Longitudinal Optical Phonon Plasmon Coupling Research Articles

Damping effect of longitudinal optical phonon—plasmon coupling on thermal radiation from surface micro-gratings on direct and indirect electronic transition type semiconductors

Thermal radiation properties of surface micro-Au-line and space structures on direct and indirect electronic transition-type semiconductors of n-type (n-) GaAs and GaP, respectively, are studied. The electron densities range from 0.62 to 1.0 × 1018 cm−3. The emission spectra of samples on n-GaAs wafers show peak energies significantly shifted from the longitudinal optical (LO) phonon energy due to the LO-phonon−plasmon coupling (LOPC) at 630 K, while partial decoupling is observed. The thermal emission from the sample on n-GaP is peaked at 395.9 cm−1, shifted only by 1.5 cm−1 from the LO-phonon energy, with a linewidth of 12.5 cm−1 at 630 K. The effect of LOPC on the emission from the sample on n-GaP is mostly suppressed because of the electron localization in the X-valley, suggesting an advantage of indirect transition-type semiconductors in LO-phonon resonant infrared emitters using the phonons generated via the electron energy relaxation.

Extraction of carrier concentration and mobility of ZnO by mid-infrared reflectance spectroscopy

As a representative of the third-generation semiconductors, ZnO has great development potential in the fields of optics and electricity, so the measurement on its electrical properties is quite important. Here, we try to extract the carrier concentration and mobility of ZnO by mid-infrared reflectance spectroscopy. Considering that the longitudinal optical phonon-plasmon coupling (LOPC) mode formed by the coupling of ZnO optical phonons and free carriers depends on carrier concentration and mobility, we conduct a test on the MIR spectra (400-1600 cm−1) of ZnO crystals with different carrier concentration (1018-1015 cm−3). By fitting the strong reflection peak shape corresponding to phonon-polaritons formed by the coupling of its LOPC and infrared light through m-CDF model, the carrier concentration and mobility of these samples are obtained, which are consistent with those got from the Hall-effect measurement.

Investigation of THz Radiation from Longitudinal Optical Phonon- Plasmon Coupling in p-i-n Diodes

Background: Terahertz radiation (THz) in infrared domain at room temperature has many applications in science and technology, especially in the technology of analyzing and processing image. This paper introduces a relatively simple method to investigate the Terahertz radiation from the coupling of Longitudinal Optical (LO) phonon and coherent plasmon in p-i-n diode structure. The frequency spectra are found from the Fast Fourier Transform (FFT) of the voltage between two neighboring points in the insulating region of this diode. Numerical calculations have been applied for GaAs semiconductor device with the photo-excited carrier density ranging from 17 1.0×10 cm-3 to 18 3.0×10 cm-3 and the insulating layer size of 500 nm. Methods: In order to study of the coupling of LO phonon in p-i-n diode based on the EMC simulations, the equation of density vibration is solved simultaneously with the simulation of carrier dynamics, this is performed with spatial resolution of 1A° and time resolution of 0.20 fs . We solve the Poisson’s equation to derive the potentials along the x-axis in both cases with and without taking into account the coupling. The frequency spectrum derived from the FFT of the voltage of two layers separated by a distance 10 nm in insulating layer (i). Results: The frequency spectrum derived from the Fourier transform of the voltage between two neighboring points with and without LO phonon–plasmon coupling is shown. We can easily observe the existence of the modes which are close to the frequency values of bulk semiconductor. It should be noted that, our calculations are reasonable agreement with experiments measured by the Ibanez et al in Phys. Rev. B 69 (7), 075314 (2004). Conclusion: In this paper, we present a relatively simple approach to investigate the THz radiation from the coupling of LO phonon-plasmon in p-i-n diode structure by taking the FFT of the voltage of two neighboring points in insulating semiconductor region. The voltage is calculated through the electric potentials which relate to the charge density via Poisson’s equation. Numerical calculations have been performed for GaAs semiconductor device with carrier density ranging from 17 1.0×10 cm-3 to 18 3.0×10 cm-3. Our simulations calculations show that with appropriate photoexcited carrier density, two strong coupling LO phonon-plasmon coherent modes are appear.

