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A series of samples consisting of alternating stripes of ZnO grown by molecular-beam epitaxy (MBE) and radio-frequency (rf) sputtered Ga-doped ZnO stripes was laterally microstructured with a self-aligned pattern transfer method. We measured as a function of temperature the Seebeck coefficient S and the electrical resistivity ρ in-plane of the samples with the transport direction perpendicular to the stripe direction. Throughout the series the bar width and hence the number of interfaces was kept constant, but the interface profile was varied yielding different interface lengths and geometries. The dependence of S, ρ and the power factor S2/ρ on the interface length at room temperature were simulated using an empirical network model and it was demonstrated that the macroscopic transport coefficients are very sensitive to the interface region and that even this rather simple modelling yields useful information about the interface region.

The E 2h and A 1 (LO) phonon modes of AlN films grown on sapphire are analyzed by Raman scattering as a function of silicon doping for concentrations covering from 5.5 x 10 19 cm ―3 to 5.2 x 10 21 cm ―3 . For high doping levels the appearance of a mode around 520 cm ―1 indicates the precipitation of crystalline silicon in the samples and its inhomogeneous incorporation to the AlN layer. The frequency of this mode shifts to lower energies with doping, indicating that the silicon crystals are embedded in the AlN lattice and under tensile strain. On the other hand, the AlN phonon modes are blue-shifted due to the compressive strain as a result of the silicon incorporation. This strain is partially relieved for very high doping levels. The coupling of the polar A 1 (LO) mode of the AlN films with the electronic cloud (phonon―plasmon coupling) is analyzed for electron concentrations up to 4 × 10 17 cm ― 3 . A 2 cm ―1 blue shift of the phonon frequencies is observed for the highest electron concentrations.

The optical properties of c-plane GaN layers grown by molecular beam epitaxy on a-plane sapphire substrate are investigated. Polarization-dependent photoreflectance spectroscopy was applied with the electric field vector of the probe beam being either parallel or perpendicular to the [10 10] direction of the GaN films. A pronounced in-plane optical anisotropy under normal incidence of light is observed which is attributed to anisotropic in-plane strain. It originates from the difference of the thermal expansion coefficients of the substrate parallel and perpendicular to its c-axis. Temperaturedependent studies and a comparison to photoluminescence and reflectance anisotropy studies indicate that the strain (anisotropy) increases with decreasing sample temperature. The dependence of transition energies and oscillator strengths of GaN as a function of strain values is calculated by k·p perturbation theory for comparison. The results emphasize the experimental data.

A detailed analysis of electroreflectance (ER) spectra for AlGaN/GaN heterostructures containing a two-dimensional electron gas (2DEG) is presented. The properties of two samples with different Al-content in the barrier layer are compared. Calculated field-dependent dielectric functions (DF) for GaN and AlGaN which take into account electron-hole interaction (exciton effects) provide the basis for the modeling of the spectra. It is demonstrated that the Seraphin coefficients exhibit a strong spectral dependence around the gap which cannot be neglected in a fit of ER spectra. The use of the calculated DFs in a multi-layer model allows to reproduce all peculiarities of experimental ER spectra for both, the GaN channel layer and the AlGaN barrier layer, despite the inhomogeneous field distribution. The determined AlGaN barrier field strengths are very close to the values obtained by applying the asymptotic formulation of the Franz-Keldysh effect. Finally, the polarization gradient at the AlGaN/GaN boundary is estimated from these field strengths. The gradient is slightly lower than theoretically predicted which confirms previous studies.

Received 24 October 2006, accepted 30 October 2006 Published online 10 November 2006 PACS 68.35.–p, 68.47.Fg, 73.30.+y, 81.07.Pr, 82.47.Rs The covalent attachment of small organic molecules and larger functional biomolecules such as DNA, en-zymes, or other proteins on semiconductors is a new field of basic research with a pronounced interdisci-plinary flavour. The ultimate goal of this endeavour is the creation of novel organic/inorganic heterostruc-tures which can provide a direct link between the complex worlds of biology and digital electronics on a nanometer scale. The purpose of the present survey is to provide an overview over basic physics aspects, preparation methods, as well as possible applications of biofunctionalized semiconductors based on dif-ferent material systems.

We performed thermo and piezoresistive measurements on GaN bulk samples grown with different techniques (PIMBE, MOCVD). By analysing the I(V) characteristics as a function of temperature, we found a temperature coefficient of resistance (TCR = a) of the same order of magnitude (a few percent per °C) as commercial thermistors, with a peak value of 19%/°C. The measured gauge factor is about five, a value comparable with that obtained for GaN by other groups. We also performed PC vs. temperature measurements on a MQW GaN/AlGaN finding a peak shift of about 0.4 meV/K.