Surface structure of GaP(110): Ion scattering and density functional theory study

The structure of the GaP(110) surface has been investigated using coaxial impact collision ion scattering spectroscopy (CAICISS) and density functional theory (DFT). CAICISS simulations based on structural parameter values of the well known buckled dimer model obtained in quantitative low energy electron diffraction studies in the 1980s were found to fit well with experimental data measured in the [1̄10] azimuth, but offered a relatively poor fit in all other incident geometries. A new surface structure derived from DFT calculations, involving small changes to bond angles and interlayer spacings, was optimized during the analysis of CAICISS data, until good fits of the data were obtained for all three azimuths ([1̄10], [001], and [1̄11]). The key feature of the new structure found to be required for this improved agreement with experiment is the inclusion of relaxations both parallel and perpendicular to the surface between the second and third layers. © 2012 American Physical Society

Organic photovoltaic cells utilising ZnO electron extraction layers produced through thermal conversion of ZnSe

In this work, a thin ZnSe layer was deposited in a vacuum and then thermally annealed in air to provide an efficient electron extraction layer for an inverted organic photovoltaic (OPV) cell. Annealing the ZnSe film at 450 °C (ZnSe(450 °C)) increased the device performance and gave an efficiency of 2.83%. X-ray photoelectron spectroscopy (XPS) measurements show that the increased device performance upon annealing at 450 °C is due to the thermal conversion of ZnSe to ZnO. ZnO has a wider band gap than ZnSe, which allows for more light to reach the photoactive layer. The electronic structures of the treated ZnSe films were explored by ultraviolet photoemission spectroscopy (UPS) which showed that the ZnSe(450 °C) films had a Fermi level close to the conduction band edge, allowing for efficient electron extraction compared to the energetic barrier for extraction formed at the ZnSe(RT)/organic interface

Influence of nonmagnetic Zn substitution on the lattice and magnetoelectric dynamical properties of the multiferroic material CuO

Dynamic magnetoelectric coupling in the improper ferroelectric Cu1-xZnxO (x=0,x=0.05) was investigated using terahertz time-domain spectroscopy to probe electromagnon and magnon modes. Zinc substitution was found to reduce the antiferromagnetic ordering temperature and widen the multiferroic phase, under the dual influences of spin dilution and a reduction in unit-cell volume. The impact of Zn substitution on lattice dynamics was elucidated by Raman and Fourier-transform spectroscopy, and shell-model calculations. Pronounced softenings of the Au phonons, active along the direction of ferroelectric polarization, occur in the multiferroic state of Cu1-xZnxO, and indicate strong spin-phonon coupling. The commensurate antiferromagnetic phase also exhibits spin-phonon coupling, as evidenced by a Raman-active zone-folded acoustic phonon, and spin dilution reduces the spin-phonon coupling coefficient. While the phonon and magnon modes broaden and shift as a result of alloy-induced disorder, the electromagnon is relatively insensitive to Zn substitution, increasing in energy without widening. The results demonstrate that electromagnons and dynamic magnetoelectric coupling can be maintained even in disordered spin systems. © 2014 American Physical Society

Surface electronic properties of In-rich InGaN alloys grown by MOCVD

The band bending, position of Fermi level at the cleaned surfaces and bulk Fermi level of In-rich InxGa1-xN alloys grown by metal-organic chemical vapor deposition with a composition of 0.20 ≤ x ≤ 1.00 have been investigated using X-ray photoemission spectroscopy, infrared reflectivity and Hall effect measurements. Wet etching of InxGa1-xN alloys in HCl successfully reduced the native oxides at the surface, allowing these measurements to be performed more accurately. Electron accumulation layers, accompanied by downward band bending, are present at the surface, with a decrease to flatband conditions occurring at x ≈ 0.2 with increasing Ga fraction. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Valence-band orbital character of CdO: A synchrotron-radiation photoelectron spectroscopy and density functional theory study

N-type CdO is a transparent conducting oxide (TCO) which has promise in a number of areas including solar cell applications. In order to realize this potential a detailed knowledge of the electronic structure of the material is essential. In particular, standard density functional theory (DFT) methods struggle to accurately predict fundamental material properties such as the band gap. This is largely due to the underestimation of the Cd 4d binding energy, which results in a strong hybridization with the valence-band (VB) states. In order to test theoretical approaches, comparisons to experiment need to be made. Here, synchrotron-radiation photoelectron spectroscopy (SR-PES) measurements are presented, and comparison with three theoretical approaches are made. In particular the position of the Cd 4d state is measured with hard x-ray PES, and the orbital character of the VB is probed by photon energy dependent measurements. It is found that LDA + U using a theoretical U value of 2.34 eV is very successful in predicting the position of the Cd 4d state. The VB photon energy dependence reveals the O 2p photoionization cross section is underestimated at higher photon energies, and that an orbital contribution from Cd 5p is underestimated by all the DFT approaches. © 2014 American Physical Society

