Embedded-atom molecular dynamics simulations are used to investigate the effects of low-energy self-ion irradiation of Pt adatoms on Pt(111). Here, we concentrate on self-bombardment dynamics, i.e., isolating and monitoring the atomic processes, induced by normally incident Pt atoms with energies E ranging from 5 to 50 eV, that can affect intra- and interlayer mass transport.. We find that adatom scattering, surface channeling, and dimer formation occur at all energies. Atomic intermixing events involving incident and terrace atoms are observed at energies 15  eV, while the collateral formation of residual surface vacancies is observed only with E>40  eV. The overall effect of low-energy self-ion irradiation is to enhance lateral adatom and terrace atom migration. ©2005 American Institute of Physics

Well-resolved photoluminescence excitation (PLE) spectra are reported for selfassembled SiGe dots grown on Si(100) by molecular beam epitaxy. The observation of two excitation resonance peaks is attributed to two different excitation/de-excitation routes of interband optical transitions connected to the spatially direct and indirect recombination processes. It is concluded that two dot populations are addressed by each monitored luminescence energy for the PLE acquisition.

The effect of the high pulse current and the duty cycle on the deposition rate in high power pulsed magnetron sputtering (HPPMS) is investigated. Using a Cr target and the same average target current, deposition rates are compared to dc magnetron sputtering (dcMS) rates. It is found that for a peak target current density I-Tpd of up to 570 mA cm(-2), HPPMS and dcMS deposition rates are equal. For I-Tpd greater than 570 mA cm(-2), optical emission spectroscopy shows a pronounced increase of the Cr+/Cr-0 signal ratio. In addition, a loss of deposition rate, which is attributed to self-sputtering, is observed.

  pH determination is a prerequisite for many biochemical and biological processes. The authors have used two methods, namely, the electrochemical potential method and the site binding method to study the sensitivity of zinc oxide (ZnO) nanorods for the use as intracellular pH sensing device. The dimensions of these nanorods were varied with radii between 50–300 nm and lengths between 2 and 10 μm. The ZnO nanorods showed a high sensitivity ≈ 59 mV per decade at room temperature for a pH range (1–14), assuming that the solution is water. This is expected due to the polar and nonpolar surfaces of the ZnO nanorods. 

The stability of rock salt structure cubic Cr1-xAlxN solid solutions at high Al content and high temperature has made it one of the most important materials systems for protective coating applications. We show that the strong electron correlations in a material with dynamic magnetic disorder is the underlying reason for the observed stability against isostructural decomposition. This is done by using the first-principles disordered local moments molecular dynamics technique, which allows us to simultaneously consider electronic, magnetic, and vibrational degrees of freedom.

The mixing thermodynamics of GdN with TiN, ZrN, and HfN is studied using first-principles methods. We find that while Ti(1-x)Gd(x)N has a strong preference for phase separation due to the large lattice mismatch, Zr(1-x)Gd(x)N and Hf(1-x)Gd(x)N readily mix, possibly in the form of ordered compounds. In particular, ZrGdN(2) is predicted to order in a rocksalt counterpart to the L1(1) structure at temperatures below 1020 K. These mixed nitrides are promising candidates as neutron absorbing, thermally and chemically stable, thin film materials.

  The effect of nitrogen substoichiometry on the isostructural phase stabilities of the cubic Ti_1−xAl_xN_1−y system has been investigated using first-principles calculations. The preferred isostructural decomposition pattern in these metastable solid solutions was predicted from the total energy calculations on a dense concentration grid. Close to the stoichiometric Ti_1−xAl_xN₁ limit, N vacancies increase the tendency for phase separation as N sticks to Al while the vacancies prefers Ti neighbors. For nitrogen depleated conditions, N sticks to Ti forming TiN (0<<1) while Al tends to form nitrogen-free fcc-Al or Al–Ti alloys.

