High-temperature Nitridation Research Articles

Effects of Low-temperature NH3 Treatment on the Characteristics of HfO2 ∕ SiO2 Gate Stack

The effects of postdeposition low-temperature treatment (LTN treatment) on the characteristics of the gate stack with the TiN gate electrode were studied in this work. After the films were deposited using an AIXTRON Tricent atomic vapor deposition system, the LTN treatment was performed prior to the postdeposition annealing (PDA) step to prevent the growth of an additional interfacial layer, which is known to accompany the traditional high-temperature nitridation technique. The effective electrical oxide thickness for the devices annealed at 700°C PDA, either with or without LTN treatment, was estimated to be about 2.2 and 2.3 nm, respectively, without considering quantum effects. It was found that the LTN treatment effectively improves the characteristics of the stack gates, such as capacitance-voltage characteristics, frequency dispersion, trap generation rate, and dielectric breakdown voltage even at the high PDA temperature of 700°C.

Electric-field generated by the combustion of titanium in nitrogen

A short temporal electrical impulse (duration of 30–150ms) was generated during the nitridation of mixtures of titanium and titanium nitride by a high temperature moving reaction front. The maximum voltage and current were generated in the combustion front region, in which the conversion of Ti to TiN was incomplete. The electric field (voltage up to 2V and current up to 60mA) decayed and vanished before the maximum combustion temperature was attained. The generation of an electric field during a rapid high-temperature nitridation is most probably due to the different diffusion velocities of charge carriers through the growing titanium nitride shell during the initial stage of the reaction. When the reactant mixture contained a high percentage of pure titanium (larger than 60wt%), partial melting led to irreproducibility in the amplitude and duration of the electrical signal.

High Temperature Rapid Thermal Oxidation and Nitridation of 4H-SiC in Diluted N<sub>2</sub>O and NO Ambient

A high temperature rapid thermal processing (HT-RTP) above 1400oC was investigated for use in the gate oxide formation of 4H-SiC by a cold-wall oxidation furnace. The gate oxide film of ~50nm can be formed for several minutes in the oxidizing atmospheres such as N2O and O2, where the oxidation rates were 8-10nm/min. After the initial oxide formation, the HT-RTPs in various ambient gases were conducted, and the dependences of their MOS interface properties on the gases were evaluated by a capacitance-voltage (CV) measurement. Based on the results, the process sequence of gate oxidation was determined as follows; the initial oxide was formed by the HT-RTO (oxidation) in N2O or in O2 with subsequent post annealing in Ar ambient, and then the HT-RTN (nitridation) in NO was conducted. The total process time becomes 20-50min. The interface trap density (Dit) of fabricated MOS capacitor shows 3-5x1011cm-2eV-1 at Ec-E~0.2eV. The field-effect channel mobility of fabricated 4H-SiC lateral MOSFETs was ~30cm2/Vs.

Characterization of Nitride Layer on 6H-SiC Prepared by High-Temperature Nitridation in NH3

The nitride layers were prepared by direct thermal nitridation of 6H-SiC substrates at 1200–1570°C in a NH3 atmosphere. The layer was characterized by using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Raman scattering spectroscopy. The thickness of the nitride layers prepared at lower than 1400°C was estimated to be less than 10 nm. The higher nitridation temperature resulted in the formation of a thicker surface layer. XPS measurement showed that the surface layer was composed of N, Si, C and O. Peaks corresponding to α-Si3N4 were detected in the Raman spectra and the XRD patterns of the sample prepared at higher than 1500°C, indicating the crystallization of the nitrided layer.

Fe-38Ni-13Co-4.7Nb-1.5Ti-0.4Si超耐熱合金の窒素ガス流雰囲気中の高温窒化挙動

Fe-38Ni-13Co-4.7Nb-1.5Ti-0.4Si超耐熱合金の窒素ガス流雰囲気中の高温窒化挙動

Role of low-temperature (200 °C) nitridation in the growth of GaN by plasma-assisted molecular-beam epitaxy

The effects of sapphire nitridation temperature on the properties of the GaN epitaxial layers grown by rf plasma-assisted molecular-beam epitaxy were investigated. It is found that a nitridation temperature as low as 200 °C acts as a singular point for producing a homogeneous AlN layer on the sapphire substrate. The optical and structural characteristics of GaN epitaxial layers were dramatically improved by exploiting a 200 °C nitridation temperature. Systematic characterization of the nitrided layer, using spectroscopic ellipsometry and x-ray photoelectron spectroscopy, reveals that the sapphire nitridation chemistry, specifically, AlN vs NO production, depends on the surface temperature. A temperature at 200 °C produces a smooth, uniform AlN layer with 6 Å thickness that is close to the critical thickness of AlN on sapphire substrate. This homogeneous AlN acts as diffusion barrier for oxygen during high-temperature growth, as confirmed with secondary ion mass spectrometry measurement. In contrast, a high-temperature nitridation produces a nonhomogenous AlN layer embedded with NO which provides a source for oxygen diffusion into the subsequent GaN epitaxial layer and induces the degraded GaN epitaxial layer. Therefore, improved GaN quality can be traced back to the chemistry of the nitridation.

