Theoretical investigation of the growth process of Boron Nitride from small clusters to bulk structures

Abstract

The aim of this dissertation is to gain a better understanding of the peculiar growth processes of boron nitride (BN) crystals. BN can grow in graphite-like or diamond-like structures. The diamond-like crystal structure is the global energy minimum at zero temperature and pressure, while the graphite-like structure is metastable. However, when one tries to grow BN at low temperature and pressure, the structure that emerges is the metastable graphite-like structure. Only under high temperature and pressure is one able to produce the diamond-like BN. The question is: Why does BN grow so easily in the (metastable) graphite-like structure at low temperature and pressure? One possible explanation is that very small clusters of BN are actually more stable in the graphite-like structure. When the crystal grows to larger and larger structures, eventually the diamond-like structure will become more stable. The assumption is that the crystal is trapped on the graphite-like side of the energy barrier, and so only high temperature and pressure will allow it to change into the more stable structure, the diamond-like BN. In this dissertation we use theoretical calculations employing density functional theory (DFT) in the local density approximation (LDA) with a plane wave basis set to examine the validity of this explanation. We therefore calculate small clusters of boron and nitrogen atoms and find a very stable building block for BN to be the hexagonal BN ring consisting of three boron and three nitrogen atoms. With this building block we create larger structures, attaching the BN rings in the plane or stacking them in columns. The graphite-like structure can easily be reproduced and is definitely (meta)stable. But we are unable to produce a diamond-like structure within the size of structures that we investigated. This means that the dense structure is not only less stable for small systems, it is actually unstable.