Nanoscale Engineering of Strain: From Alloying in Epitaxial Nanocrystals to Multijunction Solar Cells

Marina Leite, California Institute of Technology

Wednesday, March 30, 2011

Spanos Auditorium

This seminar is part of the Jones Seminars on Science, Technology, and Society series

Strain engineering at the nanoscale enables the control of a semiconductor material's bandgap energy, which can be used for optoelectronic device applications. Depending on the mechanical strain (elastic energy) between a substrate and an epitaxially grown material, different morphologies can be achieved ranging from strained planar films to 3-dimensional nanocrystals. In the first part of my talk I will show how Ge-Si:Si(001) coherently-strained islands were used to implement an open (closed) system, in which matter is (not) exchanged through surface diffusion. The driving forces and the role played by the different alloying mechanisms, while the system approaches the equilibrium, provide a better understanding of why and how alloying takes place for dome-shaped Ge-Si islands. In the second part of the talk I will discuss a new approach for multijunction solar cells based on direct bandgap III-V semiconductor alloys. Device simulations indicate that efficiency over 50 % can be achieved at 100-suns illumination by using a 4-junction cell formed by InAlAs/InGaP/InGaAsP/InGaAs. For the top subcell, InAlAs, we fabricated wide bandgap solar cells with efficiencies > 14 % and Voc = 1 V. The ideal bandgap energy combination corresponds to a lattice parameter of 5.80 Å, which is not available in bulk form. Therefore, we created a "virtual substrate" for epitaxial growth. By relieving 40 nm thick coherently-strained compressed and tensile InxGa1-xAs films from InP substrates, full elastic relaxation occurs preserving the crystalline quality of the films. This method can be implemented to achieve optoelectronic devices with unique properties, including high efficiency solar cells.

About the Speaker

Dr. Marina Leite received her B.S. degree in Chemistry from University of Pernambuco, Brazil in December 2000. She received both her M.S. and Ph.D. in Physics from Campinas State University (UNICAMP) in 2003 and 2007, while working at the Brazilian Synchrotron Light Source Laboratory. In her M.S. program she worked with the synthesis and structural characterization of colloidal nanoparticles. During her Ph.D., she investigated the growth thermodynamics of self-assembled epitaxial Ge-Si nanocrystals in both the kinetic and the quasi-equilibrium regimes. In 2006 she was a visiting researcher at the Max-Planck Institute for Solid State Physics and the Eindhoven Technical University in the Netherlands. In 2008 she joined Harry Atwater's group at Caltech as a postdoctoral scholar, working on multijunction solar cells.