Role of sapphire nitridation temperature on GaN growth by plasma assisted molecular beam epitaxy: Part I. Impact of the nitridation chemistry on material characteristics

The impact of the nitridation temperature on sapphire/GaN interface modifications and the structural, chemical, and optical properties of GaN epitaxial thin films with N plasma radicals is investigated. Based on ex situ spectroscopic ellipsometry and x-ray photoelectron spectroscopy analysis, it is found that the sapphire nitridation chemistry, specifically AlN versus oxynitride (NO) production, depends on the surface temperature. Nitridation at 200 °C produces a very thin AlN layer with 90% coverage, while high temperature nitridation leads to a 70% coverage of AlN layer containing NO. These initial stages of growth significantly impact the characteristics of the layers following the nitridation step, specifically the low temperature buffer, annealed buffer, and the GaN epitaxial layer. The annealed buffer on a 200 °C nitridation provides a homogeneous GaN thin layer covering most of the sapphire surface. This homogeneous GaN layer after annealing produces a superior template for subsequent growth, resulting in improved structural and optical properties of GaN epitaxial films. On the other hand, the annealed buffer grown on sapphire nitrided at temperatures lower or higher than 200 °C, has islands of GaN nuclei revealing the sapphire substrate, and ultimately, resulting in degraded GaN epitaxial film quality as demonstrated by photoluminescence and x-ray diffraction measurements. The results can be traced back to the chemistry of the nitridation process.

Study on Chemical Treatment and High Temperature Nitridation of Sapphire for III-Nitride Heteroepitaxial Growth

ABSTRACTWe have systematically studied the effects of wet chemical etching and high temperature nitridation on the resulting sapphire surface morphology and chemical transformation. The etching of c-plane sapphire substrates using H2SO4, H3PO4, and a 3:1 H2SO4 :H3PO4 mixture as a function of temperature and etching time was studied and compared with H2 etching at 1100°C and air-annealing at 1400°C. The surface nitridation using NH3 and N2 at 1100°C was studied as a function of NH3 concentration, nitridation time, and surface pretreatment. Atomic force microscopy and x-ray photoelectron spectroscopy were used to study the surface morphology and chemical composition. The detailed surface morphology after chemical etching was a function of the chemical composition and the specific time and temperatures. The smoothest, pit-free sapphire surface was obtained by etching in pure H2SO4 at 300°C for 30 min. Sulfuric acid etching at higher temperatures or for longer periods generated an insoluble mixture of Al2(SO4)3 and Al2(SO4)3·17H2O crystalline deposits on the surface. Phosphoric acid and the 3:1 H2SO4:H3PO4 mixture etched the sapphire preferentially at defect sites and resulted in pits formation on the surface. The high temperature sapphire nitridation resulted in nitrogen incorporation into the surface. The nitrogen content of nitridation layer depends on NH3 concentration, nitridation time, and surface pretreatment. The nitrogen contents of sapphire treated with H2SO4 and 3:1 H2SO4:H3PO4 are about the same as the as-received sapphire. While the nitrogen content of the air-annealed sapphire is ∼1.6 times higher then the nitrogen content of the as-received sapphire.

Effects of the Sapphire Nitridation on the Polarity and Structural Properties of GaN Layers Grown by Plasma-Assisted MBE

The nitridation of the (0001) sapphire surface by a nitrogen rf-plasma source used in GaN molecular beam epitaxy has been investigated. Auger electron spectroscopy measurements were used to estimate the extent of the nitridation effect. Significant nitridation occurred during 100 min at a high temperature (HT) of 700 °C and the nitridated layers usually exhibited three-dimensional islands, while weak nitridation occurred at a low temperature (LT) of 165 °C and resulted to atomically flat surfaces. Linear time dependence has been found for the amount of HT nitridation. High nitridation temperature resulted in Ga-face and low temperature in N-face polarities of overgrown GaN films. Electron microscopy observations revealed that the N-face material exhibited a superior crystalline quality although presented higher surface roughness.

