Hans Christen
Hans M. Christen
Senior Research Staff Member
Leader, Thin Films and Nanostructures Group
Materials Science and Technology Division
Oak Ridge National Laboratory
PO Box 2008, Bldg. 3150
Bethel Valley Road
Oak Ridge, TN 37831-6056
Tel. (865) 574-5965
Fax (865) 574-4814
christenhm@ornl.gov
 
Research Interests
Thin films and heterostructures of metal-oxide films (perovskites in particular) provide an ideal platform for the study of interfacial and confinement effects on the properties of complex and correlated materials. Pulsed laser deposition (see recent review article) is used to grow epitaxial films of ferroelectrics, manganites and ruthenates, high-temperature superconductors, and electro-optic materials. Superlattices with atomically abrupt interfaces allow us to study the influence of strain and local asymmetry, as well as coupling at the nanoscale, on ferroelectric, transport, and magnetic properties.

Novel implementations of a continuous compositional-spread approach and of a temperature-gradient technique avoid limitations of earlier combinatorial approaches and are used as an efficient method to the discovery and optimization of materials. These approaches are not only applicable to epitaxial oxide layers but also to the study of catalysts for nanotube synthesis.

Pulsed electron deposition has been used as an alternative to pulsed laser deposition in the work on coated conductors (high-Tc superconducting tapes).

Epitaxial oxides and heterostructures

The effect of strain on properties of perovskites.
(Collaboration with H.N. Lee, D.H. Kim, and C.M Rouleau)
Epitaxial thin films are strongly affected by lattice strain. For KNbO3 layers in superlattices with paraelectric KTaO3, an increase of the ferroelectric transition temperature by 100 degrees is observed (1998 Appl. Phys. Lett. paper). More strikingly, room-temperature ferroelectricity is observed in nominally paraelectric SrTiO3 layers when subjected to in-plane tensile strain (2003 Phys. Rev. B paper). A surprisingly weak response is observed in other types of ferroelectrics: in PbZr0.2Ti0.8O3 (2007 Phys. Rev. Lett. paper) and BiFeO3 (2008 Appl. Phys. Lett. paper), the polarization remains almost unchanged when the amount of strain is modified by varying the film thickness. In charge-ordered manganites, subtle effects of strain magnitude and symmetry are observed by comparing results on different substrates and substrate orientations (2006 Appl. Phys. Lett. paper).
Atomic-scale control in perovskite superlattices.
(Film growth with H.N. Lee and C.M. Rouleau; microscopy: M.F. Chisholm and M. Varela)
The effects of interfaces and spatial confinement play the most crucial role in determining the properties of nanostructured complex materials. Their study requires unprecedented control over interfacial quality. We are applying PLD to the growth of atomic-scale perovskite superlattices with atomically flat interfaces, obtained at sufficiently high oxygen pressures to yield bulk-like properties, without requiring post-annealing. The resulting crystalline quality was previously achieved only in low-pressure molecular-beam epitaxy approaches (see below). PLD can also yield very specific samples, such as the CaTiO3 structures used to demonstrate single-atom sensitivity of La-EELS spectroscopic imaging in a scanning transmission electron microscope (2004 Phys. Rev. Lett. article).
Non-inversion symmetric artificial crystals.
(Film growth: H.N. Lee with C.M. Rouleau; microscopy: M.F. Chisholm)
The breaking of inversion symmetry plays a key role in functional materials, yielding properties like ferroelectricity and magnetism. However, the permanent removal of inversion symmetry due to an asymmetric arrangement of elements within a material is rarely observed in conventional materials. Three-component superlattices (TCSs) (stackings of the type A-B-C-A-B-C…) allow us to study the effects of asymmetry on ferroelectric properties in perovskite materials (2005 Nature article).
Stability of SrRuO3 films.
(Film growth and AFM investigation: H.N. Lee)
SrRuO3 is widely used as bottom electrode for the measurement of physical properties in complex oxides. Our work has shown that the surface of SrRuO3 films is stable only in a limited range of the temperature – oxygen pressure phase space (2004 Appl. Phys. Lett. article).


Ferroelectricity in perovskites superlattices. Size effects and long-range interactions can be studied carefully in epitaxial heterostructures containing paraelectric and/or ferroelectric perovskites. In the case of KTaO3/KNbO3 superlattices, we have shown that the ferroelectric transition temperature converges towards that of the corresponding alloy as the layers become thinner (1998 Appl. Phys. Lett. paper), while the local crystalline structure (as probed by EXAFS) in the low-temperature phase remains distinctly different (2000 J. Electroceram. paper). In heterostructures of SrTiO3 and BaZrO3 – two non-ferroelectric materials – characteristics of room-temperature ferroelectricity and of anti-ferroelectricity are observed depending on the superlattice periodicity (2003 Phys. Rev. B paper).


