Site Competition Effects Research Articles

Characterization of Undoped and Nitrogen-Doped 4H-SiC Thin Films by CVD from Bis(trimethylsilylmethane) Precursor

High-quality 4H-SiC epi layers were grown on 8° off-axis (0001) 4H-SiC substrates by chemical vapor deposition (CVD) using a single precursor, bis(trimethylsilylmethane) at a substrate temperature of 1380°C. The background doping level of the undoped epi layers was reduced by adjusting of the CVD chamber pressure and Si/C ratio. As the chamber pressure decreased from 360 to 180 Torr, the carrier concentration of the undoped epi layers decreased from to cm−3. Moreover, addition of 10 standard cubic centimeters per minute in the chamber at 180 Torr resulted in the reduction of the carrier concentration to cm−3, which can be explained by the well-known site-competition effect. The impurity incorporation effect on macrostep bunching was also discussed based on the strong correlation of atomic force microscopy topologies and impurity concentration. As the flow rate of nitrogen gas increased, the electron concentration of the epi layers linearly increased. With variation of the flow rate, a total n-doping range from to cm−3 was achieved. © 2004 The Electrochemical Society. All rights reserved.

Implantation-induced defects in silicon carbide

The electrical activation of N and P donors in C/N-, Si/N-, C/P- and Si/P-coimplanted and subsequently annealed 4H–SiC samples is investigated by Hall effect. It turns out that doping by coimplantation, which is a process far apart from thermal equilibrium, is different from doping of SiC during growth by the chemical vapor deposition, which is largely governed by the site-competition effect. In case of N and Si coimplantation, the formation of thermally stable and electrically neutral VSi(VC)4 complexes is proposed to serve as a noticeable sink for N atoms. By illumination with photons of different energy, the electrical properties of the dominating intrinsic-related defect centers (E1/E2−/+-, Z1/Z2−/+- and Z1/Z2(3C)) are investigated in 6H–, 4H– and 3C–SiC by deep level transient spectroscopy. In 4H– and 6H–SiC, negative-U-centers are observed. The optical ionization energy of the Z1/Z2−/0-center in 4H–SiC is determined.

Effect of Be++O+ coimplantation on Be acceptors in GaN

P-type regions were produced in undoped GaN films by Be+ and Be++O+ implantation and subsequent annealing at temperatures between 1000 and 1050 °C. From thermal admittance spectroscopic measurements, the activation energy of the Be acceptor level was found to decrease from ∼240 to ∼163 meV by the implantation of additional O atoms, which is in reasonable agreement with the improvement in p-type doping characteristics determined by room-temperature Hall-effect measurements. These results indicate that Be++O+ coimplantation reduces the depth of the Be acceptor level based on a site-competition effect. Therefore, these acceptor levels are most probably attributable to Be atoms at interstitial and Ga-lattice sites.

Epitaxial growth of n-type SiC using phosphine and nitrogen as the precursors

Epitaxial growth of n-type SiC was carried out using PH3 and N2 as the dopant precursors in a chemical vapor deposition system. Thermodynamic simulations were performed to analyze the gas phase composition at the growth conditions. It was shown that the incorporation behaviors of these two dopants are quite different. Compared to nitrogen, phosphorous incorporation has a weaker dependence on flow rate and has a limited site competition effect. Also, phosphorus incorporation for a given precursor flow decreases with temperature while that of nitrogen increases. It was also observed that phosphorous incorporation increases with decreasing C/Si when the ratio is between 0.8 and 3, which suggests phosphorous might incorporate into C site when C/Si is low. Variable temperature Hall measurements were used to characterize the doped layers. Two donor levels at 82–83 and 105–114 meV for phosphorous in 6H–SiC, 50–52 meV, and 78–80 meV for phosphorous in 4H–SiC were resolved. These results were compared to those from ion-implanted samples.

Surface energy, adsorption, and wetting transitions in ternary liquid alloys

Surface energy measurements have been performed on liquid Ga-Sn binary and on Ga-Pb-Sn as well as Ga-Pb-Tl ternary alloys, by the sessile drop technique under ultrahigh vacuum conditions. The purpose of these measurements was to investigate the change in adsorption behavior of Pb in Garich alloys as a consequence of ternary additions. In order to aid in the interpretation of the results, a multilayer model of surface segregation was formulated in the regular solution approximation. The results show evidence of site competition effects between the two solutes, which reduce the surface concentration of Pb in relation to binary Ga-Pb alloys. The effects of Sn on previously studied wetting-related adsorption transitions in Ga-Pb were also investigated. It was concluded that Sn additions are likely to raise the wetting temperature of the Ga-Pb alloy.

