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We investigated transport properties of organic heterointerfaces formed by single-crystals of two organic donor-acceptor molecules, tetramethyltetraselenafulvalene (TMTSF) and 7,7,8,8-tetracyanoquinodimethane (TCNQ). Whereas the individual crystals have un-measurably high resistance, the interface exhibits a resistivity of few tens of MegaOhm with a temperature dependence characteristic of a small gap semiconductor. We analyze the transport properties based on a simple band-diagram that naturally accounts for our observations in terms of charge transfer between two crystals. Together with the recently discovered tetrathiafulvalene (TTF)-TCNQ interfaces, these results indicate that single-crystal organic heterostructures create new electronic systems with properties relevant to both fundamental and applied fields.

We applied the internal photoemission technique to the direct observation of deep levels together with barrier heights and band discontinuities at different semiconductor heterostructure interfaces. Its performances and capabilities are superior to those of established techniques: the energy resolution is excellent and it can be applied to "real" device interfaces under bias. The technique was successfully tested for the Pd/n-GaAs and Pd/a-Si:H Schottky barriers as well as for the a-Ge/n-GaAs and a-Ge/p-GaP heterojunctions. We found a Pd/n-GaAs Schottky barrier height of 0.78 eV and an a-Ge/n-GaAs valence band discontinuity of 0.42 eV; in both cases the Fermi level was "pinned" at an averaged value of 0.76+/-0.03 eV above the top of the GaAs valence band. Furthermore we directly measured a feature at 1.09 eV, due to Pd-induced empty states approximately 1.1 eV above the valence band. The Pd/n-GaAs Schottky barrier also exhibits a feature at 1.36 eV, explained by transitions from the valence band to a localized state, made possible by the band bending. In the case of a-Ge/n-GaP we found a DELTA-E(V) = 0.88 +/- 0.05 eV and a DELTA-E(C) = 0.68 +/- 0.05 eV. Finally, in the Pd/a-Si:H Schottky barrier, we demonstrated the contribution of resonant tunneling through localized states of the conduction band.

We present a highly efficient hybrid heterojunction photovoltaic (PV) cell with a colloidal inorganic nanocrystal (NC) electron donor and an organic electron acceptor. The heterojunction is formed by a thin film of cross-linked PbS NCs and a vacuum deposited C60 layer. Compared to the PbS-only PV cell, the heterojunction device has improved the power conversion efficiency from 1.6% to 2.2% under 1 sun of simulated Air Mass 1.5 Global (AM 1.5G) irradiation. The C60 layer effectively prevents exciton quenching at the NC/metal interface, as evidenced by a significant improvement in fill factor of the heterojunction devices.

We use molecular dynamics (MD) with the reactive interatomic potential ReaxFF to characterize the local strains of epitaxial Si/Ge/Si nanoscale bars as a function of their width and height. While the longitudinal strain (along the bars length) is independent of geometry, surface relaxation leads to transverse strain relaxation in the Ge section. This strain relaxation increases with increasing height of the Ge section and reduction in its width and is complete (i.e., zero transverse strain) for roughly square cross sections of Ge leading to a uniaxial strain state. Such strain state is desirable in some microelectronics applications. From the MD results, which are in excellent agreement with experiments, we derive a simple model to predict lateral strain as a function of geometry for this class of nanobars.

Cataloged from PDF version of article. Electric field dependent photoluminescence decay kinetics and its radiative component are studied in polar InGaN/GaN quantum heterostructures at low fields. Under externally applied electric field lower than polarization fields, spectrally and time resolved photoluminescence measurements are taken to retrieve internal quantum efficiencies and carrier lifetimes as a function of the applied field. Subsequently, relative behavior of radiative recombination lifetimes is obtained in response to the applied field. In these characterizations of polar InGaN/GaN structures, we observe that both the carrier lifetime and the radiative recombination lifetime decrease with increasing external electric field, with the radiative component exhibiting weaker field dependence.

In this paper, we investigate the structural properties of AlGaN/GaN heterostructures grown by metal organic chemical vapor deposition on sapphire substrates with different thicknesses using high-resolution x-ray diffraction and Raman scattering methods. We discuss the microscopic nature of spatial-inhomogeneous deformations and dislocation density in the structures. Microdeformations within mosaic blocks and the sizes of regions of coherent diffraction are determined. We reveal a gradient depth distribution of deformations in the mosaic structure of nitride layers, as well as at the interface regions of the sapphire substrate on the microscale level using confocal micro-Raman spectroscopy. We determine that an increase in substrate thickness leads to a reduction in dislocation density in the layers and an increase in the elastic deformations. The features of the block structure of nitrides layers are shown to have a significant influence on their elastic properties.

Optical properties of the GaSb/GaAs quantum dot system are investigated using a time-resolved photoluminescence technique. In this type-II heterostructure the carriers of different species are spatially separated and, as a consequence, a smooth evolution of both the emission wavelength and decay timescale is observed. A wavelength shift of 170 nm is measured simultaneously with the progressive timescale change from 100 ps to 23 ns. These phenomena are explained by the evolution of the carrier density, which brings a modification to the optical transition probability as well as the shift in the emission toward the higher energies. (C) 2009 American Institute of Physics. (10.1063/1.3202419)

We develop a new three-dimensional multiparticle Monte Carlo ({\it 3DmpMC}) approach in order to study the hopping charge transport in disordered organic molecular media. The approach is applied here to study the charge transport across an energetically disordered organic molecular heterojunction, known to strongly influence the characteristics of the multilayer devices based on thin organic films. The role of energetic disorder and its spatial correlations, known to govern the transport in the bulk, are examined here for the bilayer homopolar system where the heterojunction represents the bottleneck for the transport. We study the effects of disorder on both sides of the heterojunction, the effects of the spatial correlation within each material and among the layers. Most importantly, the {\it 3DmpMC} approach permits us to treat correctly the effects of the Coulomb interaction among carriers in the region where the charge accumulation in the device is particularly important and the Coulomb interaction most pronounced. The Coulomb interaction enhances the current by increasing the electric field at the heterojunction as well as by affecting the thermalization of the carriers in front of the barrier. Our MC simulations are supplemented by the master equation (ME) calculations in order to build a rather comprehensive picture of the hopping transport over the homopolar heterojunction., 26 pages, 11 figures, LaTeX

Deep-level transient spectroscopy (DLTS) has been used to measure the concentration and energy position of deep electronic states in CuInS2. Flat TiO2?CuInS2 heterojunctions as well as TiO2-CuInS2 nanocomposites have been investigated. Subband-gap electronic states in CuInS2 films are mostly due to antisite point defects and vacancies. Substitution of indium with copper, CuInII, leads to an acceptor state 0.15 eV above the valence band, while copper vacancies, VCuI, are acceptor states at 0.1 eV. Furthermore, indium on a copper position, InCu?, yields a donor state at 0.07 eV below the conduction band, while sulphur vacancies are donor states at 0.0 = eV. With DLTS, these states are indeed found. For flat configurations, VCuI are the dominant acceptors with a concentration of 1.83×1017?cm?3. In contrast for nanocomposites CuInII are the dominant acceptors having a concentration of 6.7×1017?cm?3. We conclude that the concentration of antisite defects in nanocomposite CuInS2 is significantly higher than that in flat films of CuInS2.

A waveguide taper design for uncladded rib or channel waveguides on InGaAsP-InP with polarization-independent coupling loss to the fiber is proposed. The spot size variation of the guided mode along the taper is investigated numerically by a two-dimensional finite difference method. By introducing thin guiding layers below the waveguide it is shown that the taper structure can be designed to have relaxed fabrication tolerances and a mode matching loss to the fiber of 1 dB.

