Molecular Beam Epitaxy of Transition Metal Nitrides for Superconducting Device Applications

Abstract

Epitaxial integration of superconductors with semiconductors is expected to enable new device architectures and to increase electronic circuit and system functionality and performance in diverse fields, including sensing and quantum computing. Herein, radiofrequency plasma molecular-beam epitaxy is used to epitaxially grow 3-200 nm-thick metallic NbN x and TaN x thin films on hexagonal SiC substrates. Single-phase cubic δ-NbN and hexagonal TaN x films are obtained when the starting substrate temperature is %800 and %900 C, respectively, and the active N to Nb or Ta ratio is %2.5-3. The films are characterized using in-situ reflection high-energy electron diffraction and ex-situ atomic force microscopy, contactless sheet resistance, X-ray diffraction, X-ray photoelectron spectroscopy, secondary ion-mass spectrometry, Rutherford backscattering spectrometry, cross-sectional transmission electron microscopy, and low-temperature electrical measurements. Smooth, epitaxial, low-resistivity films of cubic δ-NbN and hexagonal TaN x on SiC are demonstrated for films at least %50 nm-thick, and their superconducting properties are reported. Epitaxy of AlN and GaN on δ-NbN is also demonstrated, as well as integration of an epitaxial NbN x superconducting electrode layer under GaN high-electron mobility transistors. These early demonstrations show the promise of direct epitaxial integration of superconducting transition metal nitrides with group III-N semiconductors.

Dense rotational twins in superconducting (111)-orientated δ-NbN epitaxial films on 4H-SiC substrates

“…These two areas are the twins of δ-NbN grown on 4H-SiC substrate with a 60 • rotation with respect to each other along the (111) axis. This rotational twins of δ-NbN was mentioned in the film grown along (111) axis on 6 H-SiC by MEB [49]. The area III is interpreted as the overlap region of two twins.…”

Growth windows of epitaxial $\textrm{Nb}_x\textrm{N}$ films on c-plane sapphire and their structural and superconducting properties

“…The inclusion of Sc in III-N semiconductors has been shown to increase the stability of the rock-salt phase, increasing the critical thickness of the metastable rock-salt semiconductor films [14]. Another method for epitaxial integration utilizes the growth of cubic TMNs oriented along the (1 1 1) body-diagonal with hexagonal III-N semiconductors grown along the c-axis [15,[18][19][20]. Although the crystal structures of the the two materials are markedly different in this case, the similarities between the structures and lattice parameters are sufficient to permit epitaxy, with the caveat that the growth of cubic films on hexagonal substrates results in multiple domains of the cubic film which differ in lattice orientation [20][21][22].…”

Coherent epitaxial growth of superconducting NbN ultrathin films on AlN by sputtering

“…3) The superconductivity of NbN has been extensively investigated during the past few decades. [4][5][6][7][8][9][10][11][12][13][14][15][16] Because the properties of NbN are strongly dependent on the crystal structure, lattice constant, and nitrogen content, control of the structural characteristics of NbN films is crucial for realizing quantum devices based on NbN.…”