Material engineering leading to extraordinary physical property is a major theme of modern condensed matter physics. With the state-of-the-art molecular beam epitaxy (MBE) systems, we aim to design and implement heterostructure and thin films for deepening our understanding in exotic form of matters.
"What could we do with layered structures with just the right layers? What would the properties of materials be if we could really arrange the atoms the way we want them?"
-- Rychard P. Feynman, 1959
MBE is an advanced thin-film deposition system built for answering Mr. Feynman's questions. It has been widely applied as a fundamental research tool in both academia and the industry for more than 40 years, pioneering the discovery of electronic devices and new physics.
Compared to various of crystal thin film deposition methods, MBE enables high quality crystal formation in very low energy states of solid or gas sources, avoiding the formation of unwanted point defects. By high-precision shutters for individual sources and highly stabilized fluxes, the rate of crystal growth is reduced to one atomic layer per tens of seconds, enabling us to control the growth of crystals in atomic precision.
Under an ultra-high vacuum condition and very stable control of temperature, a dilute beam of atoms from elemental sources are directed toward a substrate. The surface kinetics of the substrate is also adjusted by controlling the temperature. An in-situ reflective high energy electron diffraction (RHEED) is applied to monitor the crystal growth in real time.
We develop new MBE deposition techniques for advanced ARPES studies. We have a custom built chalcogenide MBE at McCullough Building in Stanford Campus, a Veeco GEN930 Oxide MBE at the SSRL Beamline in SLAC, and a small chalcogenide MBE connected in-situ to the Oxide MBE.
Chalcogenide MBE in our group is developed for growth of ultra-thin 2D material and topological insulators. The MBE chamber is connected to our basement laser ARPES system. The system is capable of storing up to 8 elements, using 4 effusion cells and a 4-pocket electron-beam evaporator. Both the MBE and ARPES chamber are maintained at a pressure in the low e-11 torr scales, allowing us to effectively study films in-situ. We have demonstrated the viability of using a cracker effusion cell to grow high quality films of the topological insulators bismuth telluride and bismuth selenide .
ALL MBE in our group is an Oxide 930 developed for growth of high temperature superconductors. It is customized to dynamically change the deposition environment of each atomic layer. Taking advantage of shutter-controlled deposition methods, different metal oxide layers can be assembled in sequence. The gas sources in the system is ozone, which are advantageous for synthesizing cupurate compounds. The MBE chamber is connected to the Synchrotron ARPES at Stanford Synchrotron Radiation Lightsource(SSRL).
"Baby MBE", or BaMBE, is a small charcogenide MBE chamber. It is connected to the Veeco MBE and Beamline ARPES, providing complementary thin film deposition of FeSe on top of different oxide terminations.
Enhanced superconductivity in monolayer FeSe
Bulk FeSe is an iron-based superconductor that has a maximum Tc of 8K. Surprisingly, the Tc of a single layer of FeSe film grown epitaxially on the SrTiO3 substrate is enhanced to over 55K. We perform in-situ synchrotron-based MBE/ARPES experiment to understand how the transition temperature can be enhanced from the bulk superconducting transition temperature (Tc) of 8K at this 1UC FeSe/STO interface. We found the origin of enhancement of two-fold: 1) additional electron doping through interface charge transfer and 2) interfacial mode coupling. Prominent replica bands in the photoemission spectra, separated by energy on the order of the phonon of the STO substrate, indicate the existence of electron-phonon coupling across the interface. The similarity of the replica bands and main bands imply that emission or absorption of a phonon by the electron occurs with very little momentum transfer (near q=0 scattering). This forward scattering could help boost the Tc even in superconductors whose order parameter has a sign change, as is the case in cuprates, and possibly the iron-based superconductors.
Since STO exhibits a wide range of unique properties, such as superconductivity and incipient ferroelectricity, it is challenging to clarify what plays the major role in the enhancement of Tc in the 1UC FeSe/STO system. In order to further explore the substrate effect, we study 1UC FeSe grown on the rutile TiO2 (100) surface. We found that 1UC FeSe/TiO2 and 1UC FeSe/STO have very similar electronic structure, superconducting gap size and Tc. The similarities between these two systems suggest that the dielectric constant, strain and crystal symmetry from the substrate likely do not play important roles in the enhancement of Tc. The study shows that the interfacial e-ph coupling is most likely to be the key reason for enhancing superconductivity in 1UC FeSe/STO systems.
Robust Doping Level
Charge transfer across an abrupt interface between two materials with different work functions is a common phenomenon in epitaxial thin films. In recent study, monolayer FeSe is grown on top of LaTiO3(LTO)/STO heterostructure. Due to the lowered work function of LTO, additional charge transfer should be expected to occur between the 1UC FeSe and STO substrate. Surprisingly, the electron density as measured by ARPES Fermi surface maps and superconducting gaps for 1UC FeSe/LTO/STO films remain nearly unchanged with different LTO thickness. The result shows that superconductivity in 1UC FeSe thin film is robust and is accompanied with an anchored “magic” doping level.
To understand the unique phenomena of “magic” doping level under the contest of iron-based superconductivity, we consider the possible interplay of first-order phase transition and superconductivity: Above a doping level of 0.11, an insulating phase may occur and form a phase separation between the superconducting phase and the insulation phase. As a result, only ~0.11 doping is visible by the ARPES experiment.
Photo-induced 2D electron gas on SrTiO3 with different surface terminations
How does surface termination of bulk material affect surface-sensitive ARPES measurement outcome? The combination of MBE growth and high resolution ARPES measurement enables us to study the surface of layered oxide materials in great detail. For example, SrTiO3(STO) is a prototypical oxide material with alternating layers of SrO and TiO2. It exhibits a wide range of physical phenomena. In past decades, STO has been recognized as a band insulator, an incipient ferroelectric material and a superconductor. Recently, the study of STO gets renewed interests because the surface of STO hosts robust 2 dimensional electron gas .
In this study, homoepitaxial STO thin films are grown with different surface termination. A 2DEG can be generated at the surface of STO with exposure to ultraviolet light, and induces oxygen vacancy dopant. From ARPES measurement, only SrO-terminated STO films hosts clear 2DEG and not on films with a TiO2 termination. While most research on surface 2DEG of STO assumes a TiO2 termination, further research into the pristine SrO termination could lead to new insights into the system.
 J. J. Lee et al. Intrinsic ultrathin topological insulators grown via molecular beam epitaxy characterized by in-situ angle resolved photoemission spectroscopy. Appl. Phys. Lett. 101, 013118 (2012)
 J. J. Lee et al. Significant Tc enhancement in FeSe films on SrTiO3 due to interfacial mode coupling. Nature 515, 245–248(2014)
 S. N. Rebec et al. Dichotomy of the photo-induced 2-dimensional electron gas on SrTiO3 surface terminations. PNAS, 116 (34) 16687-16691(2019)