Overcoming the Miscibility Gap of GaN/InN in MBE Growth of Cubic InxGa1-xN.

The lack of internal polarization fields in cubic group-III nitrides makes them promising arsenic-free contenders for next-generation high-performance electronic and optoelectronic applications. In particular, cubic InxGa1-xN semiconductor alloys promise band gap tuning across and beyond the visible spectrum, from the near-ultraviolet to the near-infrared. However, realization across the complete composition range has been deemed impossible due to a miscibility gap corresponding to the amber spectral range. In this study, we use plasma-assisted molecular beam epitaxy (PAMBE) to fabricate cubic InxGa1-xN films on c-GaN/AlN/3C-SiC/Si template substrates that overcome this challenge by careful adjustment of the growth conditions, conclusively closing the miscibility gap. X-ray diffraction reveals the composition, phase purity, and strain properties of the InxGa1-xN films. Scanning transmission electron microscopy reveals a CuPt-type ordering on the atomistic scale in highly alloyed films with x(In) ≈ 0.5. Layers with much lower and much higher indium content exhibit statistical distributions of the cations Ga and In. Notably, this CuPt-type ordering results in a spectrally narrower emission compared to that of statistically disordered zincblende materials. The emission energies of the films range from 3.24 to 0.69 eV and feature a quadratic bowing parameter of b = 2.4 eV. In contrast, the LO-like phonon modes that are observed by Raman spectroscopy exhibit a one-mode behavior and shift linearly from c-GaN to c-InN.

Origin of the spectral red-shift and polarization patterns of self-assembled InGaN nanostructures on GaN nanowires.

The luminescence of InxGa1-xN nanowires (NWs) is frequently reported with large red-shifts as compared to the theoretical value expected from the average In content. Both compositional fluctuations and radial built-in fields were considered accountable for this effect, depending on the size, structure, composition, and surrounding medium of the NWs. In the present work, the emission properties of InGaN/GaN NWs grown by plasma-assisted molecular beam epitaxy are investigated in a comprehensive study combining ultraviolet-Raman and photoluminescence spectroscopy (PL) on vertical arrays, polarization-dependent PL on bundles of a few NWs, scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and calculations of the band profiles. The roles of inhomogeneous In distribution and radial fields in the context of optical emission properties are addressed. The radial built-in fields are found to be modest, with a maximum surface band bending below 350 meV. On the other hand, variations in the local In content have been observed that give rise to potential fluctuations whose impact on the emission properties is shown to prevail over band-bending effects. Two luminescence bands with large positive and moderate negative polarization ratios of ≈+80% and ≤-60%, respectively, were observed. The red-shift in the luminescence is associated with In-rich inclusions in the NWs due to thermodynamic decomposition during growth. The negative polarization anisotropy is suggested to result from spontaneously formed superlattices in the In-rich regions of the NWs. The NWs show a preferred orthogonal absorption due to the dielectric boundary conditions and highlight the extreme sensitivity of these structures towards light polarization.

Design of Ordered Mesoporous CeO2-YSZ Nanocomposite Thin Films with Mixed Ionic/Electronic Conductivity via Surface Engineering.

Mixed ionic and electronic conductors represent a technologically relevant materials system for electrochemical device applications in the field of energy storage and conversion. Here, we report about the design of mixed-conducting nanocomposites by facile surface modification using atomic layer deposition (ALD). ALD is the method of choice, as it allows coating of even complex surfaces. Thermally stable mesoporous thin films of 8 mol-% yttria-stabilized zirconia (YSZ) with different pore sizes of 17, 24, and 40 nm were prepared through an evaporation-induced self-assembly process. The free surface of the YSZ films was uniformly coated via ALD with a ceria layer of either 3 or 7 nm thickness. Electrochemical impedance spectroscopy was utilized to probe the influence of the coating on the charge-transport properties. Interestingly, the porosity is found to have no effect at all. In contrast, the thickness of the ceria surface layer plays an important role. While the nanocomposites with a 7 nm coating only show ionic conductivity, those with a 3 nm coating exhibit mixed conductivity. The results highlight the possibility of tailoring the electrical transport properties by varying the coating thickness, thereby providing innovative design principles for the next-generation electrochemical devices.

4D-STEM at interfaces to GaN: Centre-of-mass approach & NBED-disc detection.

4D-scanning transmission electron microscopy (4D-STEM) can be used to measure electric fields such as atomic fields or polarization-induced electric fields in crystal heterostructures. The paper focuses on effects occurring in 4D-STEM at interfaces, where two model systems are used: an AlN/GaN nanowire superlattice as well as a GaN/vacuum interface. Two different methods are applied: First, we employ the centre-of mass (COM) technique which uses the average momentum transfer evaluated from the intensity distribution in the diffraction pattern. Second, we measure the shift of the undiffracted disc (disc-detection method) in nano-beam electron diffraction (NBED). Both methods are applied to experimental and simulated 4D-STEM data sets. We find for both techniques distinct variations in the momentum transfer at interfaces between materials: In both model systems, peaks occur at the interfaces and we investigate possible sources and routes of interpretation. In case of the AlN/GaN superlattice, the COM and disc-detection methods are used to measure internal polarization-induced electric fields and we observed a reduction of the measured fields with increasing specimen thickness.

