Hexagonal Nitride Research Articles

Electrochemical behavior of the composite anodes consisting of carbonaceous materials and lithium transition-metal nitrides for lithium-ion batteries

We report composite anodes that show a high first cycle efficiency of ca. 100% and a large capacity of ca. 400–450 mAh g − 1 for Li-ion batteries. The composites are prepared by mixing or ball-milling the hexagonal lithium transition-metal nitrides with different carbonaceous materials, such as disordered carbon and the graphitic carbon. Both the lithium metal nitrides and the carbonaceous materials exhibit electrochemical activity in the electrodes within a potential window of 0–1.4 V vs. Li/Li +. Additionally, a high cycling stability is feasible due to the low volumetric effects of the composite electrodes upon Li intercalation and extraction. However, a voltage hysteresis during charge and discharge is obvious because the two active hosts react with lithium at different potentials in the electrode. Some factors influencing the electrochemical behavior of the composite anodes are discussed.

Semipolar Nitrides Grown on Si(001) Offcut Substrates with 3C-SiC Buffer Layers

The growth process of semipolar GaN(10-12) on Si(001) offcut substrates with 3C-SiC buffer layers has been investigated. From XRD analysis, the difference in the crystal orientation between GaN(10-12) and 3C-SiC(001) has been found to be around 8˚ toward the [110] direction of the 3C-SiC templates. From TEM observations, a cubic-phase AlN seed layer is found to grow on 3C-SiC(001) templates, and the swift transition from the cubic phase to a hexagonal phase leads to the stable growth of hexagonal nitrides. Using 8˚-offcut Si substrates, it is possible to obtain a mirror-like surface of GaN(10-12) using an approximately 10-nm-thick AlN seed layer, which swiftly transitions from cubic AlN to hexagonal GaN.

Synthesis of metal-nitrides using high pressures and temperatures

Technologically, high density nitrides are showing promise for both ceramic and electronic applications. In a laser-heated diamond cell we prepare high density metal-nitrides by reaction of the nitrogen pressure medium with an elemental substrate. Two of our objectives are to develop criteria governing whether denser than ambient nitride phases will form, and to in particular establish the parameters required for synthesis in a multianvil press using elemental starting materials. We have already synthesized transition metal nitrides in a multianvil press using elemental starting materials, including hexagonal nickel nitride and alkali rhenium nitrides. Unlike previous metals, we also report that Cu does not form a nitride after heating with NaN3 at 2000 K and 20 GPa. Notably, Cu3N is a semiconductor exhibiting weak directional bonds, whereas the immediately adjacent lower atomic number systems are metallic interstitial nitrides. We also briefly mention our work on processing high pressure and temperature recovered reaction products with focused ion beam methods for tailored characterization using electron microscopy.

Phase formation in the B-BN-B2O3 system at high pressures and temperatures: The wettability of the phases with copper-based melts

Multiphase materials, including the B6N boron subnitride, B6O boron suboxide, and cBN cubic boron nitride, have been obtained from the initial β-rhombohedral or amorphous boron, hBN hexagonal graphite-like boron nitride and B2O3 amorphous boron oxide at a pressure of 8 GPa and temperatures from 2100 to 2900 K in a toroid high-pressure apparatus. The hardness of the resulting materials has been measured. The formation of ternary phases in the B-BN-B2O3 system has not been observed. The wettability of the resultant materials with copper-based melts has been studied. An addition of 10 mol % Ti to the copper melt has been found to decrease the contact angle to a value lower than 20 deg.

Electrochemical behavior of the composite anodes consisting of carbonaceous materials and lithium transition-metal nitrides for lithium–ion batteries

We report the composite anodes that show a high first cycle efficiency of ca. 100% and a large capacity of ca. 400–450 mAh g − 1 for Li–ion batteries. The composites are prepared by mixing or ball-milling the hexagonal lithium transition-metal nitrides with different carbonaceous materials, such as the disordered carbon and the graphitic carbon. Both the lithium metal nitrides and the carbonaceous materials are electrochemically active in the electrodes within the potential window of 0–1.4 V vs. Li/Li +. Additionally, a high cycling stability is feasible due to the low volumetric effects of the composite electrodes upon Li intercalation and extraction. However, the voltage hysteresis during the charge and discharge is obvious because the two active hosts react with lithium at different potentials in the electrode. Some factors influencing the electrochemical behavior of the composite anodes are discussed.

