Epitaxial oxide films for electronic applications

Resume : Revolutionary advances in next-generation electronics rely on the development of truly disruptive and robust electronic materials with novel and/or enhanced physical properties. Recently, we have built up a state-of-the-art, high-throughput physical vapor epitaxy (PVE) suite capable of synthesizing extremely high quality epitaxial layers, using custom computer-controlled systems to rapidly assess and identify novel materials with enhanced physical properties. This talk will focus on the growth of high quality oxide heterostructures through unconventional PVE routes. I will show how these synthesis tools are creating exceptionally high quality and novel magnetic oxides with the unique combination of record high magnetostriction and microwave performance, and new UWBG materials such as β-Ga2O3 that can be doped with silicon and alloyed with Al2O3 to control electronic properties. We found that at 0.2 wt % Si, a carrier density and mobility of 7.6 x 1019 cm-3 and 52.2 cm2/Vs (respectively) was observed. As the silicon concentration increased, the carrier density and mobility decreased. Epitaxial films of β-(AlxGa1-x)2O3 were deposited using targets with Al composition ranging from 0.175 to 0.5. X-ray diffraction confirmed both homoepitaxial Si-doped β-Ga2O3 and β-(AlxGa1-x)2O3 films. Hence, we demonstrate that pulsed laser deposition as a viable technique to grow both Si-doped homoepitaxial β-Ga2O3 and β-(AlxGa1-x)2O3/Ga2O3 heterostructures for wide bandgap electronic devices. Finally, I will give an update on our recent acquisition of a molecular beam epitaxy tool for β-Ga2O3 and share any initial results on homoepitaxial layers achieved to date.

Resume : Due to its wide band gap of ≈4.7 eV the n-type semiconducting oxide Ga2O3 has recently been recognized as a promising material for power electronics applications and UV detectors. These advanced applications typically require single crystalline films with well-defined thickness and doping, which can be readily realized by molecular beam epitaxy (MBE). During the MBE of Ga2O3 and In2O3 the desorption of their suboxides Ga2O and In2O has been determined as growth-rate-limiting step [1,2]. We explain this behavior quantitatively by a two-step growth model that involves the intermediate suboxide formation [3]. Through metal-exchange catalysis [5], the kinetic advantage for the In2O3 formation [2] combined with the thermodynamic advantage for the Ga2O3 formation [4] lead to a strong enhancement of the Ga2O3 growth rate under an additional In-flux. For the device-application relevant homoepitaxy of Ga2O3(010) under Ga2O-desorption limited growth conditions the formation of {110} facets and a strongly enhanced growth rate by metal-exchange catalysis are demonstrated.[6] This work was performed in the framework of GraFOx, a Leibniz-ScienceCampus partially funded by the Leibniz association. [1] P. Vogt et al., Appl. Phys. Lett. 108 (2016) 072101 [2] P. Vogt et al., Appl. Phys. Lett. 109 (2016) 062103 [3] P. Vogt et al., (submitted) (2018) [4] P. Vogt et al., APL Materials 4 (2016) 086112 [5] P. Vogt et al., Phys. Rev. Lett. 119 (2017) 196001 [6] P. Mazzolini et al., in preparation (2018)

Resume : Ultrawide bandgap (UWBG) gallium oxide (Ga2O3) represents an emerging semiconductor material with excellent chemical and thermal stability. It has a band gap of 4.5-4.9 eV, much higher than that of the GaN (3.4 eV) and 4H-SiC (3.2 eV). The monoclinic beta-phase Ga2O3 represents the thermodynamically stable crystal among the known five phases. The breakdown field of beta-Ga2O3 is estimated to be 8 MV/cm, which is about three times larger than that of 4H-SiC and GaN. These unique properties make beta-Ga2O3 a promising candidate for high power electronic device and solar blind photodetector applications. More advantageously, single crystal beta-Ga2O3 substrates can be synthesized by scalable and low cost melting based growth techniques. Different from the molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) growth techniques, we have developed a low pressure chemical vapor deposition (LPCVD) method to grow high quality beta-Ga2O3 thin films on both native Ga2O3 and sapphire substrates with controllable doping and fast growth rates up to 10 um/hr. In this talk, we present the growth, material characterization and device demonstration of beta-Ga2O3 thin films grown via LPCVD. The beta-Ga2O3 thin films were grown on native Ga2O3 (010), (001) and (-201) substrates and sapphire substrates using high purity gallium and oxygen as the precursors, and argon (Ar) as the carrier gas. The growth temperature ranged between 850 ˚C and 950 ˚C. Fundamental material properties will be discussed.

Resume : Homoepitaxial β-(In,Ga)2O3 thin films were successfully grown on (100) β-Ga2O3 substrates by metal organic vapor phase epitaxy (MOVPE) using triethylgallium (TEG) and triethylindium (TEIn). Growth temperatures from 650 to 825 °C and pressures from 5 to 20 mbar have been investigated. The growth rate decreased linearly with increasing growth temperature and decreased exponentially with increasing pressure. The resulting films were investigated by atomic force microscopy (AFM), high resolution X-ray diffraction (HR-XRD), Raman spectroscopy, transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDX). AFM images of the β-(In,G)a2O3 with different Indium contents showed smooth surfaces having a surface roughness of about 1 nm which slightly increased with decreasing growth temperature. HR-XRD analysis revealed that the out-of-plane lattice parameter of β-(In,Ga)2O3 thin film increases with increasing Indium content and the incorporation of indium is slightly sensitive to the growth temperature and pressure. In the Raman spectra of the -(In,Ga)2O3 films no phonon modes of additional phases, such as In2O3, could be detected. As a signature of the In incorporation, a slight shift of the Raman peaks was observed. TEM was used to examine the structural details of the grown films and the average Indium contents were verified by HR-XRD and EDX.

Resume : In this contribution we report about the growth of homoepitaxial β-Ga2O3 layers by MOVPE on different oriented substrates. In the focus of our work is the deposition of delta doped layers structures. We investigated the beginning of growth for the (100) direction and we were able to prove a layer by layer growth. For this type the growth takes place by the formation of 2D islands nucleate on the surface, leading to defects and stacking faults. Therefore we used (100) substrates with a miscut of 6° in c-direction to generate a step flow growth resulting in a suppression of the 2D island nucleation. To compare the growth of ultra-thin layers in relation to the defect generation and the shape of the interfaces we used as an alternative (010) substrates. We realized stacking layers with the following shape: 1) 175 nm LD (low doped) layer 2) 3-4 nm highly doped (HD) ẟ-layer 3) 20 nm LD layer on (010) and (100) 6° off β-Ga2O3 substrates by MOVPE. As precursors we used triethylgallium (TEGa), oxygen as oxidant and tetraethylorthosilicate (TEOS) as source for silicon as dopant. The morphology was analyzed by AFM and SEM. To determine the layer thickness ellipsometry measurements were performed. The depth profile and the silicon concentration of the multilayer stack was measured by using secondary ion mass spectroscopy (SIMS). The shape of the interfaces between the layers and the structural perfection were analyzed by SIMS and high resolution transmission electron microscopy (HRTEM).

