Xugao Cui

Structural and magnetic properties in Mn-doped GaN grown by metal organic chemical vapor deposition

Mn-doped GaN epitaxial films (Ga1−xMnxN) were grown on sapphire (0001) by metal organic chemical vapor deposition. Mn concentration was determined by energy dispersive spectrometry. For Ga1−xMnxN with x up to 0.027, no secondary phases except for GaN were detected by high resolution x-ray diffractometer. Raman scattering spectra show that the longitudinal optical phonon mode A1(LO) of Ga1−xMnxN shifts toward lower frequency with increasing Mn concentration due to substitutional Mn incorporation. Electron spin resonance (ESR) measurements were performed and highly anisotropic sixfold hyperfine line indicates that the ionized Mn2+ substitutes for Ga3+ ions. However, magnetometry reveals that all homogenous Ga1−xMnxN show paramagneticlike behaviors. From Brillouin function fit and ESR spectra, it is concluded that Mn ions are present as isolated paramagnetic centers.

Microstructural properties of over-doped GaN-based diluted magnetic semiconductors grown by MOCVD

We have grown transition metal (Fe, Mn) doped GaN thin films on c-oriented sapphire by metal-organic chemical vapor deposition. By varying the flow of the metal precursor, a series of samples with different ion concentrations are synthesized. Microstructural properties are characterized by using a high-resolution transmission electron microscope. For Fe over-doped GaN samples, hexagonal Fe3N clusters are observed with Fe3N(0002) parallel to GaN (0002) while for Mn over-doped GaN, hexagonal Mn6N2.58 phases are observed with Mn6N2.58(0002) parallel to GaN(0002). In addition, with higher concentration ions doping into the lattice matrix, the partial lattice orientation is distorted, leading to the tilt of GaN(0002) planes. The magnetization of the Fe over-doped GaN sample is increased, which is ascribed to the participation of ferromagnetic iron and Fe3N. The Mn over-doped sample displays very weak ferromagnetic behavior, which probably originates from the Mn6N2.58.

Superparamagnetic behavior in Fe ultrathin films on GaN(0001)

In this paper, growth, structural and magnetic properties of ultrathin Fe grown on GaN(OOOl) by molecular beam epitaxy. The films and their surfaces were monitored by in-situ reflection high energy electron diffraction (RHEED) and a crystal thickness monitor. The magnetic properties of the samples were determined by a superconducting quantum interference device (SQUID) magnetometer. Superparamagnetism (SPM) of the ultrathin Fe can be activated at the ambient temperature.the hysteresis loop of an as-deposited 5 ML Fe(llO) film on GaN(0001) taken at RT and reveals that the loop on one hand has an unsaturated magnetization and on the other hand possesses tiny but noticeable Mr and Hc. These two characteristics as a whole imply a coexistence of SPM and weak FM in the ultrathin film. In order to gain further insight into this mixed magnetic state, temperature dependence of the magnetization M(T) in the form of field cooling (FC) and zero field cooling (ZFC) curves of the 5 ML sample was measured by SQUID in a T range between 5 and 300 K.