Pulsed-laser-deposited epitaxial aluminum nitride films on (111) Si for surface acoustic-wave applications

Epitaxial (001) aluminum nitride (AlN) thin films on (111) Si substrates are prepared using pulsed-laser deposition. The epitaxial structure of the as-prepared thin films is characterized by checking the X-ray-diffraction θ-2 θ scan and pole-figure, using scanning electron microscopy, infrared radiation (IR) spectroscopy and Raman spectroscopy. The surface acoustic-wave resonance at 345 MHz for a 1.5 μm thick AlN film on a (111) Si substrate is observed using an inter-digital electrode. Received: 18 September 2001 / Accepted: 29 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: liujm@nju.edu.cn     

Investigation of size-driven phase transition in bismuth titanate nanocrystals by Raman spectroscopy

Raman spectroscopy was used to investigate the lattice dynamics and structural transformations in bismuth titanate (Bi₄Ti₃O₁₂) nanocrystals prepared by a chemical coprecipitation technique. The crystal structure of the samples of different grain sizes was determined by X-ray-diffraction analysis. The evolution of the Raman spectrum with grain size was characterized by an intensity decrease, a broadening of the line width, a frequency shift, and the disappearance of the Raman mode. The results revealed the appearance of a size-driven phase transition from orthorhombic to tetragonal phases at a critical size of 44 nm. This result is quite consistent with the X-ray-diffraction measurement and differential thermal analysis. The origin was attributed to the grain-size effect and explained by the surface-energy mechanism. Received: 26 June 2002 / Accepted: 18 August 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Fax: +86-25/3595535, E-mail: msz@nju.edu.cn     

Inelastic X-ray scattering: new possibilities for Raman spectroscopy

Inelastic X-ray scattering with meV resolution has recently become available for studies of dynamical properties and elementary excitations in solids. Contrary to Raman spectroscopy at visible wavelengths, which in first order is limited to the Brillouin-zone center, the wave vectors in hard X-ray Raman scattering are very large, and the crystal-momentum transfer to elementary excitations, whose energies may range from a few meV up to several eV, can be tuned continuously across the whole Brillouin zone. This paper reviews new and unique possibilities offered by X-ray Raman spectroscopy for crystalline solids, such as phonon-dispersion measurements (GaN), the determination of phonon self-energies (isotopically mixed diamond), and resonance effects and studies of electronic excitations (copper oxides). Received: 19 October 2001 / Accepted: 12 December 2001 / Published online: 27 March 2002 / Published online: 27 March 2002 RID="*" ID="*"Present address: Agilent Technologies Deutschland GmbH, Herrenberger Str. 130, 71034 B?blingen, Germany     

Raman spectroscopy—A versatile tool for characterization of thin films and heterostructures of GaAs and AlxGa1−xAs

It is shown that Raman spectroscopy can provide useful information on characteristic properties of thin crystalline films of compound semiconductors. Crystal orientation, carrier concentration, scattering times of charge carriers, composition of mixed crystals and depth profiles can be studied in thin layers and heterostructures of GaAs and Al_xGa_1−xAs. The advantages and disadvantages of Raman scattering compared to conventional characterization methods are discussed.     

How does the substrate affect the Raman and excited state spectra of a carbon nanotube?

We study the optical properties of a single, semiconducting single-walled carbon nanotube (CNT) that is partially suspended across a trench and partially supported by a SiO₂-substrate. By tuning the laser excitation energy across the E ₃₃ excitonic resonance of the suspended CNT segment, the scattering intensities of the principal Raman transitions, the radial breathing mode (RBM), the D mode and the G mode show strong resonance enhancement of up to three orders of magnitude. In the supported part of the CNT, despite a loss of Raman scattering intensity of up to two orders of magnitude, we recover the E ₃₃ excitonic resonance suffering a substrate-induced red shift of 50 meV. The peak intensity ratio between G band and D band is highly sensitive to the presence of the substrate and varies by one order of magnitude, demonstrating the much higher defect density in the supported CNT segments. By comparing the E ₃₃ resonance spectra measured by Raman excitation spectroscopy and photoluminescence (PL) excitation spectroscopy in the suspended CNT segment, we observe that the peak energy in the PL excitation spectrum is red-shifted by 40 meV. This shift is associated with the energy difference between the localized exciton dominating the PL excitation spectrum and the free exciton giving rise to the Raman excitation spectrum. High-resolution Raman spectra reveal substrate-induced symmetry breaking, as evidenced by the appearance of additional peaks in the strongly broadened Raman G band. Laser-induced line shifts of RBM and G band measured on the suspended CNT segment are both linear as a function of the laser excitation power. Stokes/anti-Stokes measurements, however, reveal an increase of the G phonon population while the RBM phonon population is rather independent of the laser excitation power.     