Electric-dipole absorption resonating with longitudinal optical phonon-plasmon system and its effect on dispersion relations of interface phonon polariton modes in metal/semiconductor-stripe structures

Interface phonon polaritons (IPhPs) in nano-structures excluding metal components are thoroughly investigated because they have lower loss in optical emission or absorption and higher quality factors than surface plasmon polaritons. In previous reports, it is found that strong infrared (IR) absorption is based on the interaction of p-polarized light and materials, and the resonance photon energy highly depends on the structure size and angle of incidence. We report the optical absorption by metal/semiconductor (bulk-GaAs and thin film-AlN)-stripe structures in THz to mid-IR region for the electric field of light perpendicular to the stripes, where both of s- and p-polarized light are absorbed. The absorption resonates with longitudinal optical (LO) phonon or LO phonon-plasmon coupling (LOPC) modes, and thus is independent of the angle of incidence or structure size. This absorption is attributed to the electric dipoles by the optically induced polarization charges at the metal/semiconductor, heterointerfaces, or interfaces of high electron density layers and depression ones. The electric permittivity is modified by the formation of these dipoles. It is found to be indispensable to utilize our form of altered permittivity to explain the experimental dispersion relations of metal/semiconductor-IPhP and SPhP in these samples. This analysis reveals that the IPhPs in the stripe structures of metal/AlN-film on a SiC substrate are highly confined in the AlN film, while the permittivity of the structures of metal/bulk-GaAs is partially affected by the electric-dipoles. The quality factors of the electric-dipole absorption are found to be 42–54 for undoped samples, and the value of 62 is obtained for Al/AlN-IPhP. It is thought that metal-contained structures are not obstacles to mode energy selectivity in phonon energy region of semiconductors.

A study of the coupling between LO phonons and plasmons in InP p-i-n diodes

This paper reports a study investigating the coupling between longitudinal optical (LO) phonons and plasmons in InP p-i-n diodes by a numerical simulation. A significant change is observed in the Fourier transform spectra of transient electric field when taking the coupling into account. The findings show two separate peaks instead of a single plasma peak as for non-coupling case. In addition, the bulk-like dispersion relations of the frequencies of those two peaks on the carrier density are found. Therefore, it is proposed that those behaviors manifest the LO phonon-plasmon coupling in the diodes. Also, there is evidence of the peak clipping by the diode itself, a phenomenon not being seen in the bulk InP semiconductor.

A study on the synthesis, longitudinal optical phonon–plasmon coupling and electronic structure of Al doped ZnS nanorods

First principles density functional theory (DFT) calculations were employed to study the structural and electronic properties of pure and Al doped ZnS nanorods.

A novel micro-Raman technique to detect and characterize 4H-SiC stacking faults

A novel Micro-Raman technique was designed and used to detect extended defects in 4H-SiC homoepitaxy. The technique uses above band-gap high-power laser densities to induce a local increase of free carriers in undoped epitaxies (n < 1016 at/cm−3), creating an electronic plasma that couples with the longitudinal optical (LO) Raman mode. The Raman shift of the LO phonon-plasmon-coupled mode (LOPC) increases as the free carrier density increases. Crystallographic defects lead to scattering or recombination of the free carriers which results in a loss of coupling with the LOPC, and in a reduction of the Raman shift. Given that the LO phonon-plasmon coupling is obtained thanks to the free carriers generated by the high injection level induced by the laser, we named this technique induced-LOPC (i-LOPC). This technique allows the simultaneous determination of both the carrier lifetime and carrier mobility. Taking advantage of the modifications on the carrier lifetime induced by extended defects, we were able to determine the spatial morphology of stacking faults; the obtained morphologies were found to be in excellent agreement with those provided by standard photoluminescence techniques. The results show that the detection of defects via i-LOPC spectroscopy is totally independent from the stacking fault photoluminescence signals that cover a large energy range up to 0.7 eV, thus allowing for a single-scan simultaneous determination of any kind of stacking fault. Combining the i-LOPC method with the analysis of the transverse optical mode, the micro-Raman characterization can determine the most important properties of unintentionally doped film, including the stress status of the wafer, lattice impurities (point defects, polytype inclusions) and a detailed analysis of crystallographic defects, with a high spectral and spatial resolution.

