A NEW X-RAY PHASE RETRIEVAL METHOD FOR IMAGING BURIED INTERFACES

Divine P Kumah

Applied Physics Program, University of Michigan, Ann Arbor, Michigan 48109-1120, USA dkumah@umich.edu                                                       

Abstract

The Coherent Bragg Rod Analysis (COBRA) phase retrieval algorithm has recently been developed to obtain 3D electron density maps with sub-angstrom resolution of thin film structures with in-plane periodicity from x-ray diffraction measurements using synchrotron x-ray sources. The application of COBRA would be presented for two systems: LAO/STO[1] and GaAs/InGaAs[2]. The physics responsible for the formation of a high-mobility, 2-dimensional, electron gas at the interface of two insulators has been the subject of considerable research and controversy since its discovery in the LaAlO₃/SrTiO₃ system. We observe a dilated, mixed-valence interface which modifies the electronic band structure, lowering the minimum of the conduction band below the Fermi level and thereby rendering the dilated interface conducting. In particular the COBRA measurements reveal the formation of an interfacial LaSrTiO₃ layer with an accumulation of trivalent Ti at the interface which is responsible for the lattice dilation and minimizes the electrostatic energy at the TiO₂-terminated SrTiO₃ substrate surface. Using COBRA, the formation of the quasi two-dimensional electron gas is explained, based on structural considerations. For the GaAs/InGaAs system, a spectroscopic approach to COBRA reveals how an interplay between intermixing and surface roughening leads to an efficient relaxation of strain.

P. R. Willmott, S. A. Pauli, R. Herger, C. M. Schlepütz, D. Martoccia, B. D. Patterson, B. Delley, R. Clarke, D. Kumah, C. Cionca, and Y. Yacoby. Phys. Rev. Lett. 99, 155502 (October 9 2007)

[1] D. P. Kumah, A. Riposan, C. N. Cionca, N. S. Husseini, R. Clarke, J. Y. Lee, J. M. Millunchick, Y. Yacoby, C. M. Schlepütz, M. Björck, and P. R. Willmott. Appl. Phys. Lett. 93, 081910 (2008).