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Brain-on-Chip devices, which enable on-chip cultures of neurons to simulate brain functions, are receiving tremendous attention to meet the demands of both fundamental research and clinical trials. Thus, miniaturized sensors are being developed to accomplish real-time monitoring of neurotransmitters, which are the signaling molecules between neurons and benchmarks for neuron network operation. Electrochemical sensors are promising candidates for detecting a critical neurotransmitter, dopamine (DA), a biomarker for Parkinson’s disease. A key advantage of electrochemical sensors is their capability of fast, real-time recording. However, current state-of-the-art electrochemical DA sensors are suffering from various issues like limited sensitivity and cumbersome fabrication. Here, we report a novel route in monolithically microfabricating vertically aligned carbon nanofiber (VACNF) electrochemical DA microsensors by an anti-blistering slow cooling process. Thanks to the new microfabrication process, we are able to achieve VACNF microsensors with complete insulation and large surface areas. The champion device shows extremely high sensitivity of 4.52×104 μA/μM · cm2, which is at least two-orders-of-magnitude higher than current devices, and a highly competitive limit of detection of 0.243 nM. These figures of merit will open new windows for future applications of Brain-on-Chip devices such as electrochemical recording from a single neuron.

In this work, we demonstrate the potential of a gallium nitride (GaN)-based visible-blind ultraviolet (UV) photodetector (PD) on a commercially viable 150-mm Si wafer. The influence of thermionic field emission (TFE) and Poole–Frenkel (PF) mechanisms on the current transport of the PD has been analyzed. Conduction due to the TFE mechanism dominates in the moderate electric fields (1.25 kV/cm ${ kV/cm), while the influence of PF is prominent at higher electric fields. A bulk trap energy level of 0.374 eV is obtained with PF conduction analysis. A high responsivity of 33.3 A/W at 15 V with a 362-nm incident wavelength has been achieved in the presence of an internal gain. The internal gain of the PD is also assisted by TFE and PF mechanisms. The PD exhibits a low dark current of 4.7 nA as well as high detectivity of $4.6\times 10^{12}$ Jones at the abovementioned bias. The demonstrated robustness and high performance show the promise of III–nitride PDs for commercial applications.

In this article, we report the temperature-dependent transistor characteristic of Epi-Gd2O3/AlGaN/GaN metal oxide semiconductor high electron mobility transistor (MOSHEMT) and compare its properties with that of AlGaN/GaN metal-Schottky high electron mobility transistor (HEMT) grown on 150 mm Si (111) substrate. Introducing an epitaxial single crystalline Gd2O3 between the metal gate and AlGaN barrier not only improves the gate leakage current significantly but also enhances its thermal stability. We observe that there is no significant change in the gate leakage current even at 473 K compared to that measured at room temperature (RT) (298 K), and this is also evident in the transistor’s subthreshold behavior at 473 K. We have determined the electric field within the Gd2O3 as well as AlGaN and investigated the leakage conduction mechanism through Gd2O3. The ${I}_{ \mathrm{ON}}/{I}_{ \mathrm{OFF}}$ of the transistor was measured as high as ~108 even at 473 K with the lowest ${V}_{\text {TH}}$ shift (91.4 mV) with temperature. Our measurements also confirm the presence of polar optical phonon scattering, which directly affects the 2-D electron gas (2DEG) mobility at high temperatures and thus the electrical characteristics of HEMT and MOSHEMT.

Funding Information: This work was supported by JSPS KAKENHI Grant No. 16H06424 and 19H02166, and the ARPA-E PNDIODES program. S. Suihkonen acknowledges the financial support of the Academy of Finland (grant 297916) and the Foundation for Aalto University Science and Technology. A part of the research was performed at the OtaNano—Micronova Nanofabrication Centre of Aalto University. This work was carried out through the use of MIT’s Microsystems Technology Laboratories, AIST NPF, and open facility in the University of Tsukuba. Publisher Copyright: © 2022 The Author(s). Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd. We report on impurity diffusion in ion implanted AlN layers after thermal annealing. Silicon, tin, germanium, and magnesium ions were implanted into single-crystal Al-polar AlN layers grown on sapphire substrates. By annealing at 1600 °C, silicon and magnesium atoms were diffused in the AlN layer, while less change was observed in the distribution of germanium atoms. Silicon implantation introduced vacancy-related defects. By annealing at temperatures over 1300 °C, the vacancy-related defects were reduced, while oxygen atoms were diffused from the substrate due to sapphire decomposition. We reproducibly achieved silicon-implanted AlN layers with electrical conductance by controlling the annealing temperature and distribution of silicon and oxygen concentrations.

Atomic layer deposition (ALD) is a viable method for depositing functional, passivating, and encapsulating layers on top of halide perovskites. Studies in that area have only focused on metal oxides, despite a great number of materials that can be made with ALD. This work demonstrates that, in addition to oxides, other ALD processes can be compatible with the perovskites. We describe two new ALD processes for lead sulfide. These processes operate at low deposition temperatures (45-155 °C) that have been inaccessible to previous ALD PbS processes. Our processes rely on volatile and reactive lead precursors Pb(dbda) (dbda = rac-N2,N3-di-tert-butylbutane-2,3-diamide) and Pb(btsa)2 (btsa = bis(trimethylsilyl)amide) as well as H2S. These precursors produce high quality PbS thin films that are uniform, crystalline, and pure. The films exhibit p-type conductivity and good mobilities of 10-70 cm² V-1 s-1. Low deposition temperatures enable direct ALD of PbS onto a halide perovskite CH3NH3PbI3 (MAPI) without its decomposition. The stability of MAPI in ambient air is greatly improved by capping with ALD PbS. More generally, these new processes offer valuable alternatives for PbS-based devices, and we hope that this study will inspire more studies on ALD of non-oxides on halide perovskites.

