TECHNOLOGY LEDs
ALTHOUGH SALES OF LED lamps are rising fast and grabbing market share from incandescent and fluorescent bulbs, they are too pricy to dominate the market today. And that’s not their only issue: Their efficiency is not that much higher than a fluorescent and can fall fast when the drive current through the LED is cranked up – a problem known as droop.
Look more closely and you’ll find that the performance of LEDs, which feature a stack of InGaN quantum wells where electrons and holes recombine to generate light, are held back by inherent weaknesses. It is conventional to grow them on the crystal orientation (0001), and this spawns strain in the light-emitting active layers that creates strong built-in polarization fields that ultimately drive down device performance. These piezoelectric and spontaneous polarization fields, which can be as high as megavolts-per- centimetre, pull apart electrons and holes in the quantum well, impairing recombination efficiency.
Making matters worse, devices are typically grown via epitaxial processes on lattice-mismatched substrates, such as sapphire and SiC. This mismatch leads to the generation of many misfit dislocations, which hamper light- generation within the LED and shorten the device lifespan. To address these issues, many researchers are considering alternative devices.
One popular option – which our theoretical team at the University of Seoul, Korea, and the Catholic University of Daegu, Korea, is looking at – is to switch to a different growth plane for the nitride LED. This can either reduce or eliminate the internal electric fields. In addition, we are investigating the potential of an even more promising, novel device: An LED built from the alloy CuBr, CuCl and CuBrCl. Such a devices could combine an incredibly high degree of optical gain with lattice-matched growth on a silicon substrate.
Slashing field strengths Various approaches can be used to reduce the impact of the electric field in the active region of a nitride LED. These include: introducing an ultrathin, indium-rich InGaN quantum-well; inserting a very thin AlGaN layer into a thick InGaN well; employing the quaternary AlInGaN; and using non-square quantum-well structures. On top of all of this, there is also the highly popular method of today – growth on a new nitride plane.
The latter approach can be traced back to the pioneering work of Tetsuya Takeuchi, Hiroshi Amano and Isamu Akasaki from Meijo University, Japan. In 1996, they reported the significant reduction in heavy hole effective masses resulting from a switch from aligning the quantum
Figure 1. (a) Configuration of the coordinate systems (xʹ, yʹ, zʹ) in (hkil) -oriented crystals. The growth axis, or zʹ-axis, is normal to the substrate surface (hkil), and the coordinate system (x, y, z) denotes the primary crystallographic axes. The Euler angles and are the polar and azimuthal angles of the direction zʹ in terms of the coordinates. (b) Nonpolar a- and m- planes with the growth direction parallel to the c-axis. θ =p/2 with φ= p/6 corresponds to the zʹ= [1120] growth direction and θ =p/2 with φ=0 corresponds to the zʹ= [1010] growth direction
October 2013
www.compoundsemiconductor.net 57
wells on the (0001) plane to (1010) and (1012) orientations. One key consequence of reducing heavy hole effective mass is to increase recombination efficiency.
Following on from this work, researchers throughout the world have looked to trim the piezoelectric and spontaneous polarizations by growing the epitaxial nitride stack on a semi- polar plane – a plane titled with respect to the (0001) direction. Initially, those layers that were grown on non-polar and semi- polar substrates were plagued with numerous non-radiative recombination centres, because it is difficult to achieve a high crystal quality on non-polar and semi-polar planes. However, this is far less of an issue today, and now researchers are reporting brighter devices. Other recent highlights in this area include the finding of a polarization crossover in a single InGaN/ GaN quantum well grown on a semi-polar (1011) direction and a high compositional homogeneity in an InGaN quantum well grown on a semi-polar {2021} substrate and non-polar (1010) m-plane. These efforts show that there is the potential for commercial devices grown on non-polar and semi-polar substrates.
Our contribution to this field is to consider the optical gain of the LED. This is a measure of the luminous efficiency of this device. We have performed calculations for a 3 nm-thick In0.2
Ga0.8 quantum well with a carrier density of 2 x 1013 cm-2 that is
sandwiched between GaN barriers (see Figure 2, which shows the xʹ and yʹ-polarized transverse electric (TE) optical gain spectra for several crystal orientations and optical anisotropy as a function of crystal orientation).
The most striking feature of these graphs is that optical gain peaks have different strengths in different directions. As crystal angle increases, the optical gain for the yʹ- polarization shifts to
N
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