Resume : In this contribution, we investigate the influence of the stepdown direction, namely the -c and c direction of the monoclinic lattice on the structural and electronic properties of homoepitaxial layers. We will show that for substrates with a stepdown direction prepared 6° towards -c, perfect step flow growth is achieved. Such layers exhibit excellent structural and electronical properties. TEM investigations reveal no structural defects within the layer and Hall-Effect measurements showed high carrier motilities between 60 -100 cm²V-1s-1 at doping concentrations in the range of 1 - 2 x 1018 cm-³. On contrary, for substrates prepared towards the c direction the layer is characterized by a high amount of twin lamellae generated at the interface between the substrate and the layer. These twin defects generate incoherent twin boundaries that penetrate through the whole layer. As shown previously8,9 such incoherent twin boundaries act as barriers for carrier transport and cause a significantly reduced charge carrier mobility of around 10 cm²V-1s-1 in these layers. To identify the reason for the different growth behaviours, we performed high-resolution scanning transmission electron microscopic (STEM) investigations at surface of the grown layer. Our data reveals that the step down direction is always -c, independent whether the substrate miscut was towards c or -c. This means that, in the case of growth on substrates with a miscut direction towards the c direction, the whole layer growth in an orientation that can be described by a c/2 glide reflection in respect to the substrate, which is the typical twin relation as described in our previous studies. Moreover, we find that the surface is characterized by (100) terraces with (2 ?01) step edges, which also accounts for both substrate miscut directions c or -c. This suggests that these facets are energetically very favourable and thus prevailing during growth To verify this assumption, density functional theory (DFT) calculations were carried out evaluating the surface free energies of different ideal crystallographic planes of b-Ga2O3. The DFT results confirm that both - the (100) and the (2 ?01) facets - are lowest in energy thus supporting our observed growth results.

Resume : Thin films of the wide band gap semiconductor ß-Ga2O3 have promising applications in transparent electronics and high power devices. However, to date the transport, thermal and thermo-electrical properties in thin films remain unknown. Here, the electrical and thermal conductivities, Hall densities and mobilities and Seebeck coefficients were measured for various thin films. In thin homoepitaxially MOVPE grown (100)-orientated ß-Ga2O3 films the room temperature mobilities in thicker films (225 nm) were similar to that of bulk of about 115 cm2/Vs , in the thinnest film (28 nm) it was reduced by about 5.5 cm2/Vs which is discussed by finite size effects. Seebeck coefficients in the range of a few hundreds µV/K have been measured at room temperature and below. The thermal conductivity was investigated of thin MOVPE grown polycristalline ß-Ga2O3 films on sapphire-substrates and single-crystalline, electrically conductive ß-Ga2O3 films on insulating Mg-doped Czochralski-grown ß-Ga2O3 substrates. In order to measure the thermal conductivity of films and substrates, we used the different techniques (3-omega and 2-omega approaches). We observe a reduction of the bulk thermal conductivity in dependence of the crystallinity and film thickness which can be explained with the reduced mean free path of the phonons due to the film thickness and grain sizes.

Resume : The wide bandgap material (WBM), single crystal beta phase gallium oxide has emerged as a semiconductor for use in several applications which may exceed the performance of current WBM semiconductors (e.g., SiC, GaN, and diamond) [1,2]. Some WBM power/RF devices are typically configured as high electron mobility transistors (HEMTs), which can be impacted by parasitic leakage current at the Schottky junction that serves as the gate – especially at elevated temperatures. In addition, a particular challenge of beta-Ga2O3 is its low thermal conductivity. Therefore, continued research and innovation are required to improve understanding and device performance. One approach is integrating a gate dielectric to significantly reduce this leakage effect while simultaneously increasing the breakdown field. Furthermore, amorphous Ga2O3 films demonstrate some of the same characteristics with the added advantage of the deposition process being carried out at room temperature [3,4]. This enables the integration of thin films in flexible electronics along with easier thickness and carrier concentration control, thereby establishing a variety of devices with no degradation of charge transport when compared with polycrystalline films. We will evaluate single crystal, metal-oxide-semiconductor capacitors, along with amorphous un-doped Ga2O3 UV detectors, Ga2O3–based TFTs, and all-oxide NiO/Ga2O3 pn junctions. [1] K. Irmscher, et al., JAP, 110, p. 063720, 2011. [2] M. Higashiwaki, et al., APL, 100, p. 013504, 2012. [3] M. Heinemann, et al., APL, 108, p. 022107, 2016. [4] S. Cui, et al., Adv. Opt. Mat., 2017.

Resume : Beta-Ga2O3 has recently attracted a lot of attention as a novel ultra-widebandgap semiconductor for next generation power devices. First, we will discuss the fundamental transport studies on Ga2O3. The low symmetry monoclinic crystal of Ga2O3 and presence of multiple IR active optical phonon modes are a challenge in these studies as the popular methods developed for symmetric crystals cannot be used in this material system. Both low field and high field transport calculations in Ga2O3 will be discussed with implications on device performance. Next, we will discuss our recent work on atomic layer deposited SiO2 dielectrics on Ga2O3. The interface electrical characterization shows low interface state densities making it suitable both as a gate and passivation dielectric. Both depletion mode and enhancement mode devices with high breakdown voltage were demonstrated using SiO2 as a gate dielectric. A low cost and effective spin-on-doping technology for low resistance source/drain contacts was developed and integrated into Ga2O3 devices. Temperature dependent characterization of these devices shows the potential of Ga2O3 for extreme environment applications.

Resume : Monoclinic gallium sesquioxide (beta-Ga2O3) belongs to the material class of transparent semiconducting oxides. It is distinguished by its large band gap of about 4.7 eV, which is the reason for a transparency range extending deep into the ultraviolet and for a high electrical breakdown field estimated at 8 MV/cm. Combined with the feasibility of n-type doping by Sn or Si, beta-Ga2O3 has great potential as a material for solar-blind photo detection and for power electronics where it might outperform GaN and SiC. For such applications, doping control is of outermost importance. Here we report on a Raman spectroscopic investigation on highly n-type doped beta-Ga2O3 single crystals obtained by the Czochralski method. For degenerate material (? > 3 × 1018 cm-3), we observe Raman lines at 256 cm-1 and 282 cm-1, forming a double peak, and at 563 cm-1 (about twice the wavenumber of the second peak), which cannot be assigned to first-order scattering by phonons. These lines exhibit only a weak temperature dependence and are essentially independent of the shallow donor species (Sn or Si). We attribute the doping induced Raman features to electronic Raman scattering caused by excitation of electrons from an effective-mass like donor impurity band into the conduction band. This assignment is based on the facts that the high-energy edge of the double peak coincides with the ionization energy of effective-mass like donors (36 meV), and that the Raman signals only appear for doping concentrations exceeding the Mott criterion. Consequently, the high-frequency Raman line at 563 cm-1 (= 2 × 282 cm-1) is explained by second-order electronic Raman scattering.

Resume : Complex-oxide materials possess a range of interesting properties and phenomena that make them candidates for next-generation devices and applications. But before these materials can be integrated into state-of-the-art devices, it is important to understand how to control and engineer the response of these materials in a deterministic manner. Here, we will discuss the science and engineering of emergent ferroic order and phenomena. In particular we will explore the role of epitaxial thin-film growth processes and the use of new types of lattice mismatch strain to engineer classic ferroelectric systems. In recent years, the use of epitaxial strain has enabled the production of model versions of these complicated materials and the subsequent deterministic study of field-dependent response. Here, we will investigate how new manifestations of epitaxial constraint can allow us to expand beyond bistable switching responses. In this regard we will explore the use of epitaxial film orientation to produce exotic domain structures and emulate neuron-like function in materials and defect-based routes to engineer local switching behaviors so as to produce multi-state stability in ferroelectric devices. The discussion will range from the development of a fundamental understanding of the physics that lies at the heart of the observed effects, to an illustration of routes to manipulate and control these effects, to the demonstration of solid-state devices based on these materials.

