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Ultrathin materials will play a crucial role in next-generation electronic devices but defects in these materials are unavoidable. The effect of defects can permeate entire ultrathin layers and extend beyond interfaces into adjacent layers. Identifying and understanding atomic scale defects inside ultrathin electronic materials leads to new ways of controlling and eliminating defects to achieve higher performance devices and tune electronic and magnetic properties. Complex oxides and two-dimensional (2D) materials are ideal for creating ultrathin conducting layers. Both classes of materials exhibit a wide range of dielectric, optoelectronic and magnetic properties useful in a range of device applications. 2D materials can be isolated down to layers just a single atom thick leading to thinner and higher performance electronics. Virtually all applications of these materials involve interfaces with other materials which presents special challenges since defects near interfaces impact material properties on either side of the interface. Surface science techniques specialized at investigating ultrathin layers such as depth-resolved cathodoluminescence spectroscopy (DRCLS) and x-ray photoelectron spectroscopy (XPS) must be used to improve these materials. The van der Waals gap which allows individual 2D layers to be isolated from one another opens the opportunity for foreign particles such as water to reside between layers and impact electronic properties. Water sitting between layers of germanane, a graphene analogue, significantly impacts luminescence properties and points to an important effect that intercalants between layers need to be accounted for when creating heterostructures based using 2D materials. While 2D materials can be mechanically exfoliated and isolated in monolayers, better interfaces can be achieved if single 2D layers are deposited directly onto a substrate using techniques such as molecular beam epitaxy (MBE). However, conditions under which 2D materials are grown can significantly impact the interfacial properties between the 2D material and the substrate. We used XPS to study the effect of growth conditions on interface properties between MnSe2, a 2D ferromagnet and Bi2Se3, a topological insulator. The samples were transferred from MBE into XPS using a UHV suitcase to study pristine interfaces and prevent air exposure. Interfaces between complex oxides are also important such as the 2D electron gas (2DEG) that forms between two otherwise insulating complex oxides, LaAlO3 and SrTiO3. A complementary 2D hole gas (2DHG) was expected to form at these interfaces as well, but defect formation made observing that interface more challenging. Advanced growth designed to control defect formation and near-nanoscale resolved defect identification confirmed that minimizing oxygen vacancies was the key to realizing a conducting 2DHG. Cation vacancies and antisite substitutions can also play a large role in the physical properties of complex oxides and impact charge distributions extending tens of nanometers below interfaces. Identifying and controlling defects plays an important role in achieving advanced electronic features at complex oxide interfaces. DRCLS is a powerful tool to study such interfaces formed with ultrathin layers. Combining MBE with DRCLS and XPS using all UHV transfers is a powerful approach to create complex oxide interfaces and study the impact of nearby defects without external contamination. |
