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Strained defects in the antipodes

by Anne-Marie - published on

The CNRS has a number of bilateral agreements with foreign universities aimed at promoting new collaborations between CNRS laboratories and these partner institutions. One of these so-called “projet de recherche collaboratif” (PRC) is with the University of Melbourne in Australia, and it was via this call that Jeff McCallum and I received funding for our project (Spectromech) whose objective is to study the effect of mechanical stress on the properties of the intrinsic defects present at the interface between silicon and its oxide using electronic spectroscopy methods (more on that below).
Scientifically speaking, Australia is a small country that lacks the critical mass of researchers to be simultaneously effective in many areas of science. The comparison is often made with the Netherlands, a country whose population and gross national product are comparable with Australia’s, but which has a large band scientific presence thanks in part to the proximity of scientific heavyweights France, the UK and Germany. To combat this, the Australian federal government’s research strategy has been to fund certain areas particularly well (i.e. better than international standards), but to significantly reduce funding in other areas. The best funded efforts are organized into networks of institutions called ‘Centres of Excellence’, and a quick reading of the 9 names gives an idea of the country’s research strengths: ‘All Sky Astrophysics in 3 Dimensions’, ‘Australian Biodiversity and Heritage’, ‘Climate Extremes’, ‘Engineered Quantum Systems’, ‘Exciton Science’, ‘Future Low-Energy Electronics Technologies’, ‘Gravitational Wave Discovery’, ‘Population Ageing Research’, and ‘Quantum Computation and Communication Technology’.
Jeff McCallum’s group is part of the ‘Quantum Computation and Communication Technology’ (the CQC2T) which is run from the University of New South Wales in Sydney. The centre’s principal ‘raison d’être’ is to demonstrate a Kane quantum computer (see Kane’s original article here: Nature 393, 133 (1998)) in which the qubits are formed from the nuclear spins of defects in semiconductors. Two materials are being explored; phosphorous donors in silicon (Kane’s initial suggestion), and NV centres in diamond. McCallum’s role in the CQC2T is as the expert in the spectroscopy of semiconductor defects, and it is also for this expertise that I contacted him about a joint PRC submission.
At PMC we have been studying the effect of mechanical stress on the electrical properties of silicon nanostructures; nanowires and nanomembranes. Recently, Heng Li (who is in the first year of his PhD) observed a large stress-induced change in the admittance of lightly doped silicon nanomembranes. The symmetry (in applied stress) of the effect indicates that it may be related to a stress-induced charging/discharging of the intrinsic interface defects (the so-called Pb0 states) at the silicon surface. The qualitative picture that we have is that the equilibrium charge state of a defect depends on its activation energy, and that this can be modified with mechanical stress. Quantitative interpretation of Heng’s observations requires a spectroscopic measurement of the shift in activation energy when stress is applied. This is the goal of the Spectromech project on which a second student, Chris Lew, is working in Melbourne.
It should also be mentioned that mechanical stress can affect both nuclear and electronic spin dynamics, and the CQC2T is very interested in integrating mechanical stress into their qubits. Spin dynamics in semiconductors is an area in which the EPS group is expert, so there are potential collaborative opportunities in this direction also.