Investigation of deep level traps in dilute GaAsN layers grown by liquid phase epitaxy

Dilute GaAsN layers are grown by liquid phase epitaxy (LPE) technique, using polycrystalline GaN as the source of nitrogen. A band gap lowering of 100 meV is obtained from low temperature photoluminescence (PL) measurements which correspond to a little more than 0.5% nitrogen in the layer. Using 10 K photocapacitance measurements, a 0.7 eV electron trap is detected in the material which is assigned to an interstitial (N-N)_As defect. Annealing of the material at 750 °C for 1 h greatly reduced this trap and new electron traps with activation energies of 0.8 and 0.9 eV are detected. These new traps are suggested to be due to individual nitrogen-related defect complexes, produced by the thermal annihilation of the N-N bond. After a high temperature treatment of the GaAsN growth melt with 0.1 wt.% Er, nitrogen is found to be removed from the layer and no electron trap was detected.     

Valence band offset at GaN/β-Si3N4 and β-Si3N4/Si(111) heterojunctions formed by plasma-assisted molecular beam epitaxy

Ultra thin films of pure β-Si₃N₄ (0001) were grown on Si (111) surface by exposing the surface to radio- frequency nitrogen plasma with a high content of nitrogen atoms. Using β-Si₃N₄ layer as a buffer layer, GaN epilayers were grown on Si (111) substrate by plasma-assisted molecular beam epitaxy. The valence band offset (VBO) of GaN/β-Si₃N₄/Si heterojunctions is determined by X-ray photoemission spectroscopy. The VBO at the β-Si3N4 / Si interface was determined by valence-band photoelectron spectra to be 1.84 eV. The valence band of GaN is found to be 0.41 ± 0.05 eV below that of β-Si₃N₄ and a type-II heterojunction. The conduction band offset was deduced to be ~ 2.36 eV, and a change of the interface dipole of 1.29 eV was observed for GaN/β-Si₃N₄ interface formation.     

GaN buffer layer growth by MOCVD using a thermodynamic non-equilibrium model

In this work, a gallium nitride (GaN) buffer layer was grown on a sapphire substrate (α-Al₂O₃) in a horizontal reactor by low pressure metal-organic chemical vapor deposition (LP-MOCVD). Trimethylgallium (TMGa) and ammonia (NH₃) were precursors of gallium and nitrogen, respectively, and hydrogen (H₂) was used as carrier gas. TMGa and NH₃ fluxes were kept constant, with flow rates of 3.36 μmole/min and 0.05 standard liter/min, respectively. The fluence of hydrogen was also kept constant with the flux rate of 4.5 standard liter/min. GaN was deposited at 550 °C and 100 mbar. According to the X-ray diffraction spectra, a buffer layer was formed with a wurtzite structure, which is the stable phase. The thermodynamic affinities were estimated as A₁ = 175.9 kJ/mole and A₂ = 62.88 kJ/mole.     

Thermal stability of nitride coatings formed by ion-plasma deposition

Titanium nitride and chromium nitride coatings were formed on a gray cast iron by condensation from a plasma phase in vacuum with the ion bombardment of the sample surface by titanium or chromium plasma flows in a residual nitrogen atmosphere. The element and phase composition of coatings were studied before and after annealing for 1-43 h in air at temperature 700 °C using Auger electron spectroscopy (AES) and X-ray diffraction (XRD). It is established that titanium nitride coatings are single-phase TiN system with a (1 1 1) preferred growth orientation, and chromium nitride coatings—a two-phase system: CrN and Cr₂N phases. The annealing of coatings at the atmospheric pressure and the temperature of 700 °C in the range of 1-43 h results in the deceleration of the oxidation process of the material substrate for chromium nitride with respect to titanium nitride coatings.     

Graphoepitaxy of High‐Quality GaN Layers on Graphene/6H–SiC

The implementation of graphene layers in gallium nitride (GaN) heterostructure growth can solve self‐heating problems in nitride‐based high‐power electronic and light‐emitting optoelectronic devices. In the present study, high‐quality GaN layers are grown on patterned graphene layers and 6H–SiC by metalorganic chemical vapor deposition. A periodic pattern of graphene layers is fabricated on 6H–SiC by using polymethyl methacrylate deposition and electron beam lithography, followed by etching using an Ar/O₂ gas atmosphere. Prior to GaN growth, an AlN buffer layer and an Al_0.2Ga_0.8N transition layer are deposited. The atomic structures of the interfaces between the 6H–SiC and graphene, as well as between the graphene and AlN, are studied using scanning transmission electron microscopy. Phase separation of the Al_0.2Ga_0.8N transition layer into an AlN and GaN superlattice is observed. Above the continuous graphene layers, polycrystalline defective GaN is rapidly overgrown by better quality single‐crystalline GaN from the etched regions. The lateral overgrowth of GaN results in the presence of a low density of dislocations (≈10⁹ cm⁻²) and inversion domains and the formation of a smooth GaN surface.     

