Polar Semiconductors Research Articles

Optically Detected Spin-Magnetophonon Resonances in Semiconductor Superlattices

Optically Detected Spin-Magnetophonon Resonances in Semiconductor Superlattices Sang Chil Lee Research Institute for Basic Science and Department of Physics, Cheju National University, Jeju 690-756 (Received 28 September 2007) The in uence of the e ective g-factor from optically detected spin-magnetophonon resonance (ODSMPR) phenomena is investigated, including the Zeeman spin Hamiltonian term, in a shortperiod InAs-based polar semiconductor superlattice in magnetic elds. The conditions for the appearance of the ODSMPR line shape for various miniband widths as functions of the magnetic eld strength and the incident photon frequency applied to the short-period InAs-based polar semiconductor superlattice are obtained. We also suggest the possibility of nding the e ective g-factor value from the spin-magnetophonon resonance (SMPR) conditions. PACS numbers: 72.20.-i, 73.20.Dx, 73.50.-h, 72.20.Dp

Dependence of vibrational density of states and Raman spectra on size in non-polar and polar nano-crystalline semiconductors

The vibrational density of states (VDOS) of Si, diamond, SiC, and InSb clusters has been calculated using the cluster model for various cluster sizes. The results show that the peak frequency of optical-like bands of VDOSs of non-polar nano-crystalline (NC) semiconductors varies with size, but that of polar NC semiconductors does not vary with size. We attribute the origin of this different behavior of non-polar and polar NC semiconductors to different interactions in the optical-like modes of them. That is, the deformation potentials for the non-polar NC semiconductors and electrostatic potentials for the polar NC semiconductors. According to the amorphous crystal (aC) model, Raman spectra are directly related to VDOS. In this Letter, it is verified that the aC model can be applied to NC semiconductors. Calculated VDOS are compared with observed Raman spectra of corresponding samples, which show agreement.

Magnetized Plasma in Polar Semiconductors

Plasma excitations in solids have been intensively studied already at the time of the 1960’s. A significant interest was focused on doped polar semiconductors. In these systems, the nowadays text-book effect of coupling between plasmons and optical phonon modes can be clearly observed as demonstrated, for example, by the pioneering work of Mooradian and Wright [1] on inelastic light scattering in n-type GaAs samples. The properties of a plasma subjected to a static magnetic field are significantly modified. A magnetic field B applied to a metallic system introduces a characteristic frequency, namely the cyclotron frequency ωc =eB/m of motion of the carriers (with an effective mass m) in the plane perpendicular to the field direction. In polar-semiconductor plasmas, by changing the magnetic field the cyclotron frequency ωc can be made comparable to all important frequencies of the system, namely the plasma frequency ωp and the frequencies of longitudinal (LO) and transverse (TO) optical phonons. The physics of coupled plasmon phonon modes under these particular and interesting conditions has never been fully explored in Raman scattering experiments in 3D systems. On the other hand, there exist a number of works, concerning both Raman scattering and far-infrared magneto-spectroscopy which address the issue of coupling between the electronic and phonon excitations in twodimensional semiconductor structures. The physics of low energy excitations in these structures is far more complex because of the modification of the energy spectrum due to effects of spatial confinement. The results of high-magmetic-field (up to 28T) inelastic light scattering studies on coupled plasmon-LO-phonon modes in bulk, degenerate n-type GaAs will be presented. [2] The data analysis implies the use of a standard dielectric function approach. Results obtained for samples with high electron concentration are well understood. A strong interaction of coupled LO-phonon-plasmon modes with the collective cyclotron resonance excitations (Bernstein modes) is observed. In samples with lower electron concentration, an unexpected Raman scattering signal in the vicinity of the undressed optical phonon is observed at high magnetic fields. A field induced metal-insulator transition is shown to be observable in Raman scattering experiments in samples with lowest electron concentration. The plasmon-phonon modes which are characteristic of the metallic state are replaced by intradonor excitations in the case of the insulating state. It is very interesting, that the resonant magnetopolaron effect reflecting the interaction between the LO phonon and the 1s-2p intradonor excitation is observed in the insulating state but no coupling between LO-phonon and cyclotron resonance excitations is seen in the metallic state. This result allows us to discuss some aspects of an apparent controversy existing in two-dimensional systems regarding the fundamental problem of Frohlich interaction: whether it manifests itself as a resonant repulsion between the cyclotron resonance and LO-phononlike modes for a two-dimensional electron gas, or not.

Transient transport in III-nitrides: interplay of momentum and energy relaxationtimes

The ultrafast transient transport in wide-gap polar III-nitride semiconductors inelectric fields is considered. A nonlinear and time-dependent (on the evolution ofthe nonequilibrium-irreversible thermodynamic state of the system) Drude-likelaw is derived, with the conductivity related to a so-called transport time (orcurrent characteristic time which is related to a memory-dependent momentumrelaxation time). From the collision operators, present in the evolution equations forthe carriers’ energy and momentum, are obtained quantities playing the role oftime-dependent energy and momentum relaxation times. The electron drift velocityovershoot at intermediate-intensity fields in GaN, AlN and InN is evidenced, and itsonset is explained as a result of the interplay of momentum and energy relaxationtimes.

