Browsing by Subject "Heterostructures"

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    Defect Engineering in Perovskite Oxide Thin Films and Heterostructures
    (2019-03) Prakash, Abhinav
    Perovskite oxide (ABO3 type compounds) is an important class of materials exhibiting a wide range of functionalities. However, in comparison to conventional semiconductors such as silicon, they possess orders of magnitude lower room-temperature electron mobilities. For example, in doped SrTiO3, the best reported room-temperature value of electron mobility has remained below 10 cm2 V−1 s−1 for over five decades. The realization of a perovskite oxide semiconductor with high room-temperature mobility would constitute a significant advancement, enabling novel physical and perhaps even a plethora of new and more realistic device concepts. Very recently a key step in this direction was taken via the growth of bulk doped BaSnO3, where room-temperature mobilities as high as 320 cm2 V−1 s−1 was reported. Thin films of BaSnO3 show much lower room-temperature mobility values ranging between 1-180 cm2 V−1 s−1 and highly dependent on the growth method, choice of substrate, and dopants. Although these findings have been encouraging for fundamental studies and potential applications in room-temperature oxide electronics, there still remains many open fundamental questions and challenges including the role of defects on the properties of BaSnO3 and the scattering mechanisms that limit the mobility in thin films from reaching values close to bulk mobility. These questions will be addressed in this thesis by studying thin films of BaSnO3 grown by molecular beam epitaxy. One of the challenges with the growth of BaSnO3 is the high electronegativity (low oxidation potential) of tin suggesting that stronger oxidizing conditions such as ozone or high-pressure oxygen plasma are required to achieve full oxidation of Sn. Such extreme oxidation conditions in an ultra-high vacuum molecular beam epitaxy system may lead to undesirable consequences such as oxidation of elemental sources leading to flux-instabilities, filament oxidation, and potential damage to vacuum pumps. As the first step in this direction, a new radical-based hybrid MBE approach for tin-based compounds is developed. For BaSnO3 growth, Ba is supplied through effusion from a cell, Sn using a chemical precursor (hexamethylditin – (CH3)6Sn2), and oxygen using a radio frequency plasma source. The unique aspect of our approach is that hexamethylditin forms highly reactive Sn• radicals, which facilitate the growth of phase-pure, stoichiometric films even in weak oxidizing environment such as molecular oxygen. Using this approach, synthesis of phase-pure and epitaxial BaSnO3 with scalable growth rates and layer-by- layer control over thicknesses is reported. Reflection high-energy electron diffraction is used to describe the strain relaxation behavior of BaSnO3. Various characterization techniques are employed for establishing the stoichiometric growth condition such as X-ray diffraction for lattice parameter measurements, Rutherford backscattering spec- trometry for quantification of cation (Sn:Ba) ratio, atomic force microscopy for imaging the surface morphology, electronic transport for measuring the carrier concentrations, resistivity, and electron mobility in lanthanum-doped BaSnO3 films, and time-domain thermoreflectance for determining the thermal conductivity. With the combination of these techniques, existence of a self-regulating “growth-window” is demonstrated. Through controlled La-doping in BaSnO3 films, a highest room-temperature electron mobility of 120 cm2 V−1 s−1 is achieved on a -5.12 % lattice-mismatched SrTiO3 substrate. The optimal doping range for the highest mobility is found to be 5.0 × 1019 cm−3 to 5.0 × 1020 cm−3. Mobility decreases at higher or lower doping concentrations. Temperature-dependent measurements of mobility provide insights into the scattering mechanisms limiting the mobility at different doping concentrations and temperatures. While dislocation scattering is found to be dominant at low doping regime, ionized impurity scattering plays a major role at high doping levels. At intermediate doping concentrations, both scattering mechanisms control the transport behavior. Phonon scattering accounts for the decreasing trend in mobility with increasing temperature. Building upon these findings which revealed mobility-limiting mechanisms in uniformly doped BaSnO3, the final step involves the development of modulation doping approach n BaSnO3-based heterostructures. The basic idea behind modulation doping is to sep- arate electrons from their ionized donors. Favorable band offsets in BaSnO3–SrTiO3 and BaSnO3–SrSnO3 systems are established. Taking BaSnO3–SrSnO3 as the model heterostructure, electron transfer from La-doped SrSnO3 to BaSnO3 is demonstrated, resulting in dramatic changes in the transport behavior. Results are encouraging and clearly suggest that electrons in BaSnO3 can be separated from ionized dopants. The transport, however, is still limited by dislocations and defects at the interface which should be the focus of future studies.
