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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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

We report on the effect of GaN buffer threading dislocation (TD) optimization and InGaN/GaN quantum well (QW) hydrogen ðH2Þ treatment on the efficiency of GaN light emitting diodes (LEDs) operating in the spectral range from 400 to 500 nm. A tenfold reduction of the TD density in the GaN buffer increased the efficiency of blue LEDs operating at high current density, while in green LEDs it had very little effect. The reduced TD density also increased the compressive strain in the InGaN QWs, and caused blue shift to the electroluminescence (EL) peak wavelength. The H2 treatment of the QWs increased strain inside the MQW stack. It was possible to apply the H2 treatment only to UV LEDs, as the increased strain in blue and green LEDs caused relaxation of the MQW stack. Although this resulted in smooth surface morphology of the MQW stack, it did not lead to any increase in the efficiency of the UV LEDs. r 2006 Published by Elsevier B.V. PACS: 73.21.Fg; 78.55.Cr; 78.60.Fi; 85.60.Jb

Charge injection to the prevailing and emerging light-emitting devices is almost exclusively based on the double heterojunction (DHJ) structures that have remained essentially unchanged for decades. In this letter, we report the excitation of a near surface indium gallium nitride (InGaN) quantum well (QW) by bipolar carrier diffusion from a nearby electrically excited pn-homojunction. The demonstrated near surface QW emitter is covered only by a 10nm GaN capping leaving the light-emitting mesa perfectly free of metals, other contact, or current spreading structures. The presented proof-of-principle structure, operating approximately with a quantum efficiency of one fifth of a conventional single QW reference structure, provides conclusive evidence of the feasibility of using diffusion injection to excite near surface light-emitting structures needed, e.g., for developing light emitters or photo-voltaic devices based on nanoplasmonics or free-standing nanowires. In contrast to the existing DHJ solutions or optical pumping, our approach allows exciting nanostructures without the need of forming a DHJ, absorbing layers or even electrical contacts on the device surface.

InN films were grown by metal-organic vapour phase epitaxy (MOVPE). The growth was performed in a MOVPE apparatus with a vertical reactor geometry optimized for the growth of GaN. The reactor geometry is found to cause enhanced cracking of NH3, and the growth rate is limited by the amount of reactive indium in the temperature range of 550–650 °C. The grown films are characterized comprehensively. Experimental results indicate that the InN films, grown on sapphire substrates, contain metallic indium and the film surface consist of hexagonal islands. Growth temperature has a strong effect on the surface island size, optical quality and electrical properties of the InN layer. The desorption of nitrogen is assumed to cause the formation of metallic indium above 550 °C.

Native bulk gallium nitride (GaN) has emerged as an alternative for sapphire and silicon as a substrate material for III-N devices. While quasi-bulk GaN substrates are currently commercially available, single crystal GaN substrates are considered essential for future high performance light emitters and power devices. The ammonothermal method is currently considered one of the most feasible methods to grow large truly bulk crystals of GaN at low cost and high structural quality. High crystalline quality GaN substrates sliced from ammonothermally grown crystals have been demonstrated and utilized in homoepitaxy for III-N devices. However, despite the high crystalline quality the properties of as-grown ammonothermal GaN crystals and substrates are affected by the presence of impurities and other defects that hinder their use for device applications. Here, the main developments of ammonothermal growth of GaN and the effects of impurities, native point defects, and dislocations on the material properties are summarized. Additionally, measurement techniques that enable the evaluation of point defect concentration and low dislocation density distribution over a large area on bulk GaN substrates are reviewed.

We studied theoretically and experimentally plasmon-phonon polaritons and longitudinal plasmon-phonon oscillations in n-GaN epitaxial layers. The studies were carried out on the epitaxial layers with various doping levels. Simulation of the reflectivity spectra and dispersion relations of plasmon-phonon polaritons was performed in a wide frequency range. Reflectivity spectra transformation associated with phonon damping and electron relaxation processes has been revealed. Experimental studies of the reflectivity spectra have been performed in the spectral range of 8-80 meV. The experimental spectra are well fitted by the simulated ones. Results of the study can be used for contactless determination of the electron concentration and mobility in GaN epitaxal layers.