The influence of pressure on the phase stabilities of Ti_1−xAl_xN solid solutions has been studied using first principles calculations. We find that the application of hydrostatic pressure enhances the tendency for isostructural decomposition, including spinodal decomposition. The effect originates in the gradual pressure stabilization of cubic AlN with respect to the wurtzite structure and an increased isostructural cubic mixing enthalpy with increased pressure. The influence is sufficiently strong in the composition-temperature interval corresponding to a shoulder of the spinodal line that it could impact the stability of the material at pressures achievable in the tool-work piece contact during cutting operations

We present a fast, highly sensitive, and efficient potentiometric glucose biosensor based on functionalized InN quantum-dots (QDs). The InN QDs are grown by molecular beam epitaxy. The InN QDs are bio-chemically functionalized through physical adsorption of glucose oxidase (GOD). GOD enzyme-coated InN QDs based biosensor exhibits excellent linear glucose concentration dependent electrochemical response against an Ag/AgCl reference electrode over a wide logarithmic glucose concentration range (1 x 10(-5) M to 1 x 10(-2) M) with a high sensitivity of 80mV/decade. It exhibits a fast response time of less than 2 s with good stability and reusability and shows negligible response to common interferents such as ascorbic acid and uric acid. The fabricated biosensor has full potential to be an attractive candidate for blood sugar concentration detection in clinical diagnoses. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4758701]

We investigate photoelectrochemical water splitting by a spontaneously formed In-rich InGaN nanowall network, combining the material of choice with the advantages of surface texturing for light harvesting by light scattering. The current density for the InGaN-nanowalls-photoelectrode at zero voltage versus the Ag/AgCl reference electrode is 3.4 mA cm(-2) with an incident-photon-to-current-conversion efficiency (IPCE) of 16% under 350 nm laser illumination with 0.075 W.cm(-2) power density. In comparison, the current density for a planar InGaN-layer-photoelectrode is 2 mA cm(-2) with IPCE of 9% at zero voltage versus the Ag/AgCl reference electrode. The H-2 generation rates at zero externally applied voltage versus the Pt counter electrode per illuminated area are 2.8 and 1.61 mu mol.h(-1).cm(-2) for the InGaN nanowalls and InGaN layer, respectively, revealing similar to 57% enhancement for the nanowalls. (C) 2014 AIP Publishing LLC.

The optical linear polarization properties of exciton complexes in asymmetric Stranski-Krastanov grown GaN quantum dots have been investigated experimentally and theoretically. It is demonstrated that the polarization angle and the polarization degree can be conveniently employed to associate emission lines in the recorded photoluminescence spectra to a specific dot. The experimental results are in agreement with configuration interaction computations, which predict similar polarization degrees for the exciton and the biexciton (within 10%) in typical GaN quantum dots. The theory further predicts that the polarization degree can provide information about the charge state of the dot.

The dynamics of the exciton and the biexciton related emission from a single InGaN quantum dot (QD) have been measured by time-resolved microphotoluminescence spectroscopy. An exciton-biexciton pair of the same QD was identified by the combination of power dependence and polarization-resolved spectroscopy. Moreover, the spectral temperature evolution was utilized in order to distinguish the biexciton from a trion. Both the exciton and the biexciton related emission reveal mono-exponential decays corresponding to time constants of similar to 900 and similar to 500 ps, respectively. The obtained lifetime ratio of similar to 1.8 indicates that the QD is small, with a size comparable to the exciton Bohr radius.

Single GaN/Al(Ga)N quantum dots (QDs) have been investigated by means of microphotoluminescence. Emission spectra related to excitons and biexcitons have been identified by excitation power dependence and polarization resolved spectroscopy. All investigated dots exhibit a strong degree of linear polarization (∼90%). The biexciton binding energy scales with the dot size. However, both positive and negative binding energies are found for the studied QDs. These results imply that careful size control of III-Nitride QDs would enable the emission of correlated photons with identical frequencies from the cascade recombination of the biexciton, with potential applications in the area of quantum information processing.

The deposition flux obtained during reactive radio frequency magnetron sputtering of an Al target in Ar/O₂ gas mixtures was studied by mass spectrometry. The results show significant amounts of molecular AlO⁺ (up to 10% of the Al⁺ flux) in the ionic flux incident onto the substrate. In the presence of ~10–4 Pa H₂O additional OH⁺ and AlOH⁺ were detected, amounting to up to about 100% and 30% of the Al⁺ flux, respectively. Since the ions represent a small fraction of the total deposition flux, an estimation of the neutral content was also made. These calculations show that, due to the higher ionization probability of Al, the amount of neutral AlO in the deposition flux is of the order of, or even higher than, the amount of Al. These findings might be of great aid when explaining the alumina thin film growth process.

