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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Humidity effects in Fe{sub 16}N{sub 2} fine powder preparation by low-temperature nitridation

Low-temperature nitridation has been reported to produce ferromagnetic {alpha}''-Fe{sub 16}N{sub 2} by ammonia nitridation of {alpha}-Fe fine powder, which was obtained from the reduction of vapor-grown {gamma}-Fe{sub 2}O{sub 3}. The effects of humidity during this preparation were investigated in the present study. {alpha}''-Fe{sub 16}N{sub 2} was inconsistently obtained, and at low yield, from Fe{sub 3}O{sub 4} fine powder (MT-40) prepared from aqueous solution. Reducing the adsorbed water content in the iron oxide starting powder resulted in improved reproducibility of the {alpha}''-Fe{sub 16}N{sub 2} yield of the nitridation. The use of a smaller-diameter reaction tube, less than 25 mm in diameter, enabled more reproducible preparation from vapor-grown {gamma}-Fe{sub 2}O{sub 3} powder (CI-30). The reaction yield was further improved by using high-quality ammonia with a water content of {<=}0.05 ppm. Minimizing the humidity made it possible to obtain a fine powder with a high {alpha}''-Fe{sub 16}N{sub 2} content. Enhancement of the magnetization to 210 emu g{sup -1} at room temperature was observed from a nitrided mixture of {alpha}''-Fe{sub 16}N{sub 2} with residual {alpha}-Fe, compared to 199 emu g{sup -1} for an {alpha}-Fe fine powder reduced from {gamma}-Fe{sub 2}O{sub 3}. However, excess nitrogen and residual oxygen in the nitrided products reduced the magnetization below the value of {alpha}-Fe powder after nitridation. The magnetization was enhanced in nitrided products with a nitrogen content slightly below the stoichiometric amount for {alpha}''-Fe{sub 16}N{sub 2}. - Graphical abstract: Humidity should be as low as possible to obtain a fine powder with a high {alpha}{sup -}Fe{sub 16}N{sub 2} content. An enhancement of magnetization to 210 emu g{sup -1} at room temperature was observed from a nitrided mixture of {alpha}{sup -}Fe{sub 16}N{sub 2} with residual {alpha}-Fe, compared to 199 emu g{sup -1} for an {alpha}-Fe fine powder reduced from {gamma}-Fe{sub 2}O{sub 3}.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Isothermal nitridation kinetics of TiSi{sub 2} powders

The aim of the present work is to determine the kinetics of reaction between TiSi{sub 2} powder and gaseous nitrogen. Isothermal nitridation of TiSi{sub 2} powders with fine (1.4 µm) and medium (4.5 µm) particle size has been studied in pure nitrogen atmosphere from 1000 to 1200 °C for duration up to 50 h. The isothermal nitridation kinetics of TiSi{sub 2} powders were investigated by thermogravimetry. The nitridation rate strongly depends on the particle size and temperature. Smaller size particle exhibits higher nitridation rate due to its larger surface area. The conversion process is complex with nucleation and growth of TiN at the surface of the grain and Si{sub 3}N{sub 4} inside the grain promoted by the Kirkendall effect with an influence of the volume increase. - Graphical abstract: Backscattered electrons image of a transverse TiSi{sub 2} grain nitrurated at 1100 °C for 50 h. - Highlights: • Influence of grain size on TiSi{sub 2} powder nitridation. • Influence of temperature on TiSi{sub 2} powder nitridation. • Experimental measurements of the nitridation kinetics. • An explanation of the nitridation mechanism.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Advanced surface plasma nitriding for development of corrosion resistant and accident tolerant fuel cladding: Final Report

