Next generation white LEDs using hybrid inorganic/organic semiconductor nanostructures for general illumination and wireless communication

There is a significantly increasing demand for sustainable energy-efficient technologies due to the world energy crisis and climate change. The energy consumed due to general illumination accounts for about 29% of the world's total energy consumption, currently using rather inefficient technologies often containing toxic elements. It is therefore necessary to develop ultra energy-efficient solid-state lighting sources to replace these incandescent and fluorescent lights, for which the leading candidates are mainly based on white light emitting diodes (LEDs). Such white LEDs can be fabricated from inorganic or organic semiconductors, with the former leading the way for high brightness and efficiency. These are constructed from III-nitride semiconductors, which have direct bandgaps across their entire composition range, covering the complete visible spectrum and a major part of the ultraviolet. Fast modulation of the white LEDs, at speeds undetectable to the eye, allows them to also be utilised as optical transmitters for wireless data communication. This opens up the exciting possibility of white LEDs serving as lighting sources for simultaneous illumination and wireless communication. This is the emerging technology of visible light communication (VLC) and has a number of major advantages over the present-day radio frequency (RF) communication technology, such as increasing security, eliminating any RF-induced health concern, etc

However, the performance and cost of current white LEDs is not sufficiently impressive to allow replacement of conventional lighting sources at the moment. Furthermore, in terms of VLC applications, the bandwidth is currently limited to the MHz level, which is well below the practical requirements of current broadband WiFi systems. This is due to the long carrier recombination lifetime of current III-nitride based LEDs, which are conventionally grown in a "polar" orientation containing intense piezoelectric fields. These fields result in a reduced overlap between the electron and hole wavefunctions in the active regions of the LEDs, which then suffer from long radiative recombination lifetimes (10-100 ns) and also low internal quantum efficiency. In addition, the conventional phosphors used to convert the emission to white light have even longer decay times and presents an additional limitation on the available bandwidth.

The project will employ non-polar III-nitrides and integrate the two major semiconductor families (organic and inorganic semiconductors) using a novel nanofabrication technology in order to achieve ultra energy efficient LEDs with ultrafast modulation speeds for next generation III-nitride based white lighting. Structuring on a nanometre scale will be used in the growth of the III-nitride layers to achieve high quality non-polar GaN, thereby eliminating the piezoelectric fields to give faster, more efficient devices. The nanostructures will also be used to introduce extra nanocavity effects, further reducing the radiative recombination lifetime and increasing the optical efficiency. The target of the project is a novel hybrid nanostructure to achieve prototype white-LEDs with a modulation speed on a level of 10 GHz and a step change in energy efficiency compared with the current state-of-the-art. The devices will be fabricated using metal-organic vapour phase epitaxy and cleanroom processing and fully characterised using optical and electrical measurements. Each stage in the process will be optimised and close working with industry will ensure that the resulting methods are practical and scalable to high volumes.

The proposed work should deliver major impact in many areas, including technology, society, economy and environment. It has been predicted that replacing all existing light bulbs with white LEDs would save 15% of electricity generated worldwide, 15% of the fuel used, and reduce carbon emission by 15%. This will make very significant contributions to achieving an 80% reduction in carbon-emission by 2050, the challenging target required for the UK by Kyoto protocol. It will greatly contribute to minimising climate-change, and thus improving our environment. Unlike the current lighting sources which contain toxic elements white LEDs are truly eco-friendly lighting sources. In addition the expected results will lead to a revolutionary change in the concept of wireless communication. This will contribute to elimination of health concerns associated with current wireless communication, allow for wireless communication in airplanes and underwater, and significantly improve security.

The project also involves development of new technologies in advanced nanofabrication, next generation wireless communication, III-nitrides and organic materials, which are all important frontiers of current scientific research. These all align extremely well to EPSRC priority areas, such as Manufacturing, Digital Economy, Energy and Healthcare as well as the priority themes and competences of the Technology Strategy Board (TSB). The vibrancy and importance of III-V activities in the UK are demonstrated by the report from the recent Road-mapping exercise in III-V semiconductors (2012). III-nitride LEDs were ranked as one of the highest priorities, with direct impact on 4 of the top 10 technologies. The proposed devices will also have significant impact on high data rate devices for telecoms, which came out top in the applications vs technology matrix. This research on III-nitrides will contribute to the development of a sustainable, energy-consumption-efficient world with very healthy and secure communication systems over the next 10-50 years.

Both applicants have strong track records in collaboration with the semiconductor optoelectronics industry. Seren Photonics Ltd, the industrial partner of the project, was established on Professor Wang's research innovations. Both have established extensive formal and informal collaborations with the semiconductor industry in the UK and overseas, including IQE (Cardiff), Enfis (Swansea), Forge Europa Ltd (Cumbria), Zeta-control (Oxford), and Plessey semiconductor (Swindon/Plymouth), Osram (Germany), OptoGaN (Russian), etc. Exploitation opportunities of any Sheffield-IP generated will be carried out by the University's Research and Innovation Services (RIS), and commercialisation will be assisted by Fusion IP, an AIM-listed public company which has signed a longer term agreement with the University on commercialising university research.

Professor Martin has worked closely with the semiconductor industry, and has a number of patents in III-nitride technologies. He has recently competed industrial projects with SemiMetrics Ltd. and McCann Energy Ltd.. In addition to the valuable input from Strathclyde's Research Office the Physics Department is host to the KT Directorate of the Scottish Universities Physics Alliance (SUPA-KT) which provides excellent help in identifying commercial partners and opportunities. He is a lead partner in the Intelligent Lighting Centre within Strathclyde's £100 M Technology and Innovation Centre (TIC), where new laboratories for this work will be completed in early 2014, bringing together high numbers of academic and industrial researchers across a wide range of disciplines. A number of TIC partners offer high potential for engagement with the work in this project, for example, the Fraunhofer Centre for Applied Photonics.