DFT study of Pt-induced Ge(001) reconstructions

📝 Abstract

Pt deposited on a Ge(001) surface spontaneously forms nanowire arrays. These nanowires are thermodynamically stable and can be hundreds of atoms long. The nanowires only occur on a reconstructed Pt-Ge-surface where they fill the troughs between the dimer rows on the surface. This unique connection between the nanowires and the underlying substrate make a thorough understanding of the latter necessary for understanding the growth of the nanowires. In this paper we study possible surface reconstructions containing 0.25 and 0.5 of a monolayer of Pt. Comparison of calculated STM images to experimental STM images of the surface reconstruction reveal that the Pt atoms are located in the top layer, creating a structure with rows of alternating Pt-Ge and Ge-Ge dimers in a c(4x2) arrangement. Our results also show that Pt atoms in the second or third layer can not be responsible for the experimentally observed STM images.

💡 Analysis

Pt deposited on a Ge(001) surface spontaneously forms nanowire arrays. These nanowires are thermodynamically stable and can be hundreds of atoms long. The nanowires only occur on a reconstructed Pt-Ge-surface where they fill the troughs between the dimer rows on the surface. This unique connection between the nanowires and the underlying substrate make a thorough understanding of the latter necessary for understanding the growth of the nanowires. In this paper we study possible surface reconstructions containing 0.25 and 0.5 of a monolayer of Pt. Comparison of calculated STM images to experimental STM images of the surface reconstruction reveal that the Pt atoms are located in the top layer, creating a structure with rows of alternating Pt-Ge and Ge-Ge dimers in a c(4x2) arrangement. Our results also show that Pt atoms in the second or third layer can not be responsible for the experimentally observed STM images.

📄 Content

DFT study of Pt-induced Ge(001) reconstructions Danny E. P. Vanpoucke and Geert Brocks Computational Materials Science, Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands (Dated: November 11, 2021) Pt deposited on a Ge(001) surface spontaneously forms nanowire arrays. These nanowires are thermodynamically stable and can be hundreds of atoms long. The nanowires only occur on a re- constructed Pt-Ge-surface where they fill the troughs between the dimer rows on the surface. This unique connection between the nanowires and the underlying substrate make a thorough under- standing of the latter necessary for understanding the growth of the nanowires. In this paper we study possible surface reconstructions containing 0.25 and 0.5 of a monolayer of Pt. Comparison of calculated STM images to experimental STM images of the surface reconstruction reveal that the Pt atoms are located in the top layer, creating a structure with rows of alternating Pt-Ge and Ge-Ge dimers in a c(4×2) arrangement. Our results also show that Pt atoms in the second or third layer can not be responsible for the experimentally observed STM images. PACS numbers: 68.35.-p, 61.72.uf, 68.35.B-, 68.37.Ef I. INTRODUCTION Metal/semiconductor interface interactions continue to attract interest in both fundamental and applied re- search, and are of vital importance for the semiconduc- tor and microelectronics industry. With miniaturization pushing into the nanoscale regime, new ways of con- structing nanoscale devices are investigated. Bottom- up approaches such as atomic size reconstructions, self assembly and metal nanowires (NWs) are investigated as alternatives for the usual top-down techniques, such as etching and lithography.1 Generally, the focus lies on metal/Si interfaces because of the importance of Si in the semiconductor industry. Of these, the Au/Si(001) inter- face, due to the high conductivity of Au, is one of the most well studied interfaces. For this system, it has been shown that different types of reconstructions appear depending on the Au cover- age and the annealing temperature.2–4 The metal/Ge in- terface is less well studied, notwithstanding its impor- tance in the development of radiation detector systems and high speed electronic devices. Recent experimen- tal studies of the Pt/Ge(001) interface, done at the sub- monolayer Pt coverage regime, have shown the formation of self-organized Pt NW arrays after high temperature annealing.5–7 It has been suggested that this reconstruc- tion was due to a strengthened interaction of the 5d met- als with the Ge surface: relativistic mass effects which contract the s shell, reducing its energy and thus increase the s occupancy at the expense of the d electrons. This partially depletes the antibonding d states, strengthen- ing the d bonds.8 The same mechanism was thought to be responsible for the one dimensional chain structures of Au on Ge.9,10 There certainly are similarities between the growth of Au and Pt chains on Ge(001), such as the appearance of dimer vacancies before the chains appear. Yet, there are also important differences between the two systems. The Pt NWs only form after annealing at over 1000 K, while the Au chains appear after deposition at 675 K and disappear into the bulk Ge when annealed at 1000 K.5,10 Moreover, while the Pt NWs have a thickness of a single atom, the Au chains are dimer rows containing Au-Au or Au-Ge dimers.9 The Pt NWs are thermody- namically stable and their length is only limited by the size of the underlying terrace. This so-called β-terrace is a previously unknown Pt-Ge-surface reconstruction of which, to date, only a tentative model proposed by G¨url¨u et al. exists.5 The unique connection between this ter- race and the NWs makes a thorough understanding of the reconstruction desirable before one can study and understand the NW arrays and the physical phenomena associated with them.11,12 In this work, we present a geometry for the β-terrace based on the comparison of the simulated scanning tun- neling microscope (STM) images (henceforth referred to as pseudo-STM images) to experimental STM images. The current experimental information on the β-terraces is very limited. At the moment of writing only STM im- ages are available. A preliminary account of this work appeared in Ref. 11. The structure of this paper is as follows: In Sec. II we describe the theoretical method and introduce the no- tation used for the different geometries. In Sec. III we present our theoretical results starting with a compari- FIG. 1: Top view of the Ge(001) surface with symmetric (2×

1. reconstruction. 0–7 are indexes for the surface dimer atoms and are used in the nomenclature of the geometries (see text). arXiv:0901.2969v2 [cond-mat.mtrl-sci] 1 Feb 2011 2 son of the formation energies of the different geometries followed by a general review of the pseudo-STM images. We identify one pa

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