The crystal quality of bulk GaN crystals is continuously improving due to advances in GaN growth techniques. Defect characterization of the GaN substrates by conventional methods is impeded by the very low dislocation density and a large scale defect analysis method is needed. White beam synchrotron radiation x-ray topography (SR-XRT) is a rapid and non-destructive technique for dislocation analysis on a large scale. In this study, the defect structure of an ammonothermal c-plane GaN substrate was recorded using SR-XRT and the image contrast caused by the dislocation induced microstrain was simulated. The simulations and experimental observations agree excellently and the SR-XRT image contrasts of mixed and screw dislocations were determined. Apart from a few exceptions, defect selective etching measurements were shown to correspond one to one with the SR-XRT results.

Although conventional III-V compound semiconductors are often considered not to exhibit an efficiency droop, a pronounced low temperature droop was recently measured in AlGaInP/GaAs multi-quantum well structures. In this work, we investigate the efficiency droop in simple optically pumped lattice matched InGaAs/InP single well heterostructures to exclude charge transport related effects from the measurements. The results show that droop is present in this very simplistic setup and, furthermore, starts approximately at the same carrier density as in typical III-N structures. Our results suggest that in its most fundamental form, droop can be explained by Auger-like processes.

High-resolution Raman mapping of the stress distribution in an etched GaN micro-pillar template and a 5 μm thick GaN layer grown on a micro-pillar patterned GaN template is investigated. Raman mapping of the E2 (high) phonon shows differences in stress between the coalescing boundary, the top surface of the pillar region and around the GaN micro-pillar. Increased compressive stress is observed at the coalescing boundary of two adjacent GaN micro-pillars, when compared to the laterally grown GaN regions. The electron channeling contrast image reveals the reduction of threading dislocation density in the GaN layer grown on the micro-pillar patterned GaN template.

International audience; We demonstrate that an extraction enhancement by a factor of 2.8 can be obtained for a GaN quantum well structure using metallic nanostructures, compared to a flat semiconductor. The InGaN/GaN quantum well is inserted into a dielectric waveguide, naturally formed in the structure, and a silver grating is deposited on the surface and covered with a polymer film. The polymer layer greatly improves the extraction compared to a single metallic grating. The comparison of the experiments with simulations gives strong indications on the key role of weakly guided modes in the polymer layer diffracted by the grating.

Variation of absorption of terahertz radiation in lateral electric field was investigated in GaN epitaxial layers. Different behaviour of the absorption modulation in electric field was observed for radiation polarized along electric field and perpendicular to it. Joint analysis of optical and transport measurements let us obtain field dependencies of mobility, electron concentration and absorption cross-section. For terahertz radiation polarized perpendicular to the electric field, results are in accordance with Drude model of free electron absorption. Another polarization demonstrates significant deviation that is yet to be studied more thoroughly.

In this Rapid Communication, we report positron annihilation results on in-grown and proton irradiation-induced vacancies and their decoration in aluminium nitride (AlN) single crystals. By combining positron lifetime and coincidence Doppler measurements with ab initio calculations, we identify in-grown ${V}_{\mathrm{Al}}\ensuremath{-}{\mathrm{O}}_{\mathrm{N}}$ complexes in the concentration range ${10}^{18}$ cm${}^{\ensuremath{-}3}$ as the dominant form of ${V}_{\mathrm{Al}}$ in the AlN single crystals, while isolated ${V}_{\mathrm{Al}}$ were introduced by irradiation. Further, we identify the UV absorption feature at around 360 nm that involves ${V}_{\mathrm{Al}}$.

We have applied positron annihilation spectroscopy to study 400 – 500 nm InGaN-based LED structures, as well as InGaN and AlGaN materials with varying In and Al contents. We find that the effect of adding In to GaN on the annihilation parameters obeys the Vegard's law, while in the case of AlGaN the possible effect of Al is completely screened by efficient formation of cation vacancies. The results obtained in the InGaN LED structures are indistinguishable from defect-free GaN, suggesting that the positrons annihilate preferentially in the barriers of the MQW system.

We report on the observation and experimental studies of impurity breakdown and terahertz luminescence in n-GaN epilayers under external electric field. The terahertz electroluminescence is observed in a wide range of doping levels (at noncompensated donor density from 4.5×10[sup 16] to 3.4×10[sup 18] cm[sup −3]). Spectra of terahertz luminescence and photoconductivity are studied by means of Fourier transform spectrometry. Distinctive features of the spectra can be assigned to intracenter electron transitions between excited and ground states of silicon and oxygen donors and to hot electron transitions to the donor states.

