X-RAY DIFFRACTION ANALYSIS OF FREE STANDING AND OVERGROWN SURFACE NANOSTRUCTURES
U. Pietsch 1, N. Darowski1, K. H. Wang2, A. Forchel2, Q. Shen3 and S. Kycia3
1 Institute of Physics, University of
Potsdam. Germany
2 Technical Physics, University of
Wuerzburg, Germany
3 CHESS, Cornell University, Ihaca,
N.Y., U.S.A.
Lateral surface wire nanostructures defined on a MBE grown GaAs / GaInAs / GaAs quantum well structure have beeen investigated before and after overgrowing with (Al)GaAs. The lateral periodicity of the free standing nanostructure was about 250\,nm. The triangular or rectangular shaped surface wires were prepared by wet-chemical or ion-beam etching, respectively, with a depth of about 50 nm on an area of 0.4 x 2 mm2. One half of the samples were partially or completely overgrown by a second MBE step.
The strain analysis was performed by high-resolution X-ray diffraction (HRXRD) and depth-resolved X-ray grazing-incidence diffraction (GID). Running reciprocal space maps close to a fundamental Bragg reflections of substrate we determined the average out-of plane and in-plane strain acting at the surface nanostructure. Whereas the in-plane strain appears dilative for the free-standing wires we found compressive strain after overgrowing. Using GID we became able to resolve the variation of in-plane strain as a function of depth below the surface. The evaluated strain field depends on the shape of wire and from the parameters of the overgrowing process. For perfect structures we identified an in-plane strain modulation within the overlayer. It shows the same lateral periodicity as the wire nanostructure of substrate and appears still up to the completely smooth sample surface. The experimentally determined strain parameters can be compared with the results of finite-element calculations. In particular, the strain acting at the quantum well can be used to interprete the energy shift of respective PL-experiments.
Acknowledgements: This work is supported by VW Foundation (Grant: I74007) and NATO (Grant CRG 970298)