Although various surface coating techniques have been proposed to increase oxidation and corrosion resistance of fuel cladding materials, the de-bonding of the coating layer with the original cladding matrix under exposure to coolants makes such approaches unsuitable for reactor applications. Furthermore, the feasibility of techniques for large scale processing on cladding tubes remains another technological bottleneck. This project aims to develop a hollow cathode plasma nitriding technique to solve the above issues. Starting with alloys of DOE interest, the team will apply an advanced surface plasma nitriding technique to convert alloy surface layers into nitride layers for better structural integrity and compatibility with both coolants and nuclear fuels. The technique can form uniform nitride layers on both inner and outer surfaces of cylindrical hollow tubes. Both treated and untreated samples will be irradiated by using Fe self ions or dual beams of Fe ions and He ions. Various microstructural characterizations and mechanical property measurements will be performed. For outside surfaces, samples will be tested in high temperature water and liquid molten salt to evaluate their corrosion resistance and compatibility with coolants for LWR and fast reactor applications. For inside surfaces, solid-to-solid diffusion couple experiments will be used to study interface reactions of nitrided samples with uranium, to test whether a nitrided layer can slow down fuel-cladding interactions. The project will impact both the development of advanced methods for manufacturing and the development of advanced reactor in-core structural materials. The proposed technique can form nitride layers on alloys of irregular shapes without an edging effect - an issue encountered in traditional plasma nitriding processes, and is suitable for surface processing of other in-core components.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
NASA NTRS1998

Surface Area, and Oxidation Effects on Nitridation Kinetics of Silicon Powder Compacts

Commercially available silicon powders were wet-attrition-milled from 2 to 48 hr to achieve surface areas (SA's) ranging from 1.3 to 70 sq m/g. The surface area effects on the nitridation kinetics of silicon powder compacts were determined at 1250 or 1350 C for 4 hr. In addition, the influence of nitridation environment, and preoxidation on nitridation kinetics of a silicon powder of high surface area (approximately equals 63 sq m/g) was investigated. As the surface area increased, so did the percentage nitridation after 4 hr in N2 at 1250 or 1350 C. Silicon powders of high surface area (greater than 40 sq m/g) can be nitrided to greater than 70% at 1250 C in 4 hr. The nitridation kinetics of the high-surface-area powder compacts were significantly delayed by preoxidation treatment. Conversely, the nitridation environment had no significant influence on the nitridation kinetics of the same powder. Impurities present in the starting powder, and those accumulated during attrition milling, appeared to react with the silica layer on the surface of silicon particles to form a molten silicate layer, which provided a path for rapid diffusion of nitrogen and enhanced the nitridation kinetics of high surface area silicon powder.

Bhatt, R. T.↗
DOE OSTI2013

Structure and luminescence properties of thermally nitrided Ga{sub 2}O{sub 3} nanowires

Graphical abstract: The as-synthesized Ga{sub 2}O{sub 3} nanowires exhibited a broad emission band at approximately 570 nm in the green region. In contrast, the thermally nitrided Ga{sub 2}O{sub 3} nanowires exhibited a much stronger emission band at approximately 455 nm in the blue region. Display Omitted Highlights: ► The structure and luminescence properties of thermally nitrided Ga{sub 2}O{sub 3} nanowires were examined. ► A uniform GaN shell layer was formed on the surface of the nanowires by thermal nitridation. ► The as-synthesized Ga{sub 2}O{sub 3} nanowires exhibited a broad yellow emission. ► The nitrided Ga{sub 2}O{sub 3} nanowires exhibited a much stronger blue emission band due to the GaN shell layer. -- Abstract: The structure and luminescence properties of thermally nitrided Ga{sub 2}O{sub 3} nanowires were examined. Transmission electron microscopy and X-ray diffraction confirmed the formation of a uniform GaN shell layer on the surface of the nanowires by thermal nitridation. The core and shell of the nitrided nanowires were monoclinic-structured single crystal Ga{sub 2}O{sub 3} and wurtzite-type hexagonal close-packed-structured single crystal GaN, respectively. The as-synthesized Ga{sub 2}O{sub 3} nanowires exhibited a broad emission band at approximately 570 nm in the yellow region. In contrast, the nitrided Ga{sub 2}O{sub 3} nanowires exhibited a much stronger emission band at approximately 455 nm in the blue region, which must originate from the newly formed GaN shell layer.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Oxidation Protection of Uranium Nitride Fuel using Liquid Phase Sintering

