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Laser-thinning of 2D materials such as MoS2 is a promising approach for a local reduction of the number of multilayers down to a monolayer. For a precise control of the thinning process real-time m...

Ga2O3 micro- and nanowires-based optical microcavities have been obtained by patterning pairs of distributed Bragg reflectors (DBRs) with a focused ion beam (FIB) microscope. DBRs result in widely tunable high reflectivity bands. The microcavities have been designed and optimized with the aid of simulations and optically characterized by micro-photoluminescence. Tunable strong modulations are confirmed in the NUV-blue as well as in the red-NIR ranges for unintentionally doped and chromium doped wires, respectively. Experimental, analytical and simulations results will be compared and some possible applications of these cavities will be assessed.

We show that one-dimensional (1d) GaN quantum-wires (QWRs) exhibit intense and spectrally sharp emission lines. These QWRs are realized in an entirely self-assembled growth process by molecular beam epitaxy (MBE) on the side facets of GaN/AlN nanowire (NW) heterostructures. Time-integrated and time-resolved photoluminescence (PL) data in combination with numerical calculations allow the identification and assignment of the manifold emission features to three different spatial recombination centers within the NWs. The recombination processes in the QWRs are driven by efficient charge carrier transfer effects between the different optically active regions, providing high intense QWR luminescence despite their small volume. This is deduced by a fast rise time of the QWR PL, which is similar to the fast decay-time of adjacent carrier reservoirs. Such processes, feeding the ultra-narrow QWRs with carriers from the relatively large NWs, can be the key feature towards the realization of future QWR-based devices. While processing of single quantum structures with diameters in the nm range presents a serious obstacle with respect to their integration into electronic or photonic devices, the QWRs presented here can be analyzed and processed using existing techniques developed for single NWs., This work was supported by the Deutsche Forschungsgemeinschaft (Sfb 787). SMS acknowledges funding from “Programa Internacional de Becas “la Caixa”-Severo Ochoa”. SMS and JA acknowledge funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish Ministerio de Economía y Competitividad project ValPEC (ENE2017-85087-C3). ICN2 acknowledges support from the Severo Ochoa Program (Ministerio de Economía y Competitividad, Grant SEV-2013-0295) and is funded by the CERCA Programme/Generalitat de Catalunya. Part of the present work has been performed in the framework of the Universitat Autónoma de Barcelona Materials Science PhD program. Some of the research leading to these results has received funding from the European Union Seventh Framework Program under Grant Agreement 312483 - ESTEEM2 (Integrated Infrastructure Initiative - I3). This work has received funding from the European Union's Horizon 2020 Research and Innovation Programme under grant agreement No. 654360 NFFA-Europe.

We present a pH-sensitive AlGaN/GaN ion sensitive field-effect transistor (ISFET) with monolithically integrated Wheatstone bridge layout and demonstrate the possibility to identify and to compensate the influence of cross-sensitivities to environmental changes, such as temperature, without the need of an external electronic circuit. Based on this Wheatstone-approach low-drift penicillinase-modified AlGaN/GaN solution-gate field-effect transistors (PenFET) with an ultrathin Al2O3 gate coating were prepared and exhibit improved stability and sensitivity compared to PenFETs with conventional wet chemically oxidized gate surface over a course of 60 days as they yield a denser, more homogeneous enzyme layer on the gate area. Moreover, using the Wheatstone-approach can identify a loss of physisorbed enzymes as the dominant aging mechanism for shorter time-periods and stable operation conditions after a period of 10 days with an extracted average Michaelis constants of (20 ± 4) μM for the immobilized enzyme layer by quantitative evaluation applying a kinetic model. By application of the Wheatstone-approach for acetylcholinesterase-modified AlGaN/GaN solution-gate field-effect transistors (AcFET) as a model, we demonstrate that temperature-induced drift can be differentiated from substrate-induced signals and a substantial loss of immobilized enzymes over the course of 12 days after production can be identified.

We demonstrate that the complex adsorption behavior of H2O on InGaN/GaN nanowire arrays is directly revealed by their ambient-dependent photoluminescence properties. Under low-humidity, ambient-temperature, and low-excitation-light conditions, H2O adsorbates cause a quenching of the photoluminescence. In contrast, for high humidity levels, elevated temperature, and high excitation intensity, H2O adsorbates act as efficient photoluminescence enhancers. We show that this behavior, which can only be detected due to the low operation temperature of the InGaN/GaN nanowires, can be explained on the basis of single H2O adsorbates forming surface recombination centers and multiple H2O adsorbates forming surface passivation layers. Reversible creation of such passivation layers is induced by the photoelectrochemical splitting of adsorbed water molecules and by the interaction of reactive H3O+ and OH– ions with photoactivated InGaN surfaces. Due to electronic coupling of adsorbing molecules with photoactivated surf...

