Electromigration in Al thin films induced by surface acoustic waves: application to imaging

The propagation of a high amplitude surface acoustic wave in an Al thin film induces a large-scale electromigration phenomenon resulting in a permanent etching of the acoustic field in the film. The etched patterns depend on the time of propagation and on the acoustic characteristics. Preliminary observations of a few grooved structures in Al films have been performed by different techniques. A first explanation of this phenomenon based on dynamical Grinfeld instabilities is proposed. By providing permanent pictures of acoustic fields emitted by transducers, this effect could be used to perform imaging of surface acoustic wave propagation.     

Propagation of surface acoustic waves inn-type GaAs films

The effect of nonparabolicity on the amplification of surface acoustic waves in n-type GaAs films is investigated quantum mechanically in the GHz frequency region. Numerical results show that the amplification coefficient for the nonparabolic band structure is enhanced due to the nonlinear nature of the energy band in semiconductors. Moreover, the amplification coefficients in semiconductors depend on the temperature, the electronic screening effect, the frequency of sound waves, the applied electric field, and the thickness of the semiconductor film.Partially supported by National Science Council of the Republic of China.     

Laser deposition of gallium nitride films

Insulating c-oriented hexagonal epitaxial gallium nitride (GaN) films have been obtained by means of pulsed laser sputtering of a gallium target in nonactivated nitrogen atmosphere. The GaN films were deposited onto (0001)-oriented sapphire substrates either directly or above a ZnO buffer layer. The laser-deposited films exhibit edge photoluminescence at 370 nm.     

Effect of the nitrogen ion energy on the MBE growth of thin gallium nitride films

The influence of the energy of bombarding nitrogen ions on the growth of thin gallium nitride (GaN) films under molecular beam epitaxy (MBE) conditions has been simulated using the method of balance kinetic equations. The dependence of the GaN film growth rate on the ion energy is determined and changes in the structure of films grown at different ion energies are explained. Theoretical estimates satisfactorily agree with the available experimental data.     

Optical properties of nanocrystalline gallium nitride films

Nanocrystalline GaN films with different crystallite sizes were deposited onto quartz and NaCl substrates by magnetron sputtering of a GaN target in argon plasma. All the films showed predominant hexagonal phase. The band gap values were always found to be higher than that of the bulk. This blue shift in band gap could be attributed to the quantum confinement effect. The optical absorption in these films could be explained by the combined effects of phonon and inhomogeneity broadening along with optical loss due to light scattering at the nanocrystallites. Band edge luminescence is absent in these GaN nanocrystalline films. The line shapes of the photoluminescence (PL) spectra are asymmetric and broad. The film deposited at lower substrate temperature showed broader PL peak. It may be observed that no significant energy shift in the peak positions was observed with reduction in crystallite size but the intensity of the peak decreased for films with the reduction in crystallite size. Below band gap emission observed in this study may also originate due to the presence of polarization-induced electric field present in wurtzite GaN deposited here.     

Propagation of nonlinear acoustic waves in gallium

We study nonlinear phenomena which occur during strong sound signal propagation in superpure Ga and are due to the perturbation of the electron subsystem in the metal. Although the observations are in qualitative agreement with theoretical concepts in a number of aspects, there are effects which still lack adequate theoretical interpretation, in particular, an oscillatory dependence of the harmonic amplitudes on magnetic field which is linear with respect toH ^1/2, and appreciable nonlinear distortions of the sound pulse envelope accompanied by the acoustic noise generation.     

Impedance studies of free-standing,flexible thin films of PVDF filled with gallium nitride nanoparticles

Flexible, free-standing composite films of poly(vinylidene fluoride) (PVDF) with Gallium Nitride (GaN) as fillers, in varying concentrations, were synthesized by sol–gel method. Modulations in the microstructural, morphological and dielectric properties, due to the addition of fillers, were investigated. Modifications in the spherocrystal structure, their dimensions and their number density were observed. Microstructural studies confirmed the presence of GaN nanoparticles in the matrix. FTIR and Raman spectroscopy revealed the presence of the three polymorphs of PVDF in the composite films. The dielectric constant of the composite films were found to increase with the increase in the filler concentration, to almost?~?6 times that of the value for the pristine film due to the interfacial polarization playing between the polymer chains and the filler nanoparticles. Low values of dielectric constant at higher frequencies were observed due to the contribution of dipolar polarization. A peak-to-peak voltage of ~?5.4?V, from a triboelectric nanogenerator fabricated using a 1 wt% composite films, was obtained.

     

Growth and surface characterization of magnetron sputtered zinc nitride thin films

Zinc nitride films were deposited on Si(100) substrates at room temperature using RF-magnetron sputtering in pure N₂ and in Ar + N₂ atmospheres. Two active phonon modes (270.81 and 569.80 cm^− 1) are observed in Raman spectra for films deposited in Ar + N₂ atmosphere. Atomic force microscopy showed that the average surface roughness of the films deposited in pure N₂ atmosphere (1.3-3.33 nm) was less than for those deposited in a mixed Ar + N₂ atmosphere (10.3-12.8 nm). Low temperature cathodoluminescence showed two emission bands centered at 2.05 eV and 3.32 eV for both types of films.     

