Emission Of Longitudinal Optical Phonons Research Articles

High-Mobility States and Dielectric Breakdown in Polymeric Dielectrics

We introduce a new experimental technique for directly measuring the energy and momentum scattering rates of hot electrons in dielectrics. Their energy distribution and impact ionization rate at a given electrical field F is a function of \begin{equation*}F^{2} \int^{E}_{0} \gamma_{p}(E)\gamma^{L0}_\mu{ }(E)dE\end{equation*} where ϒр is the momentum scattering rate and ϒLoμ the longitudinal optical phonon emission rate. Experiments on n-C36H74 show the energy loss rate to be governed by LO phonon emission (C-H stretch mode). Above E ~ 1.5 eV, ϒР is controlled by acoustic phonon scattering.

UV emission from sputtered zinc oxide thin films

A UV emission observed in photoluminescence measurements on ZnO thin films prepared by the conventional r.f. diode sputtering and r.f. magnetron sputtering techniques. The UV emission is attributed to excitonic recombination with the simultaneous emission of longitudinal optical phonons. It was found that a correlation exists between the UV photoluminescence and the film quality. We suggest that measurements of the UV photoluminescence are useful for a characterization of sputtered ZnO films because this emission is observed in all as- deposited films.

Pulse dispersion and preheating effects in ultrafast photoconductive detectors: In0.53Ga0.47As as example

The finite longitudinal optical phonon emission time of hot carriers created by nonresonant excitation of short channel photoconductive detectors and the momentum distribution of these electrons are shown to be limiting factors for the response of the detector to an infinitely narrow pulse. Pulse dispersion of the order of 40 ps is caused in In0.53Ga0.47 As/InP detectors with 10-μm channel width after nonresonant excitation at 820 nm in agreement with most recent experiments. Resonant excitation is imperative for a photoconductive detector of ultimate time resolution and sensitivity.

Cooling of high density electron-hole plasma

We generalize for high density electron-hole plasma, the previous theories [12] of temperature cooling of non-equilibrium hot plasma. Especially we take into account the cooling by emission of mixed longitudinal optical phonon and plasmon modes, these quasiparticles described by a non-equilibrium distribution function. We show that a strong slowing of the plasma cooling occurs, at high electron-hole density. We calculate for CdSe the temperature kinetics of plasma created by Yag laser (pulse duration 30 ps).

Electron-acceptor luminescence from a non-Maxwellian distribution of photoexcited electrons in GaAs

Distribution functions are calculated for photoexcited electrons in GaAs, under conditions of continuous, monochromatic excitation. The lattice temperature is taken to be 1.2 K and the excitation intensity such that the density of photoexcited carriers is insufficient for the distribution to be affected by intercarrier scattering. A Boltzmann equation approach is used to take account of the effects of, injection of electrons into the conduction band, at an energy below the threshold for longitudinal optical phonon emission, scattering by acoustic phonons, via the deformation potential and piezoelectric interactions, and recombination. The equation is solved numerically using an iterative technique and the distribution functions are found to differ significantly from a Maxwellian form. Emission spectra due to conduction band to neutral acceptor transitions are derived from the computed distribution functions and are compared with recent experimental results.

Theory of transient high-field conductivity in polar semiconductors

The transient high-field conductivity of a simple model of polar semiconductors is studied. In this model, it is assumed that the only possible scattering mechanisms is the emission of longitudinal optical (LO) phonons. The time-dependent Boltzmann equation is solved analytically. The behavior of the distribution function is shown for a short period of time after the electric field is applied.

Ultraviolet luminescence of epitaxial zinc oxide films subjected to one- and two-photon excitation

A study was made of ultraviolet luminescence emitted from epitaxial zinc oxide films as a result of radiative annihilation of A (n =1) excitons and simultaneous emission of several longitudinal optical phonons after one- and two-photon excitation. Stimulated emission was observed at the frequencies of phonon replicas when the excitation was of the two-photon kind. The results demonstrated the possibility of constructing a tunable near-ultraviolet laser from an epitaxial ZnO film subjected to two-photon pumping.

On the momentum distribution of electrons in polar semiconductors for high electric field

AbstractAt zero temperature and low electron concentration the only important effect limiting the electric conductivity in a perfect polar semiconductor is the emission of real longitudinal optical phonons. An analytical method based on Boltzmann's equation is developed to obtain the electron momentum distribution in this situation. Resulting plots of the distribution function with parameters corresponding to InSb and E = 12 and 120 V/cm are given.

