micro-Raman Spectroscopy Measurements Research Articles

Physical properties of RF magnetron sputtered GaN/n-Si thin film: impacts of RF power

GaN thin film was successfully produced on $$n{\text{-}}Si\left({100} \right)$$ substrate by RF magnetron sputter under different RF power. Experimental measurement techniques such as UV/Vis spectroscopy, field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), and Micro-Raman Spectroscopy were used to research effects of Radio Frequency power on physical properties of produced thin film. It has been found that produced thin film was polycrystalline structure with (100) and (110) planes of hexagonal $$GaN$$ from X-ray diffraction measurement result. It also proved that increasing RF power gives rise to deterioration in crystal quality of $$GaN$$ thin film. Reason of this deterioration was discussed. It has been achieved that increasing RF power has resulted in decreasing optical band gap energy of $$GaN$$ thin film. Reasons for these changes in optical band gap energy were explained. It was seen that some thin films were grown as layer-plus-island mode (Stranski–Krastanov growth mode) and others were grown as layer-by-layer growth mode (Frank van der Merwe mode) from AFM analysis. It has been found that increasing RF power has resulted in improvement of surface morphology of thin film from field emission scanning electron microscopy analysis. However, reaching RF power to 125 W leads to start to deteriorate of surface of $$GaN$$ thin film. The reasons for this have been discussed. $$E_{1} \left( {TO} \right)$$ transverse optical phonon mode of hexagonal $$GaN$$ with different intensity was detected from Micro-Raman Spectroscopy measurement. The reasons for this difference have been discussed. It was concluded that RF power has played a significant role in growing high quality $$GaN$$ thin film. Morphological, structural, and optical properties of $$GaN$$ thin film were enhanced by controlling RF power, making them a potential candidate for LED, solar cell, diode application.

Engineering strain and conductivity of MoO3 by ion implantation

α-MoO3 lamellar crystals are implanted with 170 keV oxygen ions at room temperature and with fluences between 1 × 1012 cm−2 and 1 × 1017 cm−2, in order to modify the electrical and structural properties of the crystals. A controllable and significant increase of the electrical conductivity, over several orders of magnitude, is observed after implantation at high fluences. Based on high resolution X-ray diffraction (HRXRD) and micro-Raman spectroscopy measurements, this effect is attributed to the formation of donor-type defect complexes and new phases more conductive than the α-MoO3 orthorhombic phase. A significant expansion of the b lattice parameter, increasing with fluence, is observed as a response to the defects created by implantation. Strain build-up occurs in several steps and in distinct depth regions within the implanted layer. Contrary to the typical values reported for other implanted oxide materials, an unusually high maximum perpendicular deformation of ∼3% is verified.

Laser crystallized low-loss polycrystalline silicon waveguides

We report the fabrication of low-loss, low temperature deposited polysilicon waveguides via laser crystallization. The process involves pre-patterning amorphous silicon films to confine the thermal energy during the crystallization phase, which helps to control the grain growth and reduce the heat transfer to the surrounding media, making it compatible with CMOS integration. Micro-Raman spectroscopy, Secco etching and X-ray diffraction measurements reveal the high crystalline quality of the processed waveguides with the formation of millimeter long crystal grains. Optical losses as low as 5.3 dB/cm have been measured, indicating their suitability for the development of high-density integrated circuits.

Dynamics of a powerful deep submarine eruption recorded in H2O contents and speciation in rhyolitic glass: The 2012 Havre eruption

Constraining the syn-eruptive volatile contents of magmatic melt is critical to understanding the intensities and styles of deep submarine volcanic eruptions, for which direct observations are scarce. Quantifying residual magmatic water contents in volcanic glass is complicated by rehydration, i.e., late-stage addition of molecular water. The 2012 deep submarine silicic eruption of Havre volcano provides an unusual opportunity to quantify glass water contents from a recent, well-sampled stratigraphic sequence. Fourier-transform infrared and microRaman spectroscopy measurements of water concentration and water speciation across the Havre 2012 eruptive sequence reveal an unanticipated range of excess molecular water within pumice. This excess water requires rapid timescales of diffusion that are inconsistent with our current understanding of low temperature secondary rehydration in both subaerial and subaqueous eruptive products. Diffusion models applied to enrichment profiles at vesicle edges confirm that low temperature rehydration is an unlikely cause. We instead support higher temperature, syn-eruptive pumice rehydration by condensed magmatic water and seawater in a submarine plume. Hydroxyl concentrations suggest shallow quenching depths of Havre pumice hundreds of meters above the 900-meter-deep main vent. Our data also support the presence of a vapor-rich plume and consequent modification of ocean pressure above the vent. We combine this novel volatile data with textural information and cooling rate calculations to explore the conditions that would cause slower, shallow cooling of clasts from a deep submarine eruption. By exploring the physical conditions for the interaction between pumice and submarine plumes, we emphasize fundamental differences between subaerial and submarine clast-producing eruptions.