Two methods are proposed to increase the oxidation resistance of uranium nitride (UN) nuclear fuel. These paths are: (1) Addition of USi{sub x} (e.g. U3Si2) to UN nitride powder, followed by liquid phase sintering, and (2) 'alloying' UN nitride with various compounds (followed by densification via Spark Plasma Sintering or Liquid Phase Sintering) that will greatly increase oxidation resistance. The advantages (high thermal conductivity, very high melting point, and high density) of nitride fuel have long been recognized. The sodium cooled BR-10 reactor in Russia operated for 18 years on uranium nitride fuel (UN was used as the driver fuel for two core loads). However, the potential advantages (large power up-grade, increased cycle lengths, possible high burn-ups) as a Light Water Reactor (LWR) fuel are offset by uranium nitride's extremely low oxidation resistance (UN powders oxidize in air and UN pellets decompose in hot water). Innovative research is proposed to solve this problem and thereby provide an accident tolerant LWR fuel that would resist water leaks and high temperature steam oxidation/spalling during an accident. It is proposed that we investigate two methods to increase the oxidation resistance of UN: (1) Addition of USi{sub x} (e.g. U 3 Si 2 ) to UN nitride powder, followed by liquid phase sintering, and (2) 'alloying' UN nitride with compounds (followed by densification via Spark Plasma Sintering) that will greatly increase oxidation resistance.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

Influence of Protection Layers on Thermal Stability of Nitride Thin Films

Nitrides are commonly studied for their properties relevant to numerous coating applications, and have been recently used to synthesize newly predicted materials in thin film form. However, thermodynamic stability of such nitride materials is difficult to experimentally evaluate, due to limited thermal budget of thin films. Here we show that thermal stability of nitride films can be extended using protection layers. Specifically, we find that zirconium nitride (ZrN) thin films are stable up to at least 1200°C annealing temperature in N2 atmosphere, if aluminum nitride (AlN) diffusion barriers and capping layers are used. X-ray diffraction (XRD) measurements show the expected thermodynamically stable rocksalt structure of ZrN, and scanning electron microscopy (SEM) confirms compact microstructure of these ZrN films. Without such protection layers, transient bixbyite Zr2ON2 phase and high-temperature ZrSi2 phase are observed after annealing by XRD, SEM, and auger electron spectroscopy (AES), due to side reaction with native surface oxide (ZrOx) and substrate (Si) respectively. These results demonstrate that protection layers are beneficial to increase the thermal budget of nitride thin films, for evaluating thermodynamic stability of new nitride materials, for capturing transient metastable phases during crystallization, and for using functional nitride coatings in extreme environments.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Gas nitriding behavior of refractory metals and implications for multi-principal element alloy design

Multi-principal element alloys (MPEAs) comprise a large, flexible compositional space that enables tuning of their chemistry, structure, and properties. Here, to facilitate the development of nitriding-based surface-enhancement strategies that harness a broad compositional space, this study examined the gas nitriding behavior of Hf, Mo, Nb, Ta, Ti, and Zr as a function of time, temperature (750 and 1000 °C), and nitriding potential (i.e. ammonia-to-hydrogen ratio). These metals were selected because they have a strong driving force to form nitrides, and appear in many promising refractory MPEA compositions. The nitriding temperatures were selected based on the phase transformation temperature of Ti and Zr, and the nitriding potentials were chosen such that all elements are expected to form nitrides. Mass gain measurements indicate that all six elements follow parabolic kinetics. The microstructure observations and quantitative microchemical analysis show formation of dense and well-adhered compound layers for Mo, Nb, and Ta. Thick diffusion zones appear in Hf, Ta, Ti, and Zr, and diffusion coefficients were fit to the composition profiles. Partial delamination of the compound layer occurred for Ti and Zr. Peak hardness values above 30 GPa are obtained in the dense compound layers, and the solute hardening of the underlying alloy is correlated with the nitrogen content. The results provide insight into the dynamics of nitride compound formation relative to interstitial dissolution of nitrogen, and are discussed in the context of MPEA composition and processing design.