The aim of this paper is the study of the single-electron coherent propagation in a quantum-computing gate made of coupled quantum wires. The structure under investigation is based on a two-dimensional (2-D) electron gas realized in a modulation-doped GaAs-AlGaAs heterostructure. A number of surface electrodes are used to form one-dimensional channels. The profile of the conduction band at the heterojunction has been computed numerically by solving the three-dimensional Poisson equation on the whole structure at 300 mK. Finally, a single-electron wavefunction is propagated within the so-formed quantum wire geometry by means of a 2-D, time-dependent Schro/spl uml/dinger solver. Results are shown for a single-qubit rotation gate implementing a quantum-NOT transformation. This work is part of a feasibility study on a solid-state realization of a universal set of quantum gates.

Amorphous silicon/crystalline silicon heterojunction solar cells, deposited by the plasma-enhanced chemical vapor deposition (PECVD) technique, have been fabricated using different technologies to passivate defects at the heterointerface: without treatment, the insertion of a thin intrinsic amorphous layer or that of a thin intrinsic epitaxial layer. The open circuit voltage of heterojunction solar cells fabricated including an intrinsic amorphous buffer layer is strangely lower than in devices with no buffer layer. The structure of the amorphous buffer layer is investigated by high resolution transmission electron microscope observations. As an alternative to amorphous silicon, the insertion of a fully epitaxial silicon layer, deposited at low temperature with conventional PECVD technique in a hydrogen-silane gas mixture, was tested. Using the amorphous silicon/crystalline silicon (p a-Si/i epi-Si/n c-Si) heterojunction structure in solar cells, a 13.5% efficiency and a 605-mV open circuit voltage were achieved on flat Czochralski silicon substrates. These results demonstrate that epitaxial silicon can be successfully used to passivate interface defects, allowing for an open circuit voltage gain of more than 50 mV compared to cells with no buffer layer. In this paper, the actual structure of the amorphous silicon buffer layer used in, heterojunction solar cells is discussed. We make the hypothesis that this buffer layer, commonly considered amorphous, is actually epitaxial.

High-resolution x-ray diffraction, in surface symmetric, skew symmetric, and grazing incidence in-plane diffraction geometries, has been used to investigate the effect of an AlN interlayer between micron thick GaN and AlxGa1−xN layers grown by metalorganic vapor phase epitaxy on basal plane sapphire. No change is found in the tilt mosaic (threading screw dislocation density) with thickness or Al fraction x of the upper layer. A linear increase in the twist mosaic (threading edge dislocation density) was observed as a function of interlayer thickness and x. For all samples the twist mosaic of the AlGaN was significantly greater, by at least a factor of two, than that of the GaN layer. With increasing interlayer thickness the in-plane lattice parameter of the AlGaN decreased. The results are explained in terms of extra threading edge dislocations resulting from relaxation at the GaN/AlN interface.

We demonstrate a novel method for measuring the discrete energy spectrum of a quantum dot connected very weakly to a single lead. A train of voltage pulses applied to a metal gate induces tunneling of electrons between the quantum dot and a reservoir. The effective tunnel rate depends on the number and nature of the energy levels in the dot made accessible by the pulse. Measurement of the charge dynamics thus reveals the energy spectrum of the dot, as demonstrated for a dot in the few-electron regime., 4 pages, 4 figures

