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. Shioda revealed to delegates at the ninth International Conference on Nitride Semiconductors (ICNS) that this approach could increase the output power of green LEDs by a factor of almost ten. The engineer from Toshiba began by highlighting the big issues associated with propelling LEDs to longer wavelengths: deterioration of the crystal structure and greater 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. Shioda explained that the company 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 overlap of electrons and holes. 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 studies 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 micron by 600 micron 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 AlGaN layer with an aluminium content of 0.3 produces 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 4.6V.