This letter reports the demonstration of N-polar Al0.8Ga0.2N/AlN continuously-graded-channel polarization-doped field-effect transistors (PolFETs) on SiC. A PolFET with a source to drain distance of $12~\mu \text{m}$ exhibited a maximum drain current of 62.8 mA/mm and an ON/OFF current ratio of $1.1\times 10^{{4}}$ . The maximum drain current was stable between 20 °C and 250 °C operating temperatures. With the addition of 30-nm-thick Al2O3 gate insulator the maximum drain current increased to 126 mA/mm.

In this letter, we demonstrate a recessed-gate p-channel MISFET using a p-GaN (70 nm)/Al 0.2Ga0.8N (15 nm)/GaN heterostructure grown by metalorganic chemical vapor deposition (MOCVD) on a Si substrate. A major benefit of such epi-structure is that it contains both a 2-dimensional electron gas (2-DEG) and a 2-dimensional hole gas (2-DHG), hence making it suitable for the GaN-based complementary circuit implementation. In addition, the 2-DEG at the interface of the AlGaN/GaN heterostructure is depleted due to the p-GaN layer on top giving rise to normally- OFF operation for the n-channel transistors. A contact resistivity of $\textsf {4.9}\times \textsf {10}^{-\textsf {6}}\Omega \cdot \textsf {cm}^{\textsf {2}}$ is obtained for the ohmic contact to the hole channel for a p-GaN Mg concentration of $\textsf {2}\times \textsf {10}^{\textsf {20}}\, \textsf {cm}^{-\textsf {3}}$ . Long channel ( ${L} _{\textsf {g}}=\textsf {3}\,\,{\boldsymbol \mu } \text{m}$ and ${L} _{\textsf {sd}}=\textsf {9}\,\,{\boldsymbol \mu } \text{m}$ ) p-type transistors show an ON–OFF current ratio of ~105 and ON resistance of $\sim 2.3~\text{k}{\Omega } \cdot \textsf {mm}$ at ${V} _{\textsf {GS}}=-\textsf {11}$ V.

Due to the functional limitations of SiO2 for SOI applications, alternative dielectric materials have been investigated. Alternative SOI materials in this work include, AlN and AlGaN. The dielectrics were deposited using MOCVD, and with the aid of PECVD deposited SiO2, and the SiO2 was directly bonded to a handle Si wafer. Tensile tests were performed on the samples to examine the fracture behavior and maximum tensile stresses, with results being comparable to a traditional SOI. Characterization was undertaken using TEM to understand the microstructural and interfacial properties of alternative SOI. High crystal quality Al(Ga)N was achieved on a Si(111) substrate that generally contained well defined chemical interfaces. Finally, synchrotron X-ray diffraction topography was used to understand the topographical strain profile of the device and handle wafers. Topography results showed different strain network properties between the device and handle wafer. This work has demonstrated preliminary feasibility of using alternative dielectrics for SOI applications.

This study demonstrates all-optical Raman scattering study of dislocations in ammonothermally grown α -GaN crystal and identifies an edge a → -type threading dislocation (TD) using a confocal Raman 3-D imaging technique. These findings make possible the characterization of volumetric stress field and low TD density distributions over a large area on bulk α -GaN single crystal. The dislocation type effects on Raman shift are also discussed in detail (in order to identify the edge a → -type and mixed a → + c → -type TDs, and theorize the invisibility of the screw c → -type TDs). Authors are not aware of any previous reports using the confocal Raman 3-D imaging to identify the edge a → -type and mixed a → + c → -type TDs.

We present the effect of miscut angle of SiC substrates on N-polar AlN growth. The N-polar AlN layers were grown on C-face 4H-SiC substrates with a miscut towards 〈1¯100〉 by metal-organic vapor phase epitaxy (MOVPE). The optimal V/III ratios for high-quality AlN growth on 1° and 4° miscut substrates were found to be 20,000 and 1000, respectively. MOVPE grown N-polar AlN layer without hexagonal hillocks or step bunching was achieved using a 4H-SiC substrate with an intentional miscut of 1° towards 〈1¯100〉. The 200-nm-thick AlN layer exhibited X-ray rocking curve full width half maximums of 203 arcsec and 389 arcsec for (0 0 2) and (1 0 2) reflections, respectively. The root mean square roughness was 0.4 nm for a 2 μm×2μm atomic force microscope scan., Academy of Finland (grant 297916), Aalto University Science and Technology, Japan Society for the Promotion of Science. Grant-in-Aid for Scientific Research (Grant no. 16H06424), Japan Society for the Promotion of Science. Grant-in-Aid for Scientific Research (Grant no. 17K14110)

In this work, atomic layer etching (ALE) of thin film Ga-polar GaN(0001) is reported in detail using sequential surface modification by Cl2 adsorption and removal of the modified surface layer by low energy Ar plasma exposure in a standard reactive ion etching system. The feasibility and reproducibility of the process are demonstrated by patterning GaN(0001) films by the ALE process using photoresist as an etch mask. The demonstrated ALE is deemed to be useful for the fabrication of nanoscale structures and high electron mobility transistors and expected to be adoptable for ALE of other materials.

