Thermal stability of semi-insulating property of Fe-doped GaN bulk films studied by photoluminescence and monoenergetic positron annihilation techniques

The thermal stability of electrical resistivity (rho) is one of the crucial functions of semi-insulating (SI) substrates. In this paper, we describe the thermal stability of SI property in Fe-doped GaN (GaN:Fe) films grown by hydride vapor phase epitaxy, in view of point defect chemistry by means of monoenergetic positron annihilation and photoluminescence (PL) measurements. PL spectra of GaN:Fe at 8 K exhibited broad emission bands in UV, blue, and yellow spectral regions, as well as a series of characteristic infrared peaks with a sharp zero-phonon line at 1.300 eV. A rho value higher than 10(8) center dot cm was obtained when the doping concentration of Fe, [Fe], exceeded the major shallow donor (Si) concentration (5x10(17) cm(-3)). For those SI samples, the relative intensity of the yellow luminescence band at 2.2 eV, of which the origin has been attributed to Ga vacancies (V-Ga) and/or defect complexes composed of V-Ga and O, over the UV/blue emission was remarkably decreased. Simultaneously, the Doppler broadening S parameter for the positron annihilation measurement, which represents the size or concentration of negatively charged vacancy type point defects such as V-Ga, was decreased. The results are consistent with the increase in formation energy of V-Ga due to the downward shift of the Fermi level by Fe doping. The values of rho, S, and W parameters that represents the fraction of positrons annihilated with core electrons, in the bulk region did not change remarkably while the positron diffusion length was increased by the annealing in N-2 between 600 and 1050 degrees C. Although the defect concentration in uncapped surface region was increased remarkably by annealing at 1050 degrees C due to the surface decomposition, the present results indicate that GaN:Fe can be used as a thermally stable SI substrate for electronic devices because the surface does not decompose during the epitaxial growths of overlayers.