Reduction of Lattice Mismatch Induced Strain and Critical Thickness Identification for Long-Range Crystallinity Niobium Nitride on 6H-SiC for High Quality SNSPD Detectors

Niobium nitride (NbN) films have garnered significant attention and research recently due to their high critical temperature (T_c) and their usage in infrared-sensitive superconducting nanowire single photon detectors (SNSPDs). Devices fabricated from NbN have demonstrated single photon detection to mid-wave infrared wavelengths, which unlocks possibilities for novel applications such as long-range laser detection and ranging (LiDAR), interferometry of signals in planetary emissions, quantum key decryption, and optical communications. Due to their polycrystalline nature, NbN films are sensitive to defects stemming from the deposition and fabrication processes referred to as constrictions. These film constrictions effectively reduce the cross-sectional area of the detector nanowire, which in turn limits the critical current and resultant detection efficiency and jitter. NbN films are still, however, among the most promising for practical use and array development due to their high T_c approaching 16K, thus necessitating the development of methods to reduce film defectivity.<br/>NbN films utilized for SNSPD fabrication must be thin, typically &lt;100 nanometers. As a result, the film quality and defectivity are highly correlated to the interface between the NbN film and underlying lattice, lattice-mismatch strain, and deposition parameters of the NbN processing. To optimize this interface, a material with similar lattice structures and parameters to NbN must be chosen to minimize intrinsic defect sources. Then the deposition process can be optimized to reduce extrinsic defects.<br/>Silicon carbide (SiC) 6H polytype substrates were selected for use as the deposition substrate. 6H-SiC has an in-plane lattice parameter a=3.08Å, leading to a 3.57% lattice mismatch with hexagonal NbN with in-plane lattice parameter of 2.98Å and minimizing strain from lattice matching below the critical thickness. SiC can be later utilized to fabricate semiconductor components necessary for focal plan array (FPA) development. NbN films were sputtered onto the substrates in an AJA ATC Orion 8 sputtering tool utilizing a non-reactive RF magnetron process in an inert argon atmosphere to control resultant film stoichiometry. Deposition conditions such as temperature, gas flow, and pressure were modified, and the resultant films were characterized through x-ray diffraction to determine phase, orientation, crystallinity, and stress.<br/>The temperature of the substrate during deposition runs was increased from ambient temperature to 600C. As temperature increased, XRD peak intensity increased, and crystallite size calculated from peak FWHM increased linearly from 26.07nm to 1618.1nm for P6m2 NbN. This substantial increase in crystallinity stemming from increased energy promoting nucleation and growth of higher order crystallites reduces the number of grain boundaries and resultant constrictions within a SNSPD to improve efficiency. High temperatures, however, are not always compatible with semiconductor fabrication necessary for FPA development so pressure modifications were also explored as an alternative. The deposition pressure was increased from 3mTorr linearly to 8mTorr. Similar improvements in crystallinity to temperature were observed. The critical thickness where the film stress cannot be accommodated through lattice matching was measured to extract the stress of these films. 3mTorr films demonstrated a critical thickness of 475A, while films with higher pressures increase that to &gt;750A. This improvement points towards reduced film stress and reduced formation of threading dislocations to accommodate strain.<br/>In conclusion, we studied the impact of substrate lattice matching and non-reactive sputtering of NbN films on 6H-SiC. Modifying the deposition temperature and pressure allowed for the development of films with long range crystallinity and low stress to enable defectivity reduction and facilitate the development of high-quality long wavelength SNSPDs.