Origin of the Raman mode at 379 cm−1 observed in ZnO thin films grown on sapphire

In this work, we present a detailed Raman scattering study to clarify the origin of the mode at 379 cm⁻¹ which is observed in Raman spectra of the ZnO films grown on c‐sapphire substrates and generally attributed to the A₁‐transverse optical (A₁‐TO) mode of ZnO. The studied ZnO films were deposited by metal‐organic chemical vapor deposition on c‐sapphire and (0001) ZnO substrates. In the z(−,−)z̄ backscattering configuration, the A₁‐TO mode is forbidden, while the 379 cm⁻¹ peak is still observed in the as‐deposited film grown on sapphire substrate. However, this mode is not observed in Raman spectra of the as deposited film grown on ZnO substrate. We suggest that the peak at 379 cm⁻¹ is the E_1g mode of the sapphire substrate which is allowed in z(−,−)z̄ backscattering configuration. The effects of annealing, the substrate and the collection cross‐section on Raman active modes were analyzed. Copyright © 2011 John Wiley & Sons, Ltd.     

Effect of ZnO sacrificial layer thickness on the structure and optical properties of GaN nanoparticles prepared by RF magnetron sputtering

GaN nanoparticles were prepared on sapphire (0001) substrates with ZnO sacrificial layers by self assembly of Ga₂O₃ films in their reaction with NH₃. ZnO sacrificial layers with different thicknesses and Ga₂O₃ films were deposited on sapphire substrates in turn by a radio frequency (RF) magnetron sputtering system. Nitridation of the Ga₂O₃ films was then carried out in a quartz tube furnace. The effect of ZnO sacrificial layer thickness on the structure and optical properties of nanoparticles prepared by RF magnetron sputtering were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and photoluminescence (PL). GaN nanoparticles with ZnO sacrificial layers of different thicknesses possess hexagonal wurtzite crystal structure and have a preferred orientation with c axis perpendicular to the sapphire substrates. XRD, SEM, and AFM results reveal that the better-crystallinity, uniform, and well-dispersed GaN nanoparticles (～30 nm) without agglomeration were obtained with a ZnO sacrificial layer 300-nm thick. The PL result reveals that the optical properties of the GaN nanoparticles are improved with a ZnO sacrificial layer 300-nm thick. Therefore, we suggest that a ZnO sacrificial layer 300-nm thick is the most suitable condition for obtaining better-quality GaN nanoparticles with good luminescence performance. Moreover, the mechanism of the formation of GaN nanoparticles with ZnO sacrificial layers is also discussed.     

Light scattering study of dispersion and attenuation of hypersound in adamantane

The Brillouin scattering techniques have been used to measure the velocity dispersion of hypersonic acoustic waves in the “high temperature” disordered cubic phase of adamantane. Shear waves, characteristic of the C₄₄ elastic constant, show no significant dispersion. Longitudinal waves propagating in the (001) plane show strong velocity dispersion. The measures have been performed at the same temperature T = 295.7 K. Using a classical single relaxation time model for the dispersion as a function of frequency at temperature T, the L-mode data have been correctly fitted.The importance of the dispersion $\text{(C}{}_{\mathrm{\infty }}\text{? C}{}_{\mathrm{o}}\text{)}\text{C}{}_0$ for the elastic constants is 20% for C₁₁, 51% for C₁₂ #1% for C₄₄ and ?2.8% for $\text{(C}{}_{11}\text{? C}{}_{12}\text{)}\text{2}$. The fitted relaxation time is τ ? 9 × 10^?11 sec.     

The role of a ZnO buffer layer in the growth of ZnO thin film on Al2O3 substrate

A ZnO buffer layer and ZnO thin film have been deposited by the pulsed laser deposition technique at the temperatures of 200 C and 400 C, respectively. Structural, electrical and optical properties of ZnO thin films grown on sapphire (Al₂O₃) substrate with 1, 5, and 9 nm thick ZnO buffer layers were investigated. A minute shift of the (101) peak was observed which indicates that the lattice parameter was changed by varying the thickness of the buffer layer. High resolution transmission electron microscopy (TEM) was used to investigate the thickness of the ZnO buffer layer and the interface involving a thin ZnO buffer between the film and substrate. Selected area electron diffraction (SAED) patterns show high quality hexagonal ZnO thin film with 30 in-plane rotation with respect to the sapphire substrate. The use of the buffer can reduce the lattice mismatch between the ZnO thin film and sapphire substrate; therefore, the lattice constant of ZnO thin film grown on sapphire substrate became similar to that of bulk ZnO with increasing thickness of the buffer layer.     