Resume : Magnetic skyrmion is a topologically protected whirling spin texture in nanoscale. Its small size, topologically-protected stability, and solitonic characteristics together hold great promises for future spintronics applications. To translate such compelling features into practical spintronic devices, a key challenge lies in achieving effective control of skyrmion properties, such as size, density, and thermodynamic stability. Here, we report the discovery of ferroelectrically tunable skyrmions in ultrathin BaTiO3/SrRuO3 bilayer heterostructures. The ferroelectric proximity effect at the BaTiO3/SrRuO3 heterointerface can trigger a sizable Dzyaloshinskii-Moriya interaction, thus stabilizing robust high-density skyrmions. The skyrmion size in this system can be minimized down to approximately 10 nm. Moreover, by manipulating the ferroelectric polarization of the BaTiO3 layer, we achieve local, switchable and nonvolatile tunability of both skyrmion density and thermodynamic stability. Such ferroelectric control of skyrmion properties heralds a novel approach to simultaneously enhance in the integratability and addressability of skyrmion-based functional devices.

Resume : Controlling the functional properties of strongly-correlated system by ferroelectric field effect (Fe-FET) that allows faster and lower-power operations has sparked a surge of research activities in the last decades. Here, we report on perovskite heterostructures combining (Ca,Ce)MnO3 and BiFeO3 in its ‘‘supertetragonal’’ phase on YAlO3(001) substrates. The rich phase diagram of (Ca,Ce)MnO3 offers great potential for electrostatically induced metal-insulator transitions and the large polarization of BiFeO3 qualifies it as a suitable ferroelectric gate. BiFeO3/(Ca,Ce)MnO3 bilayers with different Ce contents are epitaxially grown by pulsed laser deposition. High quality is assessed by X Ray Diffraction, Piezoresponse Force Microscopy (PFM) and Scanning Transmission Electron Microscopy (STEM). For the Fe-FET studies, these BiFeO3/(Ca,Ce)MnO3 bilayers are patterned into Hall bars in which the polarization of the BiFeO3 gate can be switched back and forth with the PFM. The influence of the BiFeO3 polarization direction on the resistance and the carrier concentration in (Ca,Ce)MnO3 is checked by transport measurements. Nevertheless, no prominent electronic responses are observed. We carried out depth resolved mapping of ferroelectric switching by STEM. Indeed, the ferroelectric field effect is very sensitive to the interfacial state and STEM combined with Electron Energy Loss Spectroscopy (EELS) can be used to probe the interfacial electronic and structural properties between the ferroelectric and the Mott insulator. Surprisingly, atomic-resolved STEM images reveal that the polarization switching is incomplete with 31% to 91% of the volume population being effectively reversed from down to up. The switched domains seem to nucleate from the top surface of BiFeO3 and propagate toward the BiFeO3/(Ca,Ce)MnO3 interface. However, only 19% of the interface is successfully switched. While the small fraction of switched polarization at the interface causes the deterioration of the field effect at the micron scale, the charge transfer onto the adjacent (Ca,Ce)MnO3 can be clearly observed in nanometric areas which are successfully switched down to the interface. The electrostatic carrier modulation is revealed by the EELS near-edge fine structure of Mn L3 at interfaces with opposite polarization. Close to the BiFeO3/(Ca,Ce)MnO3 interface, the excess of electrons is lowered for the upward polarization with an effective charge modulation of 2.38 e- corresponding to a sheet carrier density variation of 1x1015 cm-2. This is within the range of the expected modulation for BiFeO3 with a polarization of the order of 100 μC.cm-2, showing the expected ferroelectric field-effect at the local scale.

Resume : Ag(Nb1−xTax)O3 ceramics and thin films have received a great deal of attention over the years because of their interesting dielectric/ferroelectric properties and complex phase transitions. Additionally, the microwave dielectric properties as another research hotspot of Ag(Nb1−xTax)O3 system have also been investigated due to combination of a high dielectric constant, a medium Qf value and a low temperature coefficient of resonant frequency, and can be thus integrated with various passive dielectric components like band-pass filters and resonators. In the present work we investigated the growth and dielectric properties of Ag(Nb0.5Ta0.5)O3 (ANT) thin film grown on (0001) Al2O3 single-crystalline substrates by pulsed laser deposition. Samples were characterized by reflection high-energy electron diffraction, X-ray diffraction (XRD), Rutherford backscattering spectroscopy, electron microscopy, and high-frequency dielectric measurements. The effect of laser fluence and oxygen background pressure on plume size and ANT composition were determined, together with the effect of target-to-substrate distance, deposition temperatures, and the laser frequency. Optimized deposition conditions enabled us to grow a pure-phase polycrystalline Ag(Nb0.5Ta0.5)O3 thin film with the thickness of 300 nm. For as-prepared thin film electrical properties in broad frequency range were determined and correlated with characteristics of ceramics counterparts.

Resume : Due the efficient ferroelectric polarization-driven carrier separation perovskitic thin films are increasingly being studied for applications in solar energy. The multiferoics materials, with a small band gap are potential candidates for green energy production. In this category, bismuth ferrite (BiFeO3-BFO) can have a high impact. Due the chemical stability and high leakage issues, BFO application in photovoltaics still pends. Having the possibility of tailoring the value of band gap of BFO by rare earth elements doping, this material is intensively studied including for photocatalitic apllications such as water splitting or degradation of organic pollutants. In this work we report the functional properties BiFeO3/LaFeO3 heterostructures obtained by Pulsed Laser Deposition on different substrates (Nb doped SrTiO3, SrTiO3, GdScO3 substrates). The purpose of the study is to benefit from the strong ferroelectricity of BFO in combination with the excellent chemical stability of LaFeO3 (LFO) for obtaining superior photocatalytic behavior. A parametric study on the influence of deposition parameters on the properties of the BiFeO3/LaFeO3 heterostructures was carried out. Crystalline properties and topography of surface of thin films were studied using X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM) and atomic force microscopy (AFM). The optical properties were determined by spectroscopic ellipsometry (SE) and the values of band gap (Eg) where determined from Tauc plot. We have obtained high photocurrent densities (J photocurent> 700 µA/cm2) of the heterotructures for a limited interval of the monolayer's thickness, by photoelectrochemical analysis, showing a non-monotonic dependence of the photocatalytic activity with induced structural strain.

Resume : In recent years, it has been shown that novel functionalities can be achieved in oxide heterostructures in which the interfaces are atomically controlled, in terms of atomic stacking as well as in terms of the local symmetry. In this contribution, I will highlight the recent developments in atomic controlled growth of epitaxial oxides by pulsed laser deposition, with a focus on heterostructures showing manipulated magnetic and electronic properties. Integration of epitaxial complex oxides in Si and III-V technologies has recently attracted a lot of attention in science and industry. Such complex oxides include, amongst others, ferro- and piezo-electrics and materials for resistive switching devices. In this presentation I will focus on the integration of Pb(Zr,Ti)O3 (PZT) with Si and III-V semiconductors. The epitaxial integration of PZT with Si and for instance GaN is hampered by the difference in crystal structure and large lattice mismatch. Using epitaxial buffer layers and optimized growth with pulsed laser deposition, we are able to obtain epitaxial growth of PZT on, for instance, MgO-buffered GaN. The thickness of the MgO can be lowered down to single monolayers while maintaining the high quality and good properties of epitaxial PZT films, which enable practical applications for high power FET’s and non-volatile ferroelectric controlled electronics devices. I will furthermore highlight some recent new insights in the “physics” of pulsed laser deposition of complex oxides, focusing on the influence of oxygen pressure on the deposited species during growth as well as the large-scale growth of epitaxial oxides on wafers up to 200 mm in diameter.

Resume : Ferroelectric polarization switching has been shown to persist down to about three or four monolayers of BaTiO3 when it is inserted between two metal electrodes. When it comes to a metal-insulator-semiconductor capacitive structure, the stabilization of a ferroelectric polarization in the insulator phase raises specific questions due to the inability of the semiconductor to provide free carriers to screen the surface charges. We studied the growth, structural and ferroelectric properties of BaTiO3 grown on silicon either in the form of epitaxial ultrathin films (1.6 – 4 nm) or in the form of composite material consisting of an amorphous matrix and of nanocrystallites. The crystalline structure studied by X-ray diffraction and high resolution scanning electron microscopy as well as the composition will be presented. Difference in terms of dielectric permittivity, leakage currents and ferroelectric polarization will be discussed on the basis of current-voltage measurements and investigations by scanning probe microscopy in different electrical modes.

