Rainbow Project Brings Color To LEDs (Cover Story)
A European project to develop a manufacturing base for nitride-based devices has ended with the demonstration of multicolored LEDs, writes project coordinator Marie-Antoinette di Forte-Poisson of Thomson-CSF.
Funded by the European Union, the Rainbow project, which ended in July of last year, brought together a multidisciplinary consortium from industry and academia to develop nitride-based devices and related manufacturing infrastructure. The Gallium Indium Aluminum Nitride For Multicolor Sources project ran for 42 months and was designed to address two important growing markets: first, high-brightness outdoor lighting, as used in traffic signals, large outdoor displays, automobile lighting, etc; and second, the market for high-density optical disk storage, as used in multimedia environments. With the final target being the production of LEDs and laser diodes, the major technical objective that had to be achieved in the framework of the Rainbow project was the realization of optoelectronic device demonstrators. These included LED prototypes emitting in the 400590 nm range and laser diodes operating at a wavelength of around 400 nm. This ambitious challenge required the development of a complete GaN-based technological background, from material studies to device realization and characterization. This process included four major technological milestones, which were all successfully reached:
development of a multiwafer metal-organic chemical vapor deposition (MOCVD) reactor for the growth of group III nitride materials; identification and development of optimum group III and nitrogen precursors; optimization of the LP-MOCVD growth process for group III nitrides; definition of basic technology for the fabrication of group III nitride-based devices. The optimization of these key technological process steps (material growth, device processing, etc) was made possible because of the varied, high-quality skills contained within the Rainbow consortium. These included manufacturers of electronic components (Thomson-CSF/Thales LCR), MOCVD growth reactors (Aixtron) and starting chemicals (Epichem), together with experts in device design and characterization (University of Surrey, UK), reactor design (University of Erlangen, Germany), material characterization (University of Aveiro, Portugal) and material growth (CRHEA, France). Project results During the Rainbow project, several LED demonstrators were fabricated. ( shows one such device.) The LEDs were grown on sapphire substrates and incorporated an InGaN/GaN multi quantum well (MQW) active region, sandwiched between n- and p-type AlGaN/GaN superlattices. A schematic of the structure is shown in . Satisfying one of the main objectives of the Rainbow project, a range of blue, blue/green and yellow InGaN-based LEDs were realized. The electroluminescence spectra of these devices are illustrated in , while the performance characteristics (see ) are close to being state of the art. As part of the Rainbow project, spontaneous emission was observed from a nitride-based blue laser, although stimulated emission was not recorded. In addition to the device demonstrators, significant progress was made to establish the necessary infrastructure for the epitaxial growth of nitride materials. High-purity group III and dopant precursors are now routinely produced by Epichem, while Aixtron is supplying LP-MOCVD multiwafer reactors for the growth of group III nitride materials. Future directions Thomson-CSF intends to use the results of the Rainbow project in numerous applications, and the company s central research laboratory (LCR) is already developing components and systems using nitride-based devices. Components based on blue and green resonant cavity LEDs are currently under development. Transceivers for use with plastic optical fiber will enable multimedia optical data buses for various automotive and avionics applications. Concerning the use of blue lasers, Thomson-CSF is targeting the liquid-crystal display (LCD) projection market, with applications in areas such as air-traffic control and entertainment simulation devices. Research studies are being carried out on a compact LCD projector using blue, green and red solid-state laser illumination. Electronic applications are also being considered: GaAlN/GaInN/GaN technology is being optimized for power amplifier studies in the 1218 GHz frequency range. Applications include satellites and broadband communications. Acknowledgements The author would like to thank the following for their contribution to the Rainbow project: S Rushworth and L Smith (Epichem); M Heuken and O Schn (Aixtron); D Lancefield, E O Reilly and A Adams (Surrey University); E Pereira and T Monteiro (Aveiro University); L Kadinski (Erlangen University); B Beaumont and P Gibart (CRHEA); F Huet, M Calligaro, A Romann and M Tordjman (Thales); J Di Persio (Lille University); R Kakanakov (Plovdiv Institute); and B Pecz (Budapest Institute). This work was supported by the European Union through its BRITE/ EURAM-3 program.