XPS, electric and photoluminescence-based analysis of the GaAs (1 0 0) nitridation

In this paper, nitridation process of GaAs (1 0 0) substrates was studied in-situ using X-ray photoelectron spectroscopy (XPS) and ex-situ by means of electrical method I-V and photoluminescence surface state spectroscopy (PLS³) in order to determine chemical, electrical and electronic properties of the elaborated GaN/GaAs interfaces.The elaborated structures were characterised by I-V analysis. The saturation current I_S, the ideality factor n, the barrier height Φ_Bn and the serial resistance R_S are determined.The elaborated GaN/GaAs structures are also exhibited a high PL intensity at room temperature. From the computer-aided analysis of the power-dependent PL efficiency measurements (PLS³ technique), the value of the interface state density N_SS(E) close to the mid-gap was estimated to be in the range of 2-4 × 10¹¹ eV⁻¹ cm⁻², indicating a good electronic quality of the obtained interfaces.Correlation among chemical, electronic and electrical properties of the GaN/GaAs interface was discussed.     

Nitridation under ammonia of high surface area vanadium aerogels

Vanadium pentoxide gels have been obtained from decavanadic acid prepared by ion exchange on a resin from ammonium metavanadate solution. The progressive removal of water by solvent exchange in supercritical conditions led to the formation of high surface area V₂O₅, 1.6H₂O aerogels. Heat treatment under ammonia has been performed on these aerogels in the 450-900 °C temperature range. The oxide precursors and oxynitrides have been characterized by XRD, SEM, TGA, BET. Nitridation leads to divided oxynitride powders in which the fibrous structure of the aerogel is maintained. The use of both very low heating rates and high surface area aerogel precursors allows a higher rate and a lower threshold of nitridation than those reported in previous works. By adjusting the nitridation temperature, it has been possible to prepare oxynitrides with various nitrogen enrichment and vanadium valency states. Whatever the V(O,N) composition, the oxidation of the oxynitrides in air starts between 250 and 300 °C. This determines their potential use as chemical gas sensors at a maximum working temperature of 250 °C.     

The impact of ultra thin silicon nitride buffer layer on GaN growth on Si (1 1 1) by RF-MBE

Ultra thin films of pure silicon nitride were grown on a Si (1 1 1) surface by exposing the surface to radio-frequency (RF) nitrogen plasma with a high content of nitrogen atoms. The effect of annealing of silicon nitride surface was investigated with core-level photoelectron spectroscopy. The Si 2p photoelectron spectra reveals a characteristic series of components for the Si species, not only in stoichiometric Si₃N₄ (Si⁴⁺) but also in the intermediate nitridation states with one (Si¹⁺) or three (Si³⁺) nitrogen nearest neighbors. The Si 2p core-level shifts for the Si¹⁺, Si³⁺, and Si⁴⁺ components are determined to be 0.64, 2.20, and 3.05 eV, respectively. In annealed sample it has been observed that the Si⁴⁺ component in the Si 2p spectra is significantly improved, which clearly indicates the crystalline nature of silicon nitride. The high resolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM) and photoluminescence (PL) studies showed a significant improvement of the crystalline qualities and enhancement of the optical properties of GaN grown on the stoichiometric Si₃N₄ by molecular beam epitaxy (MBE).     

Synthesis,growth mechanism and optical characterization of zinc nitride hollow structures

In this report, we describe the noncatalytic and template-free synthesis of zinc nitride (Zn₃N₂) novel microstructures with hollow interiors via simple nitridation reaction of zinc powder at optimum temperature of 600° C for 120 min in ammonia gas environment under atmospheric pressure. Hollow microstructures obtained were mostly of spherical shape with diameters in the range 8–35 μm and with open mouth on the surface. The growth mechanism has been proposed for the elucidation of hollow structures formation. Crystal structure and phase purity of the product were investigated by X-ray diffraction (XRD) characterization and energy dispersive X-ray spectroscopy (EDS) analysis confirmed the chemical composition of the product. Morphology of the as-prepared product was investigated using scanning electron microscopy (SEM). Ultraviolet–visible–near infrared (UV–vis–NIR) spectrophotometry was used to study the transmittance behaviour of zinc nitride microstructures and thereby an indirect optical band gap of 2.81 eV was calculated using Davis–Mott model. Room temperature photoluminescence (PL) studies exhibited two prominent peaks of the product; one very strong peak near band edge UV emission (395 nm) and other comparatively suppressed and broad peak at orange luminescence emission (670 nm).     

