The control of conductivity of bulk GaN crystals which are grown by Na flux method.

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The elements of impurities and the origins of impurities were investigated to control conductivity of GaN which were grown by Na flux method. The major impurity incorporated into crystals was turned out to be oxygen. The oxygen concentration in LPE-GaN was ranged from the order of 10^<18> to 10^<19> cm^<-3>. The origin of oxygen was successive corrosion of alumina crucible in the growth period. We searched materials for the crucible which can tolerate Ga-Na mixed metal at high temperature and found the yttrium aluminum garnet (YAG) and yttria to be strong enough for that system. The oxygen concentration in LPE-GaN could be reduced to the order of 10^<16> cm^<-3> using yttria crucible. As for the p-type dopants, it was clarified that Mg element can be doped in the LPE-GaN with the order of 10^<20> cm^<-3>, which is the almost same concentration that adopted in the current devices. Mg doping enabled to increase in the conductivity to the order of 10^6 Ω・cm. On the other hand, Li and Ge could reduce the conductivity to the order of 10^<-2> Ω・cm.Also, even in the Li, Ge or Mg doped system, large bulk GaN single crystal could be obtained by co-doping of carbon which could significantly suppress the growth of poly-crystals.