Order Raman Peak Research Articles

Oxygen vacancy induced anomalous Raman mode in intrinsic ZnO film

ZnO network were prepared by a simple solution procedure due to its simplicity and cost effectiveness that has been calcined at different temperatures. With increasing the calcination temperature the preferred crystallographic orientation shifts from <002> to <103> direction. Additional Raman modes have been accounted in a number of current literatures compacting with intrinsic and doped ZnO layers but no clear justification has been given. Here we propose that disorder-activated Raman scattering commences increasingly growing second order Raman peak, (B1high– B1low) originated by the breakdown of translational equilibrium of the crystal by the O-vacancy induced defects and changes direction from preferred c-axis orientation with I<103>/I<002> > 1. At higher calcination temperatures O-vacancies have the inferior formation energy. The high-concentration of O-vacancy defect which assembles the <103> crystallographic orientation and the momentous strong anomalous Raman mode are projected to move forward clarification of the mechanisms of defect induced Raman mode and raise the opto-electronic relevance of ZnO matrix.

Impact of impurities and crystal defects on the performance of CVD diamond detectors

Radiation detectors based on diamond are highly favored for particle physics research due to the superior radiation hardness. In this work, we investigate the influence of impurities and crystalline imperfections on the charge collection efficiency (CCE) of single crystal diamond. Seventeen (17) ultra-low fluorescent diamond samples grown by microwave plasma chemical vapor deposition method from IIa Technologies PTE LTD are pre-selected for this study. The measured CCE of all samples using 241Am (α-particles) as ionizing source are analyzed together with the concentration of trace impurities and crystalline imperfection in the crystal. The amounts of impurities are quantified from integrated fluorescence intensity arising from the nitrogen vacancies (NV) created during different CVD growth process conditions. The crystal imperfections are assessed by X-ray rocking curves from X-ray topography images obtained at the Cornell High Energy Synchrotron Source. The CCE decays rapidly as the intensity of NV (INV), phonon sideband approaches that of diamond 2nd order Raman peak which follows the relation: CCEINV=100/1+INV1.052. The energy resolution, ΔE/E (ratio of the energy spectrum width to the most probable peak) highly correlates with broader rocking curve width distribution. Prime novelty statementThis work provides an understanding on the most important factors that contribute to degradation of charge collection efficiency (CCE) in diamond based detectors and sensors. The CCE decays rapidly as the intensity of nitrogen vacancy phonon sideband approaches that of diamond 2nd order Raman peak which follows the relation: CCEINV=100/1+INV1.052. The energy resolution, that is the ratio of the energy spectrum width to the most probable peak, highly correlates with broader X-ray rocking curve width distribution.

Second-order resonant Raman scattering in single-layer tungsten disulfideWS2

Resonant Raman spectra of single layer WS$_{2}$ flakes are presented. A second order Raman peak (2LA) appears under resonant excitation with a separation from the E$^{1}_{2g}$ mode of only $4$cm$^{-1}$. Depending on the intensity ratio and the respective line widths of these two peaks, any analysis which neglects the presence of the 2LA mode can lead to an inaccurate estimation of the position of the E$^{1}_{2g}$ mode, leading to a potentially incorrect assignment for the number of layers. Our results show that the intensity of the 2LA mode strongly depends on the angle between the linear polarization of the excitation and detection, a parameter which is neglected in many Raman studies.

Aluminum doped ZnO sol–gel derived nanocrystals: Raman spectroscopy and solid solubility characterization

In this study, a sol–gel method was used to synthesize Al‐doped ZnO nanocrystals (AZO). For preparation of this highly concentrated sol, zinc acetate concentration was fixed at 2M. UV‐visible absorption and photoluminescence spectroscopy were used to evaluate optical properties of the prepared samples. High temperature annealing was employed to evaluate the solid solubility limit of Al ions. XRD and Raman spectroscopy are then used to evaluate the doping process of aluminum in ZnO nanopowders. Disappearing of the second order Raman peak of ZnO at 1160 cm−1 was used to confirm the solid solubility limit. It can be found that the disappearing of second order Raman peak intensity at 1160 cm−1 is observed for sample with 2 at% Al3+ ion. Experimental results have shown that the solid solubility limit of aluminium ions in sol–gel derived zinc oxide nanocrystals was achieved.