Formation mechanisms of Si3N4 and Si2N2O in silicon powder nitridation

Commercial silicon powders are nitrided at constant temperatures (1453 K; 1513 K; 1633 K; 1693 K). The X-ray diffraction results show that small amounts of Si₃N₄ and Si₂N₂O are formed as the nitridation products in the samples. Fibroid and short columnar Si₃N₄ are detected in the samples. The formation mechanisms of Si₃N₄ and Si₂N₂O are analyzed. During the initial stage of silicon powder nitridation, Si on the outside of sample captures slight amount of O₂ in N₂ atmosphere, forming a thin film of SiO₂ on the surface which seals the residual silicon inside. And the oxygen partial pressure between the SiO₂ film and free silicon is decreasing gradually, so passive oxidation transforms to active oxidation and metastable SiO(g) is produced. When the SiO(g) partial pressure is high enough, the SiO₂ film will crack, and N₂ is infiltrated into the central section of the sample through cracks, generating Si₂N₂O and short columnar Si₃N₄ in situ. At the same time, metastable SiO(g) reacts with N₂ and form fibroid Si₃N₄. In the regions where the oxygen partial pressure is high, Si₃N₄ is oxidized into Si₂N₂O.     

Gas phase nitridation of silicate fiber glass materials with ammonia:Three case studies

Three types of the leached fiber glass materials of silicate origin modified with Zr, Al and rare earth metals (REM) were nitridated with ammonia at temperatures ranging from 673 to 973 K. DRIFTS (Diffuse Reflectance Infrared Fourier-Transform Spectroscopy) and H/D exchange using ND₃ were applied for characterization of the formed NH_x species. Along with nitridation of silanol groups their dehydroxilation as well as hydrolysis of NH_x species take place. In situ DRIFTS study showed that the limiting steps are slow chemical reactions, but not a diffusion of NH₃ or H₂O molecules in the bulk of glasses. The concept on strained siloxane bridges formed due to dehydroxylation of two adjacent silanol groups was used for explanation of the reaction kinetics features.     

含氮ZSM-5分子筛骨架中氮取代位置的密度泛函计算

运用Gaussian 98程序包, 采用密度泛函理论B3LYP方法, 基于ZSM-5分子筛的8T模型, 分别通过6-31G, 6-31G(d)和6-311G(d,p)基组计算了ZSM-5分子筛中氮原子取代前后各O原子和各N原子的能量, 从而得到各O原子与各N原子在骨架中的稳定性及其对氮化取代反应的影响. 计算结果表明, N原子在骨架中的稳定性对氮取代反应的影响较大. ZSM-5分子筛晶体结构中与B酸位处于同一个四面体的O11位置, 为氮原子的最佳取代位置, 因此氮化后分子筛表面的B酸强度得到较大程度的减弱.     

Formation of charged defects during the nitridation of a metal particle

A model explaining and predicting generation of a temporal electric potential during nitridation of a single metal pellet has been developed. The model takes into account the kinetics of defects formation and assumes that the rate of the chemical reaction can be described by the shrinking-core process. The model simulations have shown that time scale of the generated electric potential depends on both the initial nitride shell thickness and heat removal from the particle surface. At thin initial shell and low rate of heat removal the maximum of the surface electric potential is attained before the temperature and surface nitrogen concentration have reached their maximums but after the maximum of nitridation has appeared. Quasi-neutral distributions of metal vacancies and electron holes are formed at the maximum temperature. At thick initial shell and/or high rates of heat removal from the particle surface the potential maximum may be observed much later: after the maximum temperature has been achieved. Correspondingly, non-equilibrium concentrations of the charged defects exist till the end of nitridation. In contrast to oxidation the nitrogen adsorption rate constants (the activation energy and pre-exponent) have negligible effect on the surface potential form and amplitude. At the ignition limit the rate of nitridation is proportional to the power of −1/2 for the ambient nitrogen pressure in the proposed scheme of defects formation. Metal vacancies and electron holes are the main charged defects in nitrides during nitrogen combustion. The nitride formation is limited by transfer of the vacancies in nitride.     

