Crystalline Formations of NbN/4H-SiC Heterostructure Interfaces

Abstract

Given the importance of incorporating various superconducting materials to device fabrication or substrate development, studying the interface for possible interactions is warranted. In this work, NbN films sputter-deposited on 4H-SiC were heat-treated at 1400 °C and 1870 °C and were examined with transmission electron microscopy to assess whether the interfacial interactions undergo temperature-dependent behavior. We report the diffusion of NbN into the SiC substrate and the formation of NbN nanocrystallites therein during the 1400 °C treatment. After the 1870 °C treatment, tiered porosity and the formation of voids are observed, likely due to catalytic reactions between the two materials and accelerated by the stresses induced by the differences in the materials’ coefficients of thermal expansion. Lastly, Raman spectroscopy is employed to gain an understanding of the interface lattices’ optical responses. INTRODUCTION Since its discovery, superconducting NbN has played a key role in several facets of materials science and device fabrication.1 The modern use of NbN is sustained by the ease with which it can be deposited on a suitable substrate as well as its ability to maintain a superconducting state until above 10 K and for some considerable amount of magnetic flux density (approximately 10 T).2 Thin films of NbN exhibit some versatility when applying them to quantum computing in the form of single photon detectors, flux quantum circuits, and qubits.3 4 5 Furthermore, these films and their similar counterparts can be deposited onto a variety of substrates including SiC for the purposes of vertical transistors,6 7 8 9 10 resistance standards,11 12 and Josephson junction devices.13 Given the extent of functionality for these thin films, knowledge on the interactions between NbN and its corresponding substrate is of vital importance for device engineering. Among available substrates used for material studies, SiC stands out in the following ways: wide bandgaps for semiconductor applications, low loss for THz frequencies, chemical compatibility with other thin films (from insulating to superconducting) fit for quantum communications, and wide commercial availability.14 For these reasons, interest remains strong in understanding the interfacial interactions between NbN and SiC. Results for NbN/3C-SiC interactions have been recently reported assessing the viability of this heterostructure for hot-electron bolometer mixers and other THz applications.15 16 Similarly, results as recent as last year reported on the interactions between NbN and 6H-SiC.10, 17 Given this new interest in examining SiC polymorphs and their interactions with thin NbN, it is only fitting that work should be done using 4H-SiC. 4H-SiC is another polymorph known to have superior electronic properties, such as a higher electron and hole mobility as well as a higher Baliga figure of merit,18 when compared to polymorphs 3C-SiC and 6H-SiC.19 20 In this work, NbN films were deposited on 4H-SiC and heat treated at 1400 °C and 1870 °C. The heat-treated and as-deposited films were examined with transmission electron microscopy (TEM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy to observe the temperature dependence of any interfacial reactions. All depositions were performed via sputtering as with similar processes,21 22 23 but it should be noted that there are several varieties of methods for depositing NbN.2,24 25 26 27 The diffusion of Nb and N into the 4H-SiC substrate and subsequent recrystallization therein is reported at the lower of the two heat treatment temperatures. For the higher temperature, we observe tiered porosity and the formation of voids, likely resulting from the differences between the coefficients of thermal expansion (CTEs) of the interface materials. Data acquired with Raman spectroscopy suggest the formation of carbon-based lattices at the interface for both treatment temperatures. RESULTS AND DISCUSSION Annealing Summary and As-Deposited Samples Various 4H-SiC substrates were diced from a wafer and subsequently placed in a sputter chamber. After the NbN layer was deposited, specimens were either left as grown, heated to 1400 °C, or heated to 1870 °C. Each of the three specimens were then prepared by focused ion beam (FIB) for examination by transmission electron microscopy (TEM), as described in detail in the Methods section. Figure 1 summarizes the results schematically as they will be shown in the following sections. In the case of a lower-temperature 1400 °C film, diffusion of NbN and the formation of NbN crystallites and voids within the near-interface region were observed, whereas in the higher-temperature 1870 °C film, larger voids with less surface area formed. In both films, the NbN formed into grains generally tens of microns in extent. Figure 1. Schematic illustration of the NbN–4H-SiC interface after deposition