Strong linear- k valence-band mixing at semiconductor heterojunctions

This paper examines linear- k terms in the gamma(8) valence-band Hamiltonian for heterostructures of zinc-blende-type semiconductors. In bulk crystals such terms are known to be extremely small, due to their origin as relativistic perturbations from d and f orbitals. However, in heterostructures there is a nonvanishing contribution from p orbitals. This contribution is an order of magnitude larger than the corresponding bulk term, and it should give rise to an optical anisotropy comparable to (although smaller than) that seen in recent experiments on the quantum-well Pockels effect.     

Influence of copolymer interface orientation on the optical emission of polymeric semiconductor heterojunctions

We have examined the Coulombic interactions at the interface in a blend of two copolymers with intramolecular charge-transfer character and optimized band offsets for photoinduced charge generation. The combination of both time-resolved measurements of photoluminescence, and quantum-chemical modeling of the heterojunction allows us to show that relative orientation across the heterojunction can lead to either a repulsive barrier ( approximately 65 meV) or an attractive interaction which can enhance the charge-transfer processes. We conclude that polymer orientation at the heterojunction can be as important as energy-band offsets in determining the dynamics of charge separation and optical emission.     

Exciton optics of a disordered semiconductor

Interband light absorption in a disordered semiconductor is considered with due allowance for exciton effects. Allowance is made for the scattering of the electron and hole in a weak static random field and also for collisions of the second kind experienced by the exciton on interaction with a random field. It is shown that the contribution of these latter to the half width of the absorption line of the exciton spectrum (even at temperatures of the order of the exciton ionization energy) is considerably smaller than the contribution arising from the scattering of the particles forming the exciton.     

Exciton relaxation in self-assembled semiconductor quantum dots

The present study focuses on the effect of excited states on the exciton–polaron spectrum for self-assembled InAs/GaAs semiconductor quantum dots. The analytical model takes into account the Coulomb interactions between the electron and the hole as well as, each carrier, the coupling with the longitudinal optical phonon field. Furthermore, the key role played by the exciton continuum in the dot spectrum is also introduced. Such an approach is well fitted to analyze recent experimental findings about single-dot spectroscopy and allows peaks assignment, line width estimation, relaxation time evaluation, etc., necessary steps toward an understanding of the internal dynamics of quantum dots.     

Exciton/plasmon mixing in metal–semiconductor heterostructures

We report on the strong coupling between surface plasmons and inorganic quantum well excitons. The sample is formed by a corrugated silver film deposited on the top of a heterostructure consisting of five GaAs/GaAlAs quantum wells grown by molecular beam epitaxy. Reflectometry experiments at low temperature (77 K) evidence the formation of plasmon/heavy-hole exciton/light-hole exciton mixed states. The interaction energies, deduced by fitting the experimental data with a coupled oscillator model, amount to 22 meV for the plasmon/light-hole exciton and 21 meV for the plasmon/heavy-hole exciton. Some particularities of the plasmon–exciton coupling are also discussed and qualitatively related to the plasmon polarization.     

Electrical characterization and fabrication of organic/inorganic semiconductor heterojunctions

Thin films of polyaniline (PANI) titanium dioxide (TiO₂) nanocomposites prepared with and without surfactant (tetradecyltrimethylammonium bromide, TTAB) were formed by spin coating onto chemically cleaned p-type silicon substrates. The current–voltage characteristics of the Au/PANI TiO ₂/p-Si/Al and Au/PANI TiO ₂ TTAB/p-Si/Al heterojunctions had rectifying behavior with the potential barrier formed between the polymeric thin films and p-Si semiconductor, and they were analyzed on the basis of the standard thermionic emission (TE) theory. Cheung functions combined with conventional forward I–V characteristics were used to obtain diode parameters such as barrier height, ideality factor and series resistance (R _s ). The values of barrier height, ideality factor and R _s were found as 0.496±0.003 eV, 2.313±0.067 and 23.633±7.554 Ω for the Au/PANI TiO ₂/p-Si/Al device; 0.494±0.003 eV, 2.167±0.018 and 12.929±2.217 Ω for the Au/PANI TiO ₂ TTAB/p-Si/Al device. In addition, the energy distributions of the interface state density of the devices were determined from the forward I–V characteristics by taking into account the bias dependence of the ideality factor and barrier height. It was seen that the PANI TiO ₂ TTAB/p-Si device had slightly higher interface state density values than those of the PANI TiO ₂/p-Si device.     