Theoretical and experimental study of the optical absorption at longitudinal phonon or phonon-plasmon coupling mode energy: An example of GaN

Reflectance loss of p-polarized infrared light at longitudinal optical (LO) phonon energies has been reported by Berreman. The origin of this loss has been discussed in view of the absorption by electric dipole moment by the polarization charges at the interfaces and propagation of interface polaritons. However, the existence of the absorption effect is not clear. In this issue, we take an example of GaN films with large polarization charges, and distinguish the two effects on optical spectra by theoretical and experimental analysis. It is found for the first time that the absorption by electric dipoles at the resonant energy with the LO phonon or LO phonon-plasmon coupling (LOPC) mode is identified separately from the interface polariton effect by varying the wavenumber component parallel to the interfaces by attenuated total reflectance analysis. We construct a theoretical expression of the pseudo-dielectric function of thin films around the LO phonon or LOPC mode energies, and show the quantitative proof of the existence of the absorption effect in the experimental spectra.

Optical spectroscopy study of damage induced in 4H-SiC by swift heavy ion irradiation

Single crystals of 4H-SiC were irradiated with swift heavy ions (332 MeV Ti, 106 MeV Pb and 2.7 GeV U) in the electronic energy loss regime. The resulting damage was investigated with UV–visible optical absorption spectroscopy and micro-Raman spectroscopy. The evolution of the Raman data with fluence shows an accumulation of isolated point defects without amorphization of the material and a partial recrystallization of the structure, but only at the lowest fluence. Furthermore, the longitudinal optical phonon–plasmon coupling mode disappears upon irradiation, suggesting a strong perturbation of the electronic structure. This evolution is consistent with the optical bandgap decrease and the Urbach edge broadening that was also previously observed for the irradiation with 4 MeV Au ions.

Hole density and anisotropic mobility of Mg‐doped InN from the analysis of LO phonon‐hole plasmon properties

AbstractInfrared reflectance and ellipsometry measurements are applied in order to study the influence of Mg‐doping on the properties of hexagonal InN films. Reflectance spectrum characteristics reveal the large effective mass and large plasmon damping rate just in the region where net acceptors have been observed by electrolyte capacitance‐voltage technique. The numerical spectrum analysis accounting for the modulation of the normal mode energies of longitudinal optical phonon–plasmon coupling (LOPC) by the large hole scattering rate yields the hole density of (0.1‐1.2)×1019 cm–3 and optical mobility of 25‐70 cm2/Vs for the direction vertical to the c axis. The properties of the bulk‐like part of the films are determined by the optical techniques. Infrared ellipsometry on the study of anisotropy of LOPC mode broadening indicates that threading dislocations or columnar grain boundaries cause the significantly larger scattering rate for holes vibrating along the c axis than those vibrating vertical to the c axis. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Infrared analysis of hole properties of Mg-doped p-type InN films

Mg-doped InN films grown by plasma-assisted molecular beam epitaxy were characterized by infrared reflectance. Signatures of p-type conductivity in the spectra were obtained in the same doping density range where the existence of net acceptors was found by electrolyte capacitance-voltage measurements. Numerical spectrum analysis, which takes into account the large broadening factor of the normal mode energies of longitudinal optical phonon–plasmon coupling yielded high hole densities in the range of (0.1–1.2)×1019 cm−3 and optical mobilities in the range of 25–70 cm2/V s.

Anisotropic damping of longitudinal optical phonon-plasmon coupling modes of InN films

The energy broadenings of the higher energy branch of the longitudinal optical (LO) phonon-plasmon coupling modes for E1(LO) and A1(LO) are analyzed for InN films by infrared reflectance and spectroscopic ellipsometry. Larger broadening for the vibration parallel to c of A1(LO)-plasmon coupling is found with the decrease in electron density. This phenomenon is caused by the plasmon lifetime anisotropy, and is attributed to the reduction in crystal defects causing isotropic carrier scattering and the remaining of defects along the c axis such as threading dislocations and columnar grain boundaries.

