Acoustic Phonon Energy Research Articles

Suppression of electron-phonon interaction in narrow-band crystals

Taking the inelastic nature of the interaction of electrons with acoustic phonons into account is crucial in crystals having a conduction band width Δɛ comparable to the maximum acoustic phonon energy\(\bar \varepsilon \). In view of this, the laws of conservation of energy and wave vector impose stringent constraints on possible electron scattering processes, and single-phonon scattering becomes impossible for\(\Delta \varepsilon< \bar \varepsilon \). The phonon contribution to the resistance may be negligible, therefore, in narrow-band conductors.

Zone-boundary phonons in hexagonal and cubic GaN

We present results of second-order Raman-scattering experiments on hexagonal and cubic GaN covering the acoustic and the optical overtone spectral region. Based on a comparison of the experimental scattering data with the calculated phonon-dispersion curves as well as the group-theoretically derived selection rules, we were able to assign the observed structures to particular phonon branches and determined the points in the Brillouin zone from which the scattering originates. Our measurements reveal the energies of acoustic zone-boundary phonons in hexagonal GaN.

Tunneling spectroscopy of resonant interband tunneling structures

We report the first study of a resonant interband tunneling (RIT) structure using tunneling spectroscopy. Peaks in the tunneling spectra are shown to correspond to energies of acoustic phonons in the constituent materials of the RIT. We conclude that these phonons are involved in inelastic tunneling through the structure and could play a role in determining the overall form of the current-voltage characteristic.

Phonon growth in a many valley model semiconductor at low lattice temperatures

It is shown how the traditional method of neglecting the energy of acoustic phonons and approximating their distribution by the equipartition law leads to significant errors in the phonon growth rate in a many-valley model semiconductor when the lattice temperature is low.

Effect of oxygen deficiency (δ) on transition temperature of yttrium cuprate superconductors

The nature of pairing mechanism as well as transition temperature of yttrium cuprates is discussed using the strong coupling theory. An interaction potential has been developed for the layered structure with two conducting CuO2(a–b) layers in a unit cell. The interaction potential properly takes care of electron-electron, electron-phonon and electron-plasmon interactions. Furthermore, the electron-phonon coupling parameter (λ), the modified Coulomb repulsive parameter (μ*) and the 2D acoustic phonon (plasmon) energy as a function of oxygen deficiency is worked out. Finally, the superconducting transition temperature (T c) is then evaluated by using these coupling parameters and obtainedT c = 95(92)K for Y(Yb)Ba2Cu3O7−δ superconductors withδ = 0·0. The model parameters estimated from the layered structure approach are consistent with the strong coupling theory. The result deduced on the variation ofT c withδ are in fair agreement with the earlier reported data on yttrium cuprates. The analysis of the above results are discussed.

Terahertz absorption by electrons and confined acoustic phonons in free-standing quantum wells

The acoustic phonon confinement in a free-standing quantum well (FSQW) results in an acoustic phonon energy quantization. Typical quantization energies are in the terahertz frequency range. Free electrons may absorb electromagnetic waves in this frequency range if they emit or absorb acoustic phonons. Therefore, the terahertz absorption reveals the characteristic features of the acoustic phonon spectrum in free-standing structures. We have calculated the absorption coefficient of an electromagnetic wave by free electrons in a FSQW in the terahertz frequency range. We took into account a time dependent electric field, an exact form of the acoustic phonon spectrum and eigenmodes, and electron interactions with confined acoustic phonons through the deformation potential. We demonstrate numerical results for GaAs FSQW of width 100 Å at low lattice temperatures in the frequency range 0.1-1 THz. The absorption coefficient exhibits several structures at frequencies corresponding to the lowest acoustic phonon modes. These features occur due to absorption of photons by electrons, which is accompanied by the emission of corresponding acoustic phonons.

Nonequilibrium carrier transport in III–V compounds at low lattice temperatures

The effect of finite energy of the deformation potential and piezoelectric acoustic phonons on the electric field dependence of the effective temperature of the nonequilibrium carriers in n-type InSb and InAs has been calculated here by a numerical method for low lattice temperatures taking the nonparabolicity of the conduction band into account. Unlike the traditional method of neglecting the phonon energy in comparison to the carrier energy the calculations are carried out here under the condition that the acoustic phonon energy can neither be neglected nor the phonon distribution be represented by the equipartition law. The results are significantly different from those that follow from the traditional approach and are found to be in good agreement with the experimental data.

Wentzel-Bardeen singularity in coupled Luttinger liquids: transport properties

The recent progress on 1 D interacting electrons systems and their applications to study the transport properties of quasi one dimensional wires is reviewed. We focus on strongly correlated electrons coupled to low energy acoustic phonons in one dimension. The Wentzel--Bardeen singularity suppresses antiferromagnetic fluctuations and pushes the system toward the metallic phase via an intermediate, metallic phase. The implications of this phenomenon on the transport properties of an ideal wire as well as the properties of a wire with weak or strong scattering are analyzed in a perturbative renormalization group calculation. This allows to recover the three regimes predicted from the divergence criteria of the response functions.