Effect of N/Ge co-implantation on the Ge activation in GaN

N-type regions have been produced in undoped GaN films by Ge and N/Ge implantation, sequentially, and subsequent annealing with a SiO2 encapsulation layer at 1300 °C. Improved Ge-doping characteristics have been achieved for GaN by N/Ge co-implantation, attaining activation efficiencies above 95%, whereas in the case of conventional Ge implantation, the activation efficiency is low owing to the generation of N vacancies. In particular, overlapping of the N-implanted region with the Ge one can make the Ge activation higher at a N/Ge ratio of ∼1. Therefore, the co-implantation of additional N atoms drastically enhances the Ge activation based on a site-competition effect.

Surface complexation modeling of carbonate effects on the adsorption of Cr(VI), Pb(II), and U(VI) on goethite.

Dissolved carbonate species are known to affect the sorption behavior of trace species. The macroscopic description of these interactions with a thermodynamic approach has been limited by the lack of data on the binary interaction between carbonate and relevant mineral surfaces. This work follows from two detailed studies of carbonate adsorption on goethite (4, 13). It shows that independent triple-layer surface complexation modeling (TLM) of carbonate adsorption allows successful descriptions of carbonate-trace element ternary sorption on this oxide, using relatively simple and optimal stoichiometries. Carbonate adsorption was considerably enhanced in the presence of Pb(II), despite an invariant total Pb(II) sorption to equilibration with up to 1% C02(g). Both the Pb(II)-carbonate system behavior and the anion-like pH adsorption behavior of U(VI) in the presence of C02 were successfully modeled using binary and ternary metal-bound surface complexes. The significant reduction of Cr(VI) adsorption edges to lower pH values in the presence of C02 was accurately simulated and explained via site competition and surface electrostatic repulsion effects on the predicted inner- and outer-sphere Cr(VI) surface complexes formed. The results of this research are highly relevant to modeling of metal transport field data and of potential soil remediation schemes using carbonate.

Radiation-induced inter-granular segregation in first wall fusion reactor materials

Experimental evidence for phosphorus segregation at grain boundaries in steels is presented. Theories for irradiation-induced inter-granular segregation are described. Non-equilibrium segregation (NES) and rate theory approaches have similar success in predicting phosphorus behaviour in the practically important temperature range although site competition and micro-structural effects are better accounted for by the NES theory. The need for better data on diffusion constants and point defect-impurity binding energy is emphasised.

Effect of C/B sequential implantation on the B acceptors in 4H–SiC

Boron (B) is one of the promising group-IIIA acceptor impurities for SiC and forms a shallow acceptor level (285–390 meV) and deep level (540–720 meV). The B-related deep defect can lead to the degradation of a devices electrical characteristics over the long terms, therefore, a suitable process is needed to suppress the B-related deep defect level, known as the D-center. The C/B sequential implantation technique, a site-competition effect, was applied to suppress the formation of the D-center. The thermal admittance spectroscopy was used to detect the electrically active shallow and deep defect levels introduced by the C/B sequential implantation of 4H–SiC. The C/B sequential implantation was found to be effective in suppressing the formation of the D-center. However, the concentration of the co-implanted C-atoms is very sensitive as the increasing C content leads to the decrease in the active hole concentration by quenching the shallow acceptor level due to the formation of a deep defect level at about 428 meV. A complex microstructure in which C-atom bonds with the B atom at the Si-lattice site was proposed to account for the experimentally observed new B-related deep defect level.

Computer simulation of surface segregation in ternary alloys

This paper addresses the use of Monte Carlo (MC) methods, in conjunction with empirical embedded atom method and Finnis–Sinclair potentials, for modeling surface segregation phenomena in ternary alloys. After a brief overview of these simulation approaches, comparisons are provided between the results of simulations and experimental data on two ternary systems: Cu–Ag–Au and Pb–Bi–Ni. Some comparisons are also made between simulated results and the behavior of the underlying binary Cu–Ag, Cu–Au, Pb–Bi and Pb–Ni systems. The ternary alloys considered are interesting insofar as they display site competition and co-segregation effects which are characteristic of systems containing more than one surface-segregating species. It is shown that the results of the simulations are in qualitative agreement with the experimentally observed trends, but that quantitative agreement in most cases is only fair. Some possible reasons for lack of quantitative agreement are discussed.

