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technology  GaN


Figure 5.One way to form semi-polar GaN wafers is to initiate GaN growth on c-plane side-walls of trenches etched into sapphire wafers with specific non-c-plane orientations


Technologies Inc. and Technologies and Devices International Inc. (at that time part of the Oxford Instruments Group, but now owned by Ostendo) announced their joint development of semi- polar (1122) GaN layers on sapphire substrates. And in November, Sumitomo Electric unveiled its large-scale production of the world’s first 2-inch semi-polar/nonpolar GaN substrates for green lasers. This platform had a dislocation density of the order of just 105


cm-2 .


investigate how the angle of the facet impacts variations in the thickness and composition of the wells. It also led to the fabrication of a more uniform surface of material, which had a greater area of semi-polar planes.


One application of such structures is luminescence conversion. A partner in that work is Osram Opto Semiconductors of Regensburg, Germany. “[Osram] are interested in getting green light by optically pumping such material with highly efficient blue LEDs. They are currently comparing their polar quantum well structures grown conventionally in c-direction with our semi-polar material,” explains Scholz.


From hills to plains From 2008 onwards, the researchers at Ulm have also been trying to form engineered substrates with a flat, semi-polar surface using c-plane growth. “Triangular shaped devices are not liked in industry, because you have to produce contacts on a very fancy surface,” explains Scholz.


His team’s efforts have followed in the footsteps of researchers in Japan: Nobuhiko Sawaki’s group from Aichi Institute of Technology, Japan, which have developed flat {1011} GaN surfaces by etching trenches with {111} sidewalls in silicon; and more recently, Kazuyuki Tadatomo’s team at Yamaguchi University, that produced pure {1122} GaN by patterning r-plane sapphire with 3 µm wide, 1 µm deep stripes running along the in- plane m-direction and separated by 3 µm-wide terraces.


If the growth on these structured substrates were perfect, material would just grow on one type of sidewall, known as the +c-wing (see Figure 5). In this case GaN would grow out of the trench, grow laterally over the ridges separating the trenches and eventually coalesce, creating a flat surface with a semi-polar nature. In practice, however, GaN also grows laterally in the opposite direction after having filled the trench in the wafer. This forms a -c-wing, which is riddled with defects (see Figures 6, 7 and 8). In comparison, the quality of material outside this area is far higher. According to Scholz, material produced by his team has: “on average, stacking faults below 104 dislocation density below 109


, per centimetre, and cm-2 .” Comparisons with companies


In 2010, the progress of the German team would have been compared to that of two companies outside of Europe that announced tremendous progress towards the manufacture of semi-polar GaN substrates. In the summer of that year, Ostendo


48 www.compoundsemiconductor.net March 2013


Figure 6.Transmission electron microscopy highlights the reduction in the crystal quality of GaN in the –c-wing


Impressive announcements from Sumitomo and TDI could have jeopardized further funding of PolarCoN, but they didn’t. “In Germany, we are in this good situation where the funders do not kill a project after such a message,” says Scholz, who believes that continued backing of the project was aided by its broad aims: Not to only make semi-polar GaN, but to also understand the nature of this material. According to him, there are still opportunities to improve the technologies for making semi-polar and non-polar GaN, and the findings that stem from PolarCoN could benefit companies from Germany or other parts of Europe that may make these materials in future. Although it is now more than two years since these announcements from both Sumitomo and the collaboration between TDI and Ostendo, little is known about the material produced by these companies.


“I would think that TDI is just growing on r-plane sapphire,” says Scholz. “Maybe they have found a method to get just one phase of semi-polar material. But there is no scientific publication about that, so it’s very hard to discuss this.” Sumitomo is just as secretive. “From what I see, there is not even a publication from groups who may have used that substrate,” claims Scholz.


He and his co-workers are continuing to develop their flat semi- polar substrates, and deposit heterostructures on them. They began with quantum wells, and the strong intensity of the photoluminescence emanating from them encouraged the development of LEDs on this semi-polar platform, which are compared to devices on sapphire. “The intensity of the electroluminescence [from these semi-polar devices] is less than that from c-plane counterparts, which is to some extent due to non-optimised p-doping,” says Scholz, who plans to continue to develop these devices.


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