Xionghui Zeng

Dislocation cross-slip in GaN single crystals under nanoindentation

The dislocation multiplication and movement mechanism in GaN single crystals has been studied using nanoindentation and cathodoluminescence. Dislocation loops can multiply and move from plane to plane by cross-slip, thus producing a wide plastic deformation in GaN during indentation. This mechanism is further supported by the remarkable movement of indentation induced dislocations during annealing. Furthermore, the so-called pop-in events, in which the indenter suddenly enters deeper into the material without the application of any additional force, can be better understood by considering the cross-slip mechanism.

Effects of annealing on the color, absorption spectra, and light yield of Ce : YAlO3 single crystal grown by the temperature gradient technique

YAlO3 single crystal doped with Ce3+ at concentration 1% was grown by the temperature gradient technique. The as-grown crystal was pink. After H2 annealing or air annealing at 1400 °C for 20 h, the crystal was turned into colorless. We concluded there were two kinds of color centers in the as-grown crystal. One is F+ center attributed to absorption band peaking at about 530 nm, the other is O− center attributed to absorption band peaking at about 390 nm. This color centers model can be applied in explaining the experiment phenomena including the color changes, the absorption spectra changes, and the light yield changes of Ce:YAP crystals before and after annealing.

Growth Mechanism of Large-Size Rubrene Single Crystals Grown by a Solution Technique

Large-size and high-quality rubrene single crystals have been grown by a solution technique. The biggest crystal has 4 mm dimension. Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy both indicate that the crystals obtained are pure rubrene and did not incorporate the solvent at the detection level. The morphology of surface and transect of the rubrene single crystals was observed by optical microscope, atomic force microscope (AFM), and scanning electron microscope (SEM). Three-dimensional nucleation was happened on the surface of single crystals. The array of small and large straight steps was observed on the surface of the rubrene single crystals. Layer-like structures were also observed on the cross-section of single crystals. All results indicate that each large step is most probably formed from the piling of small steps and the growth mechanism of rubrene crystals grown from solution is probably a three-dimensional nucleation–layer-by-layer periods growth.

Cathodoluminescence properties of Pr, Tm co-implanted GaN thin films

In this work, Pr and Tm co-doped GaN thin films were prepared by ion implantation. After a thermal annealing treatment for the lattice recovery and ions activation, temperature-dependent cathodoluminescence (CL) spectroscopy was applied to investigate the luminescent properties. At room temperature, the intensity ratio of blue emission to infrared emission of Tm ions was decreased as the implantation dose of Pr ions was increased, which is due to the energy transfer between Pr and Tm ions. In addition, the 467 nm blue emission of Tm and the 528 nm green emission of Pr were observed only at low temperature. A model was proposed to illuminate the light emission mechanism of Pr and Tm co-implanted GaN thin films.

Identifying dislocations and stacking faults in GaN films by scanning transmission electron microscopy

The application of annular bright field (ABF) and medium-angle annular dark field (MAADF) scanning transmission electron microscopy (STEM) imaging to crystalline defect analysis has been extended to dislocations and stacking faults (SFs). Dislocations and SFs have been imaged under zone-axis and two-beam diffraction conditions. Comparing to conventional two-beam diffraction contrast images, the ABF and MAADF images of dislocations and SFs not only are complementary and symmetrical with their peaks at dislocation core and SFs plane, but also show similar extinction phenomenon. It is demonstrated that conventional TEM rules for diffraction contrast, i.e. g b and g R invisibility criteria remain applicable. The contrast mechanism and extinction of dislocation and SFs in ABF and MAADF STEM are illuminated by zero-order Laue zone Kikuchi diffraction.