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The crystalline form of sp3-hybridized carbon, diamond, offers various electrolyte-stable surface terminations. The H-termination-selective attachment of nitrophenyl diazonium, imaged by AFM, shows that electrochemical oxidation can control the fractional hydrogen/oxygen surface termination of diamond on the nanometer scale. This is of particular interest for all applications relying on interfacial electrochemistry, especially for biointerfaces.

Immobilization of proteins on a solid electrode is to date done by chemical cross-linking or by addition of an adjustable intermediate. In this paper we introduce a concept where a solid with variable surface properties is optimized to mediate binding of the electron-transfer protein Cytochrome c (Cyt c) by mimicking the natural binding environment. It is shown that, as a carbon-based material, boron-doped diamond can be adjusted by simple electrochemical surface treatments to the specific biochemical requirements of Cyt c. The structure and functionality of passively adsorbed Cyt c on variously terminated diamond surfaces were characterized in detail using a combination of electrochemical techniques and atomic force microscopy with single-molecule resolution. Partially oxidized diamond allowed stable immobilization of Cyt c together with high electron transfer activity, driven by a combination of electrostatic and hydrophobic interactions. This surface mimics the natural binding partner, where coarse orientation is governed by electrostatic interaction of the protein's dipole and hydrophobic interactions assist in formation of the electron transfer complex. The optimized surface mediated electron transfer kinetics around 100 times faster than those reported for other solids and even faster kinetics than on self-assembled monolayers of alkanethiols.

International audience; Nickel doped single crystal diamond layers were grown by microwave-plasma enhanced chemical vapour deposition. Optical emission spectroscopy (OES) utilised during growth proved that the organometallic compound nickelocene is an applicable nickel source. It was possible to produce stable and adjustable nickel OES signals during growth by altering the flux of the nickelocene carrier gas. The successful incorporation of nickel into the diamond layers was verified by secondary ion mass spectrometry. Cathodoluminescence (CL) was applied to reveal optically active defects related to nickel. The signature of substitutionally incorporated nickel, namely the nickel related 1.4 eV centre, as well as the 2.369 and 1.563 eV centres were observed in CL. The latter is supposed to be a single-photon emitter on the basis of a nickel-nitrogen defect centre.

Electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR) experiments have been performed on a set of GaN epitaxial layers doped with Mg from 2.5 x 10(18) to 5.0 x 10(19) cm(-3). The samples were also characterized by secondary-ion-mass spectroscopy (SIMS), temperature-dependent Hall effect, and low-temperature photoluminescence (PL) measurements. EPR at 9 QHz on the conductive films reveals a single line with g(parallel to)similar to2.1 and g(perpendicular to)similar to2 and is assigned to shallow Mg acceptors based on, The similarity of the spin density with that found for the number of uncompensated Mg shallow acceptors from Hall effect and the total Mg concentration by SIMS. PL bands of different character are observed from these layers,, including shallow-donor-shallow-acceptor recombination at 3.27 eV from the lowest,doped sample, and, in most cases, broad emission bands with peak energy between 2.9 and 3.2 eV from the more heavily doped films. In addition, several of the films exhibit a weal, broad emission band between 1.4 and 1.9 eV. ODMR at 24 GHz on the "blue" PL bands reveals two dominant features. The first is characterized by g(parallel to),g(perpendicular to)similar to1.95-1.96 and is assigned to. shallow effective-mass donors. The second line is described by similar g tensors as found by the EPR experiments and, thus, is also attributed to shallow Mg acceptors. Although several groups have related the 2.8 eV PL in heavily Mg-doped GaN with the formation of deep donors, no clear evidence was found from the ODMR on this emission for such centers. However, based on the near-midgap PL energy and the observation of the feature assigned to shallow Mg acceptors, the strongest case from magnetic resonance for the existence of deep donors in these films is the isotropic ODMR signal with g = 2.003 found on emission

The influence of nonradiative recombination processes on the output power P of InGaN light emitting devices (LEDs) with emission wavelength between 380 and 430 nm grown on sapphire substrates have been investigated. The electroluminescence intensity was studied as a function of the forward current I and the device temperature. The P-I curves of all LEDs show a super-linear behaviour at low currents (between 80 µA and 1 mA) and a nearly linear characteristic around the operating forward current of 20 mA (equivalent to a current density of 44 A/cm2). The super-linear behaviour is attributed to the saturation of nonradiative processes which also affect the output power at operating forward current. The reduction of the output power at 20 mA with increasing temperature is correlated with the super-linear behaviour at low currents indicating that nonradiative processes are thermally activated.

