Neutron transmutation doped(NTD)-GaN, which is irradiated with fast and thermal neutrons at fluxes of 6.7 x 10 18 cm -2 and 1.4 x 10 19 cm -2 , respectively, keeps the high resistivity of 10 8 Ωcm at room temperature. The energy level estimated from the temperature dependence of carrier concentration by high temperature alternating current Hall effect measurements is 960 meV. This energy is attributed to the nitrogen interstitial (N i ) as a deep acceptor level located below the bottom of the

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... nduction band, whose level is close to that of the nitrogen split interstitial. In Rutherford backscattering spectroscopy/channeling measurements using 1.5 MeV H + ions, the lattice displacement of N atoms is observed, suggesting the existence of N i . Neutron-transmuted DX-like center of Ge as a donor is compensated by both N i and 14 C acceptors generated from the (n,p) reaction of 14 N. The origins of high resistivity after the neutron irradiation are attributed to these acceptors.