Bipolar transport in blends of a copolymer of fluorene, thiophene and electron accepting groups, and the substituted fullerene [6,6]-phenyl-C61-butyric acid methylester have been studied through charge extraction by linearly increasing voltage on solar cells and with field effect transistors. Between 10% and 90% polymer has been used and the results show a clear correlation to solar cell performance. Optimal solar cells comprise 20% polymer and have a power conversion efficiency of 3.5%. The electron mobility is increasing strongly with fullerene content, but is always lower than the hole mobility, thus explaining the low amount of polymer in optimized devices.

Temperature dependent charge carrier mobility measurements using field effect transistors and density functional theory calculations are combined to show how the conformation dependent frontier orbital delocalization influences the hole-and electron mobilities in a donor-acceptor based polymer. A conformationally sensitive lowest unoccupied molecular orbital results in an electron mobility that decreases with increasing temperature above room temperature, while a conformationally stable highest occupied molecular orbital is consistent with a conventional hole mobility behavior and also proposed to be one of the reasons for why the material works well as a hole transporter in amorphous bulk heterojunction solar cells.

We report on the polarizing electrochromic (EC) effect of a conjugated polymer. This has been achieved in a planar flexible electrochemical device cell comprised of a patterned stretch-aligned thin film of polyaniline and an electrolyte, all made on a polyethylene foil substrate. The resulting device exhibits polarized absorption characteristics, of a dichroic ratio of 4, that can be controlled by the voltage applied. Also, thin flexible EC polarizers have been realized by combining two stretch-aligned polyaniline films with orthogonal stretching direction. In the resulting EC polarizer the orientation of the polarized absorption can be switched between two orthogonal directions, depending on the voltage applied.

The role of an optical spacer layer has been examined by optical simulations of organic solar cells with various bandgaps. The simulations have been performed with the transfer matrix method and the finite element method. The results show that no beneficial effect can be expected by adding an optical spacer to a solar cell with an already optimized active layer thickness.

Field-effect transistors have emerged as NO2 sensors. The detection relies on trapping of accumulated electrons, leading to a shift of the threshold voltage. To determine the location of the trapped electrons we have delaminated different semiconductors from the transistors with adhesive tape and measured the surface potential of the revealed gate dielectric with scanning Kelvin probe microscopy. We unambiguously show that the trapped electrons are not located in the semiconductor but at the gate dielectric. The microscopic origin is discussed. Pinpointing the location paves the way to optimize the sensitivity of NO2 field-effect sensors. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4758697]

The geometry and formation energy of substitutional B and Al dopants as well as their complexes with hydrogen have been calculated in 4H-SiC using first-principles methods. Our results show that boron selecting the silicon site and, therefore, getting activated as a shallow acceptor depends on the presence of hydrogen which is promoted into the crystal by boron itself. Without hydrogen, boron would mostly be incorporated at the carbon site. Aluminum does not show this behavior: it always selects the silicon site and is incorporated independently of hydrogen. © 2001 American Institute of Physics.

Mid-infrared optical Hall effect measurements are used to determine the free charge carrier parameters of an unintentionally doped wurtzite-structure c-plane oriented In0.33Ga0.67N epitaxial layer. Room temperature electron effective mass parameters of m(perpendicular to)* = (0.205 +/- 0.013) m(0) and m(parallel to)* = (0.204 +/- 0.016) m(0) for polarization perpendicular and parallel to the c-axis, respectively, were determined. The free electron concentration was obtained as (1.7 +/- 0.2) x 10(19) cm(-3). Within our uncertainty limits, we detect no anisotropy for the electron effective mass parameter and we estimate the upper limit of the possible effective mass anisotropy as 7%. We discuss the influence of conduction band nonparabolicity on the electron effective mass parameter as a function of In content. The effective mass parameter is consistent with a linear interpolation scheme between the conduction band mass parameters in GaN and InN when the strong nonparabolicity in InN is included. The In0.33Ga0.67N electron mobility parameter was found to be anisotropic, supporting previous experimental findings for wurtzite-structure GaN, InN, and AlxGa1-xN epitaxial layers with c-plane growth orientation. Published by AIP Publishing.

