Electronic Raman Scattering Research Articles

Selective photoluminescence spectroscopy of shallow levels in wide band gap semiconductors

We present an overview of recent selective photoluminescence (PL) experiments on shallow levels in ZnSe and GaN. Through two electron transitions (TET) and electronic Raman scattering (ERS) investigations on solid phase recrystallized bulk ZnSe doped with Al, Cl and In, the spectroscopic properties of donor impurities are re-examined. It is shown that the donor spectra previously attributed to Al and Cl are in fact due to common residual impurities. The Al-donor excited states are detected up to n=6 allowing a very accurate determination of its ionization energy. We show that the central cell correction of Al is vanishing leading to a new evaluation of the Rydberg which is discussed in view of the last experimental determinations of the dielectric constant and electron effective mass. The neutral-donor bound-excitons have been studied in n-type wurtzite GaN homoepitaxial layers and heteroepitaxial layers deposited on sapphire or SiC substrates. It is demonstrated that the existence of strain distributions in heterostructures induces original resonant effects on selective photoluminescence spectra allowing to identify the bound exciton excited states on a large domain of tensile and compressive strains and in particular near the A and B valence band crossing. Finally, the donors are investigated by TET and ERS spectroscopy.

Polarized electronic Raman scattering in high-T c superconductors

We study the polarized Electronic Raman Scattering (ERS) of several classes of high-T c cuprates as functions of temperature, doping, and pressure with an emphasis on the effect of superconductivity. The doping dependence of the single-layer HgBa 2CuO 4+ δ (Hg1201) confirms our earlier observations from two-layer Bi 2Sr 2CaCu 2O 8+ δ (Bi2212) that the superconducting energy gap is highly anisotropic at optimal doping, but that 2Δ peaks associated with superconductivity are not present in underdoped crystals. Low temperature ERS measurements of the n-type (T′) Pr 1.85Ce 0.15CuO 4 at optimal doping (T c = 20K) show negligible gap anisotropy, consistent with other spectroscopic results which suggested an isotropic s-wave order parameter. Normal state Raman scattering measurements of underdoped Bi2212 at pressures up to 7.6 GPa reveal anomalies in the lattice dynamics which offer insight into local oxygen ordering and may influence the observed large hysteresis in T c (P).

Temperature induced normal state redistribution of B 1g spectral weight in underdoped La 2-xSr xCuO 4

In electronic Raman Scattering experiments carried out on underdoped crystals of La 2-xSr xCuO 4 (La214) there is a depletion of low energy B 1g spectral weight relative to that measured in optimally doped (x ≈ 0.16) samples. In lightly underdoped (x ≈ 0.13) samples the depletion is independent of temperature for 20K < T < 300K. As the doping level is decreased further, additional spectral weight is lost and, furthermore, in the more heavily underdoped (x ≈ 0.08) samples the depletion at low energies also increases with decreasing temperature. Estimates are obtained for the magnitude of the pseudogap, E g, and the onset temperature T*.

Resonant electronic Raman scattering in optimally dopedBi2Sr2CaCu2O8+x superconductor

We report experimental results on resonant electronic Ramanscattering in superconducting single crystalsof Bi2Sr2CaCu2O8+x. The change of excitationwavelength results in a shift of the position for thesuperconductivity-induced 2Δ peak in A1g andB1g symmetries. In addition, in the red-light excitation anew Raman peak arises at approximately 130 K in the B1gspectra. Various explanations of these effects are discussed.