Resume : Oxides are among the semiconductor materials with the widest tunability of physical properties and have been investigated for a long time as a candidate platform for novel electronic devices. In practice, however, silicon (ubiquitously) and other compound semiconductors (in particular, III-V (GaN (power 0,6-1,2 kV), InGaN (opto), GaAs (RF)) and IV (SiC (power > 1,2 kV)) are already well-established commercial semiconductors in their respective application domains. In many cases, therefore, a strong case for adopting an oxide semiconductor is still required. Our paper will discuss some novel and recently identified opportunities that are being opened up by wide bandgap oxides. The emerging property set of Ga2O3 is one of these opportunities. Owing to a number of advances including commercial wide area single crystal wafer production using cheap and scalable methods, good control of shallow donor doping and (our) recent demonstration of p-type conductivity, this material now presents unrivalled material properties for some power electronic applications. A key underlying reason for this is that Ga2O3’s critical electric field is ~2-3 times larger than current competitors (SiC and GaN) and probably presents a better forward/reverse trade-off despite its modest electron mobility. A new kind of oxide based device called “Solaristors” is another of these emerging opportunities. Classic perovskite oxides (in particular Pb(Zr,Ti)O3) are wide bandgap transparent oxides and the class of materials where ferroelectricity is strongest. An often-overlooked property of a ferroelectric semiconductor is that the switchable bipolar surface arrangement also implies a switchable surface band-bending. In a heterojunction with other more efficient light harvesters, we have engineered this property to define cheap and large area compact self-powered two-terminal solar transistors (or “solaristors”) for the first time. Acknowledgements. This work has been partially financially supported by Agencia Estatal de investigación (AEI) and Fondo Europeo de Desarrollo Regional (FEDER) under contract ENE2015-74275-JIN. [1] E. Chikoidze, A. Fellous, A. Pérez-Tomás, G. Sauthier, T. Tchelidze, C. Ton-That, T. T. Huynh, M. Phillips, S. Russell, M. Jennings, B. Berini, F. Jomard, Y. Dumont, “P-type β-gallium oxide: A new perspective for power and optoelectronic devices”, Materials Today Physics 3, 118-126 (2017). [2] A. Pérez‐Tomás, A. Lima, Q. Billon, I. Shirley, G. Catalan, M. Lira‐Cantú, “A Solar Transistor and Photoferroelectric Memory”, Adv. Funct. Mater. 28, 1707099 (2018).

Resume : Perovskite oxides with the ABO3 chemical formula have a very wide range of interesting intrinsic properties such as metal-insulator transition, ferroelectricity, pyroelectricity, piezoelectricity, ferromagnetic and superconductivity. For the integration of such oxides, it is of great interest to combine their properties with traditional electronic, memory and optical devices on the same silicon-based platform. In this work, we propose to analyze in-depth the relaxation mechanism and defects generation in functional oxides (such as BaTiO3 or PZT) grown by molecular beam epitaxy or pulsed laser deposition on SrTiO3/Si(001) pseudo-substrates. Based on various physical, optical and electrical characterization techniques, including reflection high-energy electrons diffraction (RHEED), X-ray diffraction (XRD) and high resolution spectroscopic transmission electron microscopy (HR-STEM), we could reveal the critical thicknesses of the different epitaxial systems. Finally, we propose to use strain mediated superlattice-based heterostructures to reduce the defect density in the structures and improve the polarization properties as confirmed by Piezo-Force Microscopy (PFM) characterization technique.

Resume : The thriving of memristive oxide devices is arousing interest in the field-enhanced hopping transport of oxygen vacancies not only because in many n-type oxides oxygen vacancies effectively act as double donors, but also, more importantly, because memristive oxide devices exhibit ultra-nonlinear switching dynamics [1]. Optimized performance requires resistive switching (SET and RESET) within tens of nanoseconds upon the application of a writing bias, and the ON and OFF resistance states should remain stable for up to ten years. Field-accelerated ion mobility constitutes one such source of ultra-nonlinearity [2]. The main problems of RS cells which need further improvements include mainly the issue of electroforming step, reproducibility in RS characteristics and/or RS material, so-called “voltage-time” dilemma and sneak paths. Electroforming-free bipolar BiFeO3 [3] and unipolar YMnO3 [4] RS have been realized. [1] N. Du, M. Kiani, C. Mayr, T. You, D. Bürger, I. Skorupa, O. G. Schmidt, and H. Schmidt, Front. Neurosci. 9, 227 (2015). [2] S. Menzel, M. Waters, A. Marchewka, U. Böttger, R. Dittmann, and R. Waser, Adv. Funct. Mater. 21, 4487 (2011). [3] T. You, S. Yao; W. Luo, N. Du, D. Bürger, I. Skorupa, R. Hübner, S. Henker, C. Mayr, R. Schüffny, T. Mikolajick, O.G. Schmidt, H. Schmidt, Adv. Funct. Mat. 24 (2014) 3357-3365 [4] A. Bogusz, A. D. Müller, D. Blaschke, I. Skorupa, D. Bürger, A. Scholz, O. G. Schmidt, H. Schmidt, AIP Advances 4 (2014) 107135.

Resume : The electronic reconstruction occurring at oxide interfaces may be the source of interesting device concepts for future oxide electronics. Among oxide devices, multiferroic tunnel junctions are being actively investigated for the possibility of independently controlling the switching of the magnetization of the electrodes and of the ferroelectric polarization of the barrier. Here, we exploit the dynamic control of the vacancy profile in the nanometer thick barrier of a ferroelectric tunnel junction to demonstrate the interplay between resistive (oxygen vacancy) and ferroelectric switching. We find a strong interplay between resistive and ferroelectric switching which highlights the coupling between electrochemical and electronic degrees of freedom triggered by the known coexistence of multiple valence states in transition metal oxides. In this talk we show that the spin reconstruction at the interfaces of a La0.7Sr0.3MnO3 /BaTiO3 / La0.7Sr0.3MnO3 multiferroic tunnel junction is the origin of a spin filtering functionality which can be turned on and off by reversing the ferroelectric polarization. The ferroelectrically controlled interface spin filter enables a giant electrical modulation of the tunneling magnetoresistance between values of 10% and 1000%, that could inspire device concepts in oxides based low dissipation spintronics. [1] Nature Nanotechnology 12, 655 (2017)

Resume : It is commonly believed that variation of the oxygen pressure during PLD growth is a direct way to control formation of oxygen vacancies (VO). Recent reports, however, address this point more critically [1],[2]. In our work we systematically studied formation of point defects in homoepitaxial SrTiO3 PLD films. For growth at chamber pressures ≤ 10-2 mbar in an oxygen flow we obtain 3 findings: (1) HRTEM reveals local lattice distortions in the film not present in the substrate. (2) These lattice distortions exhibit fluctuations on a time scale of seconds. (3) The STEM LAADF intensity is increased for such films. The image contrast between film/substrate could, however, not be reproduced by STEM image simulations even for defect concentrations of several percent. All 3 mentioned features are less pronounced for higher growth temperature. Samples grown in a pure Argon atmosphere but otherwise identical conditions showed surprisingly the same results. Consequently there is no simple relationship between oxygen partial pressure used during PLD growth and formation of VO. Our results suggests that the observed point defects originate from bombardment of the film with highly energetic species typical for PLD. An increase of the chamber pressure, irrespectively of the used gas, reduces the kinetic energy and leads to less point defects. Higher growth temperatures enable an in-situ annealing of the damaged film. The nature of the temporal fluctuations will be discussed in the paper. [1] Lee et al., Sci. Rep. 6, 19941 (2016) [2] Keeble et al. PRB 87, 195409 (2013)