Optical and electrical properties of carbon nitride films deposited by cathode electrodeposition

Carbon nitride (CN _x ) films were prepared on silicon (100) wafers and ITO conductive glasses by cathode electrodeposition, using dicyandiamide (C₂H₄N₄) in acetone as precursors. The composition ratios (N/C) were approximated to or larger than 1 from XPS. The optical properties and electrical resistivities of the films were investigated. Intense PL with two bands in the range 2.5–3.5 eV was observed on the CN _x films. The band gaps (E _opt) deduced from measurements of the optical absorption coefficients in the UV-VIS spectra were found to be in the range of 1.1–1.6 eV. From the PL and UV-VIS spectra, the nitrogen content has a large effect on the PL band gap and E _opt. The electrical resistivities of the films on Si wafers are in the 10⁹–10¹⁰ · cm range.     

Band offsets of InXGa1−XN/GaN quantum wells reestimated

Recently In_XGa_1−XN/GaN heterostructures and quantum wells (QWs) have gained immense importance in the application of III-V nitride materials. Reported values of the ratios of conduction band offset to valence band offset for In_XGa_1−XN/GaN QW structures, ΔE_c:ΔE_v, vary widely from 38:62 to 83:17. While trying to explain the unusual shifts in the photoluminescence (PL) spectra, obtained from In_XGa_1−XN/GaN QW structures, it has been found that a band offset ratio, ΔE_c:ΔE_v = 55:45, explains all the experimental data precisely. In this paper detailed theories, procedures, results and discussions to establish the newly estimated band offsets will be presented.     

Plasma nitriding of AISI 52100 ball bearing steel and effect of heat treatment on nitrided layer

In this paper an effort has been made to plasma nitride the ball bearing steel AISI 52100. The difficulty with this specific steel is that its tempering temperature (~170–200°C) is much lower than the standard processing temperature (~460–580°C) needed for the plasma nitriding treatment. To understand the mechanism, effect of heat treatment on the nitrided layer steel is investigated. Experiments are performed on three different types of ball bearing races i.e. annealed, quenched and quench-tempered samples. Different gas compositions and process temperatures are maintained while nitriding these samples. In the quenched and quench-tempered samples, the surface hardness has decreased after plasma nitriding process. Plasma nitriding of annealed sample with argon and nitrogen gas mixture gives higher hardness in comparison to the hydrogen–nitrogen gas mixture. It is reported that the later heat treatment of the plasma nitrided annealed sample has shown improvement in the hardness of this steel. X-ray diffraction analysis shows that the dominant phases in the plasma nitrided annealed sample are ε (Fe_2 − 3N) and γ (Fe₄N), whereas in the plasma nitrided annealed sample with later heat treatment only α-Fe peak occurs.     

Energy band diagram of a photosensitive Sn-<Emphasis Type="Italic">p</Emphasis>-InSe structure

Photosensitive Sn-p-InSe structures obtained by thermal deposition of Sn in vacuum onto thermally pretreated p-InSe substrates are well described within the framework of a MIS model with the insulating layer represented by a film of wide-bandgap γ-In₂Se₃ with E _g=2.0 eV formed as a result of thermal treatment of the base semiconductor.     

Deep level centers in InGaN/GaN heterostructure grown on sapphire and free-standing GaN

Deep level transient spectroscopy has been used to characterize the deep levels in InGaN/GaN grown on sapphire substrate as well as on free-standing GaN. The deep levels at E_c − E_t ∼ 0.17-0.23 eV and E_c − E_t ∼ 0.58-0.62 eV have been detected in our samples which are present in GaN samples reported by others. These two deep levels have been attributed by us to threading dislocations as they exhibit logarithmic capture kinetic behavior and are found to be substantially reduced in its trap concentration (∼ from 10¹⁴ to 10¹² cm^− 2) in GaN grown on free-standing GaN template. Other than the two deep levels, an additional level at E_c − E_t ∼ 0.40-0.42 eV has been identified in both samples, which is believed to be related to In segregation. AFM image shows region of pits formation in InGaN epilayer for sample grown on u-GaN using sapphire substrate while the latter gives a much smoother morphology. From the X-ray diffraction space mapping, the mosaicity of the sample structure for both samples were studied. Dislocations do not play a significant role in the structural properties of InGaN grown on free-standing GaN since the FWHM based on the Δ ω is relatively small (± 0.15°) in the case of InGaN/GaN on free-standing GaN substrate as compared to that on sapphire (± 0.35°). The wider spread in Δω-2θ value for InGaN layer on free-standing GaN also suggested the effect of compositional pulling with increasing InGaN layer thickness.     