Spark Plasma Sintered AlN-BN Composites and its Dielectric Properties

A series of samples of hexagonal boron nitride-aluminum nitride (10-90, 15-85, 20-80, 25-75, 30-70wt.%) ceramic composites were prepared by spark plasma sintering in a nitrogen atmosphere at 1650 °C-1800 °C for 5min. Different amounts of CaF2 were added as sintering aids. The effects of CaF2 and sintering temperature on densification and dielectric properties were discussed. The addition of CaF2 enhances relative dielectric constant and loss tangent of the samples. The increase in sintering temperature promotes the densification and decreases the dielectric loss tangent. When being sintered at 1700 °C, the relative density, dielectric constant and dielectric loss tangent of the sample with 15wt.%BN are 98.04%, 7.15 and 6.31×10-4 respectively.

Structural characterization of buried nitride layers formed by nitrogen ion implantation in silicon

The synthesis of buried silicon nitride insulating layers was carried out by SIMNI (separation by implanted nitrogen) process using implantation of 140keV nitrogen (14N+) ions at fluence of 1.0×1017, 2.5×1017 and 5.0×1017cm−2 into 〈111〉 single crystal silicon substrates held at elevated temperature (410°C). The structures of ion-beam synthesized buried silicon nitride layers were studied by X-ray diffraction (XRD) technique. The XRD studies reveal the formation of hexagonal silicon nitride (Si3N4) structure at all fluences. The concentration of the silicon nitride phase was found to be dependent on the ion fluence. The intensity and full width at half maximum (FWHM) of XRD peak were found to increase with increase in ion fluence. The Raman spectra for samples implanted with different ion fluences show crystalline silicon (c-Si) substrate peak at wavenumber 520cm−1. The intensity of the silicon peak was found to decrease with increase in ion fluence.

Molecular modeling of ionic liquid tribology: Semi-empirical bonding and molecular structure

The interactions between selected ionic liquids and either an aluminum oxide or a silicon nitride surface are modeled in this report using semi-empirical methods. The ionic liquids include a series of seven cationic imidazolium derivatives with PF 6 − as the anion. The tribological properties of these ionic liquids and their interactions with aluminum and Al–steel alloys are modeled using a smooth aluminum oxygen surface. The ionic liquids are allowed to form a complex with this surface and the enthalpies of complex formation are seen to correlate with the tribological properties of the seven ionic liquids. The seven ionic liquids are also complexed with a hexagonal silicon nitride surface and their tribological properties are predicted on the basis of complex formation enthalpies.

First PAC experiments in MAX-phases

MAX-phases are hexagonal ternary carbides and nitrides with the general formula: M n+1 AX n and n =1 to 3. 111In was implanted into the two MAX compounds Ti2InC and Zr2InC. Based on the general knowledge of previous 111In implantations one expects to find the probes on the indium lattice-site in these compounds. First experiments on the annealing behaviour and the thermal stability of the indium-containing MAX-phases are reported. The observed EFGs are interpreted and first PAC-measurements under compressive stress are shown.

Features of sputtering of nitrides and their components

The sputtering behaviour of hexagonal or wurtzite polycrystal nitrides of boron, aluminium, and gallium was explored employing methods of molecular dynamics. The sputtering yield Y of nitrides and the average energy Ē 1 of emitted particles were studied as dependent on the mass m 1 of bombarding He, Li, B, N, Ne, Al, Ar, Ni, Ga, Kr, and Xe ions with the initial energy E 0 between 200 and 10,000 eV. Y(m 1) and Ē 1(m 1) were researched upon for preferential sputtering of nitride components at low E 0. It was revealed that the ratio between the sputtering yield of the light component and that of the heavy one depends on ion energy and mass. At E 0=200 eV and m 1=40, the ratio for BN, AlN, and GaN amounts to 1.2, 2.2, and 2.4, respectively. For nitride components, an anomalous dependence of sputtering on ion mass was found, its maximum occurring at a certain ratio m 2/m 1, where m 2 is the averagè mass of two nitride components.

Dielectric Properties of Boron Nitride-Aluminium Nitride Composites Prepared by Spark Plasma Sintering

In this study, hexagonal Boron Nitride-Aluminium Nitride composites were prepared by spark plasma sintering at 1700°Cfor 4 min under the pressure of 25 MPa, varying the amount of BN from 0 to 30 wt.%. The dielectric constants and losses of the samples were measured from 100 Hz to 1 MHz at room temperature, and the results show that with the increment of BN content, the dielectric constants and losses of AlN-BN composites decrease. When the BN content is 20 wt.%, the dielectric constant and loss of the composites at 1 MHz are 6.18, 2.1 × 10-3 respectively.

