Evidence of extreme type-III band offset at buried $n$-type $\text{CdO}/p$-type SnTe interfaces

At covalent semiconductor interfaces, the band alignment is determined by the location of the band edges with respect to the charge neutrality level, but extension of this method to more ionic semiconductor systems requires further consideration. Using the charge neutrality level concept, a type-III (or broken band gap) band offset is predicted at the interface between $n$-type CdO and $p$-type SnTe. Employing hard x-ray photoelectron spectroscopy, we report on the chemical composition at the buried interface and the valence-band offset. Chemical intermixing at the interface between SnTe and CdO is found to be limited to $\sim 2.5$ nm in our heterojunction samples. We measure a valence-band offset of 1.$95(\pm 0.15$ eV) irrespective of the layer configuration. Once the degenerate hole doping of the SnTe is considered, the measured band-edge offset agrees with the type-III offset predicted from alignment of the band edges with respect to the charge neutrality level of the semiconductors.

Figure 1

(a) Tauc plot analysis of the direct band-gap absorption onset for CdO films of varying thickness (transmission spectra inset). (b) HAXPES valence-band spectra of CdO films showing Fermi-level shifting correlated to film thickness (magnified Fermi edge inset). (c) O $K$-edge XAS of CdO films.

Figure 2

Schematic diagrams and HAXPES Cd $3d_{\text{5/2}}$, O $1s$, Sn $3d_{\text{5/2}}$, and Te $3d_{\text{5/2}}$ core-level peaks for series of varying thickness (a) CdO thin films, (b) SnTe thin films, and (c) CdO films on top of SnTe films, all grown on slide glass substrate. Vertical dotted lines identify peaks corresponding to bulk charge states, while the thicker lines identify surface and interface species. Contamination/intermixing lines are color-coded to match the surface/interface they are attributed to in the sample schematics on the left. (d) A diagram showing the exponential dropoff of HAXPES spectral contributions from within a sample.

Figure 3

XAS of two samples of CdO on SnTe. Vertical dotted lines correspond to features indicative of ${\mathrm{SnO}}_2$ [35].

Figure 4

(a) The method for determining the energy separation between a core-level and valence-band leading edge, for example bulk SnTe and bulk CdO thin films, and (b) the difference between a SnTe and a CdO core level for two example heterojunctions.

Figure 5

(a) Diagram of the band bending at the interface for CdO on SnTe (top) and SnTe on CdO (bottom) heterojunction configurations. $\mathrm{\Delta }\mathrm{VB}$ is the experimentally determined offset between the valence band of CdO and Fermi level at the interface. (b) Simplistic diagram of the valence and conduction bands at the heterojunction interface, based on our experimental band offsets. Band-gap values for CdO and the CNL location for SnTe were obtained from theoretical calculations [37] and ion irradiation studies [15], respectively.