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Abstract
In this thesis, the introduction of top gates over buried phosphorus donor devices in silicon patterned with the atomic precision of a Scanning Tunneling Microscope (STM) is investigated. To achieve this a low temperature 200 ALD grown Al2O3 dielectric is introduced and a process strategy is developed to both contact the buried nanostructure and align surface gates to the devices.
The thesis describes the impact of the Al2O3 dielectric on the electrical proper- ties of the buried nanostructure and successful efforts to maintain the integrity of the dielectric whilst contacting the buried donor layers in silicon. Dose rates are optimized for the EBL resist to contact the buried donor device. Moreover, with the optimization of the etch rate of the deposited Al2O3 to minimize the dielectric undercut, the top gate was successfully implemented on 3 types of donor based devices.
In the first device a 2D δ-layer, the implementation of top gate shows a effective gate range of -4 V to 4 allowing ∼ 3 change in the carrier density of the highly doped (∼ 1014 cm−2) δ-layer in silicon.
In the second device an Al2O3 dielectric and a top gate was integrated onto a precision STM-patterned SET. An increase in the overall tunability of the SET
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quantum dot was observed using the top gate owing to its large lever arm (∼ 4 times that of the in-plane gate) with no significant reduction in the charging energy of the quantum dot. An enhanced gating range of -4 V to 4 V was observed for the SET quantum dot device. Additionally, the patterned top gated device demonstrated exceptional stability with the lowest noise and drift of all devices and a successful dynamic frequency response up to 1 MHz.
In the third device the top gate and an Al2O3 dielectric was incorporated into a precision STM-patterned 4 quantum dot device with the aim to capacitively couple two singlet-triplet qubits. Without the top gate the limited in-plane gate range did not allow any singlet-triplet inter-dot transitions in the gate space. However, after successful patterning of a top gate, two new inter-dot transitions were accessible in the gate range, with one of them being a singlet-triplet type inter-dot transition. An enhanced gate range of -4 V to 4 V was again observed for the top gate patterned on top of the 4 quantum dot.
In conclusion a successful integration of an Al2O3 dielectric on precision STM- patterned donor devices in silicon was demonstrated important for the continuing success of silicon based atomic electronics.