Giant reduction of InN surface electron accumulation: Compensation of surface donors by Mg dopants

Extreme electron accumulation with sheet density greater than 1013 cm -2 is almost universally present at the surface of indium nitride (InN). Here, x-ray photoemission spectroscopy and secondary ion mass spectrometry are used to show that the surface Fermi level decreases as the Mg concentration increases, with the sheet electron density falling to below 108 cm-2. Surface space-charge calculations indicate that the lowering of the surface Fermi level increases the density of unoccupied donor-type surface states and that these are largely compensated by Mg acceptors in the near-surface hole depletion region rather than by accumulated electrons. This is a significant step towards the realization of InN-based optoelectronic devices. © 2012 American Physical Society

High voltage hybrid organic photovoltaics using a zinc oxide acceptor and a subphthalocyanine donor

We demonstrate hybrid organic photovoltaic (HOPV) bilayer devices with very high open circuit voltages (VOC) of 1.18 V based on a sol-gel processed zinc oxide (ZnO) acceptor and a vacuum deposited boron subphthalocyanine chloride (SubPc) donor layer. X-ray photoelectron spectroscopy (XPS) and Kelvin Probe (KP) measurements of the ZnO/SubPc interface show that the ZnO preparation conditions have a significant impact on the film composition and the electronic properties of the interface, in particular the work function and interface gap energy. Low temperature processing at 120 °C resulted in a ZnO work function of 3.20 eV and the highest VOC of 1.18 V, a consequence of the increased interface gap energy. © the Owner Societies 2014

Optimal growth and thermal stability of crystalline Be 0.25Zn0.75O alloy films on Al2O 3(0001)

The influence of growth temperature on the synthesis of Be xZn1-xO alloy films, grown on highly-mismatched Al2O3(0001) substrates, was studied by synchrotron x-ray scattering, high-resolution transmission electron microscopy and photoluminescence measurements. A single-phase BexZn 1-xO alloy with a Be concentration of x = 0.25, was obtained at the growth temperature, Tg = 400 °C, and verified by high-resolution transmission electron microscopy. It was found that high-temperature growth, Tg≥600 °C, caused phase separation, resulting in a random distribution of intermixed alloy phases. The inhomogeneity and structural fluctuations observed in the BexZn1-xO films grown at high temperatures are attributed to a variation in Be composition and mosaic distribution via atomic displacement and strain relaxation. © 2014 AIP Publishing LLC

Hard x-ray photoelectron spectroscopy as a probe of the intrinsic electronic properties of CdO

Hard x-ray photoelectron spectroscopy (HAXPES) is used to investigate the intrinsic electronic properties of single crystal epitaxial CdO(100) thin films grown by metal organic vapor phase epitaxy (MOVPE). The reduced surface sensitivity of the HAXPES technique relaxes stringent surface preparation requirements, thereby allowing the measurement of as-grown samples with intrinsically higher carrier concentration (n=2.4×1020cm-3). High-resolution HAXPES spectra of the valence band and core levels measured at photon energy of 6054 eV are presented. The effects of conduction band filling and band gap renormalization are discussed to explain the observed binding energy shifts. The measured bandwidth of the partially occupied conduction band feature indicates that a plasmon contribution may be present at higher carrier concentrations. The Cd 3d5/2 and O 1s core-level line shapes are found to exhibit an increased asymmetry with increased carrier concentration, interpreted as evidence for final state screening effects from the carriers in the conduction band. Alternatively the core-level line shape is interpreted as arising from strong conduction electron plasmon satellites. The nature of these two competing models to describe core-level line shapes in metallic oxides is explored. © 2014 American Physical Society

Influence of charged-dislocation density variations on carrier mobility in heteroepitaxial semiconductors: The case of SnO2 on sapphire

In highly mismatched heteroepitaxial systems, the influence of carrier- and dislocation-density variations on carrier mobility is revealed. Transmission electron microscopy reveals the variation of dislocation density through a series of SnO2 films grown by molecular-beam epitaxy on sapphire substrates where the lattice mismatch exceeds 11%. A layer-by-layer parallel conduction treatment of the carrier mobility in SnO2 epilayers is used to illustrate the dominant role of the depth-dependent dislocation density and charge profile in determining the film-thickness dependence of the transport properties. © 2012 American Physical Society