Spreading Out Laterally Enables a Phosphor-Free White LED
Conventional white LEDs are actually blue or ultraviolet sources, with added phosphor shifting the light to white. However, the converter degrades over time, the wavelength shifting is inefficient, and the packaging steps are complex.
Researchers, therefore, have been looking into how to build a phosphor-free white LED. One approach has involved stacked blue and green multiple quantum wells constructed out of InGaN and GaN. Unfortunately, the ratio of blue to green often changes with the injection current, leading to a white light that changes hue significantly.
Now a group from Gwangju Institute of Science and Technology in South Korea has found a solution.
A combination of blue- and green-emitting multiple quantum wells produces a white light. Because the wells are laterally distributed and not serially connected, the hue of the resulting light remains fairly constant at high injection currents. Courtesy of Seong-Ju Park, Gwangju Institute of Science and Technology.
The problem with LEDs built as vertical stacks of material is that the mobility of electrons in GaN is an order of magnitude higher than that of the holes that mark the absence of electrons. Because holes are slower, their transport dominates the radiative recombination process. The result is that whichever emitters are deepest suffer from a nonuniform injection of carriers.
“We came up with the idea that the multiple quantum wells emitting different wavelengths should be connected in parallel for uniform injection of electrical carriers,” said Seong-Ju Park, a professor of materials science and engineering at the institute.
The group developed a fabrication process that laterally distributed blue and green emitters using a selective-area growth method. Park characterized the process as simple — as long as the growing conditions of the multiple quantum wells and the dry etching conditions of the GaN are well optimized.
The process begins with the growth of the blue emitters atop n-type GaN, followed by the deposition and patterning of SiO2, which is used as a mask to etch the blue devices. That layer is followed by the growth of the green multiple quantum wells. The next step caps everything with a layer of p-type GaN, with this final step completing the production of the white LED.
Tests with a fiber optic spectrometer made by Ocean Optics Inc. of Dunedin, Fla., showed that the blue emitters had a peak at 431 nm and that the green devices had a peak at 511 nm. With an injection current of >300 mA, the electroluminescent ratio between the two devices was nearly constant at about 4.9, meaning that the hue of the white light virtually was unchanged. At lower injection currents the same was not true, a result that Park said was unavoidable given the materials used.
The researchers now are working on their long-term goal of fabricating highly efficient white LEDs.
“Our research is focused on improving the efficiency and color rendering index of the white LEDs by growing more efficient and longer-wavelength green multiple quantum wells,” Park said.
Applied Physics Letters, March 3, 2008, Vol. 92, 091110.
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