Surface treatment of the foreign substrate is a critical factor influencing heteroepitaxial catalyst-free growth of nanowires, their crystal quality, their diameter and their areal density. To this end, catalyst-free growth of GaN nanowires on Al2O3(0001) by plasma-assisted molecular beam epitaxy was achieved using the following substrate surface treatments: (a) deposition of a SixNy layer on nitridated Al2O3 surface, and (b) deposition of Si on bare Al2O3 surface. The nanostructure of GaN nanowires and GaN/Al2O3 interfaces was explored by quantitative high-resolution transmission electron microscopy and related analytical methods. Spontaneous growth of GaN nanowires was realized on the amorphous SixNy layer, while a discontinuous crystalline zone in contact with Al2O3 was identified as partially strained AlN. Subsequently, GaN nanowires were directly grown on top of Al2O3 among stress-free Si islands. The orientation relation of these islands with the substrate was the [112]( 1 ¯ 1 ¯ 1 )Si//[ 1 1 ¯ 00 ](0001)Al2O3, providing the minimum lattice misfit between the two structures. Increasing the Si deposition time a higher density of Si islands was realized, leading to non-coalesced nanowires of lower density and better structural quality. Hence, the presence of Si islands induced a mask-like effect on the nucleation of GaN nanowires that can be exploited for a controlled catalyst-free growth of nanowires.

III-nitride semiconductors (AlGaN, GaN and InGaN) have received considerable attention in various fields ranging from high-frequency and high-temperature electronics [1] to LED lighting technologies [2]. [...]

The lack of mirror symmetry in binary semiconductor compounds turns them into polar materials, where two opposite orientations of the same crystallographic direction are possible. Interestingly, their physical properties (e.g., electronic or photonic) and morphological features (e.g., shape, growth direction, and so forth) also strongly depend on the polarity. It has been observed that nanoscale materials tend to grow with a specific polarity, which can eventually be reversed for very specific growth conditions. In addition, polar-directed growth affects the defect density and topology and might induce eventually the formation of undesirable polarity inversion domains in the nanostructure, which in turn will affect the photonic and electronic final device performance. Here, we present a review on the polarity-driven growth mechanism at the nanoscale, combining our latest investigation with an overview of the available literature highlighting suitable future possibilities of polarity engineering of semiconductor nanostructures. The present study has been extended over a wide range of semiconductor compounds, covering the most commonly synthesized III–V (GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb) and II–VI (ZnO, ZnTe, CdS, CdSe, CdTe) nanowires and other free-standing nanostructures (tripods, tetrapods, belts, and membranes). This systematic study allowed us to explore the parameters that may induce polarity-dependent and polarity-driven growth mechanisms, as well as the polarity-related consequences on the physical properties of the nanostructures., ICN2 acknowledges funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish MINECO coordinated project ENE2017-85087-C3. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant SEV-2017-0706) and is funded by the CERCA Programme/Generalitat de Catalunya. Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science Ph.D. program. S.M.S. acknowledges funding from “Programa Internacional de Becas “la Caixa”-Severo Ochoa”. The HAADF-STEM experiments were conducted in the Laboratorio de Microscopias Avanzadas at Instituto de Nanociencia de Aragon-Universidad de Zaragoza. A.F.i.M. thanks SNSF for funding through the NCCR QSIT. This work has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement No. 654360 NFFA-Europe. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 823717 – ESTEEM3., We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).

Neuderth, Paula et al., An experimental strategy for systematically assessing the influence of surface passivation layers on the photocatalytic properties of nanowire photoanodes by combining photocurrent analysis, photoluminescence spectroscopy and high resolution transmission electron microscopy with a systematic variation of sample structure and the surrounding electrolyte is demonstrated. Following this approach we can separate the impact on recombination and transport processes of photogenerated carriers. We apply this strategy to analyze the influence of ultra-thin TiO2, CeO2 and Al2O3 coatings deposited by atomic layer deposition on the photoelectrochemical performance of InxGa1−xN/GaN nanowire (NW) photoelectrodes. The passivation of surface states results in an increase of the anodic photocurrent (PC) by a factor of 2.5 for the deposition of 5 nm TiO2. In contrast, the PC is reduced for CeO2- and Al2O3-coated NWs due to enhanced defect recombination in the passivation layer or increased band discontinuities. Furthermore, photoelectrochemical oxidation of the InxGa1−xN/GaN NW photoelectrode is attenuated by the TiO2 layer and completely suppressed for a layer thickness of 7 nm or more. Due to efficient charge transfer from the InxGa1−xN NW core a stable TiO2-covered photoanode with visible light excitation is realized., Financial support is provided by the DFG via the GrK (Research training group) 2204 “Substitute Materials for sustainable Energy Technologies.” MC thanks Ram ́on y Cajal program RYC- 2013-12448. We also acknowledge nancial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496). This article is based upon work from COST Action MP1402 “Hooking together European research in atomic layer deposition (HERALD)”, supported by COST (European Cooperation in Science and Technology). SMS acknowledges funding from “Programa Internacional de Becas “la Caixa”- Severo Ochoa”. JA, MdlM. and SMS also acknowledge funding from Generalitat de Catalunya 2014 SGR 1638 and the Spanish MINECO e-TNT (MAT2014-59961-C2-2-R). ICN acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV- 2013-0295) and is funded by the CERCA Programme/Generalitat de Catalunya. The atomic resolution HAADF-STEM microscopy was conducted in the Laboratorio de Microscopias Avanzadas at the Instituto de Nanociencia de Aragon-Universidad de Zaragoza. JA and SMS thank them for offering access to their instruments and expertise. Part of the present work has been performed in the framework of Universitat Aut`onoma de Bar- celona Materials Science PhD program. RM gratefully acknowledges funding in the Emmy-Noether program (MA 5392/3-1) of the German Research Foundation DFG.