Raman spectroscopy of core/shell silicon nanowires grown on different substrates

AbstractCrystalline–amorphous core–shell silicon nanowires (SiNWs) grown separately on glass and stainless steel substrates were investigated by Raman spectroscopy. Raman spectra confirmed the presence of crystalline and amorphous silicon phases in both samples. With respect to bulk silicon, the crystalline Raman peaks for the nanowires grown on a glass substrate showed much larger red shift and spectral broadening as compared to that of the nanowires grown on stainless steel. The Raman shift was attributed to local heating which was further confirmed by the reduction in red shift of the Raman spectrum at lower incident powers for both samples. For a low input power of 0.8 mW, the nanowires grown on stainless steel showed no shift in the first order Raman peak as compared to bulk silicon, however a small spectral broadening was still observed. magnified imageTEM images for the nanowires grown on steel substrate.

Influence of annealing treatment on as-grown Ib-type diamond crystal at a high temperature and high pressure

In this paper, we report on the influence of annealing treatment on as-grown Ib-type diamond crystal under high pressure and high temperature in a china-type cubic anvil high-pressure apparatus. Experiments are carried out at a pressure of 7.0 GPa and temperatures ranging from 1700 °C to 1900 °C for 1 h. Annealing treatment of the diamond crystal shows that the aggregation rate constant of nitrogen atoms in the as-grown Ib-type diamond crystal strongly depends on diamond morphology and annealing temperature. The aggregation rate constant of nitrogen remarkably increases with the increase of annealing temperature and its value in octahedral diamond is much higher than that in cubic diamond annealed at the same temperature. The colour of octahedral diamond crystal is obviously reduced from yellow to nearly colorless after annealing treatment for 1 h at 1900 °C, which is induced by nitrogen aggregation in a diamond lattice. The extent of nitrogen aggregation in an annealed diamond could approach approximately 98% indicated from the infrared absorption spectra. The micro-Raman spectrum reveals that the annealing treatment can improve the crystalline quality of Ib-type diamond characterized by a half width at full maximum at first order Raman peak, and therefore the annealed diamond crystals exhibit nearly the same properties as the natural IaA-type diamond stones of high quality in the Raman measurements.

Effect of Temperature on the Growth of Vertically Aligned Carbon Nanotubes from Palm Oil

The effects of synthesis temperature on the quality and quantity of vertically aligned carbon nanotubes (VACNT) were studied using high resolution scanning electron microscopy, and micro-Raman spectroscopy. The VACNT was synthesized by Fe catalytic decomposition of palm oil deposited on silicon substrate by thermal chemical vapour deposition method. The analysis shows that the growth rate increases from 3.8 to 5.5 µm/min as the temperature was increased from 750 to 800°C. The nanotube diameters were observed bigger at low temperature range. Smaller and uniform diameter (~15 nm) was found at 750°C and the increment in diameter size was seen at higher temperature range. Smaller graphite Raman “G” peak width, low ID/IG ratio (~0.52) indicated higher crystallinity of the nanotube and moderate I2D1/ I2D2 ratio for second order Raman peak was also detected at synthesis temperature of 750°C. These results indicated that the optimum synthesis temperature for higher quality VACNT production was at 750°C.

Spectroscopic research on ultrahigh pressure (UHP) macrodiamond at Copeton and Bingara NSW, Eastern Australia

Millions of macrodiamonds were mined from Cenozoic placers across Eastern Australia, 98% from within the Copeton and Bingara area (85 km across) in the Phanerozoic New England region of New South Wales (NSW). Raman spectroscopy of inclusions in uncut diamond, from the Copeton and Bingara parcels, identifies them as ultrahigh pressure (UHP) macrodiamond formed during termination of subduction by continental collision. Infrared spectral properties of the two parcels are critically similar in terms of nitrogen abundance (low in zoned diamond, high in unzoned diamond), requiring a pair of different growth mechanisms/protoliths. Within each parcel, the degrees of nitrogen aggregation are relatively strong and coherent, but they are so different from each other (moderate aggregation for Bingara, strong for Copeton) that the two parcels require separate primary and local sources. The local sources are post-tectonic alkali basaltic intrusions which captured UHP minerals (garnet, pyroxene, diamond) from eclogite-dominated UHP terranes (density stranded at depth—mantle, lower crust). X-ray diffraction studies on Copeton diamond indicate a normal density, despite previous reports of anomalously high density. For non-fluorescent diamond, a 2nd order Raman peak, which is prominent in theoretical perfect diamond and in African cratonic diamond, is suppressed in Copeton and Bingara UHP macrodiamond. Pervasive deformation during macrodiamond growth probably causes this suppression, the strong nitrogen aggregation, and the exceptional durability documented through industrial use.