Resonant Raman scattering from Al(x)Ga(1-x)N(x

In(0.13)Ga(0.87)N/GaN double heterostructures and quantum wells (QWs) have been studied by room-temperature photoluminescence (PL) and spectroscopic ellipsometry (SE). PL revealed the presence of strong piezoelectric fields, which strongly influence the luminescence properties for InGaN QW widths in the 3-12 nm range. The pseudodielectric function spectrum derived from the SE measurements were analyzed using a multilayer approach, describing the dielectric function of the individual layers by a parametric oscillator model. The fundamental band-gap resonance in the InGaN dielectric-function spectrum was found to broaden for an InGaN layer width of 12 nm, as compared to bulk-like InGaN layers, due to piezoelectric field effects. For a much narrower QW width of 1.7 nm, however, quantum confinement was found to dominate over piezoelectric-field effects, resulting in a much sharper band-gap resonance shifted to higher energies and an increased oscillator strength.

New approaches towards high reliable thermal stable ohmic and Schottky contacts for GaN/AlGaN-HFETs have been developed and implemented in device structures. The contact technology has been systematically optimized towards i) good electrical properties, ii) superior contact morphology and contour definition and Hi) high reliability and thermal stability. HFETs employing source/drain contacts based on WSiN diffusion barriers sandwiched between an Ti/Al/Ti/Au internal layer system and an overlayer metal as well as WSiN/Au and Ir/Au gate contacts have demonstrated long term stability at 400°C, Even after temperature storage tests at 500°C no significant device degradation could be detected. Ir/Au Schottky gates have found to be stable at the rather high level of Schottky barrier height of about 1.1 eV resulting in dramatically reduced leakage currents as compared to standard Pt/Ti/Au gate contacts to GaN-HFETs.

The deep, yellow photoluminescence band well known in GaN has been studied in In(x)Ga(1-x)N (x

The blue Mg induced 2.8 eV photoluminescence (PL) band in metalorganic chemical vapor deposition grown GaN has been studied in a large number of samples with varying Mg content. It emerges near a Mg concentration of 1*10/sup 19/ cm/sup 3/ and at higher concentrations dominates the room temperature PL spectrum. The excitation power dependence of the 2.8 eV band provides convincing evidence for its donor-acceptor (D-A) pair recombination character. It is suggested that the acceptor A is isolated Mg/sub Ga/ while the spatially separated, deep donor (430 meV) D is attributed to a nearest-neighbor associate of a Mg/sub Ga/ acceptor with a nitrogen vacancy, formed by self-compensation.

InAs/AlSb/GaSb/AlSb/InAs interband-tunneling structures have been analyzed with respect to the composition and structural quality of the AlSb tunneling barriers. The addition of AlAs monolayers at the interfaces between the AlSb barriers and the InAs and GaSb layers was found to result in the expected reduction in the valley current density of the resonant interband-tunneling diode. Vibrational mode Raman spectroscopy showed that the introduction of AlAs monolayers led to the formation of pseudoternary Al(SbAs) barriers, which cause the observed reduction of the valley current density. Ellipsometric measurements indicate that the structural quality of both types of barrier layers, with and without AlAs monolayers added to the interfaces, is inferior to that of thick bulklike AlSb layers. The observation of Raman scattering from a coupled hole plasmon-phonon mode indicates the formation of a hole gas in the GaSb quantum well at the center of the tunneling structure.

We present investigations on the in-plane effective mass of conduction electrons in pseudomorphic, strained GaAs/InsubxGasub1minusxAs/AlsubyGasub1minusyAs quantum wells. The samples are modulation doped by silicon leading to electron sheet densities in the range of 10high12cmhighminus2 in the Insub1GasubqminusxAs layers. In photoluminescence experiments at low temperature we observe that all electrons of the two-dimensional electron gas up to the Fermi energy contribute to the luminescence. This leads to an asymmetric broadening of the luminescence line shape and indicates a breakdown of the k-conservation rule. This offers the possibility of determining the Fermi energy from the low-temperature spectra. From contactless microwave Shubnikov-de Haas measurements we determine a quantity correlated to the sheet carrier density. By combining both methods we deduce the in-plane effective electronic mass and investigate its dependence on confinement. We observe a slight increase of the mass due to the built-in strain of the pseudomorphic layers and a strong increase due to confinement effects by up to 40% for 2-nm wells. Self-consistent calculations of the electronic-energy levels, the wave functions, and the perpendicular effective mass show that the observed dependence of the effective mass on the confinement is supported from a theoretical point of view. We compare the in-plane effective mass with the perpendicular one.

In low-dimensional systems dynamical nuclear polarization is a sensitive tool for the study of electron-nuclear interaction. We report on Overhauser-shift experiments on the 2DEG of AlsubxGasub1minusxAs/GaAs heterostructures. The conduction electron spin resonance has been electrically detected via the associated change of the magnetoresistance Rsubxx, and dynamical nuclear polarization (the Overhauser effect) has been achieved by saturating the conduction electron spin resonance (CESR). In order to investigate the nuclear spin relaxation the samples were embedded in an RF coil and the temporal decay of the Overhauser shift was followed with and without additional NMR irradiation. Different spin-lattice relaxation times of high69Ga, high71Ga and high75As could be detected and small deviations from a pure Korringa behaviour have been found.