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Selective area epitaxial growth of III–V and III–N nanostructures and nanowires was proven efficient for synthesis of scalable and highly ordered electronic and optoelectronic nanomaterials on diss...

In order to fabricate the AlGaN/GaN-on-Si HEMTs on existing Si-CMOS production lines, manufacturing processes must meet the rigors of Si-CMOS technology eliminating Au. One of the requirements is formation of Au-free, low resistivity ohmic contacts to AlGaN/GaN heterostructures. In this work we report structural and electrical studies of Ti/Al-based ohmic contacts to AlGaN/GaN HEMTs with TiN/Cu cover layers. Ohmic contacts have been observed after annealing of Ti/Al/TiN/Cu or Ti/Al/Ti/TiN/Cu/TiN multilayers on recessed AlGaN/GaN structure at temperature of 550 °C, specific contact resistance is about 2.3 × 10−4 Ω cm2, however, lower value was obtained after annealing at 750 °C. The XRD and TEM studies reveal formation of additional phases during annealing, namely Al3Ti and Ti2AlN inside the volume of the metallization stack and AlN at metal/GaN interface. Measured dependence of contact resistance on temperature suggests a “metal-like” carrier transport mechanism in partially recessed Ti/Al/Ti/TiN/Cu ohmic contact to 2D electron gas.

In the present work we analyze the Zn1−xMgxO:Al/Ni/p-GaN system as a transparent electrode for GaN-based UV-LEDs. We first study the properties of the Zn1−xMgxO:Al films obtained by DC magnetron sputtering under various conditions and find that they exhibit anomalous crystallization behavior i.e. not all Mg substituted Zn in the ZnO crystal lattice creating Mg precipitates as well as a lack of correlation between the Mg content and film band gap. Subsequently, we study the behavior of the contact with ultrathin Ni films (2.5–10 nm) introduced at the interface with p-GaN both after deposition as well after annealing steps at temperatures from 400 °C to 750 °C. We use complementary techniques to discuss the structural and chemical properties of the material. We find that the Ni interlayer improved the properties of electrodes, especially their electrical parameters - a 2–3-fold increase in the current at a bias of 2 V was observed. Finally, in a full LED structure, we obtained an enhancement of the 380 nm irradiated power by 9.5% for the Zn1−xMgxO:Al/2.5 nm Ni electrode in comparison with a diode without the Ni interlayer.

The recent rapid development of transparent electronics, notably displays and control circuits, requires the development of highly transparent energy storage devices, such as supercapacitors. The devices reported to date utilize carbon-based electrodes for high performance, however at the cost of their low transparency around 50%, insufficient for real transparent devices. To overcome this obstacle, in this communication highly transparent supercapacitors were fabricated based on ZnO/MnO2 nanostructured electrodes. ZnO served as an intrinsically transparent skeleton for increasing the electrode surface, while MnO2 nanoparticles were applied for high capacitance. Two MnO2 synthesis routes were followed, based on the reaction of KMnO4 with Mn(Ac)2 and PAH, leading to the synthesis of β-MnO2 with minority α-MnO2 nanoparticles and amorphous MnO2 with embedded β-MnO2, respectively. The devices based on such electrodes showed high capacitances of 2.6 mF cm−2 and 1.6 mF cm−2, respectively, at a scan rate of 1 mV s−1 and capacitances of 104 μF cm−2 and 204 μF cm−2 at a very high rate of 1 V s−1, not studied for transparent supercapacitors previously. Additionally, the Mn(Ac)2 devices exhibited very high transparencies of 86% vs. air, far superior to other transparent energy storage devices reported with similar charge storage properties. This high device performance was achieved with a non-acidic LiCl gel electrolyte, reducing corrosion and handling risks associated with conventional highly concentrated acidic electrolytes, enabling applications in safe, wearable, transparent devices.

