INDUSTRY LEDs
Moving in the right direction At Osram Opto Semiconductors of Regensburg, Germany, we have been steadily improving the efficacy of our green LEDs. In 2008, our colleagues, led by Matthias Peter, reported a 1 mm2
,
ThinGaN 527 nm chip producing 100 lm at 350 mA. This corresponds to 73 lm/W. Two years’ later, we increased efficacy to 100 lm/W at 350 mA, using an optimised 1 mm2
Figure 2: Comparison of the current dependent external quantum efficiency (EQE) of a blue and green 1mm2
InGaN/GaN LED emitting at 442 nm and 530 nm, respectively
the lower bandgap. This higher drive voltage drags down power conversion efficiency. The second weakness is given by the fact that green LEDs are plagued by droop, the steady decline in internal quantum efficiency at increasing current densities. Droop occurs in blue LEDs, but its impact is far greater in green ones, leading to very low efficiencies at common operating currents (see Figure 1 and 2).
The cause of droop is a hotly debated topic within the nitride community. Since the loss rate that causes droop exhibits a cubic dependence on charge carrier density under both electroluminescence and photoluminescence excitation, Auger recombination (direct or phonon- assisted) in the active layer is one of the main suspects.
However, this is by no means the only conjecture for the cause of droop – there have been attempts to explain the origin of this mysterious malady with theories involving either dislocations, carrier spill-over (thermionic, ballistic and Auger- induced spill-over) or electron leakage. The latter is enhanced by high internal piezoelectric fields.
chip housed in a Golden Dragon Plus package. At this drive current, luminous flux is 117 lm, while cranking up the current to 1 A increases output to 224 lm.
More recently, we have raised the bar for green LED performance again. Higher efficacies have been possible with MOCVD-grown LEDs formed on c-plane sapphire that feature an active region with five to seven InGaN quantum wells embedded in GaN barriers. A 5 µm-thick, silicon-doped GaN buffer layer underpins this active region, which is covered with a 30 nm-thick, p-type magnesium-doped AlGaN electron-blocking layer and a 140 nm-thick, magnesium-doped GaN contact layer.
We have compared the photoluminescence produced by the active region of this structure with that of a device coming off our production line (see Figure 3). With the high-volume device, micro- photoluminescence reveals strong inhomogeneity in intensity, with a pattern of dark spots appearing against a meandering, bright background. The density of the dark spots corresponds to the hexagonal crystal defect (V-pits) density, leading us to suspect that there is a strong correlation between these spots and the V-pits. This view is supported by several studies by other groups, which confirm a point-to-point correlation.
Lowering the growth rate in the active region significantly improves quantum well material quality, according to micro- photoluminescence imaging. The density of the dark spots is similar to that of the sample from the production line, but the affected area is far smaller. This increases the proportion of bright areas, leading to a more homogeneous luminescence pattern.
Figure 3: Microphotoluminscence images of a device from a production (a) and a research and development sample (b). For a better contrast, the lower part of the images are shown in levels of grey only
This improvement, which results from an increase in material quality that enhances internal quantum efficiency and transport characteristics, leads to better-performing LEDs. Recent samples that have been
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www.compoundsemiconductor.net October 2013
Figure 5: Electro-optical characteristics of a 2 mm2
Figure 4: Electro-optical characteristics of a 1mm2
ThinGaN chip in a Dragon package: device from production (blue), device with improved transport (black) and optimized epitaxial structure (orange)
mounted in a Dragon package with a spherical lens produce 114 lm at 350 mA, corresponding to an efficacy of 100 lm/W (see Figure 4). In comparison, devices from the production line emit just 108 lm under the same drive current. Even better results are possible by by removing quantum wells that do not contribute significantly to light generation. In our case, that means trimming the number of wells from seven to five, and therefore improving carrier transport. With this refinement, output from the 532 nm- emitting, 1 mm2
ThinGaN chip hits 134 lm
at 350 mA, corresponding to an efficacy of 108 lm/W.
The key to further improvement in these green-emitting LEDs is to combat droop by cutting carrier density, through either an increase in chip size or the number of emitting quantum wells. The efficacy curves of Figure 4 allow us to estimate that cutting the current density by a factor of two or four increases efficacy by 25 percent or 60 percent.
We have adopted this approach, increasing chip size to 2 mm2
. This increased output power for a green LED
ThinGaN chip in an OSLON package with an improved carrier transport and an optimized epitaxial structure
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