Rabi splitting at intersubband transition assisted by longitudinal optical phonon

Rabi splitting in absorption spectra of intersubband transition of a polar semiconductor quantum well is predicted if the intersubband frequency is near the longitudinal phonon frequency. The calculated value of the splitting for a GaAs quantum well changes from $1.82\phantom{\rule{0.3em}{0ex}}\mathrm{meV}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}6.86\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, when the electron concentration varies from ${10}^{16}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}{10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. It is also predicted that the absorptance is enhanced as compared to the one for a nonpolar quantum well with the same intersubband transition. The oscillator model of the dielectric function gives the same order of the splitting.

Raman spectroscopy of optical phonons confined in semiconductor quantum dots and nanocrystals

AbstractThis paper gives an overview of theoretical and experimental results on Raman‐active optical phonon modes confined in nearly spherical nanocrystals (NCs) of polar semiconductor materials with cubic crystal structure. A systematic consideration of the effect of reduced dimensionality on both electrons and phonons is presented. The theoretical approach is based on a continuum lattice dynamics model and the effective mass approximation for electronic states in the NCs. Calculated resonant and non‐resonant Raman scattering spectra are compared to experimental data stressing the effects of phonon confinement, type of excitation, polaron correction to the exciton spectrum enhanced because of the reduced dimensionality, and individual features of the NC material. Copyright © 2007 John Wiley & Sons, Ltd.

Raman scattering of nanoporous semiconductors

AbstractThe application of Raman scattering for characterizing porous semiconductors is reviewed. For structural units of only a few nanometers in size, as observed in por‐Si or por‐Ge, phonon confinement effects appear that can be used for size determinations. In polar porous semiconductors, surface‐related phonons (Fröhlich modes) between the TO and LO phonons appear. These surface phonons couple with the plasmons of free charge carriers in the porous medium. Examples are given of porous layers on III–V semiconductors in which Raman scattering is used to study charge carrier depletion. For theoretical treatment, the effective medium theory is used, taking into account the morphology with parallel pores or nanocolumns with radii down to a few tens of nanometers. Copyright © 2007 John Wiley & Sons, Ltd.

Quasimonochromatic emission spectra in the near field by polar semiconductor thermal sources

The near-field spectra of the electromagnetic field emitted from a planar surface are theoretically investigated for a number of polar semiconductor and dielectric materials that support surface phonon polaritons. All of the studied materials, InP, GaAs, GaN, SiC, and α-Al2O3 (sapphire), exhibit quasimonochromatic thermal emission symbolized by strong peaks of evanescent modes at well-defined frequencies in the near field that correspond to the appropriate peaks in the density of states for surface phonon polaritons. It is also found that the materials with lower polariton frequencies (e.g., InP and GaAs) generally demonstrate a higher peak spectral energy density compared to those with higher frequencies (e.g., SiC). This trend is maintained over the entire range of temperature (300–600K) and the distance from the surface (⩽10μm) considered in the calculation. Thus, the results clearly indicate that among the studied materials InP and GaAs are the best candidates to provide the quasicoherent thermal emission for potential use as a nanoscale thermal source. The energy density stored in the evanescent peaks, when close to the surface, is estimated to be many orders of magnitude larger than that in the blackbody radiation.

Raman and photoluminescence spectroscopy investigations of ZnO nanostructures

We discuss Raman and photoluminescence (PL) studies of ZnO in solid, thin film and nanowire samples. Resonant laser excitations ranging from near-infrared to ultraviolet are used in order to observe and analyse ZnO nanostructures. The changes of optical emission and electronic bandgap response to the vibration modes are studied simultaneously by PL and Raman spectroscopy. Both Raman and PL spectroscopy analyses of nanostructure, electronic bandgaps and photo-emission demonstrate that nanostructure geometry, including its aspect ratio, and phonon–electron interactions play important roles in polar semiconductor structures such as ZnO.

Fano resonance in the luminescence spectra of donor bound excitons in polar semiconductors

An unusual response in the luminescence measurements of GaN thin film and ZnO bulk crystal is observed at low temperatures. The authors demonstrate theoretically that such an unusual response is due to the longitudinal optical phonon mediated Fano resonance involved in the recombination process of the donor bound exciton. The line shapes obtained by the calculation in the present mechanism are in excellent agreement with the experimental results.

Conductance of nanowires: Phonon effects

Continuum models are applied to derive phonon modes for carbon nanotubes (CNTs) of finite length; phonon-confinement and geometrically-determined mode symmetries are taken into account to derive suitable phonon modes for such nanowires. In addition, conductance values are estimated for quantum wires fabricated of a variety of polar semiconductors.