Though several methods exist to produce bulk crystals of gallium nitride (GaN), none have been commercialized on a large scale. The sodium flux method, which involves precipitation of GaN from a sodium–gallium melt supersaturated with nitrogen, offers potentially lower cost production due to relatively mild process conditions while maintaining high crystal quality. We successfully developed a novel apparatus for conducting crystal growth of bulk GaN using the sodium flux method which has advantages with respect to prior reports. A key task was to prevent sodium loss or migration from the growth environment while permitting N 2 to access the growing crystal. We accomplished this by implementing a reflux condensing stem along with a reusable capsule containing a hermetic seal. The reflux condensing stem also enabled direct monitoring of the melt temperature, which has not been previously reported for the sodium flux method. Furthermore, we identified and utilized molybdenum and the molybdenum alloy TZM as a material capable of directly containing the corrosive sodium–gallium melt. This allowed implementation of a crucible-free system, which may improve process control and potentially lower crystal impurity levels. Nucleation and growth of parasitic GaN (“PolyGaN”) on non-seed surfaces occurred in early designs. However, the addition of carbon in later designs suppressed PolyGaN formation and allowed growth of single crystal GaN. Growth rates for the (0001) Ga face (+ c -plane) were up to 14 µm/h while X-ray omega rocking (ω-XRC) curve full width half-max values were 731″ for crystals grown using a later system design. Oxygen levels were high, >10 19 atoms/cm 3 , possibly due to reactor cleaning and handling procedures.

Emission of terahertz radiation from a AlGaN/GaN/Al $_{\mathbf {2}} \mathbf {O} _{\mathbf {3\, }}$ heterostructure with a surface metal grating is investigated under conditions of 2D electron heating in a lateral electric field. The studies are performed under essentially nonequilibrium conditions when the effective temperatures of 2D electrons and 2D plasmons are several times higher than the lattice temperature. This makes it possible to observe and explore high-quality peaks of intense THz radiation corresponding to 2D plasmon resonance.

In this letter, we report the impact of epitaxial Gd2O3 on the electrical properties of an AlGaN/GaN high electron mobility transistor (HEMT) grown on a 150 mm diameter Si (111) substrate. Incorporation of epitaxial Gd2O3 grown by the molecular beam epitaxy technique under a metal gate (metal/Gd2O3/AlGaN/GaN) causes six orders of magnitude reduction in gate leakage current compared to metal/AlGaN/GaN HEMT. We observe that epi-Gd2O3 undergoes complete structural changes from hexagonal to monoclinic as the thickness of the layer is increased from 2.8 nm to 15 nm. Such structural transformation is found to have a strong impact on electrical properties whereby the gate leakage current reaches its minimum value when the oxide thickness is 2.8 nm. We find a similar trend in the density of interface traps (Dit) having a minimum value of 2.98 × 1012 cm-2 eV-1 for the epioxide layer of thickness 2.8 nm. Our measurements also confirm a significant increase in the two dimensional electron gas (2DEG) density (∼40%) at AlGaN/GaN interface with epioxide grown on AlGaN, thus confirming the contribution of epitaxial lattice strain on 2DEG modulation.

Improvements to the growth of nitride crystals in ammonothermal growth environments can be achieved through improved autoclave designs, purity, and use of in situ monitoring techniques. Given the limited data available on the stability of materials in supercritical ammonia solutions, this study intends to broaden the known dataset by experimentally investigating the mechanical and chemical stability of 35 bulk metals, 2 bulk metalloids, and 17 bulk ceramics while identifying suitable materials for future in-depth corrosion studies. This was performed by exposing each material to three different supercritical ammonia solutions in nickel-chromium superalloy autoclaves held at an external wall temperature of 575 °C for 4–12 days. The solutions were formed by initially filling the autoclave with pure ammonia (NH3), ammonia and sodium (NH3-Na), or ammonia and ammonium chloride (NH3-Cl) to achieve total system pressures of 100–250 MPa. Zirconia, silicon carbide, tungsten carbide, molybdenum and its alloys, tungsten and its alloys, and a cobalt-tungsten-aluminum alloy (Co80W10.6Al9.4) appeared stable in all three environments. Vanadium, niobium, and tantalum appeared chemically stable in all environments, though these samples embrittled and gained weight. Oxides containing high concentrations of aluminum, silicon, and/or zirconium were most stable in NH3-Cl while magnesium oxide was not stable in any environment. Silicon nitride corroded in NH3-Na, yet appeared stable in NH3 and NH3-Cl. Silver and a platinum-ruthenium alloy (Pt91Ru9) appeared to be the only noble metals stable in NH3-Na while all of the noble metals investigated, except for silver, appeared to be stable in NH3-Cl. Copper and its alloys were not stable in any environment except for Constantan (Cu53Ni47) in NH3. Exposed thermocouples of type K and N remained functional post-run in NH3 and NH3-Na. Iron and nickel and their respective alloys appeared stable in NH3 and NH3-Na but not in NH3-Cl. Most other metals and metalloids investigated in this study were not stable in any of the three solutions and lost weight. Results in this study agree with literature data, where available.

