Characterization of ultrathin insulating Al2O3 films grown on Nb(110)/sapphire(0001) by tunneling spectroscopy and microscopy

Abstract

Various structural as well as chemical properties of ultrathin Al 2 O 3 films prepared on Nb͑110͒/ sapphire͑0001͒ were analyzed. For this purpose, in a first step, 40-nm-thick Nb͑110͒ films are grown epitaxially by sputtering on top of sapphire͑0001͒. The Nb͑110͒ films are (1ϫ1) reconstructed and exhibit the epitaxial relations Nb(110)ʈAl 2 O 3 (0001), Nb͓001͔ʈAl 2 O 3 ͓1 គ 010͔ and equivalents as determined by x-ray diffraction. In a second step, a 1-nm-thick Al film is evaporated on top of the Nb͑110͒ and oxidized ex situ in an oxygen plasma resulting in a polycrystalline Al 2 O 3 film of thickness ϳ1.3 nm. This oxide film was characterized by scanning tunneling microscopy and spectroscopy ͑STS͒ as well as by x-ray photoelectron spectroscopy. Additionally, the ultrathin Al-oxide films were tested as insulators in large area metal-insulator-metal ͑MIM͒ or superconductor-insulator-metal tunneling contacts. According to these experiments, the niobium film underneath is completely protected from oxidation, while the Al film is fully oxidized without pinholes over areas of some mm 2 . The tunneling barrier heights range from 1.0 to 1.5 eV according to MIM tunneling and STS.

Studying the bulk band structure of niobium by scanning tunneling spectroscopy

“…[12]. Directly after preparation, the films were transferred in situ to a STM chamber operated at a pressure well below 10 À10 mbar.…”

Atomically Thin Al2O3 Films for Tunnel Junctions

“…In order to experimentally probe this hydroxylation process, one cycle of ALD Al 2 O 3 was performed on an Al wetting layer with an initial H 2 O pulse of variable duration. This high band gap is remarkable because it is comparable to the ultrathin (~1.3 nm) epitaxial Al 2 O 3 band gap [24]. The ALD Al 2 O 3 tunnel barrier also displayed a hard-breakdown type behavior under the STM electric field which is typical for epitaxial Al 2 O 3 thin films [25].…”

Design of an extremely stable low-temperature ultrahigh vacuum scanning tunneling microscope

“…These values are in excellent agreement with previous measurements. 13,14 To obtain an even better estimate of the energy resolution of our instrument, additionally the superconducting gap of an epitaxially grown Nb͑110͒ film on sapphire 15,16 was determined at 6 K. The corresponding result is presented in the inset of Fig. 6.…”