Resume : We investigate local electrical conduction in oxygen-deficient strontium iron oxide SrFeO3-δ (SFO) epitaxial thin films by conductive atomic-force microscope (C-AFM). The brownmillerite-structured insulating SrFeO2.5 epitaxial thin films were first grown on 0.5 wt% Nb-doped SrTiO3 substrates by pulsed laser deposition. No electrical conduction was detected in the as-grown films (Figure 1b). On the other hand, oxidizing the as-grown films by post annealing treatment in air results in a drastic change in conduction behavior. The film after annealing at 600 ºC shows spatially inhomogeneous conduction across terraces across step edges. Interestingly, with further annealing (or oxidizing), this inhomogeneous conduction disappears, resulting in a uniformly high conduction state. These observations imply that oxygen ions are preferentially incorporated near the step edge regions in the initial stage of the oxidizing process of the SrFeO2.5 films. We also reveal that such local conduction can be tuned by redox reactions under an electric field due to bias dependent local movement of incorporated oxygen ions. These results reveal the strong influence of the local oxygen content (or local oxidation state) on the electrical conduction of SrFeO3-δ.

Resume : The transition metal perovskite oxide SrMoO3 (SMO) with Mo4 in a 4d2 electronic configuration has a room-temperature resistivity of 5.1 µΩcm as single crystal and, thus, can be considered as a promising conducting electrode material for all-oxide microelectronic devices. SMO thin films with resistivity values below 20 µΩcm at room temperature have been reported. However, stabilizing the unfavorable Mo4 valence state in SMO films requires reductive growth conditions incompatible with a highly oxidative environment as required for other functional oxides such as dielectrics. In order to overcome the constrictions of contradicting conditions during thin film growth, we have investigated oxygen diffusion in epitaxial oxide heterostructures comprised of SMO films in combination with several thin perovskite oxides SrTiO3, BaTiO3, Ba0.5Sr0.5TiO3 (BST), and SrZrO3. The valence states of the transition metal cations as Ti, Zr, and Mo were investigated in these materials at different oxidation conditions using X-ray photoelectron spectroscopy. Among the investigated materials, the lowest oxygen diffusion has been observed in BST. A BST film with a thickness of only 5 unit cells preserves the Mo4 oxidation state in the SMO underlayer at the used oxygen partial pressure of 3 mTorr and a temperature of 630 °C. Thus, SMO thin films covered with atomically layered thin BST remain conducting in an oxygen environment and can be integrated into all-oxide thin-film device heterostructures.

Resume : Resistive switching oxides are promising candidates for future non-volatile memory and for functional units of neuromorphic computing. Epitaxial SrTiO3 thin films can be regarded as model system for resistive switching thin films due to its well know defect chemistry and the absence of grain boundaries. We studied in detail the impact of the Sr/Ti stoichiometry on the filament formation process and the switching performance. We observed that Sr rich thin films exhibit an improved retention of the low resistive state (LRS) at low current compliance values and an increased memory window with respect to stoichiometric thin films. We investigated the filament formation process of SrTiO3 thin films with different stoichiometry by photoelectron emission microscopy. Whereas stoichiometric thin films form a single filament, Sr rich thin films tend to form multiple filaments with increasing current compliance. We could prove explicitely, that stoichiometric thin films form a stable LRS state as soon as the current compliance is sufficient to trigger SrO segregation at the surface. We attribute this to the impeded reoxidation of the oxygen deficient filament by the presence of the SrO at the surface. We could mimic this effect by intentionally depositing additional SrO on the surface of stoichiometric thin films and were thereby able to improve the memory window as well as the LRS retention. These results provide a pathway to a rational design of resistive switching devices STO with improved reliability.

Resume : Perovskite oxides exhibit a plethora of fascinating electronic material properties covering an exceptionally wide range of phenomena in solid state and surface physics. This has led to tremendous efforts to functionalize these materials in applications for energy technology, gas sensing, and electronics. Layered in an atomically defined epitaxial heterostructures and superlattices, diverse properties of perovskites can be combined on the nanoscale level. In such structures, even new functionality can arise at interfaces of layered materials, exhibiting properties that are absent in the bare bulk materials. In our approach, we utilize atomically-defined layer growth to obtain desired material properties. However, on top of that, we employ thermodynamic engineering of crystal defects as a unique approach to functionalize material properties at surfaces and interfaces: Even at material synthesis conditions close to perfection, device properties are often determined by imperfection, hence, by lattice disorder and crystal defects. As we discuss, we can intentionally control defect structure in nanoscale devices, by developing and utilizing thermodynamic routes to trigger surface and interface reactions in confined systems. While historically defects were seen as something to be avoided, a change of paradigm is required in the field of complex oxides today: In these materials, we can promote functionality, such as metallicity in nominally insulating compounds, by atomic defect-management. Therefore, rather than avoiding defect formation, it is an essential necessity to control and to utilize defect formation in oxides on the nanoscale. Here, we discuss fundamental aspects of lattice disorder effects in bulk oxides, and elaborate the special character of defect formation in thin films, surfaces and interfaces. Focusing on SrTiO3 as a perovskite model system, we will crosslink fundamental perspectives on lattice disorder to actual applications, addressing different examples, such as resistive switching memories, high-mobility electron gases and induced magnetism, oxygen sensors, and electro-catalysts.

Resume : Although a number of studies address the topic of intrinsic atomic defects in SrTiO3 [1-3], though with contradictory results, little is known about the dynamics of point defect in SrTiO3 including diffusion and clustering. By means of TEM we found nanometer sized voids in SrTiO3 bulk crystals grown in oxygen containing atmospheres, irrespectively of the used growth method. In contrast to that, crystals grown in a nitrogen atmosphere are free of voids. The total void volume increases with the oxygen partial pressure during the growth and the size of voids increases with decreasing cooling rates after the growth. Annealing of crystals at high temperatures leads to an increase of the void size. We suggest a vacancy diffusion and clustering process to be responsible for formation of voids: Initially oxygen and metal vacancies are incorporated into the growing crystal. Their total equilibrium concentration is expected to be highest for slightly oxygen rich conditions [1]. During cool down after growth, the solubility of vacancies in the crystal decreases and eventually they become supersaturated. Since oxygen vacancies, shallow donors, and compensating metal vacancies are oppositely charged [1] and Coulomb attractive they cluster and form nuclei. Further diffusion of vacancies leads to growth of these nuclei resulting in the observed nanometer sized voids. Based on our results we give an estimation for formation energies and diffusion coefficients of vacancies in SrTiO3 crystals. [1] Varley et al., Phys. Rev. B 89, 075202 (2014) [2] Liu et al., Phys. Chem. Chem. Phys. 16, 15590 (2014) [3] Tanaka et al., Phys. Rev. B 68, 205213 (2003)

Resume : Electric field induced oxygen vacancy drift in SrTiO3 between two Schottky-type platinum electrodes is a widely used model to describe the resistive switching behaviour of the material. The permittivity of SrTiO3 is strongly dependent on the electric field strength and the temperature. Since high electric fields can occur in SrTiO3 Schottky diodes, profound knowledge on the field dependence of the permittivity is indispensable for a correct interpretation of switching experiments. Here we present an investigation of the temperature dependence of the permittivity in homogeneous high electric fields. Insulating SrTiO3 bulk crystals were used as dielectric for thick plate capacitor structures. For high electric field experiments, insulating SrTiO3 thin-films were epitaxially grown on highly conductive, niobium doped SrTiO3 substrates. Capacitance-voltage and current-voltage measurements were performed at temperatures between 15 K and 300 K and electric field strengths up to 1 MV/cm. High electric field strengths of up to 1 MV/cm can suppress the increase of the permittivity with decreasing temperatures. At fields above 400 kV/cm, the increasing permittivity is inverted to a decreasing permittivity with decreasing temperatures. The impact of the field dependence on the capacitance-voltage characteristics of Schottky diodes is visible by a deviation of the 1/C² ~ V relation. In consequence, the drift of oxygen vacancies in resistive switching devices might be influenced.

