Joanna Millunchick

Atomic Surface Structure in Compound Semiconductor Alloys: Mechanisms for Surface Segregation in Compound Semiconductor Thin Films

Collaborators: Dr. Normand Modine, Sandia National Laboratory; Prof. Anton van der Ven, University of Michigan-Ann Arbor
Sponsor: Department of Energy
Our goal is to determine the atomistic mechanisms for surface segregation in III-V semiconductor alloy systems. Surface segregation has significant technological impact, since it affects the interfacial abruptness and compositional uniformity in device structures. However, the details of atomic processes that govern surface segregation are still unknown, and a comprehensive theory for this phenomenon has not yet developed. Unlike the study of binary systems such as GaAs, InAs and InP, the ternary systems have been consistently underrepresented in scientific literature both experimentally and computationally despite its importance to optoelectronic device technology. We are investigating the surface atomistics through both experimental molecular beam epitaxy (MBE) growth coupled with in-situ STM as well as first-principle density functional theory (DFT) calculations using the Vienna Ab-initio Simulations Package (VASP). The primary focus of our simulations work is concerned with the surface reconstructions of ternary systems, mainly InGaAs and InGaSb, in both lattice-matched and strained epilayers.
Highlights (Click an image for more information)
  • Mixed Surface Reconstruction in Sb/GaAs(001)

    1.7monolayers of Sb/GaAs(001) exhibits a mixed surface reconstruction of α(4x3), common to GaSb, and α2(2x4), uncommon in GaSb. The α(4x3) appears to nucleate first forming small islands up to 30nm2. For islands larger than this critical thickness, α2(2x4) nucleates in the center of the island. We have been able to show this is due to strain relaxation and the ability of α2(2x4) to better relax the lattice mismatch strain in the system.

  • Stability of the z(4x4) reconstruction

    This graph shows the formation energy (eV) of both a straight row α2(2x4) reconstruction (R-Model) and a zigzag row α2(2x4) reconstruction (Z-Model) for GaAs slabs alloyed with In. In replaces the surface cation positions and the concentration of In is denoted XIn. Constructed using Density Functional Theory calculations, it shows that indeed the zigzag configuration, or z(4x4) reconstruction, is lower in energy under most alloying conditions.

  • GaSb/GaAs

    This shows the structure of GaAs films on GaSb with increasing thickness from (a) 0.5ML (b) 0.75ML (c) 1.0ML (d) 1.5ML (e) 2.0ML and (f) 3.0ML which shows Quantum Dots.

  • 2x8 reconstruction

    RHEED and STM of t=5.6s (2ML) Sb/GaAs grown at RSb=0.36ML/s and cooled under RSb=1.5ML/s.  (a) [1-10] RHEED pattern just after reducing Sb flux,  (b) [1-10] RHEED pattern upon completion of cooling,  (c) STM of surface taken at I=100pA and V=-3.43V.

  • DFT calculations of pure GaSb

    This image shows the results of DFT calculations of pure GaSb at different lattice parameters.  In each of the curves the lowest energy at a give chemical potential is the thermodynamically stable reconstruction.  Left indicates Sb poor conditions and right indicates Sb rich conditions.  

  • Proposed structures for the GaSb/GaAs-(2x8) reconstruction

    This is a collection of proposed structures for the GaSb/GaAs-(2x8) reconstruction.  The center column shows simulated STM images and right column shows simulated line scans of these images for comparison with experimental results.

  • Ming - nanoFET