Japanese collaboration seeks efficient white LED lighting
Other investigations include general illumination considerations such as how objects appear under lighting (known as the color rendering index or CRI), and the optimum arrangement of devices and their effects on perception. A CRI value of more than 90 (100 represents a perfect illuminator) is being sought for an LED fixture with a luminous efficacy of 80 lm/W. This is comparable to the output of a fluorescent bulb.
A team effort
The project is being jointly administered by the New Energy and Industrial Technology Development Organization (NEDO), a semi-governmental organization affiliated with Japan s Ministry of Economy, Trade and Industry, and the Japan Research and Development Center for Metals (JRCM).
JRCM hosts the collaboration, which includes Sumitomo Electric, Japan Energy and Furukawa Electric. These companies are developing suitable substrates for GaN. Sumitomo Chemical, Mitsubishi Cable Industries and Showa Denko are looking at epitaxial issues. LED fabrication is being investigated by Stanley Electric, while Kasei Optronix is developing fluorescent coatings to convert short-wavelength LED radiation to white light. Mitsubishi Electric Lighting, Omron and Agilent are addressing lamp fabrication.
Other players include lighting concern Yamada Shomei Lighting and Namiki Precision Jewel, a company that is concentrating on improving the surface preparation of sapphire substrates. The participating universities are Yamaguchi, Mie, Tokushima and Tokyo, and these institutions are joined by Japan Light Tubes, an association whose role is standardizing future LED lamps for general lighting use.
High-efficiency UV LEDs
According to Masayoshi Kubota, a NEDO project coordinator, one of the primary technical goals is to develop a UV LED lighting device with an external quantum efficiency of 40%.
"One of our main results so far has been the demonstration of an external quantum efficiency of 31% from a UV LED operating at 400 nm. This was achieved using a novel technology called lateral epitaxy on patterned substrates, or LEPS," said Kubota. The InGaN chip was designed by Mitsubishi Electric, Stanley and Yamaguchi University. "While our target for the external quantum efficiency is 40%," continued Kubota, "the development of a more efficient red phosphor as part of the tri-color phosphor scheme is presently our most important priority." Kubota adds that after the project ends in March 2004, there are currently no plans for a follow up. "Some of the companies may cooperate on various technologies after this program," he said.
An overview of the project s mid-term achievements was presented in Tokyo this March. In his overview of the project, T Taguchi of Yamaguchi University suggested that by 2003 lamps with a luminous efficiency of 80 lm/W could become a reality. The program is making some progress to achieving this end, not least with the 400 nm device mentioned above.
Taguchi says that to turn this into efficient phosphor conversion and to make white LEDs, the optimum route is a single UV chip driving tri-color phosphor combinations that provide stable color co-ordinates and a CRI value of more than 90. This approach is preferred to the combination of 470 nm blue devices with yellow YAG phosphors, which use complementary mixing of blue and yellow emission to produce white output, but have less stable color co-ordinates and a color rendering index of around 75.
UV LEDs and ZnS-based II-VI phosphors similar to those used in fluorescent lamps have been developed, so far demonstrating 20 to 30 lm/W and color rendering index values of 93. This luminous efficiency is comparable to the industry s best commercial devices, which currently offer around 30 lm/W.
To provide suitable lattice-matched substrates for subsequent growth of GaN device structures, A Yokohata et al. from Japan Energy and Yamaguchi University described a solution growth method. Using a hydrogen pressure of 1 GPa and a temperature of 1475 °C, single-crystal GaN substrates up to 25 mm in diameter have been grown (see figure 1). Characterization of these substrates revealed excellent crystallinity and a dislocation density of less than 105/cm2. According to Yokohata, these crystals are reproducibly obtained primarily near the surface of the Ga melt.
S Sakai from Tokushima University described the use of SiN buffer layers as a way to reduce the number of dislocations in AlGaN-based UV LEDs grown on SiC or sapphire substrates. Due to thermal and lattice mismatch, GaN structures grown on sapphire and SiC substrates typically contain dislocation densities up to 1010/cm2.
Such defects act as non-radiative recombination centers, but contrary to expectation the dislocations in InGaN/GaN devices do not quench the light. Instead, light emission takes place by recombination of carriers in small regions of indium-rich material formed by phase segregation in the InGaN wells, which has led to blue and green LEDs. However, Sakai notes that for near-UV devices with lower indium content, the localized carrier effect is weaker, and dislocation interactions have a more significant effect on emission.
To reduce the number of dislocations in UV devices operating below 370 nm, Sakai described a new SiN defect reduction layer. This layer is deposited on sapphire at 500 °C prior to two GaN buffer layers grown at 500 °C and 1050 °C. The SiN buffer provides a pattern of nano-scale "holes" that localize the dislocations and produce areas of defect-free material in a manner similar to epitaxial overgrowth on patterned substrates (see figure 2). The advantage is that photolithography is no longer required to mask epitaxial overgrowth regions.
Devices with AlGaN wells are also attractive because they allow devices operating in the spectrum below 370 nm, but unfortunately they suffer from poor emission.
To address this Sakai also reported a method of increasing the compositional non-uniformity in AlGaN using a Ga droplet layer prior to AlGaN deposition. This produces an effect similar to indium segregation in InGaN wells. The Ga droplet layer disturbs the growth of the AlGaN interfaces, which localizes the carrier recombination regions. This has been found to improve the efficiency at low values of current injection.
T Meada summarized work that Sumitomo Chemical and Mie University have done on a 15 x 2 inch multi-wafer system. Uniformity measurements showed that cross-wafer and wafer-to-wafer uniformity was less than ±1%. Subsequent epitaxial overgrowth of GaN layers on sapphire substrates using triangular (11-22) and square (11-20) etched facets produced smooth surfaces. In addition, the dislocation density in these structures could be reduced to around 105/cm2 in a 10 mm2 region.