High-quality EuO thin films the easy way via topotactic transformation.

Thomas Mairoser,Darrell G Schlom,Daniel Hodash,Julia A Mundy,Artur Glavic,Paul Cueva,Andreas Schmehl,Alexander Melville,David A Muller,Jürgen Schubert,Rainer Held

doi:10.1038/ncomms8716

Abstract

Epitaxy is widely employed to create highly oriented crystalline films. A less appreciated, but nonetheless powerful means of creating such films is via topotactic transformation, in which a chemical reaction transforms a single crystal of one phase into a single crystal of a different phase, which inherits its orientation from the original crystal. Topotactic reactions may be applied to epitactic films to substitute, add or remove ions to yield epitactic films of different phases. Here we exploit a topotactic reduction reaction to provide a non-ultra-high vacuum (UHV) means of growing highly oriented single crystalline thin films of the easily over-oxidized half-metallic semiconductor europium monoxide (EuO) with a perfection rivalling that of the best films of the same material grown by molecular-beam epitaxy or UHV pulsed-laser deposition. As the technique only requires high-vacuum deposition equipment, it has the potential to drastically improve the accessibility of high-quality single crystalline films of EuO as well as other difficult-to-synthesize compounds.

Highlights

Epitaxy is widely employed to create highly oriented crystalline films
europium monoxide (EuO) is scientifically the most interesting compound, mainly because it is a rare example of a high-magnetization ferromagnetic semiconductor (7 mB per Eu; TC 1⁄4 69 K) which can be epitactically integrated with silicon[6], GaAs7, GaN6, graphene[8], diamond[9] and many oxide substrates
At 700 K, europium will only be stable in its lowest oxidation state (Eu2 þ in EuO) if the oxygen partial pressure lies between 5 Â 10–80 and 2 Â 10–57 atm[29,38]

Summary

Introduction

Epitaxy is widely employed to create highly oriented crystalline films. A less appreciated, but powerful means of creating such films is via topotactic transformation, in which a chemical reaction transforms a single crystal of one phase into a single crystal of a different phase, which inherits its orientation from the original crystal. As the resulting highquality EuO film inherits its crystalline structure form the precursor film, the reduction reaction is a topotactic transformation. This instability in ambient conditions has, up to now, limited the growth of epitactic EuO films with high crystalline quality to UHV MBE6–10,30,31 and UHV PLD32,33.

Results

Conclusion