In this work, the sensing ability and the underlying reaction pathways of H2S adsorption on two nanomaterial systems, pristine zinc oxide (ZnO) nanowires (NWs) and gold functionalized zinc oxide nanowires (Au@ZnO NWs), were explored in a side-by-side comparison of optical and electrical gas sensing. The properties of optical sensing were analyzed by photoluminescence intensity-over-time measurements (PL-t) of as-grown ZnO NW samples, and the electrical gas-sensing properties were analyzed by current-over-time measurements (I-t) of ZnO NW chemically sensitive field-effect transistor (ChemFET) structures with a gas-sensitive open gate. The ZnO NWs were grown by high-temperature chemical vapor deposition (CVD) and thereafter surface-functionalized with a thin Au nanoparticle layer by magnetron sputtering. Detailed X-ray photoelectron spectroscopy (XPS) analysis, alongside an experimental estimation of activation energies (E A) involved in the H2S sensing process, and the application of a simple analytical test allowed us to propose a complete picture of the sensing mechanism on the pristine ZnO surface and the Au@ZnO surface. The combined results hint at H2S dissociation via surface interaction and irreversible adsorption dynamics for both material systems occurring already at room temperature. Our findings specifically emphasize the impact of Au functionalization morphology on sensor sensitivity and the beneficial importance of chemical affinity between Au and H2S for superior H2S sensing results, aiming at enhanced response and selectivity for potential medical H2S detection in human breath.

In this work, internal 4T1→6A1 transitions within the half-filled 3d shell of Fe3+ in extremely pure chemical vapor deposition (CVD)-grown ZnO layers were investigated by means of high-resolution, low-temperature continuous wave (cw) photoluminescence (PL), time-resolved PL, photoluminescence excitation (PLE) spectroscopy, Zeeman spectroscopy, and deep level transient spectroscopy (DLTS). For comparison, Zeeman spectroscopy measurements were also performed on commercially available, hydrothermally grown ZnO bulk crystals. Magnetic fields up to 15T were applied parallel and perpendicular to the c-axis of the ZnO crystals in order to investigate the fine structure of included states. The splitting pattern of emission lines related to 4T1→6A1 Fe3+ transitions was theoretically modeled by a Hamiltonian matrix including the crystal field in cubic and trigonal symmetries and spin–orbit interaction for the complete excited 4T1 state. The extremely pure ZnO used in this study, in direct comparison to hydrothermally grown ZnO, allows the identification, investigation, and description of single isolated Fe3+ defects in ZnO for the first time—different from literature reports hitherto, which seemingly were recording data on Fe–Li complexes. The resulting exact energy-level scheme in combination with the experimental data leads to a re-evaluation of 4T1→6A1 Fe3+ transitions in ZnO.

In this work, we report on three new extremely sharp emission lines in zinc oxide (ZnO) related to iron–lithium complexes. The identification is based on a comparison of hydrothermally grown ZnO with high lithium concentration and a lithium-free sample grown by methane based chemical vapor deposition, which both were implanted with iron. After annealing in a mixed oxygen/argon atmosphere at 800°C, the lithium-free sample showed no additional lines besides the well-known emission at 693nm (1.78734eV), whereas the hydrothermally grown sample emitted three intense and sharp lines in photoluminescence (PL) spectroscopy. These emission lines at 705nm (1.75873eV), 712nm (1.74153eV), and 732nm (1.69283eV) were characterized by temperature dependent high resolution continuous wave (cw) and time-resolved PL spectroscopy, as well as by photoluminescence excitation spectroscopy, and are assigned to different Fe–Li complexes. We find a single thermally activated excited state for each of the complex emission lines, different from the 4T1→6A1 transition of Fe3+ with its at least three excited states. While time-resolved photoluminescence measurements of the Fe3+ transition show a lifetime of 24.9±0.5ms, we find a reduced lifetime of 8.5±1ms for the new zero phonon lines, pointing to stronger mixture of the Fe 3d states with surrounding p-orbitals.

This work presents a H2S selective resistive gas sensor design based on a chemical field effect transistor (ChemFET) with open gate formed by hundreds of high temperature chemical vapour deposition (CVD) grown zinc oxide nanowires (ZnO NW). The sensing ability of pristine ZnO NWs and surface functionalized ZnO NWs for H2S is analysed systematically. ZnO NWs are functionalized by deposition of discontinuous gold (Au) nanoparticle films of different thicknesses of catalyst layer ranging from 1 to 10 nm and are compared in their gas sensing properties. All experiments were performed in a temperature stabilized small volume compartment with adjustable gas mixture at room temperature. The results allow for a well-founded understanding of signal-to-noise ratio, enhanced response, and improved limit of detection due to the Au functionalisation. Comprehension and controlled application of the beneficial effects of Au catalyst on ZnO NWs allow for the detection of very low H2S concentrations down to 10 ppb, and a theoretically estimated 500 ppt in synthetic air at room temperature.

