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ABSTRACT We have investigated the Ca dopant site within the GaN lattice using ion channeling in combination with Rutherford backscattering spectrometry (RBS), particle induced x ray emission (PIXE) and nuclear reaction analysis (NRA). Metalorganic chemical vapor deposition (MOCVD) grown GaN on c-plane sapphire substrates implanted with 40 Ca at a dose of Ix10 15 cm-2 with post-implant annealing were investigated. The channeling results indicate that more than 80 % of Ca are near Ga sites even in as-implanted samples, however, they are displaced by - 0.2 A from the Ga sites and that the Ca goes to the exact Ga sites after annealing at I 100°C. We think that the displaced Ca in the as-implanted samples are electrically compensated due to formation of complex defects with donor like point defects, such as CaGa-VN and/or CaGa-GaN, and that CaGa becomes electrically active when these complex defects are broken and the point defects diffuse away with annealing at I 100°C.

INTRODUCTION GaN has attracted great interest not only for the fabrication of blue light emitting lasers but also for high power and high temperature devices because of its outstanding thermal and chemical stability [ 1,21. It has been reported that Ca implanted GaN becomes p-type from n-type with post-implant annealing at -1 100IC. The ionization level of Ca was estimated to be -170 mV, which is as shallow as that of Mg [3]. Since Ca can be a p-type carrier without any coimplantation while Mg implantation requires P co-implantation to achieve p-type conductivity [4], Ca is expected to be a desirable p-type dopant. Ion implant damage and removal by annealing have been studied as a function of implantation dose and annealing temperature [5, 6]. However,

most of these works concerned only electrical and optical properties or crystalline quality of the GaN itself, no work on structural information on impurity lattice location has been reported other than Si implanted GaN [7]. We have studied directly the lattice location of Ca implanted GaN using ion channeling combined with Rutherford backscattering spectrometry (RBS), particle induced x ray emission (PIXE) and nuclear reaction analysis (NRA) for Ga, Ca and N detection, respectively. We estimated the substitutionality of Ca in the GaN lattice from minimum channeling yield and studied whether the Ca is in Ga or N sites by comparing channeling angular distributions of Ca, Ga and N. We will also discuss displacements of Ca from lattice sites. In this paper, we will demonstrate the lattice location of Ca. We will also discuss the mechanism of p-type conductivity of Ca implanted GaN with post-implant annealing.

EXPERIMENTAL 2 jtm-thick GaN layers were grown on c-plane sapphire substrates by metalorganic chemical vapor deposition (MOCVD) at 9900C with a 25 nm-thick GaN buffer layer grown at 520'C. 40Ca ions were implanted into the undoped GaN layer at a dose of IxI0 15 cm-2 at an energy of 200 keV. The implantation was performed at liquid nitrogen temperature. The maximum 40 Ca concentration present address