Quantum criticality in (CaxSr1-x)RuO3.
(Film growth with I. Ohkubo, now at Tokyo University; characterization with P. Khalifah, now at University of Massachusetts)
Systems exhibiting Non-Fermi Liquid (NFL) behavior, i.e. metals in which the low-temperature electronic behavior differs from that of the Fermi Liquid model, are rare exceptions to the vast majority of known materials. One potential route for finding NFL behavior is tuning a system to the vicinity of a quantum phase transition (QPT), a regime where the length scale of electronic fluctuations are diverging as the precise end point of the quantum phase transition is approached. Using PLD, thin film alloys of CaRuO3 and ferromagnetic SrRuO3 can be compositionally tuned to exhibit such a QPT (2004 Phys. Rev. B paper).

Compositional-spread and temperature-gradient approaches

High-throughput materials synthesis. Multi-sample approaches allow us to systematically investigate the effect of composition or growth temperature on simultaneously grown thin films. To this purpose, we have developed a continuous compositional-spread method based on pulsed laser deposition that is applicable to metastable materials and heterostructures, thereby going well beyond the limitations of previous combinatorial synthesis techniques. Similarly, a temperature-gradient method can be used to determine the optimal film-growth temperature in a single run (see below for more details). Click here for more details (2005 review article).

Continuous compositional spread.
Compositional-spread approaches, in which a sample of continuously varying chemical composition is obtained by the simultaneous deposition of multiple constituents, have been used for more than 35 years. However, the earliest methods were based on naturally-occurring spatial deposition-rate variations, and led to simultaneous spatial variations of film thickness and deposition energetics. Alternative approaches based on moving shutters yield well-controlled composition variations, but on length scales that are too small for standard characterization techniques. The present approach is based on a moving substrate—more difficult to implement but yielding samples that can be investigated with conventional measurement techniques. Click here for technical details (2003 RSI paper).
Temperature-gradient approach.
(Film growth with Isao Ohkubo, now at Tokyo University; ellipsometry: G.E. Jellison, Jr.; piezoresponse force microscopy: S. Kalinin)
Optimization of the growth temperature is a first – and often time-consuming – step in the development of a new material. Depositing simultaneously onto multiple samples (held at temperature from 200°C to 800°C) significantly increases productivity. Furthermore, the growth temperature-dependence of optical properties, crystallinity, and electro-mechanical behavior, can systematically be investigated. In the growth of materials for which systematic information about crystallization temperatures is lacking, this method allows us to quickly screen a large number of candidate materials for a particular application.
Examples include electro-optic materials such as SrxBa1-xNb2O6 (2004 Appl. Phys. Lett. paper) and rare-earth scandates as candidate high-k dielectrics (2006 Appl. Phys. Lett. paper).
Application to the catalysis of carbon nanotubes.
(Nanotube synthesis: A.A. Puretzky and D.B. Geohegan, microscopy: A.A. Puretzky and H.Cui)
The characteristics of nanomaterials are known to depend strongly on the size and activity of the catalyst nanoparticles responsible for their growth. Here we use the compositional-spread approach to quickly and systematically study the properties of multiwalled carbon nanotubes as a function of the metal catalyst composition. Tube height is used as first and most accessible parameter, but other properties (such as thermal conductivity and Raman spectra) are also investigated (2004 NanoLetters paper).

 Coated conductor research  


The cost-effective and reliable fabrication of superconducting tapes is a critical challenge in realizing the advantages of high-Tc cuprate applications, such as power transmission, motors and generators, transformers, flywheels, etc. (see www.ornl.gov/sci/htsc). Pulsed Electron Deposition (PED) is a relatively low-price method for depositing complex mixtures of elements in a simple process. Similar to PLD, a pulsed energy source (i.e. a 100 ns electron pulse) results in the ablation of material from a solid “target.” This technique has been used to deposit (at room temperature) fluoride-based precursors that can then be converted (by annealing at elevated temperature) into epitaxial layers of YBa2Cu3O7-x. A reel-to-reel system (see illustration) allows us to deposit onto moving tape. More details are given in a 2005 Supercond. Sci. Technol. publication. One of the motivations for this work was our analysis of the cost involved in pulsed laser deposition of YBa2Cu3O7-x (more details of the cost model can be found in the book chapter (2001) or in the updated viewgraphs (2003).
Education
Swiss Federal Institute of Technology
M.S.
1991
 Physics Engineering
Lausanne, Switzerland
Ph.D.
1994
 Physics
 
Professional Positions
2006-Present Leader, Thin Films and Nanostructures Group, Oak Ridge National Laboratory
2000-2006 Research Staff Member, Oak Ridge National Laboratory
1999-2000 Program Manager, Microwave Microscopy, Neocera, Inc., Beltsville, MD
1997-1999 Staff Scientist, Neocera, Inc., Beltsville, MD
1994-1996 ORAU and Swiss NSF Postdoctoral Fellow, Oak Ridge National Laboratory
1991-1994 Research Assistant, IBM Research Division, Zurich Research Laboratory
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