Carrier relaxation, pseudogap, and superconducting gap in high-Tccuprates: A Raman scattering study

We describe the results of electronic Raman-scattering experiments in differently doped single crystals of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}({\mathrm{Ca}}_{x}{\mathrm{Y}}_{1\ensuremath{-}x}){\mathrm{Cu}}_{2}{\mathrm{O}}_{8}.$ The data in antiferromagnetic insulating samples suggest that at least the low-energy parts of the spectra of metallic samples originate predominantly from excitations of free carriers. We therefore propose an analysis of the data in terms of a memory function approach which has been introduced earlier for the current response. Dynamical scattering rates $\ensuremath{\Gamma}(\ensuremath{\omega})=1/\ensuremath{\tau}(\ensuremath{\omega})$ and mass-enhancement factors $1+\ensuremath{\lambda}(\ensuremath{\omega}{)=m}^{*}(\ensuremath{\omega})/m$ of the carriers are obtained. It is found that a strong polarization dependence of the carrier lifetime develops towards low doping. In ${B}_{2g} (\mathrm{xy})$ symmetry selecting predominantly electrons with momenta along the diagonals of the ${\mathrm{CuO}}_{2}$ planes the Raman data compare well with the results obtained from dc and dynamical transport. In ${B}_{1g} {(x}^{2}\ensuremath{-}{y}^{2})$ symmetry projecting out momenta along the Cu-O bonds the dc scattering rates of underdoped materials become temperature independent and considerably larger than in ${B}_{2g}$ symmetry. This increasing anisotropy is accompanied by a loss of spectral weight in ${B}_{2g}$ symmetry in the range between the superconducting transition at ${T}_{c}$ and a characteristic temperature ${T}^{*}$ of the order of room temperature which compares well with the pseudogap temperature found in other experiments. The energy range affected by the pseudogap is doping and temperature independent. The integrated spectral loss is approximately 25% in underdoped samples and becomes much weaker towards higher carrier concentration. In underdoped samples, superconductivity-related features in the spectra can be observed only in ${B}_{2g}$ symmetry. The peak frequencies scale with ${T}_{c}.$ We do not find a direct relation between the pseudogap and the superconducting gap.

Near-Band Gap Selective Photoluminescence in Wurtzite GaN

Selective photoluminescence (SPL) experiments are performed at low temperature on n-type GaN layers obtained by epitaxial lateral overgrowth on sapphire. A significant impurity electronic Raman scattering (ERS) effect is observed when excited in the spectral range of excited states of A and B free excitons, giving an accurate determination of the transition energies to the n = 2 states and a straightforward estimation of the A and B Rydberg. From the donor transition energy measured on the ERS spectra we deduce a 30.5 meV binding energy for the main residual donor in our epitaxial layers. In addition, SPL experiments have been used to analyse the neutral donor bound exciton (I2) spectra. It is shown that a laser tuned near the A free exciton induces strong and narrow resonant peaks superimposed to I2. It is concluded that such effect is due to the selective excitation of inhomogeneously broadened I2 excited states. Up to five rotational I2 excited states are identified.

Pseudogap and Superconducting Gap in YBa2Cu3o6+x: A Raman Study

We present results of electronic Raman-scattering experiments in differently doped YBa 2 Cu 3 O 6+x . In B 2g symmetry, an analysis of the data in terms of a memory function approach is presented and dynamical relaxation rates Γ (ω, T) and mass-enhancement factors 1 + λ(ω, T) for the carriers are obtained. Starting from temperatures T > 180K, Γ(ω, T) and 1 + λ(ω,T) are extrapolated to lower temperatures and used to re-calculate Raman spectra. By comparison with our data, we find a loss of spectral weight between T c < T < T* at all doping levels x. T* is comparable to the pseudogap temperature found in other experiments. Below T c , the superconducting gap is observed. It depends on x and scales with T c whereas the energy scale of the pseudogap remains the same.

Surface-Enhanced Electronic Raman Scattering from Self-Assembled Alkanethiol Monolayers on Gold Surfaces

A preliminary model is presented that describes the observed surface-enhanced electronic Raman scattering spectrum of self-assembled n-alkanethiol monolayers on roughened Au surfaces. At room temperature, a series of intense resonance Raman spectral lines, superimposed on the normal surface-enhanced Raman vibrational spectrum, are produced when monolayers of CH3(CH2)mSH (m = 8 to 17) are illuminated with laser radiation between 630 and 740 nm. We describe the effects of isotopic substitution, and of varying both the alkane chain length and the temperature of the monolayer films.

