Thin Nitride Research Articles

Optimization of technology of diode laser mirror processing to maximize the threshold of catastrophic optical degradation

In this paper, optimization works on the technology of passivation and optical coatings of laser diode facets are described. The main goal is to increase the optical power at which the catastrophic optical mirror damage occurs. The coatings and passivation processes have been done in an ion source-aided electron-beam evaporator. The essence of passivation was to remove native oxides and produce a native thin nitride layer with simultaneous saturation of the dangling atomic bonds. The procedure has been realized with the help of nitrogen or forming gas (N2 + H2) beam. As a result, we present sets of technological parameters allowing to increase the catastrophic optical mirror damage threshold of diode lasers.

Synergy of dopants and defects in ultrathin 2D carbon nitride sheets to significantly boost the photocatalytic hydrogen evolution

Atomically thin 2D carbon nitride sheets (CNs) can improve the photocatalytic performance due to their high specific surface area, exposed active sites, and nano-scaled diffusion path of the charge transferred to surface. However, in practical application, 2D CNs photocatalysts always suffer the low solar absorption. Here, we present a novel approach to achieve the ultrathin 2D CNs photocatalysts with wide spectrum absorption in visible-light range. The new method, i.e. thermal annealing of atomically thin CNs in Se vapor, can dramatically extend the absorption edge of the ultrathin CNs from 418 to 574 nm. Using structural characterization and spin-polarized density functional theory, we identify the loss of the nitrogen atoms at the edges and in situ Se doping into the network of ultrathin CNs, which could change the electronic band structure of the 2D CNs. As a consequence, the ultrathin 2D CNs photocatalyst via Se vapor annealing (Se-CNs) modified by Pt cocatalyst shows a superior photocatalytic hydrogen evolution activity under visible light irradiation. More significantly, the ultrathin Se-CNs photocatalyst also exhibits an impressive photocatalytic hydrogen evolution activity under visible-light irradiation in the absence of Pt cocatalyst. We believe that our study provide a facile and environmentally friendly strategy to realize the utilization of 2D CNs photocatalyst under wide spectrum range of solar radiation to harvest solar energy efficiently, and it can be further applied in various advanced fields, including CO2 photoreduction, organic photosynthesis, and pollutant controls.

Ion-Beam Deposition of Thin AlN Films on Al2O3 Substrate

Thin aluminum nitride (AlN) films on sapphire (Al2O3) substrates were grown by means of ion-beam deposition (IBD) and studied by methods of scanning electron microscopy, Raman scattering, and optical transmission spectroscopy. Results revealed the influence of IBD process parameters (gas mixture composition, ion beam energy, and substrate temperature) on the morphology, structure, and optical properties of obtained thin AlN films on sapphire.

Cathodoluminescence enhancement and quenching in type-I van der Waals heterostructures: Cleanliness of the interfaces and defect creation

Van der Waals heterostructures give access to a wide variety of new phenomena that emerge thanks to the combination of properties brought in by the constituent layered materials. We show here that owing to an enhanced interaction cross section with electrons in a type I van der Waals heterostructure, made of single layer molybdenum disulphide and thin boron nitride films, electrons and holes created in boron nitride can be transferred to the dichalcogenide where they form electron-hole pairs yielding luminescence. This cathodoluminescence can be mapped with a spatial resolution far exceeding what can be achieved in a typical photoluminescence experiment, and is highly valuable to understand the optoelectronic properties at the nanometer scale. We find that in heterostructures prepared following the mainstream dry transfer technique, cathodoluminescence is locally extinguished, and we show that this extinction is associated with the formation of defects, that are detected in Raman spectroscopy and photoluminescence. We establish that to avoid defect formation induced by low-energy electron beams and to ensure efficient transfer of electrons and holes at the interface between the layers, flat and uniform interlayer interfaces are needed, that are free of trapped species, airborne ones or contaminants associated with sample preparation. We show that heterostructure fabrication using a pick-up technique leads to superior, intimate interlayer contacts associated with significantly more homogeneous cathodoluminescence.

High frequency guided mode resonances in mass-loaded, thin film gallium nitride surface acoustic wave devices

We demonstrate high-frequency (>3 GHz), high quality factor radio frequency (RF) resonators in unreleased thin film gallium nitride (GaN) on sapphire and silicon carbide substrates by exploiting acoustic guided mode (Lamb wave) resonances. The associated energy trapping, due to mass loading from gold electrodes, allows us to efficiently excite these resonances from a 50 Ω input. The higher phase velocity, combined with lower electrode damping, enables high quality factors with moderate electrode pitch and provides a viable route towards high-frequency piezoelectric devices. The GaN platform, with its ability to guide and localize high-frequency sound on the surface of a chip with access to high-performance active devices, will serve as a key building block for monolithically integrated RF front-ends.