A stack of five Al(Ga)N-based quantum wells is investigated by combined laterally and depth resolved cathodoluminescence (CL) spectroscopy in order to distinguish lateral and vertical inhomogeneities of these wells. Transmission electron microscopy (TEM) micrographs provide data for the real sample structure, which enters into the Monte-Carlo simulation of the depth-resolved CL measurements to refine the depth resolution. The comparison of these CL measurements to the results of electron energy loss spectra (EELS) allows to identify local thickness variations of the lower three quantum wells to be the origin of two different luminescence contributions to the overall spectrum. The differentiation of the two groups of quantum wells by depth-resolved CL is demonstrated.

We investigate the influence of different types of template micro-patterning on defect reduction and optical properties of non-polar GaN using detailed luminescence studies. Non-polar (11 (2) over bar0) (or a-plane) GaN exhibits a range of different extended defects compared with its more commonly used c-plane counterpart. In order to reduce the number of defects and investigate their impact on luminescence uniformity, non-polar GaN was overgrown on four different GaN microstructures. The micro-patterned structures consist of a regular microrod array; a microrod array where the-c-side of the microrods has been etched to suppress defect generation; etched periodic stripes and finally a subsequent combination of etched stripes and etched microrods (double overgrowth). Overall the presence of extended defects, namely threading dislocations and stacking faults (SFs) is greatly reduced for the two samples containing stripes compared with the two microrod samples. This is evidenced by more uniform emission and reduction in dark regions of non-radiative recombination in room temperature cathodoluminescence imaging as well as a reduction of the SF emission line in low temperature photoluminescence. The observed energy shifts of the GaN near band edge emission are related to anisotropic strain relaxation occurring during the overgrowth on these microstructures. A combination of stripes and microrods is a promising approach for defect reduction and emission uniformity in non-polar GaN for applications in light-emitting devices as well as power electronics.

HVPE grown layers typically show a high density of pyramidal structures on the surface. We found that a slight off-orientation of the substrate totally suppresses the development of these structures. Further we found that a misorientation toward the m-plane of GaN features a smoother surface morphology, compared to an off-orientation towards the a-plane. After the improvement of the surface morphology and other properties of the HVPE grown layers, we studied self-separation processes. Our approaches to remove the thick GaN-layer from the substrate were a low-temperature interlayer and a structured dielectric mask.

The growth of single-crystalline high-quality zinc oxide (ZnO) layers by a methane (CH4)-based chemical vapor deposition (CVD) growth process on sapphire substrates with an aluminum nitride (AlN) nucleation layer was investigated. We achieved monocrystalline ZnO layers free of rotational domains, which show in high-resolution X-ray diffraction (HRXRD) measurements a very narrow (similar to 110 arcsec) full width at halfmaximum (fwhm) in. scans for the ZnO (0002) reflection. The influence of growth time and layer thickness on crystal properties such as surface roughness, dislocation density, and optical properties was investigated. We find low edge and screw dislocation densities of around 6.4 x 10(8) and 2.1 x 10(7) cm(-2), respectively. In low-temperature photoluminescence (PL) spectra the fwhm of the donor-bound exciton emission drops to about 170 mu eV for increasing layer thickness. Moreover, these layers have a smooth surface with a surface roughness RMS value of 4 nm and a very low donor concentration of about 1.7 x 10(15) cm(-3). We also studied the influence of substrate miscut on crystal growth properties and found no significant influence. The results prove the high potential of methane-based chemical vapor deposition for the production of high-quality ZnO layers.

We investigate the influence of different types of template micro-patterning on defect reduction and optical properties of non-polar GaN using detailed luminescence studies. Non-polar ( 11 2 ˉ 0 ) (or a -plane) GaN exhibits a range of different extended defects compared with its more commonly used c -plane counterpart. In order to reduce the number of defects and investigate their impact on luminescence uniformity, non-polar GaN was overgrown on four different GaN microstructures. The micro-patterned structures consist of a regular microrod array; a microrod array where the − c -side of the microrods has been etched to suppress defect generation; etched periodic stripes and finally a subsequent combination of etched stripes and etched microrods (double overgrowth). Overall the presence of extended defects, namely threading dislocations and stacking faults (SFs) is greatly reduced for the two samples containing stripes compared with the two microrod samples. This is evidenced by more uniform emission and reduction in dark regions of non-radiative recombination in room temperature cathodoluminescence imaging as well as a reduction of the SF emission line in low temperature photoluminescence. The observed energy shifts of the GaN near band edge emission are related to anisotropic strain relaxation occurring during the overgrowth on these microstructures. A combination of stripes and microrods is a promising approach for defect reduction and emission uniformity in non-polar GaN for applications in light-emitting devices as well as power electronics.