Influence of oriented CNT forest on thermoelectric properties of polymer-based materials

Thermoelectric (TE) materials are highly important due to their ability to convert wasted heat energy into electricity. Among the different TE materials, organic-based or polymer-based TE systems are among the most promising due to their sustainability, non-toxicity and good electrical properties. In our research, we have investigated for the first time the application of vertically aligned carbon nanotubes forest (VA-CNTF) as a filler for TE composite; compared to unconnected carbon nanotubes (CNT), which are typically used in polymer/CNT composites, dry pulled VA-CNTF sheets have more ordered structure, which is supposed to improve the TE efficiency of the material. VA-CNTF and short unoriented multiwalled carbon nanotubes (MWCNT) were used as fillers of a polymeric matrix, to prepare TE composites. Various stacking configurations were explored by using CNTF. All the samples were examined by scanning electron microscopy (SEM), micro-Raman spectroscopy, and four-point probe electrical measurements; MWCNT-based samples were used as benchmarking systems.The results revealed a dramatic increase of the Seebeck coefficient up to 46 μV/K for the VA-CNTF-based sample, while the best MWCNTs-based sample (MWCNT concentration 50 wt%) provided only 21.49, which is roughly the Seebeck coefficient of pure polymer. This research represents the first application of VA-CNTF as a promising material for TE systems and demonstrates that oriented nanoforests and related CNT sheets are a very perspective material for promising developments in the field.

Residual stress measurement in a metal microdevice by micro Raman spectroscopy

Large residual stress induced during the electroforming process cannot be ignored to fabricate reliable metal microdevices. Accurate measurement is the basis for studying the residual stress. Influenced by the topological feature size of micron scale in the metal microdevice, residual stress in it can hardly be measured by common methods. In this manuscript, a methodology is proposed to measure the residual stress in the metal microdevice using micro Raman spectroscopy (MRS). To estimate the residual stress in metal materials, micron sized β-SiC particles were mixed in the electroforming solution for codeposition. First, the calculated expression relating the Raman shifts to the induced biaxial stress for β-SiC was derived based on the theory of phonon deformation potentials and Hooke’s law. Corresponding micro electroforming experiments were performed and the residual stress in Ni–SiC composite layer was both measured by x-ray diffraction (XRD) and MRS methods. Then, the validity of the MRS measurements was verified by comparing with the residual stress measured by XRD method. The reliability of the MRS method was further validated by the statistical student’s t-test. The MRS measurements were found to have no systematic error in comparison with the XRD measurements, which confirm that the residual stresses measured by the MRS method are reliable. Besides that, the MRS method, by which the residual stress in a micro inertial switch was measured, has been confirmed to be a convincing experiment tool for estimating the residual stress in metal microdevice with micron order topological feature size.

Crystal Growth and Characterization of HgBa2Ca2Cu3O8+δ Superconductors with the Highest Critical Temperature at Ambient Pressure.

We report an original procedure for the elaboration of very high-quality single crystals of superconducting HgBa2Ca2Cu3O8+δ mercury cuprates. These single crystals are unique, with very high-quality surface paving the way for spectroscopic, transport, and thermodynamic probes in order to understand the hole-doped cuprate phase diagram. Annealing allows one to optimize Tc up to Tcmax = 133 K. The superconductivity transition width of about 2 K indicates that they are homogeneous. We show for the first time that, with adequate heat treatment, Hg-1223 can be largely underdoped and its doping level controlled. Importantly, the crystal structure was studied in detail by single-crystal X-ray diffraction, and we have identified the signature of the underdoping by a detailed sample characterization and micro-Raman spectroscopy measurements.

Tunable Doping in Hydrogenated Single Layered Molybdenum Disulfide

Structural defects in the molybdenum disulfide (MoS2) monolayer are widely known for strongly altering its properties. Therefore, a deep understanding of these structural defects and how they affect MoS2 electronic properties is of fundamental importance. Here, we report on the incorporation of atomic hydrogen in monolayered MoS2 to tune its structural defects. We demonstrate that the electronic properties of single layer MoS2 can be tuned from the intrinsic electron (n) to hole (p) doping via controlled exposure to atomic hydrogen at room temperature. Moreover, this hydrogenation process represents a viable technique to completely saturate the sulfur vacancies present in the MoS2 flakes. The successful incorporation of hydrogen in MoS2 leads to the modification of the electronic properties as evidenced by high resolution X-ray photoemission spectroscopy and density functional theory calculations. Micro-Raman spectroscopy and angle resolved photoemission spectroscopy measurements show the high quality of the hydrogenated MoS2 confirming the efficiency of our hydrogenation process. These results demonstrate that the MoS2 hydrogenation could be a significant and efficient way to achieve tunable doping of transition metal dichalcogenides (TMD) materials with non-TMD elements.

