Maria Hilse1,Hemian Yi1,Cui-Zu Chang1,Roman Engel-Herbert1,2
The Pennsylvania State University1,Paul-Drude-Instit für Festkörperelektronik2
Maria Hilse1,Hemian Yi1,Cui-Zu Chang1,Roman Engel-Herbert1,2
The Pennsylvania State University1,Paul-Drude-Instit für Festkörperelektronik2
FeSe, a bulk superconductor with a T<sub>C</sub> of 9 K has attracted a high level of attention over almost one decade since a skyrocketing boost in T<sub>C</sub> was reported for a single unit cell (UC) layer of FeSe grown by molecular beam epitaxy (MBE) on SrTiO<sub>3</sub>(001) to as high as 100 K.[1] FeSe-SrTiO<sub>3</sub> heterostructures have since been fabricated by numerous groups in the field but the record T<sub>C</sub> proved extremely difficult to reproduce and thus the mechanism behind it still remains concealed. However, after extensive work in the past, the field appears to agree on certain key “ingredients” in the heterostructure sample preparation that are believed essential for the boost in T<sub>C</sub>. Those key players are; 1. an ultra-clean substrate surface of specific double layer of TiO<sub>2</sub> termination typically realized by a chemical and thermal <i>ex-situ</i> and additional thermal <i>in-situ</i> substrate preparation before growth; 2. ultra-thin – one to few unit cell thickness – limit of FeSe layer thickness; 3. a high number of Se vacancies in the FeSe film ensured through post-growth annealing steps carried out under ultra-high vacuum (UHV) for several hours; 4. followed by a suitable capping layer growth in case films need to be characterized <i>ex-situ</i> under ambient pressure due to the rapid oxidation of FeSe in air. <br/>In this talk, we present our findings on FeSe thin film growth by MBE and present a roadmap for high-T<sub>C</sub> – as high as 20 K in macroscopic <i>ex-situ</i> transport measurements – circumventing above mentioned steps 1, 2, and 3 by simple <i>in-situ</i> Se/Fe flux ratio control during FeSe growth. For this study, we have been growing FeSe films of 20-UC-thickness at varying temperature and Se/Fe flux ratios and analyzed the structural and morphological properties of the obtained uncapped FeSe films. Se and Fe fluxes were calibrated before each growth using a heated Quartz crystal microbalance (QCM) and tooling factors for the QCM calibration were extracted from physical thickness measurements by X-ray diffraction (XRD). The substrate surface temperature during growth was monitored with high precision by an infrared (IR) camera system. SrTiO<sub>3</sub> substrates were cleaned in subsequent ultrasonic baths of acetone, isopropyl alcohol, and DI-water and annealed in air to 980 °C for about 1.5 h in a box furnace to realize an atomic terrace surface morphology and additionally annealed in UHV to 600 °C for 2 h to remove adsorbates. Structural and morphological analysis was carried out <i>in-situ</i> by reflection high energy electron diffraction (RHEED) and <i>ex-situ</i> by XRD and atomic force microscopy (AFM), respectively within 20 min of taking the samples out of the UHV environment to minimize oxidation of the films. The morphology of the films showed a sensitive dependence on the growth temperature and flux ratio spanning from perfectly smooth and continuous films with atomic terraces at 450 °C growth temperature and a low flux ratio of 2.5 to exclusively disconnected island growth of large height but smooth top surfaces at lower temperatures and/or higher flux ratios. Surprisingly, the tetragonal P4/nmm crystal structure of β-FeSe was maintained for all investigated films and the <i>in-situ</i> observed diffraction pattern in RHEED also maintained the streaky pattern characteristic for smooth FeSe films even for the samples with the most pronounced island growth resulting in a root mean square (rms) AFM roughness of more than 18 nm. Smaller flux ratios than 2.5 resulted in mixed – β-FeSe/elemental Fe – phase samples. FeSe films grown under optimized conditions at 450 °C and a flux ratio of 2.5 (but without any post-growth UHV anneal) and capped with the commonly used FeTe (300 °C) and elemental Te (room temperature) layers yielded superconducting onset temperatures of about 30 K and a T<sub>C</sub> of 20 K.<br/> <br/>[1] D. Huang, J.E. Hoffman, Monolayer FeSe on SrTiO 3, Annu. Rev. Condens. Matter Phys. 8 (2017) 311–336. https://doi.org/10.1146/annurev-conmatphys-031016-025242.