LO Phonon Research Articles

Raman scattering studies of the lateral Mn distribution in MBE-grown Ga1-xMnxN epilayers

Recent interest in very thin single phase Ga1-xMnxN dilute magnetic layers increased needs for precise, non-destructive, and relatively fast characterization methods with key issues being the macroscopic lateral Mn distribution and the absolute values of Mn concentration x. We report on resonantly enhanced UV Raman scattering studies of high quality Ga1−xMnxN layers grown on GaN templated sapphire by molecular beam epitaxy with 4 < x < 9%. The main advantage of the UV excitation is the restriction of the light penetration depth to nearly a hundred nanometers, eliminating signal from the GaN buffer. Under this conditions we determine the dependence of the 1LO phonon frequency on x, what allows for a fine mapping of its lateral distribution over the entire surface of the samples. Our Raman scanning clearly confirms substantial lateral distribution of Mn atoms across the layer, which is radial with respect to its center. From the established distributions in two deliberately chosen layers the magnitude of the optimal growth temperature for most efficient Mn atoms incorporation in epitaxial GaN has been confirmed. It is shown that the combination of the 1LO line width and its energy provides assessment of the crystalline quality of the investigated layers.

Anisotropic Biaxial Strain Evaluation in Carbon-Doped Silicon Using Water-Immersion Raman Spectroscopy

Background and purpose Carbon-doped silicon (Si:C) is used in the source and drain region as a stressor to the channel of n-type MOSFET [1]. However, there are few reports directly evaluate the strain states of Si:C. Furthermore, it has not been evaluated the relaxation of the strain state in Si:C after microfabrication. In this study, we evaluated the anisotropic biaxial strain relaxation of Si:C caused by microfabrication, using water-immersion Raman spectroscopy. Experimental method Si:C films were grown on (001) Si substrate by molecular beam epitaxy. The film thicknesses were 43 and 37 nm, with C concentration of 0.600% and 1.06%, confirmed by cross-sectional TEM and SIMS measurement, respectively. Then the Si:C films were processed into patterns by electron-beam lithography and dry etching. Fig. 1 shows the schematic of the patterned Si:C structure. The pattern width (W) were varied as 1000, 500, 200, and 100 nm, while the pattern length (L) was fixed at 5 μm. The Raman shifts of the LO and TO phonon modes are essential for the anisotropic biaxial strain evaluation. We adopted the water-immersion Raman spectroscopy with appropriate polarizer to obtain both the LO and TO phonon mode Raman shifts from the patterned Si:C [2]. The numerical aperture of the objective lens and the refractive index of pure water were 1.2 and 1.3, respectively. The oblique light component induced by the high-NA lens can effectively excite TO phonon even under the forbidden backscattering geometry and enable us to evaluate anisotropic strain state. The wavelength of the excitation light source and the focal length of the spectrometer were 355 nm and 2000 mm, respectively. Here, the wave number resolution was 0.1 cm−1. Results and Discussion Figure 2 shows the W dependence of LO/TO Raman shifts in the patterned Si:C. As shown in Fig. 2, the Raman shift from the patterned Si:C shifts to the higher wavenumber side as W decreases. From Fig. 2, it is confirmed that the tensile strain is originally induced in the film and relaxed as the pattern width decreases. Furthermore, since the Raman peaks shift significantly at W ≦ 200 nm, it can be considered that the strain is relaxed considerably at 100 nm from the both pattern edges. It is clear that strain relaxation by microfabrication depends on the pattern size. Fig. 2 shows the same tendency as in the patterned SSOI [2]. From Fig. 2, since the strain in the patterned Si:C1.06% was more significantly relaxed as the W decreased, it is considered that the larger the strain applied in the film, the easier to be relaxed by microfabrication. The strain in Si:C increases as the C concentration increases, however, even if the C concentration is increased to introduce a huge amount of the strain into Si:C, the residual strain after relaxation due to the microfabrication may be smaller than the lower C concentration Si:C. We can estimate from the LO/TO peak shifts of Si:C0.600% that the strain perpendicular to the pattern length is dominantly relaxed while the strain parallel to the pattern is kept unrelaxed. Therefore, the strain relaxation in the low C concentration Si:C with small applied strain was anisotropic, while the relaxation for highly strained Si:C with higher C concentration sowed more isotropic. IN conclusion, t2his study experimentally revealed the anisotropic strain relaxation in Si:C.

