Papers by Keyword: phonon modes

Abstract: Ba [Zn_1/3(Nb_1-xTa_x)_2/3]O₃ (BZNT, x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) solid solutions were synthesized by conventional solid-state sintering technique. Fourier transform far-infrared reflection spectroscopy (FTIR) and X-ray diffraction (XRD) were employed to evaluate the correlation between crystal structures and vibration modes of these solid solutions as a function of Nb⁵⁺ ions replaced by Ta⁵⁺ ions. Spectroscopic and structural data show sensitivity to the sample structural evolution with Ta⁵⁺ concentration. XRD result shows that the peaks shift to lower angle, i.e. higher d-spacing, with the increase in Ta⁵⁺content, and the lattice structures have changed from cubic structure to hexagonal structure gradually with the increase in the unit cell volume due to the substitution of Ta⁵⁺ to Nb⁵⁺. The phase transition is also verified by the FTIR spectra. The phonon modes of the vibration spectra were assigned, the position and width were determined, and the correlation of phonon vibrations with the crystal structures for the different atoms substituted in the-site was found.

Abstract: Fourier transform infrared (FTIR) spectroscopy has been utilized to measure long-wavelength optical lattice vibrations of high-quality quaternary AlxlnyGa1-x-yN thin films at room temperature. The AlxlnyGa1-x-yN films were grown on c-plane (0001) sapphire substrates with AlN as buffer layers using plasma assisted molecular beam epitaxy (PA-MBE) technique with indium (In) mole fraction y = 0.0 to 0.10 and constant aluminium (Al) mole fraction x = 0.06. The experimental results indicated that the AlxlnyGa1-x-yN alloys had two-mode behavior, for the A1 (LO) and E1 (TO) modes.

Abstract: 500nm AlGaN thick layer with AlGaN/GaN MQW interlayer was grown on sapphire substrate for UV detector and resonant tunneling diodes by MOCVD equipment. We were strongly interesting in the stress information of QW. There are a big mismatch of lattice between AlN and GaN.The growth of thick and high quality AlGaN is difficult task. AlGaN/GaN MQW layers were designed to relax the big mismatch stress. Many researcher focused on the stress relax mechanism for the growth of AlGaN alloy. The stress in QW can change the band gap structure and carrier contents of polarize induced charge. Raman spectra were a useful tool to observe the stress of semiconductor materials without damaging the sample. Using 514nm green laser, we only obtained the phonon modes of GaN. So applying 325nm Ar ion laser, we can observed the phonon modes spectra of both AlGaN and GaN layers. According to resonance conditions, the phonon modes of 789.74 cm-1 was origin from AlGaN alloy layer. The phonon modes of 740.89 cm-1 and 575.06 cm-1 were origin from GaN layer. Compared to other results, GaN layer was compress strain. We determined that AlGaN/GaN MQW interlayer relaxed strain stress from lattice mismatch, and phonon modes were clearly observed.

Abstract: Self-assembled nanowires have attracted much attention due to their potential applications in electronics and optoelectronics. A recent interest in Mn catalyzed GaAs nanowires are due to their potential use in spintronic devices at nanoscale. High densities of Au- and Mncatalyzed self-assembled GaAs nanowires (NWs) with diameter in the range of 20 to 200 nm and length of few microns were synthesized by molecular beam epitaxy (MBE) on different substrates at varied substrate temperatures. These nanowires were investigated by means of μ-Raman spectroscopy at room temperature. The Raman spectra from NWs show an energy downshift and a broadening of the LO and TO phonon lines that differ from those of epitaxial GaAs. We suggest that those downshift and broadening are due to the relaxation of the q=0 selection rule in the presence of structural defects in the nanowires. The results indicate that the use of Mn instead of Au as growth catalyst does not affect the structural quality of the nanowires drastically.