LEDs ♦ news digest
Samsung develops nitride LEDs on 200 mm silicon
Following the lead of Bridgelux, the Korean LED maker Samsung is developing a process to produce LEDs on 200 mm silicon.
Samsung Advanced Institute of Science and Technology (SAIT) has developed a process to produce LEDs on 200 mm silicon. Using this platform instead of sapphire, the widely used substrate for making LEDs today, promises to cut the cost of LED deposition and processing.
A paper presented by SAIT at the ninth International Conference on Nitride Semiconductors (ICNS) described the method used to produce LED epi- structures with an internal quantum efficiency of 65 percent.
Growth of high-quality GaN layers on silicon is not easy, due to differences in both the lattice constant and the thermal coefficient of expansion of the two materials.
SAIT has addressed both these issues by creating a crack-free template featuring AlN/AlGaN buffer layers, intermediate layers and an n-type GaN layer.
On this template engineers have created an LED structure featuring five quantum wells and a p-type AlGaN electron blocking layer. Photoluminescence (PL) mapping of this epiwafer produced an average emission wavelength of 429.5 nm, with a standard deviation of 10.3 nm.
Temperature-dependent PL measurements suggest that the internal quantum efficiency of the LEDs is 65 percent.
The research team have also employed a substrate transfer technique to fabricate vertical-LED chips from 8-inch epiwafers.
Toshiba addresses the green gap in LEDs
Inserting a thin AlGaN layer between the InGaN quantum well and the gallium nitride barrier delivers a tremendous hike in the output power of green 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 August/September 2011
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