Acousto-optic light deflection in an Al2O3 optical waveguide structure

The total deflection of an optical guided wave by a surface acoustic wave (SAW) in a silicon-based SiO₂/AL₂O₃/SiO₂ optical waveguide structure is reported. The SAW is generated by an interdigital transducer with piezoelectric ZnO deposited on top of the optical waveguide structure.     

Influence of post-deposition annealing on the structural and optical properties of ZnO thin films prepared by sol–gel and spin-coating method

Thin films of zinc oxide have been deposited onto (0001) sapphire substrate by sol–gel and spin-coating methods. The XRD pattern showed that the crystallinity of the annealed ZnO films had improved in comparison with that of the as-grown films. Photoluminescence spectra revealed a two-line structure, which is identified in terms of UV emission and defect-related emission. The emission intensity was found to be greatly dependent on heat treatment. Host phonons of ZnO and a shift of the _E2$E_2$ (high) peak from its position have been observed from Raman spectra. The surface morphologies of the film had been improved after annealing was observed from AFM images.     

Melt growth and properties of B6P crystals

The compound semiconductor boron subphosphide has, at low temperatures, an ideal stoichiometry of 6 boron atoms to 1 phosphorus atom. This unit cell contains Bi₁₂P₂ and has a density of 2.60 $\text{g}\text{cm}{}^3$. It has been melted under inert gas pressure at 2120°C with very little loss of phosphorus. Data are presented on the diffusion coefficient of phosphorus, the refractive index, the band gap, and the optical absorption coefficient. Some optical data on B₆As are also given.     

Pulsed-laser deposition of a Nd:KGd(WO4)2 waveguide in Ar and O2 environments

Thin-film Nd-doped potassium gadolinium tungstate (Nd:KGW or KGd(WO₄)₂) waveguides are deposited on (1102)sapphire or (100)YAG substrates by KrF laser ablation of potassium-rich ceramic targets in Ar and O₂. The dependence of the stoichiometry, crystallinity and waveguide properties of the films on the environmental gas pressure and substrate temperature is studied. Highly textured crystalline (110) KGW films are grown. An optical waveguide loss as low as 3 dB/cm is obtained for the films grown in Ar. The as-grown films are optically active. Upon annealing at 900 °C in air, the crystallinity and the properties of the emission spectra are dramatically improved. PACS 81.15.Fg; 42.70.Hj; 78.20.Ek     

Deposition of nickel from Ni(CO)4 on palladium and iron surfaces studied by X-ray photoelectron spectroscopy

The interaction of nickel carbonyl, Ni(CO)₄, with evaporated palladium and iron surfaces has been studied at 90 and 290 K by X-ray photoelectron spectroscopy. The carbonyl is weakly adsorbed in molecular form at 90 K on the metals giving a $\text{Ni 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ peak at 854.6 eV, a C 1s at 287.2 eV and an O 1s at 533.8 eV. Some fraction of the carbonyl decomposes even at 90 K on iron to give deposited nickel atoms. In the interaction with palladium at 290 K, deposited nickel atoms $\text{(Ni 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{= 852.9 eV)}$ and chemisorbed CO are observed. A satellite feature of the $\text{Ni 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ peak varies depending on the quantity of the deposited nickel atoms; the main peak-satellite separation increases with increase in the quantity. The same variation is observed for evaporated nickel-palladium alloys. This can be ascribed to the difference in the electronic states of the nickel atoms. The difference is reflected in the reactivity of the atoms with O₂. With iron the deposited nickel atoms show an increase in binding energy of 0.4 eV in the $\text{Ni 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ Peak and no satellite when the number of nickel atoms is small. The oxidation of the surface is also studied.     