and the two heat treatments. For (a) the as-deposited interface, no significant reactions were observed. At 1400 °C, some of the NbN appears to diffuse into the SiC substrate from the film, and forms crystallites. At 1870 °C, the NbN film coarsens into larger grains, resulting in a porous film-substrate interface. In the first case, the as-deposited sample was examined for a baseline comparison to annealed samples. XRD data were acquired and shown in Fig. 2 (a), with the data suggesting that the NbN film is amorphous. Fig. 2 (b), shows a TEM image of the overall polycrystalline NbN film atop the 4H-SiC substrate, with the inset SAED pattern for the NbN film corroborating the polycrystallinity of the NbN. Figure 2. (a) XRD data taken on NbN suggesting an amorphous structure. (b) A TEM image crosssection of the NbN/4H-SiC film stack, with a polycrystalline SAED pattern for the NbN film inset. (c) A higher-magnification TEM image of the indicated region in (b) showing the interface more clearly Heat Treatment at 1400 °C Heating the film to 1400 °C induces a trio of major reactions, both in the NbN film and the 4H-SiC substrate. The extent of the transformations is shown in Figure 3. The NbN film has completely recrystallized into large grains from its formerly nano-crystalline state. The grains are approximately 100 nm to 300 nm in extent, with single grains spanning the full depth of the film. Also immediately evident, in contrast to the as-grown film, is the separation between the NbN film and the 4H-SiC substrate. It appears not simply delaminated but very rough and uneven at the length scale of the film. This is notable because SiC is generally stable under heat treatments at much higher temperatures. This decomposition of the interfacial region of the 4H-SiC substrate may be partially accounted for by the tendency to relieve increased interfacial energy due to a large mismatch in the CTE during cooling.28 29 30 The third reaction, though, may offer an additional explanation for the extensive degradation of the 4H-SiC at the interface. Examining the near-interfacial region of the 4H-SiC at high magnification reveals a population of faceted nanocrystallites distinct in contrast from the surrounding SiC material. EDS reveals these crystallites to comprise niobium and nitrogen either in part or in full and is likely some phase of NbxNy. The presence of these crystallites and constituent species within the transformed regions of the 4H-SiC suggest the possibility that the decomposition of the erstwhile 4H-SiC surface was catalyzed by the presence of and facilitated the migration of these crystallites away from the film and into the substrate. Figure 3. (a) Overview of a cross-section of the film heat-treated at 1400 °C by TEM and (b) a higher-magnification image of the area called out in (a). The coalescence and subsequent faceting of the NbN film is especially apparent in the grain to the left side of the image. The regions of bright contrast between the NbN film and the 4H-SiC substrate are porosity induced by the heat treatment. In the XRD data shown in Fig. 4 (a), three distinct crystalline NbN phases are present: (I) δ-NbN (225), which has a characteristic lattice constant a = 0.446 nm (II) tetragonal Nb4N3 (139), with a = 0.438 nm and c = 0.863 nm, and (III) a primitive cubic NbNx, with a = 0.694 nm, which has scant reports in the literature.31 Fig. 4 (b), shows the Nb-N crystallites in detail. Figure 4. (a) XRD data show a mixture of three crystalline NbN phases: (I) δ-NbN (225), (II) tetragonal Nb4N3, and (III) cubic primitive NbNx. The cubic response is very small compared with the other two phases. (b) A high-resolution TEM image of the near-surface region of the 4H-SiC substrate revealing NbN crystallites having formed after migration of Nb and N from the film but before the voids opened up. Heat Treatment at 1870 °C Increasing the heat treatment temperature to 1870 °C results in a more well-defined and locally smooth interface, but still features significant intrusion into the 4H-SiC surface by a newly recrystallized NbN film. Cross-sections of this specimen are shown in Figure 5 (a and b) In a similar fashion to the 1400 °C case, the NbN film has recrystallized into grains of about 300 nm to 500 nm in extent and through the full thickness of the film. Closer inspection reveals the wellfaceted nature of the recrystallized NbN. The insets in Figure 5 (b) show clear preferences in the NbN for recrystallizing along low-index planes, both at the NbN/4H-SiC interface and at the NbN/air interface. The scale of the faceting can be seen more clearly in Figures 5 (c and d), in which the film coarsened into roughly equiaxed grains in which the grains formed terraces to favor the (111) plane interface with air. Also evident are large pores, of order 100 nm, between some coarsened grains which are too dispersed to have been captured in the relatively small area of the TEM specimen.