GaN layers grown by metal organic vapour phase epitaxy on sapphire were imaged by synchrotron radiation X-ray topography. The threading dislocations could not be resolved in the topographs due to t ...

Surface plasmon polaritons are investigated in heavily doped n-GaN epitaxial layers. The grating etched on the surface of the epitaxial layer is used to convert photons into the surface plasmon polaritons and vice versa. The spectral study of reflection demonstrates the possibility of nonequilibrium surface plasmon polaritons excitation due to terahertz radiation scattering on the grating. Terahertz electroluminescence is investigated under lateral electric field. The luminescence spectrum demonstrates a significant contribution of nonequilibrium surface plasmon polariton scattering to terahertz radiation emission.

The attention towards light-emitting diode (LED) structures based on nanowires, surface plasmon coupled LEDs, and large-area high-power LEDs has been increasing for their potential in increasing the optical output power and efficiency of LEDs. In this work we demonstrate an alternative way to inject charge carriers into the active region of an LED, which is based on completely different current transport mechanism compared to conventional current injection approaches. The demonstrated structure is expected to help overcoming some of the challenges related to current injection with conventional structures. A functioning III-nitride diffusion injected light-emitting diode structure, in which the light-emitting active region is located outside the pn-junction, is realized and characterized. In this device design, the charge carriers are injected into the active region by bipolar diffusion, which could also be utilized to excite otherwise challenging to realize light-emitting structures.

We propose a direct electrical measurement method for determining the extraction efficiency (EXE) and internal quantum efficiency (IQE) of III-Nitride light-emitting diodes (LEDs). The method is based on measuring the optical output power as a function of injection current at current densities near the external quantum efficiency (EQE) maximum and extracting IQE and EXE from the measurement data. In contrast to conventional methods, our method requires no low temperature measurements or prior knowledge of the device structure. The method is far more convenient than commonly used methods because it enables measuring the EXE and IQE of different LED structures at room temperature directly in a repeatable and consistent way. This enables convenient comparison of LED structures. We apply the method to determine the IQE and EXE of one commercial LED and selected self-grown planar LED chips to compare the effects of different LED structure designs. Our results are in line with published experimental results and also give more insight to our earlier findings regarding the effects of growth parameters on the quantum efficiency. In addition, our measurement method allows estimating the Shockley-Read-Hall and radiative recombination parameters if the Auger parameter is known.

Experimental evidence on low energy electron beam induced point defect activation in GaN grown by metal-organic vapor phase epitaxy(MOVPE) is presented. The GaN samples are irradiated with a 5–20 keV electron beam of a scanning electron microscope and investigated by photoluminescence and positron annihilation spectroscopy measurements. The degradation of the band-to-band luminescence of the irradiatedGaN films is associated with the activation of point defects. The activated defects were identified as in-grown Ga-vacancies. We propose that MOVPE-GaN contains a significant concentration of passive VGa-H n complexes that can be activated by H removal during low energy electron irradiation.

doi: 10.1557/opl.2012.782, Published by Cambridge University Press, 13 April 2012.The acknowledgment that follows should have been included in the published article:THE CORRECTIONThis article was originally published on June 2012 with a mistake. The name of the third author was incorrect. The correct name of the third author is “Lukasz Kilanski.”The authors regret the error.

The Current-Voltage (IV) measurements on Pt/InN Schottky barrier diodes in the temperature range 10-280 K were done. It was found that the contact was Schottky up to 280 K, becoming irreversible ohmic for higher temperatures. The ideality factor, the saturation current and the apparent barrier height were calculated by using the thermionic emission (TE) theory. The ideality factor is temperature dependent, while the saturation current and the barrier height are not. The non conventional Richardson plot exhibits good linearity, corresponding to an activation energy of 2.08 eV and a Richardson constant of 18.7Am -2K-2. The Cheung's method to estimate the value of a possible series resistance RS yields a negligible resistance. From reverse-bias IV analysis, it is found that the experimental carrier density (ND) value increases with temperature. © 2009 American Institute of Physics.