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Overview:

The Kawakami laboratory integrates molecular beam epitaxy (MBE) materials synthesis, nanoscale device fabrication, and advanced characterization via optical spectroscopy, magnetotransport, ultrafast magneto-optic microscopy, spin-polarized scanning tunneling microscopy (SPSTM), and time-resolved micron-scale angle-resolved photoemission spectroscopy (tr-microARPES). A special aspect of our apparatus is the ability to move samples among various growth chambers and measurement stations without exposing the samples to air. Based on sample transfer using ultrahigh vacuum (UHV) suitcases and inert gloveboxes, we achieve air-free integration of synthesis and measurement not only within our own laboratory, but also within Ohio State University user facilities as well as with off-campus facilities and collaborators.

Time-Resolved Micron-scale Angle-Resolved Photoemission Spectroscopy (tr-microARPES)

Prof. Kawakami is responsible for the tr-microARPES endstation at the NSF NeXUS national user facility, which is currently under construction. This will allow optical pump, XUV probe experiments to directly measure electron dynamics with energy-resolution, momentum-resolution, spatial-resolution, and time-resolution. Our primary interests are to investigate electron dynamics and phase transitions in topological, magnetic, and 2D materials. This will be used to:

  • track the movement of electrons through the electronic band structure
  • measure the dynamic evolution of the band structure itself
  • probe the dynamics of phase transitions

The endstation is a Specs Kreios system with a hemispherical energy analyzer, momentum-microscope capabilities, and photoelectron emission microscope (PEEM) for sub-micron spatial resolution. A pulsed XUV source based on high harmonic generation will provide photon energy of 10 – 100 eV and pulse widths of 300 fs down to 40 fs. The Krieos endstation was installed in 2021.

Material Synthesis by Molecular Beam Epitaxy

  • MBE-1 chamber: This MBE system is designed for the growth of metals, binary oxides, complex oxides, and magnetic materials such as Co, Fe, Al, Ti, Sr, Mg, MgO, SrO, EuO, SrTiO3, EuTiO3, Fe­3­O­4, CoFe2O4, etc. It is currently operational with base pressure less than 3´10-11 Up to nine deposition sources and a sample size of up to 1.5” can be accommodated. It is equipped with a reflection high-energy electron diffraction (RHEED), computer controlled shutters, a quartz crystal thickness monitor, ion pump, titanium sublimation pump (TSP), and liquid nitrogen cryopanel. Beyond the typical MBE features, our system possesses three unusual features: (1) a wedge shutter allows deposition of controlled thickness gradients for systematic thickness dependent studies, (2) a transferable thermocouple (Thermionics STLC system) for accurate temperature measurement; and (3) a water-cooled manipulator to stabilize the substrate temperature.

  • MBE-2 chamber: This MBE system is optimized for the synthesis of magnetic multilayers and intermetallic materials. It has eight sources in an upward facing geometry. Sources include PID-controlled Mn, Fe, Co, Ge, Sn, Cu, CaF2 effusion cells and e-beam Pt evaporation cell. A water-cooled manipulator allows a growth temperature up to 800 ⁰C which is monitored and controlled by a PID controller. It is equipped with reflection high-energy electron diffraction (RHEED), computer-controlled source shutters, a quartz deposition monitor, and a wedge shutter which enables the growth of samples with thickness gradient. The ion pump, turbo pump and titanium sublimation pump (TSP) can support a base pressure down to 4*10-10 Torr. The loadlock of this MBE system has a 2.75” side port which can be connected to the UHV suitcase. This allows UHV transfer of the MBE-grown sample to the other characterization systems for surface-sensitive measurements (for example, spin-polarized STM and time-resolved ARPES). The research has focused on the synthesis of (1) skyrmion materials FeGe, MnGe, and related B20 compounds, (2) thin films of kagome material Fe3Sn2, CoSn, RMn6Sn6 (R = rare earth) and their heterostructures, and (3) atomically-thin metallic magnetic multilayers for size-tunable skyrmions.

  • 2D MBE chamber (in SEAL user facility): Kawakami is a primary user of this Veeco GEN930 MBE system in the Semiconductor Epitaxy and Analysis Laboratory (SEAL) facility. The system is equipped with a Se valved cracking source and a Te effusion cell from the growth of Se- and Te-based 2D chalcogenide materials. It is equipped with RHEED, cryopump, ion pump, liquid nitrogen cryopanel, and can support up to nine MBE sources. Specific materials of interest are 2D semiconductors (MoSe2, WSe2), semimetals (MoTe2, WTe2), and magnets (CrGeTe3). In addition to RHEED, this MBE chamber is also equipped with Raman and photoluminescence spectroscopy for in situ characterization.
  • Quantum Defects MBE chamber: For the synthesis and in situ characterization of quantum point defects, this ultrahigh vacuum chamber has: MBE effusion cells, variable temperature sample manipulator from ~77 K (liquid nitrogen) to ~1800 K (e-beam heating), and in situ optical characterization by photoluminescence spectroscopy and microscopy. The chamber is equipped with electromagnets with ~1200 Oe in-plane field, and ~400 Oe out-of-plane field. Our primary materials of interest are rare-earth magnetic impurities in various wide gap semiconductor hosts for potential applications in quantum memories and quantum networking.

Device Preparation

  • Exfoliation: A standard optical microscope is utilized for identifying and isolating exfoliated 2D materials.
  • Transfer tool: A home-built transfer tool consists of a Mitutoyo microscope head with two xyz sample manipulators, rotational/tilt stage, and sample heater. This is used for the alignment and dry transfer 2D materials for fabricate vertically-stacked van der Waals heterostructures.