Gallium and antimony self-diffusion experiments have been performed in undoped 69Ga121Sb/71Ga123Sb isotope heterostructures at temperatures between 571 and 708 °C under Sb- and Ga-rich ambients. Ga and Sb profiles measured with secondary ion mass spectrometry reveal that Ga diffuses faster than Sb by several orders of magnitude. This strongly suggests that the two self-atom species diffuse independently on their own sublattices. Experimental results lead us to conclude that Ga and Sb diffusion are mediated by Ga vacancies and Sb interstitials, respectively, and not by the formation of a triple defect proposed earlier by Weiler and Mehrer [Philos. Mag. A 49, 309 (1984)]. The extremely slow diffusion of Sb up to the melting temperature of GaSb is proposed to be a consequence of amphoteric transformations between native point defects which suppress the formation of those native defects which control Sb diffusion. Preliminary experiments exploring the effect of Zn indiffusion at 550 °C on Ga and Sb diffusion reveal an enhanced intermixing of the Ga isotope layers compared to undoped GaSb. However, under the same conditions the diffusion of Sb was not significantly affected., H.B. acknowledges a Feodor Lynen fellowship of the Alexander von Humboldt-Stiftung. The authors are indebted to W. Walukiewicz for valuable discussions. This work was supported in part by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098, by U.S. NSF Grant No. DMR-97 32707, by the Fond der Chemischen Industrie, and by a Max-Planck Research Award.

Ga-face AlGaN/GaN heterostructures with different sheet carrier concentrations have been studied by photoluminescence and Raman spectroscopy. Compared to bulk GaN, an energy shift of the excitonic emission lines towards higher energies was observed, indicating the presence of residual compressive strain in the GaN layer. This strain was confirmed by the shift of the E2 Raman line, from which biaxial compressive stresses ranging between 0.34 and 1.7 GPa were deduced. The spontaneous and piezoelectric polarizations for each layer of the heterostructures have been also calculated. The analysis of these quantities clarified the influence of the residual stress on the sheet electron concentration (ns). Possible causes for the discrepancies between the calculated and experimentally determined sheet carrier densities are briefly discussed. Andres.Cantarero@uv.es ; Gema.Martinez@uv.es

The indiffusion of Zn into InGaAsP layers was studied using a MOVPE-based diffusion process. Hydrogen and nitrogen were alternatively employed as carrier gas to compare their effect on the diffusion behaviour. Using nitrogen, larger diffusion coefficients of Zn were obtained under comparable conditions. Whereas in InGaAs, maximum hole concentration levels of >1×10 20 cm −3 were obtained for both N 2 and H 2 , i.e. a factor of 3–4 higher than achievable with MOVPE doping, there proved to be no enhanced doping effect in InP. As to the lateral diffusion uniformity superior results were obtained with nitrogen.

The device performance of a novel color sensitive diode (p-i-i-i-n-diode)-based on amorphous silicon is discussed. The detectors are developed with regard to the /spl mu//spl tau/-(carrier mobility/spl times/lifetime)-product and the bandgap of the different i-layers resulting in a voltage controlled spectral response. Since the linear independence of the spectral response curves and a linear response of the photocurrent on the incident light intensity is a prerequisite for the generation of a red-green-blue signal, the influence of light intensity on the color separation is examined by spectral response measurements under different monochromatic bias illumination. Additionally, numerical simulations are carried out to study the optoelectronic properties of these devices. Finally, a deconvolution method is presented which allows the transformation of the measured spectral response curves into a red-green-blue signal.

The detection of the fundamental components of the visible light (blue, green, red) is realized with a multispectral photodiode based on amorphous silicon. The preferential carrier collection region of this two-terminal device shifts upon changing the applied bias voltage, which leads to a color sensitivity. Structures with controlled bandgap and mobility-lifetime product exhibit a dynamic behavior above 95 dB. Three linearly independent spectral response curves can be extracted to generate a RGB-(red-green-blue)-signal. Bias voltage switching experiments under different monochromatic illumination conditions are carried out to investigate the time-dependent behavior. Furthermore, in order to improve the understanding of the photocurrent transients, we have characterized the voltage-dependent capacitance behavior, Thus, a Simulation Program with Integrated Circuit Emphasis (SPICE) model including the capacitance-voltage (CV) data has been developed, which describes the transient behavior of the detector. The simulations are in good agreement with the experimental data and allow an interpretation of the transient photocurrent response after bias voltage switching. Based on these results, optimization criteria to improve the transient behavior are discussed.

