There are many well-known magnetic materials among spinel ferrites. The grain orientation and size can be controlled when spinels are grown on mismatched cubic substrates. Varying the grain size has a strong effect on the magnetic structure of the film due to the appearance of spin glass or superparamagnetic states.
Ruddlesden-Popper (RP) phases form natural, thermodynamically stable, and atomically sharp interfaces that are nearly lattice matched with perovskites. Growing very thin RP layers can thus be an interesting route to fabricating structurally and electronically sharp tunnel barriers and quantum wells in oxides.
BeO is the lightest stable oxide with a band gap of over 10 eV and excellent heat conductivity. BeO is isostructural with ZnO, and it is interesting to attempt to form alloy phases, despite the huge lattice mismatch. We use alternate deposition from BeO and ZnO targets to make the alloy films.
Carrier confinement at oxide interfaces may give rise interesting electronic states. Structurally sharp quantum wells grown by PLD can be used to study the transport behavior as a function of carrier density and distribution. In doped-channel FETs, the carrier density can be tuned by a combination of doping and field effect.
One of the largest challenges in fabricating oxide FETs is to find a suitable gate insulator that can be grown epitaxially on the semiconductor channel, such as SrTiO3. One good candidate is DyScO3, but despite good lattice matching, there is still a strong low-temperature threshold bias shift, indicating that the interface is not clean.
The FET switching performance is strongly affected by trapped charge and the maximum gate field that can be applied to the device. DyScO3 films can sustain higher breakdown fields than CaHfO3 while also having cleaner interfaces when grown epitaxially on SrTiO3.
Preparing good source and drain electrodes in epitaxial SrTiO3 FETs is complicated by the high growth temperatures needed for high-quality gate insulator growth. One of the most effective ways of fabricating electrodes is ion milling, which produces a conducting, oxygen-deficient surface layer that can survive high-temperature processing.
The morphology of the insulator film has a large effect on FET performance because the typical device size is larger than the thin film grain size. Due to this, the gate insulator may hold significant fixed charge that is directly visible as a threshold shift in FETs. This problem can be mitigated by film growth optimizations and by device scaling.
Lateral fractional-layer structures, such as nanowire arrays, can be used to study the effects of inhomogeneity on the transport properties of delta-doped layers and to probe the effects of vertical interdiffusion. Metallic LaTiO3 nanowires grown along the step edges of a step-and-terrace SrTiO3 substrate is one particularly convenient model system for such studies.
La doping transforms semiconducting SrTiO3 into a good metal even when the La atoms only form a single LaO delta-doping layer. Fractional lateral structures can be used to study at what fractional coverage a percolative conduction path forms in a two-dimensional layer. Such ultrathin heterostructures show a curious competition between 2-dimensional localization and 3-dimensional metallicity.
The operation of various electronic devices and the performance of functional materials is dependent on non-equilibrium carriers. The mobility of such carriers depends on the defect density in the semiconductor. Photoconductivity is one simple way of characterizing the presence and density of structural defects and impurities that affect non-equilibrium carrier transport in oxide semiconductors.
Rh:SrTiO3 is a p-type semiconductor that works as a hydrogen evolution catalyst in photoelectrochemical water splitting powered by sunlight. Rh:SrTiO3 thin films can be used to determine the role of the Rh4+/3+ dopant valence on the electronic structure and hydrogen evolution activity under visible light irradiation.
Although the electronic structures of Rh- and Ir-doped SrTiO3 are very similar, Rh:SrTiO3 is a p-type hydrogen evolution photocatalyst, while Ir:SrTiO3 is an n-type oxygen evolution photocatalyst. A comparison of the electronic spectra can be used to draw a direct link between the electronic structure and the photoelectrochemical activity.
The photoelectrochemical activity of oxide semiconductors is dependent on the locations of occupied and unoccupied in-gap states induced by doping. The location of the unoccupied states can be seen in X-ray absorption spectra, while the occupied states can be probed by x-ray emission spectroscopy.
VO2 is well known for the metal-insulator transition that occurs slightly above room temperature. This is a charge ordering transition that also involves a structural change. The structural transition can be driven by dynamic mechanical strain by bending a crystal, inducing a change in conductivity.
Ferroelectric materials generally form complicated domain structures. An electrochemical techniques has been used to impose a high and uniform electric field on a PbTiO3 film, inducing a homogeneous domain structure over a wide area.
Thin films occasionally exhibit spontaneous phase separation. A periodic composition variation has been observed in BaSnO3 films where the periodicity of the composition modulation depends on the growth rate.
Magnetite Fe3O4 is a ferrimagnetic ferroelectric below the Verwey transition temperature. Epitaxial magnetite nanopyramids can be grown on SrTiO3 in a process where solid-phase dewetting is used to form (001)-oriented seed crystals.
Magnetic insulators can be used to construct spin-filtering tunnel junctions. Pr0.8Ca0.2Mn1-yScyO3 is an insulator with a tunable magnetic field strength.
SrTiO3 is a very common substrate material for oxide thin film growth. In heterostructure studies, it is necessary to control the surface stoichiometry of the substrate. The Sr content on the substrate surface is measured by ion scattering spectroscopy and various treatment procedures can be used to optimize the substrate surface structure and composition.