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This paper describes the deposition of superconductive Nb-N thin films in a metal-organic plasma-enhanced atomic layer deposition process using (tert-butylimido)-tris (diethylamino)-niobium and hydrogen plasma as precursors. In extension of our previous work, which investigated the possibility to deposit superconducting Nb-N, we systematically investigated the influence of different plasma parameters on superconducting and morphological properties of the niobium nitride thin film formed during the process. An initial increase of the duration of the plasma dose led to higher transitions temperatures and critical current densities, the optimum being a plasma dose time of 50 s. By decreasing plasma pressure, the resistivity at room temperature decreased, while the transition temperature increased. In addition, Nb-N thin films were deposited onto several substrates such as silicon, thermally grown silica, magnesium oxide (MgO), and r-plane sapphire. ${\rm{T}}_{{\rm{C}}}$ values from 6.2 K up to 14 K were achieved independently of the substrate materials. However, films deposited on MgO showed lower ${\rm{T}}_{{\rm{C}}}$ values. X-ray photoelectron spectroscopy measurement revealed the presence of niobium nitride but also of niobium oxide and oxy-nitride components in the films as well as the existence of a high amount of incorporated carbon impurities. X-ray diffraction measurements revealed two significant reflexes, which could be attributed to niobium nitride only. No crystalline niobium oxide or niobium oxynitride was detected. Thus, the films consisted of a matrix of polycrystalline Nb-N and amorphous or microcrystalline grains of different niobium oxide and oxynitride phases. Due to the fact that the deposited material showed superconductivity especially for ultrathin layers with thicknesses in the nanometer range, these films may be suitable for superconducting nanowire single photon detectors.

Gallium oxide nanowires were grown on different substrates using a corona plasma assisted vapor phase epitaxy process and gold catalyst. It is shown that the silicon carbide pseudo substrate in combination with the plasma excitation of the gas phase supports the growth of the gallium oxide nanowires. Analyzing the orientation of the nanowires with respect to the growth surface, it is concluded that the nanowires growth proceed along the fast growth direction of gallium oxide.

The vacuum/liquid interface of ionic liquid/metal salt binary mixtures was investigated under ultra-high vacuum conditions using photoelectron spectroscopy. In this study we compare two ionic liquids 1- B utyl-1- m ethyl- p yrrolidinium-bis( t ri f luoromethylsulfonyl)-imide, [BMP][Tf 2 N] and 1- P ropyl-3- m ethyl- im idazolium- bis( t ri f luoromethyl-sulfonyl)-imide [PMIm][Tf 2 N] – in combination with two fluoride salts – TaF 5 and NbF 5 – at different concentrations. The results show a clearly different behavior for samples, which were in contact with ambient conditions and those being kept in inert conditions. Under inert atmosphere, the experimental results suggest rather equal anion and cation concentration of the IL constituents in the near surface region. At ambient conditions and high MF 5 (M = Ta, Nb) concentration, a strong deviation from the expected stoichiometry in the near surface region was observed, obviously caused by chemical reactions of the IL/salt system with the atmospheric environment. It is expected, that these reactions have also impact on the quality of electrodeposited metal films from these solutions.

Amorphous carbon thin films were prepared by direct current hollow cathode sputter deposition in Ar discharge with the injection of small amounts of H2 and/or N2. The influence of these additives on the film properties with particular focus on the application as a coating for electron cloud mitigation in particle accelerators is characterized by optical spectroscopy, X-ray photoelectron spectroscopy, and secondary electron yield (SEY) measurements. The SEY maximum increased from initially 0.98 with pure Ar in the gas discharge up to 1.38 at 0.5% H2 while the addition of 1% of pure N2 enabled to reduce it to 0.88. The simultaneous addition of N2 to the H2 containing discharge allowed an average SEY maximum reduction of 20%. The optical bandgap revealed a correlation between the increase/decrease of the bandgap and the SEY increment/reduction for H2/N2 addition. The surface composition changes and the resulting modification of the sp2/sp3 ratio correlate with the changes in SEY and optical properties. The obtained results highlight the potential of intentionally injected N2 to counteract the detrimental effect of the inevitable H2 partial pressure in the coating systems during the production of amorphous carbon thin films for anti-multipacting applications in particle accelerators.