36 MATERIALS SCIENCE↗
DOE OSTI2012

In vitro bio-functionality of gallium nitride sensors for radiation biophysics

Highlights: Black-Right-Pointing-Pointer Gallium nitride based sensors show promising characteristics to monitor cellular parameters. Black-Right-Pointing-Pointer Cell growth experiments reveal excellent biocompatibiltiy of the host GaN material. Black-Right-Pointing-Pointer We present a biofunctionality assay using ionizing radiation. Black-Right-Pointing-Pointer DNA repair is utilized to evaluate material induced alterations in the cellular behavior. Black-Right-Pointing-Pointer GaN shows no bio-functional influence on the cellular environment. -- Abstract: There is an increasing interest in the integration of hybrid bio-semiconductor systems for the non-invasive evaluation of physiological parameters. High quality gallium nitride and its alloys show promising characteristics to monitor cellular parameters. Nevertheless, such applications not only request appropriate sensing capabilities but also the biocompatibility and especially the biofunctionality of materials. Here we show extensive biocompatibility studies of gallium nitride and, for the first time, a biofunctionality assay using ionizing radiation. Analytical sensor devices are used in medical settings, as well as for cell- and tissue engineering. Within these fields, semiconductor devices have increasingly been applied for online biosensing on a cellular and tissue level. Integration of advanced materials such as gallium nitride into these systems has the potential to increase the range of applicability for a multitude of test devices and greatly enhance sensitivity and functionality. However, for such applications it is necessary to optimize cell-surface interactions and to verify the biocompatibility of the semiconductor. In this work, we present studies of mouse fibroblast cell activity grown on gallium nitride surfaces after applying external noxa. Cell-semiconductor hybrids were irradiated with X-rays at air kerma doses up to 250 mGy and the DNA repair dynamics, cell proliferation, and cell growth dynamics of adherent cells were compared to control samples. The impact of ionizing radiation on DNA, along with the associated cellular repair mechanisms, is well characterized and serves as a reference tool for evaluation of substrate effects. The results indicate that gallium nitride does not require specific surface treatments to ensure biocompatibility and suggest that cell signaling is not affected by micro-environmental alterations arising from gallium nitride-cell interactions. The observation that gallium nitride provides no bio-functional influence on the cellular environment confirms that this material is well suited for future biosensing applications without the need for additional chemical surface modification.

63 RADIATION, THERMAL, AND OTHER ENVIRON. POLLUTAN↗
DOE OSTI2013

Nitridation Temperature Effects on Electronic and Chemical Properties of (Ga 1-x Zn x )(N 1-x O x ) Solid Solution Nanocrystals

Solid solution nanocrystals of gallium nitride–zinc oxide have been realized as potential photocatalysts for visible light driven overall water splitting. The band gap of these materials has been found to narrow further into the visible region as a function of increasing zinc oxide concentration, and thus, it is desirable to synthesize zinc oxide-rich gallium nitride–zinc oxide solid solutions. In this paper, we discuss the effects of using nitridation temperature to control zinc oxide content on the electronic and chemical properties of gallium nitride–zinc oxide solid solution nanostructures. The effect of nitridation temperature was studied using X-ray absorption fine structure (XAFS), including both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), and X-ray excited optical luminescence (XEOL). It was determined that using nitridation temperature as a method of controlling zinc oxide concentration results in solid solutions with poor crystallinity, phase separation, and mixed surface oxide formation. These findings suggest that many complications arise from using nitridation temperature to control zinc oxide concentration in gallium nitride–zinc oxide solid solutions, and thus, it is possible that the resultant materials would exhibit poor photocatalytic activity.

