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technology  GaN


always reach the surface. Similarly, in semi-polar material the planes of stacking faults are inclined at an angle to the substrate, so they also reach the surface. In both cases, no growth technology is currently capable of eliminating these faults.


Growth on triangular pyramids Scholz and his co-workers at the University of Ulm have developed a technique that slashes the density of stacking faults through modifications to the substrate. Back in 2004, they were improving the quality of GaN with a well-known technique called epitaxial lateral overgrowth. This begins by taking a sapphire wafer and depositing onto it a GaN film and a SiO2 wafer is then patterned, with SiO2


mask. The selectively removed to create


Scanning electron microscopy reveals the surface of the wafer containing inverse GaN pyramids with semi-polar facets


would still like to get such a device – a green laser is always a good demonstrator that you are successful – but we now focus on more basic relations of non-polar and semi-polar material.” Efforts have been directed at producing high-quality material – principally on semi-polar planes, but also on non-polar planes – and understanding the bandstructure of this material and how its influences electrical transport.


During the project the team has looked at the growth of bulk GaN and developed techniques to form semi-polar GaN films and novel lasers on sapphire substrates. In addition, the group has provided new insights into growth conditions on different planes and explanations for the differences in photoluminescence of quantum wells grown on different types of substrate.


The most attractive approach to forming semi-polar GaN is to deposit this on r-plane sapphire. “[This] is more expensive than c-plane, but it is still in the range of $70 for 2-inch,” explains Scholz. Producing high-quality GaN on this plane of sapphire is very tricky, however, because GaN tends to be plagued with stacking faults that propagate throughout the material.


These stacking faults occur due to local differences in crystalline structure. GaN is a hexagonal material, and its atomic units follow the sequence ABAB… The other common crystalline structure for compound semiconductor materials is cubic: GaAs crystals are an example of this, and their units align in the order ABCABC. Stacking faults arise in GaN when the units have a sequence ABC, which involves a shift in the position of the planes.


“For c-plane devices, [stacking faults] don’t matter so much,” says Scholz. “You have a lot of stacking faults close to the foreign substrate, such as sapphire, but they run parallel to the interface and you don’t find them later if the material grows nicely.” In stark contrast, in non-polar material, the planes of stacking faults are aligned perpendicularly to the plane of the wafer, and they will


46 www.compoundsemiconductor.net March 2013


Figure 1.Conventional GaInN quantum wells are embedded in GaN barriers and grown in the c-direction (left).The GaInN quantum well gets compressively strained due to the different lattice constants of the two materials.This leads to an internal piezoelectric field and a tilting of the conduction (CB) and valence band (VB) edges.One consequence of this is a spatial separation of the electrons in the CB (which move to the right) and the holes in the VB (moving to the left).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 hole wavefunctions to overlap perfectly,improving light emission from this structure (right)


open stripes in the mask, before the structure is put back in the MOCVD chamber. Using optimized growth conditions, GaN stripes with triangular cross-section selectively grow out of these windows. Hence, Scholz’s team have realized pyramid-shaped GaN, which has semi-polar facets but is formed by c-plane growth – so the problems of stacking faults are avoided.


This team has gone on to form a range of LEDs on these facets. Early results included 425 nm LEDs producing more than 3 mW when driven at 110 mA. “We later focused on a longer wavelength,” says Scholz. “You get problems with indium


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