Wistey Group Research Page
ÉPÉE Lab: Epitaxial Photonics & Electronics Engineering
What we do: Solve physical and material problems in order to build new cool devices.
We push the boundaries of growth and devices, to create "unusual but useful"
combinations of materials and semiconductor physics.
We create lasers, photovoltaics, transistors,
modulators, and detectors, and the materials for new devices,
and ultimately things like optical neural interfaces.
Silicon Photonics and Heteroepitaxy
Dilute Germanium Carbide alloys:
Growth of Ge:C can be tricky if carbon atoms are allowed to touch
each other. By synthesizing the right precursor molecules in
electronic grade purities, dilute Ge:C can be grown. Several
theoretical models predict a direct bandgap from this material
due to band anticrossing.
A project I initiated led to the discovery of techniques for growing
defect-free Ge on Si using novel gas precursors. This enables both
GaAs-based optoelectronics and III-V CMOS on silicon. The Ge was
nearly strain-neutral at typical device temperatures, providing
improved reliability for high power devices.
Developed a device process flow for GeSn photodetectors and
modulators in the mid-IR, making silicon photonics a realistic
possibility for future optoelectronics.
Silicon is notoriously unreactive with light, and the other Group IV
elements (Ge, Sn, C) are hardly better. My research has opened
several avenues which are likely to produce efficient Group IV
photodetectors, solar cells, and even lasers. Stay tuned! Or better
still, come join a group spanning from interesting physics to applied
Photovoltaics with Higher Efficiency and Lower Cost
We are extending high efficiency multijunction solar cells to
inexpensive, robust substrates such as silicon and potentially metal.
This is possible through "zero thickness" buffer layers which change
the lattice constant at the surface without introducing dislocations
in the subsequent growth.
We have recently been developing new techniques for efficient upconversion
of low-energy photons to high-energy electrons, which would markedly
increase the efficiency of existing inorganic PV designs (silicon and
III-V TFETs and MOSFETs
As a postdoc at U.C. Santa Barbara, I led the growth and fabrication teams
the first scalable III-V MOSFETs, using self-aligned techniques.
These are the first
III-V FETs which can be scaled to nanometer dimensions (20-50nm),
to extend Moore's Law for several generations beyond silicon.
This work is a delightful collaboration among multiple groups and
different universities, ranging from atomic theory to circuits and
from California to Massachusetts.
At UCSB (as at Stanford), the boundaries between departments are
fuzzy, leading to some of the most exciting research collaborations,
with different viewpoints available to tackle any problem, and
interesting new challenges every day. Our group enjoys the same spirit
of collaboration at Notre Dame.
- Self-aligned, low resistance contacts:
Molecular beam epitaxy (MBE) is a line-of-sight deposition technique,
but my work has been able to fill in recesses under the edges of the
gate in a MOSFET. We have also produced the lowest contact resistances
to InGaAs to date. Low resistance is crucial for breaking the 1 THz
barrier with transistors at 1000 GHz.
Additional Expertise: Photonics and Highly Mismatched Alloys (Dilute Nitrides, GaInNAs)
1540nm VCSELs: The first electrically-pumped, GaAs based (GaInNAsSb, dilute nitride)
VCSELs in the telecommunication band near 1550 nm, for inexpensive, high
speed fiber communications. (See reprints at left.) My
thesis (5.9 MB) contains many of the little lessons about growth of
thermodynamically unstable materials such as dilute nitrides, in order
to produce laser-quality material.