Deposition of large-area, high-quality cubic boron nitride films by ECR-enhanced microwave-plasma CVD

Large-area, 1-μm-thick cubic boron nitride (cBN) films were deposited on (001) silicon substrates by electron-cyclotron-resonance-enhanced microwave-plasma chemical vapor deposition (ECR-MP CVD) in a mixture of He-Ar-N₂-BF₃-H₂ gases. With the assistance of fluorine chemistry in the gas phase and substrate reactions, the phase purity of the sp³-configuration was improved to over 85% at a reduced substrate bias voltage of -40 V. The grown films show clear Raman transversal optical (TO) and longitudinal optical (LO) phonon vibration modes, characteristic of cBN. Such Raman spectral characteristics are the first ever observed in cBN films prepared under ECR-MP CVD conditions. Received: 3 May 2002 / Accepted: 7 May 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +852-2788/7830, E-mail: apwjzh@cityu.edu.hk     

Preparation of Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> nanowires by calcining the precursor powders synthesized in a novel inverse microemulsion

Manganese oxide (hausmannite) nanowires were prepared by annealing precursor powders at a temperature of 800 °C for 3 h, which were produced in a novel inverse microemulsion (IμE) system. The microstructures of the as-prepared Mn₃O₄ nanowires were investigated by means of X-ray diffraction, transmission electron microscopy, and Raman spectra. It has been found that the Mn₃O₄ nanowires were relatively straight and their surfaces were smooth with a typical diameter of 75–150 nm. The formation mechanism of the Mn₃O₄ nanowires is discussed. Received: 30 May 2002 / Accepted: 7 October 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: wangqun@nju.edu.cn     

A simple wet-chemical synthesis and characterization of CuO nanorods

Using a simple wet-chemical route, we synthesized CuO nanorods with diameters of ca. 5–15 nm and lengths of up to 400 nm. The purity, crystallinity, morphology, structure features, and chemical composition of the as-prepared CuO nanorods were investigated by powder X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Received: 22 March 2002 / Accepted: 12 June 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: wangqun@nju.edu.cn     

Nonlinearity in Intensity versus Concentration Dependence for the Deep UV Resonance Raman Spectra of Toluene and Heptane

Abstract: The relation between Raman scattering, resonance Raman scattering, and absorption is reviewed to determine to what extent quantitative analysis can be applied in resonance Raman spectroscopy. In addition, it is demonstrated experimentally that normal Raman spectra can be dramatically inhibited by absorption and resonance Raman effects. Raman spectra of toluene and heptane mixtures—with progressively increasing concentrations of heptane—were measured using 229-nm laser excitation. The results show that the characteristic band intensities are not directly proportional to the relative concentrations of the compounds and deviate due to absorption resonance effects. An approximated mathematical model is developed to demonstrate that the intensities of the normal Raman scattering bands are suppressed. An inhibition coefficient K_i is introduced to describe the situation and determine the penetration depth. Most remarkably, it is shown that the intensity of the resonance Raman scattering bands can be constant even when the concentration ratios differ substantially in the sampled mixtures.     

