R. Evrard

Energy levels of bound polarons

Abstract A new variational wave function to describe the ground state and the excited states of a bound polaron is proposed. It is of the form |Ψ〉 = c|O〉|o n 〉 + ∑ g k ∗ V k ∗ (e i k·r − ρ k ∗ )a k + |O〉|o n . It is argued that this form is reasonable for all electron—phonon coupling α and all strengths β of the Coulomb potential. Numerical and analytical results are derived for the energy of the ground state and compared to existing results. Results for the energy of the lowest p-type excited state of the bound polaron are obtained.

Electrical conduction by interface states in semiconductor heterojunctions

Electrical conduction in semiconductor heterojunctions containing defect states in the interface region is studied. As the classical drift-diffusion mechanism cannot in any case explain electrical conduction in semiconductor heterojunctions, tunnelling involving interface states is often considered as a possible conduction path. A theoretical treatment is made where defect states in the interface region with a continuous energy distribution are included. Electrical conduction through this defect band then allows the transit of electrons from the conduction band of one semiconductor to the valence band of the second component. The analysis is initiated by electrical measurements on n-CdS/p-CdTe heterojunctions obtained by chemical vapour deposition of CdS on (111) oriented CdTe single crystals, for which current-voltage and capacitance-frequency results are shown. The theoretical analysis is based on the numerical resolution of Poissons equation and the continuity equations of electrons, holes and defect states, where a current component corresponding to the defect band conduction is explicitly included. Comparison with the experimental curves shows that this formalism yields an efficient tool to model the conduction process through the interface region. It also allows us to determine critical values of the physical parameters when a particular step in the conduction mechanism becomes dominant.

Zwanzig’s perturbation method applied to the thermodynamic properties of dense noble gases

Zwanzig’s perturbation method leads to power series in 1/T for the thermodynamic properties and the compressibility of simple gases or liquids. To check the convergence of these series, the precision of limited 1/T expansions is obtained by comparison with the experimental results for argon and xenon. The expansion coefficients are determined by minimizing the relative mean square deviations. The expansions for the thermodynamic functions have a remarkable precision, even when restricted to three terms (two for the internal energy). The relative deviation is typically of the order of 10−4 to 10−3 for the whole range explored experimentally (from normal conditions to rather far in the liquid state). The coefficients obtained by this procedure are functions of the density. The dependence is very smooth. It is shown that this dependence can be approximated by an expansion in powers of the density, restricted to two or three terms. Analytical expressions are obtained for these terms by means of a cluster type analysis. Three of these coefficients, the expression of which is rather simple, are calculated using the Lennard‐Jones potential as an a p r i o r i potential, and compared to the results of the fitting. For each coefficient the comparison gives a value of the hard‐sphere diameter which is an arbitrary parameter in Zwanzig’s method. The three values obtained in this way are in reasonable agreement.

Zwanzig's perturbation theory for the triangular-potential fluid

The perturbation theory presented by Zwanzig is applied to the evaluation of the thermodynamic properties of a fluid in which molecules interact by a triangular-potential well. Coefficients of the Zwanzig expansion are calculated by the method of Clippe and Evrard, and expressed in terms of a reduced density. These are numerically evaluated in a particular case for which Monte Carlo data have been reported by Card and Walkley. The density dependence of the coefficients is well represented by expressions given by Clippe and Evrard. Agreement between ‘experimental’ and theoretical results is satisfactory. In some region of the thermodynamic (density-temperature) plane, some disagreement exists either because of the scarcity of data or because of the truncated nature of the expansions. The convergence of the method is discussed. The results indicate that the good agreement found by Clippe and Evrard for their extension of the Zwanzig method in the case of real fluids is of intrinsic nature and has not been d...