Metallurgical investigation of failure analysis in industrial machine components

Bearings and chains are important industrial machine components used in wide range of heavy cyclical load applications. Main aim of this research work is to conduct a metallurgical investigation of failure analysis in failed bearing and chain parts. The failure analysis is carried out by chemical analysis, optical metallography, hardness analysis and scanning electron microscopy (SEM) analysis. In failed bearing inner race ways ridged marks were perceived due to electrical pitting and outer race ball groove worn-out on one side due to axial thrust load. Bearing balls have surface spalling due to contact fatigue. In chain failed bush multiple crack fronts are observed in longitudinal and circumferential. Crack geometry and fracture surface indicates failure through impact fatigue mode. The fracture surface indicated quasi cleavage fracture mode near the crack origins. The results in both investigations suggest that the major common causes of failures are due to over load, fatigue, wear and unsuitable material selection. MoS2coating, thin dense chromium coating and thin hard titanium nitride coating in all bearing components has been suggested to further improve the wear resistance and fatigue strength. Material SAE 8620 alloy steel for chain bush has been suggested to further improve life to withstand in the impact load conditions.

The influence of columnar microstructure on diffusion reaction between Ti thin films and aluminum nitride ceramics

In this paper, Ti films with certain thickness were deposited on a polished AlN ceramic substrates by direct current magnetron sputtering at room temperature (RT) and 400 °C, respectively. After deposition, the samples were annealed at 600 °C for 1 h in the mixture gases of argon and hydrogen atmosphere. The microstructure of the Ti films and interface reaction products between Ti/AlN were investigated using X-ray diffraction, and transmission electron microscopy. The effects of deposition temperature on the microstructure and grain orientation of Ti films were studied. It is found that the decomposition of AlN would mainly take place at the AlN/Ti interface during the heat-treated process. After annealing, Al and N react with Ti to produce AlTi3N in the film deposited at 400 °C, while TiN0.3 be formed if deposited at RT. The subsequent rolling-contact-fatigue test proved that after annealing at 600 °C for 1 h, the adhesion strength of Ti/AlN interface deposited at 400 °C is superior to that deposited at RT.

Atomically Thin Boron Nitride as an Ideal Spacer for Metal-Enhanced Fluorescence.

Metal-enhanced fluorescence (MEF) considerably enhances the luminescence for various applications, but its performance largely depends on the dielectric spacer between the fluorophore and plasmonic system. It is still challenging to produce a defect-free spacer having an optimized thickness with a sub-nanometer accuracy that enables reusability without affecting the enhancement. In this study, we demonstrate the use of atomically thin hexagonal boron nitride (BN) as an ideal MEF spacer owing to its multifold advantages over the traditional dielectric thin films. With rhodamine 6G as a representative fluorophore, it largely improves the enhancement factor (up to ∼95 ± 5), sensitivity (10-8 M), reproducibility, and reusability (∼90% of the plasmonic activity is retained after 30 cycles of heating at 350 °C in air) of MEF. This can be attributed to its two-dimensional structure, thickness control at the atomic level, defect-free quality, high affinities to aromatic fluorophores, good thermal stability, and excellent impermeability. The atomically thin BN spacers could increase the use of MEF in different fields and industries.

Fabrication and Characterization of Superconducting Films for Superconductor-Topological Insulator Hybrid Devices

Proximation of s-wave superconductivity to topological materials is an effective way to realize Majorana modes in condensed matter physics. In order to fabricate a successful device, the superconducting material plays a crucial role since selection of material, growth method, applicable critical temperature, adequate critical magnetic field, transparent electrical contact, and thus proper work function, needs to be deliberately considered. Here, we report the fabrications and transport measurements for two types of superconducting thin films, tantalum (Ta) and niobium nitride (NbN), at various growth conditions. Both Ta and NbN films show applicable potentials as the superconducting electrodes. We also demonstrate the proximity effect induced supercurrent state in an InAs/GaSb bilayer device with superconducting Ta electrodes.

