The role of extended and point defects, and key impurities such as C, O, and H, on the electrical and optical properties of GaN is reviewed. Recent progress in the development of high reliability contacts, thermal processing, dry and wet etching techniques, implantation doping and isolation, and gate insulator technology is detailed. Finally, the performance of GaN-based electronic and photonic devices such as field effect transistors, UV detectors, laser diodes, and light-emitting diodes is covered, along with the influence of process-induced or grown-in defects and impurities on the device physics.

Gan : processing, defects, and devices

Cubic boron nitride (cBN) has a number of highly desirable mechanical, thermal, electrical, and optical properties. Because of this, there has been an extensive worldwide effort to synthesize thin films of cBN. Film synthesis is difficult in that without significant levels of ion bombardment during growth, only sp2-bonded BN forms, not sp3-bonded cBN. Recently there has been considerable progress in improving the deposition techniques and cBN film quality. In addition, progress has been made in understanding how energetic deposition conditions can lead to cBN formation. However, unanswered questions remain and process improvements are still needed. In this paper we critically and comprehensively review recent developments in cBN film synthesis and characterization. First, the structures and stability of the BN phases and characterization techniques are described. Next, the key experimental parameters controlling cBN film formation and synthesis techniques are discussed. Following a review of microstructure, the proposed mechanisms of cBN formation and the observed mechanical and electrical properties of cBN films are analyzed. We conclude by highlighting the current impediments to the practical realization of cBN-film technology.

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Review of advances in cubic boron nitride film synthesis

Studies involving transition-metal dichalcogenides (TMDs) have been around for many decades and in recent years, many were focused on using TMDs to synthesize inorganic analogues of carbon nanotubes, fullerene, as well as graphene and its derivatives with the ultimate aim of employing these materials into consumer products. In view of this rising trend, we investigated the cytotoxicity of three common exfoliated TMDs (exTMDs), namely MoS2, WS2, and WSe2, and compared their toxicological effects with graphene oxides and halogenated graphenes to find out whether these inorganic analogues of graphenes and derivatives would show improved biocompatibility. Based on the cell viability assessments using methylthiazolyldiphenyl-tetrazolium bromide (MTT) and water-soluble tetrazolium salt (WST-8) assays on human lung carcinoma epithelial cells (A549) following a 24 h exposure to varying concentrations of the three exTMDs, it was concluded that MoS2 and WS2 nanosheets induced very low cytotoxicity to A549 cells, even at high concentrations. On the other hand, WSe2 exhibited dose-dependent toxicological effects on A549 cells, reducing cell viability to 31.8 % at the maximum concentration of 400 μg mL−1; the higher cytotoxicity displayed by WSe2 might be linked to the identity of the chalcogen. In comparison with graphene oxides and halogenated graphenes, MoS2 and WS2 were much less hazardous, whereas WSe2 showed similar degree of cytotoxicity. Future in-depth studies should be built upon this first work on the in vitro cytotoxicity of MoS2 and WS2 to ensure that they do not pose acute toxicity. Lastly, nanomaterial-induced interference control experiments revealed that exTMDs were capable of reacting with MTT assay viability markers in the absence of cells, but not with WST-8 assay. This suggests that the MTT assay is not suitable for measuring the cytotoxicity of exTMDs because inflated results will be obtained, giving false impressions that the materials are less toxic.

Cytotoxicity of exfoliated transition-metal dichalcogenides (MoS2 , WS2 , and WSe2 ) is lower than that of graphene and its analogues.

Ion beam assisted evaporation was used to deposit cubic and hexagonal boron nitride thin films. Boron was evaporated and bombardment was by argon and nitrogen ions. The effect of preparation conditions on the resulting phase was studied, and the relationship between the phase and the energy and momentum transferred into the film through ion bombardment was examined. It is shown that for a given temperature, the controlling factor in the resulting thin film phase is the momentum transferred into the film per depositing boron atom. At 300–400 °C a sharp threshold value of momentum‐per‐atom exists below which films are hexagonal and above which they are cubic. For 400 °C this threshold occurred at 200 (eV×amu)1/2 which is equal to 3.3×10−21 m kg s−1. Depositions performed using krypton and xenon instead of argon as the second bombarding gas confirmed this momentum‐per‐atom value. A second threshold was also observed, which was bombarding species dependent, above which either complete resputtering of the deposited material or reversion to the hexagonal phase occurred. Cubic boron nitride deposition was seen to occur in a window of momentum‐per‐atom values between these two thresholds. Using this information it was possible to grow cubic boron nitride using only nitrogen bombardment, although the window of momentum‐per‐atom values for nitrogen is very narrow. The effect of substrate temperature was studied, and it was found to be difficult to grow predominantly cubic phase films below 300–400 °C. The relationship between intrinsic stress and phase of the films is also discussed. A diagram is presented showing film phase as a function of bombardment, substrate temperature, and system chemistry. The parameter of momentum‐per‐atom is shown to combine into a single value the variables of ion beam assisted deposition: deposition rate, ion energy, ion flux, and ion species. It is suggested that, in general, for properties affected by ion bombardment the momentum‐per‐atom transferred into the film is the controlling factor. The results are shown to support momentum transfer as the dominant process in cubic boron nitride thin film formation.

