Split gate tunnel barriers in double top gated silicon metal-oxide-semiconductor nanostructures
Quantum computers hold the promise of far exceeding the computational efficiency of classical computers in a variety of applications. An immense amount of interest has grown in spin-based silicon quantum computers where an electron spin represents a quantum bit (qubit). Spin qubits in silicon are attractive because of their long coherence times due to the weak spin-orbit coupling of silicon and the ability to eliminate background nuclear spins by isotopic enrichment. One way spin qubits can be realized in silicon is by using donor-bound electron spins. Individual donor atoms intentionally implanted into tunnel barriers of silicon have been examined using transport measurements. However, reliable interpretation of donor transport measurements depends critically on understanding the tunnel barriers separating the localized electron state from the source and drain reservoirs. In this thesis, split gate tunnel barriers defined in a double top-gated, enhancement-mode silicon metal-oxide-semiconductor (MOS) device structure are analyzed. Tunnel barriers implanted with a small number of antimony donor atoms and non-implanted tunnel barriers were both characterized electrically at liquid helium temperatures (T \u2248 4 K). A tunnel barrier model is presented that uses the measured values of conductance to calculate the tunnel barrier height and width for a range of bias and gate voltages. The model provides a method to quantitatively describe how the barrier changes with bias and gate voltage and a way to compare different tunnel barriers for different devices. The model also provides insight about the binding energies of electrons in the potential well within the tunnel barrier. Thus the model can provide guidance to distinguish between high probability candidates for transport through intentionally implanted donors in contrast with transport through charge defects.
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