On the possibility of streaming transport due to impact ionization in wurtzite InN

An alternative streaming transport regime in wurtzite InN is predicted by ensemble Monte Carlo simulations. In polar semiconductors the conventional streaming transport, characterized by a cyclic motion of electrons in the momentum space, is caused by electron energy relaxation via optical phonon emission. It is shown that under certain conditions a different mechanism, namely impact ionization, can be responsible for the streaming transport as well. In the present paper, we observe transient drift velocity oscillations in InN with remarkably higher frequencies (23 THz at an electric field of 400 kV cm−1) induced by impact ionization. Moreover, these oscillations occur even at room temperature and at high impurity concentrations in contrast to the conventional streaming transport. The frequency of the drift velocity oscillations depends on the strength of an applied static electric field and can be tuned over a wide frequency range.

Energy relaxation of hot carriers in single-wall carbon nanotubes by surface optical phonons of the substrate

A new mechanism is proposed for the energy relaxation of hot carriers in single-wall carbon nanotubes: scattering with the emission of surface optical phonons into the semiconductor substrate. The theory involves intrasubband and intersubband forward and backward scattering. The analytical result and numerical data indicate that intrasubband forward scattering is the main process: the corresponding lifetime comprises several femtoseconds for a quartz substrate, which allows this mechanism of energy relaxation to be considered dominating for a nanotube on the surface of a polar semiconductor or a dielectric.

Bipolaronic phase in polar semiconductor quantum dots: An all-coupling approach

An all-coupling variational calculation has been performed to explore the formation and stability of a bipolaron in a polar semiconductor quantum dot. It has been shown that quantum confinement in general leads to a broadening of the bipolaron stability region. It has been furthermore shown for the first time that stable bipolarons can exist in realistic parabolic quantum dots of polar semiconductors like GaAs, CdS, CdTe and CdSe if they are fabricated in certain range of sizes.

Nonlinear charge transport in III-N semiconductors: Mobility, diffusion, and a generalized Einstein relation

A theoretical study of nonlinear charge transport in polar semiconductors is presented. It is based on a nonequilibrium statistical ensemble formalism which provides a generalized Boltzmann-style nonlinear quantum kinetic theory. The mobility and the diffusion coefficients are obtained and, relating both, a Nernst-Townsend-Einstein relation is derived extended to the nonlinear regime (i.e., outside the Ohmic domain). Numerical calculations are performed considering the particular case of the strongly polar III nitrides, which have application in blue-emitting diodes.

Polaronic effects in asymmetric quantum wire: An all-coupling variational approach

An all-coupling variational calculation based on Lee–Low–Pines–Huybrechts (LLPH) theory is performed to study the ground state and the first excited state in an asymmetric polar semiconductor quantum wire that is valid for the entire range of the electron–phonon coupling constant and arbitrary confinement length. It is shown that the polaronic effects are very important and size dependent, if the effective width of the wire is reduced below a certain length scale. It is also shown that asymmetry in a quantum wire can be used as an extra parameter to increase the stability of the polaron. Finally the theory is applied to a realistic CdS quantum wire.

Nanoscale Resolved Infrared Probing of Crystal Structure and of Plasmon−Phonon Coupling

We show that slight variations of a crystal lattice cause significant spectral modifications of phonon-polariton resonant near-field interaction between polar semiconductor crystals and a scanning metal tip. Exploiting the effect for near-field imaging a SiC polytype boundary, we establish infrared mapping of crystal structure and crystal defects at 20 nm spatial resolution (lambda/500). By spectroscopic probing of doped SiC polytypes, we find that phonon-polariton resonant near-field interaction is also sensitive to electronic properties due to plasmon-phonon coupling in the crystals.

Preparation and Raman spectra of high quality ZnO nano-bulk materials

ZnO nano-bulk materials with high density and low brittleness is prepared on CS-1B 6×8000kN cubic high pressure apparatus. Grain sizes and microstructures of the samples have been characterized by X-ray diffraction and field emission scanning electron microscopy. The Raman spectra of polar semiconductor ZnO are studied by Raman spectrophotometer in the range of 50—500cm-1. No obvious size confinement effect is found in the polar semiconductor oxide.

Calculation of electronic properties of SrSi 2 within the framework of a band theory

Results of the electronic property calculations of SrSi 2 using the density-functional theory and the local density approximation with the generalized gradient correction are presented. The calculated structural parameters are in good agreement with the experimental values. The nearest Si–Si bond has a covalent nature. The electron density distribution belonging to the highest valence band (HVB) along this bond has a minimum in the center of this bond while that belonging to the lowest conduction band (LCB) has a maximum in the center of this bond. The calculated energy difference between the bottom of the LCB and the top of the HVB becomes smaller and changes to negative values by decreasing the cell volume. This agrees with the observed gap shrinkage by compression, though the observe gap between the HVB and LCV at ambient pressure could not be reproduced by the present band calculation as in the cases of other polar semiconductors.

Polaronic effects in a polar semiconductor quantum strip with transverse parabolic confinement

The effect of electron–phonon interaction on the ground and excited state energies of an electron in a polar quantum strip is studied by using a variational method. It is shown that polaronic effects are quite significant and also size-dependent if the effective width of the strip is reduced below a certain value. It is also shown that the longitudinal polaron effective mass is substantially renormalized even by the transverse confinement in a quantum strip.