Dr Chris Pakes - University of Melbourne
Participating University of Melbourne Researchers
Matt Lay (PhD)
Mr Giuseppe Tettamanzi (PhD)
Prof David Jamieson
Prof Steven Prawer
Dr Jeff McCallum
Collaborating Centre Researchers
Prof Andrew Dzurak - University of New South Wales
Prof Robert Clark - University of New South Wales
This program aims to study charge and spin-related effects that may influence the development of silicon-based quantum computer architectures. The key objectives of the program are:
- To make use of scanning probes to explore oxide electrostatics on the nanoscale, and for imaging buried dopants in Si.
- To develop techniques for the fabrication of a nanoscale Superconducting Quantum Interference Device (SQUID), sufficiently sensitive for single-spin measurement.
- To study isolated nanoscale magnetic particles, which may have application in developing spin-based quantum computer technology.
Silicon Surface Science
In quantum computer devices exploiting Metal-Oxide Semiconductor (MOS) architectures, the reliability of the ultra-thin SiO2 barrier is likely to be a key issue. Defects present within the oxide film may act to trap charge, influencing the local electrostatic environment, and giving rise to charge noise, which can act as a source both of decoherence to underlying 31P qubits, and of unwanted signals for SET-based qubit state readout. The ion implantation process employed in top-down quantum computer development will induce defect sites that must be removed by rapid thermal annealing (RTA). Scanning Kelvin Microscopy (SKM) is employed to image the electrostatic environment of buried dopant islands, formed by ion implantation through photoresist masks into the underlying Si surface. A typical SKM surface potential image is shown in Figure 1, for islands formed by implantation of Phosphorus ions. Following implantation, the significant SKM signal observed arises presumably from charge trapped on defects introduced in the SiO2 surface layer. Figure 2 illustrates the effect of annealing samples in argon gas at 1000 C for 5 seconds, following the RTA scheme proposed within the centre. For the P-ion implanted samples (a), annealing gives rise to a reduction in SKM signal, but the sub-surface islands are still clearly visible. This indicates a significant contribution to the SKM signal arising from ionised P dopants, which must be activated subsequent to RTA. For the Si-ion implanted sample (b) we see a dramatic reduction in SKM signal, consistent with the removal of defects.Our experiments indicate that SKM may be very sensitive to buried P donors. We are currently developing ultra-high vacuum SKM techniques to examine the electrostatic environment of a small number of P dopants in Si.
Figure 1: SKM surface potential image, revealing an implanted sub-surface array of P-islands in Si.
Figure 2: SKM surface potential profiles across single P (a) and Si (b) implanted sub-surface islands.
Development of Nanoscale SQUIDs for Sensitive Spin Detection
Single-spin measurement is a crucial goal that must be achieved if readout of quantum computer architectures employing spin-based qubits is to be realised. Miniature SQUIDs are capable of measuring the magnetic properties of nanoscale particles deposited within the active device region. Scaling of the spin sensitivity with device dimension suggests that, with further reduction in device size, SQUIDs may be a potential candidate for single-spin measurement. We are examining nanoscale Nb-based SQUID architectures by direct patterning of thin films using a crossed-beam Scanning Electron Microscope (SEM)/Focussed Ion Beam (figure 3). Devices are tested at low temperature using a low-noise measurement system, capable of voltage measurement with nV/ Hz sensitivity.
Figure 3: SEM image illustrating FIB patterning of a thin film Nb SQUID structure.
Ferritin is an iron storage protein consisting of an organic cage, about 13nm in dimension, containing an antiferromagnetic core of up to 4500Fe3+ ions in a fully packed protein. These proteins have gained much attention recently as a nanoscale magnet that demonstrates macroscopic quantum tunneling of its non-compensated spin (s~100) through the anisotropy barrier. The spin is a suitable magnitude for testing the sensitivity of SQUID devices with sub-micron loop dimension, and the particle size is appropriate for the identification of isolated surface-trapped proteins using scanned-probe techniques. Accurate positioning of individual proteins has been achieved by mechanical pushing with the tip of an AFM, offering a route to the incorporation of single ferritin particles as components in future nanoscale devices. Figure 4 illustrates a small population of ferritin particles deposited on one our our SQUIDs.
Figure 4: AFM image illustrating ferritin deposition on a FIB-patterned Nb SQUID.