Study of Periodic Surface Nanostructures Using Coherent Grating X-ray Diffraction (CGXD)
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ABSTRACT It is shown that an x-ray interference phenomenon, Coherent Grating X-ray Diffraction (CGXD), can be used to study lateral nanostructure arrays on crystal surfaces and interfaces. Compared to Fraunhofer grating diffraction of visible light, x-ray grating diffraction contains information not only about geometric profiles of a surface, but also about the internal crystalline structures and lattice strain distributions in grating features. Grating diffraction patterns can also be measured in a white-beam Laue method using highly collimated polychromatic synchrotron radiation. This provides a way for parallel data collection and is useful in in-situ studies of structural evolution of nanostructure arrays.
INTRODUCTION Nanostructured semiconductor materials developed in recent years, such as nano-scale silicon 1.26 ', porous silicon3 4' , and quantum wire and quantum dot structures of III-V compounds5' 6 , are of great interests to the scientific community and semiconductor industry. Due to the effect of quantum confinement7' 8 , these small structures possess new optoelectronic properties substantially different from their bulk counterparts, and have the potential to make significant impacts on light-emitting devices and sensors in optoelectronic technology. For indirect-gap materials such as silicon, it has been known that when the crystal sizes approach a few nanometers, as in the case of porous Si or nano-scale Si particles, the crystal light is emitted from silicon in a band structure may be significantly modified so that visible 9
wide range of spectrum depending on various conditions . For direct-gap semiconductors such as GaAs and InlixGaxAs, substantial blue shifts in photoluminescence spectra have been ' . Although most observed in laterally structured quantum wires and quantum boxes 510 researchers in the field agree that the effect of quantum confinement plays an important part, a complete understanding of these unusual optoelectronic properties, however, is still lacking; it remains one of the very active areas of semiconductor research and development. One of the fundamental elements for better understanding and better engineering of nanostructure materials is the knowledge of intrinsic crystal structures and, in particular, the state of strain in these materials. In many cases, the effect of crystal lattice contraction or relaxation on electronic band structures can be equally (or even more) important when compared to quantum confinement effects. To date there have been little attempts to incorporate the strain effects in theoretical Z(x) quantum confinement analyses, largely because of the lack of accurate experix mental information on atomic-scale lattice distortions in the nanostructured materials. Fig.1. Schematic plot of a corrugated crystal In this article we show that a recently surface with a surface profile of Z(x). developed technique, Coherent Grating XRay Diffractometry (CGXD), can help re371 Mat. Res. Soc. Symp. Proc. Vol. 405 01996 Materials Research Society
searchers obtain suc
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