Ion microscope investigations of non-uniform surfaces of thin SiO 2 films produced by high-temperature nitridation experiments

Elemental composition and surface changes of thin high-temperature nitrided SiO 2 films were studied with ion microscopy using 0.92 and 1.4 MeV deuterium ions. Surprisingly, a non-uniform artificial pattern on the surface of samples annealed above 1150°C was observed. Nuclear reaction microprobe analysis and scanning electron microscopy were applied to reveal the origin and composition of the pattern. The surface topology and the maps of the light elements were studied. The backscattered signals in the NRA data provided maps of Si and variations in the elemental compositions at the surface of the specimens. With both techniques, significant differences in the elemental make-up within the pattern were obtained. The light elements were found to be enriched differently in the centre of the non-uniform patterns and the surrounding ring-like structures, suggesting the formation of compounds of C, N, O and Si.

MBE growth of different hexagonal GaN crystal structures on vicinal (100) GaAs substrates

GaN thin films of different hexagonal crystal structures have been grown by radio-frequency nitrogen plasma source molecular beam epitaxy (RFMBE) on vicinal (100) GaAs substrates. Polycrystalline hexagonal material occurred for high temperature (630°C) nitridation of the GaAs surface or low temperatures of the initial GaN buffer layer deposition. On the contrary, initial GaN growth at 540°C gave hexagonal single crystals with [0001] axis either inclined at approximately 43° from the growth axis or aligned parallel to it. The GaN orientation depended on the annealing or not, respectively, of the initial low temperature buffer layer.

High temperature–high pressure nitridation of β-Ti 63Nb 37 by means of the chemical oven technique

High temperature nitridation of a β-Ti 63Nb 37 (47 wt.% Ti) alloy was carried out by means of the chemical oven technique at 100 and 400 bar nitrogen pressure. Afterwards nitridation samples showed a golden colour, typical of the δ-(Ti,Nb)N solid solution. Samples were characterized by XRD and EDS analysis, SEM and OM observation and superconductive transition temperature measurements. Microstructure observation reveals that both inward diffusion of nitrogen and long-range Nb–Ti interdiffusion in the β-(Ti,Nb) alloy take place during nitridation. At higher temperature an external δ-(Ti,Nb)N layer grew, while internal nitridation was confined at grain boundaries. At lower temperature internal nitridation lead to the formation of elongated Ti-rich δ-(Ti,Nb)N precipitates that grew beneath the external thin layer constituted by δ-(Ti,Nb)N. Nb enrichment of the δ-(Ti,Nb)N solid solution was detected near the gas/nitride interface.

Phase reactions in the Nb–N system below 1400°C

In continuation of a recent study on high-temperature nitridation of niobium the phase equilibria of the Nb–N system were investigated for T≤1400°C by means of diffusion couples, electron probe microanalysis (EPMA) and differential scanning calorimetry (DSC). The γ-Nb 4N 3± x → δ-NbN 1− x phase transition was investigated as a function of composition and occurs at temperatures between 1070°C (45.2 at.% N) and 1225°C (38.9 at.% N). The equilibrium composition of γ-Nb 4N 3± x in this temperature interval is—depending on the temperature—only 42–44.0 at.% N, hence the transition occurs also in samples with a non-equilibrium composition. It can only be investigated by using in situ methods such as DSC and high-temperature X-ray diffraction because of the fast transition rate which does not permit quenching and which is responsible for the contradictory results in the literature. The congruent transformation η-NbN→ δ-NbN 1− x was observed between 1300 and 1320°C. Homogeneity ranges were measured from the phase bands of the diffusion couples by means of EPMA and the results are presented in the form of a phase diagram.

High temperature gas nitridation and wear resistance of TiAl based alloys

ABSTRACTGas nitridation of TiAl based alloys in an ammonia atmosphere was carried out in the present work. The nitride layers were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The evaluation of the surface hardness and wear resistance was performed to compare with those of the non-nitrided alloys. It is concluded that the nitride layers are composed of Ti2AlN as the inward-growing layer and TiN as the outward-growing layer. The nitridation temperature and time were two major factors influencing the thickness of the nitride layers of the alloys. The high temperature nitridation raised the surface hardness and the wear resistance of the TiAl based alloys markedly. The tribological behaviors of the nitrided alloys were also discussed.

High-temperature nitridation of the intermetallic compound TiAl at 1000-1200 K

The nitridation process of MJ12 (Ti-47Al-2Nb-2Cr) and MJ47 (Ti-47Al-2Nb-2Mn + 0.8TiB 2 ) in an atmosphere of purified ammonia has been investigated over the temperature range 1000-1200 K. The nitrided alloys showed improvement of hardness and wear resistance in this environment. At 1200 K nitridation, Knoop hardness values of MJ12 and MJ47 were 1283.6 and 1391.3 kg mm -2 , respectively, and the decrease in wear rate of both alloys was as much as 99%. By using x-ray diffraction, TiN was detected on the surface of the alloys nitrided ≥1100 K.