Synthesis of monocrystal aluminum nitride nanowires at low temperature

Hexagonal monocrystal aluminum nitride (h-AlN) nanowires are synthesized through the direct reaction of AlCl3 with NaN3 in a nonsolvent system at low temperatures. The h-AlN nanowires are characterized by the high-resolution transmission electron microscope, electron diffraction, x-ray diffraction, and photoluminescence spectra. The analysis shows that the nanowire has a long straight-wire morphology with a diameter ranging from 40to60nm, the longest one up to several micrometers, and they are of pure monocrystal hexagonal or face center structure which has a relatively narrow emission peak, centered at 413nm (3.00eV). In addition, a possible growth mechanism for h-AlN nanowire is discussed.

Properties of niobium nitride coatings deposited by cathodic arc physical vapor deposition

Thin films of niobium nitride were deposited on high speed steel substrate by cathodic arc physical vapor deposition using − 150 V, − 200 V and − 250 V bias voltages at 1 Pa nitrogen pressure. X-ray diffraction data showed that hexagonal δ-niobium nitride was formed for all bias voltages. Fracture cross-sections revealed the existence of the transition zone structure in all coatings. The maximum hardness was found to be 39 GPa. The Rockwell C adhesion and the scratch test were used to compare adhesion properties. In Rockwell adhesion test the best adhesion was obtained at − 150 V bias voltage and in scratch test the cracks were occurred approximately at 3–5 Ns.

Effect of electromechanical coupling on the pressure coefficient of light emission in group-III nitride quantum wells and superlattices

We investigate the influence of the direct and converse piezoelectric effect, i.e., electromechanical coupling (EC), on the strain and the built-in electric field in hexagonal group-III nitride heterostructures under hydrostatic pressure. Particularly, we derive the analytic formulas for pressure dependences of the strain tensor components and the built-in electric field in wurtzite heterostructures, taking into account the EC effect. Then, we calculate pressure coefficients of the light emission, $d{E}_{E}∕dP$, in various $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{Ga}\mathrm{N}$ and $\mathrm{In}\mathrm{Ga}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ superlattices and quantum wells (QWs). Generally, our calculations reveal that taking into account the EC leads to the decrease of the pressure derivative of the built-in electric field in the QW region, which further causes an increase of the $d{E}_{E}∕dP$. The contribution of the EC to $d{E}_{E}∕dP$ depends significantly on the geometry, composition, and strain state of heterostructures. We have found that the largest influence of the EC on $d{E}_{E}∕dP$ is for $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ heterostructures. In $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{Ga}\mathrm{N}$ structures, the contribution of the EC to $d{E}_{E}∕dP$ grows with an increasing well width and Al concentration in the barriers. A larger influence of the EC on $d{E}_{E}∕dP$ is observed for the structures with strained barriers than with strain wells. Interestingly, for $\mathrm{In}\mathrm{Ga}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ heterostructures grown coherently on GaN substrates, the effect of the EC on $d{E}_{E}∕dP$ is negligible.

Synthesis and characterisation of hexagonal molybdenum nitrides

Four hexagonal molybdenum nitrides—three modifications of δ-MoN and Mo 5N 6—were prepared by the plasma-enhanced chemical vapour deposition (PECVD) method and ammonolysis of MoCl 5 and MoS 2. The nitrides were structurally characterised by X-ray diffraction, high-resolution transmission electron microscopy, and selected area electron diffraction. δ 1-MoN is best described by the WC-type structure with stacking faults due to nitrogen atom disorder. Ordering of nitrogen atoms results in δ 2-MoN with the NiAs-type structure. Formation of trigonal molybdenum clusters in δ 3-MoN is responsible for the doubling of the unit cell in a and b directions compared to δ 2-MoN. Mo 5N 6 can be viewed as an intergrowth structure of the WC- and NiAs-type building blocks, accompanied by vacancies on Mo sites. Influence of reaction conditions on the formation of the four nitrides is discussed; their magnetic properties are presented.