Controlling the surface formation of the group-V face of (1 1 1)-oriented III-V semiconductors is crucial for subsequent successful growth of III-V nanowires for electronic and optoelectronic applications. With a view to preparing GaP/Si(1 1 1) virtual substrates, we investigate the atomic structure of the MOVPE (metalorganic vapor phase epitaxy)-prepared GaP(1 1 1)B surface (phosphorus face). We find that upon high-temperature annealing in the H2-based MOVPE process ambience, the surface is phosphorus-depleted, as evidenced by X-ray photoemission spectroscopy (XPS). However, a combination of density functional theory calculations and scanning tunneling microscopy (STM) suggests the formation of a partially H-terminated phosphorus surface, where the STM contrast is due to electrons tunneling from non-terminated dangling bonds of the phosphorus face. Atomic force microscopy (AFM) reveals that a high proportion of the surface is covered by islands, which are confirmed as Ga-rich by Auger electron spectroscopy (AES). We conclude that the STM images of the samples after high-temperature annealing only reflect the flat regions of the partially H-terminated phosphorus face, whereas an increasing coverage with Ga-rich islands, as detected by AFM and AES, forms upon annealing and underlies the higher proportion of Ga in the XPS measurements.

Single crystalline and textured In2O3 thin films with (1 1 1) surface orientation, grown by plasma-assisted molecular beam epitaxy, were used as a model system to study the role of bulk and surface electron accumulation layer conductance for ozone sensing. Both conductance contributions, which add to the total film conductance, were systematically varied. The resulting ozone sensitivity was determined by total conductance measurements in synthetic air with defined ozone concentration using UV irradiation instead of heating to regenerate the In2O3 surface. Depletion of the surface electron accumulation by an oxygen plasma treatment, confirmed by X-ray photoelectron spectroscopy, rendered the films ozone insensitive. The ozone response of films with an accumulation layer was increased by thickness reduction or by designing the bulk of the film semi-insulating using deep acceptor doping by Mg. Our results of using electron accumulation layers for gas sensing and bulk doping by deep acceptors to increase sensitivity can be generalized to other gas sensing materials. The use of single crystalline films allows selecting the most sensitive crystallographic surface orientation and may have further advantages over polycrystalline films, such as increased stability and sensing speed.

authoren The interaction of defect-rich nanocrystalline indium oxide films, which have previously shown to exhibit excellent ozone sensing properties, with O3, O2, and H2O molecules is investigated using ultra-violet and X-ray photoelectron spectroscopy. The investigated samples are grown by metalorganic chemical vapor deposition at low temperatures resulting in high oxygen deficiency and high defect density. The ozone-induced surface oxidation and UV-induced photoreduction mechanisms of the ozone sensor active material are evaluated with respect to surface stoichiometry and electronic properties including adsorbate features, band bending and surface dipole formation. A strong interaction with ozone and water is found, whereas the interaction with O2 is relatively weak. In all cases the interaction results in the same negatively charged oxygen adsorbate species, which can either be removed by UV light or by annealing resulting in the capability of these films to be used in reversible adsorption induced oxidation and UV/thermal reduction cycles.

We report the design of a low-temperature liquid metal battery (LMB). Li and Ga as the negative and positive electrode, respectively, are used in combination with a room temperature ionic liquid as an electrolyte. 1 mol/L lithium bis(trifluoromethylsulfonyl)imide (Li[TFSI]) in 1-butyl-1-methylpyrrolidinium bis (trifluoromethylsulfonyl)imide ([BMP][TFSI]) is chosen as electrolyte. The battery operates at 220 °C which is a relatively low temperature for a LMB and shows good electrochemical performance at low current density. The cells were cycled for more than 600 h and achieved a round-trip Coulombic efficiency close to 100% and an average voltage efficiency of 66% resulting in an overall energy efficiency of 65%. At higher current densities, however, the system showed up to 75% irreversible capacity loss after three cycles. To understand the origin of this strong deterioration, we characterized the surface and the bulk properties of the Ga cathode using X-ray Photoelectron Spectroscopy. Especially at higher current densities a decomposition of the electrolyte was found. The chemical changes that occurred and the elemental distribution at the Ga cathode are analyzed based on XPS measurements at different stages of the battery charge/discharge cycling.