Applications of Huang–Rhys theory in semiconductor optical spectroscopy

A brief review of Huang–Rhys theory and Albrechtos theory is provided, and their connection and applications are discussed. The former is a first order perturbative theory on optical transitions intended for applications such as absorption and emission involving localized defect or impurity centers, emphasizing lattice relaxation or mixing of vibrational states due to electron–phonon coupling. The coupling strength is described by the Huang–Rhys factor. The latter theory is a second order perturbative theory on optical transitions intended for Raman scattering, and can in-principle include electron–phonon coupling in both electronic states and vibrational states. These two theories can potentially be connected through the common effect of lattice relaxation – non-orthonormal vibrational states associated with different electronic states. Because of this perceived connection, the latter theory is often used to explain resonant Raman scattering of LO phonons in bulk semiconductors and further used to describe the size dependence of electron–phonon coupling or Huang–Rhys factor in semiconductor nanostructures. Specifically, the A term in Albrechtos theory is often invoked to describe the multi-LO-phonon resonant Raman peaks in both bulk and nanostructured semiconductors in the literature, due to the misconception that a free-exciton could have a strong lattice relaxation. Without lattice relaxation, the A term will give rise to Rayleigh or elastic scattering. Lattice relaxation is only significant for highly localized defect or impurity states, and should be practically zero for either single particle states or free exciton states in a bulk semiconductor or for confined states in a semiconductor nanostructure that is not extremely small.

Velocity saturation in La-doped BaSnO3 thin films

BaSnO3, a high mobility perovskite oxide, is an attractive material for oxide-based electronic devices. However, in addition to low-field mobility, high-field transport properties such as the saturation velocity of carriers play a major role in determining the device performance. We report on the experimental measurement of the electron saturation velocity in La-doped BaSnO3 thin films for a range of doping densities. The predicted saturation velocities based on a simple LO-phonon emission mode, using an effective LO phonon energy of 120 meV show good agreement with the measurements of velocity saturation in La-doped BaSnO3 films. Density-dependent saturation velocity in the range of 1.8 × 107 cm/s reducing to 2 × 106 cm/s is predicted for δ-doped BaSnO3 channels with carrier densities ranging from 1013 cm−2 to 2 × 1014 cm−2, respectively. These results are expected to aid the informed design of BaSnO3 as an active material for high-charge density electronic transistors.

Effects of temperature and magnetic field on the ground state binding energy of the strong coupling magneto-polaron in an RbCl asymmetrical semi-exponential quantum well

The ground state binding energy (E[Formula: see text]) and the mean number of LO phonons (N) of the strong-coupling magneto-polaron (SCMP) in an asymmetrical semi-exponential quantum well (ASEQW) are studied theoretically. Temperature (T) effects on E[Formula: see text] and N are acquired with the quantum statistics theory (QST). By using the Lee-Low-Pines unitary transformation (LLPUT) and linear combination operation method (LCOM), the variations of E[Formula: see text] and N with T and [Formula: see text] of magnetic field are discussed. The investigated results indicate that both T and [Formula: see text] have great influence on E[Formula: see text] and N of LO phonons.

Emission of Light by CdMnTe/CdMgTe Heterostructure with Narrow Quantum Wells

The optical properties of the Cd0.4Mn0.4Te/Cd0.5Mg0.5Te heterostructure with quantum wells of 7 monolayers in thickness are studied. The temperature dependences of the integral intensity of luminescence of the barrier layer and quantum well and intracenter luminescence of manganese ions are determined, and the luminescence excitation spectra are measured. Numerous components corresponding to LO phonons of the CdTe, MgTe, and MnTe types are observed in the Raman scattering spectra.

The role of intervalley phonons in hot carrier transfer and extraction in type-II InAs/AlAsSb quantum-well solar cells

Simultaneous continuous wave (CW) photoluminescence and monochromatic current density-voltage (J-V) measurements of InAs/AlAsSb QW p-i-n diodes reveal stable hot carriers observed at relatively low power (nearly independent of power). This behavior is attributed to preferential scattering of high energy carriers to the upper satellite L- and X-valleys, which inhibits carrier thermalization via LO phonon emission. Although both high electric field and optical excitation are shown to enable stable hot carrier generation in the quantum wells (QWs), the extraction of these carriers is inhibited by the mismatch in valley degeneracy (L to Γ) across the InAs QW/n-AlInAs layers resulting in carrier localization in the QWs in the operating regime of the solar cell.

