Novel InSb quantum dots monolithically grown on silicon for low cost mid-infrared light emitting diodes

There is great worldwide interest in the mid-infrared spectral region (2-5 um) as it contains the fundamental absorption bands of a number of pollutant and toxic gases and liquids. These include gases such as carbon dioxide, carbon monoxide and hydrogen chloride which require accurate, in-situ multi-component monitoring in a number of industries such as oil-rigs, coal mines, land-fill sites and car exhausts. Strong absorption bands also exist for drug intermediates, pharmaceuticals, narcotics and biochemicals where the absorption strength is typically 2 orders of magnitude stronger than in the near-infrared allowing highly selective and sensitive detection in the fields of: environmental monitoring, bio-medicine, industrial process control and health and safety. There is also an atmospheric transmission window between 3.6 and 3.8 um which enables free space optical communication and thermal imaging in both civil and military situations as well as the development of infrared countermeasures for homeland security. However, these applications have yet to be fully exploited due to the lack of efficient and affordable light sources and detectors. This work proposes the growth and fabrication of a new light emitting diode (LED) architecture based on indium antimonide (InSb) quantum dots onto low cost silicon (Si) substrates. This will revolutionize how we utilize these devices and lead to a dramatic scaling in the cost and size of the optical systems to enable their widespread uptake. It will also enable the photonic components to be directly embedded into electronic circuits which would open up a new field of mid-infrared photonic integrated circuits. This would generate entirely new technology in areas such as integrated 'lab-on-a-chip' sensors and compact biochips bringing great commercial benefits and opportunities to the UK.

In the last few years, there has been significant progress in the development of mid-infrared devices using interband cascade lasers and type II superlattices. However these structures are extremely complex and expensive to fabricate and are grown on gallium antimonide (GaSb) substrates which are of poor quality, high cost (~50 times the cost of Si) and are only available in small sizes. Growth onto silicon would be most desireable to enable cost effective manufacture and to ensure future commercial success. The major obstacle in direct epitaxial growth of III-Vs onto Si is the large lattice mismatch between the III-V/Si interface, resulting in a large density of threading dislocations (TDs) which strongly deteriorate the device performance. This project shall overcome this by implementation of a new device design based on InSb quantum dots on low defect density GaSb buffer layers grown on Si. The key advantages are the mechanical robustness and very low sensitivity of the quantum dots to TD compared to bulk or quantum well structures, and the suppression of non-radiative Auger recombination to increase the quantum efficiency. In a quantum well device, every threading dislocation which propagates through it will act as a non-radiative centre drastically reducing the device performance. However in a QD, each TD will only 'kill' one or a few isolated dots which will not significantly affect device performance providing the TD density in the buffer layer can be reduced to moderate-to-low levels. Low defect density GaSb buffer layers shall be realized through novel 'interfacial misfit arrays (IMF)' and dislocation filtering layers designed to bend and annihilate TD generated at the III-V/Si interface.

The Si based mid-infrared LEDs will be developed in close collaboration with academic (University of Southampton and University of Montpellier) and industrial (Compound Semiconductor Technologies and Gas Sensing Solutions) project partners to evaluate device performance for use in practical applications which will help to achieve future commercialisation.

The development of new materials and technologies highlighted in this research has the potential to impact over a very wide range of communities both economically and socially. The main fields are;

1. Environmental monitoring: There is great interest in environmental monitoring of pollutant and toxic gases such as carbon dioxide, carbon monoxide and methane which have strong absorption lines in the mid-infrared region. This would address a wide range of industries including; power stations, oil-rigs, landfill sites, natural gas, waste treatment, agricultural industries and pharmaceutical processing. However the uptake of the current technology is limited by cost, size and complexity. The silicon based LEDs developed in this work will provide a route towards low cost and high volume manufacturing of high performance gas sensors. This will enable widespread and enhanced gas monitoring capabilities and will lead to improved air quality in urban and industrial areas.

2. Security and Defence: progress towards the development of sources for the 3.6-3.8 um atmospheric transmission window will enable secure free space optical communications in both civil and military applications as well as the development of infrared countermeasures for homeland security and to help combat terrorist threats. In the longer term, higher performance focal plane arrays for use in next generation thermal imaging equipment used for surveillance and explosive detection can be envisaged. Monolithic integration with Si readout integrated circuits (ROICs) will enable higher pixel count, lower cost and higher device reliability.

3. Healthcare: There are significant opportunities for exploitation in healthcare due to the specific sensitivity of various biomolecular components in the mid-infrared spectral range. These include; bio-medical analysis where mid-infrared wavelengths allow rapid analysis of proteins in the body to identify cancerous cells and infrared detection of drugs and bio agents using breath analysis. In the longer term, integration onto Si will enable these processes and measurements to be performed on a single chip enabling handheld and personalized healthcare devices.

Collaboration with relevant industrial and academic partners as highlighted in the pathways to impact document will help to achieve impact in these highlighted fields.