In this work we investigate the interaction of potassium and water with 2×2 reconstructed InN(0001) surfaces prepared by plasma-assisted molecular beam epitaxy. The influence of adsorbate-substrate-interaction on surface properties is characterized in-situ by photoelectron spectroscopy. Potassium exposure leads to a strong reduction in the work function Φ to 1.6 eV revealing a charge transfer from the adsorbate to the InN surface. In parallel, a reduction of the surface downward band bending by 0.2 eV and hence a reduced electron accumulation density is observed. While interaction of water with clean InN(0001)-2×2 surfaces induces only minor changes in the surface band bending, water adsorption at potassium covered InN(0001) leads to a reversal of the K-induced reduction in surface band bending and a slight increase of Φ to 2.4 eV. These results show that surrounding water modifies the interaction of potassium with InN(0001) surfaces. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Commercial titanium was treated in ambient atmosphere using pulsed Nd:YAG ( λ = 1064 nm ) laser irradiation. Repeated laser treatments induce a removal of surface contaminants as well as the formation of a nanostructured top layer exhibiting a large effective surface and nanometer roughness. The laser induced oxidation leads to the presence of a surface layer with strongly improved, hydrothermally stable adhesion when joined to a one-component, hot-curing epoxy-based adhesive. Changes in the material properties have been characterized with respect to the topography, the chemical composition and the crystal structure using SEM, cross-beam FIB, XPS and XRD analyses in order to correlate the adhesion behavior with the structural and chemical characteristics of the surface.

We investigated the impact of SF6 plasma treatments on the electronic transport properties of GaN/AlGaN/GaN heterostructures by employing different plasma conditions as well as annealing in nitrogen atmosphere at 425 °C. The electrical properties are characterized by Hall-effect measurements while electron spectroscopy and X-ray measurements are used to investigate changes in the surface chemical composition and in the layer structure, respectively. It is demonstrated that plasma treatments strongly affect the 2DEG properties of the heterostructure due to altering of the surface potential accompanied by the formation of a thin fluorinated amorphous film. Increasing the DC bias voltage for the plasma treatment leads to an additional degradation of the mobility caused by incorporation of fluorine into the heterostructure interface. Furthermore, the thin GaN cap layer is etched by plasma treatments with higher bias potential, which increases the carrier density at the interface (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The valence band offset (VBO) at the interface between indium nitride (InN) and selected oxide materials (Al2O3, TiO2, In2O3 and HfO2) is determined using X-ray photoelectron spectroscopy (XPS). For exact VBO determination, InN samples with oxide cap layers of varying thickness are investigated. The VBO values are extrapolated by linear regression of the thickness dependent energetic distances ΔE between the valence band maxima (VBM) at the oxide and InN surface and their corresponding heterointerface. The determined VBO values are (2.7 ± 0.2) eV for Al2O3/InN, (1.8 ± 0.2) eV for TiO2/InN, (1.5 ± 0.2) eV for In2O3/InN, (1.3 ± 0.2) eV for e-beam evaporated HfO2 on InN, and (2.0 ± 0.2) eV for atomic layer deposited HfO2 on InN. In some cases the oxide deposition process leads to an oxidation of the InN film at the oxide/InN interface (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The surface electronic properties and adsorption behaviour of as-grown and oxidized N-polar InN films are characterized by photoelectron spectroscopy (XPS, UPS). The epitaxial growth of the InN layers was performed by plasma-assisted molecular beam epitaxy on GaN/6H-SiC(000-1). After growth and in situ characterization the InN surfaces were exposed to molecular oxygen to evaluate the adsorption behaviour of O2 on N-polar InN and to study its impact on the surface electronic properties of the III-nitride material. The results are compared with studies on In-polar InN on GaN/sapphire templates. The as-grown N-polar InN surface exhibits a pronounced surface state at a binding energy of ∼1.6 eV. The valence band minimum lies about 0.8–1.0 eV below the surface Fermi level. Additionally, the XPS core level binding energies for InN(000-1) are reduced compared to InN(0001) films, indicating different surface band bending for clean N-polar and In-polar InN, respectively. The interaction of molecular oxygen with the InN(000-1) surface leads to a downward band bending by 0.1 eV compared to the initial state. Additional adsorption of species from the residual gas of the UHV chamber increases the surface downward band bending. Furthermore two pronounced oxygen related states with an energy distance of ∼5 eV could be detected in the valence band region. The adsorbed oxygen results in an additional component in the N1s core level spectra, which is interpreted as formation of NOx bonds.

