High-quality epitaxial Bi(111) films on Si(111) by isochronal annealing T. Payer, C. Klein ⁎, M. Acet, V. Ney 1 , M. Kammler 2 , F.-J. Meyer zu Heringdorf, M. Horn-von Hoegen Department of Physics and Center for Nanointegration (CENIDE), University Duisburg ‐ Essen, Lotharstrasse 1, 47057 Duisburg, Germany abstract article info Article history: Received 17 October 2011 Received in revised form 4 May 2012 Accepted 2 June 2012 Available online 7 June 2012 Keywords: Epitaxy Strain Annealing Bismuth Silicon Bi(111) films grown on Si(111) at room temperature show a significantly higher roughness compared to Bi films grown on Si(100) utilizing a kinetic pathway based on a low-temperature process. Isochronal annealing steps of 3 min duration each with temperatures up to 200 °C cause a relaxation of the Bi films' lattice parameter toward the Bi bulk value and yield an atomically flat Bi surface. Driving force for the relaxation and surface reordering is the magic mismatch of 11 Bi atoms to 13 Si atoms that emerges at annealing temperatures above 150 °C and reduces the remaining strain to less than 0.2%. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The semimetal bismuth exhibits very promising electronic proper- ties: it combines a high surface state conductivity [1] with a large charge carrier mean free path and a low carrier density [2,3]. The large Fermi wavelength [4] and the large spin mean free path make Bi and thin Bi films an interesting candidate for spintronic applications. In recent years, applications for topological isolators based on Bi and Bi alloys have also attracted much attention [5]. All these applications depend on high-quality Bi films on suitable substrates. Previous work to grow high-quality Bi(111) films on Si(100) substrates required a multi-step multi-temperature preparation process starting with a cooled substrate at 150 K [6]. Bi films grown at room temperature on Si(111) substrates undergo a phase transition from a cubic allotrope to the hexagonal phase at a thickness of 1.5–2 nm [7–9]. The high quality of these Bi(111) films is attributed to a 6:7 match between Bi and Si lattices [8–12]. Here, we discuss an additional annealing procedure to improve the Bi film quality on (111)-oriented Si substrates to achieve a quality similar to Bi films on Si(100) [13] without the need for sample cooling below room temperature. 2. Experimental details Experiments have been performed under ultrahigh vacuum (UHV) conditions in two separate molecular beam epitaxy (MBE) systems. One of the systems (MBE-I) is equipped with a low energy electron diffraction (LEED) instrument. In this system sample heating is performed using a GE Boralectric boron nitride heater. The other UHV system (MBE-II) is equipped with a high resolution spot profile analysis LEED (SPA-LEED) [14] and uses direct current heating for sample heating. The 500 μm thick silicon (111) samples (25×20 mm² in the MBE-I System and 5×15 mm² in the MBE-II system) were cut from a 4 inch Si wafer (B-doped, 30–100 Ω cm, 0.2° miscut). The samples were wiped off with ethanol before use and degassed at 600 °C in high vacuum for several hours. Repeated flash-annealing at 1250 °C removed the native oxide. Afterwards the LEED pattern showed the expected (7×7) reconstruction of the clean Si(111) surface without diffuse intensity. Temperature calibration of the MBE-I setup has been performed with a type K thermocouple in direct contact with a dedicated silicon sample. In the MBE-II system we used an emissivity-corrected infrared pyrometer with extrapolation to the used temperatures. Bi (6N from MaTeck, Germany) was deposited from a commercial multi- cell e-beam evaporator (Thermionics) at a typical rate of 2 nm/min in MBE-I, while a thermal cell was used in MBE-II. The film thickness was monitored in-situ by a quartz crystal monitor. Non-contact atomic force microscope (AFM) measurements, using a Veeco Dimension 3100 microscope in non-contact mode were performed in air immedi- ately after removal of the samples from UHV. A Phillips PANalytical X'Pert PRO working in the Bragg-Brentano (θ–2θ) geometry was used for ex-situ X-ray diffraction and X-ray reflectometry measurements. We use non-monochromatized X-rays (Cu-anode) with a weighted average wavelength of 1,5418 Å. 3. Results and discussion Fig. 1(a) shows an AFM image of a 20 nm thick continuous Bi film that was deposited at room temperature (RT) on a Si(111) (7 × 7) surface. A Thin Solid Films 520 (2012) 6905–6908 ⁎ Corresponding author. E-mail address: claudius.klein@uni-due.de (C. Klein). 1 Present address: Johannes Kepler University Linz, 4040 Linz, Austria. 2 Present address: University of Applied Science, 93049 Regensburg, Germany. 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.06.004 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf