Free Carrier Contribution Research Articles

Effect of ordering of mobile oxygen on spectra of two-magnon and electronic Raman scattering of light in YBa2Cu3O6+x crystals with different doping levels

The effect of ordering of mobile chain-site oxygen in YBa2Cu3O6+x crystals at different doping levels x on the kinetics of the intensity change of the two-magnon line and the extended structureless electronic continuum in optical Raman spectra and on the superconducting transition temperature Tc, has been studied in detail. An increase in the chain-site oxygen content x leads to a higher contribution of free carriers to the electronic continuum in Raman spectra. The kinetics of the electronic continuum becomes slower with x, whereas the relaxation rate of the two-magnon scattering is a nonmonotonic function of the stoichiometric index. Computer simulations of the relaxation of nonequilibrium states using the Monte Carlo technique qualitatively describe the kinetics observed in experiments. Our results lead us to a conclusion about local inhomogeneities in the electronic and spin systems in CuO2 planes with scales of several lattice constants.

Effect of disorder on the temperature dependence of radiative lifetimes in V-groove quantum wires

We have studied the effect of disorder on the radiative properties of semiconductor quantum wires by time-resolved photoluminescence spectroscopy. The dependence of the radiative lifetimes is measured over a temperature range extending from 8 to 150 K. At low temperatures we find that the measured dependence does not conform to the theoretical prediction for recombination of free excitons in a quasi-one-dimensional system due to the major influence of disorder. An extension of the theory is developed in order to take into account exciton ionization and the contribution of free carriers. A mean localization length for excitons is also estimated from the radiative lifetime at 8 K.

Infrared reflection in the Reststrahlen region of GaN MBE grown epitaxial layers on Si substrates

Abstract Infrared reflectivity in the region of the Reststrahlen band has been investigated for molecular beam epitaxy (MBE) grown GaN epitaxial layers on Si substrates with and without an AlN buffer layer. Deformation of the Reststrahlen band due to free carriers has been observed depending on whether the GaN layer was grown on an AlN buffer layer or directly on the Si substrate. A Lorentz oscillator for the phonon contribution and a Drude–Lorentz model for free carrier contribution to the complex dielectric function has been assumed in order to calculate and fit the reflectivity to the measured spectra. Free carrier concentration and mobility has been evaluated from fitting parameters. The layers grown on AlN buffer layer have lower carrier concentration and a by one order of magnitude higher carrier mobility than the layer grown without AlN buffer layer.

π-band contribution to the optical properties of carbon nanotubes: Effects of chirality

We study the \ensuremath{\pi}-band contribution to the optical properties of the carbon nanotubes based on the tight-binding model, including optical absorptions, optical rotatory power, and circular dichroism. By taking into account not only the interband contributions but also the free-carrier contributions, the dielectric functions and the third-rank tensor responsible for the optical activity are calculated. The following have been shown. (i) The features of the dielectric functions are consistent with the ellipsometry experiments by de Heer and co-workers [Science 268, 845 (1995)]; (ii) The calculated plasma frequency is of the same order of magnitude as the one recently observed by Bommeli et al. [Solid State Commun. 99, 513 (1996)]. (iii) Chiral nanotubes are, as expected, optically active and the spectra of their optical rotatory power (RP) and circular dichroism (CD) are highly oscillatory. (iv) Nanotubes with a diameter about 4 nm can give RP and CD that are of the same order of magnitude as those of certain organic compounds. (v) The RP and CD decrease as the diameter increases.

