GLOBAL-Promoting Research Partnership in Fabrication of Advanced III-nitride Optoelectronics With Ultra Energy Efficiency Using Nanotechnology

Our research has the potential to meet two major challenges which human beings are facing: energy crisis and climate change. Currently, the energy consumed due to general illumination accounts for 29% of the world's total energy consumption. Although the energy provided by an hour of solar radiation on the Earth is equivalent to the world's total energy consumption per year, solar cells contribute only 0.03% to the figure. Therefore, it is necessary to develop new technologies to achieve ultra energy-efficient solid-state lighting sources and solar cells. The appearance of III-nitride semiconductors provides human beings with such a unique opportunity, as the light emission from III-nitrides covers the complete visible spectrum and also a major part of the solar spectrum. It has been predicted that III-nitride LEDs if used in our homes and offices could save 15% of the electricity generated at power stations, 15% of the fuel used, and 15% reduction in carbon emission.

For more than a decade substantial efforts have been devoted to developing high-brightness III-nitride LEDs (HB-LEDs) worldwide. Consequently, major achievements have been made. However, a fatal problem has appeared, and has to be solved urgently. That is the well-known "efficiency droop": the efficiency of HB-LEDs shows the highest value only at a low injection current, and a further increase in injection current leads to a significant reduction in efficiency. This is the "efficiency droop". Under the injection current required for practical applications, the efficiency drops down to >50% of the peak value, meaning that a large amount of energy has been wasted. This also causes a severe reliability issue, as the wasted energy leads to an elevated temperature of the devices and thus severe degradation in device performance. The physical origins of the efficiency droop are very complicated and thus unclear. So far, there is not any efficient solution.

In the project, the scientists from 6 world-leading teams at University of Sheffield, Yale University (USA), Nanjing University (China) and Technology University of Braunschweig (Germany) are pooling their unique but complementary expertise, proposing to employ a number of advanced nanotechnologies and epitaxial growth techniques in order to explore the fundamental issue, and then achieve ultra energy-efficient LEDs.

For solar cells, it has been predicted that an energy-conversion efficiency of >50% can be achieved with III-nitrides, which is much higher than that of any current solar cell. The solar energy-conversion efficiency of current III-nitride solar cells is extremely low, only ~3% in the best report due to a number of technologic challenges. We will combine our complementary expertise from 6 teams to tackle the challenges by employing a similar nanotechnology to fabricate into nanorod array solar cells on the epiwafers with a thick super-lattice structure on the GaN substrates with ultra-high crystal quality.

III-nitride optoelectronics have had a major impact in a number of technologically important areas, in particular, energy conservation and clean & sustainable energy. Given concerns with the sustainability of energy supplies and carbon emission, the ability to fabricate ultra energy-efficient solid-state lighting sources and solar cells is a major driver of this research field. Estimated global sales of $4.5 billion are predicted by 2012, with a further rapid increase expected as improved efficiencies produce further advantages compared to more traditional light sources. In addition to replacing 'standard' light sources, III-nitride LEDs are finding application in more specialist areas, for example, backlighting for LCD displays.

The results of our research will have economic impact in a number of fields. Most important will be the development of improved efficiency solar cell and blue emitters with application in lighting and display technology. These will be of benefit to a number of UK companies, for example IQE (Cardiff), Enfis (Swansea), Forge Europa Ltd (Cumbria), Zeta-control (Oxford), and the Sheffield spin-out company Seren Photonics Ltd, etc. The benefits to Seren Photonics include the enhancement of their product range, allowing them to maintain competitiveness against other international companies, particularly in Japan and US.

We also have strong links with other UK companies involved in the application of III-nitrides or the development of related equipment (e.g. Thomas Swann) and the UK semiconductor/laser community via the UK Nitride Consortium and also the EPSRC National Centre for III-V Technologies. The results of our work will be communicated via regular open scientific meetings including the biannual Nitride Consortium meetings of which the Sheffield team is an active member.
The project offers an excellent training opportunity for PhD students and PDRAs, both in terms of technical training, development of research skills, etc. The training provided for PhD students and PDRAs will also have a direct economic impact via the provision of skilled workers who may subsequently be employed by relevant companies. This has been a career destination of staff previously employed/trained on our related projects and has contributed to the pool of staff trained in device fabrication, epitaxial growth, characterisation, etc., a critical requirement in maintaining the competitive edge of UK companies.

Indeed, the impact of the project would even reach to the nation's health, as the ultra energy- efficient LEDs offers the excellent ability to tune the colour temperature according to the needs of the user, which is extremely important for household/workplace lighting. There is growing evidence that light quality in terms of colour temperature can have a strong psychological impact.

The international collaboration leads to possibilities for attracting the investments from overseas, in particular, the emerging countries such as China. This can potentially boost the local economy.

The project will potentially make a great contribution to building a very clean, sustainable and energy-consumption-efficient world in the next 10-50 years.