Letter pubs.acs.org/NanoLett
Atomic-Scale Engineering of the Electrostatic Landscape of Semiconductor Surfaces David Gohlke,†,∥ Rohan Mishra,‡ Oscar D. Restrepo,‡ Donghun Lee,†,§ Wolfgang Windl,‡ and Jay Gupta*,† †
Department of Physics and ‡Department of Materials Science and Engineering, Ohio State University, Columbus, Ohio 43210, United States § Department of Physics, Yale University, New Haven, Connecticut 06510, United States ∥ Fakultät für Physik, Universität Regensburg, Regensburg, Germany D-93053 S Supporting Information *
ABSTRACT: A low-temperature scanning tunneling microscope was used in conjunction with density functional theory calculations to determine the binding sites and charge states of adsorbed Ga and Mn atoms on GaAs(110). To quantify the adatom charge states (both +1e), the Coulomb interaction with an individual Mn acceptor is measured via tunneling spectroscopy and compared with theoretical predictions. Several methods for positioning these charged adatoms are demonstrated, allowing us to engineer the electrostatic landscape of the surface with atomic precision. KEYWORDS: Surface potential, solotronics, charged adatoms, GaAs, atomic manipulation
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Coulomb potential. Using this method, the charges of Ga and Mn adatoms are directly determined for the first time and further validated with density functional theory (DFT) calculations. The charged adatoms can be positioned with an STM tip one-at-a-time or en masse to create nanoscale regions with a desired electrostatic potential. These results hold promise not only for future nanoscale electronic devices but also for spatial control over adsorption and surface chemistry processes that depend on the local potential. Figure 1a shows typical STM images of Ga and Mn adatoms. Characteristic of III−V semiconductor surfaces, metal adatoms are imaged under these conditions as a small, bright protrusion surrounded by a sharp, dark depression. Discussed further below, a longer range depression extending several nanometers away from the adatom suggests that both adatoms are positively charged.14 Adatoms can be positioned in several ways to tailor the electrostatic landscape with atomic precision. For example, both Gaad and Mnad can be nudged along the surface by ramping the bias voltage to ∼+1.3 V while keeping the tip fixed (Figure 1a). The voltage threshold for motion is independent of set current in the 20−500 pA range and is likely correlated with tunneling into in-gap electronic states observed for both adatoms (see Supporting Information Figure S1). As with other defects in III−V surfaces,12,15 motion of both adatoms is anisotropic. The trajectories shown in Figure 1a indicate that
mproved control over defects in semiconductors and the corresponding electrostatic potential is central to a variety of applications. Semiconductor devices such as transistors are now small enough that the statistical and discrete nature of atomic dopants creates a random potential in the channel, which influences switching voltages, leakage currents and device-todevice reproducibility.1 Next-generation devices thus require nanoscale control and/or characterization of individual dopants.2−5 Surface potentials are also important in semiconductor-based photovoltaics and photocatalysts, where the separation of photoinduced carriers generates current or drives chemical reactions.6,7 In TiO2 for example, the surface potential is dictated by oxygen vacancies, which readily form during thermal processing. Visible light absorption in this wide-gap system can be improved by codoping at cation and anion sites, although the atomic mechanisms are not yet well understood.8 Recently, scanning tunneling microscope (STM) studies have explored strategies for improved nanoscale control and characterization of impurity and surface potential distributions in semiconductors. STM lithography has been used to prepare devices with atomically precise placement of phosphorus dopants,4,5 and STM-based atomic manipulation has been used to tune dopant properties via tip-induced fields9−11 or by tuning the interactions with other defects.12,13 Here we demonstrate new methods for atomic-scale engineering of the electrostatic landscape of a semiconductor surface by positioning charged adatoms. First, we develop a general method for quantifying adsorbate charges by using a Mn acceptor in a GaAs surface as a probe of the adsorbate’s © 2013 American Chemical Society
Received: January 24, 2013 Revised: April 18, 2013 Published: May 6, 2013 2418
dx.doi.org/10.1021/nl400305q | Nano Lett. 2013, 13, 2418−2422
Nano Letters
Letter
Figure 1. Methods for positioning of adatoms on GaAs(110). (a) Lateral manipulation of Gaad and Mnad by applying positive voltage pulses (+1.44 and +1.25 V respectively). Circles along the trajectory indicate the adatom position after each pulse. Images taken at (−1.44 V, 100 pA). (b) A surface d-doped region realized by unidirectional motion of adatoms while imaging adjacent areas at +1.3 V (not shown). Image taken at (−1.44 V, 100 pA). (c) Vertical manipulation of eight Gaad around MnGa. Each of these atoms was deposited on the surface by approaching the tip ∼0.5 nm at constant voltage (−1.44 V). The increased brightness of MnGa with increased number of adatoms may indicate a change in the occupation of acceptor states due to band bending.11 Images taken at (−1.25 V, 100 pA).
the motion of Mnad is nearly unidirectional along [001], with few (1.5 nm, corresponding to the range of distances studied here experimentally. In conclusion, we have demonstrated new methods for engineering the electrostatic landscape of a semiconductor surface on the atomic scale by positioning charged adatoms. Tunneling spectroscopy was used to resolve bending of the host band structure and to quantify the adatom charge via changes in the hole binding energy to Mn acceptors. Firstprinciples calculations validate the quantitative analysis of the experimental data using a simple Coulomb model. Tunneling spectroscopy of the Mn acceptor state also enables a new, general, and more direct method for quantifying adsorbate charges14,28 that can be extended to molecular adsorbates as well. Methods. We used a Createc LT-STM operating in ultrahigh vacuum (
