Hydrogen-related local vibrational modes in GaN:Mg grown by molecular beam epitaxy
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Hydrogen-related local vibrational modes in GaN:Mg grown by molecular beam epitaxy D. Pastor,1 R. Cusc´o,1 L. Art´us,1 F. Naranjo,2 and E. Calleja2 1 Institut Jaume Almera (CSIC), C. Llu´ıs Sol´ e i Sabar´ıs s.n., 08028 Barcelona, Spain. 2 ISOM and Departamento de Ingenier´ıa Electr´ onica, ETSI Telecomunicaci´on, Universidad Polit´ecnica, Ciudad Universitaria, 28040 Madrid, Spain. ABSTRACT We report a Raman scattering study of local vibrational modes (LVMs) on Mg-doped GaN grown by molecular beam epitaxy (MBE). Besides Mg:Ga local vibrational modes clearly observed at 262 and 565 cm 1 , several peaks were detected in the spectral regions around 2200 cm 1 and 2900 cm 1 . The modes in the 2200 cm 1 spectral region correspond to local modes of hydrogen complexes and hydrogen-decorated defects, and indicate the presence of a fairly high concentration of H in the samples. The peaks observed in the 2900 cm 1 region are assigned to carbon-hydrogen local modes and are indicative of the presence of C impurities in the samples. These measurements show that both C and H impurities may be present in sizable amounts not only in metal-organic chemical vapor deposition (MOCVD) samples but also in MBE grown samples, and this may have an effect on the electrical conductivity of p-type GaN:Mg samples. INTRODUCTION Despite the achievement of effective p-type doping of GaN using Mg as a dopant, basic understanding of p-type doping in GaN still remains an issue [1]. It has been recognized that hydrogen plays an important role in p-type doping of GaN, strongly passivating the acceptor states of MOCVD-grown Mg-doped GaN and making post-growth annealing treatments necessary to obtain p-type conductivity [2]. Contrary, GaN:Mg samples grown by MBE usually exhibit native p-type character in the as-grown state. It has been suggested that the different behavior of MOCVD and MBE-grown samples can be related with the hydrogen incorporation occurring during MOCVD growth, as this technique usually employs H 2 as carrier gas and ammonia (NH3 ) as nitridation source. Carbon is another impurity that could be expected in MOCVD-grown GaN from decomposition of the metal-organic reactants. Raman scattering is a powerful tool to investigate the incorporation of impurities into a crystal and their bonding with the lattice atoms because they give rise to local vibrational modes (LVMs) whose frequencies are characteristic of the impurity and its local environment. In fact, previous work on MOCVD-grown GaN:Mg [2] identified a LVM that was assigned to vibrational modes of the Mg–N–H complex. In samples annealed above 600 C, the disappearance of this LVM correlated with the activation of p-type conductivity, strongly suggesting the possibility that
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Figure 1: Room-temperature Raman spectrum of a MBE-grown Mg-doped GaN epilayer obtained in the x yy x¯ scattering configuration.
Mg is passivated by the hydrogen incorp
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