Effect Of Free Carrier Screening Research Articles

Photovoltaic spectroscopic study of GaN epilayers and InGaN quantum well structures

Room-temperature photovoltaic spectroscopy was applied to study undoped GaN, n-type GaN, and InGaN quantum well structures. Clear exciton absorption was observed in the photovoltaic spectra of the undoped GaN, and polarization measurements were made to identify the exciton absorption. For the n-type GaN sample, instead of the exciton absorption we observed only bulk absorption edge, which may be due to the free carrier screening effect. For the InGaN quantum well structures, the photovoltaic spectra showed relatively complicated line shape due to the overlap of the signals from different layers. By changing the reference phase of the lock-in amplifier, we were able to suppress some of the signals and thus identify the origin of the corresponding signal.

TRANSIENT DECAY OF PHOTOEXCITED CARRIERS AND PHOTOLUMINESCENCE EFFICIENCY IN QUANTUM WELLS

We present a phenomenological model for the recombination of photoexcited carriers in quantum wells,which takes into account of the nonradiative recombination and the screening effect of free carriers in the wells.The results show,that the decay time of photoluminescence is strongly related to the sample quality,doping level,as well as the excitation intensity.

Energy loss rate of hot electrons in n-InSb in the extreme quantum limit at low temperatures

The energy loss rate of hot electrons in n-InSb in the extreme quantum limit at low temperatures has been calculated to interpret the experimental results. The theoretical calculation includes the band non-parabolicitym the non-equipartition of phonons and the Lanbdau level broadening due to electron-impurity interactions. The effect of free carrier screening on the electron-phonon interactions is also included. The modification of free carrier screening due to magnetic quantisation has also been considered. The energy loss rate is calculated assuming that the acoustic phonon scattering via a deformation potential and piezoelectric scattering are the dominant loss mechanisms at low temperatures. The theoretical values are found to be higher than the experimental values. The modified free carrier screening due to magnetic quantisation overestimates the energy loss rate compared to the classical Debye screening.

Exciton-related optical nonlinearities in semiconductors and semiconductor microcrystallites

We report on large optical nonlinearities due to specific many-body effects in high-density electron-hole pair systems. On bulk CdS we study the effect of free-carrier screening on the absorption and refraction in the vicinity of the band-gap at room temperature. Steady-state saturable absorption at mW-power levels and four-wave mixing with first-order efficiencies as large as 2% of the incoming light are demonstrated on a 1μm slab. On CuBr microcrystallites embedded in glass we investigate the changes of the exciton absorption caused by many-exciton effects both at cryo and room temperature. In contrast to bulk semiconductors, we observe a blue shift of the exciton peak at resonant optical excitation. In addition, strong saturation of absorption with very large contrast shows up. We find Lorentzian saturation intensities in the 100 kW/cm2 range.

Screening of the exciton-LO phonon interaction in [formula omitted] quantum wells

The Fröhlich interaction of an exciton with longitudinal optic (LO) phonons gives rise to phonon satellites ‘ X LO ’ of the exciton photoluminescence (PL) line ‘ X’. We report the effect of free carrier screening of the Fröhlich interaction on the strength of X LO in a Schottky gated, modulation doped, In 0.53 Ga 0.47 As InP quantum well (QW). We discuss the differences between our results and the behaviour predicted for the screening of other quantities in QWs.

Effect of free-carrier screening on the ultrafast relaxation kinetics in hot polar semiconductors.

The effect of free-carrier screening on the kinetics of evolution of relaxation processes in non- equilibrium photoexcited plasma in polar semiconductors is investigated. We compare numerical results obtained for GaAs under three different assumptions: neglecting screening effects, using a screening factor in the Debye-H\"uckel approximation, and introducing screening effects by means of a dielectric function calculated in the random-phase approximation.

Screening of polar interaction in quasi-two-dimensional semiconductor microstructures.

The effect of free-carrier screening on polarons in semiconductor heterostructures and quantum wells is considered with particular reference to GaAs-based experimental systems. It is shown that screening has an appreciable effect on quantities like the polaronic binding energy and the effective-mass renormalization as well as on the polar scattering rate. Different models for the screened polar interaction are compared quantitatively and the importance of using wave-vector-dependent screening in the theory is established.

