Bulk and surface characterization of In${}_2$O${}_3$(001) single crystals

A comprehensive bulk and surface investigation of high-quality In${}_2$O${}_3$(001) single crystals is reported. The transparent-yellow, cube-shaped single crystals were grown using the flux method. Inductively coupled plasma mass spectrometry (ICP-MS) reveals small residues of Pb, Mg, and Pt in the crystals. Four-point-probe measurements show a resistivity of 2.0 ± 0.5 × 10${}^5\Omega$ cm, which translates into a carrier concentration of ≈10${}^{12}$ cm${}^{-3}$. The results from x-ray diffraction (XRD) measurements revise the lattice constant to 10.1150(5) Å from the previously accepted value of 10.117 Å. Scanning tunneling microscopy (STM) images of a reduced (sputtered/annealed) and oxidized (exposure to atomic oxygen at 300 °C) surface show a step height of 5 Å, which indicates a preference for one type of surface termination. The surfaces stay flat without any evidence for macroscopic faceting under any of these preparation conditions. A combination of low-energy ion scattering (LEIS) and atomically resolved STM indicates an indium-terminated surface with small islands of 2.5 Å height under reducing conditions, with a surface structure corresponding to a strongly distorted indium lattice. Scanning tunneling spectroscopy (STS) reveals a pronounced surface state at the Fermi level ($E_{\mathrm{F}}$). Photoelectron spectroscopy (PES) shows additional, deep-lying band gap states, which can be removed by exposure of the surface to atomic oxygen. Oxidation also results in a shoulder at the O 1s core level at a higher binding energy, possibly indicative of a surface peroxide species. A downward band bending of 0.4 eV is observed for the reduced surface, while the band bending of the oxidized surface is of the order of 0.1 eV or less.

Figure 1

Unit cell of the In${}_2$O${}_3$ bixbyite structure. Large (red/dark gray) balls are oxygen, and small blue balls are In-b (medium gray) and In-d (light gray).

Figure 2

The investigated In${}_2$O${}_3$ single crystals. A polarized light microscope image of one of the crystals is shown at the right.

Figure 3

(a) LEED image of an In${}_2$O${}_3$(001) single crystal ($E_{\mathrm{Electron}}=100$ eV) prepared by sputtering and annealing. Squares mark the (0, n $+$ 1/2) and (n $+$ 1/2, 0) spots that are missing due to the p4g glide plane symmetry. (b) LEIS spectra (1 keV He${}^{+}$) of the sputtered/annealed (reduced) and oxidized surface.

Figure 4

STM image of a sputtered/annealed In${}_2$O${}_3$(001) surface taken with a sample bias of $V_{\mathrm{Sample}}$ $=$ −3 V and tunneling current $I_{\mathrm{Tunnel}}$ $=$ 0.23 nA. The arrow indicates the location of the line profile.

Figure 5

(a) STM image of the reduced surface with atomic resolution ($V_{\mathrm{Sample}}$ $=$ −2.6 V, $I_{\mathrm{Tunnel}}$ $=$ 0.17 nA). (b) A magnification of the framed part indicated in (a). The 3.6 Å lattice obtained from the Fourier transform is overlaid.

Figure 6

STM images of the oxidized surface taken at (a) $V_{\mathrm{Sample}}$$=$ $+$2.3 V, $I_{\mathrm{Tunnel}}$ $=$ 0.25 nA, and (b) $V_{\mathrm{Sample}}$ $=$ $+$1.8 V, $I_{\mathrm{Tunnel}}$ $=$ 0.25 nA. The calibration bar in (b) shows that the surface is rough at the atomic scale (height units: pm).

Figure 7

Scanning tunneling spectroscopy of a sputtered/annealed In${}_2$O${}_3$(001) surface.

Figure 8

(a) Normalized valence band spectra of the oxidized and sputtered/annealed In${}_2$O${}_3$(001) sample at 105 eV photon energy. Linear extrapolations for determining the leading edge of the valence band (VB) are indicated. (b) VB of the reduced surface at different photon energies.

Figure 9

PES on the sputtered/annealed (reduced) and oxidized samples of the (a) In $3d_{5/2}$ and (b) O 1s core levels, taken with a photon energy of 610 eV. (c) Line fits of the O 1s peak of the oxidized surface at 610 eV.

Figure 10

Models of the In${}_2$O${}_3$(001) surfaces with (a) D-layer, (b) M-layer (b), and (c) O-layer termination. Shown are bulk-terminated layers with all the indium and oxygen atoms present; In-b atoms are shown darker than In-b. Note that these terminations represent polar surfaces; for stoichiometric surfaces, only half of the atoms shown here are expected to be present.

Figure 11

Downward band bending of the reduced surface (a) and upward band bending of the oxidized surface (b).