Kjell Magnusson

Device performance of APFO-3/PCBM solar cells with controlled morphology

Polymer/fullerene solar cells with three different device structures: A) diffuse bilayer, B) spontaneously formed multilayer, and C) vertically homogenous thin films, are fabricated. The photocurrent/voltage performance is compared and it is found that the self-stratified structure (B) yields the highest energy conversion efficiency.

The electronic structure of alkali-metal layers on semiconductor surfaces

Alkali-metal layers on semiconductor surfaces are model systems for metal-semiconductor contacts, Schottky barriers, and metallization processes. The strong decrease of the work function as a function of alkali-metal coverage is also technically made use of. Recently, however, interest in these systems is growing owing to ongoing controversial discussions about questions like: Is the adsorbate system at monolayer coverage metallic or semiconducting, and does the metallization take place in the alkali overlayer or in the top layer of the semiconductor? Is the bonding ionic or covalent? What ist the absolute coverage at saturation? What are the adsorption sites? Do all alkali metals behave similar on the same semiconductor surface? We try to answer some of the questions for Li, Na, K and Cs on Si(111)(2×1), K and Cs on Si(111)(7×7) and on GaAs(110), and Na and K on Si(100)(2×1) employing the techniques of direct and inverse photoemission.

Atomic geometry and electronic structure of CdTe(110)

Abstract A sp 3 tight-binding model for CdTe is constructed and validated by comparison with optical absorption and X-ray photoemission data. The energy minimization method is applied to determine the surface structure of the (110) cleavage face. The predicted surface atomic geometry compares well with that obtained from low-energy electron diffraction intensity analysis. The calculated electronic surface states give a good description of angle-resolved photoemission data.

The bonding of alkali metals to semiconductor surfaces: a direct and inverse photoemission study

Abstract The bonding character (ionic versus covalent) of alkali metals on semiconductor surfaces is controversial. This concerns the amount of charge transfer and the question where a possible metallization takes place (alkali overlayer or semiconductor substrate). To give an answer we have employed direct and inverse photoemission spectroscopy to Na, K, and Cs on Si(111)2×1 and 7 × 7 and on GaAs(110), and K on Si(100)2 × 1. We find that there is no iso-electronic behavior, e.g., monolayer coverages may result in a metallic surface {Si(111)2 × 1 K} or leave the surface semiconducting {Si(111)1 × 1 Na, Si(111)√3 × √3R30° Cs} with specific surface atom re-arrangements. The reason may lie in the increasing competition between the alkali-semiconductor and alkali-alkali interaction as a function of coverage, both being dependent on the atomic radii of the alkali-metal atoms. Our results will be compared to theoretical calculations as well as to the findings of other experimental techniques.

Zincblende–CdSe on GaSb(110): Characterization of epitaxial growth and electronic structure

Substrate‐stabilized pseudomorphic growth offers the chance to study the electronic structure of a particular semiconductor in different crystal structures, and to investigate the influence of structural differences on bulk and surface states. We have grown layers of CdSe in the zincblende modification on cleaved GaSb(110) surfaces by molecular beam epitaxy. The growth mode and structure of the overlayer were studied by means of low energy electron diffraction and photoemission using synchrotron radiation. The attenuation of substrate core level intensities with CdSe deposition indicates layerwise growth. Interface reaction leads to the liberation of Sb, which floats on the growth front, and the formation of a Ga–Se compound, as signaled by changes in substrate and overlayer core level line shape. The valence band offset for this lattice‐matched interface system is 1.09 eV, such that the heterojunction is of the staggered type, in agreement with predictions based on the dielectric midgap energy model.

Photoemission from surface states on the (101̄0) and (112̄0) surfaces of CdSe

Abstract Using a reparameterized tight binding model the symmetry resolved charge densities of electronic surface-state bands for the (1010) and (1120) cleavage faces of CdSe are calculated and compared with photoelectron spectroscopy measurements. This comparison reveals that using the relaxed surface atomic geometries predicted by the model the calculations describe the angle-resolved and angle-integrated spectra semiquantitatively for both surfaces, thereby confirming explicitly the electronic-surface-state mechanism for the reconstructions of the cleavage surfaces of wurtzite structure materials.