High-temperature nitridation of Nb–Ti alloys in nitrogen

Microstructure evolution, phase composition, weight gain and layer growth kinetics of Nb–Ti alloys (10, 47, 63 and 90 at.% Ti) annealed in high-purity nitrogen atmosphere (0.3, 3 and 30 bar) were studied in the temperature range 1300–1600°C. After nitridation, the formation of an external compact nitride layer as well as extensive internal nitride precipitation was observed. The overall nitridation kinetics (weight gain) is invariably parabolic; a deviation from the initial rate law is observed at 1450 and 1600°C for the longer reaction times when the alloy core approaches nitrogen saturation and internal precipitation slows down. The parabolic rate constant is strongly affected by the Nb content in the alloy. The phases detected in the reacted samples are isostructural with those of the Nb–Ti, Nb–N and Ti–N systems. The surface nitride was δ-(Ti,Nb)N in any case. The morphology of the internal nitridation zone corresponds to the growth of large, oriented, nitride needles for the three alloys richer in titanium. The needles are composed of α-(Ti,Nb)(N) in the case of Ti90Nb10 alloy and of δ-(Ti,Nb)N1−x in the case of Ti63Nb37 and Ti47Nb53. Such a microstructure is evidence for nucleation difficulty; coarsening of the existing particles is favoured in comparison to the formation of new precipitates. Homogeneous nucleation is hindered by the small chemical Gibbs free energy available and the elastic strain energy related to volume misfit. After initial reaction, microstructure evolution is mainly determined by the fast inward diffusion of nitrogen and the slow Nb–Ti interdiffusion in the β-(Ti,Nb) alloy. Internal nitridation of Ti10Nb90 at 1450 and 1600°C leads to the formation of fine and numerous precipitates of β-(Nb,Ti)2N. In this case heterogeneous nucleation along grain boundaries and dislocation lines is the prevailing mechanism.

Transition from parabolic to linear kinetics in the high-temperature nitridation of niobium

The nitridation behavior of pure niobium wasinvestigated in nitrogen gas over the pressure range of10-2 to 1 torr and over the temperature range1600-2000°C. A surface layer of Nb2Nformed at 1 and 10-1 torr, obeying parabolickinetics for the former and linear kinetics for thelatter. This transition in time dependency firstmanifested itself at 1 torr in nonlinear plots ofln(Kc) vs. 1/T, consistent with an adsorption-controlledreaction. An evaluation of the flux of nitrogen acrossthe gas-metal interface is presented, comparing thekinetics of gas arrival based on the kinetic theory of gases with gas removal because of inwarddiffusion into the metal. Data from the literature areused to estimate these adsorptive and diffusive fluxesand justify the observed behavior. Nitridation ofniobium at 10-2 torr resulted in gasdissolution only, with no nitride layer beingformed.

Plasma cleaning and nitridation of sapphire substrates for AlxGa1−xN epitaxy as studied by x-ray photoelectron diffraction

The influence of plasma and thermal treatments on the structure and composition of sapphire (00.1) surfaces has been studied by hemispherically recorded x-ray photoelectron spectroscopy and diffraction (XPD) to better understand the surface conditioning during the growth initiation procedure for the epitaxy of GaN. A treatment involving an O2 plasma generated by electron cyclotron resonance is shown to efficiently remove surface contamination and to produce well-ordered surfaces. After a thermal treatment in vacuum the sapphire (00.1) surface becomes terminated by Al atoms. AlN films with good short-range order are obtained by a simple high temperature nitridation step in the metal organic chemical vapor deposition reactor. A novel direct crystallographic method termed CHRISDA is employed for the analysis of the XPD data. By analysis of the XPD patterns the crystal structure of sapphire is found to extend into the probed near surface region without significant perturbations. The AlN layers formed by thermal nitridation were found to crystallize in the hexagonal phase and to be terminated by nitrogen atoms.

Ti-Al合金の高温窒化に対する微量酸素とSi添加の影響

Ti-Al合金の高温窒化に対する微量酸素とSi添加の影響

Plasma Cleaning and Nitridation of Sapphire Substrates for AlxGa1-xN Epitaxy as Studied by ARXPS and XPD

ABSTRACTThe influence of plasma and thermal treatments on the structure and composition of sapphire (00–1) surfaces have been studied by hemispherically recorded x-ray photoelectron spectroscopy in view of substrate preparation for the epitaxy of GaN. Producing well-ordered surfaces, O2 plasma based treatments are found to efficiently remove surface contamination. AlN films with good short-range order are obtained by a simple high temperature nitridation step in the MOCVD reactor.