Direct measurement of polaron lifetime in degenerate InAs

We report the first direct measurement of the longitudinal optical (LO) phonon emission time, τLO, of electrons in degenerate InAs, using electron tunneling techniques on InAsoxide-Pb junctions. In the presence of a quantizing magnetic field, (H), the bias (V) dependence of the derivative of the junction conductance (d2I/dV2) shows oscillations reflecting the electron Landau levels in InAs. The amplitude of these oscillations shows a step-function decrease at V ≃ ±ħωo/e, where ħωo is the LO-phonon energy (30 meV) of InAs. This decrease is caused by a decrease in the electron relaxation time when LO-phonon emission becomes energetically allowed. If we let Al be the oscillation amplitude at |V| = ħωo/e when relaxation by LO-phonon emission is allowed and A2 be the oscillation amplitude at |V| = ħωo/e when relaxation by LO-phonon emission is forbidden, τLO is related to the ratio A2/A1 by: A2/A1 = exp[πm*c/τLOeH], where m* is the electron effective mass. We determine τLO by measuring this amplitude decrease as a function of H. For a sample having n = 5.5x1017/cm3, we obtain τ = (5.l±0.3)x10−13 sec.

Optical Pumping of Spin-Polarized Conduction Electrons and Inelastic Scattering by Neutral Acceptors

The energy relaxation mechanisms of conduction electrons in p-type GaSb at 1.9 °K are investigated by an optical pumping technique. Spin-polarized electrons are excited across the forbidden band gap with circularly polarized light. The number of photocreated electrons is obtained from the intensity of the photoluminescence; and the spin polarization from the degree of polarization of the photoluminescence. The experiment shows that the number of electrons and the spin polarization at the conduction band edge depend on the initial energy of the electrons and on the number of neutral acceptors. An explanation of the results is given in terms of two processes of energy relaxation for conduction electrons: (i) the emission of longitudinal optical phonons, and (ii) inelastic scattering by neutral acceptors.

The application of the magnetophonon effect to a study of hot electron phenomena in insb

The magnetoresistance of three samples of high purity n-type insb(nd-na varying from 5*1013 to 5*1014 cm-3) is studied at lattice temperatures of 11 and 20 k with electric field strengths sufficient (>100 mv cm-1) to heat the electron distribution appreciably above the temperature of the lattice and at magnetic field strengths such that the effect of landau quantization is apparent. Two distinct types of magnetophonon extrema are observed, each having a different dependence on electric field and temperature. The first consists of a series of minima which are only observed when the electron temperature is in excess of about 20 k and which arises from the emission of long wavelength longitudinal optical (lo) phonons.

Contribution of Excitons to the Edge Luminescence in Zinc Oxide

The photoluminescence of "pure" Zn0 at 77\ifmmode^\circ\else\textdegree\fi{}K has been investigated. Multiple-emission maxima, both "intrinsic" and "extrinsic," have been observed in the near ultraviolet. Narrow emission bands (half-widths \ensuremath{\le}3 \AA{}) near 3690 \AA{} are ascribed to the annihilation of bound (impurity) excitons. A broad asymmetrical band (half-width $\ensuremath{\approx}22$ \AA{}) near 3747 \AA{} is shown to be closely characterized by theory of the annihilation of free (intrinsic) excitons with the simultaneous emission of longitudinal optical phonons. Multiphonon emission bands, separated by approximately the energy of the longitudinal optical phonon, are observed at longer wavelengths. An emission band near 3677 \AA{} is suggested to originate from the "direct" annihilation of free (intrinsic) excitons.

Effects of optical phonons in the impurity photoconductivity of ZnTe

The effects of phonon assisted transitions and longitudinal optical phonon emission by excited carriers are observed in the impurity photoconductivity of ZnTe at low temperatures.

Oscillatory Photoconductivity in InSb

The oscillations in the low-temperature photoconductive spectral response of $p$-type InSb are found to be strongly dependent on electric field strength in both intrinsic and extrinsic photoconductivity. The oscillatory structure gradually disappears at electric fields higher the \ensuremath{\sim}20 V/cm. As a function of increasing temperature, the position of the minima of the oscillations shifts towards lower energies and the size of the oscillations decreases. A qualitative interpretation, consistent with all observations, shows that the primary reasons for oscillatory photoconductivity are: (a) the preferential momentum losses of carriers due to longitudinal optic phonon emission in the direction of the electrical potential gradient; (b) the fact that the steady-state distribution of carriers generated by photon absorption is drastically different from equilibrium.

Photoconductivity in P-type indium antimonide with deep acceptor impurities

The addition of Au, Ag or Cu impurities to InSb may be expected to produce two acceptor levels in InSb when these atoms with one valence electron replace In in the crystal lattice. The location of the energy levels has been determined by observing the extrinsic photoconductivity produced by the excitation of electrons from the valence band to the impurity levels and from the levels to the conduction band. The energy levels for Ag are in close agreement with those calculated by assuming a simple He atom model. The greater divergence for Au is attributed to the lattice distortion introduced when the larger Au atom replaces In in the crystal lattice. Oscillatory behavior of photoconductivity associated with charge carrier excitation to the lower level of all three impurities has been attributed to the successive emission of longitudinal optical phonons.