AlGaN/GaN High Electron Mobility Transistors (HEMTs) are capable of achieving high breakdown voltage, low operating resistance and large switching speed due to the excellent performance shown by III-N structures. The paper presents selected details of technological experimental work on high voltage (HV) AlGaN/GaN-on-Si HEMTs fabricated with multifinger structures and gate widths of up to 40×1 mm. The electrical isolation of individual devices was elaborated using Al+ implantation. The ions were implanted up to a depth of 200 nm in order to produce an effective damage and isolation up to the non-conducting AlGaN buffer layer. The influence of the ion energy (in the range 208-385 kV) and the ion dose (in the range 8.5x1012-1.4x1013cm-2) on the effectiveness of the fabricated isolation was found. The properties of the fabricated ohmic contacts (using Ti/Al/Mo/Au and Ti/Al/TiN/Cu metallization schemes) with emphasis put on the technology of recess etching were studied. The impact of various pretreatment, applied before deposition of the gate metallization, on electrical parameters of multifinger devices was analysed. The tested pretreatment methods included oxide removal in HCl-based solution, and O2 or BCl3 plasma treatment, with the lowest gate leakage current obtained for the latter. The results of fabrication of the HV HEMTs with single field-plate structures with various dielectrics (Si3N4 or Al2O3) are discussed. The characterization results within the paper cover electrical (I-V characteristics), structural (TEM, XRD), topographical (AFM) and elemental (EDS mapping) analyses. This work was supported by The National Centre for Research and Development under Agreement nr TECHMATSTRATEG1/346922/4/NCBR/2017 for project "Technologies of semiconductor materials for high power and high frequency electronics"

The article presents the results of development of selective etching of p-GaN over Al0.25Ga0.75N in Cl2/Ar/O2 ICP plasma for fabrication of normally-off p-GaN gate GaN HEMTs using a laser reflectometry system for precise control of the etched material thickness. By optimizing etching process parameters such as oxygen flow, ICP power and chamber pressure, high etching selectivity of p-GaN over Al0.25Ga0.75N were obtained, with values up to 56:1. High etching selectivity and the control of etch rates of p-GaN and Al0.25Ga0.75N layers withing a wide range by changing the ICP process parameters enabled application of developed etching processes in the technology of fabrication of normally-off AlGaN/GaN HEMTs with a p-GaN gate. High threshold voltage of around 1.6 V and current densities up to 400 mA/mm were obtained. Electrical measurements have shown that the shortest 1 min Al0.25Ga0.75N overetch time after selective etching of the p-GaN layer results in best electrical parameters of fabricated devices.

Detailed microstructural studies were performed on Ni/Si ohmic contacts to silicon carbide in order to investigate the effect of initial Ni:Si ratio in as-deposited structures on the occurrence of characteristic defects in Ni silicide layers, such as voids, layer discontinuities, rough surface or rough interface. The chosen range of investigated Ni:Si ratios corresponded to δ-Ni 2 Si as a dominant phase after complete annealing sequence. Strong effect of the initial stoichiometry on the ohmic contact's microstructure was observed. The highest Ni concentration significantly lowered the temperature at which roughening of the surface and the interface occurred. The middle value of investigated concentrations resulted in the rough interface after high temperature annealing, while the lowest investigated Ni content preserved smooth interface but introduced large voids and layer discontinuities. After the first annealing step, γ-Ni 31 Si 12 and/or δ-Ni 2 Si phases were detected. In the ohmic contacts (after two-step annealing sequence), beside δ-Ni 2 Si, the metastable, high temperature phase θ-Ni 2 Si was detected (also referred to as Ni 3 Si 2 ·h). This phase can exist within a relatively broad range of Ni:Si stoichiometry. The stoichiometry change toward higher Si content, which occurs during high temperature annealing, was realized through this phase. Superstructures were detected in θ-Ni 2 Si (Ni 3 Si 2 ·h) and in γ-Ni 31 Si 12 grains. The effect of the stoichiometry change on the morphology of the Ni silicide layers is discussed.

This report presents the results of fabricating GaN nanorods by inductively coupled plasma etching using BCl 3/Cl 2 chemistry. Interestingly, the GaN nanorods are formed only in the area initially masked by the sacrificial metal mask. In addition to the metallic mask, a specific feature of this process is the application of an insulating ceramic carrier for the improvement of the process performance. The authors show that using the same etching parameters but with a conductive silicon carrier significantly reduces the efficiency of nanorod formation. Auger electron spectroscopy was applied to propose and confirm the mechanism of nanorod formation ceramic carrier and properly selected metallic masks. The usefulness of the developed method of nanorod production has been confirmed by its application in the fabrication and characterization of GaN-based UV light-emitting diodes.

Examples are presented that application of amorphous Al x O y nucleation layer is an efficient way of controlling spatial distribution of GaN nanowires grown by plasma-assisted molecular beam epitaxy. On GaN/sapphire substrates Al x O y stripes induce formation of GaN nanowires while a compact GaN layer is formed outside the stripes. We show that the ratio of nanowire length h to the thickness of the compact layer d can be tailored by adjusting impinging gallium and nitrogen fluxes. Calculations of the h/d aspect ratio were performed taking into account dependence of nanowire incubation time on the growth parameters. In agreement with calculations we found that the value of h/d ratio can be increased by increasing the N/Ga flux ratio in the way that the N-limited growth regime determines nanowire axial growth rate while growth of compact layer remains Ga-limited. This ensures the highest value of the h/d aspect ratio. Local modification of GaN growth kinetics caused by surface diffusion of Ga adatoms through the boundary separating the Al x O y stripe and the GaN/sapphire substrate is discussed. We show that during the nanowire incubation period gallium is transported out of the Al x O y stripe, which delays nanowire nucleation onset and leads to reduced length of GaN nanowires in the vicinity of the stripe edge. Simultaneously the growth on the GaN/sapphire substrate is locally enhanced, so the planar GaN layers adopts a typical edge shape of mesa structures grown by selective area growth. Ga diffusion length on a-Al x O y surface of ∼500 nm is inferred from our results.

Display Omitted Three types of GaN: MOVPE-grown, MBE-grown, ammonothermal-grown were used in the experiment.Master stamp used in the experiment consisted of glass covered with metallic layer.The pattern of the master stamp was fabricated by FIB.Soft stamps were replicated from master stamps.Triple mask was used for imprinted pattern transfer into GaN. In this paper results on GaN patterning using nanoimprint technology are presented. Direct method of stamp fabrication based on FIB etching was used. Stamp with critical dimensions of 50nm was achieved. Two kinds of polymer materials were used for master stamp replication. Influence of etching parameters using chlorine based plasma on GaN etch rate and surface roughness was discussed. Triple mask consisting of TU2 resist, Cr and SiO2 used for NIL-generated pattern transfer into GaN allowed successful patterning.

The recent rapid development of transparent electronics, notably displays and control circuits, requires the development of highly transparent energy storage devices, such as supercapacitors. The devices reported to date utilize carbon-based electrodes for high performance, however at the cost of their low transparency around 50%, insufficient for real transparent devices. To overcome this obstacle, in this communication highly transparent supercapacitors were fabricated based on ZnO/MnO

The structure of carbonic layer in three samples composed of 4H polytype of silicon carbide and the following sequence of layers: carbon/nickel/silicon/nickel/silicon was investigated with Raman spectroscopy. Different thermal treatment of the samples led to differences in the structure of carbonic layer. Raman measurements were performed with visible excitation focused on two interfaces: silicon carbide/carbon and carbon/silicide. The results showed differences in the structure across carbon film although its thickness corresponds to 8/10 graphene layers.

Nanocoral ZnO structures are fabricated by means of reactive magnetron sputtering with post deposition annealing. The films are polycrystalline with highly developed surfaces. Their application for biosensing is presented in the extended-gate FET approach where a nanocoral gate electrode is used to sense the pH of the solution and then the presence of BSA molecules.

Three samples of 4H polytype of silicon carbide (4H-SiC) covered with the following sequence of layers: carbon/nickel/silicon/nickel/silicon were investigated with micro-Raman spectroscopy. Different thermal treatments of each sample result in differences of carbon layer structure and migration of carbon atoms thorough silicide layer. Two ranges of Raman shift were investigated. The first one is placed between 1000 cm−1 and 2000 cm−1. The main carbon bands D and G are observed in this range. Analysis of the positions of these bands and their intensity ratio enables one to determine the graphitization degree of carbon layer. Additional information about the changes of the carbon layer structure was derived from analysis of 2D band placed around 2700 cm−1. Application of deep ultraviolet excitation delivered information about the structure of carbon layer formed on the free surface of silicides and the distribution of the carbon inside the silicide layer.

