Epitaxial growth and magnetic properties of Cr-doped AlN thin films

Cr-doped AlN thin fi lms were epitaxially grown on Al2O3(001) substrates at lowtemperature by reactivemagnetron sputtering,and their magnetic properties were investigated. Extensive x-ray diffraction studies indicated that the fi lms have a wurtzite-type hexagonal structure and are (001) oriented, with an epitaxial relationship of the [100] direction of the fi lms along the [110] direction of Al2O3. The c axis lattice parameter of the fi lms showed a linear dependence on the Cr concentration for Cr concentrations below 0.15. Room temperature ferromagnetism was observed, and the magnetic properties showed strong dependence on the Cr concentration over a wide range. Ferromagnetic semiconductors are attracting increasing interest due to their potential applications in spintronic devices. Ferromagnetic semiconductors with Curie temperatures well above room temperature are of special interest from the viewpoints of both fundamental research and practical applications. Following theoretical predictions [1], high temperature ferromagnetism has recently been reported in doped oxides and nitrides, such as ZnO [2], TiO2 [3], SnO2 [4], GaN [5], and AlN [6]. AlN is one of the most promising materials for optoelectronic devices, and the realization of high temperature ferromagnetism in AlN-based materials makes it attractive for multifunctional devices. It has been reported that Cr-, Co-, and Mn-doped AlN are ferromagnetic at temperatures above 300K [6–12]. So far, Cr-doped AlN fi lms have been grown on Si, sapphire, and SiC substrates by magnetron sputtering [6, 7], MBE [8, 9, 12], or ion implantation [10]. Notably, the solubility of Cr in AlN is much higher than that of Mn, especially in fi lms grown at low temperatures [8]. The Cr concentration in Cr-doped AlN has been reported as high as 30% [6], which is much higher than that of Mn in the well-studied GaMnAs [13], and low substrate temperatures could favour the incorporation of Cr into AlN[8]. The high solubility of transition metals in semiconductor matrices could facilitate modulating the magnetic properties of these materials. However, there has been no systematic study of the structure and magnetic properties of Cr-doped AlN thin fi lms with varying Cr concentrations over a wide range. In this paper, we report epitaxial growth of Cr-doped AlN thin fi lms on sapphire substrate at low temperature by reactive magnetron sputtering, and the evolution of structure and magnetic properties with Cr concentration changing over a wide range from 0 to 20%.