Zn- and Cd-induced features at the GaAs(110) and InP(110) surfaces studied by low-temperature scanning tunneling microscopy

Abstract

We used a low-temperature scanning tunneling microscope to study Zn-and Cd-doping atoms near the ͑110͒-cleavage surfaces of GaAs and InP at 4.2 K. The filled-state images showed centro-symmetric elevations while the empty-state images showed circular depressions. We attribute these features to the influence of the Coulomb potential of the ionized doping atoms on the number of states available for tunneling. In a few empty-state images of the GaAs͑110͒ surface, the depressions were surrounded by maxima, which are probably direct observations of Friedel oscillations. For the InP͑110͒ surface, all depressions were surrounded by noncentrosymmetric maxima. Upon moving the tip Fermi level to the bottom of the conduction band, we observed that the depressions turned into elevations with a triangular shape for both the GaAs͑110͒ and the InP͑110͒ surface. This shape was independent of the depth of the dopants, and the chemical nature of the dopants ͑Zn or Cd͒ did not influence the triangular shape either. The orientation of these triangular features was the same for all observed doping atoms and was geometrically determined with respect to the host lattice. Furthermore, we determined the location of a triangular feature with respect to a doping atom. The features were only visible when tunneling to the impurity band suggesting that the features are a direct image of the acceptor state although the origin of the triangular shape is not clear at present.

Band structure related wave-function symmetry of amphoteric Si dopants in GaAs

“…For donors only radially symmetric features have been reported [1,2,3,4]. All investigated acceptors exhibit distinct anisotropic features for certain tunneling conditions [5,6,7,8,9,10,11]. Either triangular shaped or bow-tie like contrasts are imaged.…”

“…3. They are determined by the center-of mass of the radially symmetric contrasts in STM topographies at high positive and large negative sample biases [1,2,6].…”

“…They are mirror-symmetric over the {110} mirror planes and have an asymmetry with reference to (001). Earlier it was suggested that whether anisotropic contrasts are observed or not depends on chemical characteristics of each doping element, e.g., electronic orbitals [5] or tetragonal strain fields surrounding the dopant [6]. Recent studies indicate that the anisotropic shapes relate to intrinsic properties of the semiconductor matrix rather than a property of the specific doping element [11,12,13].…”

Atomic scale images of acceptors in III-V semiconductors

“…In literature they are sometimes refered to as Friedel oscillations. First indications of hole charge density oscillations (CDO) on p-GaAs have been reported [78]. In the dI/dV-maps for bias voltages beginning at 1580 mV a bright halo of increased conductivity surrounds the two acceptors.…”

“…Nine doping elements are known to form an acceptor state with 100meV binding energy or lower in GaAs. Six of them have been investigated with the STM: Zn [11,22,72,78,102], Cd [78], Be [102], Mn [23,103,104], C [105,106] and Si (see chapter 4.2). For most bias voltages the acceptors exhibit circular symmetric features in the STM images.…”