TECHNOLOGY SUBSTRATES
Scaling semi-polar substrates
Growth on patterned sapphire can lead to large, low-cost semi-polar substrates
By KEISUKE YAMANE FROM YAMAGUCHI UNIVERSITY, JAPAN
ONE-FIFTH OF THE WORLD’S ELECTRICITY is consumed by lighting. So, to try and trim carbon footprints, many companies and governments are funding efforts to increase the efficiency of the light bulb.
Much interest has been devoted to solid-state lighting, thanks to its potential to deliver efficacies of several hundred lumens-per-Watt, which is far higher than the best fluorescent sources of today. At the heart of this type of light source is an InGaN-based LED that typically emits in the blue and pumps one or more phosphors of longer wavelengths. White light results from colour mixing.
Most InGaN-based LEDs are fabricated on a polar {0001} (c-plane) GaN layer, which is grown on a c-plane sapphire substrate. This platform is cost-effective, and the growth technique for forming the LED epistructure on it is refined and widely used in high-volume fabs. In contract, high-performance nitride devices, such as laser diodes or very high efficiency LEDs, are fabricated on bulk GaN substrates. The reason for this is that deposition on a native substrate gives the best device performance, because it minimises interface effects and defect formation.
Regardless of the substrate, LEDs formed with c-plane GaN-based material
have a major impediment to high performance: separation of the carriers by a piezoelectric field (see Figure 1(a)). This field pulls apart electrons and holes, reducing the likelihood for radiative recombination and thereby impairing efficiency.
To reach longer wavelengths, indium content is increased, but this also increases the strength of the unwanted
electric field in the device – and is the primary reason for the low luminous efficiency of green light-emitting devices.
An attractive option for addressing these fields is to turn to a new approach for band engineering, which involves growth on different planes of GaN. Piezoelectric fields are absent on non- polar planes, while drastically cut on semi-polar planes, and in both cases this
Figure 1. Band alignment of an InGaN/GaN quantum well structure for polar and nonpolar/semi- polar planes. Conventional InGaN quantum wells are grown in the c-direction (left). The InGaN quantum well gets compressively strained due to the different lattice constants of the two materials. This leads to an internal piezoelectric field, resulting in a tilt of the energy band. Consequently, there is a spatial separation between the electrons in the conduction band and the holes in the valence band. Moreover, the effective band gap is slightly reduced. This effect is called the quantum confined Stark effect. Removing the internal electric field allows the electron and holes to spatially overlap perfectly, resulting in efficient recombination (right)
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www.compoundsemiconductor.net March 2014
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