Resume : Amorphous metal oxide semiconductor provides high performance and transparency compared to amorphous silicon, properties required for the transparent electronics. However, the high processing temperature limits the choice of substrates. Polymer substrates cannot handle high temperatures. One of the factors affecting the activation of the thin film transistor (TFT) and its properties, is the oxygen vacancies present in the oxide. Previously the oxygen vacancies were controlled by the oxygen partial pressure while deposition and post-annealing. It limits the control over transistor behavior. This work tries to address the issue by using an ionic liquid as a gate dielectric. With high electrostatic field due to ionic liquid gating, the reversible oxygen vacancies variation was achieved. This was supported by the controlled modulation of the threshold voltage and on voltage of transistor using a predefined gate biasing. This is useful to correct the defects occurring while the formation of the oxide thin film and making fabrication process more repeatable without post-annealing.

Resume : Negative capacitance in a ferroelectric material is characterized by a state of the polarization where the polarization charge and the net electric field oppose each other. In a prototypical ferroelectric this situation occurs where the polarization is significantly suppressed. Conventionally, this is the region where the material experiences a polarization catastrophe; in other words left all by itself in this state, the material will spontaneously polarize. As a result, in an isolated ferroelectric material, this state can only be accessed in a time-dependent manner. In the first part of my presentation, I shall discuss how one could go beyond this and stabilize the ferroelectric material at a state of negative capacitance in equilibrium, i.e., at the steady state, with the help of another dielectric material placed in a series connection. Interestingly, this series connection emulates what happens at the gate of a transistor where the gate insulator and the semiconductor capacitors are connected in series. Therefore if the gate oxide is replaced by an appropriate ferroelectric, this series combination can stabilize the ferroelectric material at a state of negative capacitance. At this state, the total capacitance of the series combination is enhanced, leading to more charge at the channel at the same voltage. The boost of charge, in turn, leads to larger current at the same voltage. In fact, this boost makes it possible to reduce supply voltage of transistors below the traditional Boltzmann limit --- often termed as the Boltzmann tyranny. In the recent years, many groups around the world, both in academy and in the industry, have demonstrated the fundamental effect and the Negative Capacitance Transistors. In the second part of this presentation, I shall discuss the principle of operation and potential routes for optimization of the Negative Capacitance Transistors.

Resume : The discovery of ferroelectricity in 10 nm thin polycrystalline HfO2 has revived the research on ferroelectric memories due to hafnia’s excellent compatibility with complementary metal-oxide-semiconductor (CMOS) fabrication[2,3]. Meanwhile, also the potential for sensors and actuators is being explored.[4,5] As film quality and dominant types of defects within the ferroelectric and at its interfaces are crucial, the polycrystallinity is a challenge. We show different phases within single grains and identify preferential interphase boundaries that form despite mismatches of > 4 %.[6] Moreover, we found an orientation relationship between HfO2 and the TiN electrodes. When annealing the amorphously deposited HfO2 films, some TiN grains serve as seeds for the crystallization of the HfO2. Both the interphase boundaries and the partial templating effect of the TiN can be explained by looking at the unit cells. Phase coexistence and preferential orientation are expected to be key elements for future devices in both the fields of nanoelectronics and microelectromechanical systems. [1] T. S. Böscke et al., Appl. Phys. Lett. 99, 102903 (2011). [2] J. Müller et al., IEEE Symposium on VLSI Technology (VLSIT) 2012. [3] P. Polakowski et al., Proc. IEEE International Memory Workshop (IMW) 2014, pp. 1-4. [4] C. Mart et al., Appl. Phys. Lett. 112, 052905 (2018). [5] S. Starschich et al., Appl. Phys. Lett. 110, 182905 (2017). [6] E. D. Grimley et al., Adv. Mater. Interfaces, 5, 5, 1701258 (2018).

Resume : Over the past five decades, LiNbO3 single crystals and thin films have been studied intensively for their exceptional acoustic, electro-optical, pyroelectric and ferroelectric properties [1]. Today, LiNbO3 single crystals are key materials in electro-optics and RF acoustic filters. LiNbO3 thin films are needed urgently for the development of the high-frequency and/or wide-band RF filters adapted to the 5G communications. The integration of LiNbO3 films in guided nanophotonic devices will allow higher operational frequencies, wider bandwidth, and miniaturized optical devices in line with improved electronic conversion. The challenges and the achievements in the epitaxial growth of LiNbO3 films and their integration with Si technology and to acoustic and nanophotonic devices will be discussed in detail. In the case of SAW and optical waveguides, proper propagation axis is demanded. Thus, particular effort was done to achieve the growth of films with single orientation and nearly stoichiometric Li2O composition. Preliminary results on the micro-structuring of LiNbO3 films for the fabrication of waveguides, and acoustical/optical properties will be presented. Future prospects of potential applications and the expected performances of thin film devices are overviewed, as well. [1] A. Bartasyte et al., Adv. Mater. Interfaces 1600998, 2017. [2] A. Bartasyte et al., Mat. Chem. Phys. 149-150, 622-631 (2015). [3] A. Bartasyte et al., J. Phys. Condens. Matter 25, 205901 (2013).

Resume : The exemplary story of the CMOS microelectronic technology shows that modern electronic devices require various functional layers of a thickness up to a few micrometers, combined with atomically engineered interfaces controlled at the unit cell level. Considering realization of oxide electronic devices for high-frequency applications such as low-loss tunable filters and antennas, the desired thickness of the conducting oxide electrode is determined by the electromagnetic skin depth which increases with resistivity of the electrode material and has a value of the order of several micrometers in the frequency range of a few GHz. Among the known conductive oxide perovskites, the smallest skin depth is achieved in single crystal of SrMoO3 due to its low resistivity of 5 cm at room-temperature outperforming even Pt (10.6 muOhmcm), thus, making SrMoO3 a prominent conducting electrode material for high-frequency microelectronic devices. The used low-resistive SrMoO3 thin-film oxide electrode leads to low losses and high quality factor Q of the varactors. The layer-by-layer grown epitaxial Ba0.5Sr0.5TiO3 results an unprecedentedly high capacitance (C) tunability of n (3.7 V) = C (0) / C (3.7 V) = 3.1 at the Li-ion battery voltage level of 3.7 V. Thus, the obtained high aggregate performance of the varactors expressed in the high values of the “standard” commutation quality factor CQF (1 GHz) ~ 3700 and the more relevant voltage performance factor VPF (3.7 V, 1 GHz) = n (3.7 V) Q (0V, 1 GHz) / 3.7 V ~ 65 stands out in the gigahertz frequency range of interest and is comparable.