We report a comprehensive investigation of the local structure around As in thin InAsxP1-x strined layers in InAsxP1-x/Inp superlattices by fluorescence-detected x-ray absorption fine structure; seven superlattice samples are studied as a function of composition, and compared to six unstrained, bulk samples of similar composition. Contributions up to the third coordination shell around As are clearly visible in the spectra, and are analyzed taking into account important multiple-scattering contributions. Results show that structural modifications due to tetragonal distortion appear mainly in the second and third coordination shells, while nearest-neighbor bond lengths remain closer to the values in unstrained bulk alloys. This implies that in semiconductor alloy tetragonal strain accomodation is mainly obtained through bond-angle distortions, in analogy to the situation in bulk pseudobinary alloys. A model which combines macroscopic elastic theory and the known local structure in bulk pseudobinary alloy is presented, and is found to fit the data very well.

The incongruent evaporation of GaP source material using a conventional effusion cell equipped with a PBN scavenger at the orifice is demonstrated as a simple and cost-effective way to generate a pure P2 molecular beam, suitable for the growth of InP layers in a solid-source MBE system. Low residual carrier concentrations in combination with high mobilities as well as narrow and intense photoluminescence spectra were achieved. While some arsenic incorporation from the growth environment was observed, gallium incorporation is limited to below 0.1% of the group-III lattice sites. n- and p-doping using Si and Be has been investigated. Successful growth of InP/GaInAs heterostructures indicates that the use of the GaP source technique is a viable way for growing AlGaInAs/InP device structures by solid source MBE.

In the present paper we discuss the electronic properties of semiconductor heterojunctions, focusing on the band offset problem. We address interface-specific phenomena, where the conditions of growth -- including controlled contamination and strain effects -- may significantly alter the properties of the junction. We study the effects of ultrathin intralayers (i.e., heterovalent implantation) both at homojunctions (such as GaAs/Ge/GaAs) and heterojunctions (such as GaAs/Si/AlAs). In both cases, our theory demonstrates how the intralayers control the band offset: this result is confirmed by recent experimental observation in several systems. We then consider the band-offset engineering at lattice-mismatched heterojunctions, studying the paradigmatic case GaAs/Si. We evaluate the band-offset for several fully relaxed inequivalent interfaces. Both macroscopic strain and microscopic morphology effects are considered.

We present a photoemission study of the initial growth stages of amorphous Si on the InP(110) cleaved surface, performed at 120 K and compared with the room-temperature (RT) results. Deposition at low temperature gives a larger valence-band discontinuity with respect to RT and high-temperature (280-degrees-C) growth. This effect is explained by the smaller outdiffusion of In atoms in the silicon overlayer.

We performed a photoemission study on the (a-Si)/Si(111) homojunction with a thin intralayer of calcium fluoride (one monolayer). The intralayer and the amorphous silicon were grown in situ on cleaved Si(111) single crystals. From the core-level analysis we found a valence-band discontinuity of 0.35 eV due to the electrostatic dipole induced by the intralayer ionic bonds. Our results show that the intralayer forms an abrupt interface with Si and that the overlayer valence band has a lower binding energy than the substrate one. This implies that the most probable interface configuration is Si(111)-Ca-F(a-Si), confirming the results of previous works on the CaF2/Si(111) interface.

The interface conductance of LaAlO3/SrTiO3 heterostructures was investigated under high temperature oxygen equilibrium. The dependence of the heterostructure's conductance on oxygen partial pressure (from 10(-22) to 1 bar) and temperature (800 to 1100 K) was compared to the characteristic of SrTiO3 single crystals, which is described in terms of a defect chemistry model. Up to 950 K the equilibrated heterostructures reveal an additional influence of a metallic-like conduction path with a very slight dependence on the oxygen partial pressure. Donor-type interface states which may result from either lattice distortions or A-site cation intermixing during processing are discussed as a possible origin for the exceptional interface conduction of LaAlO3/SrTiO3 heterostructures. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3457386]