Hyungjin Bang

Concentration quenching of Eu-related luminescence in Eu-doped GaN

The dependence of Europium (Eu)-related luminescence intensity on the Eu concentration in Eu-doped GaN was studied. This luminescence is observed at 622nm and originates from the intra-4f transition of the Eu3+ ion. The intensity of the luminescence increased with increasing Eu concentration, up to about 2at.%, and then abruptly decreased. It was found that polycrystalline growth began to be induced at Eu concentrations of more than 2at.%. In addition, clear evidence for the formation of EuN compounds was obtained by x-ray diffraction and extended x-ray absorption fine structure analysis. The cause of the concentration quenching is likely to be related to the polycrystalline growth as well as EuN formation.

Magnetic Properties of Eu-Doped GaN Grown by Molecular Beam Epitaxy

Magnetic properties of molecular-beam epitaxy grown Eu-doped GaN are studied. At high temperatures, clear hysteresis and clear saturation indicating the existence of a ferromagnetic-like phase are observed in the magnetization versus magnetic curves, while at low temperatures gradually saturating nonlinear field dependence of the magnetization is also observed. The easy-axis for the ferromagnetization is perpendicular to the sample plane. A theoretical model is proposed to account for the experimental results, assuming that a molecular field is acting on Eu ions via the spin-polarized valence-band of GaN. The experimental results are accounted for by assuming the coexistence of Eu2+ ions and Eu3+ ions interacting through the Ruderman-Kittel-Kasuya-Yoshida-type interaction.

Defects in Eu- and Tb-doped GaN probed using a monoenergetic positron beam

We probed defects in Eu- and Tb-doped GaN films grown on sapphire substrates by gas-source molecular-beam epitaxy with a monoenergetic positron beam. In both Eu- and Tb-doped samples, we observed vacancy clusters consisting of two or more vacancies. These defects were introduced by replacing Ga with rare-earth elements, and resulting in distortion of the host matrix. We studied the correlation between luminescence originating from the intra-4f-transitions of Eu3+ and the crystal quality of the GaN film. In film doped at 2-at. % Eu, the mean open volume of the vacancies near the interface between the GaN film and the sapphire substrate was found to be larger than that in the subsurface region. The increase in the open volume of the defects correlated with the lowering coordination symmetry of Eu3+ and the increase in the transition rate of its 4f-electrons.

Valence transition of Eu ions in GaN near the surface

The valence states of Eu ions doped in GaN were investigated by the core-level X-ray and resonant photoemission spectroscopy. Although the photoluminescence spectra show the emissions from only trivalent Eu ions, the existence of both divalent and trivalent Eu ions is suggested by the Eu 3d core-level spectra. Taking into account the results of the 4d→4f resonant photoemission measurements, it can be concluded that valence transition from Eu 3+ to Eu 2+ occurs near the surface of GaN.

Comparative Study on the Optical Properties of Eu:GaN with Tb:GaN

Sharp luminescence originating from intra-atomic 4f-4f transition from both Eu and Tb doped GaN was observed at 622 nm which can be assigned to 5 D 0 - 7 F 2 transition of Eu 3+ and at 545 nm which can be assigned to 5 D 4 - 7 F 5 transitions of Tb 3+ . However, the luminescence intensity of Eu 3+ is two orders of magnitude stronger than that of Tb 3+ though the content of the rare earth elements is almost the same. The cause of the difference in the luminescence properties was studied based on the defect-related energy transfer model, and the role of the defects observed by Fourier transform infrared (FTIR) spectra was discussed.

Incorporation site of Tb in GaN studied by Rutherford-backscattering ion channelling measurements and x-ray absorption fine-structure analysis

We report single-crystalline growth of Tb-doped GaN grown by gas source molecular beam epitaxy using NH3 as a nitrogen source, and the incorporation site of Tb in GaN. It was found that most of the Tb atoms are substitutionally incorporated into the Ga lattice site by Rutherford-backscattering ion channelling and extended x-ray absorption fine-structure analysis. Photoluminescence spectra, which show sharp peaks originating from 4f intra-atomic transitions of Tb3+ ions, with near-band-edge emission of GaN, were observed from the film whose Tb content was roughly estimated to be 2%, and their properties are discussed.

Structural and magnetic properties of Er doped GaN

In this paper, structural and magnetic properties of Er-doped GaN were studied. Er-doped GaN shows paramagnetic character, however, clear steps around zero fields were observed at all measured temperatures. These results indicates that the coexistence of ferromagnetic order.