SusChEM: Direct Bandgap Ge:C on Silicon for Optoelectronics

Texas State University - San Marcos, San Marcos TX

Investigators

Abstract

Non-technical Description: This SusChEM project explores a new approach to grow Group IV materials with high electron-photon conversion efficiency and potential for future solid state lasers and compact optoelectronics compatible with silicon. These materials could enable faster data connections between computer chips while consuming less energy. The approach is to add a small amount of carbon to germanium to make a new alloy semiconductor, Ge:C, with strong optoelectronic properties, making it a potential alternative for laser materials besides the conventional III-V materials. A new synthesis and growth technique eliminates the major sources of defects in the grown material. Ge:C-based material replaces toxic and scarce elements such as arsenic and indium in semiconductor lasers. The research project motivates first-year undergraduate students in engineering and physics, and encourages undergraduate students to pursue graduate studies. Outreach activities include involving women in science and underrepresented minority high-school students in the research. Technical Description: The goal of this project is to demonstrate Ge:C-based alloy semiconductors with a direct bandgap and high optical gain. The films are grown by molecular beam epitaxy using tetra(germyl)methane (4GeMe) as a precursor, which is synthesized in electronic grade by the team. The specific tasks are: (1) Experimental verification of band anticrossing in Ge:C and related alloys; (2) ab-initio simulations of band structure and desirable compositions for Ge:C materials; (3) Growth of highly crystalline Ge:C with minimal interstitials or other unwanted defects; and (4) Growth with pseudo/surfactants for substitutional C, and demonstrating direct bandgap emission. The advantage of the 4GeMe precursor is that it pre-bonds each C atom to four Ge atoms, preventing undesirable carbon-carbon bonds at the substrate surface. By eliminating these defects, this study provides direct experimental demonstration on highly mismatched alloys with small bandgaps and enables the predictive modeling to design related highly mismatched alloys with high optical gain and suppressed Auger recombination. This experiment and theory combined project closes the knowledge gap between the thin film growth and modeling of the Ge:C thin film systems, and provides a chemical route to grow highly crystalline Ge:C with direct bandgap and high optical gain.