We report on the control and modification of optical transitions in 40× GaN/AlN heterostructure superlattices embedded in GaN nanowires by an externally applied bias. The complex band profile of these multi-nanodisc heterostructures gives rise to a manifold of optical transitions, whose emission characteristic is strongly influenced by polarization-induced internal electric fields. We demonstrate that the superposition of an external axial electric field along a single contacted nanowire leads to specific modifications of each photoluminescence emission, which allows to investigate and identify their origin and to control their characteristic properties in terms of transition energy, intensity and decay time. Using this approach, direct transitions within one nanodisc, indirect transitions between adjacent nanodiscs, transitions at the top/bottom edge of the heterostructure, and the GaN near-band-edge emission can be distinguished. While the transition energy of the direct transition can be shifted by external bias over a range of 450 meV and changed in intensity by a factor of 15, the indirect transition exhibits an inverse bias dependence and is only observable and spectrally separated when external bias is applied. In addition, by tuning the band profile close to flat band conditions, the direction and magnitude of the internal electric field can be estimated, which is of high interest for the polar group III-nitrides. The direct control of emission properties over a wide range bears possible application in tunable optoelectronic devices. For more fundamental studies, single-nanowire heterostructures provide a well-defined and isolated system to investigate and control interaction processes in coupled quantum structures.

We present a study of GaN single-nanowire ultraviolet photodetectors with an embedded GaN/AlN superlattice. The heterostructure dimensions and doping profile were designed in such a way that the application of positive or negative bias leads to an enhancement of the collection of photogenerated carriers from the GaN/AlN superlattice or from the GaN base, respectively, as confirmed by electron beam-induced current measurements. The devices display enhanced response in the ultraviolet A ($\approx$ 330-360 nm) / B ($\approx$ 280-330 nm) spectral windows under positive/negative bias. The result is explained by correlation of the photocurrent measurements with scanning transmission electron microscopy observations of the same single nanowire, and semi-classical simulations of the strain and band structure in one and three dimensions., This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Nano Letters (2017), copyright (C) American Chemical Society after peer review. To access the final edited and published work see http://doi.org/10.1021/acs.nanolett.7b01118

We demonstrate the direct analysis of polarization-induced internal electric fields in single GaN/Al0.3Ga0.7N nanodiscs embedded in GaN/AlN nanowire heterostructures. Superposition of an external electric field with different polarity results in compensation or enhancement of the quantum-confined Stark effect in the nanodiscs. By field-dependent analysis of the low temperature photoluminescence energy and intensity, we prove the [0001̅]-polarity of the nanowires and determine the internal electric field strength to 1.5 MV/cm.

We report on an all-optical sensor system that employs GaN/InGaN nanowire heterostructures (NWH) as optochemical transducers. When exposed to ultraviolet light, such NWHs emit green luminesce light. This photoluminescence (PL) is intense and temperature stable, and persists up to transducer temperatures well beyond 200 °C. When exposed to oxidizing gases such as O 3 , NO 2 , and O 2 the PL intensity decreases. At room temperature minimum detectable O 3 -, NO 2 -, and O 2 -concentrations are 50 ppb, 500 ppb and 100 ppm, respectively. Above their minimum detectable concentrations the O 3 , NO 2 and O 2 responses increase in a quasi-logarithmic manner up to a temperature-dependent saturation level. It is shown that the observed features can be explained within a model of Langmuir adsorption and surface recombination.