Resume : Ferroelectric multi-layers were prepared by pulsed laser deposition (PLD) method on single crystal SrTiO3 (STO) substrates with bottom SrRuO3 (SRO) electrodes. Most of the studies were performed on symmetric three-layers structures of PZT-interlayer-PZT type (Zr/Ti ratio of 20/80 for PZT). The interlayer was varied from completely conductive (SRO) to insulating (STO), with BaTiO3 (BTO) and CoFe2O4 (CFO) as materials with intermediate electrical conductivity. The structural analysis performed by XRD and TEM has revealed that the multi-layers are epitaxial, with some strain gradient from the bottom PZT layer to the top one. Hysteresis loop measurements performed in capacitor geometry, with top SRO contacts of 100x100 µm2, have revealed that the shape of the hysteresis changes with the material used for interlayer separating the two PZT films. The most interesting behavior was obtained on PZT-STO-PZT structures, for which 4 distinct polarization states were obtained, each of them accessible by applying an external voltage of suitable value. The study performed on PZT-STO-PZT structures having the same thickness for PZT layers (150 nm) and different thickness for the STO interlayer has shown that there is an optimum thickness range for STO for which the 4 polarization states can be obtained. If the thickness is too small then the entire structure behaves as a normal ferroelectric layer with only 2 polarization states, and if the thickness is too high the 4 polarization states start to be blurred and un-stable. Therefore, the optimum thickness range for STO was found to be between about 10 nm and 50 nm. Further on, a five layer PZT-STO-PZT-STO-PZT was grown, with 10 nm for STO and 100 nm for PZT layers. Up to 8 distinct polarization states were found from hysteresis measurements. These results suggest that multi-layer ferroelectric-insulator structures can be used as memory cells with multiple polarization states (up to 2n states, where n is the number of ferroelectric layers). The finding is explained by successive switching of polarization in the component ferroelectric layers, predicted also theoretically by applying thermodynamic theory to these structures. The same structures were found to have distinctive capacitance values, depending on polarization orientation in the component layers, offering in this way the possibility of non-destructive reading. However, the number of distinctive states varies as 2n-1, with n the number of ferroelectric layers. It is also shown that such structures can behave not only as memory cells but also as logic gates, allowing logic operations like AND/NAND or OR/NOR if a certain sequence of voltage pulses is applied on the structure.

Resume : Understanding photoinduced properties in ferroelectric solid solutions and gaining control over them could enable design of new, more efficient photovoltaic devices or photocatalysts. In our work, we have focused on studying the optical properties of BaSnxTi1-xO3 solid solutions, a BaTiO3-ferroelectric-based system where the Sn-5s-5p electronic states at the conduction band are expected to provide increased electron mobility. We will show from our study in the bulk compounds the close relationship of the optical and electronic properties in this system, results obtained from dielectric measurements and ultraviolet-visible-near-infrared, Raman, X-ray photoelectron and photoluminescence spectroscopies, together with density functional theory calculations. We observe a significant nonlinear increase of the direct band gap (>0.3 eV) for BaSn0.8Ti0.2O3 compared to the parent compounds values, whereas for the BaSn0.1Ti0.9O3 compound, our synchrotron radiation diffraction experiments indicate that this composition is not cubic at room temperature. In view of these investigations in the bulk system, we have selected the BaSn0.1Ti0.9O3 composition as a potential candidate composition presenting room temperature ferroelectricity and enhanced photo(ferro)conduction performances. Therefore, we have grown epitaxial thin film heterostructures of BaSn0.1Ti0.9O3 and studied their structure, chemical, surface, electronic and optical properties together with photo(ferro)voltaic responses. We will discuss these preliminary results in the film heterostructures in the framework of the bulk BaSnxTi1-xO3 compounds results and of future applications. This project has received funding from the EU‐H2020 research and innovation program under grant agreement N 654360 having benefited from the access provided by ICMAB-CSIC in Cerdanyola del Vallès within the framework of the NFFA-Europe Transnational Access Activity (project ID 524)

Resume : Ferroelectric phases with monoclinic symmetry are expected to exhibit enhanced piezoelectric properties. Together with high coupling coefficients this can be exploited for thin film surface acoustic wave (SAW) applications. In this regard, KxNa1-xNbO3 represents a suitable material system that offers both high coupling coefficients and the possibility to stabilize monoclinic phases by incorporating anisotropic epitaxial strain. Thereby, their formation crucially depends on the incorporated lattice strain provided by the epitaxial growth on a lattice mismatched substrate. In this study, we have grown K0.7Na0.3NbO3 epitaxial films by metal-organic chemical vapor deposition on different (110) oriented rare-earth scandate substrates. For a thickness of 30 nm, all films on TbScO3, GdScO3 and SmScO3 exhibit pseudocubic (001)pc surface orientation, while the average strain state changes from compressive to slightly tensile. Piezoresponse force microscopy and x-ray diffraction data reveal periodically arranged ferroelectric stripe domains and domains walls that contain the [1-12] or [-112] in-plane directions of the orthorhombic substrates. With increasing thickness, elastic lattice relaxation set in by the additional formation of a (100)pc oriented monoclinic phase eventually forming a complicated herringbone domain pattern. Propagation of SAWs is demonstrated for K0.7Na0.3NbO3 thin films on all substrates, whereby, strength and propagation direction depend on the epitaxial strain.

Resume : The behavior of ferroelectric materials under an applied electric field has been the subject of investigation in order to understand the effects of both intrinsic and extrinsic contributions during device operation. Time-resolved characterization techniques allow for the investigation of structural and electrical changes associated with different loading conditions, suitable for different applications. Also, the permanent quest for energy efficient technologies drives investigations on making a ferroelectric operational under lowest switching voltage. Here we present a simultaneous study of the electrical and structural responses of lead-zirconate-titanate-based capacitor heterostructures during charging, discharging, and polarization reversal, using time-resolved X-ray diffraction. Concomitant with the ferroelectric current peak, we observe that the switching is characterized by a transient disorder evidenced by a decrease of the Bragg peak intensity. A peak width increase reveals the domain dynamics during the reversal. Our investigations show how the incomplete screening of the depolarization charges affect the piezoelectric response, measured via the Bragg peak position. We examine the interplay between charge flow and atomic motion in real time during device operation. We investigate how the device time constant can tremendously modify both the piezoelectric strain and macroscopic coercive field.

Resume : The potential of Ga2O3 and In2O3 as transparent conducting oxides, could be greatly increased by alloying them to tune their properties - especially the band-gap - over a wide range. Alloying into (InxGa1-x)2O3 is however not straightforward due to the different thermodynamically stable structures of the binaries (monoclinic β-Ga2O3 and cubic bixbyite In2O3). Recent calculations of the phase diagram show rather narrow stability windows with low solubility limits for the above-mentioned phases and an additional window for the hexagonal InGaO3(II)-phase close to x=0.5. TEM and XRD investigations of thin PLD grown (InxGa1-x)2O3 layers on c-sapphire show however that we can expand these stability windows. Under certain growth conditions, the β-phase is stable up to x=0.5 and the hexagonal phase can contain In contents up to x≈0.7. In a dedicated in-situ experiment where amorphous (InxGa1-x)2O3 layers are crystallized by annealing, we observe even the stabilization of a Ga-rich cubic phase. Additionally to the shrinkage of the miscibility gaps, it is possible to stabilize an ε-(InxGa1-x)2O3 phase at low indium contents which is characterized with Pna21 space group symmetry. The formation of the ε-phase is enabled by introducing Sn atoms during the PLD growth, and it is also enhanced by lowering the growth temperature. The existence of metastable composition windows is explored to explain these experimental observations, but they surely open up new perspectives for applications.

Resume : Computational high-throughput screening has generated much interest in materials science because of the possibility to guide the development of functional materials along with the ability to enhance the understanding of fundamental properties. Identification of stable crystalline materials for a mixture of two components requires examination of the lowest free energy of approximately 2N configurations, where N is the number of atoms in the unit cell. Cluster expansion-based energy functions offer a numerically efficient approach for estimating the stability of new potential alloys. Combining this approach with the nested sampling algorithm, which is a Bayesian Markov chain Monte Carlo method, allows for a one-shot calculation of the phase diagram as a function of composition and temperature. Our results for stable ternary and quaternary mixtures in various crystalline symmetries of group-III oxides with the formula (InxGayAlz)2O3 where x+y+z=1 will be presented. A new stable InGaO3 compound is reported as a potential transparent conductor material. In addition, the key aspects that determine the stability of these materials will be discussed.