A study was performed on the magnetic-field-induced localization of electrons in InGaN/GaN multiple quantum wells (MQW). A stepwise behavior of both the Hall coefficient and magnetoresistivity was observed. The peculiarities were explained by a magnetic-field-induced localization of electrons in a two-dimensional (2D) potential relief of the InGaN MQW.

Previous work has shown that the concentration of shallow dopants in a semiconductor can be estimated from the photoluminescence (PL) spectrum by comparing the intensity of the bound-to-the- dopant exciton emission to that of the free exciton. In this work, we study the low-temperature PL of high-quality uncompensated Al-doped p-type 4H-SiC and propose algorithms for determining the Al-doping concentration using the ratio of the Al-bound to free-exciton emission. We use three different cryogenic temperatures (2, 41, and 79 K) in order to cover the Al-doping range from mid 10(14) cm(-3) up to 10(18) cm(-3). The Al-bound exciton no-phonon lines and the strongest free-exciton replica are used as a measure of the bound-and free-exciton emissions at a given temperature, and clear linear relationships are obtained between their ratio and the Al-concentration at 2, 41, and 79 K. Since nitrogen is a common unintentional donor dopant in SiC, we also discuss the criteria allowing one to determine from the PL spectra whether a sample can be considered as uncompensated or not. Thus, the low-temperature PL provides a convenient non-destructive tool for the evaluation of the Al concentration in 4H-SiC, which probes the concentration locally and, therefore, can also be used for mapping the doping homogeneity. Published by AIP Publishing.

The tip of a borosilicate glass capillary with functionalized hexagonal ZnO nanorods was used to make a sensitive electrochemical intracellular Ca²⁺ sensor. To adjust the sensor for Ca²⁺ measurements with sufficient selectivity and stability, polyvinyl chloride (PVC) membrane containing Ca²⁺ ionophores were coated on the surface. The membrane covered ZnO nanorods exhibited a Ca²⁺-dependent electrochemical potential difference versus an Ag/AgCl reference electrode. The potential difference was linear over a large concentration range (100 nM to 10 mM). The measurements of Ca²⁺ concentrations using our ZnO nanorods sensor in human fat cells or in frog egg cells were consistent with values of Ca²⁺ concentrations reported in the literature. This nanoelectrode device paves the way to measurements of intracellular biochemical species in specific locations within single living cells.

van der Waals solids have been recognized as highly photosensitive materials that compete conventional Si and compound semiconductor based devices. While 2-dimensional nanosheets of single and multiple layers and 1-dimensional nanowires of molybdenum and tungsten chalcogenides have been studied, their nanostructured derivatives with complex morphologies are not explored yet. Here, we report on the electrical and photosensitive properties of WS2 nanowire-nanoflake hybrid materials we developed lately. We probe individual hybrid nanostructured particles along the structure using focused ion beam deposited Pt contacts. Further, we use conductive atomic force microscopy to analyze electrical behavior across the nanostructure in the transverse direction. The electrical measurements are complemented by in situ laser beam illumination to explore the photoresponse of the nanohybrids in the visible optical spectrum. Photodetectors with responsivity up to similar to 0.4 AW(-1) are demonstrated outperforming graphene as well as most of the other transition metal dichalcogenide based devices. Published by AIP Publishing.

Integration of a site-controlled semiconductor V-groove quantum wire (QWR) into a photonic-crystal (PhC) membrane microcavity is reported. Reproducible coupling of the QWR emission to a mode of the PhC cavity is evidenced by the narrower linewidth, higher intensity, and variation with temperature and PhC parameters of the QWR line. Finite difference time domain simulations of the cavity are employed for identifying the observed mode. The presented PhC-QWR coupled structures are promising for achieving very low-threshold lasers and for studies of one-dimensional photon-exciton coupled systems

We report on a reversible hydrogen annealing effect observed in platinum-silicon dioxide-silicon carbide structures at temperatures above about 650 degrees C. It appears as a decrease of the inversion capacitance in the presence of hydrogen. This phenomenon is shown to depend on hydrogen atoms, created on the catalytic metal, that pass through the oxide and interact with charge generation sites at the oxide-silicon carbide interface. The consequence of the observation for chemical sensors based on silicon carbide is discussed. The results are phenomenological, since no details of the annealing chemistry could be developed from the present experiments. We find, however, that the annealing process and its reversal have activation energies of about 0.9 eV and 2.9 eV/site,respectively.