Ward, Matthew James↗
NASA NTRS1994

Resistance of Silicon Nitride Turbine Components to Erosion and Hot Corrosion/oxidation Attack

Silicon nitride turbine components are under intensive development by AlliedSignal to enable a new generation of higher power density auxiliary power systems. In order to be viable in the intended applications, silicon nitride turbine airfoils must be designed for survival in aggressive oxidizing combustion gas environments. Erosive and corrosive damage to ceramic airfoils from ingested sand and sea salt must be avoided. Recent engine test experience demonstrated that NT154 silicon nitride turbine vanes have exceptional resistance to sand erosion, relative to superalloys used in production engines. Similarly, NT154 silicon nitride has excellent resistance to oxidation in the temperature range of interest - up to 1400 C. Hot corrosion attack of superalloy gas turbine components is well documented. While hot corrosion from ingested sea salt will attack silicon nitride substantially less than the superalloys being replaced in initial engine applications, this degradation has the potential to limit component lives in advanced engine applications. Hot corrosion adversely affects the strength of silicon nitride in the 850 to 1300 C range. Since unacceptable reductions in strength must be rapidly identified and avoided, AlliedSignal and the NASA Lewis Research Center have pioneered the development of an environmental life prediction model for silicon nitride turbine components. Strength retention in flexure specimens following 1 to 3300 hour exposures to high temperature oxidation and hot corrosion has been measured and used to calibrate the life prediction model. Predicted component life is dependent upon engine design (stress, temperature, pressure, fuel/air ratio, gas velocity, and inlet air filtration), mission usage (fuel sulfur content, location (salt in air), and times at duty cycle power points), and material parameters. Preliminary analyses indicate that the hot corrosion resistance of NT154 silicon nitride is adequate for AlliedSignal's initial engine applications. Protective coatings and/or inlet air filtration may be required to achieve required ceramic component lives in more aggressive environments.

Strangmen, Thomas E.↗
NASA NTRS1982

Method of densifying an article formed of reaction bonded silicon nitride

A method of densifying an article formed of reaction bonded silicon nitride is disclosed. The reaction bonded silicon nitride article is packed in a packing mixture consisting of silicon nitride powder and a densification aid. The reaction bonded silicon nitride article and packing powder are sujected to a positive, low pressure nitrogen gas treatment while being heated to a treatment temperature and for a treatment time to cause any open porosity originally found in the reaction bonded silicon nitride article to be substantially closed. Thereafter, the reaction bonded silicon nitride article and packing powder are subjected to a positive high pressure nitrogen gas treatment while being heated to a treatment temperature and for a treatment time to cause a sintering of the reaction bonded silicon nitride article whereby the strength of the reaction bonded silicon nitride article is increased.