Ammonothermally grown GaN is a promising substrate for high-power optoelectronics and electronics thanks to its scalability and high structural perfection. Despite extensive research, ammonothermal GaN still suffers from significant concentrations of impurities. This article discusses the evolution of impurity incorporation during growth of basic ammonothermal GaN, in specific whether the impurity concentration changes temporally along the growth direction and how the autoclave influences the impurity concentration. The effect of the impurities on the structural, electrical and optical properties of the grown crystal is also discussed. The chemical analysis is carried out by time of flight secondary ion mass spectroscopy (ToF-SIMS) and laser-ablation inductively-coupled plasma mass spectroscopy (LA-ICP-MS). Strain and dislocation generation caused by impurity concentration gradients and steps are studied by synchrotron radiation x-ray topography (SR-XRT). Fourier transform infrared (FTIR) reflectivity is used to determine the effect of the impurities on the free carrier concentration, and the luminescent properties are studied by low temperature photoluminescence (PL). The influence of the autoclave is studied by growing a single boule in multiple steps in several autoclaves. LA-ICP-MS and ToF-SIMS ion intensities indicate that the impurity concentrations of several species vary between different autoclaves by over an order of magnitude. SR-XRT measurements reveal strain at the growth interfaces due to impurity concentration gradients and steps. Oxygen is determined to be the most abundant impurity species, resulting in a high free carrier concentration, as determined by FTIR. The large variation in Mn concentration dramatically affects PL intensity.

We report on studies of electrically excited non-equilibrium 2D electrons and 2D plasmons in an AlGaN/GaN nanostructure. Optical access to 2D plasmons is provided by means of a metal grating fabricated at the nanostructure surface, while the properties of 2D electrons are examined in the samples without metal grating. The paper focuses on the creation of highly non-equilibrium conditions when the effective temperature of 2D electrons is much higher than the crystal lattice temperature. Such conditions are realized by applying short electrical pulses with a low repetition frequency. A method has been developed for independently determining the temperature of hot electrons and the temperature of the crystal lattice under an applied electric field. It has been shown that under highly non-equilibrium conditions the spectral density of terahertz electroluminescence of 2D plasmons can significantly exceed that of 2D electrons at a certain frequency.

Metalorganic chemical vapor deposited (MOCVD) aluminum nitride (AlN) on vertical sidewalls can be used to implement piezoelectric in-plane actuation and sensing in microelectromechanical system (MEMS) sensors. The AlN films should optimally cover conformally the sidewalls and have good crystal quality with c-axis oriented microstructure for optimal piezoelectric properties. Previous MOCVD AlN research has focused on using AlN as a buffer layer for other III-nitrides and so far, AlN growth has not been studied on large vertical surfaces. In this study, AlN thin films were grown using MOCVD on vertical sidewalls of fabricated templates and the conformality and crystal quality was characterized. The growth template fabrication was optimized with respect to surface roughness, the conformal coverage was analyzed by measuring the thickness profiles of the films, and the crystal quality was investigated using in-plane XRD and TEM. The AlN films have good crystal quality (FWHM 1.70°–3.44°) and c-axis orientation on vertical Si(1 1 1) sidewalls. However, the thicknesses of the films reduce approximately at a rate of 0.8–1.2 nm/ μ m down the sidewall. Lowering the reactor pressure improved the conformal coverage while changing the growth mode from columnar to step-flow, which also improved the film morphology.

We present the electrical characterization of N-polar AlN layers grown by metal-organic vapor phase epitaxy and the demonstration of N-polar AlN-channel metal-semiconductor field-effect transistors (MESFETs). A high concentration of silicon is unintentionally incorporated during the high-temperature growth of N-polar AlN, causing a high buffer leakage current. The silicon concentration decreases from 2 x 10(18) to 9 x 10(15) cm(-3) with decreasing growth temperature, reducing the buffer leakage current to 5.6 nA/mm at a 100 V bias. The N-polar AlN MESFET exhibits an off-state drain current of 0.27 nA/mm and a transistor on/off ratio of 4.6 x 10(4) owing to the low leakage of AlN buffer layers. (C) 2018 The Japan Society of Applied Physics

Terahertz radiation emission from an electrically excited AlGaN/GaN heterostructure with a surface metal grating was studied under conditions of two-dimensional (2D) electron heating by the lateral electric field. Intensive peaks related to nonequilibrium 2D plasmons were revealed in the terahertz emission spectra with up to 4 times selective amplification of the radiation emission in the vicinity of 2D plasmon resonance. This selective emission was shown to be frequency-controllable by the grating period. Exact spectral positions of the 2D plasmon resonances were preliminarily experimentally detected with the help of equilibrium transmission spectra measured at various temperatures. The resonance positions are in a satisfactory agreement with the results of theoretical simulation of the transmission spectra performed using a rigorous solution of Maxwell’s equations. The effective temperature of hot 2D electrons was determined by means of I– V characteristics and their analysis using the power balance equation. It was shown that for a given electric field, the effective temperature of nonequilibrium 2D plasmons is close to the hot 2D electron temperature. The work may have applications in GaN-based electrically pumped emitters of terahertz radiation.Terahertz radiation emission from an electrically excited AlGaN/GaN heterostructure with a surface metal grating was studied under conditions of two-dimensional (2D) electron heating by the lateral electric field. Intensive peaks related to nonequilibrium 2D plasmons were revealed in the terahertz emission spectra with up to 4 times selective amplification of the radiation emission in the vicinity of 2D plasmon resonance. This selective emission was shown to be frequency-controllable by the grating period. Exact spectral positions of the 2D plasmon resonances were preliminarily experimentally detected with the help of equilibrium transmission spectra measured at various temperatures. The resonance positions are in a satisfactory agreement with the results of theoretical simulation of the transmission spectra performed using a rigorous solution of Maxwell’s equations. The effective temperature of hot 2D electrons was determined by means of I– V characteristics and their analysis using the power balance ...

