We report on a temperature study of flip-chip mounted multi-finger AlGaN/GaN heterostructure field effect transistors (HFETs) using micro-Raman spectroscopy and infrared (IR) thermography. Flip-chip mounting can be used to improve thermal device management, in particular, for devices grown on low thermal conductivity substrates such as sapphire. In this study, we compare two flip-chip mounted HFETs of different flip-chip contact bump layout designs and a non flip-chip mounted HFET. Both temperature measurements and 3D temperature simulations are presented. The results show that minimizing the distance between the bumps and the active area of the HFET is essential for obtaining a low device operating temperature.

Thin film transistors based on high-mobility organic semiconductors are prone to contact problems that complicate the interpretation of their electrical characteristics and the extraction of important material parameters such as the charge carrier mobility. Here we report on the gated van der Pauw method for the simple and accurate determination of the electrical characteristics of thin semiconducting films, independently from contact effects. We test our method on thin films of seven high-mobility organic semiconductors of both polarities: device fabrication is fully compatible with common transistor process flows and device measurements deliver consistent and precise values for the charge carrier mobility and threshold voltage in the high-charge carrier density regime that is representative of transistor operation. The gated van der Pauw method is broadly applicable to thin films of semiconductors and enables a simple and clean parameter extraction independent from contact effects.

Objective. Understanding how neuronal assemblies underlie cognitive function is a fundamental question in system neuroscience. It poses the technical challenge to monitor the activity of populations of neurons, potentially widely separated, in relation to behaviour. In this paper, we present a new system which aims at simultaneously recording from a large population of neurons from multiple separated brain regions in freely behaving animals. Approach. The concept of the new device is to combine the benefits of two existing electrophysiological techniques, i.e. the flexibility and modularity of micro-drive arrays and the high sampling ability of electrode-dense silicon probes. Main results. Newly engineered long bendable silicon probes were integrated into a micro-drive array. The resulting device can carry up to 16 independently movable silicon probes, each carrying 16 recording sites. Populations of neurons were recorded simultaneously in multiple cortical and/or hippocampal sites in two freely behaving implanted rats. Significance. Current approaches to monitor neuronal activity either allow to flexibly record from multiple widely separated brain regions (micro-drive arrays) but with a limited sampling density or to provide denser sampling at the expense of a flexible placement in multiple brain regions (neural probes). By combining these two approaches and their benefits, we present an alternative solution for flexible and simultaneous recordings from widely distributed populations of neurons in freely behaving rats.

Previous reports on Poisson-Nernst-Planck (PNP) simulations of solid-state nanopores have focused on steady state behaviour under simplified boundary conditions. These are Neumann boundary conditions for the voltage at the pore walls, and in some cases also Donnan equilibrium boundary conditions for concentrations and voltages at both entrances of the nanopore. In this paper, we report time-dependent and steady state PNP simulations under less restrictive boundary conditions, including Neumann boundary conditions applied throughout the membrane relatively far away from the nanopore. We simulated ion currents through cylindrical and conical nanopores with several surface charge configurations, studying the spatial and temporal dependence of the currents contributed by each ion species. This revealed that, due to slow co-diffusion of oppositely charged ions, steady state is generally not reached in simulations or in practice. Furthermore, it is shown that ion concentration polarization is responsible for the observed limiting conductances and ion current rectification in nanopores with asymmetric surface charges or shapes. Hence, after more than a decade of collective research attempting to understand the nature of ion current rectification in solid-state nanopores, a relatively intuitive model is retrieved. Moreover, we measured and simulated current-voltage characteristics of rectifying silicon nitride nanopores presenting overlimiting conductances. The similarity between measurement and simulation shows that overlimiting conductances can result from the increased conductance of the electric double-layer at the membrane surface at the depletion side due to voltage-induced polarization charges. The MATLAB source code of the simulation software is available via the website http://micr.vub.ac.be.

n-GaN pillar photoanodes are fabricated by dry etching of a planar GaN epilayer. The increased surface area results in a plateau photocurrent enhancement of 84%. However, surface damage is introduced during dry etching. In this work, the surface damage is controlled by the RF chuck power. The GaN pillars fabricated using the lowest RF power show a similar current onset potential and current-potential slope as the planar GaN. In addition, the damaged GaN surface of the pillars can be removed in NaOH solution, which leads to the plateau current enhancement of 100% and the onset potential shifts -60 mV with respect to planar GaN. A pair of anodic and cathodic peaks is found in the dark cyclic voltammogram of the damaged pillars, which indicates the charging and discharging of the deep-level traps existing at 0.6 eV below the CB edge.