Mangels, John A.↗
NASA NTRS2008

Group-III Nitride Field Emitters

Field-emission devices (cold cathodes) having low electron affinities can be fabricated through lattice-mismatched epitaxial growth of nitrides of elements from group III of the periodic table. Field emission of electrons from solid surfaces is typically utilized in vacuum microelectronic devices, including some display devices. The present field-emission devices and the method of fabricating them were developed to satisfy needs to reduce the cost of fabricating field emitters, make them compatible with established techniques for deposition of and on silicon, and enable monolithic integration of field emitters with silicon-based driving circuitry. In fabricating a device of this type, one deposits a nitride of one or more group-III elements on a substrate of (111) silicon or other suitable material. One example of a suitable deposition process is chemical vapor deposition in a reactor that contains plasma generated by use of electron cyclotron resonance. Under properly chosen growth conditions, the large mismatch between the crystal lattices of the substrate and the nitride causes strains to accumulate in the growing nitride film, such that the associated stresses cause the film to crack. The cracks lie in planes parallel to the direction of growth, so that the growing nitride film becomes divided into microscopic growing single-crystal columns. The outer ends of the fully-grown columns can serve as field-emission tips. By virtue of their chemical compositions and crystalline structures, the columns have low work functions and high electrical conductivities, both of which are desirable for field emission of electrons. From examination of transmission electron micrographs of a prototype device, the average column width was determined to be about 100 nm and the sharpness of the tips was determined to be characterized by a dimension somewhat less than 100 nm. The areal density of the columns was found to about 5 x 10(exp 9)/sq cm . about 4 to 5 orders of magnitude greater than the areal density of tips in prior field-emission devices. The electric field necessary to turn on the emission current and the current per tip in this device are both lower than in prior field-emission devices, such that it becomes possible to achieve longer operational lifetime. Moreover, notwithstanding the lower current per tip, because of the greater areal density of tips, it becomes possible to achieve greater current density averaged over the cathode area. The thickness of the grown nitride film (equivalently, the length of the columns) could lie between about 0.5 microns and a few microns; in any event, a thickness of about 1 micron is sufficient and costs less than do greater thicknesses. It may be possible to grow nitride emitter columns on glass or other substrate materials that cost less than silicon does. What is important in the choice of substrate material is the difference between the substrate and nitride crystalline structures. Inasmuch as the deposition process is nondestructive, an ability to grow emitter columns on a variety of materials would be advantageous in that it would facilitate the integration of field-emitter structures onto previously processed integrated circuits.

Bensaoula, Abdelhak↗
DOE OSTI2013

Method of manufacture of atomically thin boron nitride

The present invention provides a method of fabricating at least one single layer hexagonal boron nitride (h-BN). In an exemplary embodiment, the method includes (1) suspending at least one multilayer boron nitride across a gap of a support structure and (2) performing a reactive ion etch upon the multilayer boron nitride to produce the single layer hexagonal boron nitride suspended across the gap of the support structure. The present invention also provides a method of fabricating single layer hexagonal boron nitride. In an exemplary embodiment, the method includes (1) providing multilayer boron nitride suspended across a gap of a support structure and (2) performing a reactive ion etch upon the multilayer boron nitride to produce the single layer hexagonal boron nitride suspended across the gap of the support structure.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Crystallographic Study of Product Phases of Carbothermic Reduction and Nitridation of Hafnium Dioxide

Details of the carbothermic reduction/nitridation to synthesize hafnium nitride (HfN) and hafnium carbide (HfC) are scarce in the literature. Therefore, this current study was carried out to evaluate two pathways for synthesizing these two refractory materials: direct nitridation and carbothermic reduction/nitridation. Two mixtures of hafnium dioxide and carbon with C/HfO 2 molar ratios of 2.15 and 3.1 were nitridized directly using flowing nitrogen gas at elevated temperatures (1300–1700 °C). The 3.1 C/HfO 2 molar ratio mixture was also carbothermically reduced under flowing argon gas to synthesize HfC, which was converted into HfN by introducing a nitridation step under both N 2(g ) and N 2(g) -10% H 2(g) . X-ray diffraction results showed the formation of HfN at 1300 and 1400 °C and HfC 1–y N y at ≥1400 °C under direct nitridation of samples using a C/HfO 2 molar ratio of 2.15. These phase analysis data together with lower lattice strain and greater crystallite sizes of HfC 1–y N y that formed at higher temperatures suggested that the HfC 1–y N y phase is preferred over HfN at those temperatures. Carbothermic reduction of 3.1 C/HfO 2 molar ratio samples under an inert atmosphere produced single-phased HfC with no significant levels of dissolved oxygen. In conclusion, carbothermic reduction nitridation made two phases of different carbon levels (HfC 1–y N y and HfC 1–y 'N y ', where y' < y), while direct nitridation produced a single HfC 1–y N y phase under both N 2 and N 2 -10% H 2 cover gas environments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Synthesis, Crystal Structure, and Elastic Properties of Novel Tungsten Nitrides