Optical and electrical properties of MOVPE-grown ZnSe:N using triallylamine as a nitrogen precursor

Photoluminescence (PL), C-V and I-V characteristics of MOVPE-grown ZnSe :N doped using triallylamine (TAN) have been investigated. In our experiments, the use of TAN as dopant source does not lead to the incorporation of nitrogen into the lattice at the VI/II ratio optimized for the growth of undoped ZnSe. At smaller VI/II ratios a high concentration of donor states (10 17 -10 18 cm -3 ) appears leading to the formation of a broad PL band at 2.791-2.792 eV caused by a transition from the states formed by an overlap of the conduction band and donor states to the valence band. The acceptor states with an activation energy of 80 meV are most likely formed by oxygen contamination of TAN.

Measurements of transient photocapacitance and photocurrent on MOVPE-grown Au/ZnSe/GaAs heterostructures

The results of transient photocapacitance and photocurrent measurements performed on MOVPE-grown Au/ZnSe/GaAs heterostructures are reported for different photon energies and intensities of the monochromatic radiation. Thresholds in both spectra are observed at 1.4 eV. We attribute the photocapacitance and the photocurrent to the generation of electron-hole pairs in GaAs followed by trapping of the holes on interface states at the heterojunction between ZnSe and GaAs. In our interpretation, the stationary photocurrent is due to recombination of the trapped holes with electrons of the ZnSe conduction band.

Polar interface phonons in ionic toroidal systems.

We use the dielectric continuum model to obtain the polar (Fuchs-Kliewer like) interface vibration modes of toroids made of ionic materials either embedded in a different material or in vacuum, with applications to nanotoroids specially in mind. We report the frequencies of these modes and describe the electric potential they produce. We establish the quantum-mechanical Hamiltonian appropriate for their interaction with electric charges. This Hamiltonian can be used to describe the effect of this interaction on different types of charged particles either inside or outside the torus.

Electrical characterization of doped ZnSe-based heterostructures grown by MOVPE

The electrical characterization of Cl- or N-doped layers in the ZnMgSSe material system is reported. We used biscyclopentadienylmagnesium, dimethylzinc-triethylamine, tertiary-butylthiol, diisopropylselenide and ditertiarybutylselenide to grow layers by metalorganic vapor phase epitaxy at various low growth temperatures (330-440°C). Capacitance-voltage profiling, current-voltage, Hall, and transient photocapacitance measurements in conjunction with photoluminescence spectra were used to verify the layer properties. With intentional chlorine doping using 1-chlorobutane, free electron concentrations of up to 2 X 10 18 cm -3 in ZnSe and 1 X 10 17 cm -3 in ZnS x Se 1 -x (x= 4%) have been achieved. All C-V measurements on ZnSe:N doped with bistrimethylsilylamidozinc, trimethylsilylazide, or triallylamine show n-type or semi-insulating behavior, although PL spectra show acceptor-bound excitons and DAP recombinations. Compensation due to incorporated hydrogen originating from the precursor is assumed, besides a background chlorine contamination. However, Hall measurements of several samples doped with BTM and TAN indicate p-type conduction. Maximum hole concentrations of 5 X 10 17 cm -3 and Hall mobilities of 30 cm 2 /V. s were measured. Transient photocapacitance measurements confirm the presence of traps with very long time response, probably at the Au/ZnSe interface.

Photoluminescence of p-CdTe: characterization of impurity centers by phonon sidebands

A detailed characterization of the impurity centers involved in the photoluminescence (PL) of doped and undoped CdTe has been performed. Special attention has been devoted to the temperature dependence of the band between 1.54 and 1.55 eV observed in As and Sb doped samples. The longitudinal optical (LO) phonon replicas are characterized by a Huang-Rhys factor S. The various electron-hole recombination processes are explained by means of a simple analytic model correlating the position of the zero-phonon lines to the relative intensities of the phonon side-bands. The model accounts for the chemical shift of the defect centers and describes the effect of the charge carrier LO-phonon interaction in the framework of the adiabatic approximation within the envelope function approach.

Electronic Transitions and Relaxation Phenomena in Polarons

This is a series of four lectures concerning the excitation spectrum of polarons. The polaron model serves here to illustrate the effects of the interaction between elementary excitations. In the first lecture the Hamiltonian describing the polaron is derived.