Measuring Friedel pairs in nanomembranes of GaAs (001)

Friedel pairs are susceptible to symmetry breaking in crystals. Under resonant scattering conditions, non-centrosymmetric crystals can give rise to pairs of hkl and $$\bar{h}\bar{k}\bar{l}$$ reflections with different diffracted intensities, which are quantified as an anomalous signal of the structure. In bulk crystals, the shift in the anomalous signal through an absorption edge can be measured with good accuracy regardless the crystalline quality of the sample, leading to experimental values in agreement with theoretical ones. With the advance of nanotechnology and synchrotron sources, it has been possible to produce free-standing nanomembranes of semiconductor crystals, opening the opportunity of checking the measurability of anomalous signal in nanoscale materials. In this study, we describe a successful procedure to measure the anomalous signal in nanomembranes of GaAs (001) 15-nm thick with synchrotron radiation. Different membrane processing methods and diffraction geometries were tested, and major sources of instrumental inaccuracy were identified. Relevances of this type of measurements in nanotechnology as well as in basic science are discussed.