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technology  GaN


gas diffusion effects. It is thought that as the precursor molecules diffuse down to the bottom of the stripes, the effective diffusion length of the indium molecules is shorter than that for the gallium- containing species, and this reduces the growth rate for the well and its indium content.


One upshot of these variations within the well is a broadening of the LED output. It’s not clear if this is beneficial for commercial applications, and when Scholz has discussed this with colleagues working in industry, they have been concerned that differences in drive current lead to changes in the colour emitted by this device.


Novel lasers


Figure 2.Polarization (left axis) and wave function overlap (right axis) for a GaInN quantum well between GaN barriers. 0°refers to c-plane quantum wells,90°to non-polar planes. Two low-index,semi-polar planes are marked at about 60°


incorporation, and the power drops drastically. We are currently trying to improve it.”


Another problem with this type of LED is that it is very difficult to optimise its p-type doping. “You can’t easily measure the p-type doping in such structures, because SIMS (Secondary Ion Mass Spectroscopy) and Hall measurements do not work,” explains Scholz. One technique that can be used to identify problems is transmission electron microscopy (TEM), but it can take two months to get results with this approach.


However, although TEM doesn’t provide fast feedback, it has played a very important role in characterizing these semi-polar LEDs. It is able to determine the thickness of the quantum wells grown on the facets, and revealed that this is thicker near the apex of the stripe. Compositional fluctuations are also present in the InGaN quantum wells, according to locally resolved high- resolution X-ray measurements, which show increasing indium richness near the apex. Scholz and his co-workers have attributed these variations in the composition and thickness of the wells to


What may raise a few eyebrows is that the team from Ulm are also trying to develop semi-polar lasers on these triangular pyramids (see Figure 3). “It seems to be very difficult to think about a laser,” admits Scholz, “but it depends on which kind of laser you hope to realise, and how much stripe material you need. You can consider whether you can realise a laser that runs along the stripe.”


To optimise the design, the dimensions of the stripe should be tailored for waveguiding, and the feature sizes should be reduced to minimize variations in the thickness and indium composition of the quantum well. Judicious selection of the spacing of the structures and their dimensions may also enable them to form gratings for distributed feedback lasers (see Figure 4).


The team from Ulm are working on that, and now grappling with the problem of forming good waveguides via high-quality overgrowth of AlGaN on the stripes. “Our goal is to get optically pumped lasers this year,” says Scholz. “We have some optical gain measurements, but they are not that great yet.”


In addition to forming triangular pyramids, Scholz and his co- workers have fabricated arrays of hexagonal pyramids by creating hexagonal apertures in the dielectric mask, and then growing the structures out of these holes. This work allowed the team to


Figure 3.Masking and regrowth on the c-plane leads to the formation of triangular pyramids with semi-polar facets. Researchers at the University of Ulm are trying to develop lasers on these structures


Figure 4.Lasers formed on semi-polar facets can have feature sizes that could enable distributed feedback


March 2013 www.compoundsemiconductor.net 47


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