We study the effect of nitrogen on the GaAs0.9-xNxSb0.1 (x = 0.00, 0.65%, 1.06%, 1.45%, and 1.90%) alloy dielectric function by spectroscopic ellipsometry in the energy range from 0.73 to 4.75 eV. The compositional dependences of the critical points energies for the GaAs0.9-xNxSb0.1 are obtained. In addition to the GaAs intrinsic transitions E-1, E-1+ Delta(1), and E-0, the nitrogen-induced Gamma-point optical transitions E-0 and E+, together with a third transition E-#, are identified. We find that with increasing the N content, the E-0 transition shifts to lower energies while the E+ and (E)# transitions shift to higher energies. We suggest that the origin of the E-0, E+, and E-# transitions may be explained by the double band anticrossing (BAC) model, consisting of a conduction BAC model and a valence BAC model.

Optically pumped stimulated emission and lasing in thin films of an absorbing host 8-hydroxyquinolinato aluminum(Alq) doped with small amounts of the laser dye DCM II is observed. Forster transfer of the excitation from the Alq molecules to the DCM II molecules results in a high absorption coefficient at pump wavelength (337 nm) as well as low absorption loss at the emission wavelengths (610-650 nm). (C) 1997 American Institute of Physics.

Optically pumped laser emission has been observed from thin films of 8-hydroxyquinolinato aluminum (Alq) doped with a DCM dye deposited on a diffraction,orating formed by imprinting a film of BCB with a mold. The BCB film, which is 4 mu m thick, is deposited on a silicon or a flexible plastic substrate. Laser emission occurs at a wavelength war 655 nm which corresponds to the third order of the grating, which has a periodicity of similar to 0.6 mu m. (C) 1998 American Institute of Physics.

We report electroluminescence from a regioregular alkyl-phenyl substituted polythiophene. The polymer film exists in two forms, giving widely different optical absorption, as well as photoluminescence and electroluminescence spectra. In the low-bandgap form, we observe high emission intensity centered at 1.55 eV (800 nm), well into the infrared, while the high-bandgap form gives a maximum at 1.85 eV (670 nm). The conversion from the high-bandgap form to the low-bandgap form can be done by thermal treatment of the polymer light emitting diodes.

Tungsten was incorporated in SiC and W related defects were investigated using deep level transient spectroscopy. In agreement with literature, two levels related to W were detected in 4H-SiC, whereas only the deeper level was observed in 6H-SiC. The predicted energy level for W in 3C-SiC was observed (E-C-0.47 eV). Tungsten serves as a common reference level in SiC. The detected intrinsic levels align as well: E1 (E-C-0.57 eV) in 3C-SiC is proposed to have the same origin, likely V-C, as EH6/7 in 4H-SiC and E7 in 6H-SiC, respectively.

Room-temperature optical and spin polarization up to 35% is reported in InAs/GaAs quantum dots in zero magnetic field under optical spin injection using continuous-wave optical orientation spectroscopy. The observed strong spin polarization is suggested to be facilitated by a shortened trion lifetime, which constrains electron spin relaxation. Our finding provides experimental demonstration of the highly anticipated capability of semiconductor quantum dots as highly polarized spin/light sources and efficient spin detectors, with efficiency greater than 35% in the studied quantum dots.

Electron spin dephasing and relaxation due to hyperfine interaction with nuclear spins is studied in an InAs/GaAs quantum dot ensemble as a function of temperature up to 85 K, in an applied longitudinal magnetic field. The extent of hyperfineinduced dephasing is found to decrease, whereas dynamic nuclear polarization increases with increasing temperature. We attribute both effects to an accelerating electron spin relaxation through phonon-assisted electron-nuclear spin flip-flops driven by hyperfine interactions, which could become the dominating contribution to electron spin depolarization at high temperatures.

In this work we examine the transition between positive and negative organic magnetoresistance in poly[2-methoxy-5-(3(), 7()-dimethyloctyloxy)-p-phenylenevinylene] in order to understand how different regimes of charge transport affect the organic magnetoresistance effect. To characterize the charge transport in these devices we measured the current, low frequency differential capacitance, and electroluminescence efficiency as a function of voltage. These measurements show that the sign change of the magnetoresistance corresponds with a change from a unipolar diffusive transport below the built in voltage (V(bi)) to a regime of bipolar drift transport above V(bi).

