Probing the local electronic structure of isovalent Bi atoms in InP

Cross-sectional scanning tunneling microscopy (X-STM) is used to experimentally study the influence of isovalent Bi atoms on the electronic structure of InP. We map the spatial pattern of the Bi impurity state, which originates from Bi atoms down to the sixth layer below the surface, in topographic, filled-state X-STM images on the natural $\left\{110\right\}$ cleavage planes. The Bi impurity state has a highly anisotropic bowtielike structure and extends over several lattice sites. These Bi-induced charge redistributions extend along the $〈110〉$ directions, which define the bowtielike structures we observe. Local tight-binding calculations reproduce the experimentally observed spatial structure of the Bi impurity state. In addition, the influence of the Bi atoms on the electronic structure is investigated in scanning tunneling spectroscopy measurements. These measurements show that Bi induces a resonant state in the valence band, which shifts the band edge toward higher energies. Furthermore, we show that the energetic position of the Bi-induced resonance and its influence on the onset of the valence band edge depend crucially on the position of the Bi atoms relative to the cleavage plane.

Figure 1

Filled-state X-STM images of the same region of film 3, which are acquired at a tunnel current set point of $I_T=30\mathrm{pA}$ on the $\left(\overline{1}10\right)$ InP surface. (a) X-STM image taken at a large negative sample voltage of $U_S=-2.6\mathrm{V}$, where states deep in the VB are probed. In this case, Bi atoms in the first three surface layers (0, 1, 2) are clearly visible. (b) X-STM image taken at a smaller negative sample voltage of $U_S=-2.0\mathrm{V}$, where states closer to the VBE are probed. This results in a change of the appearance of the Bi atoms in layers 1 and 2. In addition, Bi atoms in deeper layers (3, 4, 5, 6) become visible. The color scales of both images (a) and (b) are independently adjusted for best visibility. The crystal directions are indicated in (a).

Figure 2

(a) High-resolution filled-state X-STM images of charge accumulations around individual Bi atoms in the first seven layers (0, 1,..., 6) of the $\left(\overline{1}10\right)$ InP surface. The reference lines indicate the center of the Bi-related features, which lie for Bi atoms in odd- or even-numbed layers on or in-between the anion sublattice. All images are extracted from a large-scale X-STM image of film 3, which is taken at a tunnel current set point of $I_T=40\mathrm{pA}$ and a sample voltage of $U_S=-2.4\mathrm{V}$. The color scales are independently adjusted to emphasize the spatial structure of the weaker features. (b) Schematic diagram showing the extent and symmetry of the charge accumulation around the Bi atoms in even and odd layers with respect to a top view of the $\left(\overline{1}10\right)$ InP surface. Here, filled blue disks represent the group V sublattice, which is probed in filled-state X-STM images (a). (c) The positions of the Bi atoms in (a) are indicated in a side view of the relaxed $\left(\overline{1}10\right)$ surface.

Figure 3

(a) X-STM measurements of filled states of the $\left(\overline{1}10\right)$ plane of InP taken at $-2.4$ V. The layers 0, 1, and 2 correspond to the cleavage surface and the two layers below it, respectively. These correspond to the images 0, 1, and 2 of Fig. 4. (b) LDOS calculated at 120 meV below the VBE with an assumed STM tip width of 1.13 Å. The red reference lines indicate the location of the Bi atom. (c), (d) Isodensity surfaces calculated at 120 meV below VBE corresponding to a difference between the Bi atom and the background of 0.6 and 0.4 ${\text{eV}}^{-1}{\text{Å}}^{-3}$, respectively.

Figure 4

(a) Filled-state X-STM image of film 3 taken at a tunnel current set point of $I_T=40\mathrm{pA}$ and a sample voltage of $U_S=-2.4\mathrm{V}$. (b) Spatially resolved differential conductance spectra $\left[dI\right(U)/dU]$ taken at Bi atoms down to the second layer below the surface (0, 1, 2), which are marked in (a). A spectrum acquired in the clean InP matrix in-between the Bi layers (black) serves as a reference. The spectra taken at Bi atoms show in contrast to the InP matrix additional resonances near the VBE, which lie deeper in the VB the closer the Bi atom is to the surface. The CB and VB edges are indicated by dashed vertical lines. The band gap is highlighted in gray.

Figure 5

The $dI\left(U\right)/dU$ spectra are extracted from a CITS map at the positions of Bi atoms in the first (1), second (2), third (3), and fifth (5) layers, which are labeled accordingly in Fig. 1. A spectrum of the bare InP matrix (black) serves as a reference. A logarithmic scale is used to reveal the VBE, which at the Bi atoms is shifted to higher energies as indicated by an arrow. The tunneling into the VB begins below $-1.5$ V (IV). The shoulder between $-0.6$ and $-1.5$ V is related to a $I_T$ out of occupied CB states (III). In the band gap (II) tunneling is suppressed. Empty states in the conduction band are addressed at positive $U_S$ (I). The resonant states of Bi atoms in different layers are indicated by fanned arrows.

Figure 6

Depth dependency of the energetic positions of Bi impurity state in the VB. Set A, which is shown in blue, is extracted from the $dI\left(U\right)/dU$ spectra in Fig. 4. Set B, which is shown in red, is determined on the basis of the $dI\left(U\right)/dU$ spectra in Fig. 5. The energetic positions of the Bi-related resonances in the experiment are extracted after subtraction of the InP background. The error bars represent the FWHM of the Bi impurity states.