Multiphonon resonant Raman scattering in N‐doped ZnO

Multiphonon resonant Raman scattering in N‐doped ZnO films was studied, and an enhancement of the resonant Raman scattering process as well as longitudinal optical (LO) phonon overtones up to the sixth order were observed at room temperature. The resonant Raman scattering intensity of the 1LO phonon in N‐doped ZnO appears three times as strong as that of undoped ZnO, which mainly arises from the defect‐induced Raman scattering caused by N‐doping. The nature of the 1LO phonon at 578 cm⁻¹ is interpreted as a quasimode with mixed A₁ and E₁ symmetry because of the defects formed in the ZnO lattice. In addition, the previously neglected impurity‐induced two‐LO‐phonon scattering process was clearly observed in N‐doped ZnO. Copyright © 2009 John Wiley & Sons, Ltd.     

Photoluminescence and resonant Raman scattering in N-doped ZnO thin films

A combination of studies on photoluminescence and resonant Raman scattering in N-doped ZnO thin films were carried out at room temperature. In the photoluminescence spectra, a transformation of radiative recombination mechanism from free-exciton to donor-acceptor-pair transition was observed. An enhancement of resonant Raman scattering processes as well as longitudinal optical (LO) phonon overtones up to the sixth order were observed in the Raman spectra. Also, the nature of the 1LO phonon underwent a transformation from a pure A₁(LO) mode to a quasimode with mixed A₁ and E₁ symmetry. The underlying mechanisms accounting for the influences of N doping on the optical properties of ZnO were related to the incorporation of extrinsic defects in the crystal lattice.     

Electrical performance and Raman scattering characterization of GaAs planar photoconductors

Raman scattering has been used to characterize the effects of processing techniques on the photosensitive area of GaAs planar photoconductors. The longitudinal optic (LO) phonon observed in the first-order Raman spectra was studied to probe the consequences of both chemical and plasma etching on the crystalline quality of an n-doped GaAs photosentitive area. This method has also been used to examine the stress at the interface of a Si₃N₄ film and an n-doped GaAs surface. For this purpose, three specially designed GaAs planar photoconductors (SN 353, AN 344, SN 344) have been fabricated. Electrical measurements on the photoconductors support the results of Raman study. First, as expected, increasing the number of processes leads to slightly poorer electrical performance. Secondly, a significant strain-induced shift in the GaAs LO phonon frequency has been observed under the Si₃N₄ film for the AN 344 photoconductor. However, the electrical performances of this device clearly indicate no dramatic change in the value of the n-doped surface potential.     

Micro‐Raman investigation of tin dioxide nanostructured material based on annealing effect

Rutile‐structured nanocrystalline tin dioxide (SnO₂) powder was synthesized by the chemical precipitation method using the precursor SnCl₂• 5H₂O. The SnO₂ powder was annealed at different temperatures, namely, 600, 800 and 1000 °C. Micro‐Raman spectra were recorded for both the as‐grown and annealed SnO₂ nanocrystalline samples. Micro‐Raman spectral measurements on the SnO₂ nanoparticle show the first‐order Raman modes A_1g (633 cm⁻¹), E_1g (475 cm⁻¹) and B_2g (775 cm⁻¹), indicating that the grown SnO₂ belongs to the rutile structure. The first‐order A_1g mode is observed as an intense band, whereas the other two modes show low intensity. The full width at half‐maximum and band area of the Raman lines of SnO₂ nanoparticle annealed at various temperatures were calculated. The effect of high‐temperature annealing on the vibrational modes of SnO₂ was studied. The optical image of SnO₂ nanocrystalline material was used to understand the surface morphology effect. Copyright © 2011 John Wiley & Sons, Ltd.     

Morphology of InN nanorods using spectroscopic Raman imaging

We report the vibrational properties of vertical and oblique InN nanorods (NRs) grown by molecular beam epitaxy (MBE). Surface optical (SO) Raman mode at 561 cm⁻¹, belonging to E₁ symmetry [SO(E₁)], is identified along with symmetry allowed Raman modes of E₂(low), E₂(high), and E₁(LO) at 87, 489, and 589 cm⁻¹, respectively, corresponding to wurtzite InN phase. Usually, SO phonon modes arise due to breakdown of translational symmetry of surface potential at surface defects, which are attributed by the surface roughness. Intensity distribution of E₁(LO) and SO(E₁) phonon modes over a specified area have been analysed using Raman area mapping with an optical resolution of 400 nm. Imaging with E₁(LO) phonon mode, originating from the bulk of the sample, distinguishes the vertical NRs alone. We are able to resolve NR morphologies in both vertical and oblique cases with additional Raman mapping analysis of SO(E₁) phonon mode, emerging from the surface irregularities, which are confined to the tip of MBE grown NRs. Copyright © 2013 John Wiley & Sons, Ltd.     

