Non-destructive X-ray imaging of patterned delta-layer devices in silicon

Abstract: The progress of miniaturisation in integrated electronics has led to atomic and nanometre-sized dopant devices in silicon. Such structures can be fabricated routinely by hydrogen resist lithography, using various dopants such as phosphorous and arsenic. However, the ability to non-destructively obtain atomic-species-specific images of the final structure, which would be an indispensable tool for building more complex nano-scale devices, such as quantum co-processors, remains an unresolved challenge. Here we exploit X-ray fluorescence to create an element-specific image of As dopants in silicon, with dopant densities in absolute units and a resolution limited by the beam focal size (here $\sim1~\mu$m), without affecting the device's low temperature electronic properties. The As densities provided by the X-ray data are compared to those derived from Hall effect measurements as well as the standard non-repeatable, scanning tunnelling microscopy and secondary ion mass spectroscopy, techniques. Before and after the X-ray experiments, we also measured the magneto-conductance, dominated by weak localisation, a quantum interference effect extremely sensitive to sample dimensions and disorder. Notwithstanding the $1.5\times10^{10}$ Sv ($1.5\times10^{16}$ Rad/cm$^{-2}$) exposure of the device to X-rays, all transport data were unchanged to within experimental errors, corresponding to upper bounds of 0.2 Angstroms for the radiation-induced motion of the typical As atom and 3$\%$ for the loss of activated, carrier-contributing dopants. With next generation synchrotron radiation sources and more advanced optics, we foresee that it will be possible to obtain X-ray images of single dopant atoms within resolved radii of 5 nm.