Influence of Li ion implantation on LO phonon broadening and bandgap opening in ZnO thin films

Doping of nonmagnetic impurities in technologically important ZnO has opened a new window for achieving room temperature ferromagnetism, p-type carrier conduction, and enhancement of ferroelectric properties. Here, we report on the confined optical phonon and bandgap engineering in highly oriented Li implanted ZnO thin films. Using resonance Raman scattering condition, the confined longitudinal optical phonon lineshapes in uniaxial hexagonal wurtzite crystal are analyzed in detail using the phonon confinement model. We have demonstrated that phonon confinement model can yield a meaningful result for the interpretation of resonance Raman lineshapes if one considers the contribution of both the E1 (LO) and A1 (LO) modes, particularly while dealing with oriented ZnO thin films. Furthermore, with the increase in Li dose, the bandgap of ZnO is found to show a blue shift, and such blue shift in bandgap is explained using first principles calculation.

Periodicity of probability density and effects of Rashba Spin–orbit interaction on excitation energy of bound polaron in quantum pseudodot

Abstract We study the probability density and the excitation energy of the bound polaron in the quantum pseudodot under the Rashba spin–orbit interaction by using the unitary transformation of Lee–Low–Pines and the variational method of Pekar. The results of a numerical calculation show that the probability density of the bound polaron in the superposition state is periodic and oscillates with the period T 0 . Due to the Rashba spin–orbit interaction, the excitation energy splits into different branches. It also can be seen that the excitation energy changes with the phonon wave vector when the two-dimensional electron gas’s chemical potential, the electron–LO phonon coupling, the Coulomb bound potential and the Rashba coupling parameter take different values.

Effect of hydrostatic pressure on the electron-phonon scattering in GaAs

Electron-phonon (e-ph) interaction in semiconductors is responsible for many phenomena such as electron mobility and hot electron thermalization, which are important to compute charge transport in semiconductors. In semiconductors, the application of hydrostatic pressure modifies the character of conduction band valleys, changing the effective mass of carriers, and, consequently, changing the carrier mobility. This, in turn, may limit electron transport in the semiconductor. Here, we present the effect of hydrostatic pressure on the e-ph interaction in GaAs, which goes from a direct to an indirect bandgap semiconductor at high pressures. A detailed study of e-ph self-energy and the resulting scattering rates is presented for hydrostatic pressure ranging between 0 and 9.2 GPa. Individual contributions from phonon modes to the total scattering rates indicate that short wavelength LO phonon scattering is found to be the dominant mechanism at low pressures; and at higher pressures, acoustic as well as optic phonons contribute toward the overall e-ph scattering. Finally, relaxation times are evaluated and we find that at pressures above 2.9 GPa, electron relaxation times are an order of magnitude lower than at equilibrium. These results will influence transport properties at higher pressures.

Strong Hot Carrier Effects in Single Nanowire Heterostructures.

We use transient Rayleigh scattering to study the thermalization of hot photoexcited carriers in single GaAs0.7Sb0.3/InP nanowire heterostructures. By comparing the energy loss rate in single core-only GaAs0.7Sb0.3 nanowires which do not show substantial hot carrier effects with the core-shell nanowires, we show that the presence of an InP shell substantially suppresses the longitudinal optical phonon emission rate at low temperatures which then leads to strong hot carrier effects.

Photoelectrochemical properties of the composites based on TiO2 nanotubes, CdSe and graphene oxide

The photoelectrochemical properties of electrodes based on a nanocomposite consisting of TiO2 nanotubes and CdSe films modified with graphene oxide have been studied. It is shown that in such a nanocomposite, a good adhesion of the CdSe layer to the substrate is provided. In this nanocomposite, the cathodic dark leakage currents are lower, and the efficiency of photoelectrodes is higher. From studies of Raman spectra, in which 2LO and 3LO phonon lines were observed, the quality of the modified polycrystalline films of CdSe was concluded. The amount of graphene oxide in NT-TiO2/CdSe/GO composite was determined to be 1.16 mass%, which allows one to obtain optimal photosensitivity of electrodes. It was established that the introduction of graphene oxide into semiconductor electrodes leads to an increase in their photosensitivity which is associated with a decrease of the surface recombination rate. The efficiency of the investigated nanocomposites in the photoelectrochemical cell for hydrogen production was studied together with composites based on graphene materials.