Low loss optical channel waveguides for the infrared range using niobium based hybrid sol-gel material

In this work, we report the fabrication of single-mode Nb₂O₅ based hybrid sol-gel channel waveguides. Nb₂O₅ based hybrid sol-gel material has been deposited by spin-coating on silicon substrate and channel waveguides have been fabricated by a UV direct laser writing process. Optical guided modes have been observed to confirm single-mode conditions and optical propagation loss measurements have been performed using the cut-back technique. Optical propagation losses were measured to be 0.8 dB/cm and 2.4 dB/cm at 1.31 μm and 1.55 μm respectively. These experimental results demonstrate low loss optical waveguiding within the infrared range and are very promising in view of material choice for the development of integrated optical devices for telecommunication.     

Low wavenumber wall pressure measurements using a rectangular membrane as a spatial filter

The response of a rectangular membrane to a convecting random pressure field is interpreted to reveal the inherent wavenumber filtering characteristics of the device. After experimental determination of its resonant response characteristics, one such membrane is used to measure the low wavenumber components of the wall pressure fluctuations beneath a plane turbulent boundary layer. The measurements were made at wavenumbers far below the convective region (k₁ = ω/U_c) but above the acoustic region (k₁ ? ω/c_o). Possible contamination by acoustic and convective ridge effects is considered. The low wavenumber measurements are also compared with values of the wavenumber-frequency pressure spectral density $\text{Φ}{}_{\mathrm{p}}\text{(}\text{k}\text{,ω)}$ obtained by Fourier transforming cross-spectral density data obtained by Blake in the same wind tunnel. From this comparison it is seen that this transformation of Blake's data (based on a Corcos model) grossly overestimates the magnitude of $\text{Φ}{}_{\mathrm{p}}\text{(}\text{k}\text{,ω)}$ in the low-wavenumber region. The measured values of $\text{Φ}{}_{\mathrm{p}}\text{(}\text{k}\text{, c}{}_{\mathrm{o}}\text{)}$ are about 36 dB down from convective ridge levels at the same frequency. The data are also compared with earlier results obtained by Jameson. In a similar frequency range the current data levels are approximately 10 dB higher than those of Jameson.     

Acousto-elastic measurement of stress and stress intensity factors around crack tips

Acousto-elasticity predicts that the phase velocity of sound waves in a material will be changed slightly by stress. For a slightly orthotropic plate in a state of plane stress, the shear stress σ_xy can be calculated from $\text{σ}{}_{\mathrm{xy}}\text{=}\text{(B}\text{sin}\text{2 φ)}\text{2m}$ once measurements of the acoustic birefringence B and the angle φ have been made. The birefringence is the difference in phase velocity between SH waves polarized along the ‘fast’ and ‘slow’ acoustic axes, φ is the angle between the acoustic axes in stressed and unstressed state, and m is an acousto-elastic constant for the material.For symmetrical, two-dimensional crack-opening problems, σ_xy can be expressed as a series expansion of stress functions, each of which satisfies the equilibrium equation. The coefficients in the expansion allow the appropriate boundary conditions to be satisfied. The stress intensity factor K₁ is the coefficient of the leading term in the series.Values of σ_xy and K₁ for an ASTM standard test specimen made of 2024 aluminium were acousto-elastically determined. These were compared with those obtained from a similar photoelastic specimen. Good agreement was obtained for both σ_xy and K₁.     

Low energy electron spectroscopy of Pt (100) and Pt (100)/CO

Fine structure in the n_vi, _VIIVV spectrum of clean Pt (100) has been observed, and interpreted as “band like” in origin rather than quasi-atomic. Differences in the dependence of the Auger yield on primary beam energy are observed between the N_VI, _VIIVV and O_IIIVV peaks, and are associated with anomalies in the dependence of the inner shell ionization crossection of the 4f level. Low energy electron loss spectra on the clean surface have been investigated at primary energies in the range 71–774 eV and at angles of incidence of the beam 0–60°. The results are related to high energy loss and optical data, and assignments are given for inter-band and plasmon losses. With approximately $\text{3}\text{4}$ of a monolayer of CO on the surface there is a prominent additional loss at around 13.5 eV, which is interpreted as a one electron transition from a σ state below the d band to available states several electron volts above the Fermi level.     

