Molecular
Beam Epitaxy
Molecular
beam epitaxy (MBE) is a set of ultrahigh vacuum (UHV) techniques
developed since the late 1970s. This breakthrough makes possible
highly controlled coherent epitaxial growth under non-equilibrium
conditions. MBE operates in a UHV environment, typically <10-10
torr. By heating and evaporating highly purified elemental sources,
i.e. atomic beams, atomic layers can be deposited on the substrate
one-at-a-time, and they can be forced to adhere to the same lattice
structure of the substrate, i.e. epitaxy. Therefore, by choosing a
particular substrate and a set of growth parameters, one can produce
non-equilibrium crystalline structures, which in turn makes it possible
to tailor and examine materials and properties on atomic scale.
Our
implementation of the combinatorial approach involves varying
structural or compositional parameters on a
single substrate, which can dramatically enhance the speed of probing
the large
parameter space and resolve abrupt phase transitions. The
essence of controlled combinatorial MBE is the ability to
place a
tailored set of materials parameters, such as
composition, film thickness, and epitaxial strain, onto a single
substrate and to examine the corresponding properties systematically.
Each combinatorial sample could have one or two variables that
systematically change across the substrate, strategically slicing
through the parameter space to expose corresponding physical
phenomenon.
For example, by moving sample masks at a
constant speed and constant flux, one can deposit a linear composition
profile on the substrate. This way, one can “program” one
variable onto the sample. The steps can be repeated to deposit multiple
composition profiles with a number of elements.
Advanced
Combinatorial MBE System
The
advanced combinatorial MBE growth and analysis system is integrated by
an in-situ UHV sample handling, storage, and transfer system. It
consists of a growth chamber, a room temperature UHV SPM with both STM
and AFM capabilities (Park Scientific SPM VP), a variable temperature
UHV optical magnet cryostat, capable of simultaneous SPM and MOKE
experiments with a temperature range of 1.6 - 350 K and a field range
of 3 T, a loadlock, and two storage chambers. The growth
chamber is equipped with 1) a high precision, stepper motor
controlled, high temperature sample manipulator 2) stepper
motor-controlled precision sample masks 3) up to 8
independently controlled
evaporation sources, including e-beam hearths, effusion cells, and
atomic absorption spectroscopy flux monitors 4) a 30 keV
Staib/k-Space real-time scanning RHEED CCD imaging system.
Growth chamber schematic
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