M. Himmerlich

Electronic properties of organic semiconductor blends : Ambipolar mixtures of phthalocyanine and fullerene

Mixtures of electron and hole transporting organic materials are widely used for ambipolar organic field-effect transistors and photovoltaic cells. One particular material system used in both device types is a blend of n-conducting fullerene and p-conducting copper-phthalocyanine. The electronic properties of these blends were analyzed by x-ray and ultraviolet photoelectron spectroscopy in dependence on the mixing ratio. The energies of the highest occupied molecular orbitals, the core levels, and the vacuum level are found to vary linearly with the mixing ratio. This energy shift is related to a common work function in the molecular blends.

Effect of dislocations on electrical and electron transport properties of InN thin films. I. Strain relief and formation of a dislocation network

The strain-relaxation phenomena and the formation of a dislocation network in 2H‐InN epilayers during molecular beam epitaxy are reported. Plastic and elastic strain relaxations were studied by reflection high-energy electron diffraction, transmission electron microscopy, and high resolution x-ray diffraction. Characterization of the surface properties has been performed using atomic force microscopy and photoelectron spectroscopy. In the framework of the growth model the following stages of the strain relief have been proposed: plastic relaxation of strain by the introduction of geometric misfit dislocations, elastic strain relief during island growth, formation of threading dislocations induced by the coalescence of the islands, and relaxation of elastic strain by the introduction of secondary misfit dislocations. The model emphasizes the determining role of the coalescence process in the formation of a dislocation network in heteroepitaxially grown 2H‐InN. Edge-type threading dislocations and dislocati...

The role of Si as surfactant and donor in molecular-beam epitaxy of AlN

The growth of Si-doped AlN(0001) thin films on Al2O3(0001) substrates by plasma-induced molecular-beam epitaxy is reported. We have found that Si positively affects the epitaxy being an effective surfactant for AlN growth with a remarkable impact on the crystal quality. It was proven that the characteristic surface reconstruction sequences frequently related to the Al adatoms are obviously Si induced on AlN(0001) surfaces. It was also observed that heavy doping conditions result in volume segregation of Si on the threading dislocation network and in the formation of an amorphous (AlO)(SiO)N cap layer caused by surface oxidation of the accumulated Al and segregated Si. The electron affinity was measured to be smaller than 0.5eV on the clean AlN surface after removing of the cap layer using Ar+ sputtering.

A comparative study on the electronic structure of the 1 -ethyl-3 -methylimidazolium bis (trifluoromethylsulfonyl )amide RT -ionic liquid by electron spectroscopy and first principles calculations

The near-surface electronic structure of the room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide has been investigated with ultraviolet and X-ray photoelectron spectroscopy as well as metastable induced electron spectroscopy. The results have been compared with density functional theory calculations. The good agreement between the experimental and theoretical data provides detailed insight into the origin of the observed spectral features. In particular, we found that a simple composition of the spectra of the isolated ions does not suffice to fit to the experimental results, but interionic interactions have to be considered.

Morphology, Crystal Structure and Charge Transport in Donor–Acceptor Block Copolymer Thin Films

We studied structure and charge transport properties of thin films of donor-acceptor block copolymers, poly(3-hexylthiophene-block-perylene bisimide acrylate), using a combination of X-ray scattering, AFM and vertical charge transport measurements in diode devices. Block copolymer self-assembly and crystallization of the individual components are interrelated and different structural states of the films could be prepared by varying preparation conditions and thermal history. Generally the well-defined microphase structures found previously in bulk could also be prepared in thin films, in addition alignment induced by interfacial interactions was observed. Microphase separated block copolymers sustain ambipolar charge transport, but the exact values of electron and hole mobilities depend strongly on orientation and connectivity of the microdomains as well as the molecular order within the domains.

