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Fluorescent Substrate Offers Route To Phosphor-free LEDs

Using a phosphor to convert blue emission from an LED chip into white light fundamentally limits the efficiency of solid-state lighting. Now, a research team in the US is looking to remove the need for phosphors by using fluorescent, doped-zinc-oxide substrates instead, as Jeff Nause explains.
The relative immaturity of native gallium nitride (GaN) substrates currently limits high-quality GaN epitaxy for LEDs. One alternative material that ought to provide a better substrate solution is zinc oxide (ZnO). There are two reasons for this: ZnO offers better lattice-matching between the epilayer and substrate; and the material makes new device configurations feasible.

Better matching arises because ZnO shares the same hexagonal crystal structure and "P63mc" crystallographic space group as GaN and indium gallium nitride (InGaN). As a result, the lattice mismatch between ZnO and pure GaN is only 2.2%, while there is a perfect lattice match between ZnO and InGaN with an 18% indium content.

Furthermore, ZnO is conductive. This means that a bottom-contact device can be easily fabricated from the material - something that is not an option for the current generation of sapphire-based LEDs. While ZnO presents these obvious advantages, several technological hurdles remain to be overcome before the material gains acceptance as a useful substrate.

First and foremost is the fact that atomic hydrogen and dissociated ammonia both attack ZnO. Since these species are present in large concentrations during standard MOCVD growth of GaN and InGaN, alternative methods or gases must be used.

Under a project valued at $4.8 million that has been funded to the tune of $3.8 million by the US Department of Energy, Cermet and Georgia Tech s Ian Ferguson and Alan Doolittle are working on an LED technology that exploits the fluorescence of doped ZnO. The potentially disruptive result would be phosphor-free solid-state light sources.

The aim of the project is to integrate large-area ZnO fluorescent substrates with state-of-the-art, lattice-matched nitride epitaxy to address the various technological limitations of current approaches to solid-state lighting.

Using established nitride-deposition technology, white light could be produced by self-luminescence in the doped ZnO substrate. Blue emission from the GaN material would excite the fluorescence in doped ZnO, and the emission spectra of the fluorescence and the original blue emission could then be controlled to produce the desired white output.
Technical challenges

Several technological steps were identified as critical milestones for this approach to solid-state lighting to be successful. The first was the development of high-quality, doped ZnO to act as a nitride-matching substrate and as a luminescent light source. This required the production of commercial grade, doped ZnO substrates that combined excellent crystal quality with optical transparency in the visible spectrum.

The second key challenge was to deposit low-defect-density nitride structures on the doped ZnO substrate. If successful, this would yield lattice-matched nitride layers grown on ZnO substrates with excellent crystal and optical quality.

To date, the project has yielded impressive results regarding both of these developments. In meeting the first challenge, Cermet has produced large-diameter doped ZnO (figure 1) that generates light when stimulated thanks to the kinds of dopants that are incorporated into the crystal.

In tackling the second material challenge, the team demonstrated defect densities of around 1 x 104 cm-2 for lattice-matched InGaN on ZnO. This is an important milestone as it shows that the low defect density inherent in the bulk substrate can be replicated in the active nitride layers. This should improve LED performance by reducing non-radiative decay mechanisms in the active layers.

As well as the projected energy savings that all solid-state light sources offer, this self-fluorescence approach has the potential to further drive down the cost of white LEDs. Using a self-luminescent substrate should allow production of a white-light emitter without the need for a traditional phosphor.

Such a device would eliminate much of the cost, complexity and decreased yield associated with the traditional phosphor approach. LED efficiency would be enhanced by reducing the defect density in the emitters, and by increased blue-white light conversion efficiency through enhanced luminescence directly from the substrate. Costs would be driven down through cheaper bulk substrate growth and the elimination of phosphors. Lastly, economies of scale can be met by using large-area melt growth technology to fabricate ZnO substrates, and existing volume GaN epitaxy solutions.
Integration challenge

Having met the initial technological challenges, the remaining problem is to integrate these two achievements and fabricate a white-light LED. These emitters demand high carrier concentrations in both n- and p-type layers in the diode, as well as quantum-well compatibility and wavelength-tunable emission.

The precise nature of the light emitted by white LEDs is an important issue, and is often defined by the color temperature. By employing this self-fluorescence approach, there are two ways to control color temperature. Since the white light emitted is actually a combination of the blue "pump" emission from the GaN material and the substrate s fluorescence, there are two control mechanisms available.

The pump wavelength can be tuned at the chip level by growing structures emitting at longer wavelengths suitable for optically pumping bulk ZnO with a fixed dopant content. Alternatively, the dopant content in the bulk ZnO can be tuned to fit a fixed pump wavelength. By tweaking both of these variables it is possible to cover a broad range of "white" color content.

To date, the greatest technical challenge in this LED development has been to prevent hydrogen reacting with the ZnO substrate. The alternative approach has been to use nitrogen as the carrier gas instead, to employ lower growth temperatures and to shield the substrate. An additional option is to develop alternative sources of nitrogen. Compounds with a significantly lower dissociation temperature than ammonia would allow lower growth temperatures to be employed.

At the conclusion of this project in October 2006, Cermet plans to market the developed technology to the semiconductor lighting industry. The product offerings are set to include lattice-matched device layers integrated onto Cermet s doped ZnO substrates. It is Cermet s intent to develop revolutionary LED sources for solid-state lighting that will become an industry standard.

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