Atomic Level Simulation of Ridge Reconstruction and Passivation in GaAs Nanopillars

State-of-the-art semiconductor nanopillars are an exciting new topic, due to their unique properties that can be used in a number of microelectronic applications.  Nanopillars can be grown into microarrays that can maximize surface to volume ratio, ideal for photo-voltaics^{1, 2}.  Recently, it was shown that sulfur passivation with sulfur, improved GaAs nanopillar conversion efficiency 2-3 times².  A theoretical understanding on how sulfur can remove mid-gap states in GaAs nanopillar is needed to further improve this process.

    In this work, we report our density functional theory (DFT) modeling³ result of 18 different hexagonal shaped GaAs nanopillar that is periodic in the vertical direction.  Our model examines energetics and band gaps of the pillars to form a theory on the chemical mechanism for sulfur passivation on nanopillars.  Three different stacking are studied, wurtzite, zinc blende, and polytype (ABCB) which contains 50% wurtzite and 50% zinc blende.  The nanopillars also differ by their diameter and the way they terminate at the ridges.  We theorize that sulfur passivation is mostly effective on the ridges of the nanopillars and that stacking faults play a role in the mid-gap states that exist before passivation. 

    Further study of our electronic structures, looking at the density of states, show mid-gap states inherent in GaAs ridges passivated by the presence of sulfur.  This is because there are dangling bonds along the ridges of the nanopillar that exists when there are stacking faults.  We look at many different ways sulfur can react with the ridges.  The energetically favorable configuration satisfies the octet rule and eliminates most of the midgap states.

Figure 1:  Geometry and density of State for polytype (ABCB) nanopillar before and after sulfur passivation.

References:

1.            G. Mariani, A. C. Scofield, C. H. Hung and D. L. Huffaker, Nat. Commun. 4 (1497), doi:  10.1038/ncomms2509 (2013).

2.            G. Mariani, R. B. Laghumavarapu, B. T. de Villers, J. Shapiro, P. Senanayake, A. Lin, B. J. Schwartz and D. L. Huffaker, Appl Phys Lett 97 (1), 013107 (2010).

3.            T. H. Yu, L. Yan, W. You, R. B. Laghumavarapu, D. L. Huffaker and C. Ratsch, Appl Phys Lett 103, 173902 (2013).