Indium rich (In-rich) InGaN films were grown on Ge (111) substrate by plasma assisted molecular beam epitaxy with thin GaN as a buffer layer. The effects of annealing temperature and annealing time on the structural properties of In-rich InGaN films were investigated by X-ray diffraction (XRD). XRD results indicate that the as-grown InGaN films annealed at different temperatures for 1 min and 1 h respectively did not improve the film crystalline quality. But with the annealing at 750 °C and 800 °C for 1 min respectively the metallic indium was desorbed from the InGaN structure. The InGaN films annealed at higher than 660 °C for 1 h also showed the indium desorption. The InGaN film has the best film quality after annealed at 660 °C for 6 h with the full-width at half-maximum of InGaN (002) peak to be 879 arcsec. The InGaN crystalline quality started to degrade after annealed at the temperatures higher than 660 °C for 6 h.

Indium-rich InGaN films were grown on Ge(111) substrate by plasma-assisted molecular beam epitaxy (PAMBE). The influence of the indium flux on the structural properties, surface morphology, and photocurrent for water splitting has been investigated. Before the InGaN growth, 20 nm of GaN was deposited as a buffer layer. A streaky reflection high-energy electron diffraction (RHEED) pattern was observed for the GaN buffer growth. At the onset of InGaN growth, the streaks became spotty, indicating roughening of the surface and three-dimensional (3D) growth due to the lattice mismatch between GaN and InN. The indium composition in the InGaN structure was roughly fit to be around 50% from x-ray diffraction (XRD) ω–2θ measurements. Growth with excess indium supply led to segregation of metal indium on the surface. During cooling down, this metal indium transformed partially into InN. The crystal quality of the InGaN film decreased with increase of the indium flux. The photocurrent of the InGaN films used as photoelectrodes for water splitting also decreased with increase of the indium flux.

Porous n-GaN has been fabricated using electrochemical anodic etching in a 0.5 M H2SO4 solution in the dark for different biases (5.5–18.0 V). The pore morphology of the porous GaN shows distinctive differences: from narrow branching pores to wide parallel pores for increasing applied bias. The pore formation process has been investigated using cyclic voltammetry and chronoamperometry. The photoelectrochemical properties of these porous n-GaN layers have been examined. For the porous GaN etched at 5.5–15.0 V, the plateau photocurrent increases over 4 times, and the potential difference between the current onset and the plateau is reduced by 0.24 V with respect to unetched, planar n-GaN.

GaN nanopillar arrays are fabricated by inductively-coupled-plasma dry etching of a GaN epitaxial layer, using self-assembled Ni clusters as mask. Pillars of 0.4-1.6 μm height were prepared and were investigated photoelectrochemically. After the roughening, the surface area increases up to 6 times and the plateau photocurrent increases by 84% with respect to the planar GaN. The enlarged GaN/electrolyte interface promotes charge transfer and photocarrier separation in the pillar array. The defect-mediated carrier recombination becomes more important after the dry etching process. The impact of surface damage is studied by the photoelectrochemical and photoluminescence measurements.

Low-cost GaN-on-Si-based transistors are targeted to function at high ambient temperatures. With this perspective, it is aimed to evaluate the high-temperature (HT) capabilities of GaN-on-Si double-heterostructure field-effect transistors. It is highlighted that HT device operation degrades both ON and OFF states that are directly related to the increase in the on-resistance and the decrease in device breakdown voltage; 2-DEG mobility drops with increasing temperature and is responsible for ON-state degradation. Regarding the OFF-state operation, it is observed that at low-voltage operation and with increasing temperature, there is an increase in the OFF-state leakage current because of thermal-assisted electrical conduction across the III-N layers and various interfaces. The main breakdown limiting mechanism at any temperature is, however, buffer leakage along the AlN/Si interface. Because this parasitic conduction, a negative temperature coefficient of breakdown voltage of approximately -1 V/degrees C is observed. For devices after Si removal, the leakage across the AlN/Si interface is interrupted and therefore HT OFF-state characteristics show high potential to be used at high operating voltage. A breakdown voltage as high as similar to 1800 V is observed after Si removal compared with similar to 500 V with Si at 150 degrees C.

InxGa1−xN (InGaN) alloys are predominantly grown by heteroepitaxy on foreign substrates. Most often Al2O3, SiC and Si are used as substrates, however this complicates vertical conduction from the InGaN surface to the substrate backside. Therefore we investigate the heteroepitaxial growth of InGaN layers on Ge substrates. Single crystalline InGaN was obtained and domain formation was suppressed by using a thin GaN buffer layer. The InGaN shows compressive strain, which follows from the lattice mismatch with the GaN buffer layer. The In distribution is uniform throughout the InGaN layer, with no significant In segregation within the layer. Only at the surface, in a very thin layer of 20 nm, strong In segregation is observed with about 50% In. InGaN/GaN/Ge diodes show vertical current conduction of 1 A cm−2 at −2 V. InGaN grown on Ge is therefore promising for device applications with preferred vertical conduction.

