Epitaxial growth and thermodynamic stability of SrIrO3/SrTiO3 heterostructures

Abstract

Obtaining high-quality thin films of 5d transition metal oxides is essential to explore the exotic semimetallic and topological phases predicted to arise from the combination of strong electron correlations and spin-orbit coupling. Here, we show that the transport properties of SrIrO 3 thin films, grown by pulsed laser deposition, can be optimized by considering the effect of laser-induced modification of the SrIrO 3 target surface. We further demonstrate that bare SrIrO 3 thin films are subject to degradation in air and are highly sensitive to lithographic processing. A crystalline SrTiO 3 cap layer deposited in-situ is effective in preserving the film quality, allowing us to measure metallic transport behavior in films with thicknesses down to 4 unit cells. In addition, the SrTiO 3 encapsulation enables the fabrication of devices such as Hall bars without altering the film properties, allowing precise (magneto)transport measurements on micro-and nanoscale devices.The intriguing electronic structure of 5d transition metal oxides arises from the delicate interplay between competing energy scales. Iridium compounds display a particularly large spin-orbit coupling (SOC) of the order of 0.4 eV, which leads to the formation of novel J eff = 1/2 and J eff = 3/2 states 1 . The combination of this strong SOC and slight lattice distortions has recently drawn attention to SrIrO 3 as a promising candidate to realise topological (semi)metallic phases 2-6 . Perovskite SrIrO 3 is a member of the Pbnm space group, featuring two glide planes and a mirror plane which are crucial in determining its band structure 7,8 . At atmospheric pressure, SrIrO 3 crystallises in a 6H-hexagonal structure, while its perovskite form can be obtained by applying high pressure and temperature and subsequent quenching 9 . This requires particular care due to the high volatility of iridium oxides and competition with other phases such as Sr 2 IrO 4 and Sr 3 Ir 2 O 7 10 . These extreme conditions can be avoided by resorting to thin film growth, where epitaxial constraint can be used to synthesize perovskite SrIrO 3 films 8,[11][12][13][14][15][16][17][18][19] . SrIrO 3 films are generally grown by pulsed laser deposition (PLD), where a relatively high oxygen pressure (0.01 -1 mbar) is required to control the Ir oxidation state19 . In such high pressure conditions, the interaction dynamics between the expanding plume and the background gas are very complex 20 . This can readily result in slight deviations from the ideal film stoichiometry, which can strongly affect the electrical properties through the formation of crystal defects. Electrical transport measurements of SrIrO 3 films reported in literature show a rather large variability, which brings to question the role of disorder and secondary phase formation on a) Electronic mail: d.j.groenendijk@tudelft.nl the film properties [13][14][15]21,22 .In this Letter, we identify key issues related to the growth and stability of SrIrO 3 thin films and study how these affect their electrical properties. F...

Spin-Orbit Semimetal SrIrO3 in the Two-Dimensional Limit

“…week ending 22 DECEMBER 2017 0031-9007=17=119(25)=256403 (6) 256403-1 © 2017 American Physical Society degradation in ambient conditions and enable lithographic processing [18]. Hall bars were patterned by e-beam lithography, and the SrIrO 3 layer was contacted by Ar etching and in situ deposition of Pd and Au, resulting in Ohmic contacts.…”

“…More details regarding the growth and sample characterization can be found in the Supplemental Material [19] and in Ref. [18]. First-principles DFT calculations were performed within the generalized gradient approximation using the plane wave VASP [25] package and PBEsol for the exchange-correlation functional [26] with SOC.…”

Synthesis and electronic properties of Ruddlesden-Popper strontium iridate epitaxial thin films stabilized by control of growth kinetics

“…51 Furthermore, under high background pressure (sample (iv) P O2 ∼ 10 −1 Torr), lighter species are likely to experience back-scattering by background oxygen. 29,51,58,59 ; Eventually a crossover to the regime where the angular distribution of lighter species is broader than that of the heavier ones takes place. This mechanism gives rise to the formation of a Irrich phase in the film.…”

Growth and engineering of perovskite SrIrO 3 thin films

“…Epitaxial Pv-SrIrO3 films are also sensitive to air, and their electrical properties degrade while exposing the film to ambient conditions and after lithographic processing [40]. Therefore, an alternative solution is to use a capping layer for precise measurements for future device fabrication.…”