An ensemble of spatially distributed III-nitride quantum dots can produce the broad, visible emission that is desirable for ambient lighting and the growth of crops, says Soh Chew Beng, Chua Soo Jin and Liu Wei from the Singapore Institute of Materials Research and Engineering.

GaN-based LEDs have made tremendous progress in the last decade. In particular, the high brightness, phosphor-coated white variety has come on in leaps and bounds, and it is now starting to penetrate solid-state lighting. In this market it offers a far more efficient alternative to the incandescent bulbs that are being phased out in many countries through government legislation, and it does not employ the toxic materials used in compact fluorescent bulbs.

However, light generated by phosphor-coated LEDs is far from perfect. Its color-rendering index is inferior to that of a compact fluorescent, there are production yield issues, and the phosphor coating tends to degrade with usage, leading to unwanted changes in the spectral output of this lighting source.

Shifts in emission profile are a major impediment to the deployment of LEDs for in-door ambient lighting and horticultural lighting for eco-conscious greenhouse and indoor growers. To mimic sunlight and enable plants to perform photosynthesis, it is essential for horticulture lighting to have dominant peak wavelengths at 430- 460 nm and 650-700 nm, the spectral ranges where Chlorophyll A and B have the highest responsivity. If strong emission can be produced at these wavelengths, LEDs can then tap into the horticultural lighting market that currently uses sources such as the HID lamp, which has low conversion efficiency and only outputs a small proportion of its emission spectrum in the ranges useful for photosynthesis.

At Singapore Institute of Materials Research and Engineering – which is a member of the Agency for Science, Technology and Research (A*STAR) – we are developing light sources for horticultural and ambient lighting. These feature a long broad spectrum in the yellow to red regime, plus a narrow blue emission peak.

To produce this form of emission, we have developed a novel technique for incorporating quantum dots (QDs) into LEDs that can tune their emission spectrum. These QDs, which are embedded in the multiple quantum wells of an LED, are clearly visible in transmission electron microscopy images. They produce an internal quantum efficiency of 40 percent, according to photoluminescence measurements at 4K and 300K with an excitation source at 325 nm

Ambient sources

Incorporation of quantum dots into the active layers ofdevices yields two major advantages over theconventional InGaN well: It increases the recombinationefficiency of the emitting layer, thanks to the strongexciton binding energy and large band-offsets; and itreduces the electroluminescence shift due to superiorcarrier confinement in three dimensional space.

While many researchers have turned to QDs to make a narrow linewidth source, we have taken an entirely different tack, using them to create a broad emission spectrum that covers 450 - 750 nm and mimics daylight. To make such a source requires the fabrication of dots with a broad range of sizes, which we realize by variations in growth temperature and trimethylgallium flow (see Figure 2).

 



Figure 2 (Top left) TEM images of the dual stacked MQWs in quantum dots incorporating white LEDs. (Top right) Sample structures of the LEDs. (Bottom left) Electroluminscence spectra of a packaged warm white LEDs (Bottom right) I-V curve of the LEDs

 

Our white LED is based on dual-stacked InGaN/GaN multiple quantum wells with QDs embedded in one of the multi-quantum wells. The lower part of the structure comprises long-wavelength emitting, indium-rich QDs incorporated in quantum wells and the upper set features cyan-green emitting multiple quantum wells. By controlling growth temperature and the precursor flows, we can realize LEDs with many different shades of white.

LED performance has been evaluated with current-voltage and electroluminescence tests. These measurements revealed that there is minimal change in the LED emission peak as injection current increases from 100 mA to 280 mA – it shifts by just 5 nm. This suggests that the piezoelectric field effect does not have a major influence on the enerfgy levels of QDs embedded in quantum wells.

Graphing light output as a function of injection current shows that droop is more prevalent in conventional LEDs than it is in our novel emitters. This characteristic makes our QD-based LEDs attractive candidates for high-power device applications.

LEDs for horticulture

Studies have shown that ultraviolet and far red radiation isbetter for driving photosynthesis than green emission.Against the backdrop of efforts to reduce carbon dioxidefootprints, greenhouse growers are exploring differentcombination of lighting for effective vegetation andflowering during different stages of the plant’s growthcycles.

However, the different type of materials used for growth of UV LEDs (III-nitrides) and red LEDs (III arsenides) will pose a reliability issue for LED lighting units for horticulture. What’s more, the integrated electroluminescence spectrum tends to deviate from its optimum profile after a period of use.