Structure and properties of Ta/Al/Ta and Ti/Al/Ti/Au multilayer metal stacks formed as ohmic contacts on n-GaN

Formation of ohmic contacts to GaN is of high practical importance for device fabrication. Due to the wide band gap, formation of multilayer metal structures is required to make electrical connections with low contact resistance. The paper presents a study on structure, composition, adhesion and electrical properties of Ti/Al/Ti/Au and Ta/Al/Ta metal stacks fabricated by e-beam evaporation and thermal annealing in order to provide ohmic contacts to n-type GaN films grown on Si. For the Ti-based case, an interdiffusion of Au and Ga into the stack is found, which is probably caused by a granular structure of the top Ti layer making no proper barrier. Ti of the bottom layer is observed to diffuse into GaN, forming a thin layer of titanium nitride with a low Schottky barrier at GaN interface allowing ohmic contact as shown by electrical measurements. The Ta-based stacks have the expected layered structure with minor interdiffusion of Al and Ta at the interfaces of these two metals. No direct microscopic evidences for Ta diffusion into GaN is observed. However, the formation of a thin tantalum nitride layer at the GaN interface can be deduced from the current–voltage measurements showing ohmic electrical contacts with low contact resistivity of 1.2 × 10−3 Ω cm2. Four-point bending tests on the both types of samples show that cracks always develop at the interface between Si and buffer layers of GaN. No exfoliation of the metallization layers is observed allowing us to conclude about good adhesion of the metal stacks. Thus, the fabricated Ti- and Ta-based multilayer structures show good electrical performance and reliable adhesion making them promising for the formation of ohmic contacts to n-type GaN.

A High Efficiency Osmotic Energy Harvester with Vertically Aligned Boron-Nitride-Nanopore Membrane

Application of the abundant salinity-gradients in the environment, motivated by the Gibbs free energy of mixing fresh and salt water, has been under intensive investigation for clean-energy harvesting. Globally, there is a potential of at least 2.6 TW of power that could be generated at coastal estuaries from the ion-concentration gradient. Recent investigation on an energy conversion efficiency of single boron nitride nanotube (BNNT) has revealed the huge potential of an efficient exploitation of electrokinetic phenomena in nanofluidic systems for energy harvesting. Here, for the first time, we demonstrate rationally designed nanostructured vertically aligned boron-nitride-nanopore membranes (VA-BNNP) which can efficiently harness osmotic power from salinity gradient. A thin hexagonal boron nitride (hBN) layer was uniformly deposited within the pores of anodized alumina substrates by low-pressure chemical vapor deposition which produced first-ever macroscopic VA-BNNP membrane with high nanopore density, up to ~108 pores/cm2. Cross-sectional scanning-electron-microscope (SEM) images show the hBN layer (~35 nm) was uniformly deposited along the pores without excessive hBN on the top surface, ensuring that most pores remained open. In addition, the results of scanning confocal Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) showed the high quality of the hBN layers in the AAO pores. We investigated the power generation of the macroscopic VA-BNNP membranes at different pH and salinity concentrations. The power generation per unit pore area increased as the salt concentration and pH increased. The highest power density of the membrane was up to ~100 W/m2 which is two orders of magnitude higher than that of other macro-scale, salinity gradient driven, power generation system reported so far. In addition, we also elucidate the fundamental ion transport mechanism in BN nanopore using the molecular dynamic simulations to support to the experimental power density values. These findings indicate the great potential of large-area VA-BNNP membranes as next-generation nanostructured membranes for renewable energy harvesting.

Increasing Solar Absorption of Atomically Thin 2D Carbon Nitride Sheets for Enhanced Visible‐Light Photocatalysis

Atomically thin 2D carbon nitride sheets (CNS) are promising materials for photocatalytic applications due to their large surface area and very short charge-carrier diffusion distance from the bulk to the surface. However, compared to their bulk counterpart, CNS' applications always suffer from an enlarged bandgap and thus narrowed solar absorption range. Here, an approach to significantly increase solar absorption of the atomically thin CNS via fluorination followed by thermal defluorination is reported. This approach can greatly increase the visible-light absorption of CNS by extending the absorption edge up to 578 nm. The modulated CNS loaded with Pt cocatalyst as a photocatalyst shows a superior photocatalytic hydrogen production activity under visible-light irradiation to Pt-CNS. Combining experimental characterization with theoretical calculations shows that this approach can introduce cyano groups into the framework of CNS as well as the accompanied nitrogen vacancies at the edges, which leads to both narrowing the bandgap and changing the charge distribution. This study will provide an effective strategy to increase solar absorption of carbon-nitride-based photocatalysts for solar energy conversion applications.