Niobium nitride films formed by rapid thermal processing (RTP): a study of depth profiles and interface reactions by complementary analytical techniques

The nitridation of niobium films approximately 250 and 650 nm thick by rapid thermal processing (RTP) at 800 °C in molecular nitrogen or ammonia was investigated. The niobium films were deposited by electron beam evaporation on silicon substrates covered by a 100 or 300 nm thick thermally grown SiO₂ layer. In these investigations the reactivity of ammonia and molecular nitrogen was compared with regard to nitride formation and reaction with the SiO₂ substrate layer. The phases formed were characterized by X-ray diffraction (XRD). Depth profiles of the elements in the films were recorded by use of secondary neutral mass spectrometry (SNMS). Microstructure and spatial distribution of the elements were imaged by transmission electron microscopy (TEM) and energy-filtered TEM (EFTEM). Electron energy loss spectra (EELS) were taken at selected positions to discriminate between different nitride, oxynitride, and oxide phases. The results provide clear evidence of the expected higher reactivity of ammonia in nitride formation and reaction with the SiO₂ substrate layer. Outdiffusion of oxygen into the niobium film and indiffusion of nitrogen from the surface of the film result in the formation of oxynitride in a zone adjacent to the Nb/SiO₂ interface. SNMS profiles of nitrogen reveal a distinct tail which is attributed to enhanced diffusion of nitrogen along grain boundaries.     

Nitride layers on uranium surfaces

Uranium as an important energy material plays a significant role within the field of material sciences and nuclear industrial applications. However, metallic uranium is chemically active in ambient environment and is easily oxidized and corroded, leading to not only deterioration of its properties and failure of performance as working components but also nuclear pollution of the environment. Therefore, the development of corrosion protection systems for metallic uranium is an issue of prime importance. In view of the nitridation technology in Ti and Fe-based alloys, the successful application to improve the surface wear hardness and corrosion resistance, several nitridation methods have been developed for the surface modification of metallic uranium. Many studies have shown that the surface nitridation of metallic uranium can efficiently improve its corrosion resistance. The surface oxidation layer thickness is as thin as several nanometers even if placed 4?years in the atmosphere. At the present, nitridation of uranium surface is considered as the most promising surface modification way to protect uranium from corrosion. To design and fabricate nitride layers on uranium surface with reliable long-term protective effects, however, one needs deep understanding on the relationships among the physical and chemical properties of the nitride layers, the composition and structure of the layers, and the dependence on the techniques and the processing parameters. One also needs deep understanding on the corrosion behavior of the prepared nitride layers in the environment, and the related corrosion mechanism.In this review, we bring to the readers the achievements and recent advances on the uranium nitridation in the world, including the processing techniques and the related studies on the formation mechanism of the nitride layers, and the understanding on the property-processing-corrosion performance relationship of the layers, aiming at the development of high-performance resistance layers for metallic uranium by the surface nitridation technique. In the review (1) the surface nitridation techniques developed recently, the relationship between the preparation parameters and the composition as well as the structure of the surface layer are summarized; (2) the fundamental physical properties of the uranium nitrides are summarized, depicted and discussed; (3) the influence of the nitrides structure and composition and of the environment on resistance to corrosion as well as the formation mechanism of corroded products in oxidizing environments are depicted and discussed; (4) the potential application of uranium nitrides in other application field such as the application of thermal-electrical conversion is also discussed. Finally, the prospective on the investigations of nitride layers is suggested.