Probing Semiconductor Gap States with Resonant Tunneling

Abstract

Tunneling transport through the depletion layer under a GaAs {110} surface is studied with a low temperature scanning tunneling microscope (STM). The observed negative differential conductivity is due to a resonant enhancement of the tunneling probability through the depletion layer mediated by individual shallow acceptors. The STM experiment probes, for appropriate bias voltages, evanescent states in the GaAs band gap. Energetically and spatially resolved spectra show that the pronounced anisotropic contrast pattern of shallow acceptors occurs exclusively for this specific transport channel. Our findings suggest that the complex band structure causes the observed anisotropies connected with the zinc blende symmetry.

Surface Induced Asymmetry of Acceptor Wave Functions

“…Acceptors with a smaller binding energy have a larger effective Bohr radius and thus their wave functions couple more strongly with the surface strain than the energetically deep acceptor states which are more spatially localized. This argument addresses why all the shallow acceptors such as C, Zn, and Be in GaAs appear as triangular features in STM measurements [2,4,6,10].…”

“…Observations of the asymmetry of true Coulombic impurities like C, Be, and Zn [with ground state binding energies (E a ) around 30 meV in InP and GaAs] show much more pronounced triangular shapes than Mn. It is known that these shallow acceptors display a triangular contrast which extends laterally $5 lattice constants on the cleavage surface [2,4,6,10]. Proposed explanations for the triangular appearance of the shallow acceptors include the electronic configuration of the outer shell d electrons of different acceptor species [10], wave function mapping of the excited states retaining the zinc blende tetrahedral (T d ) symmetry [4], and a resonant tunneling process involving evanescent states [6].…”

“…The surface, however, is known to strongly influence the properties of an impurity close to the surface [8], and thus a central question for all such surface measurements via cross-sectional scanning tunneling microscopy (XSTM) is how closely the local density of states (LDOS) is related to properties of an impurity in the bulk. Several groups have interpreted the XSTM contrast of acceptors as being directly related to the wave function of the impurity state [2,3,5,6], while others have proposed that highly asymmetric observed acceptor shapes arise from hybridization of the impurity state with excited states [4] or intrinsic surface states [9].…”

“…In the past decade many groups have scrutinized at the atomic scale the electronic and spectroscopic details of individual impurities, such as Si, Zn, and Mn, embedded in the topmost layers of a cleaved semiconductor crystal like GaAs or InAs [1][2][3][4][5][6]. A correct interpretation of these atomistic properties is essential for a proper understanding of the macroscopic properties of doped materials.…”

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

“…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].…”