Epitaxial graphene was grown on Si-face 4H-SiC. A SiC pretreatment with a carbon cap¬ping technique was used as well as slow heating rates and a temperature of 1800 °C under atmos¬pheric argon pressure. The surface morphology was investigated by atomic force microscopy and Raman spectroscopy was performed for samples with different graphitization times.

We investigated the surface chemistry and valence band (VB) structure of as-grown thin InN(0001)-2 × 2 films as well as their change upon the exposure to oxygen and water. The InN films were grown by plasma-assisted molecular beam epitaxy (PAMBE) and in situ characterized by reflection high electron energy diffraction (RHEED) and photoelectron spectroscopy (UPS, XPS). The oxygen and water exposure was directly performed on the as-grown, contamination-free InN surfaces at room temperature and leads to changes in the chemical surface states as well as the electronic properties. For 2 × 2 reconstructed InN surfaces one observes directly after growth a surface state at the Fermi-edge which decreases continuously with oxygen and water exposure. Furthermore, two oxygen related electronic states develop in the VB at binding energies at around 5 and 10 eV. For water exposure a third weak state around 8 eV is additionally observed. The impact of oxygen and water on the work function Φ as well as the variation of surface band bending was investigated. In both cases for initially 2 × 2 reconstructed surfaces a reduction in the downward band bending is found, while Φ increases in the case of oxygen exposure but in the case of interaction with water a reduced work function is observed. The oxygen uptake rates reveal a higher reactivity of water with InN surfaces compared to oxygen. Furthermore, during oxidation and water exposure different chemical oxygen bonds are formed, but a direct assignment to In–O or N–O bonds is difficult due to changes in the In3d and N1s XPS core level peak shape.

