Thorsten U. Kampen

Experimental investigation and simulation of hybrid organic/inorganic Schottky diodes

We have investigated electronic transport in hybrid organic/inorganic Schottky diodes. In order to derive from basic principles the transport properties of the organic semiconductors, we have use a two-dimensional drift-diffusion simulator which properly accounts for transport in both organic and inorganic layers. We have calculated the I–V characteristics of Ag/PTCDA/GaAs Schottky diodes as a function of PTCDA thickness and compared the results with experimental in situ measurements. The interplay between barrier height, PTCDA thickness, space-charge-limited current, and image charge is outlined.

Barrier heights of GaN Schottky contacts

Abstract Silver and lead contacts prepared by evaporation onto clean n-GaN(0001) surfaces are rectifying. Their zero-bias barrier heights and ideality factors were determined from the current-voltage characteristics. The observed linear correlation between the barrier heights and the ideality factors is attributed to nonuniform distributions of barrier heights along the interfaces. The barrier heights of ideal Schottky contacts depend on the applied voltage due to the image-force lowering only and their ideally factors nif are approximately 1.01. By extrapolation of our experimental data to n = 1.01, we obtain barrier heights of 0.82 eV and 0.73 eV for uniform Ag- and Pb/n-GaN(0001) contacts, respectively. By applying the idea of metal-induced gap states (MIGS), the barrier heights of ideal Schottky contacts have been predicted to vary linearly as a function of the difference of the metal and the semiconductor electronegativities. The zero-charge-transfer barrier height and slope parameter are characteristic of the respective semiconductor. The zero-charge-transfer barrier heights have been calculated using an empirical tight-binding approach and the slope parameters are given by the optical dielectric constants. The experimental barrier heights of GaN Schottky contacts confirm the predictions of the MIGS-and-electronegativity model.

Study of the interaction of tris-(8-hydroxyquinoline) aluminum (Alq3) with potassium using vibrational spectroscopy: Examination of possible isomerization upon K doping

The geometrical structure of potassium-doped Alq3 [tris-(8-hydroxyquinoline) aluminum] and the interaction between the Alq3 molecule and potassium were studied using infrared reflection absorption spectroscopy (IRRAS), surface-enhanced Raman scattering (SERS), and density functional theory calculations. A major aim of this study was to examine the theoretically predicted isomerization of Alq3 molecules from the meridional form to the facial form upon alkali-metal doping. The observed spectra show significant changes with the deposition of potassium on a thin Alq3 film. The calculated IR spectra of the K-Alq3 complex differ significantly between the meridional and facial forms, and the calculation for the meridional form agrees fairly well with the observed spectrum. This demonstrates that (1) the Alq3 molecule does not change to a facial isomer with the deposition of potassium, but retains the meridional form, in contrast to the reported theoretical prediction, and (2) the structure of the complex as eval...

Energy level alignment driven by electron affinity difference at 3,4,9,10-perylenetetracarboxylic dianhydride/n-GaAs(100) interfaces

Ultraviolet photoemission spectroscopy (UPS) was employed to investigate the electronic structure upon deposition of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on differently treated n-GaAs(100) surfaces. Interface dipoles are found to form according to the electron affinities (EA) of the substrates and PTCDA films at the interfaces and, consequently, the vacuum level alignment rule does not hold. The results demonstrate that the energy offset between the conduction band minimum of n-doped inorganic semiconductors and the lowest unoccupied molecular orbital of organic molecular films at the interfaces can be obtained using UPS by systematically varying the EA of substrates with a known band gap.

Electronic properties of cesium-covered GaN(0001) surfaces

Abstract Cesium was deposited on clean n-GaN(0001)-1 × 1 surfaces at 150 K using Cs dispensers. X-ray photoemission spectroscopy showed that the Cs-films grow layer by layer. Cesium-induced variations of electronic surface properties were observed using photoemission spectroscopy with monochromatized He I radiation (UPS) and a Kelvin probe (CPD). The UPS data recorded with clean GaN(0001) surfaces give their ionization energy as 6.8 ± 0.1 eV, so that their electron affinity measures 3.35 ± 0.1 eV. The work function of clean GaN(0001) surfaces was determined as 4 ± 0.2 eV. Cesium deposition first reduces the work function by up to 2.7 eV and then increases it until eventually a work function of 2 eV is reached. A simple point-charge model easily explains this decrease of the work function by the formation of surface dipoles. The latter value is characteristic of bulk cesium.

