RESEARCH REVIEW Nitride LEDs emit in the red
Aluminium-rich interlayer boosts efficiency at longer wavelengths RESEARCHERS from Toshiba are claiming to have produced the first red-emitting LED with an output power exceeding 1 mW at 20 mA.
Under that drive current, a 629 nm LED delivered 1.1 mW at an external quantum efficiency of 2.9 percent.
Producing efficient emitters in the green, yellow, orange and shorter-wavelength red is challenging, but this success should help the development of white light sources that combine a very high colour-rendering index with an absence of energy-sapping, colour-converting phosphors.
Reaching shorter wavelengths with today’s commercial, AlGaInP-based red-emitting LEDs is not possible, due to a transition from a direct to an indirect bandgap. So what is needed is to stretch the emission of nitride LEDs, which can deliver external quantum efficiencies in excess of 80 percent in the blue, to longer wavelengths. Efficiency of the nitride LED declines at longer wavelengths due to the quantum-confined Stark effect (QCSE): an internal electric field pulls apart electrons and holes, hampering radiative recombination.
Turning to semi-polar or non-polar substrates can reduce or eliminate these fields, but a higher indium content is then be needed, because the QCSE leads to a red-shift in emission. Increasing the indium content in the InGaN quantum well is far from trivial, because significant lattice mismatch between InGaN and GaN can cause the well to be riddled with light-sapping defects.
Toshiba’s engineers have been able to overcome this by introducing an AlGaN interlayer into the active region.
Production of these devices involved atmospheric MOCVD growth on c-plane sapphire. Construction of the LEDs began with a 2 μm-thick undoped GaN layer and a 3 μm-thick silicon-doped GaN layer, which had an estimated threading dislocation density of 4 x 108
cm-2 . The four-period multi- quantum well active region that followed
consisted of a 3 nm-thick InGaN quantum well layer, a 1 nm-thick AlGaN interlayer, and a 10 nm-thick InGaN barrier. Well and interlayer were grown at 755 °C, while the barrier was grown at 100 °C higher. Addition of a magnesium-doped p-type GaN layer and heavily-doped cap completed the growth.
In these devices, the indium content in the well is estimated to be 35 percent, while in the barrier it is below 1 percent. Meanwhile, the aluminium content in the AlGaN layer is varied: in one type of device it is around 30 percent, but in another type it is about 90 percent.
N layer. These imperfections are present, but far less pronounced, in the device with an Al0.9
To test device performance, simple device structures were formed with a transparent indium-tin-oxide p-type contact and an n-type electrode formed from a Ti/Pt/Au stack. Ray tracing calculations indicate that the light extraction efficiency from these chips should exceed 60 percent. Dark spots and dotted emission are seen in fluorescence images of the LED with the Al0.3
Ga0.7 Ga0.1 N layer.
To understand why this is, the team evaluated surface morphology of the quantum well, interlayer and barrier with atomic force microscopy (AFM), scrutinising samples with growth stopped on top of each of the layers. This approach uncovered an atomic-step-and- terrace structure in the quantum well layer, which featured V-shaped defects with a
density of 5 x 108 cm-2 – this is comparable to the threading dislocation density.
Further revelations from the AFM study included a smoothing of the surface with the addition of the Al0.3 did not occur for the Al0.9
Ga0.7 Ga0.1
that had a three-dimensional structure. Differences in lattice mismatch are thought to be behind this.
Depositing the barrier improved the surface morphology of the structure with the Al0.9
Ga0.1 Ga0.7
Efforts by Toshiba’s engineers could help to lead to the commercialization of red, nitride-based LEDs.
N layer, which N layer
N layer, while V-shaped
defects with a larger diameter than seen before appeared after a barrier covered the Al0.3
defects was 1.5 x 109 with 7 x 108
cm-2 Al0.9 Ga0.1 N layer.
The team speculates that the density of V-shaped defects exceeds that of threading dislocations due to the generation of misfit dislocations. It is possible that the misfit dislocations are suppressed with the Al0.9
Ga0.1 N layer, thanks to strain compensation.
Based on these observations of surface morphology, the team constructed LEDs with an Al0.9
Ga0.1 , compared
N layer. The density of these cm-2
for the structure with an
N layer. Driven at 20 mA,
devices produced 1.1 mW at 629 nm, while cranking the current up to 250 mA increased the output to 7.8 mW, and blue-shifted the emission to 607 nm.
J-Il. Hwang et. al. Appl. Phys. Express 7 071003 (2014) Copyright Compound Semiconductor Issue VI 2014
www.compoundsemiconductor.net 69
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