Characterization of MOVPE grown InPSb/InAs heterostructures

InPSb was grown by low pressure metal-organic vapour phase epitaxy (LP-MOVPE) on InAs at temperatures between 520°C and 570°C. The samples were analysed by Raman and photoluminescence (PL) spectroscopy and high resolution X-ray diffraction (XRD). XRD as well as PL spectroscopy demonstrate the high quality of the deposited material. Sb contents of x = 0.31–0.34 in InP1−xSbx were investigated and lattice matching on InAs was achieved as controlled by XRD. Raman spectra of micron thick InPSb layers display InSb-like and InP-like phonon features and an additionally weak structure related to liberated Sb. p-type doping of the InPSb with diethylzinc or H2S causes a change of the lattice constant as indicated by XRD. The Raman spectra of the doped samples reveal changes in the lineshape of especially the InP-like phonon structure. Moreover, these changes differ for n- and p-type doping, which is interpreted in terms of free carrier contributions to the spectra via plasmon-phonon coupling. The predicted miscibility gap and its consequences for future opto-electronic applications are discussed with respect to the growth conditions and the doping.

Exciton saturation in room temperature GaAs/AlGaAs multiple quantum wells

We report time resolved studies of optically induced circular dichroism in room temperature GaAs/AlGaAs multiple quantum wells to resolve spin-dependent and spin-independent contributions to exciton saturation. Phase-space filling and Coulomb contributions were separated by spin dependence while the effects of broadening and screening were distinguished using different pulse bandwidths. Bound and free carrier contributions were compared by observing the temporal dependence of the circular dichroism on femtosecond timescales. The spin independent contribution to exciton saturation was found to be independent of whether the carriers were bound or free.

Femtosecond time-resolved Z-scan investigations of optical nonlinearities in ZnSe

Degenerate (single-color) and nondegenerate (two-color) time-resolved Z-scan techniques, with femtosecond time resolution,have been implemented to measure the magnitude and dynamical processes of the optical nonlinearities of ZnSe. In the time-resolved Z-scan spectra a reversal of sign in the refractive-index change Deltan as a function of time delay was observed. A bound electronic contribution with a resolution limited response s,200 fs) gave rise to a positive Deltan at zero time delay, whereas a free-carrier contribution resulted in a negative Deltan at longer time delays. The free-carrier contribution to Deltan has two initial rapid relaxation components with time constants of a few picoseconds and a few tens of picoseconds, respectively. The longlived carriers also contributed to a slow relaxation component in Deltan on a nanosecond time scale.

Origin of the resonant optical Kerr nonlinearity in Cd(S, Se)-doped glasses and related topics

After having reported preliminary results related to saturation, we first theoretically consider the various mechanisms contributing to the resonant optical Kerr effect in Cd(S, Se)-doped glasses. We obtain the expression for the expected effective susceptibility in different possible cases. This nonlinearity is studied experimentally using optical-phase conjugation in the low-intensity regime. We show that, by time resolving the nonlinear response of such glasses having experienced various degrees of photodarkening, we can clearly assess the origin of the resonant optical Kerr effect in these materials. Usually, a combination of a fast free-carrier contribution due to particles without traps and of a slow trapped-carrier one due to particles with traps is observed. For the free-carrier contribution, induced absorption is observed to be almost as important as absorption saturation. We also report frequency-dependent measurements and discuss the change in absorption spectrum and the increase of the nonradiative decay rate that accompany darkening.

Anisotropy of the thermal conductivity in blue bronze

We have measured the thermal conductivity of K 0.3MoO 3 (blue bronze) in the chain direction and in both perpendicular directions from 80 to 300 K. As expected, it shows an anisotropic behaviour in lattice contribution as well as in free carrier contribution. These results are in favor to our previous explanation that there are residual anomalies in thermal conductivity resulting from the contribution of low-frequency phasons of rather large velocities.

Recombination processes and photoluminescence intensity in quantum wells under steady-state and transient conditions.