We report the observation of transverse-magnetic-polarized infrared absorption assigned to the s-p(z) intraband transition in Ge-doped GaN/AlN nanodisks (NDs) in self-assembled GaN nanowires (NWs). The s-p(z) absorption line experiences a blue shift with increasing ND Ge concentration and a red shift with increasing ND thickness. The experimental results in terms of interband and intraband spectroscopy are compared to theoretical calculations of the band diagram and electronic structure of GaN/AlN heterostructured NWs, accounting for their three-dimensional strain distribution and the presence of surface states. From the theoretical analysis, we conclude that the formation of an AlN shell during the heterostructure growth applies a uniaxial compressive strain which blue shifts the interband optical transitions but has little influence on the intraband transitions. The presence of surface states with density levels expected for m-GaN plane charge-deplete the base of the NWs but is insufficient to screen the polarization-induced internal electric field in the heterostructures. Simulations show that the free-carrier screening of the polarization-induced internal electric field in the NDs is critical to predicting the photoluminescence behavior. The intraband transitions, on the other hand, are blue-shifted due to many-body effects, namely, the exchange interaction and depolarization shift, which exceed the red shift induced by carrier screening.

InGaN/GaN nanowire heterostructures are presented as nanophotonic probes for the light-triggered photoelectrochemical detection of NADH. We demonstrate that photogenerated electron-hole pairs give rise to a stable anodic photocurrent whose potential- and pH-dependences exhibit broad applicability. In addition, the simultaneous measurement of the photoluminescence provides an additional tool for the analysis and evaluation of light-triggered reaction processes at the nanostructured interface. InGaN/GaN nanowire ensembles can be excited over a wide wavelength range, which avoids interferences of the photoelectrochemical response by absorption properties of the compounds to be analyzed by adjusting the excitation wavelength. The photocurrent of the nanostructures shows an NADH-dependent magnitude. The anodic current increases with rising analyte concentration in a range from 5µM to 10mM, at a comparatively low potential of 0mV vs. Ag/AgCl. Here, the InGaN/GaN nanowires reach high sensitivities of up to 91µAmM-1cm-2 (in the linear range) and provide a good reusability for repetitive NADH detection. These results demonstrate the potential of InGaN/GaN nanowire heterostructures for the defined conversion of this analyte paving the way for the realization of light-switchable sensors for the analyte or biosensors by combination with NADH producing enzymes.

arXiv:1604.07978v2, We have characterized the photodetection capabilities of single GaN nanowires incorporating 20 periods of AlN/GaN:Ge axial heterostructures enveloped in an AlN shell. Transmission electron microscopy confirms the absence of an additional GaN shell around the heterostructures. In the absence of a surface conduction channel, the incorporation of the heterostructure leads to a decrease of the dark current and an increase of the photosensitivity. A significant dispersion in the magnitude of dark currents for different single nanowires is attributed to the coalescence of nanowires with displaced nanodisks, reducing the effective length of the heterostructure. A larger number of active nanodisks and AlN barriers in the current path results in lower dark current and higher photosensitivity and improves the sensitivity of the nanowire to variations in the illumination intensity (improved linearity). Additionally, we observe a persistence of the photocurrent, which is attributed to a change of the resistance of the overall structure, particularly the GaN stem and cap sections. As a consequence, the time response is rather independent of the dark current., Financial support from the EU ERC-SG “TeraGaN” (#278428) and ANR JCJC COSMOS (ANR-12-JS10-0002) is acknowledged. Furthermore, the groups in Grenoble and Giessen received traveling support from the DAAD/Campus France program Procope. P.H., J.S., and M.E. acknowledge financial support within the LOEWE program of excellence of the Federal State of Hessen (project initiative STORE-E). M.d.l.M. and J.A. acknowledge funding from Generalitat de Catalunya 2014 SGR 1638 and the Spanish MINECO MAT2014-51480-ERC (e-ATOM) and Severo Ochoa Excellence Program.

This paper assesses nonpolar m-oriented GaN:Si/Al(Ga)N heterostructures grown on free-standing GaN for intersubband optoelectronics in the short-wavelength, mid- and far-infrared ranges. Characterization results are compared with reference c-plane samples and interpreted by correlation with self-consistent Schrödinger–Poisson calculations. In the near- and mid-infrared regions, we demonstrate m-GaN/Al(Ga)N multi-quantum-wells exhibiting room-temperature intersubband absorption tunable in the range of 1.5–5.8 µm (827–214 meV), the long wavelength limit being set by the second order of the Reststrahlen band in the GaN substrates. Extending the study to the far-infrared region, low-temperature intersubband transitions in the 1.5–9 THz range (6.3–37.4 meV) are observed in larger m-plane GaN/AlGaN multi-quantum-wells, covering most of the 7–10 THz band forbidden to GaAs-based technologies.