Mobility characteristics of non-equilibrium carriers in III–V compounds at low lattice temperatures

The effect of the finite energy of the deformation potential and piezoelectric acoustic phonons on the high-field mobility characteristics of n-type InSb and InAs at low lattice temperatures is calculated by a numerical method taking the non-parabolicity of the conduction band into account and under the conditions that the acoustic phonon energy cannot be neglected in comparison with the carrier energy and the phonon distribution cannot be represented by the equipartition law. The results are interesting in that they are significantly different from those that follow from the traditional approximation of negligible phonon energy and also provide good agreement with the available experimental data. The inadequacies of the present theory and the scope for further improvements are discussed.

Phonon Raman scattering in disordered silicon carbides

Abstract A study has been conducted of the spectra of Raman scattering in silicon carbide crystals containing stacking disorders. The Raman bands of these crystals exhibit broadening, distortion and asymmetry in their band shape. For heavily disordered Sic, a broad low-frequency tail appears in the region of the transverse acoustic phonon energy. Raman intensity profiles are calculated for models of the disordered structure, which consider the structure as an extreme case of long-period polytypes. The calculated Raman spectra are compared with the experimental spectra of the disordered Sic crystals. The comparison indicates that the broadening and distortion of the Raman bands in the disordered SiC are dominated by the relaxation of the wave-vector conservation rule. Also the spectral shape was found to depend on the stacking structure of the domains existing in the crystals, on the average size of the domains and also on the distribution of the domain sizes. X-ray diffraction measurements support the Ram...

Effect of hot electrons on exciton luminescence of ultrapure GaAs in a magnetic field

Exciton photoluminescence in ultrapure GaAs (total shallow impurity concentration ND approximately 1012 cm-3) was studied in magnetic fields up to 20 T. The observed magnetic field enhancement of free-exciton luminescence and exciton cooling were found to depend strongly on the excitation energy, giving direct evidence for the role of hot photoexcited electrons. The observed effect in fact reflects cooling of electrons in a magnetic field due to their increasing acoustic phonon energy relaxation rate and an efficient exciton-electron interaction. In a magnetic field parallel to the sample surface a suppression of the exciton-electron interaction was found because of the spatial separation of excitons and electrons.

Images of the Response Signal of a 2D Gas of Carriers to a Pulsed Beam of 3D Phonons

The interaction of a 2D electron gas (2DEG) and a 2D hole gas (2DHG) with bulk acoustic phonons has been studied by different techniques in recent years (for a review cf. [1] and [2]). Due to the reduced dimensionality of a 2D gas of charge carriers (2DCG), the interaction with bulk phonons differs considerably from their interaction with 3DCGs. Another interesting feature of a 2DCG is the smallness of the Fermi wave vectors kF (and consequently the smallness of the Fermi momentum, the Fermi velocity υF , and the Fermi temperature TF). For a 2DCG the Fermi wave vector is comparable or even less than the typical wave vector of small energy acoustic phonons. Using phonon beams one can experimentally sweep relatively large volume of the 2D momentum space. There are very practical reasons for doing experiments of this type, because the electron-phonon interaction plays an important role in limiting the mobility, i.e. the performance of low-dimensional devices. Collisions with phonons limit the room temperature mobility of electrons in 2D high electron mobility transistors (HEMTs). In a series of papers (cf. [2]) Dietsche and collaboration (the Stuttgart group — SG) published the results of measurements of the time integrated response of a 2DEG to pulsed beams of acoustic phonons. In their experiments phonons propagated in a GaAs crystal and interacted with the 2D electrons lying in a {001} plane. They used sources of monochromatic phonons (a superconducting Al

Effect of Finite Energy of Deformation Potential Acoustic Phonons on the Low‐Temperature Breakdown Characteristics in n‐Ge

AbstractThe effect of the finite energy of acoustic phonons on the breakdown characteristics is investigated here for a sample of n‐Ge at lattice temperature 5K. Impact ionization and thermal recombination are the two kinetic processes which need be taken into account for the phenomenon phenomenon of impurity breakdown in n‐Gc. The electric field dependence of impact ionization coefficient A1(E) and thermal recombination coefficient B1(E) is investigated here using the newly developed field dependence of electron temperature where the effect of finite energy of the acoustic phonons is taken into account. Then the impurity breakdown characteristics of the material are investigated. The calculations are carried out for a parabolic law of dispersion and a scalar effective mass. In comparison with other calculations and with available experimental data, the results obtained here show a better agreement in some of the characteristics.

Effect of finite-energy acoustic phonons on the zero-field mobility characteristics of high-purity semiconductors at low lattice temperatures.