Direct measurement of momentum relaxation time in wurtzite InN

In this paper, we demonstrate that the momentum relaxation time of photoexcited carriers in wurtzite InN can be directly determined by investigating the rapid dephasing nature of coherent upper-branched longitudinal optical phonon-plasmon coupling (L+) mode. It is found that coherent L+ mode is a plasmonlike vibration mode and its dephasing time reduces from 120to60fs when the photoexcited carrier density increases from 2×1017∕cm3to2×1018∕cm3. The measured dephasing time is attributed to the momentum relaxation of photoexcited carriers, which is governed by the screened Coulomb interaction among photoexcited and background carriers in the bulk region of InN.

Infrared measurements of InN films at low temperatures

InN crystals with the electron concentration Ne of 4–11 × 1018 cm−3 are grown on sapphire substrates by molecular beam epitaxy. Optical reflectance and transmission measurements are performed in a temperature range of 5 to 296 K. The reflectance spectra are analysed by simulation including the longitudinal optical phonon–plasmon coupling scheme. At a Ne value of 4 × 1018 cm−3, the electronic states are found to be still degenerated. An anomalous temperature dependence of the bandgp energy Eg is found; with decreasing temperature Eg increases, and then decreases below 50 K. This decrease is smaller for samples with smaller Ne. In spite of this energy shift the decrease of Ne is too small to be detected. The effective electron mass of the intrinsic region is estimated to be 0.1m0.

Comparative optical studies of p-type and unintentionally doped GaN: The influence of annealing

We present Raman studies of p-type and unintentionally doped GaN epitaxial layers grown by metalorganic vapor phase epitaxy onto c-plane sapphire substrates. The E2 (high) Raman mode from a series of thermally annealed p-type samples shows that a compressive lattice distortion is induced with increasing annealing temperature. This is further corroborated by our photoluminescence measurements which show that the blue luminescence at 2.8 eV undergoes a redshift upon increasing the annealing temperatures beyond 650 °C. In comparing the Raman and photoluminescence spectra from the various samples we discuss the importance of two possible mechanisms: local distortion and longitudinal optical phonon–plasmon coupling.

Detection of terahertz radiation from longitudinal optical phonon–plasmon coupling modes in InSb film using an ultrabroadband photoconductive antenna

Terahertz radiation from longitudinal optical (LO) phonon–plasmon coupling modes in InSb films is observed using an ultrafast photoconductive antenna detector. We demonstrate a response frequency of up to 7 THz for a low-temperature-grown GaAs-based photoconductive antenna gated with 25 fs laser pulses. It is found that the emission frequencies of the coupling modes are dependent only on the residual carrier density, not on the excitation carrier density. It is also found that the LO phonon–plasmon oscillations in semiconductors can serve as an efficient THz source.

Raman studies of longitudinal optical phonon–plasmon coupling in GaN layers

Micro-Raman measurements have been performed on GaN epilayers to investigate longitudinal optical phonon–plasmon coupling. For planarly and axially coupled modes, the electron plasmon frequencies and damping constants have been evaluated from lineshape analyses and compared. The results showed a larger electron mobility along the c-direction. Near-resonant Raman experiments have been carried out to measure the scattering strength of coupled modes and their overtones.

Characterization of strontium titanate ceramics by infrared reflectivity spectroscopy and electron paramagnetic resonance

Undoped strontium titanate ceramics prepared by organic complexation route exhibits a sharp paramagnetic line near g = 2 the intensity of which varies rapidly with the Sr/Ti ratio. This paramagnetic center vanishes in the presence of free carriers introduced either by doping with Nb5+ or reduction under COCO2 or ArH2 mixtures at high temperature. High carrier concentrations may be deduced from the analysis of infrared reflection spectra that display longitudinal optical phonon-plasmon coupling behavior. This natural paramagnetic center is interpreted as a hole trapped on a titanium site. Its study allows accurate control of one of the main factors which govern the electrical conductivity of the ceramics, i.e. the Sr/Ti ratio which is otherwise difficult to control.