Temperature‐depending Raman line‐shift of silicon carbide

Silicon carbide (SiC) is often used for electronic devices operating at elevated temperatures. Spectroscopic temperature measurements are of high interest for device monitoring because confocal Raman microscopy provides a very high spatial resolution. To this end, calibration data are needed that relate Raman line‐shift and temperature. The shift of the phonon wavenumbers of single crystal SiC was investigated by Raman spectroscopy in the temperature range from 3 to 112°C. Spectra were obtained in undoped 6H SiC as well as in undoped and nitrogen‐doped 4H SiC. All spectra were acquired with the incident laser beam oriented parallel as well as perpendicular to the c‐axis to account for the anisotropy of the phonon dispersion. Nearly all individual peak centers were shifting linearly towards smaller wavenumbers with increasing temperature. Only the peak of the longitudinal optical phonon A₁(LO) in nitrogen‐doped 4H SiC was shifting to larger wavenumbers. For all phonons, a linear dependence of the Raman peaks on both parameters, temperature and phonon frequency, was found in the given temperature range. The linearity of the temperature shift allows for precise spectroscopic temperature measurements. Temperature correction of Raman line‐shifts also provides the ability to separate thermal shifts from mechanically induced ones. Copyright © 2009 John Wiley & Sons, Ltd.     

InGaN薄膜的紫外共振喇曼散射

利用325nm紫外光激发,对不同组分的In_xGa_1-xN薄膜的喇曼散射谱进行了研究.在光子能量大于带隙的情况下,观察到显著增强的二阶A₁(LO)声子散射峰.二阶LO声子峰都从一阶LO声子的二倍处向高能方向移动,移动量随样品In组分的增加而增大,认为是带内Frhlich相互作用决定的多共振效应引起的.分析了一阶LO声子散射频率和峰型与In组分的关系.在喇曼谱中观察到样品存在相分离现象,并与X射线衍射的实验结果进行 关键词： xGa_1-xN合金')" href="#">In_xGa_1-xN合金 紫外共振喇曼散射 二阶声子 相分离     

Use of various growth catalysts for laser sputtering of ZnO micro- and nanorods

Pulsed laser deposition and various catalysts are used to fabricate ZnO micro- and nanorods at temperatures close to the optimum temperature for each catalyst. A comparative analysis of the optical and structural properties of the rods shows that, as the temperature of growth on Al₂O₃(11–20) substrates decreases, the internal stresses in the rods decrease, which improves their structure and optical properties. This effect is not observed for GaN/Si substrates because of the high stresses induced by the difference in the lattice parameters. An increase in the synthesis temperature leads to an increase in the lattice strains and the concentration of point defects, the relaxation of selection rules, and the appearance of numerous phonon A ₁(LO) overtones. The lattice strains calculated from unit cell parameter a and the shift in the phonon A ₁(LO) frequency agree qualitatively. The study of the photoluminescence of the rods shows that a decrease in the synthesis temperature decreases the imperfection of the ZnO rods and improves their optical properties for UV applications.     

Structural modifications of GaN after cerium implantation

Among the family of rare earth (RE) dopants, the doping of first member Ce into GaN is the least studied system. This article reports structure properties of Ce‐doped GaN realized by technique of ion implantation. Ce ions were implanted into metal organic chemical vapor deposition grown n‐ and p‐GaN/sapphire thin films at doses 3 × 10¹⁴ and 2 × 10¹⁵ cm⁻². X‐ray diffraction scans and Raman scattering measurements exhibited expansion of lattice in the implanted portion of the samples. First order Raman scattering spectra show appearance of several disorder‐activated Raman scattering modes in addition to typical GaN features. A dose‐dependent decrease in intensity of E₂ mode was observed in Raman the spectra of the implanted samples. Ultraviolet Raman spectra of implanted samples show complete quenching of photoluminescence emission and appearance of multiple A₁(LO) phonon scattering modes up to fifth order. Moreover, a decrease in intensity and an increase in line width of LO modes as a function of wavenumber were observed for implanted samples. Copyright © 2012 John Wiley & Sons, Ltd.     

Comparative Analysis of Temperature-dependent .Raman Spectra of GaN and GaN/Mg Films

The Raman spectra of unintentionally doped gallium nitride (GaN) and Mg-doped GaN films were investigated and compared at room temperature and low temperature. The differences of E₂ and A₁(LO) mode in two samples are discussed. Stress relaxation is observed in Mg-doped GaN, and it is suggested that Mg-induced misfit dislocation and electron–phonon interaction are the possible origins. A peak at 247 cm⁻¹ is observed in both the Raman spectra of GaN and Mg-doped GaN. Temperature-dependent Raman scattering experiment of Mg-doped GaN shows the frequency and intensity changes of this peak with temperature. This peak is attributed to the defect-induced vibrational mode. Translated from Chinese Journal of Semiconductors, 2005, 26(4) (in Chinese)     

