Free Carrier Screening Research Articles

The hot-phonon effect on hot-electron energy relaxation in Hg0.8Cd0.2Te under magnetic quantisation at low temperatures

The energy relaxation of hot electrons in Hg0.8Cd0.2Te at 1.5 K is studied theoretically in the extreme magnetic quantum limit. The calculation includes band nonparabolicity, free-carrier screening, nonequipartition of acoustic phonons, and Landau level broadening due to electron-impurity interactions. It is shown that the nonequilibrium acoustic phonons have to be included to explain the experimental values of the energy relaxation time. The phonon lifetime is found to lie in the range 32 to 112 ns.

Optical properties of strained-layer superlattices with growth axis along

We show that large piezoelectrically induced electric fields strongly modify the optical properties of [111] growth axis strained-layer superlattices with growth axis along [111]. These internal fields reduce the superlattice band gap, change optical matrix elements, and cause a spatial separation of photoexcited electrons and holes so as to screen the fields. Very large (${\mathrm{\ensuremath{\chi}}}^{3}$\ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}1}$ esu) optical nonlinearities occur because of the free-carrier screening of the strain-generated internal electric fields.

Theory of photoabsorption in modulation-doped semiconductor quantum wells.

A systematic study of the electronic and optical properties of modulation-doped GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum wells is undertaken. We consider cases in which the ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As barriers are doped either n or p type and the GaAs wells are filled with a gas of free carriers. The electronic band structure, exciton binding energy, exciton oscillator strength, and interband absorption are studied as functions of well width and doping concentration. A multiband effective-mass method is used which takes coupling between heavy- and light-hole states into account. In our model for excitons we include effects of valence-subband nonparabolicity, free-carrier screening, and the k dependence of optical matrix elements. The interband optical absorption is obtained using Fermi's golden rule with a correction made for the screened final-state interaction. Our theoretical results are compared with available experimental data.

Theory of energy relaxation of 2D hot carriers in GaAs quantum wells

Abstract The energy relaxation rate of two-dimensional (2D) hot carriers due to LO phonon emission is explored theoretically by assuming a simple quantum-well structure for GaAsAl x Ga 1− x As superlattices. The differences of energy loss rate between electrons and holes are shown with emphasis on the roles of free carrier screening and form factors. The picosecond cooling behavior of photoexcited 2D carriers is presented on the basis of the polar carrier-LO phonon coupling process.

Theory of energy relaxation of 2D hot carriers in GaAs quantum wells

The energy relaxation rate of two-dimensional (2D) hot carriers due to LO phonon emission is explored theoretically by assuming a simple quantum-well structure for GaAsAl x Ga 1− x As superlattices. The differences of energy loss rate between electrons and holes are shown with emphasis on the roles of free carrier screening and form factors. The picosecond cooling behavior of photoexcited 2D carriers is presented on the basis of the polar carrier-LO phonon coupling process.

Improvement of debye‐hückel evaluation of free carrier screening effects in semiconductors

AbstractMost of the applications of the “energy loss” method for free carrier mobility calculations are based on Debye‐Hückel approximation for the evaluation of the free carrier screening properties. However, it is well known that, indeed, this approximation is fully available in a relatively high temperature range. The aim of this paper is to estimate, from Lindhard's dielectric function and in the effective mass approach some corrective terms which allow us to improve this approximation and consequently to extend the range of applicability of mobility calculations by the “energy loss” method.

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.

Optical phonon interaction effects in layered semiconductor structures

Various aspects of the electron-LO-phonon interaction effects on the electronic properties of a single two-dimensional electron layer (as occurring, for example, in artificially structured single quantum wells or heterojunctions made of III–V or II–VI semiconducting materials) are discussed theoretically. In particular, perturbation theory is carried to the second order in the coupling constant to obtain the two-dimensional polaron energy in the weak-coupling limit. Intermediate coupling (the so-called Lee-Low-Pines theory) and strong coupling theories for the two-dimensional polaron problem are developed and interpolation (Padé approximations) formulae valid for arbitrary coupling are derived. Effects of the band non-parabolicity and of the free-carrier screening on the weak-coupling theory are discussed. The real and the imaginary parts of the two-dimensional polaron self-energy are obtained in a many-body perturbation calculation. Comparison with the known three-dimensional results is made wherever possible, showing that the electron-LO-phonon interaction effects are substantially enhanced in confined structures. Explicit formulas valid for two-dimensional systems are given for various polaron parameters like the binding energy, the effective mass, the scattering rate, the average phonon density in the polaron cloud, etc.

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.

Evidence of bandgap-narrowing in the space-charge layer of heavily doped silicon diodes

The gradient voltage has been measured for seven heavily doped, graded-junction silicon diodes at 300 K. Experimental values up to nearly 0.5 V lower than conventional theoretical predictions have been observed. The lowering is attributed to bandgap-narrowing in the space-charge region. This narrowing is expected to be much larger than in neutral material of the same doping density because of the absence of free-carrier screening.

