Lead Research Organisation: Imperial College London
Department Name: Dept of Physics


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.

Planned Impact

This proposal will accelerate the development of ultra high ultra sensitive multifunctional room temperature sensors. It is about exploring the underlying physical concepts and establishing the constraints on each separate sensing function in order to facilitate multifunctional capability. Industries that may benefit from these sensors include those that currently utilise the functional properties separately. Magnetic sensing is used in a whole range of industries, ultra high resolution sensing is particularly important for the recording industry and all those who depend on it for information storage and retrieval and it is also important for the those industries that use sub micron magnetic beads for cancer detection, image enhancement and drug discovery and require precise information on bead location within biological environments. Electric field, charge or PH sensing are of relevance to industries that work on new methods for chemical detection or detection of chemical activity in specific environments. Optical sensors are clearly of importance to industries developing new imaging modalities. Industrial beneficiaries of the multifunctional sensor would be any manufacturer of nano-circuits who would benefit from the availability of our sensors in scanning or array geometries, which could be used to assess circuit defects associated with current (magnetic field), charge or opto-electronic function. Multifunctionality will greatly enhance the ability to conduct high throughput screening studies of biological materials for research and clinical activities, e.g. for cancer detections. The potential wider socio-economic benefits would be in the areas of information storage, ICT, drug discovery, early cancer diagnosis and security. Our sensors may be utilised in monitoring environmental degradation due to air or soil pollution, building erosion, car emissions etc. Multiple industries will directly benefit via the incorporation of these sensors into new products that will provide a competitive edge, enable new technologies and products and aid commercial output. By implication this effort will also contribute to wealth creation and employment. Our work is directed towards areas of recognised UK industrial strength and capability as identified in the recent on line survey run by the Electronics, Sensors and Photonics ESP KTN (http://vimeo.com/26765852).
Description This proposal aimed to develop nanostructured multifunctional sensors. We have studied InSb quantum well structures, epilayers and CVD graphene hybrid structures to understand their magnetic, electric and light sensitivity. InSb quantum well material when fashioned into a simple Hall cross structure is capable of performing as an ultra sensitive sensor of these three perturbative fields. The work was carried out collaboratively with the National Physical Laboratories combining the development of scanning modalities at the NPL and at Imperial College.
Exploitation Route The work has been completed. Sensors of graphene or other high mobility 2D structures will benefit from the research conducted in this grant - with implications for biohealth, in the area of intra-cell imaging.
Sectors Electronics

URL https://www.imperial.ac.uk/people/l.cohen/research.html