High pressure Raman scattering of silicon nanowires

We study the high pressure response, up to 8 GPa, of silicon nanowires (SiNWs) with ∼ 15 nm diameter, by Raman spectroscopy. The first order Raman peak shows a superlineartrend, more pronounced compared to bulk Si. Combining transmission electronmicroscopy and Raman measurements we estimate the SiNWs’ bulk modulusand the Grüneisen parameters. We detect an increase of Raman linewidth at ∼ 4 GPa, and assign it to pressure induced activation of a decay processinto LO and TA phonons. This pressure is smaller compared to the ∼ 7 GPa reported for bulk Si. We do not observe evidence of phase transitions, such asdiscontinuities or change in the pressure slopes, in the investigated pressure range.

Micro-spectroscopy on silicon wafers and solar cells

Micro-Raman (μRS) and micro-photoluminescence spectroscopy (μPLS) are demonstrated as valuable characterization techniques for fundamental research on silicon as well as for technological issues in the photovoltaic production. We measure the quantitative carrier recombination lifetime and the doping density with submicron resolution by μPLS and μRS. μPLS utilizes the carrier diffusion from a point excitation source and μRS the hole density-dependent Fano resonances of the first order Raman peak. This is demonstrated on micro defects in multicrystalline silicon. In comparison with the stress measurement by μRS, these measurements reveal the influence of stress on the recombination activity of metal precipitates. This can be attributed to the strong stress dependence of the carrier mobility (piezoresistance) of silicon. With the aim of evaluating technological process steps, Fano resonances in μRS measurements are analyzed for the determination of the doping density and the carrier lifetime in selective emitters, laser fired doping structures, and back surface fields, while μPLS can show the micron-sized damage induced by the respective processes.

Submicron resolution carrier lifetime analysis in silicon with Fano resonances

AbstractDefect rich regions in multicrystalline silicon are investigated by Raman spectroscopy at high and low injection levels. By analyzing the Fano type asymmetry and the spectral position of the first order Raman peak crucial properties such as recombination lifetime, doping density and stress can be extracted simultaneously. Due to the small wavelength of the excitation laser the spatial resolution of these measurements is significantly below 1 µm, which gives new insight into the impact of defects on the carrier recombination lifetime. The results are evaluated by comparing them to micro‐photoluminescence and synchrotron X‐ray fluorescence measurements. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Superheating of silicon nanocrystals observed by Raman spectroscopy

In the Raman spectra of silicon nanocrystals a new anomalous component was detected. Close to the usual first order Raman peak situated for a bulk crystal at 521 cm −1 at room temperature, two peaks arise shifting towards lower energy and demonstrating a huge temperature increase, as measured by the ratio of the Stokes/anti-Stokes peak intensities. This behavior is dependent on the laser power and on the morphology of the nanocrystals. We can exclude, however, confinement effects, although surface enhanced phonon modes could be responsible of such superheating. Alternative explanations are also suggested and discussed.

Raman scattering characterization of Si(100) implanted with mega-electron-volt Sb

Dose dependent structural modifications in Si(100) due to 1.5 MeV implantation of Sb have been characterized using Raman spectroscopy and Rutherford backscattering spectrometry/channeling (RBS/C) techniques. With increasing fluence, an intensity reduction of the first order Raman peak, characteristic of crystalline Si, is observed. The amorphicity in Si lattice appears at a dose of 1×1013 ions/cm2 and it increases with each dose. For a dose of 5×1014 ions/cm2 the Raman spectrum resembles that of amorphous Si. RBS/C studies also support a fully amorphized lattice at this dose though for smaller doses it suggests lower disorder. For the fluences of 1×1013 and 1×1014 ions/cm2 a coexistence of undamaged crystalline Si regions and amorphous zones is observed. Consequently, phonon confinement is observed. Lattice recovery achieved by subsequent annealing has also been investigated using Raman spectroscopy. By annealing at 600 °C, sample crystallinity is fully recovered in all the cases up to the fluence of 5×1014 ions/cm2. For higher doses small amorphicity still remains. Depth dependent measurements of the shifts in the Raman peaks demonstrate a gradient in stress which is of compressive nature near the surface region but is tensile in deeper layers. Maximum stress in the lattice appears for a dose of 1×1012 ions/cm2 which gets relaxed by the incorporation of amorphous zones at higher fluences.

