Session U5: Epitaxial Oxide/Semiconductor Systems

Structural and Electronic Properties of Epitaxial Complex Oxide-Silicon Interfaces

Following the discovery of a method to deposit a crystalline complex oxide, SrTiO$_{3}$, directly on Si, a significant effort has been devoted to understand the nature of crystalline oxide-semiconductor interfaces. This research has in part been motivated by the desire to find a suitable high dielectric constant (high-k) insulator to replace SiO$_{2}$ in Si-based field effect transistors. In this talk, we present results on the detailed structural and electronic properties of epitaxial SrTiO$_{3}$-Si and LaAlO$_{3}$-SrTiO$_{3}$-Si heterostructures grown by oxide molecular beam epitaxy. Using synchrotron crystal truncation rod analysis and reflection high energy electron diffraction, we place constraints on the detailed positions of the atoms at the oxide-semiconductor interface. These results are compared with predictions based on first-principles calculations of the interface. We also discuss capacitance-voltage and inelastic tunneling spectroscopy measurements of the electronic and dielectric properties of these high-k oxide-semiconductor systems.   

From GaAs MOSFETs to epitaxial oxides on silicon : old and new MBE stories.

50 years of intense development in chip technology did not fundamentally change the initial concept: the capability to modulate charges right at the interface between two dissimilar materials. This concept allowed the whole microelectronic industry to develop exponentially and to disseminate its products all over our environment. Two simple reasons can be given to such a success: i) device scaling was a simple and cost-effective method to make chip faster; ii) faster chips simply allowed our computing environment to perform new functions. None of the two reasons given will remain true in the next few years. Scaling has come to an end. The materials properties will be scaled instead of the device itself. The recent introduction of high-k materials perfectly illustrates such a transition. The future success for chip makers might then depends on new rules: i) many new materials will be developed, and interfaces, still a key element for a device to perform better, will multiply; ii) The future technology developments will be more expensive and generate smaller performance gains. The added value might be then in the integration of functions implemented in these new materials. A few years ago, molecular beam epitaxy allowed band-gap engineering in compound semiconductors to build new devices and, more recently, was successfully used to explore the physics and chemistry of complex perovskites. During the last years, new developments have been made to combine oxides and semiconductors. In particular, many groups have reported the growth of epitaxial oxides on silicon surfaces. The recent and renewed interest in compound semiconductor MOSFETs structures might indeed be seen as a logical conclusion for this evolution. This presentation will review the latest developments in the field, with a focus on the activities taking place at IBM Zurich. It will also put them in perspective with the new rules the microelectronic industry might follow.   

First-principles modeling of functional oxides-semiconductor interfaces

Search is ongoing for new classes of materials and structures based on complex oxides. One important example is the multiferroics which combine ferromagnetism or antiferromagnetism and ferroelectricity in a single phase. The coexistence, and occasionally the coupling of the two order parameters, has opened new opportunities for multifunctional applications. A promising route for practical applications is to employ thin films and multilayered structures, the properties of which can be readily manipulated at the nanoscale. Epitaxial multiferroic films are currently being developed through integration with semiconductors. Despite the progress in synthesis and experimental characterization, the roles of the interface phenomena, including strains, chemistry, etc., on the ferroelectric and magnetic properties of multiferroic thin films is not fully understood and difficult to differentiate experimentally. In this talk we illustrate the utility of theoretical methods based on density functional theory in understanding these technologically relevant structures. We describe examples of oxide-semiconductor interfaces based on YMnO$_3$ on GaN that have been synthesized recently and show how interfacial spins behave differently from those in the bulk. The interfacial effects lead to an intriguing behavior of the band offsets. We also discuss our ongoing investigation of electric field doping interfaces.   

Thin Film Synthesis of New Complex Titanates.

Thin film deposition methods allow for one to synthesize rationally specific compositions in targeted crystal structures. Because most of the thermodynamic and kinetic variables that control the range of materials that can be synthesized are unknown for specific compounds/processes, epitaxial stabilization and design of artificially layered crystals are driven through empirical investigations. Using examples taken primarily from the family of complex titanates, which exhibit a range of interesting physicochemical behaviors, the thermodynamic and kinetic factors that control materials design using thin film deposition are discussed. The phase competition between the pyrochlore and the (110) layered perovskite structure in the \textit{RE}$_{2}$Ti$_{2}$O$_{7}$ family (\textit{RE} = rare-earth, Bi) will be explored, using pulsed laser deposition as a synthesis method. For \textit{RE} = Gd, Sm, Nd, and La, the phase stability over a wide range of conditions is dictated entirely by substrate choice, indicating that the free energies of the phases are similar enough such that by controlling nucleation one controls the phase formation. In a related fashion, the growth of \textit{AE}Ti$_{2}$O$_{5}$ films (\textit{AE} = Ba or Sr) will be discussed with respect to the formation of single-phase films or films that phase separate into \textit{AE}TiO$_{3}$ and TiO$_{2}$. The entire Ba$_{1-x}$Sr$_{x}$Ti$_{2}$O$_{5}$ series was grown and will be discussed with respect to growth technique (using MBE and PLD) and/or substrate choice. In this case, rock-salt substrates, which are not expected to interact strongly with any phase in the system, allow for the formation of single-phase films. Finally, several examples will be discussed with respect to the (SrO)$_{m}$(TiO$_{2})_{n}$ system, which includes the perovskite SrTiO$_{3}$ and the Ruddlesden-Popper phase Sr$_{2}$TiO$_{4}$, grown using layer-by-layer molecular beam epitaxy. The solid phase epitaxial formation of the perovskite SrTiO$_{3}$ from superlattices of rock-salt SrO and anatase TiO$_{2}$ is discussed from both a kinetic and thermodynamic perspective by exploring the growth of a range of $m$ and $n$ values. Using similar arguments for stability, new layered intergrowths in the Sr$_{m}$TiO$_{2+m}$ family are presented and their structures are discussed.   

Semiconductor-on-epitaxial insulator: towards ultrathin and nonclassical semiconductor devices

The microelectronics industry is currently moving from bulk Si field-effect transistors (FETs) with silicon-dioxide gate insulators to high-k gate dielectric FETs and semiconductor-on-insulator (SOI) substrates, with alternative non-Si channel materials and nonplanar device layouts on the horizon. The possibility of integrating epitaxial insulator layers with well-controlled bandgaps and near-monolayer thickness control may open up new opportunities for nonclassical devices and possibly optical sources. Unlike their III-V counterparts, where epitaxial heterostructures have been available for decades, epitaxial oxide-based SOI devices have the crucial advantage of potential integrability with dominant silicon technology. This talk will discuss the examples of tunneling FETs and real-space transfer devices, as well as a proposed tunneling-based SOI intersubband laser. At this point, all of the proposed devices require epitaxial control and material quality that exceeds the state-of-the-art. Still, the unique characteristics deriving from quantum mechanical tunneling make such devices an interesting playground for innovative device research, essentially replicating the III-V heterostructure device platform in the silicon-dominated microelectronics industry just as standard Si FET technology heads towards the long-predicted end of the miniaturization paradigm.