Local vibrational modes in Mg-doped GaN grown by molecular beam epitaxy

Local vibrational modes in the region of the acoustic and optical phonons are reported for Mg-doped GaN grown by molecular beam epitaxy. The modes, studied by Raman spectroscopy, appear in addition to the known modes in the high-energy region around 2200 cm−1. We suggest disorder-activated scattering and scattering from Mg-related lattice vibrations to be the origin of the low-energy modes. Our assignment is supported by calculations based on a modified valence-force model of Kane. Temperature-dependent measurements between 4 and 300 K exclude an electronic Raman-scattering mechanism. We also report a new line at 2129 cm−1 and discuss the origin of all five observed high-energy modes.

Electronic Raman Scattering from Mg-Doped Wurtzite GaN

AbstractElectronic Raman scattering experiments have been carried out on both MBE and MOCVD-grown Mg-doped wurtzite GaN samples. Aside from the expected Raman lines, a broad structure (FWHM ≅ 15cm−1) observed for the first time at around 841cm−1 isattributed to the electronic Raman scattering from neutral Mg impurities in Mg-dopedGaN. From the analysis of the temperature-dependence of this electronic Ramanscattering signal binding energy of the Mg impurities in wurtzite GaN has been found to be Eb ≅ 172 ± 20meV. These experimental results demonstrate that the energy between the ground and first excited states of Mg impurities in wurtzite GaN is about 3/5 of its binding energy.

Polarized Electronic Raman Scattering Studies of Underdoped and Overdoped High-T c Superconductors

We report on polarized Raman scattering from high-T c superconductors in the Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) and HgBa 2 CuO 4+δ (Hg1201) classes, with an emphasis on the effect of superconductivity on the low frequency electronic continuum. The Hg-based materials confirm earlier observations from Bi2212 that the superconducting energy gap 2Δ peak is not present in underdoped samples, while its Raman shift is maximum in B 1g symmetry at optimal doping, and decreases dramatically with overdoping. These similarities despite the different numbers of bands crossing the Fermi surface argues against a multiple-band explanation for the anomalous doping dependence of the gap. We also discuss an anomalous B 1g feature observed at 600 cm −1 in Y underdoped Bi2212. This “pseudogap-like” feature has several properties which suggest it is of electronic - rather than phononic - origin.

Gapless superconductivity in Raman studies of cation substituted Bi 2Sr 2CaCu 2O 8+ δ

The electronic Raman scattering (ERS) spectrum of the high-temperature superconductor Bi 2Sr 2CaCu 2O 8+ δ is studied as a function of cation substitution and oxygen content. In unsubstituted crystals with δ near and above that for the optimal T c, the ERS spectra show a redistribution of spectral weight below T c consistent with the opening of a superconducting energy gap at 2 Δ. At low oxygen concentrations and in cation substituted samples with optimal oxygen content, the pair-breaking peak is not observed, displaying instead “gapless” or “pseudogap” spectra. When cation substituted samples are post-annealed in high pressure oxygen, the 2 Δ peak remarkably appears.

Raman and optical spectroscopic studies of small-to-large polaron crossover in the perovskite manganese oxides

We present an optical reflectance and Raman-scattering study of the ${A}_{1\ensuremath{-}x}{A}_{x}^{\ensuremath{'}}{\mathrm{MnO}}_{3}$ system as a function of temperature and doping $(0.2l~xl~0.5).$ The metal-semiconductor transition in the ${A}_{1\ensuremath{-}x}{A}_{x}^{\ensuremath{'}}{\mathrm{MnO}}_{3}$ system is characterized by a change from a diffusive electronic Raman-scattering response in the high-temperature paramagnetic phase, to a flat continuum scattering response in the low-temperature ferromagnetic phase. We interpret this change in the scattering response as a crossover from a small-polaron-dominated regime at high temperatures to a large-polaron-dominated low-temperature regime. Interestingly, we observe evidence for the coexistence of large and small polarons in the low-temperature ferromagnetic phase. We contrast these results with those obtained for ${\mathrm{EuB}}_{6},$ which is a low-${T}_{c}$ magnetic semiconductor with similar properties to the manganites, but with a substantially reduced carrier density and polaron energy.

