Innovative Materials, Processes, and Tools Improve Performance, Quality of White LEDs
Lumileds Lighting joined forces with Sandia National Laboratories to investigate critical materials issues related to solid-state lighting technology. The results of their collaborative, cost-shared efforts include breakthroughs in three key areas:
- Investigation of low dislocation density GaN templates for LEDs led to the development of an improved growth process that reduces defects by a factor of 1000.
- Development of in situ monitoring tools for production reactors offers improved color control of the light emitted by the LEDs, increasing production yields and producing a better quality product.
- Investigation of semiconductor nanoparticles ("quantum dots") as luminescent down-converting materials for white LEDs produced conversion yields up to 76 percent, a world record.
Cantilever Epitaxy Approach Simplifies Fabrication, Improves Performance
Investigation of low dislocation density GaN templates was based around "cantilever epitaxy" (see Fig. 1), an MOCVD-based process developed at Sandia that provides a dramatic reduction in threading dislocation density at the GaN epitaxial surface. Unlike other dislocation density reduction techniques, cantilever epitaxy relies on sapphire substrate patterning and the choice of GaN growth conditions to force the horizontal bending of vertical threading dislocations, which can then annihilate upon coalescence, all in a single growth step.The resulting overall dislocation density is in the low 107 cm-2 regime (and ~ 106 cm-2 or less in the cantilever "wing" region).
The potential simplicity of the cantilever approach is attractive from a manufacturing point of view, because the reduced dislocation density substrate and LED device structure can be fabricated in the same MOCVD growth run. High power Luxeon™ LEDs fabricated by Lumileds on cantilever GaN templates show improved efficiency at low currents and improved temperature dependence. Work is ongoing to optimize the LED growth and quantify the comparison with baseline conventional MOCVD, which causes threading dislocation densities in the low 109 cm-2 regime.
In Situ Tools Allow Improved Control During Growth Process
The development and use of in situ tools (see Fig. 2) was established to understand more about the growth environment during GaN MOCVD and to provide the means to analyze film properties during growth, rather than after the fact. For example, chemistry models developed at Sandia explain the different reaction pathways between GaN vs. AlN growth, and a predictive method for determining AlN mole fraction was provided for AlxGa1-xN for x = 0 to 0.20. At Lumileds, a multi-beam optical stress sensor (MOSS) tool was installed on an R&D reactor, and used to show that Si concentration and AlN mole fraction can be determined in situ by monitoring the strain state of the epitaxial film.
Also, an ultraviolet-based pyrometer was developed at Sandia and transferred to Lumileds for implementation on an R&D reactor. This tool was used in conjunction with a new infrared pyrometer (developed by Lumileds) for in situ measurement of the wafer temperature, which is a critical parameter for controlling InN mole fraction in InGaN quantum wells.
Using these new pyrometry tools, Lumileds was able to demonstrate a factor of five increase in improved color targeting for green (~540 nm) LEDs over multiple growth runs.
Quantum Dots Offer High Efficiency, Greater Control
Semiconductor nanoparticles, or "quantum dots," offer potential advantages over conventional phosphors as luminescent down-converting materials. For example, the emission spectra of quantum dots can be "tuned" by controlling the particle size distribution and/or surface chemistry, unlike phosphors where the emission spectra is largely fixed by nature. At the beginning of the program, the researchers found that the quantum efficiencies of state-of-the-art semiconductor nanoparticles were far behind that of phosphors. Work at Sandia focused then on improved synthesis processes for CdS quantum dots, which ultimately resulted in a quantum efficiency of 76% (in solution) for a blue emission (see Fig. 3).
This result, believed to be a world record, compares favorably with the quantum efficiencies of some phosphors (80 to 100%). Furthermore, a process for encapsulating the nanoparticles was developed at Sandia and resulted in CdS quantum dots being successfully incorporated into epoxy — a first step towards applying them directly in LEDs. Detailed optical characterization at Lumileds' Luminescence Laboratory showed these CdS-loaded epoxy films to maintain quantum efficiency to 70%. The encapsulation process was transferred from Sandia to Lumileds and quantum dot materials were incorporated in high power Luxeon™ LEDs for the first time. This enabled experimentation regarding the stability of quantum dots at the fluence levels existing in high power LEDs. While much further work is necessary to bring quantum dots to a level to compete with phosphors, this program has achieved substantial strides forward in understanding the use of semiconductor nanoparticles as luminescent down-converting materials.