In this work, we investigate the catalytic effects of gold (Au) and platinum (Pt) nanoparticle layer deposition on highly sensitive zinc oxide (ZnO) nanowires (NWs) used for selective H2S detection in the sub-ppm region. Optimum quality pristine ZnO NWs were grown by high temperature chemical vapor deposition (CVD) in the vapor liquid solid growth (VLS) mode on silicon with a thin Au layer acting as a growth catalyst. The surface of pristine ZnO NWs was modified by systematic magnetron sputtering of discontinuous Au and Pt layers of 0–5 nm thickness. Resistive gas sensors based on the gas sensing mechanism of a chemical field effect transistor (ChemFET) with open gate, which is formed by hundreds of parallel aligned pristine Au-modified or Pt-modified ZnO NWs, were measured toward H2S diluted in dry nitrogen (N2) or in dry synthetic air at room temperature. Gas sensing results showed a largely improved response due to the catalytic effects of metal deposition on the ZnO NW surface. Controlled application of ZnO NW growth under optimized conditions and metal catalyst deposition showed a clear response enhancement toward 1 ppm H2S from the initial 20% achieved with pristine ZnO to over 5000% with ZnO NWs covered by 5 nm of Au, and, hence, significantly lower than the limit of detection.

In this work, we report on the innovative growth of semipolar “bow-tie”-shaped GaN structures containing InGaN/GaN multiple quantum wells (MQWs) and their structural and luminescence characterization. We investigate the impact of growth on patterned (113) Si substrates, which results in the bow-tie cross section with upper surfaces having the (101¯1) orientation. Room temperature cathodoluminescence (CL) hyperspectral imaging reveals two types of extended defects: black spots appearing in intensity images of the GaN near band edge emission and dark lines running parallel in the direction of the Si stripes in MQW intensity images. Electron channeling contrast imaging (ECCI) identifies the black spots as threading dislocations propagating to the inclined (101¯1) surfaces. Line defects in ECCI, propagating in the [12¯10] direction parallel to the Si stripes, are attributed to misfit dislocations (MDs) introduced by glide in the basal (0001) planes at the interfaces of the MQW structure. Identification of these line defects as MDs within the MQWs is only possible because they are revealed as dark lines in the MQW CL intensity images, but not in the GaN intensity images. Low temperature CL spectra exhibit additional emission lines at energies below the GaN bound exciton emission line. These emission lines only appear at the edge or the center of the structures where two (0001) growth fronts meet and coalesce (join of the bow-tie). They are most likely related to basal-plane or prismatic stacking faults or partial dislocations at the GaN/Si interface and the coalescence region.

The growth of high-quality single-crystalline zinc oxide (ZnO) layers on silicon (Si(111)) substrates with an intermediate aluminum nitride (AlN) nucleation layer was investigated. The ZnO layers were grown using a methane (CH4)-based chemical vapor deposition (CVD) growth process, while the AlN nucleation layers were grown by metal-organic vapor-phase epitaxy (MOVPE). We investigate the influence of nucleation layer thickness and growth temperature on the final ZnO layer quality and also vary the ZnO growth parameters to obtain smooth surfaces and the best crystal quality. The ZnO layers were extensively characterized by various methods such as atomic force microscopy (AFM), high-resolution X-ray diffraction (HRXRD), electron backscatter diffraction (EBSD), and scanning/transmission electron microscopy (SEM/TEM). We find that AlN nucleation layers grown at 1150 degrees C for 15 min yielded the best final ZnO layer quality with fully c oriented ZnO layers free of rotational domains. In HRXRD omega scans full width of half maximum (fwhm) values of about 710 and 1240 arcsec for the (0002) and (10 (1) over bar2) reflections, respectively, were achieved. Dislocation densities of rho(edge) = 1.6 x 10(10) and rho(screw) = 9.1 x 10(8) cm(-2) and a lateral coherence length (LCL) of L-parallel to = 300 nm are deduced, values which are among the best reported in the literature for ZnO layers grown on Si(111). Additionally, we carried out high-resolution photoluminescence (PL) measurements at liquid-helium and room temperatures, which also prove low defect densities. The low-temperature PL spectra were also used to study the stress within the ZnO layer and compared to HRXRD stress measurements. Both measurements confirm a stress reduction in the ZnO layer with increasing thickness.