The effect of free-carrier screening on indirect optical transitions in semiconductors

The effect of free-carrier screening on the indirect optical transitions in semiconductors, both intra- and interband, is studied in detail. The formalism allows for slowly and rapidly varying (in space) static perturbations of the crystal potential (ionised impurities, chemical disorder in alloys, etc.), as well as for the difference between the static and high-frequency dielectric constant of the crystal.

Effect of free carrier screening on LO-phonon emission time

The decay of optically pumped LO phonons in far from equilibrium polar semiconductors is analyzed. We define a LO-phonon emission time in terms of the thermodynamic variables which describe the nonequilibrium state of the system. It is discussed its dependence on effective carrier temperature and mode wavevector, and the influence of carrier screening on Fröhlich interaction.

Effect of free carrier screening on the electron mobility of GaAs: A study by field-effect measurements

By using a field effect technique, the electron mobility in GaAs at 300K has been studied in two systems where the free electron concentration may be varied over a wide range while the density of ionized impurities is kept constant. These are (a) a thin, almost fully depleted n-type layer on a semi-insulating substrate and (b) a GaAs/Al xGa 1−xAs modulation doped heterojunction. In both cases the mobility falls by more than a factor of two as the free electron concentration is reduced, due to loss of screening.

Effect of free carrier screening on the binding energy of D − centers in polar semiconductors

A calculation of the binding energy of a D − center, defined as the energy required to remove one of the two electrons from the D − center to infinity, in the presence of other free carriers is reported in polar crystals. It is assumed that the effective interaction between each of the two electrons and the positive ion is described by Thomas-Fermi potential. The effective interaction between the two electrons in polar crystals is, however, described by two recently proposed potentials. The binding energy of a D − ion is calculated variationally in several polar crystals as a function of the screening parameter δ and the value of δ at which the binding energy goes to zero ( D − Mott transition) is determined. A possible experimental situations where this system may be studied is discussed.

Low-field electron mobility and thermoelectric power inIn1−xGaxAsyP1−ylattice-matched to InP

Electron drift and Hall mobility, and thermoelectric power in In 1-x Ga x As y P 1-y , lattice-matched to InP, are calculated for different temperatures and compositions. The two-mode nature of the polar optic phonons is considered jointly with deformation-potential acoustic, piezoelectric, alloy, ionized-impurity, and electron-electron scattering. Band nonparabolicity, admixture of p functions, arbitrary degeneracy of the electron distribution, and the screening effects of free carriers on the scattering probabilities are incorporated. The Boltzmann equation is solved by an iterative method using the currently established values of the material parameters. The agreement with the available experimental data is found to be satisfactory.

Photoluminescent Properties of CdSe Single Crystals Doped with Donor Impurities

The photoluminescent properties of CdSe single crystals have been investigated in the vicinity of the fundamental absorption edge, varying the concentration of In or Cl impurity over a wide range. Band-to-band emission is observed in crystals with carrier concentrations above 1×1018 cm-3. Both the peaks of the band-to-band emission and the edge emission shift toward shorter wavelengths with increasing carrier concentration. These peak shifts are discussed by taking into consideration the movement of the Fermi level, the shrinkage of the band gap, the screening effect of free carriers, and so on. The emission intensity in the high energy region has the maximum value when the carrier concentration is around 1×1018 cm-3. The emission intensity increases noticeably on annealing the crystal in molten Cd.

Lattice Dynamics ofMg2Pb at Room Temperature

The phonon-dispersion curves along the [001], [110], and [111] directions in ${\mathrm{Mg}}_{2}$Pb at room temperature have been measured by neutron inelastic scattering. Although these curves are qualitatively similar to those for ${\mathrm{Mg}}_{2}$Sn, marked differences exist in some of the longitudinal branches which show large dips at small $q$ for ${\mathrm{Mg}}_{2}$Pb, suggesting large screening effects of free carriers. A reasonable fit to the measured dispersion curves has been obtained with a nine-parameter shell model. The model was used to calculate the phonon density of states and the temperature dependence of the Debye temperature. The calculated Debye temperature agreed well with the Debye temperature obtained from heat-capacity measurements.

Exciton luminescence and the screened coulomb interaction in CdS

A simple theoretical model describing the screening effects of free carriers on the electronhole Coulomb interaction predicts a decrease in exciton binding energy with increasing free carrier concentration. Within the framework of this model, then, one would expect to observe shifts in the energy of the exciton photoluminescence for different levels of exciting light. Experimental observations of photoluminescence as a function of exciting light intensity for CdS reveal that such shifts exist, thus supporting the screening model.