Optical properties and electronic structure of copper atoms adsorbed on a platinum electrode

The optical properties of Cu atoms deposited in the underpotential region from electrolytic solution onto a polycrystalline Pt electrode have been studied by differential reflectance spectroscopy. Spectra of the normalized reflectance change ($\text{ΔR}\text{R}$) have been obtained in the photon energy range between 1.5 and 5.5 eV for p- and s-polarized light as a function of coverage θ and angle of incidence ?₁. Evaluation of the adsorbate optical constants by a simple 3-phasemodel reveals three absorption bands in the range studied, one of them being strongly angular dependent. From an assignment of these transitions the positions of the 3d-, 4s- and 4p-derived levels in the energy level diagram of the Cu adatom is determined. As expected these levels are considerably shifted and broadened in comparison to their unperturbed atomic counterparts. Optical evidence for the occurrence of a structural change in the adsorbate layer at about θ = 0.6 is also presented.     

Study of Ag/Si(111) submonolayer interface: I. Electronic structure by angle-resolved UPS

Angle-resolved ultraviolet photoelectron spectra have been measured for well defined Ag/Si(111) submonolayer interfaces of (1) $\text{Si(111)(}\text{3}\text{×}\text{3}\text{)R30°-Ag}$, (2) “Si(111)(6 × 1)-Ag”, and (3) Ag/Si(111) as deposited at room temperature. Non-dispersive and very narrow (FWHM ～ 0.4–0.5 eV) Ag 4d derived peaks are found at 5.6 and 6.5 eV below the Fermi level for surface (1) and at 5.3 and 6.0 eV for surface (2). Dispersions of sp “binding” states in the energy range between E_F and Ag 4d states have been precisely determined for surface (1). Electronic structures similar to those of the Ag(111) surface, including the surface state near E_F, have been observed for surface (3).     

Effects of Mn‐doping on surface acoustic wave properties of ZnO films

Surface acoustic wave (SAW) filters based on Mn‐doped ZnO films have been fabricated and effects of Mn‐doping on SAW properties are investigated. It is found that the electromechanical coupling coefficient (K²) of Zn_0.913Mn_0.087O films is 0.73 ± 0.02%, which is 73.8% larger than that of undoped ZnO films (0.42 ± 0.02%). Zn_0.913Mn_0.087O film filters also exhibit a lower absolute value of insertion loss (|IL|) of 16.1 dB and larger bandwidth (BW) of 5.9 MHz compared with that of undoped ZnO film filter. However, Zn_0.952Mn_0.048O film filters exhibit a smaller K² of 0.34 ± 0.02%, larger |IL| of 26.9 dB and smaller BW of 3.5 MHz. It is suggested that the SAW properties can be improved by appropriate Mn‐doping and Mn–ZnO/Si multilayer structure with large d₃₃ is promising for wide‐band and low‐loss SAW applications. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Two-photon induced stimulated Raman scattering in cesium vapor

Two-photon induced stimulated Raman scattering has been observed for the first time. The observations have been made in cesium vapor. In the process cesium atoms are excited from the 6S_{$\text{3}\text{2}$} ground state to the 6P_{$\text{3}\text{2}$} excited state through the absorption of two laser photons at 1.064 μm and the emission of one scattered photon at 1.416 μm. The spectrum of the scattered light reveals the optical Stark shifts of the initial and final levels.     

A combined LEED,AES and XPS study of the ZnO {0001} polar surfaces: I. LEED,AES and XPS of contaminated surfaces

The {0001} polar surfaces of ZnO single crystals have first been examined after a chemical treatment involving HCl and H₃PO₄ and a 24 hr bakeout at 250 °C. The impurities detected on the (000$\text{1}$)-O surface with AES were carbon, chlorine, phosphorus and to a lesser extent sulphur. On the (0001)-Zn surface, carbon, chlorine and sulphur were the dominant impurities, while the phosphorus signal was less important. These results were confirmed by XPS measurements on frehsly etched surfaces. The AES spectra were recorded as distribution curves N(E). Averaging, curve-fitting and related numerical techniques were used to obtain high resolution spectra, enabling the identification of the phosphorus L₁-transitions. The etched surfaces were cleaned progressively using argon ion bombardment and ohmic heating. It has been consistently observed that the clean surfaces exhibit primitive (1 × 1) structures. Superstructures such as $\text{(}\text{3}\text{×}\text{3}\text{)}$ on the (000$\text{1}$)-O surface, and $\text{(4}\text{3}\text{× 4}\text{3}\text{)}$ and (3 × 3) on the (0001)-Zn surface, were repeatedly observed at discrete spots of contaminated surfaces. A clear correlation with impurities as observed by AES however could not be found. Facetting was observed after prolonged heating.