Characterization of free-standing InAs quantum membranes by standing wave hard x-ray photoemission spectroscopy

Free-standing nanoribbons of InAs quantum membranes (QMs) transferred onto a (Si/Mo) multilayer mirror substrate are characterized by hard x-ray photoemission spectroscopy (HXPS), and by standing-wave HXPS (SW-HXPS). Information on the chemical composition and on the chemical states of the elements within the nanoribbons was obtained by HXPS and on the quantitative depth profiles by SW-HXPS. By comparing the experimental SW-HXPS rocking curves to x-ray optical calculations, the chemical depth profile of the InAs(QM) and its interfaces were quantitatively derived with angstrom precision. We determined that: i) the exposure to air induced the formation of an InAsO$_4$ layer on top of the stoichiometric InAs(QM); ii) the top interface between the air-side InAsO$_4$ and the InAs(QM) is not sharp, indicating that interdiffusion occurs between these two layers; iii) the bottom interface between the InAs(QM) and the native oxide SiO$_2$ on top of the (Si/Mo) substrate is abrupt. In addition, the valence band offset (VBO) between the InAs(QM) and the SiO$_2$/(Si/Mo) substrate was determined by HXPS. The value of $VBO = 0.2 \pm 0.04$ eV is in good agreement with literature results obtained by electrical characterization, giving a clear indication of the formation of a well-defined and abrupt InAs/SiO$_2$ heterojunction. We have demonstrated that HXPS and SW-HXPS are non-destructive, powerful methods for characterizing interfaces and for providing chemical depth profiles of nanostructures, quantum membranes, and 2D layered materials.