The focus of my group’s research is investigating and perfecting the properties of oxide materials for electronic uses. To do this, we grow oxide thin films on single crystal substrates of closely related substances. The single crystal substrate provides a structural template for the thin films that we grow. The films follow this atomic template and are thus said to be epitaxial (inheriting their crystalline arrangement from the underlying substrate). Our focus on oxides is due to the tremendous promise that these materials hold for electrical applications. Oxides exhibit an unparalleled variety of electronic properties. Insulating, semiconducting, and even superconducting oxides all exist within the set of structurally compatible oxides known as perovskites. This structurally related family also includes oxides that are magnetic, ferroelectric, or even both at the same time. In short, this family of oxides contains the full spectrum of electronic properties. A major challenge, however, is to prepare these materials with sufficient quality and integrate them with adequate control so that these properties can be fully utilized in electronic devices. This is our research goal.

Exploiting the capabilities of these materials for the most demanding electronic applications will require the synthesis of custom-made stacks of single crystal films, each attached epitaxially to the one beneath it and prepared in such a way that composition and structure can be controlled at the level of single atomic layers. To achieve this customized layering capability, our research group utilizes a thin film growth method known as molecular-beam epitaxy (MBE).

MBE amounts to atomic spray painting, and allows us to prepare customized thin film structures of oxide materials in a very controlled manner. In this process several beams, each of a different atomic or molecular type, travel through a vacuum of such emptiness (ultra-high vacuum) that collisions on the way to the substrate are exceedingly rare, and chemical reactions occur exclusively on the substrate. This allows beams of highly reactive or even metastable species to reach the deposition surface undisturbed. A wide range of growth conditions are accessible using MBE, and such flexibility is often key to achieving controlled growth at the atomic layer level. Several molecular beams may be sprayed onto the surface to be coated, either simultaneously or sequentially. The crystalline arrangement of the surface of the film is studied during MBE growth with electron diffraction and after growth by various characterization methods, which allow us to see what the composition of the film is, and what its properties are (e.g., is it insulating, semiconducting, ferroelectric, ferromagnetic, superconducting, or does it offer performance advantages to existing materials and devices?). The nanometer-scale layering control allows the growth of customized structures in which the sequence of atomic layers can be changed at will, enabling the full spectrum of the electronic properties of oxides to be combined in novel epitaxial heterostructures.

A second thin film growth method we employ is a vacuum deposition method called pulsed laser deposition (PLD), in which we vaporize a target made of selected elements with an ultra-violet laser and condense them on top of a single crystal substrate. Although not as precise a thin film growth method as MBE, PLD offers a rapid means of preparing custom-made stacks of single crystal films. Using PLD, we were the first group in the world to grow thin films of Sr2RuO4, the only known copper-free superconductor with the same structure as high-temperature superconductors. By growing this and other related ruthenate materials and studying their properties, we are helping to establish the common features of high-temperature superconductors, which will aid in their theoretical understanding and improvement.

Our research has relevance. For example, a materials revolution recently occurred in computers—after over 40 years, the SiO2 gate dielectric of Si-based transistors has been replaced by a material with a much higher dielectric constant (K), HfO2. A important materials science contribution to this revolution that now dominates the marketplace was laid over a decade ago by our group when we suggested the thermodynamic stability of HfO2 in contact with silicon.[1] This publication has become the fifth most highly cited paper in the Journal of Materials Research and demonstrates the power of thermodynamics in drastically reducing the number of high K candidates suitable for this application, i.e., only those that are stable in contact with silicon. A comparison of the materials listed as being appropriate in the 1997National Technology Roadmap for Semiconductors vs. the 1999 International Technology Roadmap for Semiconductors makes the impact of this work clear.[2],[3] In 1997 the high‑K materials listed were (Ba,Sr)TiO3, Ta2O5, and TiO2.2,[4] All of these materials react with silicon (as our thermodynamic analysis published in 1996 predicted1) and extensive research by semiconductor companies around the world revealed the futility of this approach. These materials were then abandoned in favor of the materials suggested by our thermodynamic analysis3. This materials revolution saves power at the same time that it boosts speed. For our role in this high-K revolution allowing Moore’s law and growth of the semiconductor industry to continue,[5] Schlom was awarded the 2008 MRS Medal.[6]

Together with collaborators from around the world we have studied the effect of strain on the properties of functional oxides that are ferroelectric or multiferroic. Conventional wisdom was that the best possible properties of any such material was to be found in a single crystal of that material. But our work over the past decade has dispelled this dogma and unearthed a new paradigm in how to control and enhance the properties of ferroelectric and multiferroic materials through strain-engineering. Our ferroelectric films have properties far better than single crystals of the same materials. They are more structurally perfect, have improved figures of merit, and can retain these enhanced properties to temperatures hundreds of degrees higher than their single crystal counterparts. [7]

Our research has so far concentrated on the growth and characterization of layered combinations of oxide superconductors, ferroelectrics, ferromagnets, and related phases and integrating these oxides with conventional semiconductor materials, e.g., silicon. Some of these materials are literally engineered at the atomic-layer level and have only been synthesized by MBE. In addition to making these materials, we evaluate their properties with the goal in mind of designing even better materials using the atomic-layer control possible with MBE. We are studying the crystal growth process of oxides at the atomic scale and investigating fundamental phenomena of oxide materials. Our long-range goal is apply the layering capability of MBE to the fabrication of novel electronic devices. Understanding how to manipulate atoms with ever-greater control to provide enhanced electronic devices is a major goal of our research.

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