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.

A contribution to the theory of the open-circuit photovoltage of an abrupt p- n junction

The theory for calculating the open-circuit photovoltage of an abrupt p- n junction, as originally given by Parrott [5], is generalized to include degeneracy and heavy doping effects. A useful approximation is given which simplifies the general theory considerably. Results are presented for silicon assuming rigid bands with parabolic density of states functions and assuming the free-carrier screening model for the bandgap narrowing.

Electronic power transfer in pulsed laser excitation of polar semiconductors

A time-resolved transport analysis of highly nonequilibrium photoexcited carrier-phonon systems is presented. It is shown that amplification of the most strongly coupling optical-phonon modes leads to a drastic reduction of the intraband cooling rates of the electron-hole plasma. As a consequence and in contrast to earlier expectations, free-carrier screening of the long-range Fr\"ohlich couplings turns out to play only a secondary role. The theory is free of adjustable parameters and in qualitative agreement with hitherto unexplained experimental results from time-resolved transmission spectroscopy of gallium arsenide.

Dynamic dielectric response to electron-hole and electron-electron interactions

The interaction potential between two charged particles in a dielectric medium is shown to have the form $U(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}},{\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}}_{\mathrm{c}.\mathrm{m}.})=\frac{V(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}})}{\ensuremath{\epsilon}(q,\ensuremath{\omega}=\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}},{\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}}_{\mathrm{c}.\mathrm{m}.})}$, where $V(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}})$ is the bare Coulomb potential, ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}}_{\mathrm{c}.\mathrm{m}.}$ is the center-of-mass velocity, and $\ensuremath{\epsilon}(q,\ensuremath{\omega})$ is the dielectric constant of the medium. An analysis of the dielectric screening is performed, incorporating core electron, lattice ion, and free-carrier contributions. The random-phase-approximation dielectric constant for arbitrary degeneracy is employed in calculating the free-carrier component of $\ensuremath{\epsilon}(q,\ensuremath{\omega})$. It is found that although screening of the bare potential by lattice ions may sometimes be evaluated in the static or high-frequency limits, it is virtually always a poor approximation to treat the free-carrier screening in either of these limits. Dynamic screening processes have been incorporated into a calculation of relaxation times for electron-hole scattering in semiconductors for which ${m}_{h}\ensuremath{\gg}{m}_{e}$. Sample calculations for GaAs at a wide variety of temperatures and photoexcited carrier densities show that dynamic screening has a significant effect on the relaxation time in most regimes. It is equally important to treat the screening of electron-electron interactions dynamically when calculating transport properties which are sensitive to electron-electron scattering.

Free carrier screening of the Fröhlich interaction in GaAs

Picosecond luminescence studies of hot carrier thermalization are reported for GaAs. We find a slowing of the thermalization of the photoexcited hot carriers at highest excitation intensities. This reduced cooling is explained by free carrier screening of the Fröhlich interaction of electrons and holes with polar optical phonons.

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.

Electron mobility in low-temperatureHg1−xCdxTeunder high-intensity CO2laser excitation

A photo-Hall technique has been employed to determine the dependence of the low-temperature electron mobility in narrow-gap ${\mathrm{Hg}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}\mathrm{Te}$ ($x\ensuremath{\approx}0.2$) on optically generated carrier density. Excitation is provided by C${\mathrm{O}}_{2}$ laser pulses of 200-nsec or 25-\ensuremath{\mu} sec duration. At low excitation levels the mobility is found to increase due to the neutralization of ionized acceptors by the photoexcited holes. At higher excitation levels the mobility decreases due to electron-hole scattering. Comparison is made to a theory which fully incorporates the Kane band model in treating the scattering of electrons by ionized impurities, photoexcited holes, and compositional disorder. The adaptation of the partial-wave phase-shift method to a nonparabolic band structure is discussed. The treatment of electron-hole scattering incorporates a recently developed theory which accounts in detail for the dynamic dielectric response of the lattice polarizability and free carrier screening. With the use of the random-phase-approximation dielectric constant $\ensuremath{\epsilon}(q,\ensuremath{\omega})$ for arbitrary degeneracy, it is found to be particularly important that the screening by photoexcited holes be treated dynamically. Because the mobility increase at low excitation levels is highly sensitive to the number of acceptors present in the sample, the photo-Hall technique is quite promising as a means of accurately determining compensation densities in narrow-gap ${\mathrm{Hg}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}\mathrm{Te}$.

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.

A Neutron Scattering Study of Lattice Dynamics of HgTe and HgSe

The dispersion relations for the acoustic and optic phonons in HgTe and for the acoustic phonons in HgSe were determined by neutron inelastic scattering in three high symmetry directions. The effect of the free-carrier screening of the long-range electric field of LO phonons in HgTe was observed. The formalism of the rigid ion model is used for numerical calculations of the phonon dispersion relations and the phonon densities of states in HgTe and HgSe.