Disorder‐induced Raman scattering effects in one‐dimensional ZnO nanostructures by incorporation and anisotropic distribution of Dy and Li codopants

In Dy³⁺ and Li⁺ codoped ZnO nanowires, the additives accumulate preferentially in {0001} planes, resulting in serious breakdown of the translational symmetry in ab plane and modification of the phonon oscillation field. Not only acoustic overtones, silent optical modes, surface optical (SO) phonon modes, and multi‐phonon processes can be effectively observed in the nonresonant Raman scattering (RS) and the Fourier‐transform infrared (FTIR) spectra, but the quasi‐LO and TO modes of mixed A₁ and E₁ symmetry also show a noticeable red shift from E₁ symmetry (in ab plane) to A₁ symmetry (along c axis). The presence of dislocations and internal strain at the surface layer rich in additives, coming from the segregation of additives, forms a quasi‐bilayer system, resulting in the appearance and enhancement of SO phonon modes in RS and FTIR spectra. The Fano interference, originating from the interaction between the discrete scattering from phonons and the continuum scattering from laser‐induced electrons in the doped nanostructures, leads to typical asymmetric lineshapes on the lower wavenumber sides. Copyright © 2010 John Wiley & Sons, Ltd.     

Phonon freedom and confinement in GaAsAlxGa1−xAs superlattices

Phonon modes in GaAsAl_xGa_1?xAs superlattices simplify when the phonon wavevector q is perpendicular to the plane of the layers. We have studied such modes using a Raman back-scattering technique on SL's grown by MBE. The results are consistent with simple ideas of LA phonon freedom and LO phonon confinement suggested by one-dimensional lattice dynamical calculations. The longitudinal acoustic (LA) modes show zone folding due to mini-zone formation. Their frequencies occur in doublets linearly dependent on q and show little mini-gap formation. This is consistent with a picture of approximately free plane wave propagating through the interfaces with Raman coupling due to SL layering of the photoelastic coefficient. By contrast, Raman data on LO modes in small period GaAsAlAs SL's suggest that these modes are standing waves strongly confined in either GaAs or AlAs.     

Comparative Analysis of Temperature-dependent Raman Spectra of GaN and GaN/Mg Films

The Raman spectra of unintentionally doped gallium nitride (GaN) and Mg-doped GaN films were investigated and compared at room temperature and low temperature. The differences of E₂ and A₁(LO) mode in two samples are discussed. Stress relaxation is observed in Mg-doped GaN, and it is suggested that Mg-induced misfit dislocation and electron–phonon interaction are the possible origins. A peak at 247 cm^?1 is observed in both the Raman spectra of GaN and Mg-doped GaN. Temperature-dependent Raman scattering experiment of Mg-doped GaN shows the frequency and intensity changes of this peak with temperature. This peak is attributed to the defect-induced vibrational mode.     

Prediction of superconductivity of Ta2AlC: in situ Raman spectrometry and density functional investigations

In this paper, in situ Raman spectra of Ta₂AlC are measured in the temperature range of 80–500 K at ambient pressure. The frequencies of the Raman modes decrease with increasing temperature, which have been explained by the anharmonic and thermal expansion effects. The line‐width of E_2g (ω₃) mode increases at elevated temperatures, which is found to be due to the anharmonic phonon–phonon scatterings. On the other hand, the line‐widths of E_2g (ω₁) and A_1g (ω₄) modes decrease continuously with increasing temperature, which is explained by the electron–phonon couplings of these two phonon modes with the Ta 5d electrons. The electron–phonon coupling strengths are obtained both in experiments and density functional calculations. Finally, Ta₂AlC is predicted to be a new superconductive MAX phase. Copyright © 2014 John Wiley & Sons, Ltd.     

Phonon confinement and substitutional disorder in Cd1−xZnxS nanocrystals

1‐longitudinal optical (LO) phonons in free‐standing mixed Cd_1−xZn_xS nanocrystals, synthesized using chemical precipitation, are investigated using Raman spectroscopy. As expected for the nanocrystals, the 1‐LO modes are found to appear at slightly lower wavenumbers than those in the bulk mixed crystals and exhibit one‐mode behavior. On the other hand, the line broadening is found to be much more than that can be accounted on the basis of phonon confinement. From the detailed line‐shape analysis it turns out that the substitutional disorder in the mixed crystals contributes much more to the line broadening than the phonon confinement. The linewidth arising from these mechanisms are also extracted from the analysis. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis and optical properties of single crystalline GaN nanorods with a rectangular cross-section

GaN nanorods were synthesized from the reaction of a Ga/Ga₂O₃ mixture with NH₃ on Si substrates by chemical vapor deposition. The synthesized products were characterized by scanning and transmission electron microscopy, X-ray diffraction, photoluminescence and Raman spectroscopy. The nanorods are highly single crystalline and possess uniform smooth surfaces. PL revealed only a strong emission at 3.268 eV, ascribed to free exciton (FX) transitions, at room temperature; while the well-known yellow luminescence band centered at 2.2-2.3 eV was not detected. Four first-order phonon modes, corresponding to the A₁(TO), E₁(TO), E₂(high), and A₁(LO) at ∼531, 554, 564, and 721 cm⁻¹, respectively, were observed by Raman backscattering. The red-shift of the FX emission peak and the down-shifts of the Raman modes by a few wave numbers are attributed to the presence of tensile strain inside GaN nanorods.     