Nonlinear elasticity in III-N compounds:Ab initiocalculations

We have studied the nonlinear elasticity effects in zinc-blende and wurtzite crystallographic phases of III-N compounds. Particularly, we have determined the pressure dependences of elastic constants in InN, GaN, and AlN by performing ab initio calculations in the framework of plane-wave pseudopotential implementation of the density-functional theory. The calculations have been performed employing two exchange-correlation functionals, one within the local density approximation and the other within the generalized gradient approximation. We have found that ${\mathrm{C}}_{11}$, ${\mathrm{C}}_{12}$ in zinc-blende nitrides and ${\mathrm{C}}_{11}$, ${\mathrm{C}}_{12}$, ${\mathrm{C}}_{13}$, ${\mathrm{C}}_{33}$ in wurtzite nitrides depend significantly on hydrostatic pressure. Much weaker dependence on pressure has been observed for ${\mathrm{C}}_{44}$ elastic constant in both zinc-blende and wurtzite phases. Further, we have examined the influence of pressure dependence of elastic constants on the pressure coefficient of light emission, $d{E}_{E}∕dP$, in wurtzite $\mathrm{In}\mathrm{Ga}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ and $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{Ga}\mathrm{N}$ quantum wells. We have shown that the pressure dependence of elastic constants leads to a significant reduction of $d{E}_{E}∕dP$ in nitride quantum wells. Finally, we have considered the influence of nonlinear elasticity of III-N compounds on the properties of hexagonal nitride quantum dots (QDs). For typical wurtzite $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ QDs, we have shown that taking into account pressure dependence of elastic constants results in the decrease of volumetric strain in the QD region by about 7%. Simultaneously, the average $z$ component of the piezoelectric polarization in the QDs increases by $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{MV}∕\mathrm{cm}$ compared to the case when linear elastic theory is used. Both effects, i.e., decrease of volumetric strain as well as increase of piezoelectric field, decrease the band-to-band transition energies in the QDs.

Electronic states and physical properties of hexagonal β-Nb 2N and δ′-NbN nitrides

The electronic structure of hexagonal β-Nb 2N and δ′-NbN, and cubic fcc δ-NbN, grown by magnetron sputtering, have been investigated by X-ray photoemission spectroscopy and ellipsometric measurements. The valence band (VB) energy distribution curves (EDC) of these nitrides significantly differ each from each other. The hybridized N2p–Nb4d bands of β-Nb 2N originate a featureless peak centered at 6 eV below the Fermi level, those of the δ′-NbN are characterized by two narrow peaks centered at 5 eV and 6.5 eV. Striking changes are also observed in the EDCs near the Fermi level, these features are associated with the nearly metallic Nb4d states. The dielectrical function of these nitrides reveals structures near the screened plasma edge which correlate well with their associated electronic structure. The dielectric function spectra can be used in the phase identification of the hexagonal and fcc phases. Comparing hexagonal and fcc electronic structures, both β-Nb 2N and δ′-NbN are more covalent that the cubic δ-NbN. The prominent covalent bonding in these hexagonal nitrides can be related to their higher hardness values compared to that of the cubic phase.

Temperature dependence of Raman scattering in hexagonal indium nitride films

We report on Raman spectroscopy study of hexagonal InN thin films grown by metal-organic vapor-phase epitaxy, with the emphasis on frequencies and linewidths of E2(high) and A1(LO) modes in the temperature range from 93to443K. The present InN exhibits a fundamental band gap of ∼1.2eV from photoluminescence and optical transmission spectra, which is in good agreement with the recent suggested parameter for intrinsic InN. The temperature dependence of the E2(high) and A1(LO) phonons can be described well by a model which has taken into account the contributions of the thermal expansion of the crystal lattice, the strain between InN thin films and sapphire substrates, as well as three- and four-phonon coupling. Micro-Raman mapping results also demonstrate the high uniformity of the studied InN.

Single crystal growth of gallium nitride in supercritical ammonia

Gallium nitride single crystals with 2 µm diameters and lengths up to several millimeters were grown in supercritical ammonia. The colorless transparent hexagonal nitride crystals were grown at 600 °C for 4 days with a vertical temperature gradient of 150 °C. Several amide catalysts and temperature gradients were studied. The wurtzite structure (space group P63mc) of the crystals was confirmed by X-ray powder diffraction. The chemical mechanisms of the nitride growth and the transport in supercritical ammonia are discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Raman scattering in hexagonal InN under high pressure

The behavior of the ${E}_{2}$ and ${A}_{1}(\mathrm{LO})$ optical phonons of hexagonal indium nitride under hydrostatic pressure was studied using Raman spectroscopy. Linear pressure coefficients and the corresponding Gr\uneisen parameters for both modes were determined for the wurtzite structure up to $11.6\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, close to the starting pressure of the hexagonal to rocksalt phase transition of InN. Raman spectra acquired within the $11.6\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}13.2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ pressure range suggest that wurtzite InN undergoes a gradual phase transition, and the reverse transformation exhibits a strong hysteresis effect during the downstroke.