On boundary condition-induced states in low-dimensional semiconductor structures

We present here the results on electron transport and noise (diffusion) in low‐dimensional semiconductor structures obtained self‐consistently by the Monte Carlo technique. Electron scattering by confined and localized optical phonons in quasi‐one‐dimensional quantum wires and quasi‐two‐dimensional quantum wells is taken into account. In spite of the considerable differences in phonon spectra and their coupling with electrons, and in spite of electron quantization, the diffusivity (noise) and transport parameters in low‐dimensional structures only slightly differ from corresponding bulk material characteristics at room temperature. At low lattice temperatures the existence of several optical phonon modes in low‐dimensional structures causes electron streaming at several frequencies.

The ratio of the spontaneous emission and Auger recombination rates for quantum wells and wires is compared theoretically with that for bulk structures. Dramatic improvements in radiative efficiency are not predicted for these novel structures unless carrier densities are greatly reduced.

One of the most interesting parameters of semiconductors and devices is the carrier mobility in the valence and conduction band because the mobility can result in a limitation of the speed of the device. From the development of ultrafast measuring techniques new questions arise — because the carrier mobility is different at different energy states in the band or subband and the mobility can be influenced by external parameters like field strength applied, temperature, stress etc. Especially in low-dimensional semiconductor structures (quantum wells, quantum wires or quantum dots) we have a quantized energy structure. Furthermore, there are many physically different dynamical processes (intraband and interband transitions, different scattering mechanisms etc.). To perform mobility measurements it seems to be necessary to develop measuring techniques running on a time scale faster than the fastest dynamical process to be studied. Time-resolved photoconductivity seems to be a powerful tool to solve these problems down to a picosecond time scale.KeywordsCarrier DensityCarrier MobilityElectrical PulseShallow TrapMobility EdgeThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The absorption of linearly polarized light in low-dimensional semiconductor structures is investigated. It is shown that the absorption under consideration can give rise to spin orientation of free carriers. A theory of this optical orientation by linearly polarized light is developed for resonant intersubband optical transitions in n-type quantum wells. It is demonstrated that, in the vicinity of the resonance, the optical orientation undergoes spectral inversion, namely, the electron spin orientation reverses sign with increasing frequency. This behavior can be accounted for by the spin-orbit subband splitting, which is linear in the wave vector, and by the energy and quasi-momentum conservation laws.

Nuclei with Z<40 and 36<N<50 show large changes in deformation and coexistence both between prolate and oblate and spherical and deformed shapes. Theoretical interpretation of experimental data in this region is rather complicated by the presence of the N,Z $#x223C; 38,40 subshell closures, which is deformation dependent and its manifestation is a function of the number of protons and neutrons involved. The region has been thoroughly studied in numerous works (see e.g. 1–17) with special interest in features of rotational bands up to high-spins. Various nuclear models, including shell model, particle - rotor (axial, triaxial) with and without variable moment of inertia, cluster vibrational model and IBM have been used in efforts to understand experimental results in this area.

The theoretical interpretation of experimental data from the previous papers (1,2) are extended with the purpose to demonstrate the possibilities of a qualitative characterisation of flocculated emulsions by means of dielectric spectroscopy. Both low and high frequency limits of dielectric spectra are analysed, which is an extension of our previous paper (1) where only low frequency values were interpreted. Attention is paid to the emulsion stability with regard to flocculation in a dielectric spectrum. The influence of salt concentration inside the aqueous droplets in the dispersed phase is interpreted. The theoretical interpretation is based on a generalised model of the flocculated state, where floes are characterised not only by their shapes, but also by their volumes. The information of floe properties obtained by means of dielectric spectroscopy, and different hydrodynamic estimations characterising the flocculation process is compared and correlations are established.

