Doping of GaN by ion implantation
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Doping of GaN by ion implantation Eduardo J. Alves 1,2, C. Liu1,2, Maria F. da Silva 1,2, José C. Soares1,2, Rosário Correia3 and Teresa Monteiro3 1 Instituto Tecnológico e Nuclear, EN 10, 2686-953 Sacavém, Portugal 2 Universidade da Lisboa, CFN, Av. Gama Pinto 2, 1649-003 Lisboa, Portugal 3 Universidade da Aveiro, Dept. Física, 3810-193 Aveiro, Portugal ABSTRACT In this work we report the structural and optical properties of ion implanted GaN. Potential acceptors such as Ca and Er were used as dopants. Ion implantation was carried out with the substrate at room temperature and 550 °C, respectively. The lattice site location of the dopants was studied by Rutherford backscattering/channeling combined with particle induced X-ray emission. Angular scans along both [0001] and [10 1 1] directions show that 50% of the Er ions implanted at 550 oC occupy substitutional or near substitutional Ga sites after annealing. For Ca we found only a fraction of 30% located in displaced Ga sites along the [0001] direction. The optical properties of the ion implanted GaN films have been studied by photoluminescence measurements. Er- related luminescence near 1.54 µm is observed under below band gap excitation at liquid helium temperature. The spectra of the annealed samples consist of multiline structures with the sharpest lines found in GaN until now. The green and red emissions were also observed in the Er doped samples after annealing. INTRODUCTION One of the challenges in GaN research is to introduce shallow p-type dopants. A primary limitation originates from the high ionization energy of acceptor species in GaN that limits the number of ionized free holes at room temperature. To date, Mg has been most commonly used as an acceptor in GaN with an ionization energy level ca. 200 meV [1,2]. Compared to acceptor levels in other III-V semiconductors, however, this level is still too deep. Therefore, the search for alternative acceptor species with relatively lower ionization energy is of particular interest. Ca has been proposed as a potentially superior shallow acceptor in GaN considering d-electron core relaxation effects [3] and its ionization energy was reported to be about 170 meV in GaN [4]. It is well known that both the electrical and optical properties of impurities in semiconductors depend strongly on the locations of impurities in the lattice. Thus, it is important to study the lattice site location of different dopants in GaN. Previous studies report a fraction of 95% of Ca in Ga lattice sites [5]. On the other hand for Er we observe that for low fluence implantations all the Er occupies Ga sites [6]. However, the optical activation requires the annealing of the implanted samples. The near-IR photoluminescence at 1.54 µm was first observed by Wilson et al. [2] from Er-implanted GaN thin films. Moreover Er-doped GaN during the growth can emit not only IR light, but also visible light [7] which has sparked great interest in developing optoelectronics made from rare earth doped GaN [8]. In this work, we report the results of l
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