Almost all modern inorganic light-emitting diode (LED) designs are based on double heterojunctions (DHJs) whose structure and current injection principle have remained essentially unchanged for decades. Although highly efficient devices based on the DHJ design have been developed and commercialized for energy-efficient general lighting, the conventional DHJ design requires burying the active region (AR) inside a pn-junction. This has hindered the development of emitters utilizing nanostructured ARs located close to device surfaces such as nanowires or surface quantum wells. Modern DHJ III-N LEDs also exhibit resistive losses which arise from the DHJ device geometry. The recently introduced diffusion-driven charge transport (DDCT) emitter design offers a novel way to transport charge carriers to unconventionally placed ARs. In a DDCT device, the AR is located apart from the pn-junction and the charge carriers are injected into the AR by bipolar diffusion. This device design allows the integration of surface ARs to semiconductor LEDs and offers a promising method to reduce resistive losses in high power devices. In this work, we present a review of the recent progress in gallium nitride (GaN) based DDCT devices, and an outlook of potential DDCT has for opto- and microelectronics.

The optical and electrical properties of silicon-doped epitaxial gallium nitride layers grown on sapphire have been studied. The studies have been performed over a wide range of silicon concentrations on each side of the Mott transition. The critical concentrations of Si atoms corresponding to the formation of an impurity band in gallium nitride (∼2.5 × 1018 cm−3) and to the overlap of the impurity band with the conduction band (∼2 × 1019 cm−3) have been refined. The maximum of the photoluminescence spectrum shifts nonmonotonically with increasing doping level. This shift is determined by two factors: (1) an increase in the exchange interaction leading to a decrease in the energy gap width and (2) a change in the radiation mechanism as the donor concentration increases. The temperature dependence of the exciton luminescence with participating optical phonons has been studied. The energies of phonon-plasmon modes in GaN: Si layers with different silicon concentrations have been measured using Raman spectroscopy.

Devices based on nanostructures hold great potential to improve the performance of present light emitter technologies. In this paper, we examine a buried multiquantum well III-nitride diffusion injected light-emitting diode (DILED), where the active region is located outside the p-n junction and current injection to the active region takes place through bipolar diffusion. We study the current–voltage behavior and light emission characteristics of the DILED as a function of temperature experimentally and theoretically. We show that in contrast to conventional LEDs, the light output efficiency of the DILED increases when temperature is increased from 0 °C to 100 °C. This anomalous temperature behavior is shown to be linked to the strong temperature dependency of ionized acceptor density in the p-doped region, which increases the hole diffusion current into the active region. This highlights the fundamental difference in the operating principle of the DILED compared with conventional LEDs. In addition to optical and electrical characterization of a DILED, we also study the relation of the observed yellow-band luminescence to Shockley–Read–Hall recombination, compare the measurements to charge carrier transport simulations, and present an equivalent circuit model of the DILED structure for additional insight into the new current injection scheme.

The batch mode UVA-LED photocatalytic reactor with recirculation was tested for formic acid (0.22 mM) and phenol (0.1 mM) degradation using commercially available titanium dioxide. Decomposition of model molecules was monitored by means of high pressure liquid chromatography (HPLC), total organic carbon analyzer (TOC) and chemical oxygen demand (COD). Apparent quantum yields for formic acid and phenol were estimated to be 0.8% and 0.98%, respectively. Also, plywood mill effluent treatment was performed using chosen reactor. Primary degradation of tannic acid, as well as TOC and COD removal were monitored in the course of an experiment. About 72% and 60% of COD and TOC removal was achieved, respectively, after 60 min of irradiation at TiO2 loading 0.5 g L−1. The evolution of average oxidation state of carbon atoms was estimated for plywood mill wastewater.

Optical transmission measurements were performed on high quality bulk gallium nitride (GaN) crystals grown by sodium flux, hydride vapor phase epitaxy, and the ammonothermal method with varying free electron concentrations ranging from 4×10 16 cm −3 to 9×10 18 cm −3 . The quality of the crystals was analyzed by x-ray diffraction, threading dislocation density determination, impurity concentrations, and Hall mobility measurements. The sub-bandgap absorption coefficient and index of refraction was determined based on illumination wavelengths ranging from 360 nm to 800 nm. Phonon-assisted free carrier absorption was determined to be the dominant absorption mechanism above approximately 0.1 cm −1 . The absorption coefficient at 450 nm varied linearly from 0.1 cm − 1 to 5 cm −1 for free electron concentrations ranging from 1×10 17 cm −3 to 9×10 18 cm −3 . The ammonothermal GaN samples exhibited a strong defect related onset of absorption above 2.9 eV which can be explained by the presence of appreciable hydrogenated gallium vacancies having defect states close to the valance band within the electric bandgap of GaN. The presence of hydrogenated gallium vacancies was experimentally confirmed by Fourier transform infrared absorbance measurements and double hydrogenated gallium vacancy defect are speculated to be prominent in ammonothermal GaN.

