. We further find that increasing temperatures will lower the values of the mechanical properties. Lastly, two nanowire heterostructures are constructed using diamond cubic Si and either SiC (3C) and SiC (2H). The potential energy of the interface is estimated and compared to the Si and SiC (3C)/(2H) sections of the heterostructure. We find that due to dislocations the energy is highest at the interface. The dislocation pattern of the interface is analyzed, and edge dislocations of the type 1/2<110> and 1/6<11-2> are found.

Popular Abstract

How modeling nanoscale devices is like playing with LEGO bricks.

Nanoscale devices are all around us. It is very likely that you – right now– have several billion nanodevices lying in your pocket. They live inside your smart phone, ready to follow your command at every swipe. You may not think about them, but they are constantly doing thousands of small tasks just to make your life easier.
So why don’t we know more about these tiny helpers? An obvious reason is that we never see nanodevices. They are so small that they are invisible to the human eye. All this talk about invisible nanodevices doing this and that, might seem complicated or may leave you a little overwhelmed –but don’t worry! – because in this paper, I will try to make the argument that building nanodevices is not that different from playing with Lego bricks.
Now, imagine you have a big pile of Lego brick in front of you. They have different colors and sizes; some are made from hard plastic and some are made from soft plastic. Instead of Lego bricks, material scientists will have a lot of different semiconductor materials at their disposal. Just like your Lego bricks they also have different sizes and hardness. However, unlike your Lego bricks, they also have different electronic and chemical properties.
Your first task is to create a heterostructure. A heterostructure is created by taking two different materials and putting them together. This may not sound very exciting, but trust me, heterostructures are the heart of almost any nanodevice we have today. In solar cells, they produce power from the light of the sun, and in computers, they are the cornerstones of the logical gates which make your computer able to compute. Creating a heterostructure using Lego bricks is simple: you take two differently colored bricks and stack them on top of each other – easy! However, for material scientists it is not that easy; we must grow the materials together using a technique called Chemical Vapor Deposition. It’s a bit like when water vapor solidifies and becomes frost on the windshield of your car. What further complicates the matter, is the fact that different materials will have different lattice spacing. In the analog of Lego bricks this would correspond to trying to put two bricks with differently spaced knobs and holes together. You might succeed, but in the interface there will be tiny cracks and other kinds of plastic deformations.
When playing with Legos, these cracks might not really matter. But for material scientists finding cracks, or crystal defects as we call them, in our heterostructures is a true nightmare. In solar cells, they will act as electron traps and lower the efficiency of the cell. In computer chips, they will lower the performance of the transistors and thereby make your computer slower.
To avoid making these mistakes, scientists use advanced computer modeling to simulate how the semiconductor materials respond to various types of mechanical loading and deformations. Computer models are needed since the size of the nanodevices makes it very complicated to perform real-life experiments.
This is exactly what we will do in this report, where certain nanostructures, called nanowires, made from the semiconductor SiC are tested in order to find their mechanical properties and limits.

• author: Vesti, Anders
• supervisor: Jonas Johansson; Per Hansson
• organization: Solid State Physics; Department of Physics
• course: FYSM30 20182
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: atomistic modeling, nanowires, molecular dynamics, simulation, SiC, silicon carbide, fracture behavior, nanomechanics
• language: English
• id: 8977878
• date added to LUP: 2019-05-27 23:48:05
• date last changed: 2019-05-27 23:51:34