First-Principles Study of Initial Growth of InP Nanowires: Self-Catalytic Effect and Nucleation Mechanism of In Adatoms

We perform systematic density-functional calculations on the self-catalytic effect and the nucleation mechanism of In adatoms on InP(111)B substrate during the initial growth of self-catalyzed InP nanowires. We first determine the equilibrium structure and stability of pure InP(111)B surface. The calculated surface formation energies show that the (2 x 2) surface with P trimer is a stable phase for a wide range of P chemical potential. Based on the P trimer structure, the adsorption of In atoms can induce the desorption of P trimer on InP(111)B surface. The adsorption-desorption phase diagrams as functions of temperature and P-2 pressure indicate that the desorption of P trimer on the (2 x 2) surface without In adatom occurs beyond 970-1190 K, while the desorption with the In adatom does beyond 570-700 K. The P trimer desorption promoted by In adatom suggests the self-catalytic role of In adatom. It can be interpreted by the electron-counting rule. The structural relaxations and the calculated Gibbs free energies of In adatoms for a wide range of coverage indicate that the In adatoms can be stabilized on InP(111)B surface only at a low coverage (Theta(In) <= 1 ML). In contrast, the In adatoms prefer to form the In droplets on the surface when the In coverage is continuously increased. The In adatoms of InP(111)B surface can be nucleated because the bonding interaction between the In adatoms and surface P atoms becomes weak with the increase of In coverage. Once the substrate is heated under In-rich condition, the surface In atoms easily form the In droplets. Our results offer a valuable information to insight into the initial growth of self-catalyzed InP nanowires by the vapor liquid solid mechanism on InP(111)B substrate.