Charge States of DX Centers and Electronic Raman Scattering in n-Ga1-xAlxAs

The observation of the electronic Raman scattering in n-Ga1-xAlxAs (x > 0.45) is reported and the charge state of DX centers in III-V alloy semiconductors is discussed. The result shows clearly that the electronic Raman scattering in n-Ga1-xAlxAs of indirect band-gap and the persistent photoconductivity in n-Ga1-xAlxAs of direct band-gap are equivalent to each other. Both of them originate from the optical induced metastable state of donors in n-Ga1-xAlxAs (x > 0.22) at low temperatures. At higher temperatures, this hydrogen-like level is depopulated to the benefit of a stable DX-like deep state.

Defect-induced Raman scattering in resonance with yellow luminescence transitions in hexagonal GaN on a sapphire substrate

Strong defect-specific low-frequency peaks are detected in low-temperature Raman spectra of hexagonal GaN grown by molecular beam epitaxy on sapphire substrate. The intensity of these peaks is found to be enhanced by excitation in resonance with yellow luminescence transitions. The validity of the assignment to electronic Raman scattering (ERS), as proposed before for their counterparts in cubic GaN on GaAs [M. Ramsteiner, J. Menniger, O. Brandt, H. Yang, and K. H. Ploog, Appl. Phys. Lett. 69, 1276 (1996)], is confirmed. Our results imply that the observed ERS peaks are related to shallow donors which are not necessarily hydrogenic. One Raman peak at very low frequency (11.7 meV) is alternatively explained by a pseudo-localized vibrational mode.

Electronic Raman-scattering study of low-energy excitations in single and doubleCuO2-layer Tl-Ba-(Ca)-Cu-O superconductors

Low energy Raman continuum and the redistribution of the continuum to a peak (2\Delta-peak) in the superconducting state have been studied in Tl-Ba-(Ca)-Cu-O superconductors with single (Tl-2201) and double (Tl-2212) CuO_2 layer. The 2\Delta/k_BT_c ratios in A_1g and B_1g symmetries are larger for Tl-2212 than for Tl-2201. The B_1g/A_1g gap ratio is also larger in Tl-2212. The A_1g intensities of the continuum and the 2\Delta-peak are significantly weaker than the B_1g intensities in Tl-2201, but are comparable in Tl-2212. This shows that the Coulomb screening is much stronger in Tl-2201. The change from Tl-2201 to Tl-2212 of the normalized A_1g 2\Delta-peak intensity is identical within experimental error to that of normalized A_1g continuum intensity. This suggests that the excitations forming the 2\Delta-peak and the continuum couple to light by the same mechanism.

Electronic Raman Scattering on High-Temperature Superconductors in High Magnetic Fields

physica status solidi (b)Volume 201, Issue 2 p. R3-R4 Research Letter Electronic Raman Scattering on High-Temperature Superconductors in High Magnetic Fields I. Loa, I. Loa Ingor@physik.TU-Berlin.de Technische Universität Berlin, Institut für Festkörperphysik, Sekr. PN 5-4, Hardenbergstr. 36, D-10623 Berlin, GermanySearch for more papers by this authorJ. Hofmann, J. Hofmann Technische Universität Berlin, Institut für Festkörperphysik, Sekr. PN 5-4, Hardenbergstr. 36, D-10623 Berlin, GermanySearch for more papers by this authorC. Thomsen, C. Thomsen Technische Universität Berlin, Institut für Festkörperphysik, Sekr. PN 5-4, Hardenbergstr. 36, D-10623 Berlin, GermanySearch for more papers by this author I. Loa, I. Loa Ingor@physik.TU-Berlin.de Technische Universität Berlin, Institut für Festkörperphysik, Sekr. PN 5-4, Hardenbergstr. 36, D-10623 Berlin, GermanySearch for more papers by this authorJ. Hofmann, J. Hofmann Technische Universität Berlin, Institut für Festkörperphysik, Sekr. PN 5-4, Hardenbergstr. 36, D-10623 Berlin, GermanySearch for more papers by this authorC. Thomsen, C. Thomsen Technische Universität Berlin, Institut für Festkörperphysik, Sekr. PN 5-4, Hardenbergstr. 36, D-10623 Berlin, GermanySearch for more papers by this author First published: 16 November 2001 https://doi.org/10.1002/1521-3951(199706)201:2<R3::AID-PSSB99993>3.0.CO;2-KCitations: 2AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume201, Issue2June 1997Pages R3-R4 RelatedInformation

Electronic Raman Scattering on High-Temperature Superconductors in High Magnetic Fields

Electronic Raman Scattering on High-Temperature Superconductors in High Magnetic Fields

Raman and electronic Raman spectra of lanthanide ions in elpasolite lattices

Raman spectra have been recorded from 300--10 K for lanthanide hexachloroelpasolites, ${\mathrm{Cs}}_{2}$${\mathrm{NaLCl}}_{6}$. Electronic Raman-scattering intensity ratios have been calculated using Judd-Ofelt-Axe theory and are in reasonable agreement with experimental results, except for those cases where electron-phonon coupling has important consequences. Bands resulting from temperature-dependent electron-phonon coupling phenomena have been observed not only in the Raman spectrum of ${\mathrm{Cs}}_{2}$${\mathrm{NaYbCl}}_{6}$ but also in that of ${\mathrm{Cs}}_{2}$${\mathrm{NaTmCl}}_{6}$, and the interaction mechanism is discussed. The coupling element between the coupled a${\mathrm{\ensuremath{\Gamma}}}_{5}$\ensuremath{\sim}${\mathrm{\ensuremath{\Gamma}}}_{1}$+${\ensuremath{\nu}}_{5}$ phonon-electron states of $^{3}\mathrm{H}_{6}$ in ${\mathrm{Cs}}_{2}$${\mathrm{NaTmCl}}_{6}$ is estimated to be 20 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, and the corrected wave functions for these coupled states enable a consistent reinterpretation to be made of the optical spectra of ${\mathrm{TmCl}}_{6}^{3\mathrm{\ensuremath{-}}}$. Excited-state electronic Raman scattering has been observed in the 120-K Raman spectrum of ${\mathrm{Cs}}_{2}$${\mathrm{NaTmCl}}_{6}$. Using fixed wavelength argon-ion laser lines, there is evidence for a small resonance enhancement of electronic Raman-scattering intensity, and the unusual mechanism for this is discussed. The mixed crystal system ${\mathrm{Cs}}_{2}$${\mathrm{NaGd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Yb}}_{\mathrm{x}}$${\mathrm{Cl}}_{6}$ exhibits unimodal behavior for the ${\ensuremath{\nu}}_{1}$ and ${\ensuremath{\nu}}_{5}$ vibrations, and the more complex behavior of features in the neighborhood of ${\ensuremath{\nu}}_{2}$ is described. The vibrational energies of the gerade internal ${\mathrm{LCl}}_{6}^{3\mathrm{\ensuremath{-}}}$ modes (determined from Raman spectra) vary linearly with atomic number of L, except where electron-phonon coupling effects occur.

Polarization dependence of two-photon absorption and electronic Raman scattering intensitiesin crystals

A formalism using the properties of the irreducible representation of the scattering tensors has been developed to predict up to the third order the polarization-dependent behavior of the electronic Raman-scattering intensities and the two-photon absorption intensities of transitions between Stark levels. The polarization behavior of ions in crystals are tabulated for the 32 crystallographic point groups. The theory provides a stringent test for the Judd-Ofelt-Axe theory for two-photon intensities between Stark levels.