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technology  GaN


Figure 7.Cathodoluminescence measurements by a team at Otto-von-Guericke-University Magdeburg show that basal plane stacking faults (BSFs) are only present in the –c wing.Near band edge (NBE) emission is strong outside this region,due to far higher crystal quality


Growth mechanics


One of the biggest questions surrounding the growth of non-polar and semi-polar material is this: How do the deposition conditions compare to those for the growth of conventional GaN? Some groups claim that they are considerably different, but in general terms, Scholz believes that they are broadly similar.


“It was a very controversial discussion in our group, and to some extent the results were different in different groups,” explains Scholz, who admits that the details of the growth conditions play


a major role in drawing conclusions. He believes that the growth conditions for most facets are very similar, but studies conducted by TU Berlin have confirmed earlier studies done in Ulm that incorporation of indium is markedly enhanced on the {1011} plane, compared to other semi-polar orientations.


Another important contribution from a PolarCoN team partner – TU Braunschweig – is an explanation for the substantially shorter emission wavelength for InGaN quantum wells grown on m-plane SiC, compared to c-plane sapphire and c-plane and m-plane SiC. X-ray diffraction measurements reveal very similar levels of indium incorporation for all the wells, and Scholz and his co-workers argue that the shift in emission wavelength stems from growth on SiC. This leads to high levels of stacking faults, which create quantum-wire-like structures. “That means you produce a local different bandgap,” explains Scholz, with emission from this structure depending on the alignment of this bandgap to that of the host material. In this case, a type-II bandstructure results.


Funding for this study and other efforts within PolarCoN runs until 2015, which gives the team some more time optimise its substrates and devices. Scholz has set his team in Ulm two targets for the remainder of the programme: “One is to produce a DFB laser, making use of triangular stripes, operating at least by optical pumping, if not electrical pumping. And the other is to produce a nice wafer, which means optimising HVPE growth of semi-polar material.” Success on both fronts would be a fitting way to end this six-year effort.


Figure 8.Inserting SiN interlayers improves the quality of GaN


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March 2013 www.compoundsemiconductor.net 49


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