We performed an in-situ analysis of the interaction of water with clean 2×2 reconstructed GaN(0001) surfaces grown by plasma assisted molecular beam epitaxy. The as-grown surfaces were exposed to molecular water by backfilling. Photoemission spectra reveal an extremely high reactivity and dissociative adsorption with an oxygen sticking coefficient of 0.3 and a saturation coverage of approximately 1 ML. Core level spectra of the O1s state show three individual oxygen components with different temporal behavior. Furthermore, the initial reconstruction quickly vanishes and surface states in the valence band of the as-grown surface are removed during the exposure of the first few Langmuirs. For higher exposures, two oxygen-related states appear. The initial upward band bending of 0.4 eV for the as-grown surface is effectively reduced during oxidation resulting in flat band conditions. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We studied InN (0001) grown by plasma assisted molecular beam epitaxy (PAMBE) with high-resolution electron energy-loss spectroscopy (HREELS) after vacuum transfer thus avoiding any further surface preparation. We were able to detect for the first time besides the Fuchs-Kliewer surface phonon ω0 two plasmarons ω– and ω+, which originate from the coupling between surface optical phonons ωSO and plasma oscillations of free electrons resulting from the surface accumulation layer and/or the bulk conduction band. Using dielectric theory we get new insight into the space charge regime of InN grown by MBE. We choose an approach that reaches far beyond the accumulation layer regime. Using a very wide primary beam energy regime from 1 eV up to 200 eV, the depth of information can be varied from extremely surface sensitive to bulk sensitive. Best agreement has been obtained for a downward band bending of 0.9 eV and a peak accumulation layer density of 6.5 × 1019 cm-3 at 2 nm beneath the surface with an unintentional doping of the bulk of 1 × 1019 cm-3. The sheet density of the ionized surface states is determined to Nss = 1.8 × 1013 cm-2 from the self-consistent calculation of the conduction band profile. The presented results provide important information for future InN-based devices. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The surface properties of thin InN(0001) films grown by plasma assisted molecular beam epitaxy (PAMBE) were characterised. Two stable surface reconstructions (a (2 × 2) and a (√3 × √3) R30°) are identified which develop depending on the used preparation conditions. The structural, compositional and electronic surface properties were analysed in-situ using reflection high energy electron diffraction (RHEED) and photoelectron spectroscopy (PES). From the absence of surface contaminants as well as excess indium it can be concluded that these superstructures are formed by single adatom layers. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The structural, chemical and electron transport properties of In2O3/InN heterostructures and oxidized InN epilayers 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. The epitaxial In2O3/InN heterojunctions show an increase in the electron concentration due to the increasing band banding at the heterointerface. The oxidation of InN results in improved transport properties and in a reduction of the sheet carrier concentration of the InN epilayer very likely caused by a passivation of surface donors. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The surface properties of indium tin oxynitride films prepared by rf-sputtering in nitrogen atmosphere were investigated by X-ray and ultraviolet photoelectron spectroscopy as well as electron energy loss spectroscopy and Auger electron spectroscopy depth profiling. The results are compared to reference measurements on conventional rf-sputtered indium tin oxide films. The incorporated nitrogen is present in different chemical environments. Employing these different spectroscopic techniques, it was found that desorption of nitrogen from the ITON structure upon annealing is the origin of the observed drastical changes in the surface composition and electronic structure. The formation of oxygen vacancies and Sn surface segregation upon annealing is linked to improvements in the physical properties (larger spectral range of transmittance and higher conductivity) of the films.

The morphology and surface electronic structure of indium nitride films grown by plasma-induced molecular beam epitaxy have been studied using atomic force microscopy as well as X-ray and ultraviolet photoelectron spectroscopy. Valence band and In4d core level spectra were measured as a function of excitation energy (He I, He II and AlK α radiation) on samples with minimised exposure to ambient conditions after growth (30 s). A work function of 4.1 eV was determined and the onset of the valence band is located 1.6 eV below the Fermi level which is a strong indication of the already discovered electron accumulation at InN surfaces. In addition, the theoretically predicted InN valence states were observed. At the outermost InN surface the In4d level seems to consist of two components, where the existence of a metallic phase can be ruled out by a comparison with sputtered InN samples and measurements performed on an indium foil.

The impact of typical device processing steps (KOH, HCl, HF wet chemical etching, SF 6 and Cl plasma etching) on the surface properties (roughness, chemical composition, contact angle to water) of group III-nitride based chemical sensors is investigated with emphasis on the electrical performance of the sensor and the biocompatibility. An AlGaN/GaN high electron mobility transistor serves as basic sensing device. For our studies, the widely distributed mammalian cell cultures HEK 293FT and CHO-K1 are used as biological model systems. The processing of the devices has only little influence on the cell growth onto the sensor, which is in all cases superior to that on silicon surfaces. Fluorine dry etching leads to oxidation and smoothing of the surface, thus, improving the electrical properties of the AlGaN/GaN sensor. In contrast, autoclave treatment enhances the carbon contamination with negative impact on the sensor properties and increased the contact angle to water, which can be used as indicator for the state of the sensor surface. For all other treatments the contact angle recaptures a stable value of about 50 ± 5° after exposure to air or water droplets for some hours due to contamination by hydrocarbons.

The evolution dynamics of nanoscale Ge islands on both Si (001) and (113) surfaces is explored using ultraviolet photoelectron emission microscopy (UV-PEEM). Real-time monitoring of the in-situ growth of the Ge island structures can allow us to study the variation of the size, the shape and the density of the nanostructures. For Ge depositions greater than monolayer (ML) with a growth rate of at temperatures of , we observed island nucleation on both surfaces indicating the transition from strained layer to island structure. During continuous deposition the circular islands grew larger via ripening processes. AFM measurements showed that the islands grown on Si (001) were dome-shaped while the islands on Si (113) were multiple-side faceted with flat tops of (113)-orientation. In contrast, for Ge deposition with a lower growth rate of on Si(113), we observed the shape transition from circular into elongated island structures. The elongated islands grew longer along the [] during continuous Ge deposition. The shape evolution of the islands is discussed in terms of strain relaxation and kinetic effects.