White beam synchrotron radiation X-ray topography (SR-XRT) and X-ray diffraction (XRD) measurements were used to non-destructively study the defect structure of a bulk GaN wafer, grown by the ammonothermal method. SR-XRT topographs revealed high crystal quality with threading dislocation density 8.8×10 4 cm −2 and granular structure consisting of large, slightly misaligned grains. The threading dislocations within grains were identified as mixed and screw type, while no pure threading edge dislocations were observed.

We report on the studies of the surface plasmon polaritons in n-GaN epitaxial layers. The grating etched on the surface of the epitaxial layer is used to excite surface plasmon polaritons by means of terahertz photons. The experimental reflectivity spectrum for p-polarized radiation demonstrates a set of resonances associated with excitation of different surface plasmon polariton modes. Spectral peculiarities due to the diffraction effect have been also revealed. Emission of terahertz radiation is investigated under epilayer temperature modulation by electric current. The emissivity spectrum of the epitaxial layer with surface-relief grating shows peaks in the frequency ranges corresponding to the decay of the surface plasmon polariton modes. The characteristic features of the reflectivity and emissivity spectra are well described theoretically by a differential method with explicit integration scheme.

High-power operation of conventional GaN-based light-emitting diodes (LEDs) is severely limited by current crowding, which increases the bias voltage of the LED, concentrates light emission close to the p-type contact edge, and aggravates the efficiency droop. Fabricating LEDs on thick n-GaN substrates alleviates current crowding but requires the use of expensive bulk GaN substrates and fairly large n-contacts, which take away a large part of the active region (AR). In this work, we demonstrate through comparative simulations how the recently introduced diffusion-driven charge transport (DDCT) concept can be used to realize lateral heterojunction (LHJ) structures, which eliminate most of the lateral current crowding. Specifically in this work, we analyze how using a single-side graded AR can both facilitate electron and hole diffusion in DDCT and increase the effective AR thickness. Our simulations show that the increased effective AR thickness allows a substantial reduction in the efficiency droop at large currents, and that unlike conventional 2D LEDs, the LHJ structure shows practically no added efficiency loss or differential resistance due to current crowding. Furthermore, as both electrons and holes enter the AR from the same side without any notable potential barriers in the LHJ structure, the LHJ structure shows an additional wall-plug efficiency gain over the conventional structures under comparison. This injection from the same side is expected to be even more interesting in multiple quantum well structures, where carriers typically need to surpass several potential barriers in conventional LEDs before recombining. In addition to simulations, we also demonstrate selective-area growth of a finger structure suitable for operation as an LHJ device with 2µm distance between n- and p-GaN regions.

We report on the epitaxial growth of ZnO on GaN template by atomic layer deposition (ALD). Diethylzinc (DEZn) and water vapour (H 2 O) were used as precursors. The structure and the quality of the grown ZnO layers were studied with scanning electron microscope (SEM), X-ray diffraction (XRD), photoluminescence (PL) measurements and positron annihilation spectroscopy. The ZnO films were confirmed epitaxial, and the film quality was found to improve with increasing deposition temperature in the vicinity of the threshold temperature of two dimensional growth. We conclude that high quality ZnO thin films can be grown by ALD. Interestingly only separate Zn-vacancies were observed in the films, although ZnO thin films typically contain fairly high density of surface pits and vacancy clusters.

The ammonothermal method is one of the most promising candidates for large-scale bulk GaN growth due to its scalability and high crystalline quality. However, emphasis needs to be put on understanding the incorporation and effects of impurities during growth. This article discusses how impurities are incorporated in different growth zones in basic ammonothermal GaN, and how they affect the structural, electrical and optical properties of the grown crystal. The influence of growth time on the impurity incorporation is also studied. We measure the oxygen, silicon, and carbon impurity concentrations using secondary ion mass spectrometry, and measure their effect on the lattice constant by high resolution x-ray diffraction (HR-XRD). We determine the resulting free carrier concentration by spatially resolved Fourier transform infrared spectroscopy and study the optical properties by spatially resolved low-temperature photoluminescence. We find that oxygen is incorporated preferentially in different growth regions and its incorporation efficiency depends on the growth direction. The oxygen concentration varies from 6.3×1020 cm-3 for growth on the 112-2 planes to 2.2×1019 cm-3 for growth on the (0001) planes, while silicon and carbon concentration variation is negligible. This results in a large variation in impurity concentration over a small length scale, which causes significant differences in the strain within the boule, as determined by HR-XRD on selected areas. The impurity concentration variation induces large differences in the free carrier concentration, and directly affects the photoluminescence intensity.

GaP0.98N0.02 layers having a thickness from 75 nm to 600 nm are grown on GaP substrates by metalorganic vapor phase epitaxy. Back-reflection X-ray topographs made with a synchrotron radiation of the thick GaP0.98N0.02 layers reveal images of misfit dislocations. The critical thickness of the grown GaP0.98N0.02 layer on GaP substrate is evaluated from the topographs to be about 200 nm. The strain relaxation is also observed from high resolution X-ray diffraction curves. X-ray diffraction curves and atomic force microscopy studies indicate that the GaP0.98N0.02 layers with thickness smaller than 200 nm possess a very good crystal quality, an atomically smooth surface with a roughness less than 0.3 nm and a well-defined interface without misfit dislocations between the layer and the substrate.

