Thermal stability of Nb thin films on sapphire

Abstract

The Nb/α−Al2O3 system has been used as a model study for investigating the stability of different MBE grown epitaxial Nb films on α−Al2O3 substrates. The films were grown at 800 °C in ultrahigh vacuum. The growth process was monitored in situ by reflection high energy electron diffraction (RHEED). After deposition the structure of the film was investigated by x-ray diffraction (XRD) and conventional transmission electron microscopy (CTEM) which encompasses also selected area diffraction (SAD). Both techniques revealed the following orientation relationship between the Nb film and the α–Al2O3 substrate: (0001)α–Al2O3‖ (111)Nb; [2110]α–Al2O3‖ [110]Nb. The stability of the niobium films was investigated by annealing the Nb-film/α–Al2O3 system to temperatures up to 1500 °C for different periods of time. Surprisingly, the orientation relationship between the Nb film and the substrate changed to (0001)α–Al2O3‖ (110)Nb; [0110]α–Al2O3‖ [001]Nb. A model will be developed which shows that above a critical film thickness the growth orientation is metastable with respect to its crystallographic orientation. Furthermore, high resolution transmission electron microscopy (HREM) was performed to investigate the defect structure of the annealed Nb/α–Al2O3 interface.

High temperature epitaxial growth and structure of Nb films on α–Al2O3(0001)

“…[7][8][9] No detailed information is available in the literature on the growth mode of Nb on the basal plane at high temperatures. [7][8][9] No detailed information is available in the literature on the growth mode of Nb on the basal plane at high temperatures.…”

“…1,3,12 The orientation in which Nb clusters nucleate on the (0001)a -Al 2 O 3 surface will be strongly IP address: 152.14.136.77 influenced by differences in interfacial energies. As shown in a recent publication, 7 For a single island, a critical thickness or height h crit can also be defined. 7 the sapphire substrate.…”

“…1,3 With increasing island size coalescence will start and finally result in a continuous film. 7 For films grown by MBE at ϳ900 ± C, the Nb grows on the sapphire substrate in such a fashion that planes of the same symmetry in Nb and sapphire are parallel [Nb(111) k a -Al 2 O 3 (0001); also reported by Durbin et al 8 and Mayer et al] 9 However, as shown by TEM investigations, small grains having a different epitaxial orientation to the sapphire can be found embedded in the epitaxial Nb film. Provided all islands have the same epitaxial orientation to the substrate, a single crystalline epitaxial film will be formed by island coalescence.…”

Comment on “Twin symmetry texture of energetically condensed niobium thin films on sapphire substrates (a-plane Al2O3)” [J. Appl. Phys. 110, 033523 (2011)]

“…4 One feature of the niobium/sapphire system that makes it unique is the three-dimensional relationship between metal and substrate, first discovered by Durbin et al 5 and confirmed by a number of others, This relationship is by no means universally observedunder certain growth and annealing conditions other orientational relationships have been found. [11][12][13][14] However, Zhao et al go one step further, calling into question the validity of this relationship and instead proposing their own threedimensional registry. My comment begins with a summary of their arguments.…”

Mechanisms of microstructure development at metallic-interlayer/ceramic interfaces during liquid-film-assisted bonding

“…49 Similar results have been found for niobium thin films (150 nm) grown on c-plane sapphire using MBE techniques at 800°C, and may be useful in explaining the mechanism of film reorientation. 50 After film deposition the orientation relationship found between the niobium film and the sapphire using Reflection High Energy Electron Diffraction (RHEED) showed the (111) planes of niobium parallel to the (0001) planes in the sapphire. During anneals at 1500°C for times ranging between 1 min and 4 h small grains of niobium embedded in the film with the (110) planes parallel to the (0001) planes in sapphire completely consumed the film.…”