Herein, the impact of high‐temperature (HT) annealing on the crystalline structure of metal organic vapor phase epitaxy (MOVPE)‐grown boron‐containing AlN layers is investigated. High‐resolution X‐ray diffraction studies reveal AlBN in the wurtzite configuration for nonannealed 300 nm‐thick layers containing several percent of boron. After 3 h of annealing at 1700 °C, the AlBN‐related reflex is weakened, showing a strong impact of the HT treatment on the crystalline structure of this material. After annealing, high‐resolution transmission electron microscopy micrographs reveal grain formation with moiré patterns, giving strong evidence of different crystal phases or orientations, alongside well‐oriented wurtzite regions. High‐angle annular dark‐field (HAADF) imaging and electron energy loss spectroscopy indicate stronger compositional inhomogeneities for the annealed sample in comparison with the as‐grown layer, most likely related to phase separation between AlN and BN. In addition, a significant diffusion of B out of the surface region is observed. AlBN with about ten times a lower boron content, for which defect propagation from the AlN template into the AlBN layer is visible, shows a much more homogeneous contrast in HAADF investigations after annealing, although the formation of granular structures is still observed.

Here, we report on an integrated sensor system based on InGaN heterostructures for (bio)chemical sensing. The system is compact in size and fits into a relatively small volume, which makes it versatile for many applications including liquid biomolecule and gas sensing in hospitals or doctors' offices. A GaN-based heterostructure with an InGaN quantum well close to its surface excited by a 405 nm diode laser reacts to molecules adsorbing on the surface (quantum-confined Stark effect, QCSE) by shifting its photoluminescence (PL) emission wavelength, thus acting as a chemical sensor. The PL signal is guided to a linear wavelength selecting filter and then detected by a split Si photodiode. This simplification yields reduction in size and cost; possible limitations and challenges are discussed. Simulation calculations about the split diode detector responsiveness indicate that best results can be obtained when fitting the diode spectral resolution to the width of the PL signal of the InGaN sensor and the expected PL shift. The system's utility is applied to different analytes, such as isopropanol, ferritin and apoferritin. For ferritin, a wavelength shift sensitivity of 9.6 nm/mg / ml was found.

Piezoelectric AlGaN/GaN FETs on SiC with high carrier mobility have been fabricated yielding IDS=450 mA/mm and gm=200 mS/mm. In the on-state, under UV-illumination, the devices sustain a drain voltage of VDS=49 V, corresponding to a power dissipation of 26.5 W/mm. On turn-on of the device from the pinch-off state, a significant delay in the drain current build-up is observed. This effect depends on the pinch-off time and the pinch-off voltage and can be removed by either a brief UV-illumination or a VDS>25 V applied in the on-state. The drain current transients are characterized by a relaxation time τ, which is in the order of several hundred seconds. From the temperature dependence of τ, an activation energy of about 280 meV and a capture cross section of 4.4·10−18cm2 were determined. The devices show pronounced persistent photoconductivity (PPC) and the drain current ID is sensitive to illumination.

The development of sensitive biosensors, such as gallium nitride (GaN)-based quantum wells, transistors, etc., often makes it necessary to functionalize GaN surfaces with small molecules or even biomolecules, such as proteins. As a first step in surface functionalization, we have investigated silane adsorption, as well as the formation of very thin silane layers. In the next step, the immobilization of the tetrameric protein streptavidin (as well as the attachment of chemically modified iron transport protein ferritin (ferritin-biotin-rhodamine complex)) was realized on these films. The degree of functionalization of the GaN surfaces was determined by fluorescence measurements with fluorescent-labeled proteins; silane film thickness and surface roughness were estimated, and also other surface sensitive techniques were applied. The formation of a monolayer consisting of adsorbed organosilanes was accomplished on Mg-doped GaN surfaces, and also functionalization with proteins was achieved. We found that very high Mg doping reduced the amount of surface functionalized proteins. Most likely, this finding was a consequence of the lower concentration of ionizable Mg atoms in highly Mg-doped layers as a consequence of self-compensation effects. In summary, we could demonstrate the necessity of Mg doping for achieving reasonable bio-functionalization of GaN surfaces.

