• Jian-Min Zhang

      Articles written in Pramana – Journal of Physics

    • Surface segregation of the metal impurity to the (100) surface of fcc metals

      Jian-Min Zhang Bo Wang Ke-Wei Xu

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      The surface segregation energies for a single metal impurity to the (100) surface of nine fcc metals (Cu, Ag, Au, Ni, Pd, Pt, Rh, Al and Ir) have been calculated using the MAEAM and molecular dynamics (MD) simulation. The results show that the effect of the surface is down to the fourth-layer and an oscillatory or monotonic damping $(|E_{1}| > |E_{2}| > |E_{3}| > |E_{4}|)$ phenomenon in segregation energy has been obtained. The absolute value of the segregation energy $E_{1}$ for a single impurity in the first atomic layer is much higher than that in the nether layers. Thus, whether the surface segregation will work or not is mainly determined by $E_{1}$ which is in good relation to the differences in surface energy between the impurity and host crystals $\Delta Q = Q_{\text{imp}} - Q_{\text{hos}}$. So we conclude that an impurity with lower surface energy will segregate to the surface of the host with higher surface energy.

    • Self-interstitial configuration in molybdenum studied by modified analytical embedded atom method

      Jian-Min Zhang Zhang Fang Wang Ke-Wei Xu

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      The stability of various atomic configurations containing a self-interstitial atom (SIA) in a model representing Mo has been investigated using the modified analytical embedded atom method (MAEAM). The lattice relaxations are treated with the molecular dynamics (MD) simulation at absolute zero of temperature. Six relatively stable self-interstitial configurations and formation energies have been described and calculated. The results indicate that the [1 1 1] dumbbell interstitial S111 has the lowest formation energy, and in ascending order, the sequence of the configurations is predicted to be S111, C, S110, T, S001 and O. From relaxed displacement field up to the fifth-NN atoms of six configurations, we know that the relaxed displacements depend not only on separation distances of the NN atoms from the defect centre but also strongly on the direction of the connected line between the NN atoms and the defect centre. The equilibrium distances between two nearest atoms in the core of the S111, C, S110, T, S001 and O configurations are $0.72a, 0.72a, 0.71a, 0.72a, 0.70a$ and $0.70a$, respectively.

    • Structural stability and theoretical strength of Cu crystal under equal biaxial loading

      Jian-Min Zhang Zhong-Liang Lin Yan Zhang Vincent Ji

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      Cu has been used extensively to replace Al as interconnects in ULSI and MEMS devices. However, because of the difference in the thermal expansion coefficients between the Cu film and the Si substrate, large biaxial stresses will be generated in the Cu film. Thus, the Cu film becomes unstable and even changes its morphologies which affects the device manufacturing yield and ultimate reliability. The structural stability and theoretical strength of Cu crystal under equal biaxial loading have been investigated by combining the MAEAM with Milstein-modified Born stability criteria. The results indicate that, under sufficient tension, there exists a stress-free BCC phase which is unstable and slips spontaneously to a stress-free metastable BCT phase by consuming internal energy. The stable region ranges from −15.131 GPa to 2.803 GPa in the theoretical strength or from −5.801% to 4.972% in the strain respectively.

    • Atomistic simulation of the point defects in TaW ordered alloy

      Zhong-Liang Lin Jian-Min Zhang Yan Zhang Vincent Ji

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      Combining molecular dynamics (MD) simulation with modified analytic embeddedatom method (MAEAM), the formation, migration and activation energies of the point defects for six-kind migration mechanisms in B2-type TaW alloy have been investigated. The results showed that the anti-site defects TaW and W$_{\text{Ta}}$ were easier to form than Ta and W vacancies owing to their lower formation energies. Comparing the migration and activation energies needed for six-kind migration mechanisms of a Ta (or W) vacancy, we found that one nearest-neighbour jump (1NNJ) was the most favourable because of its lowest migration and activation energies, but it would lead to a disorder in the alloy. One next-nearest-neighbour jump (1NNNJ) and one third-nearest-neighbour jump (1TNNJ) could maintain the ordered property of the alloy but required higher migration and activation energies. So the 1NNNJ and 1TNNJ should be replaced by straight [100] six nearestneighbor cyclic jumps (S[100]6NNCJ) (especially) or bent [100] six nearest-neighbour cyclic jumps (B[100]6NNCJ) and [110] six nearest-neighbor cyclic jumps ([110]6NNCJ), respectively.

    • Electronic structures of the F-terminated AlN nanoribbons

      Yu-Ling Song Dao-Bang Lu Ben-Liang Cui Jian-Min Zhang

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      Using the first-principles calculations, electronic properties for the F-terminated AlN nanoribbons with both zigzag and armchair edges are studied. The results show that both the zigzag and armchair AlN nanoribbons are semiconducting and nonmagnetic, and the indirect band gap of the zigzag AlN nanoribbons and the direct band gap of the armchair ones decrease monotonically with increasing ribbon width. In contrast, the F-terminated AlN nanoribbons have narrower band gaps than those of the H-terminated ones when the ribbons have the same bandwidth. The densityof-states (DOS) and local density-of-states (LDOS) analyses show that the top of the valence band for the F-terminated ribbons is mainly contributed by N atoms, while at the side of the conduction band, the total DOS is mainly contributed by Al atoms. The charge density contour analyses show that Al–F bond is ionic because the electronegativity of F atom is much stronger for F atom than for Al atom, while N–F bond is covalent because of the combined action of the stronger electronegativity and the smaller covalent radius.

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