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

Abstract

In order to confirm the superconductivity observed in hexagonal ϵ-NbN reported recently, we have deposited NbN film on (0001) 4H-SiC substrate, with good lattice match, intentionally to grow hexagonal NbN phase. The detailed structural analysis show that the deposited films are (111)-orientated δ-NbN phase with dense rotational (60°) twins. Double diffraction effect in selected area electron diffraction and Moiré fringes in high resolution transmission electron microscope images confirm the existence of rotational twins in nanometer scale. The growth of highly twined film results from no energy preference for the two rotational twins of (111)-orientated δ-NbN to bond with the Si-faced (0001) 4H-SiC substrate. These highly twined δ-NbN epitaxial films exhibit high normal state resistivity, lower T C, but good spatial uniformity of superconducting energy gap. No ϵ-NbN phase was observed in this study which is might due to a low growth temperature of thin film.

Superconducting NbN thin films on various (X/Y/Z-cut) lithium niobate substrates

“…The interplanar distance value (2.55 Å) was consistent with the value (2.558 Å) from the XRD result of the 15 nm thick film. The result confirmed that the growth model is the same as previously mentioned and in the [20,26,30]. NbN atoms are periodically well-ordered with (111) planes parallel to (001) planes of the Z-cut LN substrates.…”

“…Due to the thinness of films and the HRXRD method, the peak intensity is low. Only the (111) peaks of NbN were clearly observed, same as those in [20,26,30,31]. Besides, thickness of the ultrathin films limits the grain size, resulting in a large full width half maximum value of the XRD peaks [32].…”

“…(60 • ) twins structure grown on Al 2 O 3 [26,30] and 4H-SiC substrates [20]. According to the model in the [20], NbN films grown on Z-cut LN substrates have similar dense rotational twins, and the lattice mismatch was 4.7% (figure S1 (available online at stacks.iop.org/SUST/35/025012/mmedia)).…”

“…The tiled planes are attributed to observation from the cross-section of (111) planes in the direction <−110>. The details of the analysis of crystallography orientation can be referred to the references of NbN grown on other hexagonal substrates [20,26].…”

“…Furthermore, most NbN SNSPDs with high-detection efficiency are fabricated on Si substrates. And NbN films are single crystalline and have relatively high transition temperatures on MgO substrates [17] as well as other substrates such as SiC [19][20][21][22], GaN [23][24][25], and Al 2 O 3 [26]. The fabrication of SNSPDs on such substrates is possible; however, detectors with high efficiency are rarely reported [27].…”