Radiation-Induced Grain Boundary Segregation

Experimental evidence for phosphorus segregation and chromium depletion at grain boundaries in steels is presented. Theories for irradiation-induced inter-granular segregation are described. Non-equilibrium (NES) and rate theory approaches have similar success in predicting phosphorus behaviour in the practically important temperature range although site competition and microstructural effects are better accounted for by the NES theory. NES theory predicts chromium behaviour reasonably well but there is a need to consider site competition more closely in relation to large atom effects from elements like Zr and Hf. The need for better data on diffusion constants and point defect-impurity binding energy is emphasised.

A model of compositional surface phase transitions in ternary alloys

A model for calculating the surface excess free energy of segregated ternary alloy surfaces is presented. It is based on nearest neighbor interactions, and also accounts for strain energy effects associated with segregated solutes at the surface. In particular, the model is exercised for the case of PbBiNi alloys, for which compositional surface phase transitions have been observed experimentally. The model displays the observed phase transitions, and is useful in interpreting other features of the experimental temperature dependence of solute surface concentration, such as site competition effects.

Biphasic Synthesis of Poly(2,6-dimethyl-1,4-phenylene oxide) Using a Surface-Active Coupling Catalyst. 2. Process Improvements, Additional Kinetic Results, and Proposed Reaction Mechanism

Significant process improvements, additional kinetic findings, and a proposed mechanism are presented for a previously reported biphasic synthesis of poly(2,6-dimethyl-1,4-phenylene oxide) that avoids the need for a toxic organic solvent. A switch from magnetic stirring to the more vigorous mechanical stirring coupled with the use of an emulsifier increases the rate of formation and molecular weight of the polymer product each by more than 500%. New data indicate that the reaction has an activation energy of 14.7 kcal/mol and a first-, second-, and 0.15-order dependence on the monomer, catalyst, and oxygen concentrations, respectively. The optimal phase ratio for a fast reaction is around 1:1, and the optimal aqueous ammonia concentration is around 0.26 M. The reaction profile shows an unusual accelerating-rate feature that suggests a shifting balance between chain-growth and emulsion stabilization effects on one hand and a site competition effect at the catalyst on the other hand. A mechanism is proposed which gives a rate law in good agreement with the experimental rate law. A successful extension of the biphasic technique to several carbonylation and autoxidation reactions suggests a general utility of the technique for chemical synthesis

Dopant Incorporation Efficiency in CVD Silicon Carbide Epilayers

ABSTRACTIn order to ensure reproducible and reliable SiC semiconductor device characteristics, controlled dopant incorporation must be accomplished. Some of the many factors which greatly influence dopant incorporation are the site-competition effect, SiC(0001) substrate polarity, substrate temperature, and the dopant-source reactor concentration. In this paper, dopant incorporation is considered and compared for various dopants in the context of dopant incorporation efficiency. By using secondary ion mass spectrometry (SIMS), the relative dopant incorporation efficiencies were calculated by dividing the SIMS determined dopant concentration in the resulting epitaxial layer by the intentional gas phase dopant concentration used during the SiC CVD. Specifically, the relative magnitudes of dopant incorporation efficiencies for nitrogen, phosphorus, and boron in 6H-SiC (0001) Si-face epitaxial layers are compared as a function of the site-competition effect and the dopant-source reactor concentrations. This serves as a first approximation for comparison of the relative “doping potencies” of some common dopants used in SiC CVD epitaxial growth.

First results on silicon carbide vapour phase epitaxy growth in a new type of vertical low pressure chemical vapour deposition reactor

In this paper a new concept for silicon carbide vapour phase epitaxy (VPE) will be presented. It is based on the rotating disk technology well known in the field of III–V epitaxy. First results will be given showing excellent structural quality of the epitaxial layers together with high growth rates up to 5 μm/h and good uniformity of layer thickness over 1 inch wafers. The CSi-ratio in the process can be varied over a wide range (0.35-2.3 at 3 μm/h) without loss of surface quality. This enables a precise control of the site competition effect and, therefore, of the incorporation of acceptor and donor impurities. Thus, non-intentionally doped p- (CSi > 1) and n-type (CSi < 0.7) epilayers with carrier concentrations considerably below 1016 cm−3 can be grown.