We present a theoretical and experimental study of recombination processes in quantum wells. Our model calculations, which include free-excitonic, free-carrier, and defect-mediated nonradiative recombination, describe the dependence of the photoluminescence intensity on excitation density under steady-state conditions and on time under transient conditions. For the former conditions, it is not, in principle, possible to distinguish between excitonic and free-carrier contributions to the photoluminescence intensity. However, an accurate determination of the relative weight of nonradiative and radiative contributions can be made. For transient conditions, on the other hand, excitonic contributions may be identified, while the discrimination between radiative and nonradiative contributions is---though possible---possessed with ambiguities. We finally apply our model to a set of experimental data, taken at 300 K from a single ${\mathrm{In}}_{0.1}$${\mathrm{Ga}}_{0.9}$As/${\mathrm{Al}}_{0.33}$${\mathrm{Ga}}_{0.67}$As quantum well under both steady-state and transient conditions. The results of this analysis demonstrate the consistency of our model and its potential for the quantitative understanding of the recombination dynamics in quantum wells.

Electron-irradiated silicon: An optimized material for gyrotron windows

The special potential of high resistivity (HR) silicon for electron cyclotron wave transmission components for nuclear fusion is related to the dominant contribution of free charge carriers to the mm-wave absorption. It is demonstrated by dielectric property measurements that electron irradiation can practically overcome the problems of special concern for applications, such as the sensitivity of dielectric loss (tan δ) to ionizing radiation and the complex temperature dependence of tan δ. Also the loss levels are lowered at room temperature. The improvements are correlated with the total dose of electron irradiations and are of long term stability. It is inferred that new structural defects are formed, which act on the charge carriers as centers of trapping, recombination or scattering. As a consequence, tan δ values are measured at 145 GHz that fall below those of sapphire in the range of 120–350 K.

Optical studies of free carrier system in high- Tc superconductors

The optical response of high- T c superconductors (HTSC) in mid- and near-IR region has been investigated by spectroscopic ellipsometry in order to reveal the free carrier contribution. The experimental data have been analyzed in pseudodielectric function (PDF) approximation. A relationship between atomic microstructure and free carrier system has been studied in two groups of HTSC, Bi-based (2212)- type and Tb-doped (123)-type single crystals.

Excitonic and free-carrier polarizations of bulk GaAs studied by femtosecond coherent spectroscopy.

The transient third-order polarization at the band gap of undoped and p-doped GaAs is investigated by spectrally and temporally resolved four-wave mixing. Excitonic and free-carrier contributions simultaneously excited within the bandwidth of the 100-fs pulses are clearly distinguished by their different spectral envelopes. The excitonic part dominates at carrier densities below ${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ and shows a time evolution governed by exciton--free-carrier scattering and by many-body effects. At higher density, the free-carrier polarization has a strength similar to the exciton contribution and exhibits a spectrum resonant to the femtosecond pulses with a photon-echo-like temporal behavior. The data are analyzed by a numerical solution of the semiconductor Bloch equations including an ensemble Monte Carlo simulation of the scattering dynamics of the carriers. The theoretical model is in good agreement with the experimental results.

Optical bistability in an InGaAs/InP multiple quantum well waveguide Fabry–Perot cavity

We report measurements of quasi-cw optical bistability in an InGaAs/InP multiple quantum well waveguide Fabry–Perot cavity at 1.47 μm wavelength. This operating wavelength is detuned from the band-edge resonance by 0.07 μm, where an enhanced free-carrier contribution to the nonlinear refractive index due to 2D confinement of carriers in the quantum wells accounts for the large index variations. The effect of any thermally induced nonlinear response is negligible.

Coherent optical polarization of bulk GaAs studied by femtosecond photon-echo spectroscopy.

The nonlinear polarization close to the band gap of GaAs is studied by spectrally and temporally resolved four-wave mixing. Excitonic and free carrier contributions both excited within the bandwidth of the 100 fs pulses are distinguished for the first time. The excitonic part dominates at carrier densities below ${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. At higher density, nonthermalized free carriers give rise to an additional component resonant to the pulse that shows a photon-echo-like time behavior. Monte Carlo simulations including the coherent polarization and the scattering dynamics of the carriers account for the data.

