Synthesis of Complex Semiconductors from Atoms That Don't Want to Mix

General Overview:  The
Wisconsin MRSEC is investigating innovative methods to incorporate a greatly
expanded diversity of atom types into semiconductors, thus yielding materials
with a new range of electronic properties.  Semiconductors are the foundation of modern electronic
and photonic devices, and are widely used in technologies such as solar cells,
LEDs, and microprocessors.  Incorporating
new atom types into semiconductors will expand the range of semiconductor
materials and potentially lead to a new generation of devices.  The Wisconsin MRSEC has focused on developing
methods that permit incorporation of atomic bismuth (Bi) into a widely used
semiconductor comprised of gallium arsenide (GaAs).  Incorporating Bi into GaAs is challenging due
to the significant size differences between the atoms, which results in Bi migrating
to the surface rather than forming a mixture.  By carefully controlling the rate at which Bi
atoms were added to GaAs and using special imaging tools to characterize the
level of Bi incorporation, the Wisconsin MRSEC unexpectedly discovered a growth
process that leads to incorporation of high concentrations of Bi.  This finding promises to make the process of growing
complex
semiconductor materials more
efficient such that these materials could eventually be synthesized in the
quantities needed for more efficient commercial semiconductor-based devices from
solar cells to cell phones. 

Technical
Description: Wisconsin MRSEC researchers grew metastable
GaAs_1-yBi_y materials using metalorganic vapor phase
epitaxy (MOVPE), a process utilizing surface chemical reactions  to enable controlled growth of complex multilayer
crystals. The Bi concentration profile that developed during growth of the GaAs_1-yBi_y
materials was analyzed using high-angle annular-dark-field (“Z-contrast”)
imaging in an aberration corrected STEM.  Surprisingly, these studies show
that surface adsorbed Bi atoms incorporate into the growing alloy more readily
when the Bi precursor is absent from the growth environment.  Consequently, the most Bi-rich material in the
superstructure grew when GaAs barrier growth was intended. Pulsing of the Bi
source has proven effective in enhancing Bi incorporation during growth.
Recognizing when Bi is incorporated is essential when designing superstructures
and their growth.  Wisconsin MRSEC
researchers also grew GaAs_1-yBi_y films using molecular
beam epitaxy (MBE) which is a physical rather than a chemical deposition
process.  These results could lead to increased
efficiency in Bi incorporation resulting in growth methods that can be used to
in commercial production of GaAs_1-yBi_y containing
devices.