Optical properties and molecular orientation in organic thin films

The optical properties and the molecular orientation in thin films of 3,4,9,10perylenetetracarboxylic dianhydride (PTCDA) and N,N � -dimethyl-3,4,9,10perylenetetracarboxylic diimide (DiMePTCDI) were studied by means of variable angle spectroscopic ellipsometry (VASE), atomic force microscopy (AFM), near edge x-ray absorption fine structure (NEXAFS) spectroscopy, and infrared (IR) and Raman spectroscopy. VASE reveals that both kinds of film exhibit a strong optical anisotropy. For PTCDA, the optical constants are found to have much higher values in the substrate plane than perpendicular to it. While th ea nisotropy measured in the substrate plane on passivated GaAs(100) is very small for PTCDA a giant anisotropy is observed for DiMePTCDI. This difference in the optical properties is attributed to the different orientation of molecules in the thin organic films. While the PTCDA molecules lie flat on the substrate with their molecular plane parallel to the substrate surface, the DiMePTCDI molecules are tilted with respect to the substrate surface and are predominantly oriented with their long axis parallel to the [011] direction of the substrate as confirmed by VASE, NEXAFS, and Raman and IR results.

GaAs surface passivation by ultra-high vacuum deposition of chalcogen atoms

Abstract This paper presents a review of the last studies of the GaAs surface passivation by an ultra-high vacuum deposition of chalcogen atoms. The passivation of GaAs(100) surfaces using the chalcogen atoms selenium and sulfur was investigated using high-resolution soft-X-ray photoemission spectroscopy (XPS) at the synchrotron radiation source BESSY in Berlin. Clean homoepitaxial GaAs layers were exposed to selenium or sulfur at elevated temperatures in ultra-high vacuum (UHV) conditions. This chalcogen treatment leads to the formation of gallium sulfide or gallium selenide like layers. The surfaces are terminated with chalcogen dimers and reveal a (2×1) reconstruction as indicated by low-energy electron diffraction (LEED). These surfaces were found to be very stable with their reconstruction surviving considerable exposure to atmosphere. In addition, the chalcogen modification leads to a reduction in band bending on the n-type doped samples while the band bending on the p-type doped samples is further increased.

Energy band dispersion in well ordered N,N′-dimethyl-3,4,9,10-perylenetetracarboxylic diimide films

The electronic properties of well ordered N,N′-dimethyl-3,4,9,10-perylenetetracarboxylic dimide (DiMe-PTCDI) films prepared on sulfur passivated GaAs(001) substrates were studied by means of photoemission spectroscopy. From the photon energy dependence of normal emission spectra an intermolecular energy band dispersion of about 0.2eV was determined for the highest occupied molecular orbital (HOMO). Simulation of the density of states reveals that the HOMO band has a single π -character. The observed energy band dispersion thus originates from the intermolecular π-π interaction and is modeled using the tight binding model. The analysis provides a value of 0.04eV for the transfer integral. The inner potential was treated as a fitting parameter such that the expected periodicity of the dispersion in the reciprocal space was obtained.

Synchrotron radiation studies of inorganic-organic semiconductor interfaces

Abstract Organic semiconductors (polymers and small molecules) are widely used in electronic and optoelectronic technologies. Many devices are based on multilayer structures where interfaces play a central role in device performance and where inorganic semiconductor models are inadequate. Synchrotron radiation techniques such as photoelectron spectroscopy (PES), near-edge X-ray absorption fine structure (NEXAFS) and X-ray standing wave spectroscopy (XSW) provide a powerful means of probing the structural, electronic and chemical properties of these interfaces. The surface-specificity of these techniques allows key properties to be monitored as the heterostructure is fabricated. This methodology has been directed at the growth of hybrid organic–inorganic semiconductor interfaces involving copper phthalocyanine as the model organic material and InSb and GaAs as the model inorganic semiconductor substrates. Core level PES has revealed that these interfaces are abrupt and chemically inert due to the weak bonding between the molecules and the inorganic semiconductor. NEXAFS studies have shown that there is a preferred orientation of the molecules within the organic semiconductor layers. The valence band offsets for the heterojunctions have been directly measured using valence level PES and were found to be very different for copper phthalocyanine on InSb and GaAs (0.7 and −0.3 eV respectively) although an interface dipole is present in both cases.

PTCDA film formation on Si(111):H-1×1 surface: total current spectroscopy monitoring

Abstract The process of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) film formation on the Si(111):H-1×1 surface was studied by means of total current spectroscopy (TCS) and low-energy electron diffraction (LEED). Film deposition was performed under ultrahigh vacuum (UHV) conditions at room temperature. Analysis of the evolution of total current (TC) spectra, combined with average film thickness measurements performed by atomic force microscopy (AFM), revealed island growth under the given experimental conditions. This growth type is the result of a very weak interaction between PTCDA molecules and the passivated silicon surface. A preliminary explanation for the fine structure in the TC spectra is given in terms of variation of the elastic electron reflection on the edges of empty electronic bands located above the vacuum level of PTCDA.