Progress in the Fabrication and Imaging of Scanning Tunneling Microscopy Patterned Dopant Nanostructures in Silicon

Condensed Matter Seminars

Speaker: Taylor Stock, London Centre for Nanotechnology, University College London, UK
Date & Time: Fri, 2017-08-25 14:00 - 15:00
Location: AMPEL #311
Local Contact: Sarah Burke
Intended Audience: Graduate

Device structures consisting of 2D patterned dopant atoms in silicon can be fabricated with near atomic precision using the technique of scanning tunnelling microscopy (STM) hydrogen desorption lithography. These types of devices, such as atomic scale wires and single atom transistors, can exhibit a variety of interesting quantum phenomena due to atomic scale spatial confinement of electrons within the structures. Traditionally, the patterned dopant of choice for this technique has been phosphorus (P). Expanding this technique to include a second species of donor impurity atom will provide new possibilities for device structure and function. Here we describe a detailed study of the STM patterning of arsenic (As) atoms in silicon. This is achieved by replacing the P precursor gas (PH₃) with the As analogue, AsH₃. We find that AsH₃ and As are compatible with the multiple process steps involved in the STM fabrication, but also report a number of important differences in the surface chemistry, solid state diffusion, and electrical transport of the 2D As delta-layers.

The atomic scale control achievable in the fabrication of STM dopant nanostructures in silicon is of potential importance for future CMOS technologies and emerging quantum computing architectures. Non-invasive imaging and electrical characterization of such buried nanostructures, with precise lateral and depth resolution, would therefore be of significant value for the development and inspection of integrated circuits and quantum devices. Scanning microwave microscopy is such a technique, and we have used STM fabricated single and multilayer 2D phosphorus nanostructures to demonstrate the quantitative subsurface imaging capabilities of SMM. The SMM measurements, which are completely non-destructive and sensitive to as few as 1900 to 4200 densely packed P atoms 4 to 15 nm below a silicon surface, yield electrical and geometric properties in agreement with those obtained from electrical transport and secondary ion mass spectroscopy (SIMS) for un-patterned phosphorus delta-layers in silicon. The SMM spatial resolution is demonstrated at 37±1 nm in the lateral and 4±1 nm in the vertical direction, both values depending on SMM tip size and depth of dopant layers. These results open up exciting opportunities for dopant nanostructure fabrication by providing a measurement capability not currently possible with any other technique.