Surface Diffusion Control Enables Tailored-Aspect-Ratio Nanostructures in Area-Selective Atomic Layer Deposition.

Area-selective atomic layer deposition is a key technology for modern microelectronics as it eliminates alignment errors inherent to conventional approaches by enabling material deposition only in specific areas. Typically, the selectivity originates from surface modifications of the substrate that allow or block precursor adsorption. The control of the deposition process currently remains a major challenge as the selectivity of the no-growth areas is lost quickly. Here, we show that surface modifications of the substrate strongly manipulate surface diffusion. The selective deposition of TiO2 on poly(methyl methacrylate) and SiO2 yields localized nanostructures with tailored aspect ratios. Controlling the surface diffusion allows tuning such nanostructures as it boosts the growth rate at the interface of the growth and no-growth areas. Kinetic Monte-Carlo calculations reveal that species move from high to low diffusion areas. Further, we identify the catalytic activity of TiCl4 during the formation of carboxylic acid on poly(methyl methacrylate) as the reaction mechanism responsible for the loss of selectivity and show that process optimization leads to higher selectivity. Our work enables the precise control of area-selective atomic layer deposition on the nanoscale and offers new strategies in area-selective deposition processes by exploiting surface diffusion effects.

Electrical Polarization in AlN/GaN Nanodisks Measured by Momentum-Resolved 4D Scanning Transmission Electron Microscopy.

We report the mapping of polarization-induced internal electric fields in AlN/GaN nanowire heterostructures at unit cell resolution as a key for the correlation of optical and structural phenomena in semiconductor optoelectronics. Momentum-resolved aberration-corrected scanning transmission electron microscopy is employed as a new imaging mode that simultaneously provides four-dimensional data in real and reciprocal space. We demonstrate how internal mesoscale and atomic electric fields can be separated in an experiment, which is verified by comprehensive dynamical simulations of multiple electron scattering. A mean difference of 5.3±1.5 MV/cm is found for the polarization-induced electric fields in AlN and GaN, being in accordance with dedicated simulations and photoluminescence measurements in previous publications.

Photoluminescence Detection of Surface Oxidation Processes on InGaN/GaN Nanowire Arrays.

InGaN/GaN nanowire arrays (NWA) exhibit efficient photoluminescence (PL) in the green spectral range, which extends to temperatures well beyond 200 °C. Previous work has shown that their PL is effectively quenched when oxidizing gas species such as O2, NO2, and O3 abound in the ambient air. In the present work we extend our investigations to reducing gas species, in particular to alcohols and aliphatic hydrocarbons with C1 to C3 chain lengths. We find that these species give rise to an enhancing PL response which can only be observed when the NWAs are operated at elevated temperature and in reactive synthetic air backgrounds. Hardly any response can be observed when the NWAs are operated in inert N2 backgrounds, neither at room temperature nor at elevated temperature. We attribute such enhancing PL response to the removal of quenching oxygen and the formation of enhancing water adsorbates as hydrocarbons interact with oxygen species coadsorbed on the heated InGaN surfaces.

Optical emission of GaN/AlN quantum-wires - the role of charge transfer from a nanowire template.

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.

Bias-Controlled Optical Transitions in GaN/AlN Nanowire Heterostructures.

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.

Bias-Controlled Spectral Response in GaN/AlN Single-Nanowire Ultraviolet Photodetectors.

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 (≈ 330-360 nm)/B (≈ 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 semiclassical simulations of the strain and band structure in one and three dimensions.

UV Photosensing Characteristics of Nanowire-Based GaN/AlN Superlattices.

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.

Doping-Induced Universal Conductance Fluctuations in GaN Nanowires.

The transport properties of Ge-doped single GaN nanowires are investigated, which exhibit a weak localization effect as well as universal conductance fluctuations at low temperatures. By analyzing these quantum interference effects, the electron phase coherence length was determined. Its temperature dependence indicates that in the case of highly doped nanowires electron-electron scattering is the dominant dephasing mechanism, while for the slightly doped nanowires dephasing originates from Nyquist-scattering. The change of the dominant scattering mechanism is attributed to a modification of the carrier confinement caused by the Ge-doping. The results demonstrate that the phase coherence length can be tuned by the donor concentration making Ge-doped GaN nanowires an ideal model system for studying the influence of impurities on quantum-interference effects in mesoscopic and nanoscale systems.

Probing the internal electric field in GaN/AlGaN nanowire heterostructures.

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.