Study of GaN layer crystallization on GaAs(100) using electron cyclotron resonance or glow discharge N2 plasma sources for the nitriding process

Two kinds of N2 plasma source, ECR (electron cyclotron resonance) and GDS (glow discharge source) generating mostly N-radical atoms and N-cationic species respectively, were used to grow a thin nitride layer on a GaAs(100) substrate. It was found that this nitridation followed by annealing at 620 °C permits the crystallization of the nitride layer. Pyramidal Zinc Blende GaN nanostructures (zb-GaN) with four facets were obtained using GDS plasma. Surprisingly, a planar and pure wurtzite structure (w-GaN) was obtained using the ECR source. This w-GaN structure shows low photoluminescence intensity and a biaxial tensile strain due to lattice mismatch. Accordingly, the operator can select which phase is formed, simply by switching plasma source. The valence band discontinuity ΔEv has been determined to be 1.74 eV for the zb-GaN/GaAs and w-GaN/GaAs junctions by X-ray photoelectron spectroscopy. As a consequence the conduction bands of the GaAs substrate and the elaborated GaN thin layer are aligned for a zb-GaN/GaAs junction giving efficient electron transport at the zb-GaN/GaAs interface. For w-GaN/GaAs junction, the conduction band discontinuity ΔEc is 0.23 eV inducing an electron confinement in the GaAs(100) which can be an effective way to improve the electronic or optical properties of GaAs devices.

Supercontinuum second harmonic generation spectroscopy of atomically thin semiconductors.

Two-dimensional semiconductors have recently emerged as promising materials for novel optoelectronic devices. In particular, they exhibit favorable nonlinear optical properties. Potential applications include broadband and ultrafast light sources, optical signal processing, and generation of nonclassical light states. The prototypical nonlinear process second harmonic generation (SHG) is a powerful tool to gain insight into nanoscale materials because of its dependence on crystal symmetry. Material resonances also play an important role in the nonlinear response. Notably, excitonic resonances critically determine the magnitude and spectral dependence of the nonlinear susceptibility. We perform ultrabroadband SHG spectroscopy of atomically thin semiconductors by using few-cycle femtosecond infrared laser pulses. The spectrum of the second harmonic depends on the investigated material, MoS2 or WS2, and also on the spectral and temporal shape of the fundamental laser pulses used for excitation. Here, we present a method to remove the influence of the laser by normalization with the flat SHG response of thin hexagonal boron nitride crystals. Moreover, we exploit the distinct angle dependence of the second harmonic signal to suppress two-photon photoluminescence from the semiconductor monolayers. Our experimental technique provides the calibrated frequency-dependent nonlinear susceptibility χ(2)(ω) of atomically thin materials. It allows for the identification of the prominent A and B exciton resonances, as well as excited exciton states.

Avalanche characteristics in thin GaN avalanche photodiodes

A Monte Carlo model using random ionization path lengths describing the carriers quantum transport in thin gallium nitride (GaN) avalanche photodiodes (APDs) for ultraviolet detection in industry is developed. This work simulated avalanche characteristics such as impact ionization coefficients, mean multiplication gain and excess noise factor of GaN APDs at 0.05 μm, 0.1 μm, 0.2 μm and 0.3 μm multiplication widths in an electric field. The model simulates higher electron impact ionization coefficients than that of the hole for an electric field greater than 4.04 MV cm−1. Mean multiplication gain and excess noise factor are simulated based on the electric field dependent impact ionization coefficients. Our results show that electron-initiated multiplication gives higher multiplication gain and lower excess noise than hole-initiated multiplication for a multiplication width below 0.3 μm. Devices with dead space in general give a lower excess noise.

High-Q Localized Phonon Polaritons in Thin hBN Discs with Omnidirectional Strong Light Absorptions

Based on numerical calculations, we have found that thin hexagonal boron nitride nanodiscs support high-quality localized phonon polariton modes in the upper Reststrahlen band. Near the resonances, single disc can be phenomenologically considered as an electric dipole. By arranging discs into a square lattice, strong light absorptions can be achieved. Modeling such periodic systems as dipole lattices, simple reflecting substrates can be designed which lead to omnidirectional strong, even nearly perfect, light absorptions. Under the condition of nearly total absorption, the electric fields at the hexagonal boron nitride surfaces are largely enhanced due to low-loss localized phonon polaritons. Our investigations might be useful for phonon polariton–based nanophotonic devices, such as sensors.

