Growth of one-dimensional III–V structures on Si nanowires and pre-treated planar Si surfaces H. Detz a,Ã , P. Klang a , A.M. Andrews a,b , A. Lugstein b , M. Steinmair b , Y.J. Hyun b , E. Bertagnolli b , W. Schrenk a , G. Strasser a,c a Center for Micro- and Nanostructures, Vienna University of Technology, Floragasse 7, 1040 Wien, Austria b Institute for Solid State Electronics, Vienna University of Technology, 1040 Wien, Austria c Departments of Electrical Engineering and Physics, The State University of New York at Buffalo, Buffalo, NY, USA article info Available online 17 November 2008 PACS: 61.46.Km 81.10.Aj 81.15.AÀ 68.55.ag 68.55.AÀ Keywords: A1. Low dimensional structures A1. Nanostructures A3. Molecular beam epitaxy B2. Semiconducting III–V materials B2. Semiconducting gallium arsenide abstract This paper presents a technique that allows the growth of epitaxial GaAs nanowires that are aligned to the crystal directions of the Si substrate. Low-pressure chemical vapor deposition (LP-CVD) grown Si nanowires were used as templates for molecular beam epitaxy (MBE) growth. The growth direction of the nanowires aligns with the [111] direction perpendicular to the Si substrate. After deposition of 200 nm GaAs in a solid source MBE system, we observed the formation of GaAs whiskers perpendicular to the {11 2} sidewalls of Si nanowires. By TEM analysis, the crystal structure of the GaAs nanostructures could be identified as wurtzite, where the growth axis of the wires was in the [0 0 0 1] direction. The whiskers are of good crystalline quality as no dislocations or stacking faults were observed in large areas by TEM, which makes these structures potential candidates for III–V integration on Si. In parallel to these experiments, we found a growth technique that allows GaAs nanowires to grow along the [111] direction of the planar Si [11 2] substrates. III–V nanowires with comparable geometry but higher density could be realized. & 2008 Elsevier B.V. All rights reserved. 1. Introduction Recent advances in electronic and optoelectronic devices were based on both downscaling the geometry and reducing the dimensionality. Nanowires are promising candidates to enter the field of nanoscaled devices in the near future. The epitaxial growth of one-dimensional nanostructures allows more flexibility in the combination of materials [1]. Such structures are of potential interest because they allow integration of materials that are suitable for optoelectronic applications on Si substrates and also take benefit of well-established fabrication techniques [2,3]. Because of the small contact area, a mismatch in lattice constants or crystal structure is a minor issue. However, the challenge is to achieve control over the positioning as well as diameters and length of these nanowires. The locations of the nanowires are most often defined by lithographic steps [4], while the orientation of the wires depends on growth parameters catalyst size and precursor pressure [5,6]. 2. Experiment Si nanowires, which were later used as templates, were grown in a hot-wall low-pressure chemical vapor deposition (LP-CVD) reactor. A layer of sputtered Au with a thickness of 0.5–2nm acted as a catalyst to enable the vapor–liquid–solid (VLS) growth mode. A solution of 2% SiH 4 in He was used as a precursor being cracked at the sample surface, which was kept at 550 1C [3]. By varying the growth time, nanowire lengths up to 3 mm with diameters around 100 nm could be realized. In the next step GaAs was deposited on these templates in a solid-source molecular beam epitaxy (MBE) system. Conditions for the III–V growth were a layer-by-layer equivalent growth rate of 0.55 mm/h and an As 4 flux with a beam equivalent pressure of 1.5e–5Torr. The substrate was heated under As 4 overpressure and kept at a constant temperature of 450 1C. 3. Results and discussion Structural investigations by scanning electron microscopy (SEM) revealed the formation of GaAs branches on the Si nanowires. As shown in Fig. 1 they are oriented normal to the six sidewalls of the Si nanowire trunks, which leads to a star-like ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.11.022 Ã Corresponding author. Tel.: +43 15880136244, fax: +43 15880136299. E-mail address: hermann.detz@tuwien.ac.at (H. Detz). Journal of Crystal Growth 311 (2009) 1859–1862