Impact of strain on the band offsets of important III–V quantum wells: InxGa1−xN/GaN, GaAs/InxGa1−xP and InxGa1−xAs/AlGaAs

The photoelectrochemical behaviour of n-GaN in contact with 1 M H2SO4 and with acidic solutions containing Cl− ions was studied using rotating-ring-disk voltammetry, electrochemical impedance spectroscopy and etching experiments. It was found that n-GaN is stabilized against photoanodic decomposition in the presence of Cl− ions due to the competing oxidation of Cl− to Cl2. The competition kinetics were interpreted on the basis of a mechanism, in which intrinsic surface states take part in the photoanodic oxidation of Cl−. The participation of such intrinsic surface states may explain the discrepancies found in the literature concerning the photoelectrochemical behaviour of n-GaN.

First-principles study of electron trapping by intrinsic surface states on β-Si3N4 (0001)

Interband tunnelling of carriers through a forbidden energy gap, known as Zener tunnelling, is a phenomenon of fundamental and technological interest. Its experimental observation in the Esaki p-n semiconductor diode has led to the first demonstration and exploitation of quantum tunnelling in a condensed matter system. Here we demonstrate a new type of Zener tunnelling that involves the resonant transmission of electrons through zero-dimensional (0D) states. In our devices, a narrow quantum well of the mid-infrared (MIR) alloy In(AsN) is placed in the intrinsic (i) layer of a p-i-n diode. The incorporation of nitrogen in the quantum well creates 0D states that are localized on nanometer lengthscales. These levels provide intermediate states that act as “stepping stones” for electrons tunnelling across the diode and give rise to a negative differential resistance (NDR) that is weakly dependent on temperature. These electron transport properties have potential for the development of nanometre-scale non-linear components for electronics and MIR photonics.

The linewidths and the energy shifts of the resonant states of the impurity electron in GaAs-based quantum wells (QWs) with infinite barriers are calculated. The two-dimensional Schrödinger equation for the charge impurity in the QW is solved by the developed finite-difference method combined with the complex-scaling technique. A dependence of the linewidths and energy shifts on the QW width for the impurity localized in the center of QW is studied. The calculated results extend and improve theoretical estimations of these quantities in the GaAs-based QW by Monozon and Schmelcher [Phys. Rev. B 71, 085302 (2005)]. In particular, in agreement with their studies we obtain that resonant states originating from the second quantum-confinement subband have negligibly small linewidths. In contrast to previous estimations, we show that for the QW widths of the order of the impurity's Bohr radius the linewidths of resonant states associated to the third quantum-confinement subband linearly depend on the thickness of the QW. We show how the previous theoretical predictions can be improved for such QW widths. We also calculate linewidths for the case when the electron impurity is localized away from the center of the QW.

Recently, low dimensional semiconductor structures such as metallic nanoparticles, have attracted significant attention for photovoltaics. In this work we study the low-temperature formation of metallic nano-dots in an indium tin oxide (ITO) antireflection coating for solar cells after an appropriate hydrogen plasma treatment. Atomic force microscopy (AFM), scanning spreading resistance microscopy (SSRM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used for the analysis of the morphology, electrical properties and composition of ITO layers deposited on Corning glass at different temperatures and subsequently subjected to hydrogen plasma. Formation of highly conductive nano-structures at hydrogenation temperatures below 250degC is observed. It is shown that the nano-structures formation can be controlled by the time and the temperature of the hydrogenation

: Our research on electron-phonon and phonon-phonon processes in low-dimensional semiconductor nanostructures has been focused on the effects of a surfaces and interfaces on the electron energy momentum relaxation rates due to acoustic phonon emission, new specific mechanism of electron interaction with acoustic phonons in semiconductor heterostructures, and phonon-induced processes of charge can loosing phase coherence in heterostructures. Our research provides the necessary basis for the control of 1 dissipation and for the design of nanodevices with electron-phonon relaxation and dephasing rates required specific applications. .

926. A technique for the vacuum deposition of alkali metals: W Y Shiu et al,J Vac Sci Technol, 11 (6), 1974, 1131–1132

A simple unified theory has been developed for predicting three different types of electronic states at semiconductor surfaces: (i) point–defect states, (ii) core excitons, and (iii) intrinsic surface states. The predictions are in agreement with experiment for each class of states. The theory indicates that Fermi level pinning at III–V semiconductor interfaces is usually caused by surface antisite defects.