Doping Studies of Gallium Oxide Wide Band Gap Semiconductors
Lauren M. Garten
1^
, Joseph Waters
1
, Isa Ferrall
1
, Laura T. Schelhas
2
, Michael F. Toney
2
,
Brian Gorman
3
, Paul Ndione
1
, Stephan Lany
1
, Andriy Zakutayev
1
, David Ginley
1
1.National Renewable Energy Laboratory, Golden, CO 80401
2. SSRL Materials Science Division, SLAC National Accelerator Laboratory, Menlo Park CA
3. Colorado School of Mines, Golden, CO 80401
E-mail
Gallium oxide is a wide band gap semiconductor with the potential to be
transformative in a number of optoelectonic and power electronic markets. However, the
inter-relationship between defects, dopants, and processing in this material is not well
understood and is critical before gallium oxide based technologies can have significant
technological application. Recent DFT calculations show gallium vacancies as the dominant
defect, and silicon and tin as viable dopants in Ga
2
O
3
. Silicon is expected to act as a shallow
donor in Ga
2
O
3
, avoiding issues of Fermi level pinning that could limit the performance of tin
doped materials. The theory indicates the range of potential process space where dopants
could be activated. This work reports on work trying to realize the theoretical predictions for
silicon or tin doping on the optical, electrical, and structural properties of Ga
2
O
3
thin films as
a function of processing. The films were deposited from either a Si or Sn doped Ga
2
O
3
target
onto (0001) sapphire substrates from room temperature to 750
°
C using combinatorial pulsed
laser deposition.
The conductivity and transparency of the films grown during deposition
were found to be dependent on the processing temperature and p
O2
, with films processed
below 450 °C in reducing environments exhibiting decreased transparency and increased
conductivity. STEM-EDS results show the segregation of tin at concentrations greater than 2
wt% Sn doping. Thus the traditional approach of deposition in reducing environments results
in phase decomposition, and therefore alternative approaches of deposition and processing
are required. DFT calculations predict routes to increased dopant activation through post-
annealing films with lower dopant concentrations in highly reducing atmospheres. To test
this, amorphous doped gallium oxide films were crystallized in-situ during electrical
measurements using a custom made low p
O2
, electrical testing furnace to provide insight into
the defect compensation and dopant activation in this system as a function of temperature and
oxygen partial pressure. Additionally, in-situ x-ray diffraction was taken on these amorphous
films during rapidly thermally annealing under the same temperature and atmospheric
conditions as the in-situ electrical measurements, which shows p
O2
dependent phase
transformation pathways into the β-Ga
2
O
3
phase.
This work was supported, in part, by the Center for the Next Generation of Materials
by Design, an Energy Frontier Research Center funded by the U.S. Department of Energy,
Office of Science, Basic Energy Sciences.
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