Effects of carbon on the weak ferromagnetism in doped GaN Lin Yu, Zeyan Wang, Meng Guo, Donghong Liu, Ying Dai * , Baibiao Huang School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China article info Article history: Received 8 October 2009 In final form 27 January 2010 Available online 1 February 2010 abstract The magnetic properties of carbon doping GaN have been examined in combination of first-principles density functional calculations and experiments. The calculated results indicate that a single C dopant produces a spin magnetic moment of about 0.7 l B and the two-C-atoms form a stable ferromagnetic cou- pling when their distance lies around 5–6 Å, which can be explained by the C2p magnetic orbitals over- lapping via the nearest Ga p-d orbitals and next-nearest N2p orbitals. The ferromagnetism is also observed in our C-doped GaN samples with high C/Ga ratio. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Dilute magnetic semiconductor (DMS) has attracted lots of attention since the discovery of ferromagnetism with a high Curie temperature in Mn-doped gallium nitride (GaN) [1]. A large num- ber of studies have been done for the DMS’s based on TiO 2 , ZnO, SnO 2 , HfO 2 , GaN doped with a few 3d transition metal [2–7]. In particular, GaN-based semiconductor has been widely studied due to its potential applications such as blue optoelectronics, high-power and high-frequency devices, and photocatalysts [8– 11]. As DMS, the transition metal doped GaN had been investigated by experiments and theoretical calculations. Mn-doped GaN was reported to have nonzero magnetization 320 K above room-tem- perature [12], and Gd- [13], Ni- [14], Cu-doped [15] GaN also exhibited ferromagnetism at room-temperature. Recently, C-doped ZnO films deposited by pulsed-laser deposi- tion showed ferromagnetism (FM) with Curie temperatures higher than 400 K [16]. Pure HfO 2 film was also observed to have FM with- out any transition metal doping [17] and the following theoretical investigation suggested that the cation defect may induce the mag- netism [18]. Furthermore, first-principles density functional theory (DFT) calculations reported that C-doping induces ferromagnetism in CdS [19] and TiO 2 [20]; spin-polarizations were also predicted in BN nanotubes where C replaces either a single B or N atom [21] and in C-doped BN nanotubes with injected carriers [22]. Our previous investigation indicated that the ferromagnetism of Mn-doped GaN has been enhanced by the codoping with carbon [23]. In fact, resid- ual carbon impurity usually exists in GaN films, especially grown by metal oxide chemical vapor deposition (MOCVD). Although lots of work has been done to look for an alternative p-type dopant for GaN by doping C into GaN, carbon doping effects on magnetic property of GaN was neglected. Therefore, it is necessary to explore the possibility to form ferromagnetism in C-doped GaN. In the present work, we examined the magnetism of C-doped GaN by means of spin-polarized DFT calculations and experiments. The weak room-temperature ferromagnetism in the C-doped GaN with high C/Ga ratio is observed in our samples, and our calculated re- sults confirm that C doping may induce weak magnetism in C- doped GaN. The work will be helpful to its potential applications including biodetectors, ultralow-power memories, and integrated magnetic sensors. 2. Computational details In the present work, the first-principles spin-polarized DFT elec- tronic calculations were carried out by using the Vienna Ab-initio Simulation Package (VASP) [24,25]. Our calculations employed the generalized gradient approximation (GGA) [26], the Perdew-Wang 91 exchange–correlation functional [27], the ultrasoft pseudopo- tential [28], the plane wave cutoff energy of 400 eV for the basis set, and the sampling of the irreducible Brillouin zone with the 3  3  3 Monkhorst–Pack k-points for the supercell [29]. The convergence threshold for self-consistent-field iteration was set at 10 –6 eV. The structure of C-doped GaN was constructed on the basis of the 72-atom 3  3  2 supercell by replacing Ga or N atom with C atom (Fig. 1a). The cell parameters were fixed at the exper- imental values (i.e., a = b = 3.189 Å, and c = 5.185 Å for undoped GaN), and all atomic positions were optimized until all forces become smaller than 10 –4 eV/Å. 3. Electronic structure of C-doped GaN Firstly, we estimate the relative stability of two possible C-doped structures, i.e., carbon replaces nitrogen (C@N) and carbon 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.01.068 * Corresponding author. E-mail address: daiy60@sina.com (Y. Dai). Chemical Physics Letters 487 (2010) 251–255 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett