Quantum information device performance in semiconductors and superconductors is limited by the quality of materials and interfaces, particularly the interface to the oxide layer. By merging semiconductors and superconductors in a single crystal material, the oxide layer can be eliminated, and the advantage of both systems can be realized. To explore interface free circuits in quantum computing, I have synthesized and studied a new two-dimensional hole gas in silicon using aluminum layers sandwiched between single crystal Si layers. At high enough Al density, this system is expected to behave as a superconductor in single crystal Si.
The samples were fabricated by low temperature molecular beam epitaxy (MBE) with modulation doping of elemental Al. Scanning tunneling microscope (STM) and scanning transmission electron microscope (STEM) images show epitaxial Si layers with low surface defects and no crystalline defects in the Al enriched region. Electrical measurement shows that holes are the dominant carrier in this system with charge carrier densities of ≈ 1.39 x 1014 cm−2, and Hall mobilities of ≈ 20 cm2(Vs). The charge carrier density corresponds to ≈ (0.93 ± 0.1) hole per Al dopant atom. Unfortunately, no superconductivity was observed down to 300 mK. The likely reason for this is found to be re-distribution of Al dopants over ≈ (17 to 25) nm due to thermal annealing up to 550 °C, which decreases the peak Al concentration in Si below the critical density for superconductivity.
Al has not been well studied as a dopant in Si due to its low solid solubility, low vapor pressure, and tendency to segregate. To better understand Al as a dopant, the structures and electrical properties of incorporated Al in Si(100) are studied using STM. The scanning tunneling spectroscopy (STS) spectra show shifts of band edges on incorporated Al compared to (2 × 1) Si(100) dimers.
To test the compatibility of elemental Al for STM lithography with a hydrogen resist layer, a standard experimental protocol is tested. Elemental Al is evaluated using 3 different metrics: 1) sticking coefficient contrast, 2) effective enthalpy of sublimation contrast, and 3) surface diffusivity by deposition rates. Elemental Al is shown incompatible with STM lithography and hydrogen masking. Our study suggests that other dopants may overcome this difficulty.
Finally, a new method using ion implanted wires is a promising technique for making electrical contacts to devices in Si fabricated with STM lithography. Here, I report a new in situ method for detecting ion implanted wires using STM and STS with a novel lock-in technique. Using the ion implanted wires, a-first-of-its-kind STM-patterned nano-wire made of P dopants is demonstrated.
|Advisor:||Iliadis, Agis, Pomeroy, Joshua M.|
|Commitee:||Newcomb, Robert, Peckerar, Martin, Daniels, Kevin, Salamanca-Riba, Lourdes|
|School:||University of Maryland, College Park|
|School Location:||United States -- Maryland|
|Source:||DAI-B 81/8(E), Dissertation Abstracts International|
|Subjects:||Quantum physics, Materials science, Nanoscience|
|Keywords:||Delta doping, Silicon, Si MBE, Solid-state quantum information, STM|
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