Non‐Destructive X‐Ray Imaging of Patterned Delta‐Layer Devices in Silicon

Abstract

The ability to build nanometer-scale dopant structures buried in Si has led to great progress in classical and quantum technologies. [1] As the patterned structures become increasingly small and complex, it becomes indispensable to develop techniques to non-destructively image the dopant structures for device inspection and quality control. [2][3][4] Scanning tunneling microscopy (STM) can be used to pattern acceptors and donors into Si with atomic resolution using hydrogen resist lithography. [5,6] The technique has created complementary metal-oxide-semiconductor compatible structures, including 2D conductive sheets, [7] 3D structures, [8] nano-wires, [9] and quantum dots. [10] Precisely measuring the location of buried dopants patterned by STM is challenging and can only be accomplished with STM itself for patterns extremely near to the surface. [11,12] Techniques capable of imaging such nanoscale structures such as secondary-ion mass spectrometry (SIMS) [13] and atomThe progress of miniaturization in integrated electronics has led to atomic and nanometer-sized dopant devices in silicon. Such structures can be fabricated routinely by hydrogen resist lithography, using various dopants such as P and As. 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, X-ray fluorescence is exploited to create an elementspecific image of As dopants in Si, with dopant densities in absolute units and a resolution limited by the beam focal size (here ≈1 µm), 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 tunneling microscopy and secondary ion mass spectroscopy, techniques. Before and after the X-ray experiments, we also measured the magneto-conductance, which is dominated by weak localization, a quantum interference effect extremely sensitive to sample dimensions and disorder. Notwithstanding the 1.5 × 10 10 Sv (1.5 × 10 16 Rad cm −2 ) exposure of the device to X-rays, all transport data are 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, the authors foresee that it will be possible to obtain X-ray images of single dopant atoms within resolved radii of 5 nm.