Study of beam-fanning hysteresis in photo- refractive SBN:Ce: light-induced and primary scattering as functions of polar structure

We study light-induced scattering (beam-fanning) in the photorefractive crystal SBN:Ce as a function of the polar structure of the crystal. The spatial structure of the beam-fanning is measured at different externally applied electric fields, and an optical hysteresis is found in the scattering. It is shown that the scattering hysteresis results from a polarization hysteresis typical for ferroelectrics in the polar phase. New information about primary scattering in SBN is obtained, and a corresponding model of its origin is proposed. It is shown that the intensity and angular distribution of the primary scattering strongly depend on the polar structure of the crystal and can be affected by the subsequent action of an external field and coherent illumination. Received: 27 August 2002 / Revised version: 19 December 2002 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. E-mail: mirco.imlau@uni-osnabrueck.de     

Analysis of semiconductor surface phonons by Raman spectroscopy

Only recently Raman spectroscopy (RS) has advanced into the study of surface phonons from clean and adsorbate-covered semiconductor surfaces. RS allows the determination of eigenfrequencies as well as symmetry selection rules of surface phonons, by k-conservation limited to the Brillouin zone-center, and offers a significantly higher spectral resolution than standard surface science techniques such as high-resolution electron energy loss spectroscopy. Moreover, surface electronic states become accessible via electron–phonon coupling. In this article the fundamentals of Raman scattering from surface phonons are discussed and its potential illustrated by considering two examples, namely Sb-monolayer-terminated and clean InP(110) surfaces. Both are very well understood with respect to their atomic and electronic structure and thus may be regarded as model systems for heteroterminated and clean semiconductor surfaces. In both cases, localized surface phonons as well as surface resonances are detected by Raman spectroscopy. The experimental results are compared with surface modes predicted by theoretical calculations. On InP(110), due to the high spectral resolution of Raman spectroscopy, several surface modes predicted by theory can be experimentally verified. Surface electronic transitions are detected by changing the energy of the exciting laser light indicating resonances in the RS cross section. Received: 7 April 1999 / Accepted: 25 June 1999 / Published online: 16 September 1999     

Excimer-laser-induced permanent electrical conductivity in solid C60 films

After being irradiated in air by a XeCI (308 nm) excimer laser, the electrical conductivity of solid thin-film C₆₀ has been improved by more than six orders of magnitudes. The products resulting from laser irradiation of C₆₀ films have been investigated by Raman scattering and the onset of conductivity can be attributed to laser-induced oxygenation and disintegration of the fullerene. Irradiated by 40 ns laser pulses with different fluence, products with different microstructure were observed. At lower fluence, the Raman features of microcrystalline graphite and fullerene polymer were observed. At a fluence just below the ablation threshold (36 mJ/cm²), the fullerene molecules in the film were disintegrated completely and transformed to amorphous graphite.     

Stress effect and evidence of ferroelectric weakening in highly c-axis-oriented PbTiO3 thin films

3 were successfully grown on Pt-coated SrTiO₃ single-crystal substrates by metalorganic chemical vapor deposition (MOCVD) and were investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. The as-deposited thin films were found to be highly (001)-oriented with an average grain size of about 0.3 μm. Both a decrease of the tetragonality and a frequency downshift of the long-wavelength optical phonons were observed and attributed to the effect of compressive stress in the thin films. However, Raman scattering studies estimated a stress value of 2.6 GPa, which is much larger than the value of 0.75 GPa obtained from the XRD analyses. Raman spectroscopic studies also confirmed the grain-size-related disorder feature in the as-grown PbTiO₃ thin films. Structural investigations implied the weakening of ferroelectricity in the heteroepitaxial ferroelectric thin films. Received: 1 April 1997/Accepted: 14 July 1997     

Surface chemical modification of fullerene by mechanochemical treatment

In this study different encapsulating agents have been used for chemical modification of fullerenes. Fullerenes have reacted with tetrahydrofuran, sodium dodecyl sulfate, sodium dodecylbenzene sulfonate and ethylene vinyl acetate-ethylene vinyl versatate at room temperature under mechanical milling. The obtained powder has been dispersed in water by ultrasonication. The fullerene based colloids have been characterized by UV-vis, FTIR, Raman spectroscopy and atomic force microscopy. FTIR and Raman analysis have shown the presence of C₆₀ after surface functionalization.     

