Band-offsets scaling of low-index Ge/native-oxide heterostructures

We investigate, through XPS and AFM, the pseudo layer-by-layer growth of Ge native oxide across Ge(001), (110) and (111) surfaces in ambient environment. More significantly, our study reveals a universal set of valence and conduction band offset (VBO and CBO) values observed for Ge(001), Ge(110), and Ge(111) surfaces as a function of Ge-oxide concentration. We find that the band offsets appear to be the same across these low-index Ge surfaces i.e., for Ge-oxide/Ge heterostructures with the same Ge-oxide overlayer concentration or thickness. In contrast, different oxidation rates for Ge(001), Ge(110), and Ge(111) surfaces were observed, where the oxidation rate is fastest for Ge(001), compared to Ge(110) and Ge(111). This can be attributed to the different number of unsatisfied Ge dangling bonds (2 vs 1) associated to the respective ideal Ge surface in forming Ge-oxide. Thus, at any given oxidation time, the oxide concentration or thickness for each type of low index Ge surface will be different. This in turn will lead to different band offset value observed for each type of Ge surface. More significantly, we show that while oxidation rates can differ from different Ge surface-types, the band offset values can be estimated simply based on the Ge-oxide concentration regardless of Ge surface type.