G. Shao

On the oxidation behaviour of MoSi2

Abstract The oxidation behaviour of MoSi 2 has been analyzed on the basis of thermodynamics and kinetics. Thermodynamically speaking silica is in equilibrium with any of the stable molybdenum silicides. At low temperatures, the limited bulk diffusion in MoSi 2 makes it impossible to sustain a sufficient supply of silicon for the formation of a continuous silica layer. This leads to the simultaneous-oxidation of silicon and molybdenum, forming a scale of mixed oxides. The formation of molybdenum oxides leads to ruptures owing to the significant volume change and accelerates the oxidation, leading to the pest phenomenon.

Thermodynamic reassessment of the Mo–Si and Al–Mo–Si systems

The thermodynamic properties and phase diagrams of the Mo‐Si and Al‐Mo‐Si systems are assessed, and a complete thermodynamic description is obtained. The calculation results show good agreement with the experimental data. The high temperature phase equilibria involving the C40 and C11b MoSi2 structures in the Mo‐Si binary and Al‐Mo‐Si ternary systems have been clarified. # 2000 Elsevier Science Ltd. All rights reserved.

On the ω phase formation in Cr-Al and Ti-Al-Cr alloys

Abstract The interaction between Al and the transition metals Ti and Cr on the stability of the ω phase in metastable β-based structures was studied. Alloys were quenched from the melt to retain at room temperature a metastable β phase (B2 structure), which is stable at high temperatures. The structural study of the ω phase was carried out by correlating the deviation of ω structure from the ideal ω phase to the compositions of the parent β phase. Deviation of ω structures from the ideal one was related to the electron concentration of the parent β phase. A diffuse ω structure is reported in the Cr2Al phase (C11b structure) for the first time. The results are consistent with our previous suggestions that Al stabilises the ω phase in transition metals by lowering the spatial conduction electron concentration in the parent β phase and by enhancing p–d hybridisation of valence electrons. In the ternary Ti–Al–Cr alloys, prolonged annealing of the Ti–30Al–10Cr and Ti–20Al–10Cr alloys at 450°C led to the formation of two types of ordered crystalline ω structure.

ω-phase formation in V[sbnd]Al and Ti[sbnd]Al[sbnd]V alloys

Abstract ω-phase formation in V-50 at.% A1 alloy and Ti[sbnd]Al[sbnd]V alloys has been studied using electron diffraction. It is shown that both electron and size factors are important in affecting the magnitude of the deviation of diffuse ω maxima from the commensurate ω maxima positions. It is suggested that the addition of the electron-deficient element Al to the later transition metal V will destabilize the high-temperature β-phase and favour ω formation. It is expected by analogy that the w transformation may occur in other later transition-metal based alloy systems such as Nb[sbnd]Al, Mo[sbnd]Al, Cr[sbnd]Al and Mn[sbnd]Al. Structural models have been proposed to interpret the heavy streaking effects in β + ω and B2 + ω structures.

Engineering of boron-induced dislocation loops for efficient room-temperature silicon light-emitting diodes

We have studied the role of boron ion energy in the engineering of dislocation loops for silicon light-emitting diodes (LEDs). Boron ions from 10to80keV were implanted in (100) Si at ambient temperature, to a constant fluence of 1×1015ions∕cm2. After irradiation the samples were annealed for 20min at 950°C by rapid thermal annealing. The samples were analyzed by transmission electron microscopy and Rutherford backscattering spectroscopy. It was found that the applied ion implantation∕thermal processing induces interstitial perfect and faulted dislocation loops in {111} habit planes, with Burgers vectors a∕2⟨110⟩ and a∕3⟨111⟩, respectively. The loops are located around the projected ion range, but stretch in depth approximately to the end of range. Their size and distribution depend strongly on the applied ion energy. In the 10keV boron-implanted samples the loops are shallow, with a mean size of ∼30nm for faulted loops and ∼75nm for perfect loops. Higher energies yield buried, large, and irregularly shape...

On the crystallographic characteristics of ion beam synthesised β–FeSi2

Nanometre-scale β–FeSi2 precipitates were introduced in a single crystal silicon substrate by low-dose ion-beam synthesis (IBS). The crystallographic relationship between these nanometre β precipitates and the silicon substrate has been studied by high resolution electron microscopy (HREM). The results show that the orientation relationship (OR) between the nanometre β precipitates and the silicon substrate is [100]β//[110]Si and (001)β//(11)Si, with abnormally large strain between the precipitates and the substrate. This OR is important for the formation of 90°-OD boundaries within β–FeSi2 grains. Also, the relationship between various reported low-index ORs has been analysed and a new low-index OR is predicted.

Effect of implantation temperature on dislocation loop formation and origin of 1.55-μm photoluminescence from ion-beam-synthesized FeSi2 precipitates in silicon

Iron implantation into Si using a metal vapor vacuum arc ion source has been performed at various temperatures to synthesize nanometer scale β-FeSi2 precipitates in Si. Transmission electron microscopy (TEM) results showed that for high-temperature-implanted samples, there were a large number of dislocation loops formed. For intermediate-temperature-implanted samples, only a row of dislocation loops located at the end of implantation range was observed. For low-temperature-implanted samples, however, no dislocation loops were observed at all. From the differences in the photoluminescence spectra, in conjunction with the TEM results, the origins of the photoluminescence peaks in different samples could be distinguished and identified to be from β-FeSi2 precipitates or from crystal defects in the samples.

Metastability of the o-phase in transition-metal aluminides: First-principles structural predictions

Abstract A systematic total-energy study has been performed on the ω-phase of transition-metal-aluminide-based alloys using first-principles electronic structure calculations. The calculated o-phase heat of formation for ω-phase against the electron per atom ratio e/a is found to show the same trends as the measured diffuse ω peak for alloys with values of e/a between 3·3 and 4·7. Moreover, we predict that the ω-phase is the most stable amongst competing metastable phases for NbAl. ω collapse studies show a strong correlation between this transformation and a mechanical instability in the related B2 alloys at low temperatures. A partial ω-type shuffle is also predicted for Ni2Al alloys with e/a values close to 7 in the B82 structure type. As a result of these calculations, we are now able to study the phase diagrams of structurally important ternary alloys such as Ti[sbnd]Al[sbnd]V of Ti[sbnd]Al[sbnd]Nb.

Metastable γ phase in ion beam synthesized FeSi2

Ion beam synthesized iron silicide (FeSi2) (200 keV, 350 °C, 4×1017 Fe+/cm2) was studied by transmission electron microscopy. Face‐centered cubic FeSi2 (γ phase) was observed in the as‐implanted sample and the samples annealed at temperatures up to 600 °C. It is suggested that the formation of the equilibrium semiconducting FeSi2 (β phase) is preceded via a transition γ phase due to its better lattice match with the silicon matrix. The absence of metallic FeSi2 (α phase) can be attributed to the high iron dose implantation.

Transmission electron microscopy observation of high-temperature γ-FeSi2 precipitates formed in Si by iron implantation using a metal vapor vacuum arc ion source

This work reports the observation of high-temperature γ-FeSi2 precipitates of tens of nanometers in diameter embedded in silicon formed by iron implantation using a metal vapor vacuum arc ion source followed by a dual step annealing process. It was found that the implantation temperature and annealing conditions played important roles on the shape and phase formation of the FeSi2 precipitates. When the implantation temperature was high (about 380 °C), only β-FeSi2 precipitates were formed. When the implantation temperature was low (about −100 °C), after the dual step annealing, in addition to β-FeSi2, γ-FeSi2 precipitates coherent with the silicon substrate were formed.