Determining the directional strain shift coefficients for tensile Ge: a combined x-ray diffraction and Raman spectroscopy study

In this work the calibration of the directional Raman strain shift coefficient for tensile strained Ge microstructures is reported. The strain shift coefficient is retrieved from micro-Raman spectroscopy measurements in combination with absolute strain measurements from x-ray diffraction using focused synchrotron radiation. The results are used to fit the phonon deformation potentials. A linear dependence of the phonon deformation potentials p and q is revealed. The method can be extended to provide strain calibration of Raman experiments also in other material system.

Impact of N-plasma and Ga-irradiation on MoS2 layer in molecular beam epitaxy

Recent interest in two-dimensional materials has resulted in ultra-thin devices based on the transfer of transition metal dichalcogenides (TMDs) onto other TMDs or III-nitride materials. In this investigation, we realized p-type monolayer (ML) MoS2, and intrinsic GaN/p-type MoS2 heterojunction by the GaN overgrowth on ML-MoS2/c-sapphire using the plasma-assisted molecular beam epitaxy. A systematic nitrogen plasma (N2*) and gallium (Ga) irradiation studies are employed to understand the individual effect on the doping levels of ML-MoS2, which is evaluated by micro-Raman and high-resolution X-Ray photoelectron spectroscopy (HRXPS) measurements. With both methods, p-type doping was attained and was verified by softening and strengthening of characteristics phonon modes E2g1 and A1g from Raman spectroscopy. With adequate N2*-irradiation (3 min), respective shift of 1.79 cm−1 for A1g and 1.11 cm−1 for E2g1 are obtained while short term Ga-irradiated (30 s) exhibits the shift of 1.51 cm−1 for A1g and 0.93 cm−1 for E2g1. Moreover, in HRXPS valence band spectra analysis, the position of valence band maximum measured with respect to the Fermi level is determined to evaluate the type of doping levels in ML-MoS2. The observed values of valance band maximum are reduced to 0.5, and 0.2 eV from the intrinsic value of ≈1.0 eV for N2*- and Ga-irradiated MoS2 layers, which confirms the p-type doping of ML-MoS2. Further p-type doping is verified by Hall effect measurements. Thus, by GaN overgrowth, we attained the building block of intrinsic GaN/p-type MoS2 heterojunction. Through this work, we have provided the platform for the realization of dissimilar heterostructure via monolithic approach.

Bandgap measurements and the peculiar splitting of E2H phonon modes of InxAl1-xN nanowires grown by plasma assisted molecular beam epitaxy

The dislocation free InxAl1-xN nanowires (NWs) are grown on Si(111) by nitrogen plasma assisted molecular beam epitaxy in the temperature regime of 490 °C–610 °C yielding In composition ranges over 0.50 ≤ x ≤ 0.17. We study the optical properties of these NWs by spectroscopic ellipsometry (SE), photoluminescence, and Raman spectroscopies since they possesses minimal strain with reduced defects comparative to the planar films. The optical bandgap measurements of InxAl1-xN NWs are demonstrated by SE where the absorption edges of the NW samples are evaluated irrespective of substrate transparency. A systematic Stoke shift of 0.04–0.27 eV with increasing x was observed when comparing the micro-photoluminescence spectra with the Tauc plot derived from SE. The micro-Raman spectra in the NWs with x = 0.5 showed two-mode behavior for A1(LO) phonons and single mode behavior for E2H phonons. As for x = 0.17, i.e., high Al content, we observed a peculiar E2H phonon mode splitting. Further, we observe composition dependent frequency shifts. The 77 to 600 K micro-Raman spectroscopy measurements show that both AlN- and InN-like modes of A1(LO) and E2H phonons in InxAl1-xN NWs are redshifted with increasing temperature, similar to that of the binary III group nitride semiconductors. These studies of the optical properties of the technologically important InxAl1-xN nanowires will path the way towards lasers and light-emitting diodes in the wavelength of the ultra-violet and visible range.