Faraday rotation analysis of HgCdTe

Faraday rotation was measured in liquid-phase epitaxial (LPE) Hg1-xCdxTe (x approximately=0.22) at CO2 laser photon energies over the temperature range of 77 K to 300 K for magnetic fields up to 16 kG. The authors find that the Faraday rotation is composed of free-carrier, interband and spin-induced contributions. At 77 K, the interband and spin contributions dominate, resulting in large observed Faraday rotation, i.e. of the order of 102 deg cm-1 kG-1. All three contributions are needed to interpret the temperature and magnetic field dependence of the Faraday rotation in the samples. Resonant behaviour observed in the rotation angle is successfully described by impurity-to-band magneto-optical absorption. They conclude that Faraday rotation could serve as a viable technique for the non-contact measurement of carrier concentration in Hg1-xCdxTe.

Phason contribution to the thermal conductivity of CDW systems

We claim that there is an additional contribution to the thermal conductivity in excess of the normal phonon and free carriers contribution in charge-density-wave (CDW) systems below Peierls transition which we attribute to the phase oscillations of (CDW) order parameter. We demonstrate for two chosen CDW systems, K 0.3MoO 3 and (NbSe 4) 10I 3, that the observed increase in the thermal conductivity corresponds to the temperature range where the screening of the Coulomb interactions is sufficient to allow the contribution of the weakly damped phasons with large velocity.

Lattice vibrations of Y1− xPrxBa2Cu3O7: theory and experiment

We report measurements of the far-infrared reflectance of Y 1− x Pr x Ba 2Cu 3O 7 thin films. Detailed understanding of the experimental lineshapes is achieved with the proper treatment of the SrTiO 3 substrate, the finite film thicknesses, the background contribution of free carriers and the mid-infrared band, and the phonon modes. Shell-model calculations predict the frequencies of the infrared-active phonons to be very similar for YBa 2Cu 3O 7 and PrBa 2Cu 3O 7 with the exception of rare-earth-related vibrations. The experimental frequencies of the in-plane as well as z-polarized infrared-active phonons of PrBa 2Cu 3O 7 agree satisfactorily with the calculated ones. An in-plane mode at 270 cm -1, assigned to vibrations confined to the Y/Pr and neighboring oxygen ions, is found to be sensitive to the rare-earth-ion substitution. We report also the frequencies of the Raman-active B 2g and B 3g phonons of PrBa 2Cu 3O 7, thus giving a nearly complete picture of all k=0 eigenmode frequencies in the RBa 2Cu 3O 7 crystal structure.

Carrier-dependent nonlinearities and modulation in an InGaAs SQW waveguide

The authors report measurements of optically induced carrier-dependent refractive index changes and their saturation in an InGaAs single quantum well centered within a linear multiple quantum well guided-wave Fabry-Perot resonator using diode laser sources. A low-excitation nonlinear refractive cross-section, sigma /sub n/=-1*10/sup -19/ cm/sup 3/, was deduced for probe wavelengths near the TM (transverse magnetic) absorption edge, falling only to sigma /sub n/=-3.1*10/sup -20/ cm/sup 3/, at over 0.16 mu m from the band edge. For an incident irradiance of 18 kW/cm/sup 2/, refractive index changes in the InGaAs quantum well as large as -0.16 were deduced near the absorption edge, while the index change at a wavelength 0.16 mu m from the absorption edge was -0.055. This large off-resonant index change is attributed to an enhanced free-carrier contribution within a 2D system.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Free carrier contribution to dynamic dielectric function of heavily doped semiconductors. Application to n‐type silicon

AbstractA new approximation to calculate the free carrier contribution to the infrared dielectric function in n‐type silicon at room temperature that interpolates the quantum and classical absorption regimes is proposed. This approximation generalizes the Drude‐Lorentz approximation by introducing a frequency dependence of the relaxation parameter which is calculated via Mori formalism. Calculations of carrier concentration and wavelength dependences of the relaxation parameter, dielectric constant and optical constants for concentration intervals from 1.3 × 1015 to about 1.3 × 1020 cm−3 and in the wavelengths range from 1 to 30 μm are performed. The deviations from the Drude‐Lorentz approximation are discussed.