The paper presents investigations concerning the analysis of photonic structures with grating couplers. In the paper basic theoretically information on photonic structures with grating couplers is presented. The results of numerical investigations on photonic structures with grating couplers are discussed, too. Investigations show an essential influence of the geometrical parameters of grating couplers on the eectiveness of the input and output of optic power into and out of this photonic structure. In the paper the selected results of experimental realizations of photonic structures with grating couplers based on zinc oxide ZnO are presented.

AlGaN/GaN high electron mobility transistors on semi-insulating bulk ammonothermal GaN have been investigated. By application of regrown ohmic contacts, the problem with obtaining low resistance ohmic contacts to low-dislocation high electron mobility transistor (HEMT) structures was solved. The maximum output current was about 1 A/mm and contact resistances was in the range of 0.3&ndash, 0.6 &Omega, &middot, mm. Good microwave performance was obtained due to the absence of parasitic elements such as high access resistance.

Zinc oxide ZnO is a semiconductor material with a wide broad-band gap of 3.4 eV. It displays attractive properties permitting to apply it in sensor techniques as well as in systems of integrated optics. Data quoted in literature indicate that it is transparent in a visible range. It is characterized by a high value of the refractive index (n ≈ 2) [1–6], which is a favorable feature of the waveguide in systems of integrated optics [3]. The primary aim of the investigations is to develop in future a photonic sensor structure attained in compliance with the techniques of integrated optics applied as sensors of selected gases, e.g. NH3, NOx, . . . Gas sensors constructed on the basis of integrated optics are particularly desirable in environments liable to considerable hazards of explosion, i.e. in industry [1]. An innovation of the solution suggested by us is the construction of a sensor structure in compliance with the techniques of integrated optics, comprising a waveguide layer, a sensor layer and input-output elements, viz. Bragg’s gratings, all mounted on common substrate. The subject matter of the present paper is to investigate the optical waveguide properties of ZnO layers depending on the applied deposition technology and their thermal treatments [7–18].

GaN nanorods have been formed by inductive coupled plasma (ICP) etching in BCl3/Cl2 plasma. Nanostructures were formed only in the initially titanium masked area, wherein the dimensions of mask pattern do not define dimensions of nanorods, but the region of structure formation only. It has been found that density of nanorods strongly depends on process parameters, such as RF and ICP power, and gas pressure. Possible mechanisms of nanorods formation have been suggested, without however decisive conclusion.

In-Ga-Zn-O thin films fabricated by reactive RF magnetron sputtering have been investigated for their compositional, structural, morphological, electrical and optical properties. All resulting films present the amorphous microstructure, and root mean square roughness below 0.6 nm. The variation of the oxygen content in the deposition atmosphere from 0% to 0.9% results in the formation of a-IGZO thin films consisting of 15-29% indium, 16-28% gallium, 10-13% zinc and 30-60% oxygen, which significantly differs from the InGaZnO 4 target composition. IGZO thin films present the transmittance in range of 75% to 90% for VIS-NIR wavelengths. Mechanism of free electrons generation via oxygen vacancies formation is proposed to determine the relation between oxygen content in the deposition atmosphere and the transport properties of the IGZO of the thin films.

We report on the fabrication and performance of amorphous oxide thin-film transistors with In-Ga-Zn-O deposited by RF magnetron reactive sputtering for semiconductor channel layer. The influence of the electrical transport properties of the channel on the electrical parameters of thin-film transistors has been determined. By optimizing process parameters depletion-mode n-channel devices with maximum field-effect mobility (μ FE ) 10.1 cm 2 /Vs, threshold voltage V th =-4.85V and on to off current ratio (I on /I off )=2.1x10 2 have been demonstrated.

