Facet-Dependent and Adjacent Facet-Related Electrical Conductivity Properties of SrTiO₃ Crystals

Abstract

Electrical conductivity measurements show that the {100} faces of a perfect SrTiO3 cube are insulating, but the {110} faces of a SrTiO3 truncated rhombic dodecahedron are considerably more conductive. Interestingly, compared to electrodes touching the proximal {110} faces, adjacent {110} face contacts give notably higher current. Unexpectedly, while the {110} faces of a truncated rhombic dodecahedron remain much more conductive than its {100} faces, the adjacent conductive {110} facets may cause the poorly conductive {100} facets to become more conductive through slight current leakage to the adjacent conductive faces. Consistent with previous insulating behavior observed for a {110}-bound Cu2O rhombic dodecahedron, the {110} faces of a Cu2O rhombicuboctahedral microcrystal remain insulating. Thus, the influence of adjacent conductive facet may be avoided using sharper electrodes. Still, the adjacent facet effect may always be present even with the use of sharp electrodes. Current-rectifying asymmetric I–V curves were recorded with electrodes contacting the {100} and {110} faces of a SrTiO3 truncated rhombic dodecahedron. The electrical facet effects can be understood with different degrees of band bending at these crystal surfaces and thus different barrier heights to charge carrier transport across these surfaces. Finally, high-resolution transmission electron microscopy (HR-TEM) images over the surfaces of SrTiO3 truncated rhombic dodecahedra and cubes were taken, showing notable shifts in atomic positions within the few layers of surface lattice planes relative to the atomic positions of the interior lattice, suggesting that the lattice deviations within the thin surface layer as predicted by density functional theory (DFT) calculations may be visually observable.

Current Rectification and Photo-Responsive Current Achieved through Interfacial Facet Control of Cu2O–Si Wafer Heterojunctions

“…To explain the emergence of the observed electrical facet effects, density functional theory (DFT) calculations have revealed the presence of an ultrathin surface layer (∼1 nm or less in thickness) with dissimilar band structures for various surface planes, which should lead to different degrees of surface band bending and facilitate or prevent charge transport across a particular crystal face. ,,,− Furthermore, bond length and bond directions, as well as frontier orbital electron energy distribution, within the thin surface layer can show deviations or variations for the highly conductive crystal faces such as the Si (111) and Cu 2 O (111) planes, which should yield changes in the surface band structure. , These DFT results provide a more physical picture of the thin surface layer. Interestingly, the predicted structural deviations in the surface lattice planes may be observable, as seen in the high-resolution transmission electron microscopy (HR-TEM) images of SrTiO 3 crystals with slight atomic position deviations and noticeable peak shifts in the X-ray diffraction (XRD) patterns of polyhedral Cu 2 O crystals. , To gain further insights, the most conductive Si and Ge {111} wafers were found to have the lowest surface trap state population and the shortest carrier lifetime, matching with their best electrical conductivity behavior. , GaAs wafers, however, do not show such surface trap state and carrier lifetime correlation to their conductivity properties …”

“…Interestingly, the predicted structural deviations in the surface lattice planes may be observable, as seen in the high-resolution transmission electron microscopy (HR-TEM) images of SrTiO 3 crystals with slight atomic position deviations and noticeable peak shifts in the Xray diffraction (XRD) patterns of polyhedral Cu 2 O crystals. 7,16 To gain further insights, the most conductive Si and Ge {111} wafers were found to have the lowest surface trap state population and the shortest carrier lifetime, matching with their best electrical conductivity behavior. 17,18 GaAs wafers, however, do not show such surface trap state and carrier lifetime correlation to their conductivity properties.…”

“…Various semiconductor materials including Cu 2 O, Ag 2 O, TiO 2 , PbS, and SrTiO 3 crystals have displayed strongly facet-dependent electrical conductivity properties. − Ag 3 PO 4 crystals, however, do not possess clear electrical facet effects . Moreover, intrinsic, or undoped, Si, Ge, and GaAs wafers also exhibit large facet dependence in their electrical conductivity responses. − In the case of Cu 2 O crystals, the {111}-bound octahedron is highly conductive, but the {110}-terminated rhombic dodecahedron is actually insulating .…”

Origin and manifestation of semiconductor facet effects

“…High‐resolution TEM images were taken over the edge region of a {100}‐truncated rhombic dodecahedral SrTiO 3 particle to probe the presence of variations in the surface atom positions (Figure 9). [ 6 ] Upon initial examination, the images suggest a perfect crystal. A line drawn across the surface atoms indicates perfect atom alignment (green lines).…”

“…Semiconductor particles including Cu 2 O, Ag 2 O, TiO 2 , PbS, and SrTiO 3 have been shown to present strongly face‐dependent electrical conductivity behaviors. [ 1–6 ] That is, one face of a crystal can be highly conductive, while another face of the same crystal is poorly conductive or insulative. Ag 3 PO 4 crystals, however, show a less pronounced electrical facet effect.…”