INDUSTRY MBE FOR NITRIDES


Slashing defect densities in nitride films grown by high temperature ammonia MBE


Grown in an MOCVD reactor, nitride films tend to yield fewer defects than when they are formed in MBE chambers. But this gap in material quality can disappear with high-temperature ammonia MBE, which produces epitaxial structures with outstanding electrical characteristics, argues Alexey Alexeev and Stanislav Petrov from SemiTEq.


THERE IS A FUNDAMENTAL difference between the growth of most nitride devices and those based on the arsenide and phosphide families. While the latter are grown on native substrates with very low defect densities, nitride devices tend to be formed on the likes of sapphire, silicon and SiC, because GaN substrates are so difficult to make, pricey, limited in availability, and – in relative terms – not of that high a quality.


The biggest downside of growth on mismatched substrates is that it leads to high dislocation densities. If the more widely used deposition technology, MOCVD, is deployed, dislocation density in GaN films is typically in the range 108


-109 cm-2 . However, if this approach is


combined with an epitaxial overgrowth technology, dislocation density can fall towards 107


cm-2 . In comparison, due


to the lower temperature traditionally associated with MBE, this growth technology leads to a lower surface mobility of adatoms and ultimately an inferior material quality. Dislocation densities for nitride samples grown by MBE are typically 109


-1010 cm-2 .


Although MBE tends to yield inferior dislocation densities, it does offer several advantages over MOCVD. Some of them occur for all material


SemiTEq’s approach to slashing defect densities in nitride films October 2013 www.compoundsemiconductor.net 27


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