Resume : Wide band gap semiconductors have applications in fields like power electronics, UV-dosimetry, transparent electronics, flexible circuitry and so on. Depending on the application appropriate doping and/or alloying is used to tailor the material properties. We have developed a novel approach for combinatorial material exploration by pulsed-laser deposition (PLD) allowing growth of thin films with defined lateral variation of doping and/or the alloy composition [1]. This approach was used to utilize wavelength-selective UV and deep UV photo-detectors. Narrow bandwidth down to 7 nm (29 meV) was achieved for MgZnO-based detectors [2]. Next we demonstrate room temperature deposition of the ternary amorphous oxide zinc tin oxide (ZTO) [3] focusing on the dependence of material properties and that of simple devices on the cation composition. Further the influence of oxygen deficiency on the properties of ZTO-based MESFETs, JFETs and inverters is discussed. [1] H. von Wenckstern, Z. Zhang, F. Schmidt, J. Lenzner, H. Hochmuth, M. Grundmann, CrystEngComm 15, 10020 (2013). [2] Z. Zhang, H. v. Wenckstern, M. Grundmann, IEEE Journal of Selected Topics in Quantum Electronics 20, 106 (2014). [3] S. Bitter, P. Schlupp, M. Bonholzer, H. v. Wenckstern, M. Grundmann, ACS Comb. Sci. 18, 188 (2016).

Resume : A high electron concentration is typically observed at In2O3 surfaces. It is a drawback for electronic devices since fabrication of rectifying metal contacts is hindered. However, variation of the electron accumulation layer can be implemented as sensing principle in gases or liquids. The influence of surface adsorption processes, plasma oxidation and doping on the electronic surface properties is studied by in-vacuo photoelectron spectroscopy and 4-point probe resistivity measurements. Using model gas adsorption experiments, a clear correlation is observed between adsorbate-semiconductor charge transfer, density of surface charge carriers, and the conductivity of the material. In general, a modification of the surface electron density by electronegative (acceptor-type) oxygen adatoms is a key factor that increases the film resistivity. Furthermore, acceptor doping by Mg and Ni also reduces the surface electron concentration after growth, but both approaches are not capable to fully deplete the surface electron channel. On the other hand, an oxygen plasma treatment induces upward band bending even for highly n-doped films, generating an energy barrier applicable to produce Pt-based rectifying contacts. The oxygen-rich Pt/In2O3 interface is critical in stability and oxidation of Pt contacts is beneficial to enhance electronic barriers. Combining interface oxygen plasma modification and fabrication of PtOx contacts enables a significant improvement of In2O3 Schottky diodes.

Resume : Wide-bandgap metal oxide semiconductors are an important class of electronic materials in emerging applications such as transparent and power electronics. The perspectives of miniaturization for such applications depend critically on the ability of tuning the optoelectronic properties, which, in turn, depend on controlling the defect and dopant composition. The main goal of this study is to investigate the effect of the metal oxide dimensionality on the defect and free carrier concentration in equilibrium. The current work is focused on two of the most relevant oxide semiconductors nowadays: ZnO and Ga2O3. The thermodynamic analyses of defect and free carrier concentration equilibrium are performed using the Kroger method of quasi-chemical equations for the thermodynamic system: oxide crystal – gas of one component. In this way the concentration of free carriers and defects can be ascertained as functions of gas pressure and temperature from few nanolayers to bulk crystal. The n- and/or p-type dopability is mainly conditioned by the electronic structure of the system: band gap, ionization energies of donors, acceptors and their compensating centers; as well as by the formation enthalpies of defects. These quantities may undergo substantial changes when dimensionality is reduced and reveal strong size dependence. Ionization energies of donors and acceptors, which are needed for thermodynamic analyses, are calculated from few monolayers to a bulk crystal. The variation of defect creation energies in low dimensional systems is also considered.

Resume : β-Gallium Oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells and sensors with capabilities beyond existing technologies due to its large bandgap and the availability of high quality, large diameter bulk crystals and epitaxial layers of Ga2O3 with a range of controllable n-type doping levels by edge-defined film-fed, Czochralski, Bridgman or float zone. The high energy bandgap of Ga2O3, ∼4.9 eV, yields a very high theoretical breakdown electric field (∼8 MV/cm). For power electronics, the Baliga figure-of-merit proportional carrier mobility, critical electric field and breakdown voltage, is almost four times higher for Ga2O3 than for GaN. In this work, we report demonstrations high currents or high breakdown voltages of Ga2O3 rectifiers, diode area up to 0.2 cm2, with and without edge termination fabricated on a Si-doped n-Ga2O3 drift layer grown by halide vapor phase epitaxy on a Sn-doped n+ Ga2O3 (001) substrate. For high current diodes, current level of 1 to 2 A were achieved with the breakdown voltages ranging 150 to 645 V. For high breakdown voltage diodes, breakdown voltage ranging from 1600 to 2300 V were demonstrated.

Resume : The pursuit of Ga2O3 as an ultra-wide-bandgap (4.5–4.9 eV) semiconductor for next-generation power-switching and harsh-environment electronics has catalyzed the rapid development of Ga2O3 metal-oxide-semiconductor field-effect transistors (MOSFETs) in recent years [1]. Field-plated lateral depletion-mode devices demonstrated a high off-state breakdown voltage of 755 V, a large on/off current ratio of over nine orders of magnitude, dispersion-free output characteristics, stable high temperature operation, and strong gamma-ray tolerance [2,3]. Enhancement-mode operation with a six-order-of-magnitude on/off current ratio was enabled by an unintentionally-doped epitaxial Ga2O3 channel that was fully depleted at zero gate bias due to a low background carrier density [4]. Planar-gate vertical Ga2O3 MOSFETs, wherein a current blocking layer provided electrical isolation between source and drain except at an aperture opening through which drain current was conducted, demonstrated successful transistor action [5]. Advanced transistor architectures, notably normally-off vertical devices, will further enhance the impact of Ga2O3 power electronics. [1] M. Higashiwaki et al., Appl. Phys. Lett. 112, 060401 (2018). [2] M. H. Wong et al., IEEE Electron Device Lett. 37, 212 (2016). [3] M. H. Wong et al., Appl. Phys. Lett. 112, 023503 (2018). [4] M. H. Wong et al., Appl. Phys. Express 10, 041101 (2017). [5] M. H. Wong et al., Proc. IEEE Device Res. Conf. (2017).

Resume : As an electronic material, β-Ga2O3 has received significant attention in the last several years due to a combination of unique material properties, namely high-critical electric field strength (Ecrit), large range of controllable n-type doping, and ability to grow large area crystals from a melt. The large Ecrit offers substantial advantages in terms of power switching losses over existing commercial solid state technologies for both dc conduction losses and dynamic switching. The large range of controllable n-type doping allows operation over a large range of voltages, and the large area substrate availability offers significant cost advantages that are compounded by the high energy density capabilities of the material enabling more parts to be fabricated in a smaller area. The scalability enabled by the large Ecrit also carries over into the RF operation space in terms of increased power×frequency product. The focus of this presentation will be the evolution of lateral β-Ga2O3 technology and critical enabling material and process developments that have occurred since the first MOSFET demonstrations to include increased mobility, lower ohmic contact resistance, gate recess, and short gate-length. Emphasis will be placed on more recent efforts by AFRL to characterize high-speed switch loss metrics for β-Ga2O3 devices, which are now beyond the theoretical limit of Si, as well as pushing the RF performance of the devices beyond maximum frequency of operation of 20 GHz.