May 8, 2013…Researchers from the Chinese Academy of Sciences’ Institute of Semiconductors, Beijing, and University of Hong Kong have used strain engineering to improve the light output of Green LEDs. The researchers improved the light output of a 530nm green LED operating at 150 mA by 28.9 percent [Hongjian Li et al, Appl. Phys. Express, vol6, p052102, 2013].
The researchers note that green-emitting nitride semiconductor LED structures tend to suffer from low light output due to the difficulty in producing the high-indium-content indium gallium nitride (InGaN) needed for longer-wavelength light emission. In addition to the material quality challenge, strain induced by the lattice mismatch with pure GaN leads to large piezoelectric effects, giving electric fields that tend to pull electrons and holes apart, reducing rates of recombination into photons (i.e. the quantum-confined Stark effect, or QCSE), thus reducing quantum efficiency.
The Chinese team inserted a layer of lower-indium-content InGaN before the high-In-content light-emitting layer. Simulations suggested that such a layer could reduce the strain-dependent electric fields in the active light-emitting multiple quantum well (MQW) structure.
MOCVD on C-plane sapphire was used to produce epitaxial material with a low-In-content InGaN shallow quantum well (SQW) step. A 325nm helium-cadmium laser was used to excite the photoluminescence spectra of the materials at low temperature (85K) and room temperature (298K). One effect of the SQW was to reduce the width of the spectral peak full-width at half maximum (FWHM) at 85K from 16.7nm for the conventional LED material to 13.1nm for the SQW material. The 298K measurement reduced the conventional FWHM of 20.1nm to 15.7nm. The peak intensity was also higher with the SQW structure, therefore the SQW material had improved crystal quality.
The peak height for the SQW material at 298K was 55.1% that at 85K. The corresponding ratio for the conventional structure was 24.1%. The higher ratio for the SQW material indicates a higher rate of radiative recombination and higher internal quantum efficiency (IQE).
The electroluminescence was measured in an integrating sphere, giving light output power–current–voltage (L–I–V) results. The voltage performance is similar in the SQW and conventional devices. However, the light output at 150mA is 28.9% greater in the SQW LED (49.3mW) over the conventional device (38.4mW).
The researchers point out that improved overlap of the electron and hole wavefunctions in the device leads to improved recombination into photons. The external quantum efficiency (EQE) increased from 10.2–13.3% over the conventional LED performance.