Among transition metal nitrides, tungsten nitrides possess unique and/or superior chemical, mechanical, and thermal properties. Preparation of these nitrides, however, is challenging because the incorporation of nitrogen into tungsten lattice is thermodynamically unfavorable at atmospheric pressure. To date, most materials in the W-N system are in the form of thin films produced by nonequilibrium processes and are often poorly crystallized, which severely limits their use in diverse technological applications. Here we report synthesis of tungsten nitrides through new approaches involving solid-state ion exchange and nitrogen degassing under pressure. We unveil a number of novel nitrides including hexagonal and rhombohedral W{sub 2}N{sub 3}. The final products are phase-pure and well-crystallized in bulk forms. For hexagonal W{sub 2}N{sub 3}, hexagonal WN, and cubic W3N4, they exhibit elastic properties rivaling or even exceeding cubic-BN. All four nitrides are prepared at a moderate pressure of 5 GPa, the lowest among high-pressure synthesis of transition metal nitrides, making it practically feasible for massive and industrial-scale production.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Early stages of plasma induced nitridation of Si (111) surface and study of interfacial band alignment

We report here a systematic study of the nitridation of the Si (111) surface by nitrogen plasma exposure. The surface and interface chemical composition and surface morphology are investigated by using RHEED, X-ray photoelectron spectroscopy, and atomic force microscopy (AFM). At the initial stage of nitridation two superstructures—“8 × 8” and “8/3 × 8/3”—form, and further nitridation leads to 1 × 1 stoichiometric silicon nitride. The interface is seen to have the Si{sup 1+} and Si{sup 3+} states of silicon bonding with nitrogen, which suggests an atomically abrupt and defect-free interface. The initial single crystalline silicon nitride layers are seen to become amorphous at higher thicknesses. The AFM image shows that the nitride nucleates at interfacial dislocations that are connected by sub-stoichiometric 2D-nitride layers, which agglomerate to form thick overlayers. The electrical properties of the interface yield a valence band offset that saturates at 1.9 eV and conduction band offset at 2.3 eV due to the evolution of the sub-stoichiometric interface and band bending.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Electron/phonon coupling in group-IV transition-metal and rare-earth nitrides

Transport electron/phonon coupling parameters and Eliashberg spectral functions α{sub tr}{sup 2}F(ℏω) are determined for group-IV transition-metal (TM) nitrides TiN, ZrN, and HfN, and the rare-earth (RE) nitride CeN using an inversion procedure based upon temperature-dependent (4 < T < 300 K) resistivity measurements of high-crystalline-quality stoichiometric epitaxial films grown on MgO(001) by magnetically-unbalanced reactive magnetron sputtering. Transport electron/phonon coupling parameters λ{sub tr} vary from 1.11 for ZrN to 0.82 for HfN, 0.73 for TiN, and 0.44 for CeN. The small variation in λ{sub tr} among the TM nitrides and the weak coupling in CeN are consistent with measured superconducting transition temperatures 10.4 (ZrN), 9.18 (HfN), 5.35 (TiN), and <4 K for CeN. The Eliashberg spectral function describes the strength and energy spectrum of electron/phonon coupling in conventional superconductors. Spectral peaks in α{sup 2}F(ℏω), corresponding to regions in energy-space for which electrons couple to acoustic ℏω{sub ac} and optical ℏω{sub op} phonon modes, are centered at ℏω{sub ac} = 33 and ℏω{sub op} = 57 meV for TiN, 25 and 60 meV for ZrN, 18 and 64 meV for HfN, and 21 and 39 meV for CeN. The acoustic modes soften with increasing cation mass; optical mode energies remain approximately constant for the TM nitrides, but are significantly lower for the RE nitride due to a lower interatomic force constant. Optical/acoustic peak-intensity ratios are 1.15 ± 0.1 for all four nitrides, indicating similar electron/phonon coupling strengths α{sub tr}(ℏω) for both modes.

36 MATERIALS SCIENCE↗