Preparation of stoichiometric GaN(0001)1x1: an XPS study

We report on the investigation of GaN(0001)−1×1 using synchrotron radiation x-ray excited photoelectron spectra from the core levels Ga 3p and N 1s, as well as from the contaminants O 1s and C 1s. Measurements were done after outgassing (a) and during three surface treatment methods performed in sequence; (b) ammonia (NH 3 ) flux anneals, (c) Ga deposition with sample held at room temperature followed by vacuum anneal, and (d) Ga deposition on a heated sample followed by Ga desorption during NH 3 flux anneal. We have found that the initial NH 3 flux anneals increased the amount of present N on the surface and enabled the formation of a well-ordered surface structure, according to the low energy electron diffraction (LEED) pattern. After treatment (b) and (d) the core level spectra of Ga 3p are much improved showing clearly distinct features indicative of increased Ga-N bonding. The Ga to N concentration ratio decreases during the surface treatments from 4.0 to 1.1, hence towards stoichiometry. The amounts of C (and O) present on the surface after outgassing corresponded to 1.1 (0.9) monolayers (ML) but reduced to 0.1 (0.1) ML after the final treatment (d). The Fermi level position in the band gap shifts down by 0.55 eV during the surface treatments, indicating a change of states present in the band gap. We have also found strong support that this Ga-polar sample is initially Ga-terminated.

Surface core level shifts on CdS(101̄0)

Abstract The surface shifts of the Cd 4d and S 2p core levels of the wurtzite CdS(1010) surface have been measured. The shifts for the Cd4d and S2p levels were 0.39 and −0.43 eV, respectively. The experimental surface shifts are compared to calculated initial state surface shifts from a simple model due to Monch. It is found that the reduced Madelung potential at the surface alone can explain the experimental observations. A comparison to recently measured surface shifts of cubic CdTe(110) is performed within the same model and the structural aspects of the different surfaces are found to be fully reflected in the magnitudes of the shifts.

Rigid band behavior of Nd2-xCexCuO4-y : A direct and inverse photoemission study

Abstract We have employed ultraviolet photoemission spectroscopy at hν = 21.2 and 40.8 eV and inverse photoemission spectroscopy at hν = 9.5 eV to study Nd 2- x Ce x CuO 4- y ( x = 0.0, 0.1, 0.15, 0.2). We observe a shift of the Fermi level to higher energies as a function of x . The total shift is 0.3 eV and saturates at x = 0.2. As suggested by existing band-structure calculations, this result seems to point to a rigid-band filling of the Ce-derived electrons in these “electron-doped” superconducting cuprates.

ZnO nanocrystals on SiO2/Si surfaces thermally cleaned in ultrahigh vacuum and characterized using spectroscopic photoemission and low energy electron microscopy

Thermal cleaning in ultrahigh vacuum of ZnO nanocrystals distributed on SiO2/Si surfaces has been studied using spectroscopic photoemission and low energy electron microscopy (SPELEEM). This study thus concern weakly bound ZnO nanocrystals covering only 5%–10% of the substrate. Chemical properties, crystallinity, and distribution of nanocrystals are used to correlate images acquired with the different techniques showing excellent correspondence. The nanocrystals are shown to be clean enough after thermal cleaning at 650 °C to be imaged by LEEM and x-ray PEEM as well as chemically analyzed by site selective x-ray photoelectron spectroscopy (μ-XPS). μ-XPS shows a sharp Zn 3d peak and resolve differences in O 1s states in oxides. The strong LEEM reflections together with the obtained chemical information indicates that the ZnO nanocrystals were thermally cleaned, but do not indicate any decomposition of the nanocrystals. μ-XPS was also used to determine the thickness of SiO2 on Si. This article is the first ...