Low energy electron beam processing of YBCO thin films

Effects of low energy 30keV electron irradiation of superconducting YBa2Cu3O7−δ thin films have been investigated by means of transport and micro-Raman spectroscopy measurements. The critical temperature and the critical current of 200nm thick films initially increase with increasing fluency of the electron irradiation, reach the maximum at fluency 3−4×1020 electrons/cm2, and then decrease with further fluency increase. In much thinner films (75nm), the critical temperature increases while the critical current decreases after low energy electron irradiation with fluencies below 1020 electrons/cm2. The Raman investigations suggest that critical temperature increase in irradiated films is due to healing of broken CuO chains that results in increased carrier’s concentration in superconducting CuO2 planes. Changes in the critical current are controlled by changes in the density of oxygen vacancies acting as effective pinning centers for flux vortices. The effects of low energy electron irradiation of YBCO turned out to result from a subtle balance of many processes involving oxygen removal, both by thermal activation and kick-off processes, and ordering of chains environment by incident electrons.

Effect of doping concentration and temperature on the morphology, crystallinity and electrical conductivity of Al:ZnO nanostructured films grown from aqueous solution

Al-doped ZnO nanostructures were grown on glass substrate by hydrothermal method, in the temperature range of 70–95°C, from aqueous solutions of zinc nitrate he333xahydrate and aluminum chloride hexahydrate. Based on scanning electron microscopy, X-ray diffraction, micro-Raman spectroscopy and electrical measurement data, the effect of 2–6wt.% Al doping on the morphology, microstructure and electrical properties was investigated. The morphology and crystalline structure are strongly dependent on the Al-doping and growth temperature. When Al doping increases from zero to 2 and 4wt.%, the morphology changes from nanowires with higher optical phonon confinement, to combined morphologies including nanowires, nanoblades and transparent nanosheets. Lowering the temperature of the growth solution and/or increasing the concentration of aluminum dopant above 4wt.%, 1D (nanowires) and 2D (nanoblades and nanosheets) nanostructures collapse into an amorphous structure with totally changed morphology. Micro-Raman spectra and X-ray diffraction patterns confirmed the wurtzite structure with preferential c-axis growth direction of samples containing up to 4wt.% aluminum. The average crystallite size decreases continuously from 23.8 to 21.1nm with increasing the Al concentration from 0 to 4wt.%. The electrical conductivity of the obtained nanostructures significantly depends on the aluminum doping concentration. 2wt.% Al-doped ZnO nanostructured layer grown at 95°C showed the best electrical conductivity of 140.9Ω−1cm−1. The ambivalent role of aluminum, donor or acceptor, depending on doping concentration was highlighted.

Thermally stable device isolation by inert gas heavy ion implantation in AlGaN/GaN HEMTs on Si

Multiple energies of heavy ion implantation with inert-gas ion (84Kr+) were carried out on AlGaN/GaN high-electron-mobility transistors (HEMTs) for planar device isolation. Thermal stability of the implantated samples were also investigated by isochronal annealing at 500, 600, 700, and 800 °C (each temperature for 1 h.). Due to the damages created by heavy ions (84Kr+) in the GaN lattice, the implant-isolated Al0.27Ga0.73N/GaN HEMT samples exhibited better thermal stability than 40Ar+-implant-isolation. This was also confirmed by Rutherford backscattering spectrometry in channeling condition and ultraviolet micro-Raman spectroscopy measurements. With reference to mesa-isolated AlGaN/GaN HEMTs, the buffer breakdown voltage is also stable in the implant-isolated AlGaN/GaN HEMTs. An enhanced OFF-state breakdown voltage was also realized in the implant-isolated AlGaN/GaN HEMTs. The inert gas heavy ion implantation (84Kr+) is a viable solution for the fabrication of thermally stable planar AlGaN/GaN HEMTs even up to 800 °C under long-term isochronal annealing.

Controllable synthesis and photocatalytic properties of spherical and flower-like Ce2(MoO4)3 hierarchical architectures

Spherical and flower-like Ce2(MoO4)3 hierarchical architectures were controllably synthesized via a facile microwave-assisted hydrothermal method. The phase structures and morphologies were characterized by X-ray diffraction, field-emission scanning electron microscopy (FESEM), Fourier transform infrared, UV–vis absorbance spectra, micro-Raman spectroscopy and X-ray photoelectron spectra measurements. FESEM showed that the Ce2(MoO4)3 hierarchical architectures with distinctive morphologies could be selectively obtained by adjusting the reaction parameters of the hydrothermal process. The flower-like Ce2(MoO4)3 architectures showed higher photocatalytic activity than the spherical Ce2(MoO4)3. According to experimental results, the possible photocatalytic mechanism of the Ce2(MoO4)3 photocatalysts was proposed.