Effect of surface oxidation on electron transport in InN thin films

The chemical and electron transport properties of oxidized indium nitride epilayers and indium oxide/indium nitride heterostructures are reported. It is shown that the accumulation of electrons at the InN surface can be manipulated by the formation of a thin surface oxide layer using an ozone-assisted oxidation processing. It results in improved transport properties and in a reduction of the electron sheet concentration of the InN epilayer caused by a passivation of the surface donors and a shift of the electron density distribution peak from the surface toward the bulk InN. Using the ensemble Monte Carlo simulation method, the electron mobility for different dislocation densities and surface band bending values has been calculated. The theoretical results correlate well with our experimental data. In opposition to the ozone treatment, in epitaxial oxide/nitride heterojunctions the electron sheet concentration of InN raises due to the increasing band bending at the heterointerface affecting adversely the ...

Effect of Annealing on the Properties of Indium-Tin-Oxynitride Films as Ohmic Contacts for GaN-Based Optoelectronic Devices

Indium-tin-oxynitride (ITON) films have been fabricated by rf sputtering from an indium-tin-oxide target in nitrogen plasma. The influence of postdeposition annealing up to 800 degrees C is analyzed by electrical, optical, and surface characterization of the films in comparison to indium-tin-oxide (ITO) films fabricated in argon plasma. High-temperature annealing resulted in ITO(N) films with similar carrier concentrations. However, the resistivity and optical transmittance of the ITON films were higher than those of the ITO films. Photoelectron spectroscopy revealed that nitrogen is incorporated into the ITON structure in an unbound state as well as through the formation of metal-nitrogen and oxynitride bonds that decorate oxygen vacancies. When the core level electron spectra of ITO and ITON films are compared, a correlation between carrier concentration and the incorporated nitrogen is found. Changes in ITON electrical properties are mainly induced by the release of nitrogen at temperatures above 550 degrees C. In this context, ohmic contact behavior was achieved for ITON on p-type GaN after annealing at 600 degrees C, while no ohmic contact could be realized using ITO.

Confirmation of intrinsic electron gap states at nonpolar GaN(1-100) surfaces combining photoelectron and surface optical spectroscopy

The electronic structure of GaN(1–100) surfaces is investigated in-situ by photoelectron spectroscopy (PES) and reflection anisotropy spectroscopy (RAS). Occupied surface states 3.1 eV below the Fermi energy are observed by PES, accompanied by surface optical transitions found in RAS around 3.3 eV, i.e., below the bulk band gap. These results indicate that the GaN(1–100) surface band gap is smaller than the bulk one due to the existence of intra-gap states, in agreement with density functional theory calculations. Furthermore, the experiments demonstrate that RAS can be applied for optical surface studies of anisotropic crystals.

Surface properties of stoichiometric and defect-rich indium oxide films grown by MOCVD

The influence of metalorganic chemical vapor deposition growth conditions on the indium oxide surface properties is investigated using photoelectron spectroscopy. Particular attention is paid to nanocrystalline samples grown at fairly low temperatures which are known for their high sensitivity to ozone. The results are compared to measurements on In2O3 films in cubic and rhombohedral crystal structure. It is shown that the growth conditions have a strong impact on the physical properties and that samples grown at 200 °C or below are highly oxygen-deficient and rich in defects, influencing the surface chemical and electronic properties and resulting in high ozone sensitivity.

Birefringence and refractive indices of wurtzite GaN in the transparency range

Birefringence and anisotropic refractive indices of wurtzite GaN within the spectral range from 0.58 eV to 3.335 eV were determined combining optical retardation and spectroscopic ellipsometry measurements on a series of undoped m- and c-plane GaN bulk substrates grown by hydride vapor phase epitaxy. It is observable that the birefringence has a maximum close to the absorption edge and a weak broad minimum in near-IR range. A quantitative explanation of the whole data is given in terms of contributions to the optical response of GaN due to discrete excitons, Coulomb enhanced band-to-band optical transitions near the E0 critical point of the band structure, high-energy optical transitions, and infrared active optical phonon modes which are different for the ordinary and extraordinary waves both in magnitude and in spectral dependence.