Raman scattering and high resolution electron microscopy studies of metal-organic chemical vapor deposition-tungsten disulfide thin films

Metal-organic chemical vapor deposition (MOCVD) using tungsten hexacarbonyl (W(CO) 6) and H 2S as precursors was employed to prepare tungsten disulfide (WS 2) thin films on Si substrates. Various degrees of preferred orientation as characterized by the WS 2 crystallites with their basal planes grown parallel to the substrate ( c(=)) and non-parallel to the substrate ( c(∥)) were obtained. Raman scattering was applied to study the effects of preferred orientation in WS 2 thin films on the Raman processes. As the fraction of c(∥) crystallites increases in the thin film structure, a large second order Raman peak appeared on a shoulder of the A 1g mode to the lower wavenumbers. Based on the analysis previously provided (C. Sourisseau, F. Cruege, M. Fouassier, Chem. Phys. 150 (1991) 281), this peak was characterized by two phonon-coupling originating from longitudinal acoustic (LA) and transverse acoustic (TA) phonons at the K point of the Brillouin zone. Cross-section high resolution electron microscopy (HREM) revealed that the WS 2 thin films grew with c(=) near the interface, followed by the formation of non-parallel crystallite ( c(∥)) co-mixing with c(=). Transition from c(=) to c(∥) in the microstructure occurred smoothly, showing curvature in the lattice image. Occurrence of the localized curvature in the thin film microstructure seems to be responsible for the enhancement of the two phonon-coupling process.

Control of Enhanced Optical Absorption in μc-Si

ABSTRACTThe influence of grain size on the enhanced optical absorption of μc-Si has been investigated using films of various grain sizes prepared by solid phase crystallization. We show that we can control this grain size and therefore the degree of absorption changes. For grain sizes below a threshold range significant absorption enhancement can be seen in the photon energy range of 1 to ∼3 eV and the absorption characteristics of these films show that the dominant mode of optical transitions is indirect. A correlation between first order Raman peak broadening and enhanced absorption was found suggesting both effects are related to confinement. A simple model was developed to see how confinement in the crystallites could influence indirect optical transitions.

Raman spectra in a broad frequency region of p− type porous silicon

Throughout the 50–1050 cm−1 range, in addition to the first order Raman peak of optical phonon, a weak structure in the vicinity of 147 cm−1 and two intense peaks (bands) in the vicinities of 632 and 956 cm−1 have been observed. The structure at 147 cm−1 is believed to be mainly due to scattering by the transverse acoustic single phonon mode; the intense peak at 632 cm−1 is thought to be mainly due to the transverse acoustic and optical combination phonon modes and the intense peak at 956 cm−1 to the transverse optical overtone double phonon mode of the p− type porous silicon (PS) layer. A comparison of spectral features indicates that, in PS Raman spectra, there is no substantial contribution due to the a-Si component and for PS the crystalline characteristic still remains to a great extent. Contrary to what would be expected using the conventional and simplified microcrystal model, Raman frequency downshifts of the aforementioned structures near 632 and 956 cm−1 were clearly observed, which indicates that, unlike the case of the first order Raman spectrum of optical phonon, the microcrystal model is too simple to give an interpretation of PS second order Raman spectra.

Pressure dependence of the second-order Raman feature at 2769cm−1in diamond

Abstract The sharp peak at 2769cm−1 in the two-phonon Raman spectrum of diamond has been studied at pressures up to 51 GPa in nearly hydrostatic conditions. We found a continuous change of this feature as well as a decrease of its frequency shift with respect to the doubled frequency of the first order Raman peak studied simultaneously. This result is tentatively ascribed to the change of the phonon dispersion curves under pressure.