Extreme Epitaxy
Supervisor: Dr. Gavin Bell
Background
Crystals are regular arrays of atoms. Epitaxy – from the Greek for
“arrangement on” – refers to the relationship between two crystals
across a joining interface. A suitable match between the sets of
principal crystal axes gives rise to an epitaxial system. Layers of a
foreign crystalline material can be grown on a crystalline substrate
using a technique called molecular beam epitaxy (MBE). This is
conceptually simple – we throw atoms and molecules at a surface, they
interact, and the new crystal grows. Less simple is the multi-scale
nature of MBE: atomic-scale interactions with frequencies ~1013 s-1 lead
to the formation of crystal structures over many nm to microns in
scale, taking seconds to hours. Rigorous experiments coupled with
modelling can tease out the important processes and allow us to
optimise MBE to control the properties of the grown crystal. A simple
example with huge technological importance in semiconductor
optoelectronics is the pairing of AlAs and GaAs, which have the same
cubic crystal structure and nearly the same interatomic spacing, and
allow sandwich-like quantum well structures to be grown with atomiclayer precision. Remarkably, this process of heteroepitaxy can produce
not just flat layers of the new crystal, but self-assembled
nanostructures such as quantum dots (QDs). The nicest example is InAs
on GaAs – in contrast with AlAs, InAs is has a 7% larger lattice than
GaAs. The elastic energy produced by squashing the InAs lattice is too
great, and the structure relaxes to form islands (the QDs) rather than
flat layers. Cases of extreme heteroepitaxy involve no match between
two or more of the key bulk crystal properties: symmetries, elastic
properties, bonding type or lattice parameters. Examples are MnSb on
GaAs or Ge, which are combinations of materials we are developing for
spintronic applications. Further examples we are working on are
graphene or hexagonal boron nitride (h-BN) on Cu and Ni. In this
project you will investigate several cases of extreme heteroepitaxy,
using MBE. The aim is to understand the fundamental atomic-scale
processes and control the growth to optimise materials properties of
the grown crystals. Scanning tunnelling microscopy (STM) will be the
main technique used to image growing surfaces at atomic resolution.
Example atomic lattice images are shown in Fig. 1 (the top left 100 nm
image shows a threading dislocation at a MnSb surface).
Fig. 1 – sample STM results from
MnSb on GaAs (top, blue) and
graphene on Cu (middle, orange).
The bottom panel is a low energy
electron diffraction pattern
showing the complex epitaxial
relationship between the faceted
Cu (100) + (210) surface and the
graphene overlayer (hexes).
Project outline
This project is principally experimental, although there will be plenty of opportunities to theoretically model
the structures and growth processes (see below). You will use MBE and chemical vapour deposition (CVD)
systems in the Surface, Interface & Thin Film Group in Warwick. Growth by MBE will be investigated in situ
using electron diffraction and STM. We will use X-ray diffraction (XRD) and transmission electron microscopy
(TEM) to measure the resulting film structures ex situ. The XRD experiments will be performed in Warwick
and at synchrotron radiation (SR) sources such as Diamond Light Source (Oxfordshire), NSLS (New York State)
and SPring-8 (Japan). We may also do some STM experiments using a unique “STMBE” system in Japan built
by a long-term collaborator, Professor Shiro Tsukamoto. The plan is to examine fundamental growth
mechanisms in several interesting extreme heteroepitaxial systems (graphene, h-BN, MnSb and related
magnetic materials) which link to other on-going projects. In parallel, we will try some more high-risk MBE
growth systems. One example is MnSb on salt (ordinary NaCl) which allows the substrate to be dissolved
away to release a free-standing ultra-thin film magnetic crystal. Another is magnetic nanowires grown on
SiO2 /Si(111). The most interesting systems will be studied with detailed STM and supporting techniques, and
suitable growth models constructed.
Fig. 2 – example nanostructures which can be
grown by MBE using “bottom-up” self-assembly. On
the left are InAs nanowires grown on SiO2/Si(111)
and centre InAs QDs grown on GaAs(001). Green
images are by scanning electron microscopy. The
STM image on the right is a 3D render of a single
InAs QD.
Techniques such as MBE and STM, which rely on ultra-high vacuum technology, CVD, microscopy and SR
experience are all highly valuable skills for a future career in science. You will be able to collaborate with
both growth modelling specialists and the Warwick Microscopy Group. The thin-film and nanostructured
crystals we will be growing are applicable to a range of technologies (e.g. magnetic pnictides for spintronics)
and collaborations with academic and industrial partners will very likely flow from the fundamental growth
studies in this project.
Theoretical work (optional but encouraged)
Static layer structures can be understood by modelling with density functional theory (DFT). We use the
CASTEP and Munich SPR-KKR packages but may investigate the linear scaling SIESTA package for
nanostructures with large numbers of atoms. The aims of the DFT work will be to understand nano-scale
strain and energy landscapes for the structures formed in MBE. Growth dynamics can be simulated using
kinetic Monte Carlo (kMC) or molecular dynamics (MD) approaches. All theoretical work will be done in
collaboration with specialists, e.g. Dr. Paul Mulheran (Strathclyde University – kMC) or Dr. David Quigley
(Warwick – MD), and would strengthen a principally experimental Ph.D.
Project goals


Understand growth dynamics and optimise MBE/CVD for key extreme epitaxy systems (magnetic
pnictides on semiconductors; graphene and h-BN on metals).
Investigate unusual heteroepitaxial growth modes in more exotic systems (e.g. nanowires, salt).
Further reading
Atomistic insights for InAs quantum dot formation on GaAs(001) using STM within a MBE growth chamber
S. Tsukamoto, T. Honma, G.R. Bell, A. Ishii, Y. Arakawa and N. Koguchi, Small (2006), vol. 2, p. 386
Transformation kinetics of homoepitaxial islands on GaAs(001)
M. Itoh, G.R. Bell, B.A. Joyce and D.D. Vvedensky, Surface Science (2000), vol. 464, p. 200
Paper in preparation on graphene epitaxy – available on request.