We report on an all-optical sensor system that employs InGaN/GaN nanowires (NWs) as opto-chemical transducers. The NWs, grown by plasma-assisted molecular beam epitaxy on low-resistivity n-type Si (1 1 1) substrates, exhibit an efficient room-temperature photoluminescence (PL) that persists up to about 200 °C. After deposition of a thin (5 nm) catalytic Pt-film onto the NWs the PL intensity rises when the NWs are exposed to small concentrations of hydrogen and hydrocarbons. The gas response of the NWs was analyzed using an integrated sensor system with fiber-coupled excitation from a GaN-based power LED emitting at 365 nm and a fiber coupled photo multiplier tube for detection. With this setup, H2 concentrations as low as 200 ppb and C2H2 concentrations as low as 5 ppm could be detected when the transducers were operated at temperatures around 80 °C. This opto-chemical transducer principle is best suited for safety-critical applications where a reliable media separation is required.

The photoelectrochemical responses of GaN, GaNP, and InGaN nanowire ensembles are investigated by the electrical bias dependent photoluminescence, photocurrent, and spin trapping experiments. The r ...

The pH response of a GaN/AlN/GaN solution-gate field effect transistor (SGFET), with a GaN/AlN barrier of 7.5 nm thick, is analyzed and compared with standard GaN/AlGaN/GaN SGFETs with total barrier thicknesses of 19 and 23 nm. While all types of SGFETs show a similar surface sensitivity to H + ions, a significant improvement in the transducive sensitivity of the SGFET source-drain current under pH changes is found when decreasing the barrier thickness, due to the increased transconductance of the FET structure. Resolution better than 0.005 pH can be estimated in the case of the ultrathin SGFET. Moreover, the maximum transconductance value shifts to gate-drain voltage close to 0 V, which eventually involves no need of reference electrode in less demanding applications, simplifying the final design of the device and making AlN barrier-based SGFETs highly recommended in the broad field of chemical sensors.

Usually, the fabrication of microelectromechanical systems (MEMS) requires unstrained or tensile strained active layers on a selectively removable sacrificial layer, since compressive strain causes instabilities due to buckling effects. For group III-nitride based MEMS, AlN is a promising material for sacrificial layers since it can be epitaxially overgrown and etched selectively to GaN. However, due to the larger lattice constants GaN is growing compressively strained on AlN. Nanoheteroepitaxy opens a way to yield fully unstrained, high quality epitaxial GaN layers on nanocrystalline AlN thin film by means of a 3D strain relaxation mechanism. For this purpose sputtered nanocrystalline AlN films were overgrown with single crystalline GaN and AlGaN/GaN layers by metalorganic chemical vapor deposition. The high quality of the layers is proven by an atomically flat surface and a 2D electron gas at the interface of the AlGaN/GaN heterostructure (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An etching technology for the patterning of suspended GaN structures on lattice matched AlInN sacrificial layers is studied. Selective dry RIE etching of GaN to AlInN is demonstrated, which allows effective pattern transfer without affecting lower layers. On the other hand, wet etching of AlInN in highly concentrated KOH solutions has also been studied. We show that high vertical and lateral etch rates are possible. For a fast and selective etching suitable for MEMS technology, a high density of defects at the surface is required due to their positive influence on the lateral etch rate. (c) 2007 Elsevier B.V. All rights reserved.

This paper assesses intersubband (ISB) transitions in the 1-10 THz frequency range in nonpolar m-plane GaN/AlGaN multi-quantum-wells deposited on free-standing semi-insulating GaN substrates. The quantum wells (QWs) were designed to contain two confined electronic levels, decoupled from the neighboring wells. Structural analysis reveals flat and regular QWs in the two perpendicular in-plane directions, with high-angle annular dark-field scanning transmission electron microscopy images showing inhomogeneities of the Al composition in the barriers along the growth axis. We do not observe extended structural defects (stacking faults or dislocations) introduced by the epitaxial process. Low-temperature ISB absorption from 1.5 to 9 THz (6.3-37.4 meV) is demonstrated, covering most of the 7-10 THz band forbidden to GaAs-based technologies.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., GaN/AlN nanowire heterostructures can display photoluminescence (PL) decay times on the order of microseconds that persist up to room temperature. Doping the GaN nanodisk insertions with Ge can reduce these PL decay times by two orders of magnitude. These phenomena are explained by the three-dimensional electric field distribution within the GaN nanodisks, which has an axial component in the range of a few MV/cm associated to the spontaneous and piezoelectric polarization, and a radial piezoelectric contribution associated to the shear components of the lattice strain. At low dopant concentrations, a large electron-hole separation in both the axial and radial directions is present. The relatively weak radial electric fields, which are about one order of magnitude smaller than the axial fields, are rapidly screened by doping. This bidirectional screening leads to a radial and axial centralization of the hole underneath the electron, and consequently, to large decreases in PL decay times, in addition to luminescence blue shifts., This work is supported by the EU ERC-StG “TeraGaN” (#278428) project, the LOEWE program of excellence of the Federal State of Hessen (project initiative STORE-E), and by the Spanish MINECO MAT2014-51480-ERC (e-ATOM) and Generalitat de Catalunya 2014SGR1638. J.A. thanks ICN2 Severo Ochoa Excellence Program. MdlM thanks CSIC JaePredoc program.

