In this manuscript, we demonstrate a new process for
the atomic
layer deposition (ALD) of gallium oxide (Ga2O3) thin films using trimethylgallium (TMGa) and ozone. We evaluated
a variety of oxygen sources for Ga2O3 ALD using
TMGa but found that only ozone was effective. We explored the mechanism
for Ga2O3 ALD using in situ quartz crystal microbalance,
Fourier transform infrared spectroscopy, and quadrupole mass spectrometry
studies. We found that TMGa dissociatively adsorbs onto the Ga2O3 surface to form Ga(CH3)2 surface species and liberate ∼20% of the methyl ligands as
CH4. Next, the ozone reacts with these methyl species to
form hydroxyl and formate surface groups and liberate CH2O. We prepared ALD Ga2O3 films on Si(100) and
fused SiO2 substrates and analyzed the films using a variety
of techniques. We found the Ga2O3 growth to
be self-limiting with a growth rate of ∼0.52 Å/cycle between
200 and 375 °C. Moreover, the Ga2O3 films
were stoichiometric, free of residual carbon, and exhibited properties
similar to bulk Ga2O3. Scanning electron microscopy
revealed smooth films with good step coverage over trench structures,
and X-ray diffraction showed that the films were amorphous as-deposited
but crystallized to β-Ga2O3 upon annealing
at 900 °C.
Scanning ion conductance microscopy (SICM) has developed into a powerful tool for imaging a range of biophysical systems. In addition, SICM has been integrated with a range of other techniques, allowing for the simultaneous collection of complementary information including near-field optical and electrophysiological properties. However, SICM imaging remains insensitive to electrochemical properties, which play an important role in both biological and nonbiological systems. In this work, we demonstrate the fabrication and application of a nanopipet probe with an integrated ultramicroelectrode (UME) for concurrent SICM and scanning electrochemical microscopy (SECM). The fabrication process utilizes atomic layer deposition (ALD) of aluminum oxide to conformally insulate a gold-coated nanopipet and focused ion beam (FIB) milling to precisely expose a UME at the pipet tip. Fabricated probes are characterized by both scanning electron microscopy and cyclic voltammetry and exhibit a 100 nm diameter nanopipet tip and a UME with an effective radius of 294 nm. The probes exhibit positive and negative feedback responses on approach to conducting and insulating surfaces, respectively. The suitability of the probes for SECM-SICM imaging is demonstrated by both feedback-mode and substrate generation/tip collection-mode imaging on patterned surfaces. This probe geometry enables successful SECM-SICM imaging on features as small as 180 nm in size.
Figure S1. XP spectra of the Si 2p region of MnO|Si photoelectrodes with 100 cycles (~10 nm) of MnO (filled circles and blue line) and 25 cycles (~2.5 nm) of MnO (open circles).
Out-of-phase boundaries (OPBs) are translation boundary defects characterized by a misregistry of a fraction of a unit cell dimension in neighboring regions of a crystal. Although rarely observed in the bulk, they are common in epitaxial films of complex crystals due to the physical constraint of the underlying substrate and a low degree of structural rearrangement during growth. OPBs can strongly affect properties, but no extensive studies of them are available. The morphology, structure, and nucleation mechanisms of OPBs in epitaxial films of layered complex oxides are presented with a review of published studies and new work. Morphological trends in two families of layered oxide phases are described. The atomic structure at OPBs is presented. OPBs may be introduced into a film during growth via the primary mechanisms that occur at film nucleation (steric, nucleation layer, a-bmisfit, and inclined-cmisfit) or after growth via the secondary nucleation mechanism (crystallographic shear in response to loss of a volatile component). Mechanism descriptions are accompanied by experimental examples. Alternative methods to the direct imaging of OPBs are also presented.
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