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conference report  nitrides


surrounded by areas of weaker emission.


With the blue-emitting sample, the researchers found that the emission from the brighter areas saturated when they collected the photoluminescence through the aperture of their probe. This was not the case when a lens, which collects photoluminescence over a greater sample area, captured the emission. The conclusion of Kaneta and his co-workers: Photo-excited carriers overflow from localisation centres and are not captured by non-radiative recombination centres, thanks to the potential barrier surrounding them. This explanation is consistent with blue LED droop caused by a decline in carrier injection efficiency. In contrast, photoluminescence mapping of the green-emitting sample showed


Toshiba addresses the green gap in LEDs


One of the biggest problems facing the nitride community is the ‘green gap’ – the rapidly declining efficiency of green light emitters at longer and longer wavelengths. But this issue can be combated, according to Toshiba’s Tamonari Shioda, by inserting thin AlGaN layers in the active region of a conventional device. This approach can increase the output power of green LEDs by a factor of almost ten.


Shioda explained that there are several issues associated with propelling LEDs to longer wavelengths: Deterioration of the crystal structure and increased phase separation, which can be addressed by improving the growth process; and an increase in electron-hole separation via the quantum-confined Stark effect, which can be mitigated by switching the growth platform to a semi-polar or non-polar orientation.


Toshiba wants to improve its green devices on c-plane sapphire, and to do this its engineers have worked to improve the band structure of the device. The primary goal of this effort has been to increase radiative recombination efficiency through greater electron-hole overlap.


Initial efforts in this direction involved the growth of multiple quantum well structures featuring 1.5 nm-thick AlGaN layers with a range of aluminium compositions up to 30 percent. Cross-sectional transmission electron microscopy analysis on this set of samples revealed no degradation in any of the structures. And probing these structures via photoluminescence showed that the greater the aluminium composition in the layer, the greater the suppression of the decline in efficiency at longer wavelengths.


Shioda and his co-workers have produced 600 µm by 600 µm LEDs with an active region featuring AlGaN layers, which were grown at the same


temperature as the InGaN quantum wells. The output power of these LEDs increases with the proportion of aluminium in the interlayer. Driven at 20 mW, a 532 nm LED incorporating an Al0.3


Ga0.7 N layer produced an


output of 12 mW at an external quantum efficiency of 25.9 percent. One downside of this structure is its higher operating voltage – insertion of this aluminium layer increases the forward voltage from 3.5 V to 4.6 V.


20 www.compoundsemiconductor.net August / September 2011


Soraa’s efforts have not been limited to increasing output power – the West-coast start-up has also focused on improving the beam-quality of its single-mode green lasers. Divergence along the fast axis is 14-22 degrees,


saturation everywhere, indicating that carriers move from radiative domains to areas that are non-radiative, probably due to threading dislocations.


Lasers for displays Another highlight of ICNS-9 was the talks from representatives of Osram Opto Semiconductors and the UCSB spin-off, Soraa, which provided updates on their company’s performance of their laser chips for display applications, including picoprojectors. According to James Raring from Soraa, lamps and LEDs are alternatives for the light source, but they deliver an inferior optical throughput, typically by a factor of three.


Raring explained that the vast majority of commercial green lasers on the market today and capable of serving picoprojectors employ some form of frequency doubling of an infrared source. Replacing such devices with single, green-emitting chips will lead to improvements in efficiency, compactness, ruggedness and speckle. However, according to Uwe Strauss from Osram OS, in order to produce an image with sufficient brightness, these green laser chips must have: An emission wavelength of at least 515 nm; output power of 50 mW or more; a minimum wall plug efficiency of 5 percent; and, in both the lateral and vertical directions, a single mode output. If a shorter wavelength source is used – for example, a 505 nm laser – the power output requirements are higher.


Osram can exceed these requirements. Its lasers, which it has developed on the conventional c-plane, produce 70 mW at 522 nm with a wall plug efficiency of 5-6 percent, and have a spectral width of 1.8 nm. What’s more, reliability – defined as the time taken for the operating current to increase by 30 percent – is more than 1000 hours. In comparison, the continuous-wave output of Soraa’s 516 nm green lasers that are grown on unconventional planes of GaN now exceed 100 mW. Other characteristics of these packaged diodes included threshold currents and voltages of 125 mA and 5.9 V, a slope efficiency of 0.4 W/A and a wall-plug efficiency that peaks at 4.1 percent.


In Raring’s talk, he explained that one of the benefits of using semi-polar and non-polar planes is an increase in the radiative recombination rate, which stems from increased overlap of the electrons and holes in the quantum wells. These orientations also aid hole injection, thanks to a reduction in the effective mass of this carrier. However, he claimed that the most exciting aspect of these novel planes is the far greater design freedom that they enable.


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