Journal of the Korean Physical Society, Vol. 55, No. 3, September 2009, pp. 1013∼1017 Origin of Ferromagnetism and Long-range Interactions of Cu in GaN: Chemical Bonding and Electronegativity Approaches Seung-Cheol Lee, * Jung-Hae Choi and Kwang-Ryeol Lee Computational Science Center, Korea Institute of Science and Technology, Seoul 136-791 Pil-Ryung Cha † School of Advanced Materials, Kookmin University, Seoul 136-702 (Received) The diluted magnetic semiconductors (DMS) used in microelectronics should exhibit ferromag- netism mediated by delocalized electrons. Only Mn-doped GaN has passed this requirement, al- though the Curie temperature of this materials is at most 170 K, which is substantially lower than room temperature. Therefore, the development of a novel DMS with a higher Curie temperature is still an important issue. Recently, the potential of Cu-doped GaN has been demonstrated both theoretically and experimentally. However, it is not clear why Cu shows ferromagnetism. Using first principles density functional calculations, this study shows that the onset of magnetism of Cu in GaN results from the tendency of electrons to leave anti-bonding states. The delocalized electrons, which can stabilize the ferromagnetic alignment of the Cu ions, are generated at the Fermi level due to the small difference in electronegativities between nitrogen and Cu ions. For comparison, local- ized electrons are formed in Mn-doped GaN due to the large difference in electronegativities. The difference in electronegativities between the anion and the transition-metal impurity is expected to be helpful in the design of new DMS materials. PACS numbers: 31.15.Ar, 31.10.+z, 51.60.+a, 75.50.Pp Keywords: Diluted magnetic semiconductor, Cu-doped GaN, First-principles calculation, Electronegativity I. INTRODUCTION Diluted magnetic semiconductors (DMSs) are usually made by the substitutional alloying of the host semicon- ductor (SC) cation site with transition metals (TMs). The TMs are assumed to be uniformly dispersed in the host SC. When TMs substitutionally occupy cation sites, the d states of the TM hybridize with the p states of the neighboring host anions. This p-d hybridization results in strong magnetic interactions between localized spins and carriers in the host’s valence band [1]. As a result, the system exhibits stable ferromagnetism, and the spin- polarized carriers are used for spin manipulation. To our knowledge, Mn-doped GaAs is the only DMS material to fulfill the abovementioned criteria [2]. However, this successful DMS material exhibits a low Curie tempera- ture, which prohibits practical application of the mate- rial. Studies aimed at increasing the Curie temperature while maintaining the DMS properties using other host semiconductors have been performed. Although there have been some reports of high-temperature ferromag- netism, the origin of the ferromagnetism is still being * E-mail: leesc@kist.re.kr; † E-mail: cprdream@kookmin.ac.kr debated [3,4]. No conclusive evidence has been reported. Therefore, the discovery or design of a new DMS ma- terial is still an important issue in the research area of spintronics. Recently, experimental studies of the ferromagnetism of Cu-doped GaN have been reported. Lee et al. [5] observed room-temperature ferromagnetism of Cu- implanted GaN thin films. According to their results, the magnetic moment of Cu was 0.27 μ B , based on the M-H curve. They also found that due to the clustering of Cu atoms the ferromagnetism was sensitive to the annealing temperature. Seong et al. [6] observed ferromagnetism of Cu in GaN nanowires. In their experiment, the mag- netic moment was 0.86 μ B per Cu atom, and the Curie temperature was far above room temperature. Based on anomalous X-ray scattering and X-ray diffraction re- sults, they claimed that the Cu atoms substitutionally occupied the Ga site. Theoretical calculations regarding the possibility of ferromagnetism of Cu-doped GaN have been also reported. Lee et al. [7,8] studied the valence band splitting of transition-metal-doped GaN and sug- gested that Co- or Cu-doped GaN was the most probable candidate for DMS applications. Similar results were re- ported by Wu et al. [9] They reported that Cu in GaN showed ferromagnetism and that the conduction carrier -1013-