Over the past decade, the medical role of hydrogen sulfide (H2S) for therapeutic applications and diagnostics was extensively investigated and revealed the importance of H2S for medical breath analysis. It was suggested that decreased production of the endogenous gaseous mediator H2S in tissues of the human respiratory system can be interpreted as an early detection biomarker for inflammatory diseases like asthma etc. In order to detect the H2S concentration levels of interest, a reliable selective device sensitive to H2S in the very low parts per billion (ppb) range is needed. The main goal of our project is the development of such a highly sensitive, reproducible and reliable H2S gas sensor, acting as a future integrated component of a multiple-sensor array in an “electronic nose” type device for medical applications or for detecting trace concentrations in chemical industry. We present here a planar resistive gas sensor design based on the gas sensing mechanism of a chemical field effect transistor (ChemFET) made from very high quality zinc oxide (ZnO) nanowires (NW) with gas sensitive open gate. The open gate operates as a transducer between adsorption/desorption of the ambient target gas and a measurable resistivity change in the NWs. This resistivity change results from the band bending effect of metal oxide surface exposed to different gas atmospheres and is analyzed by current-time (I-t) measurements. To improve the selectivity of our ZnO NW sensor towards a certain target gas, we investigated the impact of surface modification by addition of catalytic metal layers on the detection limit of our ZnO NWs towards H2S. In particular, we studied the chemical affinity between gold (Au) and sulfur (S) which leads to catalytic effects between the sputtered gold nanoparticle layer and the target gas H2S. Accordingly, H2S molecules are expected to adsorb more efficiently on Au modified ZnO NWs and drastically improve the overall sensor performance towards H2S. Our modified ChemFETs showed higher signal-to-noise ratio (SNR), improved sensitivity, faster response time, and an exceptionally low detection limit of less than 10 ppb for H2S diluted in synthetic air at room temperature.

Using a new high-temperature chemical vapor deposition (CVD)-based growth process for high-quality zinc oxide (ZnO) layers, the initial stages for heteroepitaxial growth on sapphire substrates with an aluminum nitride nucleation layer was investigated. A series of samples were grown with various supplies of zinc vapor, which can easily be controlled by the amount of the precursor gas methane (CH4) used to reduce the ZnO powder. In the substrate region, the zinc vapor was reoxidized by pure oxygen, which initially led to the formation of ZnO islands on the substrate, and for longer growth duration to the desired highly crystalline ZnO layers. To determine the details about this initial layer formation process, atomic force microscopy and scanning electron microscopy were used. We find that the ZnO microcrystals coalesce very fast and form a smooth and closed layer after a growth time of 10 min only. Electron backscatter diffraction measurements on this early stage of the layer formation show a perfect c-orientation of the ZnO microcrystals. Also high- resolution X-ray diffraction measurements support the perfect alignment of the ZnO layer and show a drastic increase in crystal quality over growth time. This increase in crystal quality is also demonstrated by low-temperature photoluminescence (PL) measurements, in which the spectra show sharp lines for the donor-bound excitons already for the ZnO microcrystals. The PL spectra also show clearly that the tendency of forming basal plane stacking faults is quite high when the ZnO microcrystals are starting to coalesce but anneal out very fast after coalescence.

AlGaN-based LEDs for UV-C-light emission still suffer from relatively poor efficiency. Besides problems with carrier injection and light extraction, strong piezoelectric fields in the optically active region originating from lattice mismatch between quantum wells and barrier material are a major issue. Mixing only few percent of boron into the AlGaN active region may be sufficient to achieve lattice matched conditions, thus decreasing the influence of the quantum-confined Stark effect on the radiative recombination efficiency. However, the epitaxial growth of AlBGaN layers with sufficient crystalline quality is still a challenge, particularly due to the low solubility of boron in AlGaN and the low mobility of boron ad-atoms on the surface. Consequently, only extremely weak luminescence has been reported on layers containing few percents of boron. By thoroughly optimizing the metalorganic vapor phase epitaxial growth of AlBGaN layers with a boron content of some percent, we could achieve similar luminescence intensities as for reference AlGaN layers along with smooth hetero-interfaces and low surface roughness as measured by TEM and AFM. Besides studying the influence of basic growth parameters like temperature, V-III ratio etc., we investigate possible improvements by an optimized pulsed precursor supply sequence. To reduce the unintentional doping with impurities like oxygen or carbon, typically attributed to the standard boron precursor tri-ethyl boron (TEB), we investigate the novel metalorganic precursor tri-isopropyl-boron (TiPB). Its lower vapor pressure as compared to TEB facilitates a controlled incorporation of small B amounts. First PL spectra of AlBGaN layers grown with TiPB show promising data.