The effect of the finite energy of the acoustic phonons on the zero-field-mobility characteristics in high-purity Si and InSb at low lattice temperatures has been investigated by a numerical technique taking the nonparabolicity of the conduction band into account and under the condition that neither the acoustic-phonon energy can be neglected in comparison to the carrier energy nor the phonon distribution be represented by the equipartition law

Field Dependence of the Effective Electron Temperature for Finite Energy of the Deformation Potential Acoustic Phonons

Field Dependence of the Effective Electron Temperature for Finite Energy of the Deformation Potential Acoustic Phonons

Correlation of photoluminescence and nuclear characterization of In-implanted silicon

Photoluminescence measurements on silicon samples implanted with 115In and annealed in the range 700–1000 K reveal the presence of an optical spectrum not previously reported in the literature. The spectrum is characteristic of exciton recombination at electrically neutral defects. A strong sharp zero-phonon line at 1096.6 ± 0.1 meV is accompanied by a structured phonon sideband dominated by low energy acoustic phonon modes. Uniaxial stress measurements and comparisons with a comprehensive literature on perturbed angular correlation (PAC) data suggest that the defect observed corresponds to one of three defects observed by PAC. These results show that photoluminescence provides a suitable complementary technique to nuclear techniques used in semiconductor characterisation.

Effect of finite energy of deformation-potential acoustic phonons on the temperature of non-equilibrium carriers

The effect of the finite energy of the acoustic phonons on the temperature of the non-equilibrium carriers in pure semiconducting materials was investigated here by solving the energy balance equation for the electron phonon system at low lattice temperatures, without making the simplifying approximations of the traditional theory. The calculations were carried out for an isotropic material with a parabolic law of dispersion and a scalar effective mass. The field dependence of the electron temperature is now significantly different from what follows from the traditional theory, in which the phonon energy is neglected in comparison with the carrier energy. It is noted that the effect of the finite energy of the phonons on the carrier temperature at any field is greater the lower the lattice temperature. In comparison to the results of the traditional theory, the agreement of our results with the available experimental observations is found to be distinctly better. For a more detailed fit of the experimental results, the necessity of further refinements of the theory is discussed.

Manifestation of inelastic scattering of free high-energy carriers in heavily doped Cd xHg 1−xTe

In this paper we cite the results of investigations of optical absorption spectra in In-doped n-Cd x Hg 1− x Te having concentrations of N In = 10 17−10 18 cm −3, in the range of wavelengths λ = 2.5–25 mm, at temperatures of T = 77 and 300 K. The theoretical calculation of the free carrier absorption (FCA) coefficient, made with regard to the nonparabolicity of the conduction band for scattering by longitudinal optical phonons and ionized impurities, does not lead to satisfactory agreement with experiments and indicates the presence of an additional mechanism, weakly dependent on λ and T. The possible inelastic electron-phonon interaction with acoustic phonons is discussed as the likely mechanism of this kind. The theoretical calculation of the FCA coefficient for this mechanism in nonparabolic bands is carried out and the adjustable parameters—the inelastic interaction constant D add = 10 eV and the interaction acoustic phonon energy hω add.ph = 10 −6−10 −7 eV, are evaluated by comparison with the experiments.

Phonon anharmonicity near the melting point and the glass transition

Abstract The vibrational anharmonicity of long-wavelength acoustic phonons has been determined for vitreous selenium in the vicinity of its glass transition and for monocrystalline indium as it approaches its melting point. The experimental method used was to measure the hydrostatic pressure dependences of ultrasonic wave velocities as a function of temperature. For amorphous selenium, both dC 11/dP and dC 44/dP increase substantially as the temperature is raised towards T g, indicating that vibrational anharmonicity rises as this glass nears T g. Well below T g the Grüneisen parameters for purely internal modes of vibration are small, whereas those for external modes are larger; this is because the inter-molecular volume is much more compressible than the intramolecular volume. The experimental results for indium provide the first complete set of the pressure derivatives of elastic constants of a material up to its melting point. The acoustic-mode Grüneisen parameters in the long-wavelength limit show that there is a substantial increase in vibrational anharmonicity as T m is approached. Finally, measurements of the wave-vector dependences of the transverse acoustic phonon energy widths are reported for single-crystal lead up to T m. It has been found that there is a change from a zero-sound collision-free mode of high-frequency phonon propagation to a first-sound collision-dominated regime at about 500–550 K. Hence the phonons which have wavelengths on the scale of the interatomic spacing are heavily damped in the crystal prior to melting.

Negative differential conductivity in shallow-impurity hopping.

Charge carriers in shallow-impurity states in semiconductors are weakly coupled to the phonons. As such, hopping impurity conduction proceeds via a succession of jumps in which a single phonon is either emitted or absorbed. Hops can only occur between states which differ in energy by less than the energy of the maximum energy phonon which can be emitted or absorbed. With the application of a large electric field, hops nearly parallel to the direction of the applied electric field are precluded since their attendant energy difference is too great. As a result, at high electric fields energetically allowed hops are nearly perpendicular to the applied field. This results in a drift velocity which falls with electric field strength at large electric fields. The criterion for being in this ``high-field'' regime is that the electric-field-induced drop in potential energy between a typical pair of sites be greater than the energy of the highest-energy phonon with which the carrier interacts appreciably. For a large-radius impurity state the maximum energy acoustic phonon with which the impurity state interacts is a very small fraction of the Debye energy. With a large-radius impurity concentration of ${10}^{15}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ in highly compensated Ge, high-field behavior is expected to occur for internal electric fields above 2.4\ifmmode\times\else\texttimes\fi{}${10}^{2}$ V/cm.