Irradiation of the amorphous carbon films by picosecond laser pulses

The effect of a picosecond laser irradiation on structure modification of diamond-like carbon (DLC) and graphite-like carbon (GLC) films was analyzed in this work. The DLC films were irradiated by Nd:YVO4 laser operating at the 532nm wavelength with the picosecond (10ps) pulse duration at the fluence in the range of (0.08–0.76) J/cm2. The GLC films were irradiated only at the fluence of 0.76J/cm2. The different pulse number (1, 10, and 100) was used for irradiation the films. The micro-Raman spectroscopy measurements indicated that the laser irradiation led to rearrangement of the sp3 C–C bonds to the sp2 CC bonds in the DLC films. The formation of silicon carbide (SiC) was found in the irradiated spot after 10 and 100 pulses. Modifications in the structure of the DLC film took place even in the areas with low intensity of the Gaussian beam wings (heat affected areas). The increase in the oxygen concentration up to ten times was detected in the heat affected areas after 100 pulses. Opposite to that, the laser irradiation decreased the oxygen concentration and smoothened the surface microrelief of the GLC films. The bonding type remained unchanged in the GLC films even after irradiation with 100 pulses per spot.

Power Dissipation and Electrical Breakdown in Black Phosphorus.

We report operating temperatures and heating coefficients measured in a multilayer black phosphorus device as a function of injected electrical power. By combining micro-Raman spectroscopy and electrical transport measurements, we have observed a linear temperature increase up to 600 K at a power dissipation rate of 0.896 K μm(3)/mW. By further increasing the bias voltage, we determined the threshold power and temperature for electrical breakdown and analyzed the fracture in the black phosphorus layer that caused the device failure by means of scanning electron microscopy and atomic force microscopy. The results will benefit the research and development of electronics and optoelectronics based on novel two-dimensional materials.

Photocatalytic TiO2 sol–gel thin films: Optical and morphological characterization

TiO2 thin films have been deposited onto glass substrates by sol–gel dip coating technique. The morphological and optical characterization of the as-deposited and annealed films at different temperatures in the range 150–700°C have been carried out by UV–vis spectroscopy and ellipsometry. An original rigorous optical characterization method based on both the transmission spectrum of a tri-layer optical system and the Tauc-Lorentz dispersion model, has been developed and successfully applied to calculate the thickness, optical constants, absorption edges and optical bandgaps of the dip-coated TiO2 films. Micro-Raman spectroscopy and thermoanalytical measurements have been also performed to complete the characterization of the films and support the morphological and optical results. As-deposited films have been proved to be amorphous and the onset of the formation of anatase phase has been found at annealing temperature of 400°C. The occurrence of anatase-phase and the increase of crystal size with increasing annealing temperature have been both evidenced by Raman spectroscopy measurements performed in the samples. Such evolution has been accompanied by larger values of the refractive index and a slight decrease in the values of the optical bandgap. Results indicate that samples annealed at 500°C are organic residues free, and they shows small anatase crystals, highly porous network, as well the lower electron–hole recombination rate within the annealing temperature series.

Sputter grown sub-micrometer thick Cu2ZnSnS4 thin film for photovoltaic device application

Structural properties and solar cells performance of sputter grown sub-micrometer thick (~600nm) Cu2ZnSnS4 (CZTS) films are investigated for low cost and large scale photovoltaic deployment. The CZTS films are deposited using single quaternary stoichiometric target at room temperature. X-ray diffraction, micro-Raman spectroscopy, and scanning electron microscopy measurements were carried out to investigate the structural property and secondary phase formation. The intense phonon peak at 334cm−1 in the visible Raman spectra confirms the deposition of single phase CZTS films. CZTS-based thin film solar cells show excellent performance with open circuit voltage ~600mV and over all cell efficiency of ~3.07%, exhibiting commercial potential toward large scale fabrication of photovoltaic devices.

High Quality 100 mm 4H-SiC Substrate

High-quality 100 mm 4H-SiC boules were grown using the physical vapor transport (PVT) method with optimized growth parameters. Micro-Raman spectroscopy measurement shows that the 4H-SiC polytype of the entire wafer is uniform. The micropipe density is measured to be less than 1cm-2. The 1c threading screw dislocation (TSD) density can be suppressed to the magnitude of 102 cm-2. After chemical mechanical polishing (CMP), it is found that the warp of the substrate is less than 10 μm, and the surface roughness (Ra) is as low as 0.063 nm.