Wide band gap materials such as SiC, AlN, GaN, ZnO, and diamond have excellent properties such as high operation temperature when used as field effect devices and a high resonating frequency of the substrate materials used in piezoelectric resonator devices. Integration of FET and resonating sensors on the same chip enables powerful miniaturized devices, which can deliver increased information about a gas mixture or complex liquid. Examples of sensor devices based on different wide band gap materials will be given.

Electroreflectance studies of Pt/GaN Schottky diodes were performed in the temperature range between 5 and 300 K. The data were analysed using the electric field-dependent dielectric function of GaN and a multi-layer formalism. We observed a thermal activation of electric fields underneath the Schottky contact. The results are explained in terms of temperature-dependent ionised impurity concentration by a model with two donor levels.

The first aluminum nitride/diamond heterojunction light-emitting diode is realized by plasma-induced molecular beam epitaxy. The pn heterojunction consists of a silicon doped AlN (n-type) epitaxial film on (100) naturally boron-doped (p-type) diamond substrate. The diode shows excellent rectifying properties with intense light emission in the spectral range of approximately 2.7 and 4.8 eV. Heterojunctions manufactured from AlGaN alloys and diamond hold great promise for new applications in optoelectronics as well as in high frequency and high power electronics.

Electronic transport in semiconductors that possess high internal spontaneous and piezoelectric polarization opens up a new field of pyroelectronics and pyrosensors. The pyroelectric character of group-III-nitrides with wurtzite crystal structure yields a novel degree of freedom in designing and tailoring devices for modern micro- and nanoelectronic applications. Furthermore, spontaneous and piezoelectric polarization induced surface and interface charges can be used to develop very sensitive but robust sensors for the detection of ions, gases and polar liquids. We present a review of both theoretical and experimental studies of spontaneous and piezoelectric polarization present in AlGaN/GaN heterostructures as well as the electronic transport properties of polarization induced two-dimensional electron gases which are formed at the AlGaN/GaN interface due to the difference in the total polarization of two adjacent III-nitride layers. We demonstrate that the two-dimensional electron gases (2DEGs) achieved without modulation doping are very suitable as channel of high electron mobility transistors optimally suited for high power and high frequency applications (PART A) as well as for various kinds of sensors which can be operated in harsh environments (PART B). (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In the present article recent results concerning sensor applications of AlGaN layers and AlGaN/GaN heterostructures are summarized. The piezoresistive effect in piezoelectric AlGaN layers is investigated and the dependence of the piezoresistive gauge factor on the Al content is attributed to the influence of strain induced piezoelectric fields. An enhancement of this effect is observed in AlGaN/GaN heterostructures resulting in high longitudinal gauge factors. The response of gas sensitive Pt:GaN Schottky diodes to hydrogen and hydrogen containing gases is analyzed up to temperatures of 600 °C and employed to realize gas sensitive field effect transistors which are demonstrated to operate up to 400 °C. In addition, ion sensitive field effect transistors (ISFETs) have also been fabricated on the basis of AlGaN/GaN heterostructures. The GaN surface shows a high pH sensitivity which is attributed to the presence of a thin native metal oxide layer on the surface. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Hexagonal aluminum nitride (AlN) has revealed as an excellent candidate to be used as substrate for surface acoustic wave (SAW) devices. Its high SAW propagation velocity, electromechanical coupling constant and an almost negligible temperature coefficient of delay (TCD) promise the fabrication of low-cost SAW devices operating in the microwave region of the spectrum, useful under harmful environment conditions of temperature and radiation. In this paper, a revision of the excellent SAW-related characteristics of AlN grown by molecular beam epitaxy on sapphire is presented. Several technological issues related to the fabrication of high frequency (>1 GHz) SAW devices in this semiconductor using an e-beam lithographic system are discussed. On the other hand, design issues like the effect introduced by the generation of bulk acoustic waves (BAW) will be analyzed using a novel method based on the inverse Fourier transform. These approaches will be presented both from theoretical and experimental perspectives.

We report on the observation of channel current modulation by exposing AlGaN/GaN high electron mobility transistors (HEMT) to fluxes of ions of different signs generated by an ion spray technique. In these experiments, the gate was directly exposed to the ion flux without intermediate insulating or metallic layers. We were able to vary the channel current over several orders of magnitude in a reversible manner. The effect is likely to be caused by the compensation of bound ions at the GaN surface. Using this effect, we were able to realize a miniaturized charge-amplifying device sensitive to the sign and quantity of ion fluxes, with an amplification factor of about 1000.