Application of a molecular precursor to the preparation of tungsten and beta-tungsten nitride coatings and powders

The use of the tungsten imido/amido complex (N-H-t-Bu)₂W(N-t-Bu)₂ (Complex 1) (CAS number 72207-45-5) as a precursor to tungsten and tungsten nitride films, coatings and powders is reported. The alkane-soluble Complex 1 exhibits high solubility in solvents such as hexane and toluene, making it amenable to application to substrates via dip-coating or spotspraying. This avoids the necessity of vapour deposition methods and the, presently common, use of tungsten fluorides. Thermolysis in an ammonia/nitrogen, ammonia/argon or exclusively nitrogen atmosphere at temperatures between 600 and 1200°C provided a coating or powder comprised of elemental tungsten (75–95 mol%) and tungsten nitride (25-5 mol%). Oxygen content of these materials is less than 0.5%. Thermolysis in an argon atmosphere produced a coating on AIN substrates which were found to contain no -W₂N (as detectable by X-ray diffraction), only tungsten metal. The coating was successfully applied to sintered aluminium nitride electronic and structural substrates. A coated aluminium nitride substrate exhibited improvement in erosion resistance when compared to an uncoated specimen.     

Thermally stable and low-resistance W/Ti/Au contacts to n-type GaN

We report on the formation of thermally stable and low-resistance Ti/Au-based ohmic contacts to n-type GaN (4.0 × 10¹⁸ cm⁻³) by using a W barrier layer. It is shown that the electrical characteristic of the sample is considerably improved upon annealing at 900 °C for 1 min in a N₂ ambient. The contacts produce the specific contact resistance as low as 6.7 × 10⁻⁶ Ω cm² after annealing. The Norde and current–voltage methods are used to determine the effective Schottky barrier heights (SBHs). It is shown that annealing results in a reduction in the SBHs as compared to that of the as-deposited sample. Auger electron spectroscopy (AES), scanning transmission electron microscopy (STEM) and X-ray diffraction (XRD) examinations show that nitride and gallide phases are formed at the contact/GaN interface. Based on the AES, STEM and XRD results, a possible ohmic formation mechanism is described and discussed.     

Unusual changes observed in the photoluminescence of annealed InxGa1 − xN/GaN quantum wells explained

In_xGa_1 − xN/GaN heterostructures and quantum wells (QWs) are particularly important in the application of III-V nitride materials for light emitting diodes and laser diodes. The photoluminescence (PL) emissions from In_xGa_1 − xN/GaN QW structures have been reported, where, for successive annealing operations, the PL peak suffers a primary red shift, followed by a blue shift. The observed phenomenon remains unexplained because of its complexity. This paper is intended towards a proper explanation of the observed experimental results through suitable quantum mechanical models and computations, whether the band gap of InN is 1.95 eV or 0.7 eV.     

Examination of the phase and chemical composition of ion-plasma coatings on R18 high-speed tool steel after short-term heating

Conclusions Analytical thermodynamic examination and subsequent experimental verification by the methods of local analysis, including x-ray and Auger spectroscopy, showed that it is possible to control the oxygen, carbon, nitrogen content of the coating based on titanium nitride on the high-speed tool steel in short-term thermal (to 800°C) shocks in air. Results show that in addition to TiN, compounds TiO_1.75, TiO₂, and TiC can also form in the coating, whereas Cr₂N can form in the substrate. the possibility of formation of the new phases in the surface layer makes it possible to change its properties in a wide range, including the friction coefficient, hardness, corrosion resistance, etc.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 28, No. 1, pp. 123–125, January–February, 1992.     

In situ Auger electron spectroscopy investigation of the chemical bonding of ion-beam-deposited silicon nitride

The necessity of knowing the atomic ratio of nitrogen to silicon and the contamination of silicon nitride to obtain layers with reproducible physical, chemical and electrical properties is reviewed. Ion-beam-sputtered thin films of various nitrogen-to-silicon ratios with an oxygen content of less than 1% are analysed by in situ Auger electron spectroscopy. By deliberately allowing a pure silicon-rich nitride layer to become oxidized, the different signals contributing to the line shape of the derivative Si LVV peak are unambiguously identified. Assuming the peak-to-background ratio to be an intrinsic measurement, we propose a method to evaluate the composition of non-stoichiometric silicon nitride layers from the study of the Si LVV peak recorded in the EN(E) mode. This peak is reconstituted by using a linear combination of experimental Si LVV peaks from pure silicon and completely nitrided silicon. A realistic background is deduced from a step-by-step deposition of Si₃N₄ onto carbon. The results are compared with Rutherford backscattering spectrometry measurements and discussed.     

