Complex Oxide Materials: Surface Chemical Characterization for Wet Processing

Delafossites are complex oxides that have recently gained significant attention due to their diverse and remarkable properties. These compounds have typically a layered structure, consisting of alternating metal and oxygen planes. The metal atoms are arranged in triangular lattices, stacked in different sequences along the perpendicular direction. The general formula is ABO₂, where A is a noble metal (e.g. Pt, Pd, Ag, or Cu) and B is a transition metal (Cr, Co, Fe, Al, or Ni). Due to the weak interlayer coupling, delafossites are considered quasi–two-dimensional materials. By varying A and B combinations delafossites can be made insulating, p-type conducting or metallic with ultrahigh conductivity. These properties make delafossites excellent candidates for the study of new physics as well as promising materials for future device technologies in the field of magnetoelectric insulators, thermoelectrics, transparent conductors, and band insulators.

Implementing complex oxides in device architectures poses new challenges. After growth, patterning is a critical step. Various nanofabrication techniques need to be investigated for this purpose. These are mostly based on dry plasma etching methods due to the high etch rate yield. However, etch products tend to be non-volatile and redeposit within active regions of the device. In addition, the continuous ion bombardment that occurs during the etch induces damage in the form of amorphous surface layers of uncontrolled stoichiometry and composition. To remove these detrimental layers, wet-chemical etching processes need to be explored. With the ever-decreasing device dimensions angstrom-scale control is needed. This stringent requirement is further challenged by the presence of electrochemical driving forces within device stacks, as multiple materials are exposed simultaneously during immersion in the wet etchant.

In this PhD proposal, we will investigate wet-chemical atomic layer etching (ALE) as a method to characterize the surface chemistry of delafossites in aqueous solutions. ALE processes are typically based on a 2-step mechanism that consists of a surface modification and a surface product removal step that are time-separated and self-limiting by nature. Depending on the chemical properties of the material, an alternative 1-step ALE mechanism will also be investigated as a layer-by-layer etching method. The electrochemistry of complex oxides studied on blanket layers and relevant device stacks will further provide fundamental insight into the surface stability of these new materials so that the feasibility of wet-chemical processing can be evaluated for the removal of dry etch residues and damaged surface layers.  

As a PhD student, you will work in a dynamic and multicultural environment. You will get familiar with a large variety of advanced characterization techniques. Inductively coupled plasma mass spectrometry (ICP-MS) will be used to study the etching kinetics with atomic-scale precision. Electrochemical measurements will be combined with ICP-MS to follow the electrochemistry at solid/electrolyte interface in situ in the parameter space. Surface chemical composition and physics will be analysed both ex situ and post operando by x-ray photoelectron spectroscopy (XPS), high-resolution synchrotron radiation photoemission spectroscopy (SRPES) as well as extended X-ray absorption fine structure (EXAFS) to better understand the bonding structure of these new materials. Other complementary physical characterization methods such as elastic recoil detection analysis (ERDA), electrical measurements, conductive atomic force microscopy (c-AFM), scanning and transmission electron microscopy (SEM, TEM) are available to support your mechanistic studies on atomic layer etching and cleaning.