Site competition effect of impurities and grain boundary stability in iron and tungsten

Fe-base alloys and W are among the most important ultra-high-strength materials. The purpose of this paper is to elucidate the energetics of impurities in Fe and W GBs, and analyze the effect of impurities on the intergranular cohesion at the electron-atom level. A deeper understanding of the cohesion-decohesion processes on the microscopic level will lay a foundation for a 'smart design' of more ductile and tough alloys. In particular, the theoretical analysis of the electron structure and the energetics of Fe and W GBs, both clean (CL) and with impurities, enables one to make important predictions. The calculations have shown that impurities, such as N, O, P, S, Si weaken the intergranular cohesion resulting in 'loosening' of the GBs in Fe and W. The presence of B on the contrary, enhances the interatomic interaction across the GB. The so-called 'site-competition effect' should play an important role affecting impurity distribution in GBs. Among the impurities analyzed, both in Fe and W, B in the GB has the lowest energy, and thus would tend to push other impurity atoms off the GB, thus improving resistance to brittle fracture.

Effect of Impurities on the Electronic Structureof Grain Boundaries and Intergranular Cohesionin Tungsten

ABSTRACTThe cohesion of a grain boundary (GB) is believed to be the controlling factor limiting theductility of high-strength metallic alloys, and particularly W. Intergranular embrittlement isusually associated with segregation of impurities at the GBs. Impurities present in ppmconcentrations can result in a dramatic decrease in plasticity. This paper reviews recent results onboth semi-empirical and first-principles modelling of the energetics and the electronic structuresof impurities on a Σ3 (111) GB in W. Our calculations have shown that impurities, such as N, O,P, S, and Si weaken the intergranular cohesion resulting in “loosening” the GB. The presence ofB and C on the contrary, enhances the interatomic interaction across the GB. The so-called site-competition effect should play an important role affecting impurity distribution in W GBs.Among the impurities analyzed, B in the GB has the lowest energy, and thus would tend todisplace other impurity atoms from the GB. Microalloying with 10-50 ppm B may be an effectiveway of improving tungsten's ductility. These results are important for understanding thefundamental physics of intergranular embrittlement.

Effect of sulphur on the hydrogen trapping phenomena at the grain boundary in iron

The hydrogen trapping phenomena at grain boundaries in the iron-sulphur system were investigated by the hydrogen thermal analysis technique. The specimens were heat treated to produce the same grain size, and the concentration of sulphur at the grain boundaries was controlled by isochronal treatment at various temperatures. The peak height related to the hydrogen released from the grain boundaries decreased with increasing sulphur content at the grain boundaries. It is suggested that the variations in peak height may be attributed to the lowering of the trap binding energy due to sulphur segregation and the site competition effect between the segregated sulphur and hydrogen which are trapped at the grain boundaries.

Adsorption and excess fission XE: Adsorption of XE on vacuum crushed minerals

We report the results of Xe adsorption studies at low partial pressures of Xe (∼2×10−13 atm) for a terrestrial labradorite and lunar anorthosite 15415 which were crushed under high vacuum. The creation of fresh surfaces resulted in a distribution of surface adsorption potentials, with the most energetic, but least abundant, fractions of the surface dominating adsorption behavior at lower temperatures. Atmospheric exposure tends to eliminate the most energetic surface fractions. For sample 15415, surfaces with an adsorption potential as great as 13.7 kcal/mole were created, but these constituted only about 10−6 of the surface area. Observation of the adsorption behavior of this sample over a period of days revealed the gradual disappearance of the most energetic fractions of the surface, probably because of site competition effects caused by sorption of active gases in our high vacuum manifold. Our results consist of values of the Henry constants of Xe adsorption at various temperatures. We have used extrapolations based upon these data to evaluate the possible role of adsorption in the excess fission Xe effect observed in lunar highland gas‐rich breccias, for a model in which excess fission Xe is concentrated by adsorption while diffusing through the regolith. Our experimental results, and estimations of the sorptive properties of the in situ regolith based on the time scale of lunar surface Rn emanations, indicate that adsorption falls about five orders of magnitude short of accounting for the effect. We also infer that the adsorption characteristics of our vacuum‐crushed lunar sample are reasonable analogs to those of in situ lunar soil, so that the failure of adsorption to account for excess fission Xe cannot be attributed to significant differences in sorptive behavior of these materials.

Estimation of tree growth losses caused by pest activity

An improved method of quantitative evaluation of pest-caused reduction of tree growth is described. Radial and height growth rates were measured from discs along the bole of the tree, and growth losses were evaluated by a computer program (IMPACT). The method is independent of site characteristics or competition effects. Use of the method was demonstrated for growth losses in Pseudotsugamenziesii attributable to defoliation by Choristoneuraoccidentalis, and with slight modification it may be applied to a range of tree-pest systems. A three-dimensional graphical procedure was used to display the growth pattern.