 



Figure 3 (Top left) Electroluminescence of quantum dot incorporated red LEDs (Top right) The relative quantum efficiency curves as determined by the average plants response for photosynthesis. The red box shows the quantum (PPF) response when all photons are weighted equally between 400 nm to 700 nm. PPF overestimate the photosynthetic value of photons between 400 nm to about 550 nm (from McCree, 1972a). (Bottom Left) Processed LEDs at wafer level with violet to green obtained from conventional InGaN/GaN MQWs while yellow to red from quantum dots incorporating MQWs (bottom right) EL from QD LEDs

 

We believe that our novel LEDs that incorporate QDs can address these issues. Our red-emitting versions — which have dominant emission peak at 652 nm with a full-width half maximum of 200 meV — display minimal shift in wavelength with increasing injection current and can cover the longer wavelength (far red) emission required by plants for photosynthesis. Combining this output with the violet/blue emission from conventional InGaN/GaN LEDs enables the production of a lighting source that covers the whole photosynthetic response of plants.

Despite the favorable characteristic and properties of QDs, this system has its limitation. When these dots are incorporated into LEDs they tend to suffer from outdiffusion into the surrounding matrix of InGaN wells and GaN barriers during the growth of high temperature ptype GaN and chamber annealing for magnesium activation.

We have investigated the extent of this degradation and found that it is possible to prevent out diffusion by capping the structure with an AlN layer. This is possible thanks to the low mobility of AlN adatoms on the film surface at a low growth temperature of typically 780 °C and formation of stable Al-N bonds. Aluminum also possesses a lower vapor pressure than indium, which effectively reduces the diffusion length of subsequent indium deposited on the first quantum well layer. In turn, this increases the quantum dot density at the second quantum wells.

Another benefit of the addition of a stable thin AlN encapsulation layer is a reduction of piezoelectric polarization charge accumulation at the interface to the compressively strained InGaN well. This lowers the blue shift of emission wavelength with injection current.

Getting the light out

To improve the efficiency of LEDs and maximize theirenergy saving potential, chip manufacturers andresearchers are looking into ways to resolve efficiencydroop and improve light extraction. We are going downthis road too, and have developed the cheap patterningtechnique using nanosphere lithography. This involvespolystyrene nanospheres, anodized alumina oxide and UVenhancedelectrochemical etching to generatenanoporous GaN.

Our phosphor-free, apple-white LEDs unite a dual stack of InGaN/GaN multiple quantum wells. The lower set contains long-wavelength-emitting, indium-rich nanostructures incorporated in quantum wells, and the upper set comprises cyan-green emitting multiple quantum wells. The LEDs were grown on a nanoepitaxially lateral overgrown (nano-ELO) GaN template, which was formed through re-growth of embedded GaN nanopillars over a SiO2 film. The SiO2 film was patterned by carrying out ICP etching with an anodic aluminium oxide mask featuring an array of holes that were 125 nm in diameter and spaced 250 nm apart.

Anodization of aluminum film or foil using various acids and applied voltages produces hole arrays with diameters ranging from 60 nm to 200 nm. This serves as a natural surface patterning technique. The periodicity of the embedded array of GaN nanopillars enhanced light extraction of the LEDs by 34 percent.

Higher gains of 50 percent of more should be possible by improving internal quantum efficiency through reductions of threading dislocations and stress relaxation. This is a realistic goal, because the separation between dislocation lines is about 200 nm for a GaN sample with a dislocation density of 108 cm-2. Since in our case the diameter of the holes is just 125 nm, it should be possible to prevent many of the threading dislocations from further propagation and annihilate them at the SiO2 mask via bending at the GaN-SiO2 interface.

While our QD LEDs enjoy the advantages associated with zero dimensional structures, such as strong confinement of excitons and color tuning via size and composition control, they are held back by restrictions in the choice of growth temperatures for the top p-type GaN layer. The AlN encapsulation layer can reduce the indium outdiffusion from quantum dots, but it cannot eliminate it. So we are currently exploring alternative techniques, such as the implantation of acceptor into p type GaN, followed by laser annealing for activation.

Another goal of ours is to develop vertical quantum dot LEDs. This could be realized by either an existing laser liftoff technique or electrochemical wet etching of our embedded sacrificial SiO2 film in surface patterning. A patterned n-face GaN aids light extraction, bringing us a step closer to achieving high brightness QD LEDs.

 



Figure 4 (Top) SEM image of the anodized alumina oxide mask used to pattern a hole array on SiO2 and GaN nanopillars grown from SiO2 layer (Middle left) Electroluminescence spectra of quantum dots LEDs grown on GaN nanopillars (with nanohole array of SiO2 mask) and conventional GaN (Middle right) cross-section SEM image of overgrown GaN buffer layer (Bottom left ) Images of packaged apple white LEDs chip with GaN epitaxy overgrown on the GaN nanopillars (Bottom right) sample structures for the apple white LEDs

 

Further Reading

K. Inada et al. Plant Cell Physiol 17 355 (1978)

V. Bafetti “White Paper: Selecting LED Lighting for horticultural Application”, LumiGrow, Inc , 1-12K (2008)

J. Mccree Agric. Meteorol. 9 191(1972)

C. B. Soh J. Appl. Phys. 108 093501(2010)

C. B. Soh Nanoscale Research Letters 5 1788 (2010)