MBE Growth of GaN
General
GaN molecular
beam epitaxy (MBE) growth is a non-equilibrium
process where a Ga vapor beam from an
effusion cell and an activated nitrogen beam
from a plasma source are directed toward a
heated substrate. Under suitable conditions,
layer-by-layer deposition of Ga and N atomic
planes is possible. The MBE procedure is
performed in an ultra-high vacuum chamber,
minimizing film contamination. Other
advantages of MBE are its capability to
create heterostructures with sharp
interfaces, and to form metastable phases
such as zincblende-structure GaN.
The Riber
MBE growth camber
In addition to
Ga, our MBE chamber is equipped with Si and
Mg cells for n- and p-type doping, as well as
In for surfactant studies and InGaN growth.
We also have available a Bi cell for
surfactant studies, and plan to install an Al
source for AlGaN growth in the near future.
The Nitrogen Plasma Source
The activated nitrogen
beam is supplied by a custom
DC plasma source designed and built by
the team of
Dr. Andre Anders here at LBNL. In this
source, excited neutral nitrogen molecules
are formed in an electric field-free region
and accelerated toward the substrate by the
pressure gradient with the vacuum chamber.
Because the activated nitrogen is not
accelerated by electric fields, it arrives at
the substrate with low kinetic energy on the
order of 1 eV. This approach allows reaction
to form GaN, but prevents film damage
associated with high kinetic energy species
such as those produced by RF plasma sources.
Our GaN layers exhibit low levels of defect
luminescence compared to layers grown with
other types of plasma sources.
Schematic
of the DC discharge plasma source
Surfactant Growth
The major
drawback to the MBE technique for GaN growth
is that it must be carried out at relatively
low temperatures (~700-800 C for MBE vs. 1000-1100
C for CVD techniques). As GaN is
thermodynamically unstable under vacuum, the
film actually can undergo decomposition into
Ga and nitrogen gas during MBE growth. At
high temperatures the decomposition rate
becomes faster than the deposition rate,
setting an upper limit on MBE substrate
temperature. Low substrate temperature
reduces surface atom mobility, leading to
increased defect densities in the GaN
epilayer. One way to get around this problem
is to increase the surface adatom mobility by
means of a surfactant impurity. Atoms such as
As, In, and Mg have exhibited surfactant
effects for GaN MBE growth. Below are some
results in our chamber using the Bi impurity
as a surfactant (LBNL patent
IB-1290). GaN layers
grown with optimized Bi surfactant flux
exhibit much larger grain sizes as observed
by AFM, and somewhat improved asymmetric x-ray
rocking curves.