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 E(0) 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.

In this work we have investigated the impact of typical device processing steps on the surface properties (roughness, chemical composition, contact angle to water) of group III-nitride based chemical sensors with emphasis on the electrical performance of the sensor and the biocompatibility. Basic sensing device is an AlGaN/GaN high electron mobility transistor. The widely distributed mammalian cell cultures HEK 293FT and CHO-K1 served as biological model systems. The processing of the devices had only little influence on the cell growth onto the sensor. In all cases it was superior to silicon surfaces. Fluorine dry etching smoothes the surface and forms an oxide, which improves the electrical properties of the AlGaN/GaN sensor. In contrast, autoclave treatment enhances the carbon contamination with negative impact on the sensor properties and increased the contact angle to water. For all other treatments the contact angle recaptures a value of about 50 ± 5° after exposure to air or water droplets for some hours due to the contamination by hydrocarbons. Einfluss von Prozessen der Bauelementtechnologie auf die Oberflacheneigenschaften und die Biokompatibilitat von Gruppe III-Nitrid-basierenden Sensoren In dieser Studie wird der Einfluss von typischen Prozessen der Bauelementtechnologie auf die Oberflacheneigenschaften (Rauheit, chemische Zusammensetzung, Kontaktwinkel zu Wasser) von Gruppe III-Nitrid-basierenden chemischen Sensoren untersucht mit besonderem Schwerpunkt auf die elektrischen Sensoreigenschaften und die Biokompatibilitat. Zentrales Bauelement ist ein AlGaN/GaN high electron mobility transistor und als biologisches Modellsystem fanden HEK 293FT und CHO-K1 Kulturen Anwendung. Die Bauelementeprozessierung hatte nur geringe Auswirkungen auf das Zellwachstum auf dem Sensor. Alle untersuchten Sensoren erwiesen sich als den Siliziumoberflachen uberlegen. Fluorbasierendes Trockenatzen ebnet die Oberflache und erzeugt ein Oxid, dass die elektrischen Eigenschaften des AlGaN/GaN-Sensors verbessert. Im Gegensatz dazu fuhrt Autoklavieren zur Kontamination mit Kohlenstoff mit negativen Auswirkungen auf die Sensoreigenschaften und angestiegenen Kontaktwinkel zu Wasser. Nach allen anderen Behandlungen tendiert der Kontaktwinkel zu einem Wert von 50 ± 5° nach dem Kontakt der Oberflache mit Umgebungsluft oder Wasser aufgrund einer Kontamination mit Kohlenwasserstoffen.

The growth of fullerene films on the InP(001)-(2 ? 4) surface and the formation of the C60/InP(001)-(2 ? 4) interface were studied by x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, electron energy loss spectroscopy and low energy electron diffraction. C60 adsorption causes weak (~0.15?eV) upward band bending at the interface. Thick C60 films form an fcc (111) structure on the InP(001) surface. The (2 ? 4) reconstruction is preserved beneath the C60 film. The photoelectron measurements yield a valence band discontinuity of 0.88 ? 0.20?eV at the C60/InP(001)-(2 ? 4) interface.

The strain-relaxation phenomena and the formation of a dislocation network in 2 H -InN epilayers during molecular beam epitaxy are reported. The proposed growth model emphasizes the dominant role of the coalescence process in the formation of a dislocation network in 2 H -InN. Edge type threading dislocations and dislocations of mixed character have been found to be the dominating defects in wurtzite InN layers. It is demonstrated that these dislocations are active suppliers of electrons and an exponential decay of their density with the thickness implies a corresponding decay in the carrier density. Room temperature mobility in excess of 1500 cm2 V −1 s−1 was obtained for ∼800 nm thick InN layers with dislocation densities of ∼3×109 cm−2.