Spectroscopic ellipsometry measurements in the visible to vacuum-ultraviolet spectra (3.5-9.5 eV) are performed to determine the dielectric function of epitaxial graphene on SiC polytypes, including 4H (C-face and Si-face) and 3C SiC (Si-face). The model dielectric function of graphene is composed of two harmonic oscillators and allows the determination of graphene quality, morphology, and strain. A characteristic van Hove singularity at 4.5 eV is present in the dielectric function of all samples, in agreement with observations on exfoliated as well as chemical vapor deposited graphene in the visible range. Model dielectric function analysis suggests that none of our graphene layers experience a significant degree of strain. Graphene grown on the Si-face of 4H SiC exhibits a dielectric function most similar to theoretical predictions for graphene. The carbon buffer layer common for graphene on Si-faces is found to increase polarizability of graphene in the investigated spectrum.

We demonstrate an alternative to Kelvin Probe Force Microscopy for imaging surface potential. The open-loop, single-pass technique applies a low-frequency AC voltage to the atomic force microscopy tip while driving the cantilever near its resonance frequency. Frequency mixing due to the nonlinear capacitance gives intermodulation products of the two drive frequencies near the cantilever resonance, where they are measured with high signal to noise ratio. Analysis of this intermodulation response allows for quantitative reconstruction of the contact potential difference. We derive the theory of the method, validate it with numerical simulation and a control experiment, and we demonstrate its utility for fast imaging of the surface photo-voltage on an organic photovoltaic material.

Photoluminescence spectroscopy has been used to study single crystalline ZnO samples systematically annealed in inert, Zn-rich and O-rich atmospheres. A striking correlation is observed between the choice of annealing ambient and the position of the deep band emission (DBE) often detected in ZnO. In particular, annealing in O₂ results in a DBE at 2.35±0.05 eV, whereas annealing in the presence of metallic Zn results in DBE at 2.53±0.05 eV. The authors attribute the former band to zinc vacancy (V_Zn) related defects and the latter to oxygen vacancy (V_O) related defects. Additional confirmation for the V_O and V_Zn peak identification comes from the observation that the effect is reversible when O- and Zn-rich annealing conditions are switched. After annealing in the presence of ZnO powder, there is no indication for the V_Zn- or V_O-related bands, but the authors observe a low intensity yellow luminescence band peaking at 2.17 eV, probably related to Li, a common impurity in hydrothermally grown ZnO.

There is an attractive Casimir-Lifshitz force between two silica surfaces in a liquid (bromobenze or toluene). We demonstrate that adding an ultrathin (5-50 angstrom) metallic nanocoating to one of the surfaces results in repulsive Casimir-Lifshitz forces above a critical separation. The onset of such quantum levitation comes at decreasing separations as the film thickness decreases. Remarkably, the effect of retardation can turn attraction into repulsion. From that we explain how an ultrathin metallic coating may prevent nanoelectromechanical systems from crashing together.

We show experimentally that few layer graphene (FLG) grown on the carbon terminated surface (C-face) of 3C-SiC(111) is composed of decoupled graphene sheets. Landau level spectroscopy on FLG graphene is performed using the infrared optical Hall effect. We find that Landau level transitions in the FLG exhibit polarization preserving selection rules and the transition energies obey a square-root dependence on the magnetic field strength. These results show that FLG on C-face 3C-SiC(111) behave effectively as a single layer graphene with linearly dispersing bands (Dirac cones) at the graphene K point. We estimate from the Landau level spectroscopy an upper limit of the Fermi energy of about 60 meV in the FLG, which corresponds to a carrier density below 2.5 x 10(11) cm(-2). Low-energy electron diffraction mu-LEED) reveals the presence of azimuthally rotated graphene domains with a typical size of amp;lt;= 200 nm.mu-LEED mapping suggests that the azimuth rotation occurs between adjacent domains within the same sheet rather than vertically in the stack. Published by AIP Publishing.

The energy level alignment at interfaces, in stacks comprising of (4, 4′ -N, N′ -dicarbazolyl-biphenyl) (CBP), (4,4, 4″ -tris[3-methyl-phenyl(phenyl)amino]-triphenylamine) (m -MTDATA), and a conductive substrate, has been studied. We show that the alignment of energy levels depends on the equilibration of the chemical potential throughout the layer stack, while any electronic coupling between the individual layers is of lesser importance. This behavior is expected to occur for a broad class of weakly interacting interfaces and can have profound consequences for the design of organic electronic devices. © 2007 American Institute of Physics.

