Molecular Beam Epitaxy
A major theme in modern condensed matter physics is the engineering of new materials, with the goal of producing extraordinary physical properties. With our state-of-the-art molecular beam epitaxy (MBE) system, we aim to design and investigate heterostructures and thin films in order to deepen our understanding of exotic forms of matter.
"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?" — Richard P. Feynman, "There's Plenty of Room at the Bottom", 1959
MBE is an advanced thin-film deposition system built to answer these questions posed by Feynman. It has been widely used as a fundamental research tool in both academia and industry for more than 40 years, pioneering the discovery of new physics and accelerating the production of novel electronic devices.
Compared with other crystal thin-film deposition methods, MBE prevents the formation of unwanted point defects by enabling high quality crystal formation in very low energy states from solid or gaseous sources. By using high-precision shutters for individual sources and highly stabilized fluxes, the rate of crystal growth is throttled down to one atomic layer every few dozen seconds. This enables us to control crystal growth with atomic precision.
Such precision requires ultra-high vacuum and very stable temperature control. Under these conditions, a dilute beam of atoms from elemental sources is directed toward a substrate, whose surface kinetics is controlled by adjusting its temperature. In situ reflective high energy electron diffraction (RHEED) is used to monitor crystal growth in real time.
We develop new MBE deposition techniques for advanced ARPES studies. We have a custom-built chalcogenide MBE system in the McCullough building at the Stanford campus, a Veeco GEN930 Oxide MBE at the SSRL Beamline at SLAC, and a small chalcogenide MBE connected in situ to the Oxide MBE.
In our group, chalcogenide MBE is used to grow ultra-thin 2D materials and topological insulators. The MBE chamber at McCullough is connected to our basement laser ARPES system. The system is capable of storing up to eight elements, using four effusion cells and a four-pocket electron-beam evaporator. Both the MBE and ARPES chambers are maintained at pressures in the low 10-11 Torr scale, effectively allowing us to 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 (Bi2Te3) and bismuth selenide (Bi2Se3) .
Oxide Atomic Layer-by-Layer MBE
Our Atomic Layer-by-Layer (ALL) MBE system employs an Oxide 930 developed for the growth of high temperature superconductors. It is customized to allow the deposition environment of each atomic layer to be dynamically tailored. Using shutter-controlled deposition methods, different metal oxide layers can be assembled sequentially. The system uses ozone gas sources, which are advantageous for synthesizing cuprate compounds. This MBE chamber is connected to our Synchrotron ARPES system at the Stanford Synchrotron Radiation Lightsource (SSRL).
Our 'Baby MBE' — or BaMBE — is a small charcogenide MBE chamber. It is connected to the Veeco Oxide 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 transition temperature (Tc) of 8 K. Surprisingly, the Tc of a single layer of FeSe film grown epitaxially on an SrTiO3 substrate is enhanced to over 55 K. We perform an in situ synchrotron-based MBE/ARPES experiment to understand how Tc can be enhanced from the bulk Tc of 8 K at this 1UC (one unit-cell) FeSe/STO interface. We found two reasons for its enhancement: (1) additional electron doping through interface charge transfer, and (2) interfacial mode coupling. Prominent replica bands in the photoemission spectra, separated by energies on the order of phonon modes on the STO substrate, indicate the existence of electron-phonon coupling across the interface. The similarity of replica and main bands imply that emission/absorption of phonons by electrons occurs with very little momentum transfer (nearly q = 0 scattering). This forward scattering could help boost Tc even in superconductors whose order parameter possess a sign change, as is the case in the cuprate and possibly iron-based superconductors.
STO exhibits a wide range of unique properties, such as superconductivity and incipient ferroelectricity. It is therefore challenging to identify what plays the major role in the enhancement of Tc in the 1UC (one unit-cell) FeSe/STO system. We explored the substrate effect by studying 1UC FeSe grown on a rutile TiO2 (100) surface. We found that 1UC FeSe/TiO2 and 1UC FeSe/STO have similar electronic structure, superconducting gap size, and Tc. The similarities between these two systems suggest that the dielectric constant, strain, and crystal symmetry of the substrate do not play important roles in the enhancement of Tc. Instead, it shows that interfacial e-ph coupling is most likely crucial 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 our recent study, monolayer FeSe is grown on top of a LaTiO3 (LTO)/STO heterostructure. Due to the lower work function of LTO, additional charge transfer is expected between 1UC FeSe and an STO substrate with more LTO layers. Surprisingly, 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. This shows that superconductivity in 1UC FeSe thin films is robust and anchored by a 'magic' doping level.
To understand the unique phenomena of a 'magic' doping level in the context of iron-based superconductivity, consider the possible interplay of first-order phase transition and superconductivity. Above a doping level of 0.11, an insulating phase may form, resulting in phase separation between superconducting and insulating phases. As a result, only ~0.11 doping is visible in the ARPES experiment.
Photo-induced 2D electron gas on SrTiO3 with different surface terminations
ARPES is surface-sensitive. How, then, does the surface termination of a bulk material affect its measurement outcome? By combining MBE growth and high resolution ARPES, we can study the surfaces of layered oxide materials in great detail. SrTiO3 (STO) is a prototypical oxide material with alternating layers of SrO and TiO2 and exhibits a wide range of physical phenomena. STO has previously been recognized as a band insulator, an incipient ferroelectric material, and a superconductor. Recently, there has been renewed interest in the study of STO as its surface was found to host a robust two-dimensional electron gas (2DEG) .
In this study, homoepitaxial STO thin films are grown with different surface termination. Upon exposure to ultraviolet light, a 2DEG is generated at its surface, and oxygen vacancy dopants are induced. Using ARPES, we found that only SrO-terminated STO films host a clear 2DEG. Films with a TiO2 termination do not. While most research on surface 2DEGs of STO assume 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)