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.
  • Heterogeneous integration: 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 photonics: 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 devices.

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 multijunctions).

III-V TFETs and MOSFETs

  • As a postdoc at U.C. Santa Barbara, I led the growth and fabrication teams which made 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 PhD 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.