ULTRA-HIGH-RESOLUTION, ULTRA-SENSITIVE MULTIFUNCTIONAL BALLISTIC NANO SENSORS FOR THE SIMULTANEOUS DETECTION OF MAGNETIC, ELECTRIC AND OPTICAL FIELDS

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At macroscopic length scales, charge carriers in semiconductors can be described by diffusive transport properties and sensor concepts such as the Hall Effect are applicable. When materials are fabricated at the nanoscale, new properties emerge and quantum effects dominate. In high-mobility materials such as narrow-gap semiconductors (NGS), charge carriers exhibit ballistic properties, when device length scales are smaller than the electron mean free path. In InSb (and InAs) quantum well structures, room-temperature mobilities exceed 40,000 (and 25,000) cm2/Vs, respectively, yielding electron mean free paths in excess of 500nm, which are accessible in principle using current processing technology. Nevertheless, room-temperature ballistic effects, even in high mobility NGS have remained elusive. As a result there has been no significant exploration and exploitation of this interesting and potentially important transport regime. As a result of current funding (EPSRC EP/F067216 - EP/F065922 - end date November 2011) we have made significant theoretical and experimental developments, resulting in the demonstration of room temperature ballistic effects in NGS heterostructures. We have shown that it is possible to create collimated ballistic electrons in a simple cross-structure, which enhances the so called negative bend resistance (NBR). We have also shown that collimated NBR sensor responsivity (the change in four terminal device resistance to the perturbing field) scales with inverse device size. Remarkably, this means that there is no significant loss in sensor performance as the dimensions shrink, a highly desirable property for nanoscale electronics applications.The focus of our new proposal is to build on these considerable experimental and theoretical developments. We plan to use the NBR geometry as a natural platform to realise high sensitivity multifunctional NGS ballistic nanosensors operating at room temperature, which utilise the change in electrical resistance that results when the device is exposed to magnetic, electric and/or optical fields. As part of this vision we plan to integrate two other key device concepts that will enable the multifunctional character of the devices and boost sensitivity. The first is our discovery that in the appropriate device geometry, perturbing external fields, such as optical fields, can convert carriers from the ballistic to the diffusive regime. The second is that a metal shunt appropriately placed within the device architecture, provides a low resistance path and access to that path via a Schottky barrier is tunable via external perturbing fields. This latter property has been used to great effect in diffusive devices known as EXX sensors, which were invented by a coinvestigator and visiting academic on our proposal, Prof Stuart Solin. Apart from our own preliminary investigations, the integration of a metallic shunt in the ballistic limit, is a completely new concept and aspects such as plasmonic effects and hot carrier effects will need to be investigated. Up to this time our achievements are based on InSb heterostructures. The motivation to examine the smallest possible devices, means that the grant activities will include exploration of the properties of InAs quantum well devices. InAs offers similar room temperature mean free paths to InSb, but has attractions including lower overall resistance, the option to be more heavily doped and greater potential for tunability as far as controlling collimation because of negligible side wall depletion.The study of interface effects in the ballistic regime at room temperature is a field almost completely unexplored and because of the recognised demand for high-resolution high-sensitivity sensors for applications spanning biosensing, point of care diagnostics using magnetic bead detection, ICT, and Security, our work is timely and fits well within the EPSRC research strategic areas.