[en] The influence of annealing on the AgGaS₂ films grown by pulsed laser deposition has been investigated. The X-ray diffraction results show the AgGaS₂ films were found with preferential orientation (1 1 2) normal to the surface and silver droplets were diminished after the post-annealing. Photoluminescence (PL) measurements reveled the exciton energy is slightly red shifted that is possibly due to the thermal strain effect. The binding energy of the shallow donors is ∼28 meV determined from temperature dependent PL spectra. In addition, the A-exciton and the B/C-exciton could be observed in the transmittance spectra at room temperature

[en] We report Raman and x-ray diffraction studies of polycrystalline Ba_xSr_1-xTiO₃ (BST) prepared by the sol-gel technique in the range of x=0 to 1.0. Other than a tetragonal to cubic phase transition at composition x≅0.75 of the BST powders detected by x-ray diffraction and Raman spectroscopy, a new structural ordering transition around x≅0.4-0.5 is observed due to the atomic arrangement. By taking the anharmonic coupling between the three A₁(TO) modes into account, the observed repulsion of the giant LO-TO splitting is a result of decreasing cell dimension and strengthening the electron-phonon coupling. The results help us to explain the weakening of the ferroelectricity in the BST tetragonal phase as x decreases. The anomalous inconsistency in our experimental results and theoretical prediction are attributed to the lack of detailed information of dynamical effective charges in the Ba_xSr_1-xTiO₃ compounds

[en] Investigation of the structural and vibrational properties of the Pb_xCa_1-xTiO₃ (PCT) system exhibits two phase transitions in the range of x=0-1 by using x-ray powder diffraction and Raman spectroscopy. The first transition occurring at x∼0.65 corresponds to the tetragonal-to-cubic phase and the second one occurs near x=0.35 is attributed to the orthorhombic-to-cubic phase. The absence of the intermediate tetragonal phase between orthorhombic and cubic phases (0< x<0.35) may be mostly attributed to the very restricted region in the PCT system. In addition, a decreasing giant LO-TO splitting behavior similar to that exhibited in Pb_xSr_1-xTiO₃ system for lower Pb concentration was also observed, which has been attributed to the highly Pb-O covalent bonding to reduce the long-range Coulomb interaction. Comparison of the change of line shapes of A₁(1TO) mode among three PbTiO₃-based perovskites indicates that the anharmonicity will become more conspicuous due to the larger high-order potential terms of Ba²⁺ and Ca²⁺ substitution than Sr²⁺ substitution for Pb²⁺

[en] The InGaN/GaN light emitting diodes (LEDs) with a nano-roughened top p-GaN surface fabricated by using Ni nano-masks and laser etching methods were demonstrated and analyzed. The experiment results observed the maximum light output power of the InGaN/GaN LED with a nano-roughened top p-GaN surface etched with the laser energy of 300 mJ/cm² was 1.55 times higher than that of a conventional LED, and the wall-plug efficiency was enhanced 68% at 20 mA. The series resistance of InGaN/GaN LED was reduced by 32% by the increase in the contact area of the nano-roughened surface