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    Electronic Properties of Oxide and Semiconductor Heterostructures
    (2019-08) Sammon, Michael
    The modern world's dependency on electronics provides a constant need to discover new materials and devices. A promising technique to fabricate a new device is to create a heterostructure; a device consisting of two bulk crystals joined at an interface. These materials often support a low dimensional electron gas confined to the interface, which exhibits properties different than both the parent materials. These materials have led to the creation of MOSfets, the discovery of the quantum Hall effect, and in recent years the discovery of Majorana edge modes in nanowires. In this thesis, we study several different heterostructures. We begin with one of the most famous heterostructures, AlGaAs/GaAs. Modern AlGaAs/GaAs heterostructures support a high mobility two-dimensional electron gas (2DEG) in a quantum well. The 2DEG is provided by two remote donor $\delta$-layers placed on both sides of the well. Each $\delta$-layer is located in the midplane of a narrow GaAs well, flanked by narrow AlAs layers which capture excess electrons from donors. We show that each excess electron is localized in a compact dipole atom with the nearest donor. The excess electrons screen both the remote donors and background impurities, and are responsible for the observed high mobility. Still, we find that the mobility is substantially lower than theoretical estimates, which may be due to significant disorder in the donor layers, most likely roughness of the interfaces or spreading of the donors out of the midplane of the layer. Thus one should take care to make sure that the donor layers are as ideal as possible. We next move on to oxide heterostructures involving SrTiO$_3$ (STO). More specifically, we study the electron gas in accumulation layers of these heterostructures characterized by a density profile $n(x)$, where $x$ is the distance from the STO surface. SrTiO$_3$ at liquid helium temperatures has the highest dielectric constant which strongly enhances the role of nonlinear dielectric effects. It was recently shown that the nonlinear dielectric response results in an electron density profile $n(x)$ that slowly decays as $1/x^{12/7}$. We show that such a long tail of $n(x)$ causes the magnetization and the specific heat of the accumulation layer to diverge at large $x$. We explore the truncation of the tail by the finite sample width $W$, the transition from the nonlinear to linear dielectric response with dielectric constant $\kappa$, and the use of a back gate with a negative voltage $-\abs{V}$. We find that as a result both the magnetization and specific heat are anomalously large and obey nontrivial power law dependences on $W$, $\kappa$, or $\abs{V}$. In the linear dielectric regime under a strong magnetic field, the large dielectric constant of STO makes it easy to reach a quasi-one-dimensional state known as the extreme quantum limit (EQL) in which all electrons occupy the lowest Landau level. We present a theory of the EQL phase in STO accumulation layers. We find a phase diagram of the electron gas in the plane of the magnetic field strength and the electron surface concentration for different orientations of the magnetic field. In addition to the quasi-classical metallic phase (M), there is a metallic EQL phase, as well as an insulating Wigner crystal state (WC). Remarkably, the insulating Wigner crystal phase depends on the orientation of the magnetic field. We show that these effects can be measured through quantum capacitance measurements of the STO accumulation layer. The third material we study is semiconducting quantum wires. Though it is not a heterostructure, it supports a low dimensional electron gas which is often tuned with an external gate, making it similar to many of the devices we have studied. We have theoretically investigated the influence of interface roughness scattering on the low temperature mobility of electrons in quantum wires when electrons fill one or many subbands. We find the Drude conductance of the wire as a function of the linear concentration $\eta$ has a sharp peak. The height of this peak grows as a large power of the wire radius $R$, so that at large $R$ the conductance $G_{max}$ exceeds $e^2/h$ and a window of concentrations with delocalized states (which we call the metallic window) opens around the peak. Thus, we predict an insulator-metal-insulator transition with increasing concentration for large enough $R$. Furthermore, we show that the metallic domain can be sub-divided into three smaller domains: 1) single-subband ballistic conductor, 2) many-subband ballistic conductor 3) diffusive metal, and use our results to estimate the conductance in these domains. Finally we estimate the critical value of $R_c(\mathcal{L})$ at which the metallic window opens for a given length $\mathcal{L}$. We conclude the thesis with a discussion of a newer class of materials known as transition metal dichalcogenides (TMDs). We study a capacitor made of three monolayers TMD separated by hexagonal boron nitride (hBN). We assume that the structure is symmetric with respect to the central layer plane. The symmetry includes the contacts: if the central layer is contacted by the negative electrode, both external layers are contacted by the positive one. As a result a strong enough voltage $V$ induces electron-hole dipoles (indirect excitons) pointing towards one of the external layers. Antiparallel dipoles attract each other at large distances. Thus, the dipoles alternate in the central plane forming a 2D antiferroelectric with negative binding energy per dipole. The charging of a three-layer device is a first order transition, and we show that if $V_1$ is the critical voltage required to create a single electron-hole pair and charge this capacitor by $e$, the macroscopic charge $Q_c = eSn_c$ ($S$ is the device area) enters the three-layer capacitor at a smaller critical voltage $V_{c} < V_{1}$. In other words, the differential capacitance $C(V)$ is infinite at $V = V_{c}$. We also show that in a contact-less three-layer device, where the chemically different central layer has lower conduction and valence bands, optical excitation creates indirect excitons which attract each other, and therefore form antiferroelectric exciton droplets. Thus, the indirect exciton luminescence is red shifted compared to a two-layer device.