Multi-photon Raman scattering and yellow–green-light emission from feather-like Cd1–xZnxS nanostructures

Cd1–xZnxS (x ~ 0.5) alloy was synthesized on a silicon substrate by chemical vapor deposition in a horizontal tube furnace. Traditional analytical techniques such as scanning electron microscopy (SEM), transmission electron microscope, and Raman and photoluminescence (PL) spectroscopy were employed to characterize the samples. SEM analysis indicates that the branched nanorods are neatly perpendicular to the trunk and constitute a feather-like structure. Raman spectroscopy shows that the LO phonon modes of the feather-like Cd0.5Zn0.5S structures are blue-shifted about 3 cm−1 with respect to pure CdS. PL properties of the samples show that the feather-like Cd0.5Zn0.5S structures have an asymmetric broad emission peak at 490–570 nm. However, the pure CdS has only one narrow emission peak at 510 nm. The Gauss fitting curves show that the broad emission peak at 490–570 nm can be fitted to four peaks at 465, 519, and 543 nm, respectively. The blue-light emission peak at 465 nm is considered to be the near-band-edge emission of feather-like Cd0.5Zn0.5S structures. Furthermore, the luminescence mechanisms of peaks 519 and 543 nm are discussed, respectively.

Structural and optical study of Zn-doped As2Se3 thin films: Evidence for photoinduced formation of ZnSe nanocrystallites

Amorphous Zn-doped As2Se3 films with a nominal zinc content x up to 10 at.% were prepared by thermal evaporation. Their structure is characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoemission spectroscopy (XPS), and Raman spectroscopy. The AFM data show a considerable increase of the film surface roughness for films with x &amp;gt; 5 at.%. A strong gradient of the Zn content decreasing into the film depth is confirmed by the EDX and XPS data. Heavily Zn-doped (above 7 at.%) As2Se3 films reveal photostructural changes in the course of the Raman measurements. New Raman features are attributed to TO and LO vibrations of ZnSe nanocrystallites formed in the film under laser illumination. Depending on the laser wavelength and power density, the ZnSe nanocrystallites can experience tensile strain in the film due to a non-thermal photoplastic effect in the As2Se3 film resulting in a partial removal of the material from the laser spot. The tensile strain value, estimated from the TO and LO phonon frequency shift, is shown to reach up to 2.9 GPa.

Influence of LO and LA phonon processes on thermal-nonequilibrium excitation and deexcitation dynamics of excitons in GaN, AlN, and ZnO

The 1S-exciton density NX(1) in GaN, AlN, and ZnO is calculated for thermal-nonequilibrium states where the temperatures of an electron–hole system and phonons are different in the range of 10–400 K. The ratio of NX(1) in AlN to that in ZnO reaches 21 despite the similar exciton-binding energies of AlN and ZnO, which is due to the higher rate of excitation by LO-phonon absorption in ZnO. This result reveals that thermal-nonequilibrium states significantly affect the validity of evaluation methods for physical parameters such as internal quantum efficiency of radiation. The ratio of NX(1) in AlN to that in ZnO is enhanced from 2.2 to 18 by the occupation of states of the principal quantum number n from 2 to 5. This result demonstrates the importance of the discussion on the n≥3 states which have not been taken into account in other analyses. The main reason for the decrease in NX(1) is found to be the increase in the temperature of LO phonons rather than LA phonons, which indicates the importance of LO-phonon control in light-emitting devices. The results for general thermal-nonequilibrium states are nontrivial because the mechanisms of the population balance are complicated owing to the several-n occupation and the transition rates determined by various factors. Our analyses and discussions quantitatively unveil the LO- and LA-phonon effects on the thermal-nonequilibrium excitation and deexcitation dynamics of excitons and provide the basis for design of highly efficient light-emitting devices particularly in the ultraviolet region.

Statistics of excitonic energy states based on phononic-excitonic-radiative model

Excitation and deexcitation dynamics of excitons in GaN are analyzed by theoretical simulation using a set of rate equations based on a phononic-excitonic-radiative (PXR) model, which is applied to the analysis of experimentally-observed photoluminescence (PL) properties using a short pulse excitation. In phononic processes, deformation and piezoelectric interactions of the LA phonon and Fröhlich interaction of the LO phonon are taken into account. This model is successfully applied to the analysis of experimentally-observed emission line intensity ratios for excitons. This analysis reveals that the strong population exchange between the state of the principal quantum number n = 2 and the continuum takes place due to the increase in temperature. Further, the long experimental radiative lifetime component in the temporal PL decay curve up to 100 ns at room temperature (RT) is attributed to the shift of the population distribution to higher n states, which work as population reservoirs of the n = 1 state. Theoretical calculation using this model suggests that the dominant phonon mode in the excitation transfers from the n = 1 and 2 states shifts from the LA phonon to the LO phonon due to the increase in temperature from 130 K to 240 K. The PXR simulation model is feasible for the analysis of exciton-carrier dynamics and radiation efficiency analyses.