Localized and propagating surface plasmon resonances are known to show very pronounced interactions if they are simultaneously excited in the same nanostructure. Here, we study the Fano interference that occurs between localized surface plasmon resonance (LSPR) and propagating surface plasmon polariton (SPP) modes by means of phase-sensitive spectroscopic ellipsometry. The sample structures consist of periodic gratings of gold nanodisks on top of a continuous gold layer and a thin dielectric spacer, in which the structural dimensions were tuned in such a way that the dipolar LSPR mode and the propagating SPP modes are excited in the same spectral region. We observe pronounced anti-crossing and strongly asymmetric line shapes when both modes move to each other's vicinity, accompanied of largely increased phase differences between the respective plasmon resonances. Moreover, we show that the anti-crossing can be exploited to increase the refractive index sensitivity of the localized modes dramatically, which result in largely increased values for the figure-of-merit which reaches values between 24 and 58 for the respective plasmon modes.

Various multiple quantum well structures with three main geometries of triangular, isosceles trapezoidal, and right-angled trapezoidal shape have been designed and fabricated on 4-in. sapphire substrates by metal organic vapor phase epitaxy. Photoluminescence measurements reveal the influence of the quantum well structures on the emission wavelength and the internal quantum efficiency. We observe less decrease in the internal quantum efficiency with increasing emission wavelength for isosceles trapezoidal shaped structures with respect to conventional structures. A significant enhancement in efficiency of more than 50% is observed at 475 nm emission for the isosceles trapezoidal shaped structures compared to the conventional structures.

Measurements of low-temperature magnetotransport in lithographic wires of submicron widths fabricated from high-mobility AlGaSb/InAs/AlGaSb two-dimensional electron system heterostructures are presented. The dependence of the spin and phase coherence lengths on wire width and diffusion constant is investigated by analyzing the conductance in low applied magnetic fields with antilocalization models. Predominantly diffusive boundary scattering is deduced from the magnitude and wire width dependence of the conductance. Diffusive boundary scattering leads to a diffusion constant decreasing with wire width and hence allows the dependence of spin coherence on wire width and diffusion constant to be investigated concurrently. The spin coherence lengths are experimentally found to be proportional to the ratio of the diffusion constant to wire width. The phase coherence lengths follow Nyquist decoherence for low-dimensional wires.

In order to enable an oxide-free Cu-to-Cu bonding in a (dual) damascene process, 3-aminopropyltrimethoxysilane- and decanethiol-derived self-assembled monolayers are selectively deposited in a dielectric-Cu based metal–insulator–metal (MIM) capacitor used as a test vehicle, which represents a dual damascene architecture environment. A two-steps SAM coating sequence is investigated for this purpose. In a first step, a “sacrificial” SHSAM is deposited on the Cu areas at the bottom of the vias. In a second step, a “barrier” NH2SAM is deposited on the dielectric areas in the field region and via’s sidewalls. This deposition sequence followed by the selective thermal ablation of the “sacrificial” SAM vs. the “barrier” SAM, enable an oxide-free Cu-to-Cu connection at via’s bottom. The differential in thermal stability between the amino and thiol SAMs has been studied by water contact angle and cyclic voltammetry. While the sacrificial SAM is selectively desorbed by thermal ablation already at ∼200°C, the barrier SAM on the dielectric sidewall and field regions withstands a thermal budget as high as ∼350°C. The substrate-selective SAMs depositions are revealed by XPS chemical characterization on the Cu and dielectric areas of the MIM structures supported by the SEM visualization of the Au nanoparticles that selectively decorate the NH2 functionalities of the barrier SAM.

The influence of nanostructuring on the photoelectrochemical (PEC) properties of GaN is investigated. GaN nanopillar arrays are fabricated by inductively-coupled-plasma dry etching of a GaN epitaxial layer, using a self-assembled Ni cluster mask. Pillars of 0.4-1.6 mu m in height were prepared and were investigated photoelectrochemically. After nanoroughening, the surface area increases up to 6 times and the plateau photocurrent increases by 84% with respect to planar GaN. The pillar structure provides abundant depletion area and therefore enhances the photocarrier separation. Surface recombination becomes more important after the dry etching process, as confirmed by the PEC and photoluminescence measurements. (C) 2013 The Electrochemical Society. All rights reserved.