The near-surface electronic structure of the room-temperature ionic liquid (RT-IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf(2)N]) has been investigated with the combination of the electron spectroscopies metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS (HeI and HeII)), and monochromatized X-ray photoelectron spectroscopy (XPS). We find that the top of the valence band states originates from states of the cation (see also ref 1). The ultimately surface-sensitive technique MIES proves that the surface layer consists of both cations and anions. The temperature dependence of the spectra has been measured between about 160 and 610 K. Information on the glass transition and the possibility for low-temperature distillation of [EMIM][Tf(2)N] at reduced pressures is derived from the present results.

The growth of Si-doped 2H-AlN thin films 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 also observed that heavy doping conditions result in the volume segregation of Si at the threading dislocation network and in the formation of an amorphous (AlO)(SiO)N cap layer caused by post-growth surface oxidation of the accumulated Al and segregated Si. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nano-crystalline AlN:Si cold cathodes were grown by molecular beam epitaxy on prefabricated silicon μ-tip field emission arrays (FEAs). The investigated FEAs showed an uniform tip distribution and reproducible emission properties. Current densities of up to 0.7 mA for a 4 x 10 6 tips array (1 tip/μm 2 ) have been achieved at a relatively modest voltage of ∼ 60 V. The emitters revealed a good current stability with fluctuations of about 4% at constant voltage. The FEAs demonstrated a 40-fold decrease in operating voltage for a given current following the "activation" of the surface by high field conditioning. The corresponding changes in slope of the Fowler-Nordheim plot and UV photoemission spectra confirm that the surface cleaning causes a reduction of the surface emission barrier at least by factor of 3. The observed large electron affinity of the as loaded surface is presumed to be caused by the adsorption of oxygen and carbon containing species.

Non intentionally doped and Mg-doped InN layers were analyzed by sputter depth profiling in an Auger electron spectroscopy (AES) equipment and by Ultra-violet photoelectron spectroscopy (UPS). On the surface of both types of layers a high concentration of oxygen and a strong accumulation of electrons was observed, however, in contrast to the undoped layers the conductivity profile of Mg doped InN shows a strong discontinuity close to the surface. The depth of this discontinuity strongly depends on the oxygen concentration. The energy shift of the In MNN peak was estimated during the depth profiling in order to obtain information about the position of the Fermi level. In the bulk of the Mg-doped InN the In MNN peak shift of about 0.15 eV demonstrates the influence of the Mg on the Fermi level, while the surface is clearly n-type. A strong shift of the Fermi level close to the surface was observed, which might be attributed to the formation of In 2 O 3 . By the combination of AES and UPS a model for the band bending is proposed, which demonstrates that Mg doping indeed can compensate the n-type conductivity in the bulk and is therefore a prospective candidate to achieve p-type doping in InN.

In this study InN films with wurtzite structure grown by plasma induced molecular beam epitaxy were investigated by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), and electron energy loss spectroscopy (EELS) in order to determine the chemical composition of the sample surfaces before and after ion bombardment. Samples which were investigated without any cleaning procedures showed some contaminations (O and C) due to the previous exposure to air. The ratio between In and N was examined by XPS. The surface contaminations were removed by bombarding the surface with Ar + ions ( E kin =500 eV). Due to preferential sputtering an In rich surface is formed. The degree of In enrichment is strongly dependent on the incident angle of the ions.