Raman characterization of single-crystalline Ga0.96Mn0.04As:Zn nanowires realized by ion-implantation

Recent progress in the realization of magnetic GaAs nanowires (NWs) doped with Mn has attracted a lot of attention due to their potential application in spintronics. In this work, we present a detailed Raman investigation of the structural properties of Zn doped GaAs (GaAs:Zn) and Mn-implanted GaAs:Zn (Ga0.96Mn0.04As:Zn) NWs. A significant broadening and redshift of the optical TO and LO phonon modes are observed for these NWs compared to as-grown undoped wires, which is attributed to strain induced by the Zn/Mn doping and to the presence of implantation-related defects. Moreover, the LO phonon modes are strongly damped, which is interpreted in terms of a strong LO phonon-plasmon coupling, induced by the free hole concentration. Moreover, we report on two new interesting Raman phonon modes (191 and 252 cm−1) observed in Mn ion-implanted NWs, which we attribute to Eg (TO) and A1g (LO) vibrational modes in a sheet layer of crystalline arsenic present on the surface of the NWs. This conclusion is supported by fitting the observed Raman shifts for the SO phonon modes to a theoretical dispersion function for a GaAs NW capped with a dielectric shell.

Effect of substrate polarity on the optical and vibrational properties of (311)A and (311) B oriented InAlAs/InP heterostructures

In this report, we use the photoluminescence (PL) and micro-Raman scattering techniques to systematically study the optical and vibrational properties of InAlAs/InP heterostructures grown by metal organic chemical vapor deposition (MOCVD) on either Indium (A-face) or Phosphorous (B-face) terminated (311) oriented substrates. PL measurements show that the InAlAs band gap redshifts with sample polarity switching from B to A, which is attributed to the change of the indium composition with substrate polarity. Then, micro-Raman investigations have been realized with the aim to comprehend the influence of substrate polarity on the shifts in phonon frequencies in Raman spectra. By using the modified random-element-isodisplacement (MREI) theory, we have explained the alloying effect in the InxAl1-xAs layers. A good consistency between InAs and AlAs-like phonon frequencies from the fitting of Raman spectra and calculated is achieved. AlAs-like LO phonon frequency has been employed to calculate the In composition and residual strain in our samples. In addition, the In composition of the InxAl1-xAs layer has been also determined by PL measurements. A good agreement with the indium contents obtained by both methods and those measured during growth of the epilayers has been attained. Our findings suggest that optical and crystalline quality are better for the (311)A sample.

Raman Scattering in InSb Spherical Nanocrystals Ion-Synthesized in Silicon-Oxide Films

The Raman spectra of SiO2 films containing InSb spherical nanocrystals produced by ion-beam synthesis are studied. TO- and LO-like modes in the spectra of the InSb nanocrystals are detected at frequencies of 187 and 195 cm–1, respectively. The shift of these modes to high frequencies with respect to the corresponding frequencies in InSb bulk crystals is analyzed from the viewpoint of the influence of the quantum-confinement effect, strains in nanocrystals, the surface phonon frequency, and scattering at the frequency corresponding to stretched anion–cation modes at the surface of polar spherical nanocrystals. The position of the 195-cm–1 mode corresponds to LO phonons in InSb nanocrystals hydrostatically compressed in the SiO2 matrix at pressures of about 10 kbar. The 187-cm–1 mode corresponds to resonance at the Frohlich frequency.

Influence of the electromagnetic field on the transition of a two-level system in a Gaussian confinement potential with LO phonon and of thickness effects

In this paper, the asymmetric Gaussian confinement potential and the parabolic confinement potential (PCP) are selected to describe the along growth direction and perpendicular to growth direction confinement potential (PC) of electron in a disk quantum dot, respectively. The phonons mean number and the energies of the ground and first excited states of the electron in a quantum dot with the polarization and the thickness effect are investigated by using Lee–Low–Pines unitary transformation and the Pekar-type variational method, and on the basis of the Fermi Golden Rule, the transition of the electron caused by the electromagnetic field is discussed. Our numerical results have displayed that the electric field and the magnetic field are an indispensable condition for the induced polaron transition. The weak electric field has a strong regulatory function on the transition. The influence of the well width $$L$$ and the well depth $$V_{0}$$ of the AGFCP on the mean number of phonons, energies and transition are important and interesting; it is also found in this work that the AGFCP is better than PCP in explaining the quantum size effect of QDs and modulating electron transition properties, respectively. This quantum system can be employed as a two-level quantum qubit.