Towards superconductivity in p-type delta-doped Si/Al/Si heterostructures

In pursuit of superconductivity in p-type silicon (Si), we are using a single atomic layer of aluminum (Al) sandwiched between a Si substrate and a thin Si epi-layer. The delta layer was fabricated starting from an ultra high vacuum (UHV) flash anneal of Si(100) surface, followed by physical vapor deposition of Al monolayer. To activate the Al dopants, the sample was then annealed in-situ at 550 °C for 1 min. The Si capping layer was electron-beam evaporated in-situ at room temperature, followed by an ex-situ anneal at 550 °C for 10 min to recrystallize the Si capping layer. Low temperature magnetotransport measurements yield a maximum hole mobility of 20 cm2/V/s at a carrier density 1.39 × 1014 holes/cm2, which corresponds to ≈ (0.93 ± 0.1) holes per dopant atom. No superconductivity was observed in these devices even at T < 300 mK. Atom probe tomography and energy-dispersive X-ray spectroscopy analyses suggest that the Al dopants become distributed over ≈ (17 to 25) nm thickness. Redistribution of Al dopants reduces Al atomic concentration in Si matrix below the critical density to observe superconductivity.