We investigated the impact of fluorine and nitrogen plasma treatments on the electronic transport properties of GaN/AlGaN/GaN heterostructures by employing different plasma processes and subsequent annealing in nitrogen atmosphere at 425 °C. It is demonstrated that the plasma treatments affect the 2-dimensional electron gas (2DEG) properties of the HEMT structure resulting in a decrease of the surface potential. In order to understand the physical mechanisms we have undertaken an extensive study of the influence of plasma processing on the 2DEG properties. The electrical properties of the 2DEG are characterized by Hall measurements while X-ray photoelectron spectroscopy (XPS) is used to investigate changes in surface chemical composition. Changes of the DC bias voltage have a strong effect on the mobility presumably by incorporation of fluorine and nitrogen close to the channel. To confirm this assumption, Monte Carlo simulations were carried out, taking into account surface potential and negatively charged fluorine acceptors acting as additional scattering centers within the 2DEG (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report a strong dependence of open circuit voltage on the altered morphology of block copolymer (P3HT-b -PPerAcr) based solar cells. The open circuit voltage increases dramatically by about 300 mV by increasing the amount of acceptor homopolymer within the block copolymer/homopolymer blends. The change in open circuit voltage is found to be in correlation with the enrichment of acceptor moiety at the film surface as identified by Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Based on this fact, an additional increase in open circuit voltage to its maximum values is achieved by introducing an acceptor buffer layer at the cathode interface. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN(0001)-2×2 surfaces were investigated by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) as well as X-ray photoelectron spectroscopy (XPS). Contamination- and metal-free GaN thin films with a 2×2 reconstruction and a rms roughness below 1 nm were grown on 6H-SiC(0001) by plasma assisted molecular beam epitaxy (PAMBE). The valence band structure of the surface was investigated in-situ with ARUPS along the and directions of the surface Brillouin zone. Weak dispersive surface states related to the unreconstructed GaN surface or to the 2×2 superstructure as well as the dispersion of electron states of the bulk band structure are identified and compared to available results from density functional theory (DFT) calculations [Phys. Rev. B 77, 115120 (2008)] for GaN(0001). (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The valence band offset (VBO) at a InN/In0.3Ga0.7N(0001) as well as HfO2/InN(0001) heterojunction is investigated by X-ray photoelectron spectroscopy using monochromated AlKα radiation. The InN and In0.3Ga0.7N films were grown using plasma-assisted molecular beam epitaxy, whereas HfO2 layers were deposited by plasma-assisted electron beam evaporation. The VBOs were determined by analysing the core level binding energy and valence band maxima of bulk-like films as well as of In0.3Ga0.7N and InN layers covered with 5 nm thick overlayers of InN and HfO2, respectively. The resulting VBO values are ∼0.5 eV for the InN/In0.3Ga0.7N heterojunction and ∼0.9 eV in the case of the HfO2/InN heterointerface.

Gallium nitride thin films were grown on silicon carbide (0001) by plasma-assisted molecular beam epitaxy (PAMBE). The samples were cooled down in nitrogen plasma and characterized in situ by reflection high energy electron diffraction (RHEED), photoelectron spectroscopy (XPS/UPS), and atomic force microscopy (AFM) revealing stoichiometric and smooth GaN films virtually free of contaminations. We present valence band data obtained by UPS with strong emission from surface states inside the fundamental band gap. These states and the observed 2 × 2 surface reconstruction are highly sensitive towards residual molecules. Once these surface states have disappeared the original state could not be recovered by surface preparation methods underlining the necessity of in situ investigations on as-grown surfaces. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A thermally induced functionalization process for gallium nitride surfaces with 1-alkenes is introduced. The resulting functionalization layers are characterized with atomic force microscopy and X-ray photoelectron spectroscopy and compared to reference samples without and with a photochemically generated functionalization layer. The resulting layers show very promising characteristics as functionalization for GaN based biosensors. On the basis of the experimental results, important characteristics of the functionalization layers are estimated and a possible chemical reaction scheme is proposed.

Different thin fluorocarbon (FC) films were deposited on Si(111) using plasma polymerisation and then exposed to X-ray radiation. Changes in the chemical composition of the deposited fluorocarbon films as a function of irradiation time were investigated in situ using X-ray photoelectron spectroscopy. The evaluation of the C1s and F1s core level induced emission as a function of exposure to X-ray radiation (Mg Kα, hν = 1253.6 eV) reveals changes in the surface chemical composition of the FC polymer structure. The presented results indicate a high defluorination under X-ray irradiation. Additionally, binding energy shifts of the F1s and C1s peaks during the exposure associated with surface charging effects were observed. With ongoing exposure the surface charging decreases continuously and the FC surfaces become more conductive due to changes in the polymer structure. Different models have been used to describe the decomposition kinetics and surface composition.

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 ...