Epitaxial Ti3SiC2 (0001) thin film contacts were grown on doped 4H-SiC (0001) using magnetron sputtering in an ultra high vacuum system. The specific contact resistance was investigated using linear transmission line measurements. Rapid thermal annealing at 950 degrees C for 1 min of as-deposited films yielded ohmic contacts to n-type SiC with contact resistances in the order of 10(-4) Omega cm(2). Transmission electron microscopy shows that the interface between Ti3SiC2 and n-type SiC is atomically sharp with evidence of interfacial ordering after annealing.

Temperature dependent cw- and time-resolved photoluminescence combined with absorption measurements are employed to evaluate the origin of radiative recombination in ZnCdO alloys grown by molecular-beam epitaxy. The near-band-edge emission is attributed to recombination of excitons localized within band tail states likely caused by nonuniformity in Cd distribution. Energy transfer between the tail states is argued to occur via tunneling of localized excitons. The transfer is shown to be facilitated by increasing Cd content due to a reduction of the exciton binding energy and, therefore, an increase of the exciton Bohr radius in the alloys with a high Cd content. © 2007 American Institute of Physics.

 Doping efficiency and thermal stability of intrinsic modulation doping in InP/InGaAs heterostructures, where intrinsic defects (PInantisites) are used as an electron source, are investigated. A high efficiency of the intrinsic doping is demonstrated from a comparison between the intrinsically doped and conventional extrinsically doped structures. The thermal stability of the intrinsically doped heterostructures is shown to be largely affected by the thermal stability of the InP surface.

 Raman measurements in backscattering configuration are employed to characterize the effect of nitrogen on the structural properties of a GaNxP1-x alloy with x<=3%. The following effects of N incorporation on the vibrational spectra of GaNP are observed. First, frequencies of GaP-like and GaN-like longitudinal optical phonons exhibit strong compositional dependence, due to a combined effect of alloying and biaxial strain. Second, a dramatic quenching of two-phonon Raman scattering and an emergence of zone-edge GaP-like vibrations are observed. These effects are tentatively attributed to a local distortion of the GaNP lattice and/or compositional disorder in the alloy.

Secondary ion mass spectrometry and photoluminescence are employed to evaluate the origin and efficiency of hydrogen passivation of nitrogen in GaNAs and GaNP. The hydrogen profiles are found to closely follow the N distributions, providing unambiguous evidence for their preferential binding as the dominant mechanism for neutralization of N-induced modifications in the electronic structure of the materials. Though the exact number of H atoms involved in passivation may depend on the conditions of the H treatment and the host matrixes, it is generally found that more than three H atoms are required to bind to a N atom to achieve full passivation for both alloys. © 2007 American Institute of Physics.

 The effect of growth temperature on the optical properties of GaAs/GaNxAs1-x quantum wells is studied in detail using photoluminescence (PL) spectroscopies. An increase in growth temperature up to 580 °C is shown to improve the optical quality of the structures, while still allowing one to achieve high (>3%) N incorporation. This conclusion is based on: (i) an observed increase in intensity of the GaNAs-related near-band-edge emission; (ii) a reduction in band-edge potential fluctuations, deduced from the analysis of the PL line shape; and (iii) a decrease in concentration of some extended defects detected under resonant excitation of the GaNAs. The thermal quenching of the GaNAs-related PL emission, however, is almost independent of the growth temperature and is attributed to a thermal activation of an efficient nonradiative recombination channel located in the GaNAs layers.

 The mechanism for low-temperature photoluminescence (PL) emissions in GaNAs epilayers and GaAs/GaNxAs1 - x quantum well (QW) structures grown by molecular-beam epitaxy is studied in detail, employing PL, PL excitation, and time-resolved PL spectroscopies. It is shown that even though quantum confinement causes a strong blueshift of the GaNAs PL emission, its major characteristic properties are identical in both QW structures and epilayers. Based on the analysis of the PL line shape, its dependence on the excitation power and measurement temperature, as well as transient data, the PL emission is concluded to be caused by a recombination of excitons trapped by potential fluctuations in GaNAs.