Magnetic properties and thermal modification of nanostructured films of nickel nitrides

Nanostructured films of nickel nitrides (from Ni to Ni₂N) have been synthesized by magnetron sputtering of a nickel target in an argon atmosphere with the addition of 0.2–70 vol % nitrogen. Magnetization saturation 4πM _S and Curie temperature T _C of the solid solution of nitrogen in nickel and nitride Ni4N have been determined. With an increase in nitrogen concentration, 4πM _S and T _C decrease, respectively, to 800 G and 490 K. When heated in air, nickel nitride films undergo transformation with nitrogen loss up to phase transitions with structural changes. In particular, heating to 600 K leads to transformation of paramagnetic nitride Ni₂N into the ferromagnetic Ni phase in the form of solid solution of nitrogen in nickel.     

Ion-nitriding behaviour of Fe-Ti alloys in theα-phase region

The ion-nitriding behaviour of four iron alloys containing between 0.11 and 1.48 wt% titanium was investigated in the-phase region to discuss kinetics of the growth of the nitriding layer. The ion-nitriding experiments have been made at 823 K. Two nitriding layers were observed: a thin surface layer which mainly consists of Fe₄N; an internal nitriding layer beneath the surface layer, where the nitride formed was found to be TiN. The growth of the internal nitriding layer is controlled by a diffusion process of nitrogen in the matrix metal. The apparent diffusion coefficient of nitrogen in the nitriding layer, evaluated using the rate equation proposed for internal oxidation, increases linearly with the volume fraction of titanium nitride. Furthermore, by excluding the effect of the titanium nitride from the apparent diffusion coefficient, the diffusion coefficient of nitrogen in-iron was calculated, being in good agreement with that reported so far. In addition, the increase in hardness in the internal nitriding layer has been discussed.     

Diffusion kinetics of explosively treated and plasma nitrided Ti-6Al-4V alloy

Ti-6Al-4V alloys, which were exposed to an explosive shock process, were nitrided in nitrogen plasma in the temperature range of 700-900°C for 3-12 h. During the plasma nitriding, the surface layer consisted of TiN (δ), Ti₂N (ε) and nitrogen solid solution layers (α-Ti). The growth rate of nitride and solid solution layers were found to be controlled by the diffusion of nitrogen. An effective nitriding was achieved due to high dislocation density and vacancy concentration. Based on the present layer growth data, an analytical model for multiphase diffusion was used to estimate the effective nitrogen atom diffusion coefficient in the nitride layers. The interface velocity equations were derived from Fick's law and a numerical method has been used to compute the diffusion coefficients of nitrogen in a binary multiphase Ti-TiN system. Depending on temperature and layer thickness, the activation energies of nitrogen in TiN and Ti₂N phases were found to be 18,950 (±2116) and 27,925 (±1105) cal/mole, respectively.     

Co,Ni-base alloy thin films deposited by r.f. magnetron sputtering in Ar/N<Subscript>2</Subscript> atmosphere

Phase composition and microstructure of CoNiCrAlY thin films deposited by r.f. Magnetron Sputtering in reactive Ar/N₂ atmosphere were determined by X-ray Diffraction. Three basic phases were observed, for increasing nitrogen partial pressure: (a) a nanocrystalline supersaturated solid solution of the alloy elements with fcc structure and interstitial nitrogen (up to 10–15%), with a sharp [111] fibre texture turning into a broad [200] component for increasing N₂ content; (b) an amorphous phase with nominal composition M₂N (with M as the alloy elements); and (c) a nanocrystalline nitride approaching the nominal composition MN. Nanocomposite coatings made of nanocrystalline fcc metal phase embedded in an amorphous matrix can be formed in a relatively narrow N₂ partial pressure range, whereas at high nitrogen content the thin films tend to form an increasing fraction of a nanocrystalline nitride in addition to the amorphous matrix.Scratch test results are different for the various systems: thin films made of (a) behave as typical plastic metals, with an increased scratch test resistance for increasing N₂ content; amorphous (b) films show a very good scratch test behaviour and tend to fail in a plastic mode, with optimal properties for systems made of fcc metal nanocrystalline/amorphous nanocomposites; thin films with a high N₂ content tend to behave in a brittle way for increasing content of the nanocrystalline nitride phase.     

Ion-nitriding of an Fe-19 wt % Cr alloy

The ion-nitriding behaviour of an Fe-18.75wt% Cr alloy was investigated at 803 K under constant plasma conditions. Both a thin surface layer of-Fe₄N and an internal-nitriding layer were observed. The nitride formed in the internal-nitriding layer was found to be CrN, rather than Cr₂N. The hardness of the nitriding layer rises to Hv=1200 due to small CrN precipitates. The growth rate of the internal nitriding layer, in the present alloy is controlled by a nitrogen diffusion process in the matrix metal,-iron. Because such ion-nitriding behaviour is analogous to that of internal-oxidation, the growth rate of nitriding was discussed according to the rate equation to that of internal-oxidation. The nitrogen diffusion in the present alloy is scarcely affected by the CrN precipitates.