  • UHV transfer tool: Kawakami has developed a unique tool to transfer 2D van der Waals materials from a polymer support onto a freshly grown MBE sample for an ultraclean 2D-to-MBE interface. The transfer process produces junctions free of bubbles and has a nearly 100% yield.
  • Glovebox exfoliation and transfer tools (in NSL user facility): This transfer tool is located in a glovebox in the NSL user facility, which makes it available to on-campus and off-campus users. This enables the creation of van der Waals heterostructures using air-sensitive 2D materials such as many 2D magnets and 2D superconductors.
  • Electron beam lithography and nanoscale device fabrication (in NSL user facility): We utilize the NSL cleanroom for nanoscale device fabrication.

Optical and Transport Characterization

  • Magnetotransport measurement system (Transport-1): This is an Advanced Research Systems cryogen-free cryostat equipped with a vacuum space, voltage-controlled current source, lock-in amplifier, source-meter, high-precision multimeter, and 3000 Gauss 2-axis vector magnet.
  • Magneto-optic Kerr effect (MOKE-1): This is equipped with a He-Ne laser, optical chopper, lock-in amplifier, photodiode bridge, polarization optics, and 1500 Gauss electromagnet. This is used to measure magnetic films synthesized by MBE.
  • Photocurrent microscopy and spectroscopy (ARS Microscopy 1): This system is designed for spin/charge photocurrent microscopy and is also capable of MOKE, PL and magnetotransport measurements. The three key elements are:
    • Fianium supercontinuum laser is a white light supercontinuum fiber laser (Fianium SC400-8). The output power is 8 W and the spectral range is 400 nm to 2300 nm.
    • ARS low vibration optical cryostat is an Advanced Research Systems cryogen-free cryostat with a temperature range of 5-300 K. There are 5 fused silica optical windows which permit magneto-optic access in the longitudinal and transverse directions.
    • 2-axis vector magnet is a homemade electromagnet with a maximum field of 200 mT.
  • Ultrafast Suite: Two ultrafast laser systems, two cryostats, and spectrometer are used interchangeably for time-resolved magneto-optic measurements, MOKE microscopy, PL microscopy, photocurrent microscopy, white-light absorption and reflection, and magnetotransport measurements in various magnetic field and temperature environments.
    • Coherent Mira HP Ti:sapphire laser/Optical Parametric Oscillator (OPO) laser system: This laser system consists of a commercial OPO (“OPO-1”) pumped by a high power Ti:sapphire oscillator (“Ti:S laser-1”).  The Mira is pumped by an 18 W Lighthouse Sprout solid state laser and can output 4 W.  The Mira wavelength tuning range is 700-980 nm and the pulse duration is ~150 fs with a repetition rate of 76 MHz.  The Mira HP can be used independently, or to pump the OPO, which provides optical pulses with a wavelength range of 1000-1500 nm.  The OPO also has an idler which provides wavelengths from 1800-3000 nm, and a SHG crystal which provides wavelengths of 500-750 nm.
    • Coherent Mira 900 Ti:sapphire laser (Ti:S laser-2): This is a commercial Ti:sapphire oscillator pumped by a 8 W Coherent Verdi solid state laser. Wavelength tuning range is 700-980 nm and the pulse duration of ~150 fs. The repetition rate is 76 MHz.
    • Oxford Spectromag Cryostat: This is a magneto-optic cryostat with magnetic field range from -7 T to +7 T, and temperature range of 1.6 K – 300 K. Four fused silica optical windows permit magneto-optic access for both transverse and longitudinal access.
    • Montana Instruments Magneto-optic Cryostation: This is a low vibration Montana Instruments cryogen-free cryostat with a temperature range of 5-300 K and a magnetic field from -700 to 700 mT.  There are 5 fused silica optical windows which permit magneto-optic access in the longitudinal and transverse directions.  Attocube piezo stages allow 3D motion of the sample.
    • 0.55 m optical spectrometer with thermoelectric-cooled CCD: This triple-grating imaging spectrometer is a Horiba iHR550 system with 150 g/mm, 1200 g/mm, and 2400 g/mm gratings installed. The spectral resolution is 0.025 nm and a spectral dispersion of 1.34 nm/mm.

Spin-polarized STM

STM and non-contact AFM measurements are carried out using a Createc LT-STM/AFM system, which was primarily funded by FY2014 ARO-DURIP award (Award #W911NF-14-1-0457). The instrument is being managed jointly by Prof. Kawakami and Prof. Gupta. The instrument is customized for spin-polarized STM measurements, with operation at low temperatures (5-300K) and with two-axis magnetic field capability (1T in/out of sample plane). Energy resolution in tunneling spectroscopy is thermally limited to ~ 0.2 meV, which sets a lower limit for magnetic anisotropy barriers that can be measured with inelastic electron tunneling spectroscopy. We use bulk Cr or Cr-coated W tips for SPSTM experiments.  The instrument also has high bandwidth cabling allowing GHz-scale spin dynamics measurements with STM, as pioneered by S. Loth at IBM. AFM operation is based on the qPlus tuning fork tips designed by F. Giessbl, and is also capable of atomic resolution imaging and force spectroscopy of e.g., contact potential differences on the surface. Electrical isolation allows simultaneous STM/AFM imaging for complementary topographic, current and force images of the surface.

The UHV system for this instrument is outfitted with Auger/LEED for initial characterization of surface contamination and crystallinity. Home-built e-beam sources are used to deposit a low coverage (~ 1% of a ML) of transition metal impurities (e.g., Fe, Co, Mn), and atomic H can be deposited using a thermal cracker (Omicron) onto the cold sample in STM, allowing before/after imaging of the same nanoscale area. MBE grown samples can be transferred into the SPSTM system via a Ferrovac UHV suitcase. This allows samples to be transported between UHV systems in different labs, while maintaining a UHV environment (10-10 mbar) for up to 60 hrs.

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