Oxygen Plasma-assisted Molecular Beam Epitaxy (MBE)

Molecular beam epitaxy (MBE) growth, processing and characterization are carried out at the Ohio State University’s Electronic Materials and Nanostructure’s Laboratory. Central to the growth of complex oxides and their interfaces is a Veeco GEN 930 oxide MBE system. This growth tool is equipped with nine shuttered effusion cells, a load-locked three-pocket Thermionics electron beam evaporator, a Unibulb RF plasma source for oxygen, and 1-30 keV reflection high energy electron diffraction (RHEED) optics. A 3″ substrate manipulator assembly with continuous azimuthal rotation and dual filament heater enables crystal growth up to 1200°C and O₂ partial pressures up to 10^-5 Torr. A load locked entry/exit chamber, a buffer/preparation chamber, and an extension chamber transfer tube assembly permit UHV sample transfer between growth, processing, and analysis chambers without air exposure. The analysis chambers include XPS/UPS and DRCLS to monitor electronic, chemical, and structural properties of the epitaxial oxide films and their interfaces during the growth process and subsequent treatment processes like remote oxygen or hydrogen plasma.

X-ray & Ultraviolet Photoemission Spectroscopy (XPS/UPS)

The Versaprobe 5000 XPS facility uses an Al Kα (1486.6 eV) monochromatized emission line from an Al anode of the X-ray source and spherical capacitor analyzer with 0.477 eV energy resolution and minimum spatial resolution of 18.9 μm for measurements of surface and near-surface chemical composition, chemical bonding, and Fermi level position. An Omicron HIS 13 vacuum UV He lamp (He II photon line = 40.8 eV) is mounted within the XPS analysis chamber in order to perform higher resolution (overall resolution = 0.158 eV (He I)), angle-dependent UV Photoelectron Spectroscopy (UPS) measurements of valence bands, Fermi level positions, and valence band offsets. For thicknesses below a few nanometers, this XPS provides measurements of heterojunction band offset for the semiconductor-on-semiconductor junctions. A differentially-pumped Ar ion gun (<1.2 x10^-8 Torr) with energies controllable from <10 eV to 4 keV, focused at the XPS/UPS focal point, positioned and calibrated in-situ with a Faraday cup, permits both surface cleaning as well as introduction of lattice defects with controllable energies and fluences. The XPS/UPS system is connected with our MBE growth chamber through vacuum interconnects and transfer system.

Depth-resolved Cathodoluminescence Spectroscopy (DRCLS)

A new low temperature (80 K) DRCLS minichamber with Kimball Physics EGL-2022/EPGS-2022 (50 eV-5 keV) electron gun will be connected via vacuum interlocks with the XPS/UPS and MBE chambers. Fiber optics with near – IR to UV transmission efficiency collect and transmit the emitted photons to compact, multiple grating monochromators positioned outside the chamber. A metal evaporator with deposition masks to form 300-500 μm diameter Schottky diodes is also mounted in a connecting vacuum interlocked chamber. Also connected via UHV interlocks is a remote oxygen plasma (ROP) chamber to remove hydrocarbons, remove sub-surface hydrogen, and reduce subsurface oxygen vacancies without degrading rms surface roughness. These facilities enable electronic properties of thin metal overlayers and epilayers to be characterized incrementally during deposition or growth processes. The combination of DRCLS and angle – dependent XPS spectroscopies in the same chamber enables concurrent, nanometer – depth – resolved measurement of band gaps, deep trap levels, and chemical composition.