Research objectives and approaches


  • Fabrication of new high quality and stable (Si)GeSn alloys relies on developing a better understanding of the material system: There is little knowledge of equilibrium growth, let alone non-equilibrium growth, of the group IV alloys as well as no comparison between the atomistic ordering and even less information on the trade-offs made in stability, strain energy, and quality of current and higher Sn compositions. Our approach is to directly focus a “closed-loop” growth-modeling-properties research effort on (i) comparing and understanding the difference in material quality and stability from different non-equilibrium growth methods for the exact same high Sn composition; (ii) investigating the (Si)GeSn alloy system stability, by observing and studying atomistic positions and Sn nucleation using in-situ STM; and (iii) exploring different techniques for achieving up to 20% to 40% Sn, such as, the use of surfactant, migration enhanced MBE, ordered growth, nanoscale structures, atomic hydrogen enhancements, and superlattices.
  • Determine their fundamental structural, electrical, optical and stability properties: Most of the basic material parameters critical for designing, characterizing and modeling infrared devices do not exist for different compositions, such as, bandgap, optical transition probability, carrier mobility, type of defects, stability, and band offset. Our approach relies on: (i) we will measure these parameters directly building a needed database that is also shared with the community. (ii) newly calculated equilibrium phase diagram, tested by experiment, will provide an understanding of what material stability is possible; (iii) atomistic level material modeling to predict properties observed by RHEED, XRD, XTEM, STM and atomic probe tomography; (iv) an iterative parameter tuning will be used to simultaneously calibrate the model and determine the fundamental material property parameters; and (v) techniques including substrate removal will be used to eliminate any possible ambiguity due to strain/interface for data interpretation.
  • Demonstrate the group IV material system’s ability to compete and beat state-of-the-art infrared detectors: A key factor needed to implement the (Si)GeSn material system is to demonstrate its potential as a stable and high-quality detector technology. In our approach, with material parameters determined in objective 2 we will: (i) research intrinsic, extrinsic properties and different device structures to cover the MWIR band; (ii) establish baseline characteristics to benchmark the quality of new fabricated detectors; (iii) explore aspect ratio trapping to eliminate defects from the substrate; (iv) build on the success of “nBn” structures to extend the carrier blocking mechanism in the (Si)GeSn system by developing a “pBp” structure to block holes for superior performance; (v) study a new MWIR CMOS imaging sensor utilizing built-in circuits with time correlated double sampling to eliminate thermal noise.