This paper presents the results of investigations concerning input-output systems of an electromagnetic wave in the visible and near visible spectrum for their application in structures of integrated optics. The input-output structures used in described planar optical waveguides are in a form of prism and grating couplers. The first part of the paper contains numerical analysis of grating couplers aiming at an optimization of their geometrical parameters, strictly — the depth of the grooves in the grating coupler. The second part presents the practical realization, as well as experimental tests of the planar optical waveguide with the hybrid input-output system. As the input system of the electromagnetic wave, a prism coupler was used, and in the case of the output system — a photonic structure with grating coupler was applied. The investigated planar wave guides with the input-output structures were made of a wide energy band gap semiconductor — zinc oxide (ZnO).

AlGaN/GaN high electron mobility transistors (HEMTs) seem to dominate the market and research in the area of high-power RF technology. The dominant substrate material for GaN epitaxy is SiC. The 3.5% lattice mismatch between GaN and SiC is relatively small but still may be a considerable source of lattice defects. The main reason for use of foreign substrates was for years, lack of high quality GaN wafers. However, recently it came a breakthrough in this field. A leading GaN substrates manufacturer, Ammono S.A., has shown 2” truly bulk highly-resistive GaN wafers grown by ammonothermal method. The next key problem related to the use of foreign substrates is a management of the heat transfer inside multilayer epitaxial structure of GaN HEMTs. Due to the reliability and performance issues, this aspect is especially important for high power devices. While the thermal conductivity of 4H-SiC is higher than for GaN, the heat flow inside typical GaN HEMT on SiC substrate is significantly limited because of presence of buffer or nucleation layers (e.g. AlN) between epilayers and SiC substrate. That effect is commonly called thermal boundary resistance (TBR). Dislocations at the interfaces have a large share in the TBR. It can be generalized, that buffer layers grown on the other substrates that GaN have a significant contribution to total thermal resistance of GaN HEMT. In case of AlGaN/GaN structure grown on the bulk gallium nitride, the thermal resistance of GaN HEMT is only determined by the conductivity of bulk GaN. This paper presents first AlGaN/GaN HEMT structures prepared on novel generation Ammono-GaN semi-insulating substrates. Such substrates characterized by FWHM value of X-ray rocking curve 20 arcsec, curvature radius of several tens of meters, and dislocation density 2x104 cm-2 were chosen for epitaxial growth of GaN-based semiconducting thin film materials by MOCVD method on c-plane. The HEMT epitaxial structure consisted of 1 nm GaN-cap, 27 nm Al0.27Ga0.73N barrier layer, 0.8 nm AlN spacer, and 3 μm unintentionally doped GaN layers. High resolution 2θ-ω and rocking curve XRD scans of HEMT structure prove excellent crystal quality of epilayers grown on Ammono-GaN with FWHM=0,0090o. Sheet resistivity, sheet carrier concentration and Hall mobility were 417 Ω/□, 1.23×1013 cm-2, 1308 cm2/Vs, respectively. Fabrication of transistors starts by formation of ohmic contacts by using the regrown n+-GaN:Si layers. Isolation of adjacent devices was done by using Al+ double ion implantation. By this approach the contact resistance Rc of 0.3-0.6 Ωmm was obtained. The fabricated transistors had rectangular Ni/Au gate with 0.8µm length and 2x200 µm width. The source to gate and gate to drain distance was 1.2 and 4 µm respectively. The devices were passivated by 100 nm SiNxlayer deposited by PECVD. Finally, contact pads were opened and thickened by Au deposition. The transistor was designed as a test fixture and its topology was adapted to requirements of the "on-wafer" measurement station Cascade M150. The measurements performed have shown that the maximum output current density of fabricated transistors reaches 1000 mA/mm at VGS=2V. The extracted on-state resistance Ron is 4.4 Ω/mm. Transistor transconductance is about 220 mS/mm and reaches maximum value for the expected range of operating points of transistors. No measurable leakage current through the buffer layers or the substrate has been observed. The frequency performances of fabricated transistors were evaluated by measuring the S-matrix parameters over 45 MHz to 24 GHz frequency range at quiescent point VDSQ=28V and IDSQ=46 mA. The maximum frequency (fmax) and cut-off frequency (fT) was 30 GHz and 21 GHz as obtained by linear extrapolation with -20dB/dec slope of unilateral gain (U) and small-signal current gain |H21| characteristics. The insertion gain |S21| attain 0dB for frequency (fs) of 22 GHz. The maximum available gain (MAG) and |S21| was 22.7 dB and 15.3 dB at 2GHz and 19.8 and 12.7 dB at 4 GHz. The microwave measurements indicate the lack of significant parasitic elements and confirm the high quality of fabricated HEMTs. In overall the GaN HEMT transistor manufactured on a truly bulk monocrystalline GaN wafer was shown to have similar parameters as the ones manufactured on a SiC wafer available from a leading commercial manufacturer. In particular we have obtained similar current densities and thermal resistivity. The frequency characteristics were limited mostly by a relatively long gate resulting from application of the photolithography. That limitation is expected to vanish in the planned next round of experiments were electron-lithography is to be applied. The research was supported by the National Centre for Research and Development PolHEMT Project, Contract Number PBS1/A3/9/2012.

