Surprises from Growth of Highly Mismatched Semiconductor Alloys
Wednesday, Apr 1st, 9:30-10:30 am
GLT 4.102
https://utexas.zoom.us/j/82683336638
Center for Dynamics and Control of Materials: MRSEC Seminar
In conventional semiconductors, adding small atoms decreases the lattice constant and increases the bandgap. But adding very small atoms like C or N actually decreases the bandgap, violating decades of intuition. The band anti-crossing (BAC) model explains this: the new atom introduces a state above the conduction band edge, and the two states repel each other. Ge:Cis especially intriguing because it has a direct bandgap and grows on Si, enabling on-chip lasers and silicon photonics.
Adding a larger atom might seem to restore conventional trends, but the bandgap sometimes increases and sometimes decreases, with contradicting results across research groups. Ab-initio (VASP) simulations show that highly mismatched atoms can be canceled out by atom arrangement, whether the mismatched atom is an anion, cation, or purely covalent.
Highly mismatched alloys (HMAs) want to segregate, and thermodynamics alone suggests they can't be grown. But kinetically-limited techniques like molecular beam epitaxy (MBE) have produced world record-setting lasers and solar cells. Photoluminescence from Ge:Cwith no detectable threading defects will also be shown.
HMAs are sensitive to contamination and surface damage during growth. Inattention produces unusual features such as nano-geodes (voids lined internally with tin) and "nanomarshmallows."
Assumptions about material growth and properties deserve periodic re-examination, no matter how widely accepted. AI may not be the only source of wrong intuition in the room.