MEASUREMENTS OF SPATIAL DISTRIBUTION OF STRAIN IN QUANTUM WIRE STRUCTURES BY COHERENT GRATING X-RAY DIFFRACTION. Qun Shen, Cornell High Energy Synchrotron Source (CHESS), Cornell University, Ithaca, New York

One of the fundamental elements for better understanding and better engineering of nanostructure materials, such as quantum wires and dots, is the knowledge of intrinsic crystalline quality and the state of strain in these ultra small structures. In many cases, the effect of crystal lattice distortion on electronic band structures can be equally or more important compared to quantum confinement effects.

In recent years there are increasing interests in using high-resolution x-ray diffraction to study the structures of these quantum wire and dot arrays. Constructive interference among the wires or the dots within the coherence width of an x-ray beam produces diffraction satellite peaks around each crystal Bragg reflection. This phenomenon of Coherent Grating X-ray Diffraction (CGXD) enhances the scattering signal from individual wires or dots, much like the case of large-unit-cell crystallography. The grating x-ray diffraction pattern contains the information not only on geometric surface profiles of the wires or dots, but also on possible imperfections in the array, and crystalline registry with respect to substrate. In addition, the coherent interference within the individual wire or dot can give rise to distinct satellite peak intensity modulations in reciprocal space that are determined by the spatial distribution of the interfacial strain field. Thus this method can provide a way of directly mapping the strain distribution in a quantum wire. It also eliminates or greatly reduces the usual problem of poor strain sensitivity due to the inherent size-broadening in diffraction from small crystals.

We have applied the CGXD technique to various nanostructure arrays, including a series of 10 nm-thick InGaAs/GaAs (001) quantum wire samples, with wire widths ranging from 50 to 300 nm and array period from 400 to 1000 nm. Our measured values of strain in these wires show a substantial increase when the wire width becomes smaller. This result strongly suggests that the strain is the principle cause for the extra blue shifts in the photoluminescence spectra of the quantum wires.