Cerussite,PbCO3 – a new Stimulated Raman Scattering (SRS)‐active crystal with high‐order Stokes and anti‐Stokes lasing

Orthorhombic PbCO₃, known as natural crystal cerussite, is presented as a new Stimulated Raman Scattering (SRS)‐active crystal. With picosecond laser pumping high‐order Raman‐induced χ⁽³⁾ generation is observed. All registered Stokes and anti‐Stokes sidebands in the visible and near‐IR are identified and attributed to the SRS‐promoting phonon mode A_1g of the carbonate group, with ω_SRS ≈ 1054 cm⁻¹. The first Stokes steady‐state Raman gain coefficient in the visible spectral range is estimated as well to a value not less than 4.6 cm·GW⁻¹.     

Ultraviolet Raman scattering in ZnO nanowires: quasimode mixing and temperature effects

We use near‐resonance Raman scattering to investigate zinc oxide (ZnO) nanowires grown by chemical vapor deposition on Si substrates. We discuss the role of quasimode mixing on the wavenumber of the longitudinal optical (LO) bands, and we perform Raman measurements with different excitation powers to investigate possible laser heating effects. We find that in the Raman spectra of as‐deposited nanowires grown along the c‐axis of wurtzite, the LO bands are located slightly below the E₁(LO) mode of bulk ZnO. We perform a calculation of the expected LO wavenumber in an ensemble of randomly oriented nanowires. Our analysis shows that light refraction, together with the orientation‐dependent cross‐section of the nanowires for the incoming light, counterbalances quasimode mixing effects in the as‐grown product, giving rise to LO bands that are barely redshifted relative to the E₁(LO) mode. In the case of ZnO nanowires that have been mechanically removed (scratched) and subsequently deposited onto separate Si substrates, we observe clear laser‐induced heating. Temperature effects account well for the Raman wavenumber shifts displayed by the LO bands in the Raman spectra of the scratched nanowires. Copyright © 2010 John Wiley & Sons, Ltd.     

Temperature dependence of Raman scattering in Co-doped AlN whiskers

Raman studies are reported for the A ₁ (TO), E ₂ (high) and E ₁ (LO) phonons of Co-doped AlN whiskers from 78 K to 778 K. The temperature dependence of the Raman shifts and line widths of these phonons can be well described by an empirical model which takes into account the contribution of the thermal expansion of the lattice and the symmetric decay of phonons into two and three identical phonons with lower energy. Our results show that the three-phonon process is the dominant decay channel in A ₁ (TO), E ₂ (high) and E ₁ (LO). The symmetric decay of A ₁ (TO) may be attributed to the production of two LA phonons near the M symmetry point of the Brillouin zone. For E ₂ (high), the symmetric decay may be near the M and L symmetry points. The E ₁ (LO) phonon, whose decay behavior has not been identified in AlN, may decay near the H and K symmetry points of the Brillouin zone. Compared with undoped AlN, the stronger temperature dependence of the A ₁ (TO) and E ₂ (high) phonons in Co-doped AlN whiskers was observed, which is probably due to the distortion of the lattice and the much larger tensile stress after doping Co into AlN.     

Raman spectroscopic studies of ZnSe/GaAs interfaces

ZnSe/semi‐insulating GaAs interfaces were studied by observing photogenerated plasmon–LO (PPL) coupled modes by nonresonant micro‐Raman spectroscopy. The effect of the carriers generated by the focused laser beam was investigated for a series of different thicknesses of ZnSe epitaxial layers. The PPL mode in GaAs was observed in the micro‐Raman spectra for all samples, but with different magnitude. The plasma is believed to be an electron gas as a result of the negative nature of the interfacial region that contains predominantly hole traps. The free carrier concentration is estimated to be > 10¹⁸ cm⁻³ and their lifetime ∼0.1 ns. This relatively long lifetime suggests that the ZnSe/GaAs interface has to be of high structural quality leading to a low recombination velocity. ZnSe/GaAs heterostructures of less crystalline quality (as determined by resonant Raman measurements) shows the effect of photogenerated carriers only to lesser extent. Copyright © 2007 John Wiley & Sons, Ltd.