Evolution of microstructures during rapid crystallization of liquid GaAs
2017
The technological importance of compound semiconductor GaAs are
increasing because of their use in optoelectronic and microelectronic
applications. Due to the high conversion efficiency and carrier mobility,
GaAs can also be applied in solar cells and the recent study upon
GaAs nanowires and their heterostructures has revealed that the conversion
efficiency of GaAs nanowire array solar cells conversion is high up
to 15.3%. Early the liquid and amorphous properties of GaAs were investigated
by employing the first-principles calculations. The emergence of semi-empirical
potential and the improvement of computer level have promoted the
research and application of molecular dynamics (MD) simulation. MD simulation has now become one of the typical modeling methods
at the molecular scale. The simulation is based on the known physical
approximation of all particles in the system to solve the equation
of motion, and obtain the atomic motion trajectory. Analytical potentials
is very important in MD simulation as it is not feasible to solve
the Hamiltonian by means of quantum-mechanical methods with huge computational
complexity. Abell-Tersoff potential function is a short-ranged bond-order
algorithm, which depends on bond lengths and bond angles and hence
accesses information about the atomic structure. So it is suitable
for simulating covalent bond species. Generally used for the IV elements
and compounds like silicon, carbon, and others, but for the III-V
compound semiconductor it is not very accurate due to the ionic bonds.
Usually the modified tersoff potential, by the addition of Coulomb
term, the modified exclusion potential and the truncation parameter,
is used to simulate such semiconductor materials. Many studies on the bulk, surface and elastic properties of GaAs
by means of MD method, are in good agreement with the experimental
results. In this paper Karsten Albe’s Tersoff potential is
adopted as it allows one to model a wide range of properties of GaAs
compound structure. GaAs has two kinds of tetrahedral crystal structure,
namely, Zinc-blende and Wurtzite, the former structure is more stable
under normal conditions. But when reduced to a nanoscale scale, Wurtzite
structure becomes stable. Different structures have distinct properties,
similar to carbon and grapheme. But so far, there is no report on
the evolution of the microstructure and the specific crystalline structure
of GaAs during crystallization under rapid cooling. In this study, MD simulation was performed for liquid GaAs at the
cooling rate 1×10 10 K/s. The pair distribution function,
the total energy per atom, the bond angle distribution function, the
dihedral angle distribution and visualization method were used to
analyze the variations of microstructure during the solidification
process. Results show that the onset temperature of crystallization
of GaAs liquid is 1460 K. The random network is the essential structural
feature of liquid. The rapidly cooled crystallization is Zinc-blende
based polycrystalline structure, with the grain boundary in a eutectic
twin structure is a layer of wurtzite structure. At temperature below
520 K, part of As atoms segregate into simple cubic structure As 8 .
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