Microstructure and mechanical properties evaluation of cathodic arc deposited CrCN/ZrCN multilayer coatings

For the development of advanced nanoscale multilayer protective coatings, proper design of microstructure and chemical composition of carbon containing sequential transition metal nitride nanolayers is an important issue. In this work, five different nanostructured CrCN/ZrCN multilayer coatings were deposited periodically by cathodic arc evaporation. The bilayer period of the CrCN/ZrCN multilayer coatings was kept at 20 nm. The C2H2 gas flow ratio was adjusted from 6.3 to 20.0% for achieving CrCN/ZrCN multilayer coatings with 2.3–4.2 at.% carbon content. Nanolaminated CrCN and ZrCN nitride layers and thin amorphous carbon nitride mixed nanolayers ∼5 nm thick were obtained as the carbon content reached 4.2 at.%. It was found that the hardness and adhesion quality were strongly improved by the carbon addition to the CrCN/ZrCN multilayer coatings. An increase of 2.6–4.6 GPa in hardness was found for the CrCN/ZrCN multilayer coatings due to the balance of solution hardening effect of carbon atoms and the softening by the amorphous mixed nanolayer. An optimal combination of high hardness, 28.9 GPa, and good adhesion, 41 N of upper critical load were achieved when the carbon content was 4.2 at.% for the CrCN/ZrCN multilayer coatings.

Isolating hydrogen in hexagonal boron nitride bubbles by a plasma treatment

Atomically thin hexagonal boron nitride (h-BN) is often regarded as an elastic film that is impermeable to gases. The high stabilities in thermal and chemical properties allow h-BN to serve as a gas barrier under extreme conditions. Here, we demonstrate the isolation of hydrogen in bubbles of h-BN via plasma treatment. Detailed characterizations reveal that the substrates do not show chemical change after treatment. The bubbles are found to withstand thermal treatment in air, even at 800 °C. Scanning transmission electron microscopy investigation shows that the h-BN multilayer has a unique aligned porous stacking nature, which is essential for the character of being transparent to atomic hydrogen but impermeable to hydrogen molecules. In addition, we successfully demonstrated the extraction of hydrogen gases from gaseous compounds or mixtures containing hydrogen element. The successful production of hydrogen bubbles on h-BN flakes has potential for further application in nano/micro-electromechanical systems and hydrogen storage.

Minkowski functional characterization and fractal analysis of surfaces of titanium nitride films

The aim of this study is to gain a deeper understanding of the micromorphology characteristics of thin titanium nitride (TiN) films sputtered on glass substrates by using ion beam sputtering (IBS). For that purpose, TiN samples were deposited onto glass substrates from gas mixtures with different contents of molecular nitrogen and argon atoms. Atomic force microscopy (AFM) was used to characterize the surface microtexture of obtained thin films at high magnification. The detailed analysis of AFM images using Minkowski functionals and fractal analysis reveals a significant effect of the preparation conditions on the surface features with non-monotonic dependences. Presented results suggest that non-classical spatial characteristics of the variability of the surface topography, including fractal dimension, corner frequency, roughness, and feature shape and size, can be tuned having control over the relative flow rates of the gas mixture during film deposition.

Enhancing thermal properties of few-layer boron nitride by high-k Al2O3 capping layer

Atomically thin boron nitride (BN) film has attracted increasing attention among two-dimensional materials for the potential application in electronics devices. The thermal properties of few-layer BN nanosheets (∼2.27 nm) on SiO2/Si substrates have been investigated without and with high-k Al2O3 capping layer, using the temperature-dependent and polarized-laser power-dependent Raman spectroscopy measurements. Due to the effect of Al2O3 capping layer, Raman spectrum illustrates the blue-shift of frequency from 1364.9 cm−1 to 1367.9 cm−1, and the first order temperature coefficient for E2g mode of BN layers increases from −0.02243 cm−1/K to −0.06544 cm−1/K. Furthermore, the room-temperature thermal conductivity of BN with Al2O3 capping layer is found to be 332.57 W/mK, which is much larger than that of BN without Al2O3 capping layer (∼94.51 W/mK). The enhancement is attributed to the interface charges and compressive stress at the interface between BN and Al2O3 capping layer, which has been clarified by the first principle calculations. This work is aimed at expanding the applications of BN materials and improving the performances of BN-based devices in thermal properties.