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Direct imaging of quantum wires nucleated at diatomic steps

S. I. Molina1,2, M. Varela1, D. L. Sales2, T. Ben2, J. Pizarro3, P. L. Galindo3, D. Fuster4,
Y. González4, L. González4 and S. J. Pennycook1

Appl. Phys. Lett. 91, 143112 (2007)

Controlled growth of semiconductor quantum nano-object arrays is important for reproducible fabrication of optoelectronic devices, and therefore understanding the nature of the nucleation sites during growth, and the role of strain, is of critical need. The sublattice sensitivity of aberration-corrected Z-contrast imaging, combined with elasticity calculations, provides new and quantitative insights. A cross sectional view of an InAsxP1-x nanowire sandwiched between InP layers is easily visible because of the extra intensity from the As. In contrast, the InP barrier layers show weak intensity from the P columns. This contrast difference reveals the exact location of the interface between the InP substrate and the nanowire. It can be seen that the nanowire nucleated at a surface step. The location of the step is slightly off-center from the nanowire, indicating that the initial nucleation took place on the upper terrace of the step.

Nucleation on the upper terrace is entirely in accord with expectations based on the 3.2% lattice mismatch between InAs and InP. Because of the existence of the step, a few unit cells next to the step on the upper terrace can relax sideways, reducing the lattice mismatch, and therefore making this area the preferential nucleation site. Finite element elasticity calculations showed that the magnitude of the relaxation was indeed significant. With the nucleation taking place on the upper terrace the surface of the growing film will roughen, so this represents the onset of the Stranski-Krastanov transition from a two-dimensional to a three-dimensional growth mode. As concentration maps were used as input into finite-element elasticity calculations. The anisotropic As distribution in the nanowire also causes canting in stacked nanowires.The microscopic asymmetry of the As distribution leads to an anisotropy in the stress on the surface of the next InP barrier layer, the peak stress appearing slightly offset from the geometric center of the wire. Therefore, the favorable nucleation site for the next nanowire is also slightly offset from the center of the underlying wire. With carefully controlled conditions all nanowires line up at a specific angle to the growth direction and the observed angle is in quantitative agreement with the finite element calculations.

Molina, S. I., T. Ben, D. L. Sales, J. Pizarro, P. L. Galindo, M. Varela, S. J. Pennycook, D. Fuster, Y. Gonzalez and L. Gonzalez, "Determination of the strain generated in InAs/InP quantum wires: prediction of nucleation sites," Nanotech., 17, 5652-5658 (2006).

1Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge TN 37831, USA.
2Departamento de Ciencia de los Materiales e I.M. y Q.I., Facultad de Ciencias, Universidad de Cádiz, Campus Río San Pedro, s/n, 11510 Puerto Real, Cádiz, Spain.
3Departamento de Lenguajes y Sistemas Informáticos, CASEM, Universidad de Cádiz, Campus Río San Pedro, s/n, 11510 Puerto Real, Cádiz, Spain.
4Instituto de Microelectrónica de Madrid (CNM, CSIC), Isaac Newton 8, 28760 Tres Cantos, Madrid, Spain.


 Oak Ridge National Laboratory