In this paper we report on the MOCVD growth and characterization of GaN structures and InGaN single quantum wells grown on pillar patterned GaN/sapphire templates. During the regrowth a network of voids was intentionally formed at the interface of sapphire substrate and GaN epitaxial layer. The regrowth process was found to decrease the threading dislocation density of the overgrown layer. The quantum well sample grown on patterned template showed significantly higher optical output in photoluminescence measurements compared to the reference sample with identical internal quantum efficiency characteristics. We attribute the increase to enhanced light extraction efficiency caused by strong scattering and redirection of light from the scattering elements.

Optical spectra and electrical conductivity of silicon-doped epitaxial gallium nitride layers with uncompensated donor concentrations N D — N A up to 4.8 × 1019 cm−3 at T ≈ 5 K have been studied. As follows from the current-voltage characteristics, at a doping level of ∼3 × 1018 cm−3 an impurity band is formed and an increase of donor concentration by one more order of magnitude leads to the merging of the impurity band with the conduction band. The transformation of exciton reflection spectra suggests that the formation of the impurity band triggers effective exciton screening at low temperatures. In a sample with N D — N A = 3.4 × 1018 cm−3, luminescence spectra are still produced by radiation of free and bound excitons. In a sample with N D — N A = 4.8 × 1019 cm−3, Coulomb interaction is already completely suppressed, with the luminescence spectrum consisting of bands deriving from impurity-band-valence band and conduction-band-valence band radiative transitions.

Carrier lifetimes of the recombination processes in light emitting diodes (LEDs) depend strongly on the operating point. We measure the carrier lifetimes of InGaN/GaN LEDs at low current densities by applying the differential carrier lifetime analysis. In the measurement setup the LED is driven with constant current modulated with small amplitude alternating current (AC) and the carrier lifetime is obtained from the phase difference between input AC and light output. In addition to the optical method, we also extract the lifetimes directly from the capacitance and differential resistance of the LED obtained using an impedance analyzer and compare the results to lifetimes obtained by the optical setup. The carrier lifetimes were found to increase from hundreds of nanoseconds to microseconds when the current density was decreased from tens of A/cm2 to tens of mA/cm2. Both measurement methods resulted in similar lifetimes (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we demonstrate self-alignment of microchips on a simple-to-fabricate hybrid template with both water and UV-curing adhesive (EPO-TEK UVO-114). The hybrid template contains receptor sites with solid edges for droplet confinement and large wetting contrast between the receptor sites and the substrate for microchip manipulation. Nanostructured black silicon surface functionalized with fluoropolymer is used as a substrate, while protruded silicon dioxide patterns covered with fluoropolymer serve as receptor sites. A simple and fast process consisting only of one pass of photolithography, cryogenic deep reactive-ion etching (RIE), and RIE steps is used to fabricate the hybrid template. The self-assembly tests are carried out in a hybrid microassembly setup. Dummy microchips of sizes 200 μm × 200 μm × 50 μm are self-aligned on 200 μm × 200 μm receptor sites in ambient air with both water and adhesive.

We study theoretically and experimentally transverse and longitudinal electromagnetic waves in n-GaN epitaxial layers. The studies are carried out on the epitaxial layers with various doping levels. Simulation of the reflectivity and absorptivity spectra is performed in a wide frequency range. Additionally, the radiation emission spectra are simulated for various temperatures of the GaN epitaxial layer. It is shown that the resonance peculiarities of the optical spectra are located closely to the frequencies of the coupled plasmon phonon modes. Experimental studies of the reflectivity spectra have been performed in the spectral range 2-20 THz. The experimental spectra are well fitted by the simulated ones and can be used for contactless determination of the electron concentration and mobility in GaN epitaxial layers. The experimental and theoretical results of the present work provide insights for the development of GaN-based devices aimed to absorb/emit terahertz or mid infrared radiation selectively.

We present the results of experimental and theoretical studies of the surface plasmon polariton excitations in heavily doped GaN epitaxial layers. Reflection and emission of radiation in the frequency range of 2-20 THz including the Reststrahlen band were investigated for samples with grating etched on the sample surface, as well as for samples with flat surface. The reflectivity spectrum for p-polarized radiation measured for the sample with the surface-relief grating demonstrates a set of resonances associated with excitations of different surface plasmon polariton modes. Spectral peculiarities due to the diffraction effect have been also revealed. The characteristic features of the reflectivity spectrum, namely, frequencies, amplitudes, and widths of the resonance dips, are well described theoretically by a modified technique of rigorous coupled-wave analysis of Maxwell equations. The emissivity spectra of the samples were measured under epilayer temperature modulation by pulsed electric field. The emissivity spectrum of the sample with surface-relief grating shows emission peaks in the frequency ranges corresponding to the decay of the surface plasmon polariton modes. Theoretical analysis based on the blackbody-like radiation theory well describes the main peculiarities of the observed THz emission.