The paper reports on novel gas-sensing devices based on group III-nitride materials. Both platinum (Pt)–GaN Schottky diodes as well as high-electron-mobility transistors formed from GaN/AlGaN heterostructures with catalytically active platinum gates were investigated. The performance of these devices towards a number of relevant exhaust gas components such as H2, HC, CO, NOx was tested. Test gas concentrations were chosen to simulate exhaust gas emissions from lean-burn 4-stroke petrol engines. We found that GaN-based devices with platinum electrodes are mainly sensitive to hydrogen and unsaturated hydrocarbons with a sizeable cross-sensitivity to CO and NO2. These performance characteristics are similar to those of comparable SiC devices. With GaN devices this performance, however, can be obtained at a reduced complexity of the device processing and a greater freedom in the choice of sensor architectures.

The potential of AlGaN/GaN heterostructures for novel sensor devices is discussed. The two-dimensional electron gas which is formed at the AlGaN/GaN interface due to the difference in the spontaneous polarisation of the two adjacent III-nitride layers is shown to respond very sensitively to changes in the electrostatic boundary conditions caused by the adsorption of ions, wetting by polar liquids, exposure to gases, or the piezoelectric polarisation due to mechanical strain. Possibilities to use III-nitride heterostructures in future biosensors or integrated sensor devices are also mentioned.

In order to fully profit from the increased sensitivity of nanoscale transducer structures appropriate readout techniques are required. In this context optical methods are highly promising as they allow parallel probing of large ensembles of nano-objects without manipulating or processing single nanostructures. Furthermore, optical readout techniques feature significant advantages for certain applications as, e.g., measurements in potentially hazardous environments that prohibit any form of electricity at the point of measurement. In this article we discuss a novel type of gas-sensitive nanophotonic probes that are based on the photoluminescence response of group III-nitride nanostructures. This material class offers manifold possibilities for the synthesis of nano-objects as well as for the realization of complex semiconductor heterostructures. The latter is inevitable to fully exploit this optical approach by utilizing band engineering methods for a targeted control of the emission properties (such as the absorption behavior, the emission wavelength and intensity, as well as the temperature stability of the emission). In recent years, significant progress in terms of gas sensing applications has been obtained using GaN/AlN quantum dot systems as well as GaN nanowire ensembles with embedded heterostructures of either GaN/AlGaN or GaN/InGaN. These nanoscale transducer structures are highly suitable for the development of optical sensor systems and proved to be fully competitive to conventional sensor concepts. The state of the art of group III-nitride opto-chemical gas sensors is reviewed in the present chapter.

For the realization of good quality 3C–SiC films epitaxially grown on Si, the perfection of the film during the early stage of growth is substantial. In this paper, the beneficial role of flash lamp annealing (FLA) for the elimination of the defects in the SiC films and the strain reduction at the SiC/Si interface is discussed. FLA is a highly transient process having a flash duration of a few milliseconds. When the energy density is sufficiently high, it melts the silicon at the SiC/Si interface increasing the temperature there well above the melting point of silicon. The melted Si dissolves the 3C–SiC near the interface. Additionally, the uppermost part of the 3C–SiC film is annealed due to the heat dissipation during the flash duration and the solidification of the molten region. During the solidification of the C-rich Si melt, SiC grows by liquid phase epitaxy at the annealed uppermost 3C–SiC film which acts as a seed. This process results in a substantial improvement of the SiC film eliminating also the cavities and the stress at the interface.

A new heated microgas-sensor with only 80 μm ×80 μm active heater area and excellent long-term stability is presented. Using a previously presented SiC based microhotplate and depositing an In 2 O 3 thin film layer on top of the interdigital electrode (IDE) structure an NO 2 sensor for the MAK-range (German maximum workplace concentration limit: 5 ppm) was fabricated. The microhotplate allows a maximum operating temperature of up to 650°C, which is limited only by the IDE metallisation stability. When using In 2 O 3 layers for NO 2 detection, the hotplates’ low operating power of 20 mW allows the continuous operation at operating temperature with an AAA battery cell for 75 h. More than 20,000 work cycles of 1 min could be realised before device failure leading to a life time of over half a year when sampling the ambient gas every 10 min.

A new SiC-based heated micro gas sensor with a 100 μm ×100 μm heated membrane and excellent heater long-term stability is presented. Combining a previously presented micro hotplate with new gas sensitive (2 1 1) oriented In 2 O 3 thin film layers yields a very stable low power NO 2 gas sensor. The modular design allows its operation at operating voltages of either 1–2 or 12–24 V for battery and grid powered automotive applications. A total of 20 mW of power is sufficient for the operation at 250°C. The sensitivity and signal reproducibility in the MAK-range (5 ppm, maximum workplace concentration according to German federal law) is good. The response time τ 50 =50 s needs improvement.