The paper presents investigations concerning the design and realization of photonic structures with grating couplers. The first part of the paper deals with basic theoretical information on photonic structures with grating couplers and their application in optoelectronics. The further part presents the results of numerical investigations on photonic structures with grating couplers and shows the influence of geometrical parameters on the effectiveness of the input and output of optic power into and out of this photonic structure. The paper also provides the results of experimental investigations on a wideband gap semiconductor, viz. zinc oxide ZnO, as well as its application in planar waveguide structures and photonic structures with grating couplers.

The results of modelling of the influence of photonic crystal on the performance of VCSEL-type semiconductor laser structure are shown and indicate that the use of those structures would significantly improve the working parameters of the devices. The method of fabrication of photonic crystals in the Bragg mirrors of GaAs/AlGaAs-based VCSELs is presented.

Zinc oxide layers deposited on quartz substrates by means of RF reactive magnetron sputtering with subsequent RTP annealing in a nitrogen flow at 400°C and in an oxygen flow at 500°C have been investigated in applications to waveguide structures. The ZnO films reveal a highly c-oriented columnar structure with a surface roughness of 4.3 nm. Annealing causes a significant increase of the lattice constant to the value of 5.210±0.001 Å suggesting the relaxation of the stress in the film. The annealing process causes a significant improvement of propagation properties of the fabricated waveguide structures in comparison to structures using as-deposited ZnO films. The minimal attenuation coefficient of the 630 nm thick films was found to be 2.8 and 3.0 dB/cm for TE0 and TM0 modes respectively.

We discuss the phenomenon of the moire fringe phase multiplication encountered under incoherent superimposition of quasiperiodic structures. Its application to increase the in-plane displacement sensitivity of the AFM nano-moire technique is demonstrated. The principle is based on the spatial beating effect between the Mth harmonic of the nonsinusoidal profile quasiperiodic specimen and the first harmonic of the reference grating. Implementations using digitally generated reference grating and resampled AFM image are described. Numerical simulations and experiments with intermediate polymer stamps used in nanoimprint lithography illustrate and corroborate the proposal.

This study focuses on the relationships between the process parameters during magnetron sputter deposition of MnO2 and the resulting film properties. Three MnO2 phases were identified - γ, β and λ and the dependence of MnO2 phase presence on the oxygen content in the sputtering atmosphere was found. Selected MnO2 phases were subsequently applied as ultrathin coatings on top of nanostructured ZnO electrodes for transparent supercapacitors with LiCl-based gel electrolyte. The films containing λ-MnO2 exhibited both the highest optical transparency of 62% at 550 nm as well as the highest specific capacitance in the supercapacitor structure, equal to 73.1 μF/cm2. Initially lower, the capacitance was elevated by charge-discharge conditioning.

The paper presents investigations concerning the numerical design and research of photonic structures with grating couplers based on wide band-gap oxide semiconductor – ZnO for input/output system and gas sensors structures applications.

In the paper, the results of technological investigations on planar optical waveguides based on high band gap oxide semiconductors were presented. Investigations concerned the technologies of depositing very thin layers of: zinc oxide ZnO, titanium dioxide TiO2 and tin dioxide SnO2 on substrates of quartz glass plates. There were investigated both morphological structures of the produced layers and their optical properties. The paper also presents investigations on the technology of input-output light systems in the Bragg grating structures.