Exciton phase transitions in semiconductor quantum wells with disc-shaped electrode

Phase transitions in a system of indirect excitons in semiconductor double quantum wells are studied for a set-up when one of the electrodes is of finite size and, in particular, has the shape of a disc. At voltage a region under the rim of the disc is created where excitons have lower energy, thus providing a macroscopic trap attractive for excitons while being repulsive for charged particles. The theory of the formation of patterns of the excitonic condensed phase under the disc is built based on the assumption of the existence of the inter-exciton range where the interaction between them is attractive. The finite value of the exciton lifetime is taken into account serving as a limiting factor for the size of the islands of the condensed phase. The calculations reveal complex restructuring of the patterns of the spatial distribution of exciton density with increasing pumping intensity: from the structureless gaseous phase to separate islands of the condensed phase within the gaseous phase, then to islands of the gaseous phase in the bulk of the condensed phase and finally to the continuous condensed phase.     

Exciton broadening of the angle-resolved photoemission spectra of a semiconductor

Broadening of the angle-resolved photoemission spectra caused by the electron-hole attraction is considered. For the case of a semiconductor with the model parabolic dispersion law the spectral-line shape is found at arbitrary k_e and ?ω, the energy dependence of the spectrum width is presented too. For the real crystals of silicon, germanium, and grey tin the results of numerical calculations of these values are given in some points k_e. The accounting of the electron-hole interaction gives rise to shifts and widths up to 0.1 eV order of magnitude and results in an oscillating spectrum lineshape.     

Exciton spectra of semiconductor superlattices in a parallel magnetic field

Low-temperature photoluminescence and photoluminescence excitation spectra of GaAs/AlGaAs semiconductor superlattices having different potential barrier widths (b=20, 30, 50, and 200 Å), i.e., degrees of tunnel coupling between quantum wells, are studied in magnetic fields up to 5 T oriented parallel and perpendicular to the layers of the structure. The changes in the qualitative character of the photoluminescence excitation spectra observed in a parallel magnetic field with increasing tunnel transparency of the barrier correspond to a transition from a quasi-two-dimensional to a quasi-three-dimensional electronic spectrum as a miniband develops in the superlattice. In the photoluminescence excitation spectra of the superlattice with b=50 Å, as the parallel magnetic field is increased, a new line appears in the violet wing of the spatially indirect exciton excitation line, which is absent in a perpendicular field. A similar line was also observed to arise in the photoluminescence spectra. It is shown that the indirect exciton luminescence line can be suppressed by both parallel and perpendicular magnetic fields.     

Excitonic processes at organic heterojunctions

Understanding excitonic processes at organic heterojunctions is crucial for development of organic semiconductor devices. This article reviews recent research on excitonic physics that involve intermolecular charge transfer (CT) excitons, and progress on understanding relationships between various interface energy levels and key parameters governing various competing interface excitonic processes. These interface excitonic processes include radiative exciplex emission, nonradiative recombination, Auger electron emission, and CT exciton dissociation. This article also reviews various device applications involving interface CT excitons, such as organic light-emitting diodes (OLEDs), organic photovoltaic cells, organic rectifying diodes, and ultralow-voltage Auger OLEDs.     

Band discontinuities for the (110)-interfaces of semiconductor heterojunctions

We present a consistent tight-binding calculation for different (110) semiconductor heterojunctions. We compare the valence-band discontinuities obtained in a full consistent treatment with the corresponding ones obtained by imposing the local charge neutrality conditions. Our results show a good agreement with the experimental evidence.     

Exciton polaritons at surfaces of GaAs

The optical reflectance associated with excitons in GaAs is found to be strongly influenced by the repulsive surface potential acting on excitons. This potential explains the anomalous shape of the experimental spectra and is much more important than spatial dispersion in determining the reflectance of GaAs.     

Reduced charge transfer exciton recombination in organic semiconductor heterojunctions by molecular doping

We investigate the effect of molecular doping on the recombination of electrons and holes localized at conjugated-polymer-fullerene interfaces. We demonstrate that a low concentration of p-type dopant molecules (<4% weight) reduces the interfacial recombination via charge transfer excitons and results in a favored formation of separated carriers. This is observed by the ultrafast quenching of photoluminescence from charge transfer excitons and the increase in photoinduced polaron density by ~70%. The results are consistent with a reduced formation of emissive charge transfer excitons, induced by state filling of tail states.     

Exciton gas compression and metallic condensation in a single semiconductor quantum wire

We study the metal-insulator transition in individual self-assembled quantum wires and report optical evidence of metallic liquid condensation at low temperatures. First, we observe that the temperature and power dependence of the single nanowire photoluminescence follow the evolution expected for an electron-hole liquid in one dimension. Second, we find novel spectral features that suggest that in this situation the expanding liquid condensate compresses the exciton gas in real space. Finally, we estimate the critical density and critical temperature of the phase transition diagram at n{c} approximately 1 x 10;{5} cm;{-1} and T{c} approximately 35 K, respectively.