Typical industrial high temperature sensor applications are reviewed and a short overview of the different high temperature sensor technologies is given. The pros and cons are weighted. Silicon carbide on insulator (SiCOIN) technology comes out to be the most attractive, provided the state of development can be brought up to the one of silicon and silicon on insulator (SOI). Due to the lack of commercially available SiC on SOI wafers, a new SiC on SOI technology has been developed. It is based on the precursor gas methylsilane. The low temperature growth process is described and in-situ n-type doping, which is necessary for sensor applications, has been carried out successfully over a wide range of concentrations without loosing the good crystal properties. Actually the full process is being transferred from a test reactor to a 4 inch machine. This should provide 3C-SiC on SOI wafers for commercial sensor applications. A demonstrator of combustion pressure sensor dedicated to pressure-based engine control is shown. Results of the pressure sensor fitted in a motor-test setup are summarized.

A high temperature pressure sensor with 3C–SiC piezoresistors as sensing elements was prepared. For the first time the sensing elements were structured by selective deposition of 3C–SiC on a patterned Si/SiO 2 surface. To ensure dielectric isolation SOI substrates were used. The effectiveness of the selective deposition process is demonstrated by REM-photographs. Characterisation of the sensing elements shows the good crystal quality of the sensing elements as indicated by the gauge factor of −18 at room temperature which decreases to −10 at 200°C. As a benefit of the deep dry etching process the related sensitivity is 3.5 mV/V bar at room temperature decreasing to 2.1 mV/V bar at 200°C for a 100-μm thick circular center boss diaphragm.

The fabrication of freestanding GaN microstructures using AlN sacrificial layers (SLs) is reported. GaN layers were grown by plasma assisted molecular beam epitaxy (PAMBE) on polycrystalline AlN sacrificial layers that have been deposited at 600 °C. Isotropic wet chemical etching of the AlN film released GaN microbridges and – cantilevers. The stress gradient and the compressive stress in the GaN-film was extracted by analysis of the relation between beam geometry and displacement. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The paper presents a novel concept for the realization of optochemical sensor systems which are capable of operating in harsh environments. Key components in such sensors are nanostructures formed from gallium nitride (GaN) and its alloys with aluminum (Al) and indium (In). Nanostructures of this kind emit an efficient, visible-light photoluminescence (PL) which can be excited with low-cost ultraviolet light sources and which extends up to temperatures in the order of 200°C. When exposed to various chemical environments, changes in the PL intensity occur which constitute valuable sensor signals. Due to the all-optical approach, the PL can be excited and its chemically induced changes be read out without requiring electrical wiring at the point of measurement. The present paper presents this innovative sensor concept, the nanostructures and optochemical transducer structures that form its material base, as well as several applications of such transducers in the fields of gas and fluid sensing. The applications addressed here range from the sensing of ppb concentrations of H 2 , NO 2 and O 3 in gaseous environments to the pH monitoring in aqueous solutions.

The optical properties of wurtzite GaN nanowires containing single Al0.14Ga0.86N/GaN quantum discs of different thickness have been investigated. The dependence of the photoluminescence (PL) transition energy on the quantum disc thickness and the thickness of a lateral AlGaN shell has been simulated in the framework of a three-dimensional effective mass model, accounting for the presence of a lateral AlGaN shell, strain state and the piezoelectric and spontaneous polarization. The predicted transition energies are in good agreement with the statistics realized on more than 40 single nanowire emission spectra and PL spectra of ensembles of nanowires. The emission spectra of the single quantum discs exhibit a Lorentzian shape with a homogeneous line width as low as 3 meV. Finally, we discuss the dependence of the interband transition energy on diameter.

The highly sensitive photoluminescence (PL) response of group III-Nitrides (III-N) nanowire heterostructures (NWHs) to hydrogen (H2) and oxygen (O2) allows for the realization of reliable gas detectors. For industrial real time gas monitoring applications, e.g. in the field of aerospace, a large scale laboratory setup was miniaturized by integrating electro-optical components and the NWHs within a robust micro optical system. As a result of the all optical addressing and read out the detection periphery can be completely isolated from the investigated environment which significantly increases the detection sensitivity. The optical design and fabrication techniques as well as an experimental investigation of the system performance are the main topics discussed in this paper.

We investigate the carrier relaxation from (ZnMg)O barrier layers into a ZnO quantum well (QW) by following the dynamic screening of its built-in electric fields. The respective emission lines shift in energy as the carriers populate the QW, spectrally shifting the time-resolved photoluminescence. At low temperatures, the carrier capture into the QW is found to occur on the same or an even faster time scale than the carrier-trapping processes within the barriers.