Spin wave dispersion of FeBO3 at small wavevectors

Brillouin scattering from thermally excited magnons and ferromagnetic resonance are used to determine the spin wave dispersion of the low-frequency spin wave branch in FeBO₃, a transparent weak ferromagnet. In addition to the dominant exchange and Zeeman contributions, the investigation takes into account magnetic dipole and magnetoelastic interactions. Due to the antisymmetric exchange enhancement the material exhibits a broad spin wave band and a large gap energy at small magnetic fields. Competing directional dependences of the dipole and the exchange energy produce a degeneracy of spin waves with a certain magnitude of the wavevector propagating in different directions. The gap energy is shown to be due to magnetoelastic coupling, whereas the contribution of the anisotropy in the easy plane is negligible atT=300 K.     

Dual-broadband rotational CARS modelling of nitrogen at pressures up to 9 MPa. II. Rotational Raman line widths

Rotational coherent anti-Stokes Raman spectroscopy (CARS) is a well-established spectroscopic technique for thermometry at pre-combustion temperatures and atmospheric pressure. However, at pressures of several MPa, a previous investigation revealed large discrepancies between experimental data and the theoretical model. A re-evaluation has been made of these data (at room temperature and in the range 1.5–9 MPa) with two improvements to the spectral code. The first is the inclusion of an inter-branch interference effect, which is described in detail in Paper I. The second is the use of experimental S₁-branch Raman line widths measured at 295 K, with a temperature dependence extracted from semi-classical calculations following the Robert–Bonamy formalism. It is shown that these two modifications significantly improve the theoretical model, since both the spectral fits and the accuracy of the evaluated temperatures are considerably improved. Received: 18 February 2002 / Revised version: 9 July 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +46-46/2224542, E-mail: mikael.afzelius@forbrf.lth.se     

Modification of the fused silica glass network associated with waveguide fabrication using femtosecond laser pulses

Atomic-scale structural changes have been observed in the glass network of fused silica after modification by tightly focused 800-nm, 130-fs laser pulses at fluences between 5 and 200 J cm^-2. Raman spectroscopy of the modified glass shows an increase in the 490 and 605-cm^-1 peaks, indicating an increase in the number of 4- and 3-membered ring structures in the silica network. These results provide evidence that densification of the glass occurs after exposure to fs pulses. Fluorescence spectroscopy of the modified glass shows a broad fluorescence band at 630 nm, indicating the formation of non-bridging oxygen hole centers (NBOHC) by fs pulses. Waveguides that support the fundamental mode at 633 nm have been fabricated inside fused silica by scanning the glass along the fs laser beam axis. The index changes are estimated to be approximately 0.07×10^-3. Received: 17 December 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-925/423-2463, E-mail: dmkrol@ucdavis.edu     

Raman spectroscopy of pentyl‐functionalized carbon nanotubes

We present Raman scattering on carbon nanotubes functionalized with pentyl groups. Studies of the intermediate frequency region and the C–H bond stretching signal along with the D mode show evidence of the addition reaction by Raman spectroscopy. From the resonance profiles of the radial breathing mode (RBM) we assign the chiral indices of the tubes and study the influence of the functionalization on the transition energies, shift and intensity of the RBM signal. The largest effect we observe is on the Raman intensity of the radial breathing mode. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

     

Preparation and characterization of a poled nanocrystal and polymer composite PbTiO3/PEKc film for electro-optic applications

Composite thin films of PbTiO₃ nanocrystals and high-transparency PEKc polymer for applications in electro-optical devices were prepared using the spin coating technique. The size of the PbTiO₃ nanocrystals was estimated to be 30–40 nm using a transmission electron microscope. The transmission technique, a simple method for measuring the electro-optic coefficients of poled composite polymer films was developed. The electro-optic coefficient γ₃₃ of poled PbTiO₃/PEKc composite polymer films was measured to be 18.34 pm  V^-1 at 633 nm under room temperature. The index at 633 nm and the dielectric constant at 100 kHz under room temperature were determined to be 1.65248 and 7.32, respectively. The figure of merit F₂=n⁷γ²/ε was estimated to be 1546, showing very good electro-optical properties. Received: 5 February 2002 / Accepted: 12 March 2002 / Published online: 19 July 2002 RID="*" ID="*"Corresponding author. Fax: +852/2788-7791, E-mail: eeytc@cityu.edu.hk