Phase control of cubic boron nitride thin films

Group 6 transition metal dichalcogenides (G6-TMDs), most notably MoS2, MoSe2, MoTe2, WS2 and WSe2, constitute an important class of materials with a layered crystal structure. Various types of G6-TMD nanomaterials, such as nanosheets, nanotubes and quantum dot nano-objects and flower-like nanostructures, have been synthesized. High thermodynamic stability under ambient conditions, even in atomically thin form, made nanosheets of these inorganic semiconductors a valuable asset in the existing library of two-dimensional (2D) materials, along with the well-known semimetallic graphene and insulating hexagonal boron nitride. G6-TMDs generally possess an appropriate bandgap (1–2 eV) which is tunable by size and dimensionality and changes from indirect to direct in monolayer nanosheets, intriguing for (opto)electronic, sensing, and solar energy harvesting applications. Moreover, rich intercalation chemistry and abundance of catalytically active edge sites make them promising for fabrication of novel energy storage devices and advanced catalysts. In this review, we provide an overview on all aspects of the basic science, physicochemical properties and characterization techniques as well as all existing production methods and applications of G6-TMD nanomaterials in a comprehensive yet concise treatment. Particular emphasis is placed on establishing a linkage between the features of production methods and the specific needs of rapidly growing applications of G6-TMDs to develop a production-application selection guide. Based on this selection guide, a framework is suggested for future research on how to bridge existing knowledge gaps and improve current production methods towards technological application of G6-TMD nanomaterials.

Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives

Sputter-deposited MoS2 films have been often used as dry lubricant in various industrial fields, such as space applications. Most investigations have been focused on the reduction of the friction coefficient. However, mechanical components require not only stable lubricating films for friction reduction, but also excellent wear resistance for prolonged endurance life. Therefore, in this work, anti-wear properties of WS2/MoS2 nanometer-scale multilayers have been investigated, because a nanometer-scale multilayer film, so-called superlattice structured film, is expected to show good mechanical properties, such as high hardness and high stiffness, resulting in a low friction and long endurance lives. WS2/MoS2 nanometer-scale multilayer films have been prepared by multi-target RF sputtering. Single-layer MoS2 and WS2 films were also prepared for comparison. Ball-on-disk wear tests and nono-indentation tests were performed on the films grown onto Si substrates. WS2/MoS2 multilayer film showed a significantly improved tribological performance in air compared to the single-layer MoS2 or WS2 film, with an improvement in wear life of approximately seven times, and with a low friction coefficient of approximately 0.05.

Tribological characteristics of WS2/MoS2 solid lubricating multilayer films

Boron nitride films prepared by reactive ion plating from a boron evaporation source were characterized structurally using three independent electron optical techniques: energy filtered electron diffraction with radial distribution function analysis; electron energy‐loss spectroscopy of the near‐edge structure of the boron K edge; and spectroscopy of the plasmon region of the energy‐loss spectrum. Both specimens had a graded BNx layer between the BN layer and the silicon substrate and in addition one specimen had a titanium bonding layer underneath the BNx layer. The presence of c‐BN in both specimens was confirmed by all techniques. The specimen with the titanium bonding layer was examined in cross section and showed essentially pure c‐BN on the surface. A model for the formation of c‐BN assisted by the compressive stress generated during deposition is proposed.

Electron optical characterization of cubic boron nitride thin films prepared by reactive ion plating

A method of synthesizing cubic boron nitride (c-BN) films by the activated reactive evaporation process was investigated. A special feature of the gas activation process is that a hot cathode plasma discharge in a parallel magnetic field is used. A conventional type of ion plating apparatus, with an electron beam gun for the evaporation of boron, was used to synthesize c-BN films on a single-crystal silicon plate. A stoichiometric boron nitride film was obtained by reacting boron vapour with high density nitrogen ions, prepared using a hot cathode plasma discharge in a parallel magnetic field around the substrate. Argon gas was mixed with the reactant nitrogen gas to enhance BN formation. An r.f. bias potential was applied to the substrate in order to obtain the cubic BN phase. The optimum value of the total gas pressure in the evaporation chamber was in the range 2.6 × 10 -2 -1.1 × 10 -1 Pa. The structures of the films obtained were characterized using IR absorption spectroscopy and X-ray diffraction analysis.

The synthesis of cubic BN films using a hot cathode plasma discharge in a parallel magnetic field

We fabricated WS 2 /MoS 2 nanometer-scale multilayer films (superlattice-structured multilayer films) with dry lubricating characteristics using multitarget RF sputtering. Superlattice structures are obtained by layering two or more materials in a regular periodic structure at a thickness of several atoms or several tens of atoms. Superlattice structured film is expected to exhibit good mechanical properties, such as high hardness and high stiffness, resulting in low friction and long endurance life in comparison to those of the individual single-layer materials. In the previous study, this WS 2 /MoS 2 multilayer film showed significantly improved tribological performance in ambient air in comparison to the single-layer MoS 2 or WS 2 films. To clarify the tribological properties of this multilayer further, sliding friction experiments have been conducted with WS 2 /MoS 2 films formed on silicon substrates in contact with 440 C stainless steel balls under vacuum at room temperature and at a high temperature of 523 K. The results obtained showed clearly that the friction characteristics and friction endurance of these multilayer films were more excellent under a vacuum of 10 −5 Pa than under atmospheric conditions.

S. Watanabe

Papers

Tribological characteristics of WS2/MoS2 solid lubricating multilayer films

Electron optical characterization of cubic boron nitride thin films prepared by reactive ion plating

The synthesis of cubic BN films using a hot cathode plasma discharge in a parallel magnetic field

Friction properties of WS2/MoS2 multilayer films under vacuum environment

Tribological study of cubic boron nitride film