Variation in band offsets at ZnO/Sn:In2O3 heterojunctions measured by x-ray photoelectron spectroscopy
(Color online) XPS reference spectra of thick ZnO on F:SnO2 (FTO)/glass and thick ITO on fused quartz for determination of separation between a shallow core level (Zn 3d or In 4d) and the respective VB maximum. The low energy electron flood gun was utilized during both measurements to compensate for sample charging, so the BE values exhibit an arbitrary shift.
(Color online) In 3d core level and VB region for (a) ITO/quartz exposed to the oxygen plasma for 60 min, and (b) the same sample after heating to 200 °C in vacuum for 10 min. A metallic clip was placed in contact with the sample surface to eliminate sample charging, so BE values are absolute. Both In 3d peaks (open symbols) were fit with symmetric Voigt peaks (solid lines). VB maxima were determined by linear extrapolation of the edge to zero.
(Color online) AFM images of (a) ITO/quartz and (b) 20 Å Ga:ZnO/ITO/quartz. Images are 2 × 2 μm2; height scale is 8 nm.
(Color online) Measured valence band offset from XPS vs calculated ZnO thickness for nominally 40 Å ZnO/ITO/quartz as-grown, after oxygen plasma exposure for 30 min, after oxygen plasma exposure for 60 min, and after subsequent heating to 200 °C in vacuum for 10 min. Inset shows the C 1s region from the XPS survey spectra collected at a take-off angle of 12° for each processing step. Solid: no plasma exposure; dotted: plasma 30 min; dash-dot: plasma 60 min; dashed: heated 200 °C.
(Color online) Conduction band offset vs calculated ZnO thickness for pure ZnO, Al:ZnO, and Ga:ZnO heterojunctions with ITO. All heterojunctions were exposed to the oxygen plasma for 60 min before the XPS spectra were collected.
(Color online) (a) VB maximum position determined from the respective shallow core level position for various thicknesses of pure ZnO and Ga:ZnO. (b) In 3d core level position as a function of Ga:ZnO thickness. (c) Zn 2p core level position and FWHM as a function of Ga:ZnO thickness.
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