Polarization controlled Fourier transform infrared (FTIR) absorption measurements were performed on a high quality m-plane ammonothermal GaN crystal grown using basic chemistry. The polarization dependence of characteristic absorption peaks of hydrogen-related defects at 3000-3500 cm(-1) was used to identify and determine the bond orientation of hydrogenated defect complexes in the GaN lattice. Majority of hydrogen was found to be bonded in gallium vacancy complexes decorated with one to three hydrogen atoms (V-Ga-H-1,H-2,H-3) but also hydrogenated oxygen defect complexes, hydrogen in bond-center sites, and lattice direction independent absorption were observed. Absorption peak intensity was used to determine a total hydrogenated V-Ga density of approximately 4 x 10(18) cm(-3), with main contribution from V-Ga-H-1,H-2. Also, a significant concentration of electrically passive V-Ga-H-3 was detected. The high density of hydrogenated defects is expected to have a strong effect on the structural, optical, and electrical properties of ammonothermal GaN crystals. Published by AIP Publishing.

In this article, we analyze the behavior of threading dislocations in GaN layers re-grown on hexagonally patterned mask-less GaN. The growth mode of the material with patterned hexagonal morphology changes with the diameter and the periodicity of the hexagonal patterns. The growth mode directly affects the shape of the voids that are formed in this kind of lateral epitaxy. Transmission electron microscopy has been used to study threading dislocations in GaN layers with voids having different sizes and sidewall angles. The results show that a significant number of threading dislocations near the tapered void's surface undergo a 90° reorientation in their propagation trajectory whereas almost no dislocations bend in the case of smaller voids having more vertical sidewalls. Different types of dislocations in the vicinity of the voids have also been identified using the invisibility g·b criteria. The full width at half maximum values for XRD ω -scan recorded in (002) reflection drop from 256″ to 181″ as the void sidewall inclination changes from 85° to 60°. A similar dropping trend in the full width at half maximum values for asymmetric diffraction reflections has also been observed.

Owing to its great potential in optoelectronic devices, a lot of effort has recently been put into improving the crystal quality of heteroepitaxial GaN. In this study, GaN layers grown on patterned GaN/sapphire substrates are non-destructively investigated by X-ray diffraction techniques. The effect of the patterning is characterized by determining the strain and lattice tilt and twist in the overgrown GaN layers. High resolution X-ray diffraction (HRXRD) scans and reciprocal space maps (RSMs) were utilized for this purpose. Synchrotron radiation X-ray topography (SR-XRT) was used to examine the microstructure of the samples. The patterning was found to significantly improve the crystal quality of the GaN layers, especially by reducing the amount of edge dislocations. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present the results of GaN re-growth on hexagonally patterned GaN templates. Sapphire was used as the original substrate and the samples were grown by metalorganic vapor phase epitaxy (MOVPE). The re-growth on the patterned templates results in the formation of voids at the GaN/sapphire interface. Our extensive scanning electron microscopy (SEM)-based experimental investigations show that the void shape can be controlled from nearly vertical to fully inclined configurations. It was found that the initial hexagon hole diameter plays a key role in determining the final profile of the void sidewalls. X-ray diffraction analysis of the GaN layers indicates that the layers with inclined sidewall voids have an improved crystalline quality. Knowledge of the void configurations in the GaN layers and a possibility to control their shape can help in enhancing light extraction from the light emitting structures.

The commercial significance of superlattice structures is increasing due to greater demand of optoelectronic devices, such as the light emitting diode (LED). In order to optimize these devices, an accurate and reliable characterization method is needed. This paper describes in detail the characterization of superlattices with X-ray scattering techniques. The thicknesses of the individual layers are determined by X-ray reflectivity (XRR) measurements and the state of strain, the lattice constants and the compositions of ternary compounds by X-ray diffraction (XRD) measurements. The method is non-destructive, and yields unique results, unlike characterizations based on simulation of symmetric XRD scans. These simulations were used for verification of results. The method was tested on InGaN and AlGaN superlattice structures. The measured and simulated parameter values agreed very well. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the electrical characterization of Si-ion implanted AlN layers and the first demonstration of metal-semiconductor field-effect transistors (MESFETs) with an ion-implanted AlN channel. The ion-implanted AlN layers with Si dose of 5 × 1014 cm-2 exhibit n-type characteristics after thermal annealing at 1230°C. The ion-implanted AlN MESFETs provide good drain current saturation and stable pinch-off operation even at 250°C. The off-state breakdown voltage is 2370V for drain-to-gate spacing of 25μm. These results show the great potential of AlN-channel transistors for high-temperature and high-power applications.

GaN films were fabricated by metal organic vapor phase epitaxy (MOVPE) on patterned sapphire substrates (PSSs) with either direct or inverse type patterned structures. Both of these two types of PSSs had their own unique GaN growth process which depart from the standard growth on the planar c-plane. GaN films on PSSs showed decreased threading dislocation (TD) density. However, differences between the crystal quality of the GaN films grown on PSSs were observed. It was also found out with one of the pattern type that the TD density varied laterally and followed the periodicity of the pattern on the sapphire surface.

The effects following ion irradiation of GaN-based devices are still limited. Here we present data on the photoluminescence (PL) emitted from InGaN/GaN multiple quantum well (MQW) structures, which ...

A simple wet etching method based on the use of hot sulphuric (H 2 SO 4 ) acid for roughening of the backside of the sapphire substrates for creating light scattering objects without any lithography processes is described. Scanning electron microscope images show that this method offers a possibility to tailor the size of the scattering objects by varying the treatment time. A metal organic vapor phase epitaxy (MOVPE) grown light emitting diode (LED) structure having a roughened sapphire backside exhibits a 20–25% increase of the electroluminescence output power compared to a reference sample on the standard c -plane sapphire at the operating wavelength of 460 nm.