Surface acoustic waves in a gallium arsenide-conducting layerstructure

The electron attenuation and SAW velocity versus the surface conductivity and magnetic field induction for the structure consisting of the (001) cut anisotropic GaAs substrate and an isotropic conducting layer was calculated. The SAW propagated along the [110] piezoactive direction distinguished by the greatest electromechanical coupling coefficient     

Surface acoustic waves in thin films of barium strontium titanate on magnesium oxide substrates

It is established that, using a thin barium strontium titanate (BST) film as the active element in a surface acoustic wave (SAW) device, it possible to double the working frequency of the converter due to the formation of a periodic domain structure in the BST film. A thin ferroelectric film device with a standard electrode structure is capable of effectively exciting the SAW second harmonic. Variation of the external polarizing voltage applied to the electrodes ensures field-controlled electromechanical feedback in the converter.     

Deposition parameter studies and surface acoustic wavecharacterization of PECVD silicon nitride films on lithium niobate

Silicon nitride films were deposited by a plasma enhanced chemical vapor deposition technique using silane-ammonia as the reactant gas mixture. The influence of the process parameters such as flow ratio of the reactant gases, pressure, substrate temperature, RF power, time of deposition and electrode spacing on the deposition and etch rates were investigated. From the matrix of deposition conditions, the deposition parameters for high quality films applicable to surface acoustic wave (SAW) technology were found. Experimental results on the acoustic loss, reflectivity and velocity dispersion for the fabricated devices are presented     

Microstructure of amorphous tantalum nitride thin films

The main purpose of the present microstructural analysis by transmission electron microscopy (TEM) and X-ray diffraction was to investigate whether amorphous TaN films are a potential candidate as diffusion barrier for Cu wiring used in Si devices. The TaN thin films were prepared by a sputter-deposition technique using Ar and N₂ mixed gas, and the film structure was found to be sensitive to the gas flow ratio of N₂ vs. Ar during sputtering. Polycrystalline TaN films were obtained when the N₂/(Ar+N₂) ratio was smaller than 0.10 and amorphous TaN films were obtained when the ratio was larger than 0.15. Cross-sectional TEM observations revealed that the amorphous films had columnar structure with fine grains and that nano-scaled voids segregated at the boundaries. In addition, two-layered structures were observed in the amorphous TaN films and high density of the grain boundaries was formed close to the substrate. The present results suggested that the amorphous TaN films would not have high resistance against interdiffusion between two different materials because the density of grain boundaries with small voids was extremely high.     

Basal plane-oriented gallium nitride films on fused silica via acetate dip coating

c-axis-oriented gallium nitride (wurtzite GaN) thin films were fabricated by nitridation of acetate derived precursor films deposited on fused silica substrates without any buffer layer on the top of the substrate. The acetate derived precursors were obtained by (i) preparing a gallium-acetate sol by reacting Ga metal with acetic acid, (ii) coating cleared fused silica substrate with the sol and (iii) after drying the coated films at 100 °C, annealing them in air at 300°, 500° and 900 °C. Only films showing crystallization of α-GaO(OH) (300 °C) and (α + β)-Ga₂O₃ (500 °C) were selected for nitridation. In spite of the amorphous nature of the substrate, the GaN films showed a strong preferred orientation for the basal plane (002) under selected conditions of precursor annealing (300°, 500 °C) and subsequent nitridation (under flowing NH₃) temperature and time. In other cases formation of an additional plane, i.e. (101) was indicated as a weaker peak in X-ray diffraction (XRD) patterns. The precursors and nitride films were characterized by Fourier transform infrared spectroscopy, UV-Visible spectroscopy, XRD, high resolution transmission electron microscopy and atomic force microscopy analyses.     

Amorphous carbon buffer layers for separating free gallium nitride films

The possibility of using amorphous diamond-like carbon (DLC) films for self-separation of gallium nitride (GaN) layers grown by hydride vapor-phase epitaxy has been analyzed. DLC films have been synthesized by plasma-enhanced chemical vapor deposition under low pressure on sapphire (Al₂O₃) substrates with a (0001) crystallographic orientation. The samples have been studied by the methods of Raman scattering and X-ray diffraction analysis. It is shown that thin DLC films affect only slightly the processes of nucleation and growth of gallium nitride films. Notably, the strength of the “GaN film–Al₂O₃” substrate interface decreases, which facilitates separation of the GaN layers.     

Electrical and optical properties of silicon-doped gallium nitride polycrystalline films

Si-doped GaN films in polycrystalline form were deposited on quartz substrates at deposition temperatures ranging from 300–623 K using r.f. sputtering technique. Electrical, optical and microstructural properties were studied for these films. It was observed that films deposited at room temperature contained mainly hexagonal gallium nitride (h-GaN) while films deposited at 623 K were predominantly cubic (c-GaN) in nature. The films deposited at intermediate temperatures were found to contain both the hexagonal and cubic phases of GaN. Studies on the variation of conductivity with temperature indicated Mott’s hopping for films containing c-GaN while Efros and Shklovskii (E-S) hopping within the Coulomb gap was found to dominate the carrier transport mechanism in the films containing h-GaN. A crossover from Mott’s hopping to E-S hopping in the ‘soft’ Coulomb gap was noticed with lowering of temperature for films containing mixed phases of GaN. The relative intensity of the PL peak at ∼2·73 eV to that for peak at ∼3·11 eV appearing due to transitions from deep donor to valence band or shallow acceptors decreased significantly at higher temperature. Variation of band gap showed a bowing behaviour with the amount of cubic phase present in the films.