Direct growth of crystalline SiGe nanowires on superconducting NbTiN thin films

Novel heterostructures created by coupling one-dimensional semiconductor nanowires with a superconducting thin film show great potential toward next-generation quantum computing. Here, by growing high-crystalline SiGe nanowires on a NbTiN thin film, the resulting heterostructure exhibits Ohmic characteristics as well as a shift of the superconducting transition temperature (T c). The structure was characterized at atomic resolution showing a sharp SiGe/NbTiN interface without atomic interdiffusion. Lattice spacing, as calculated from large-area x-ray diffraction experiments, suggests a potential preferred d-spacing matching between (200) NbTiN and (110) SiGe grains. The observed out-of-plane compressive strain within the NbTiN films coupled with SiGe nanowires explains the downward shift of the superconductivity behavior. The presented results post scientific insights toward functional heterostructures by coupling multi-dimensional materials, which could enable tunable superconductivity that benefits the quantum science applications.


Introduction
Advances in device research have brought significant progress toward precise control of materials characteristics and utilization of quantum phenomena to deliver novel functionalities. A promising approach is semiconductor-superconductor hybrid structure enabling controllability of semiconductor device characteristics and usage of quantum states of superconductors. For example, Josephson junctions enable many applications by developing semiconductorsuperconductor hybrid structures. Moreover, recent progress in quantum computing, quantum sensing, and cryogenic electronics has shown that semiconductor-superconductor hybrid structures are needed for artificial intelligence, enhanced sensor performance, next-generation computing, signal processing, and quantum optoelectronic devices [1][2][3][4][5][6][7][8].
The architecture of coupling semiconductor and superconductor has been accomplished by deposition of superconducting electrodes on active elements made of semiconductor materials, which is scalable and applicable for various devices [8][9][10][11]. Another production compatible and scalable integration method is the direct deposition or synthesis of semiconducting materials on a superconductor. However, the preparation of crystalline semiconductors on superconducting materials has rarely been studied due to the Nanotechnology Nanotechnology 34 (2023) 155705 (8pp) https://doi.org/10.1088/1361-6528/acb49e * Author to whom any correspondence should be addressed.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. material incompatibility, such as mismatch of lattice spacing or thermal expansion coefficients. Nanostructuring can be a promising way to overcome such incompatibility issues because small cross-sections at the interface between incommensurate materials and large surface-to-volume ratios provide pathways to minimize strain or extended defect formation.
Semiconductor nanowires (NWs) serve as crucial building blocks in semiconductor-superconductor hybrid architectures because they fulfill the requirements for nanostructuring and crystallinity. Moreover, crystalline semiconductor NWs have been grown on various metal and ceramic films showing superconductivity at cryogenic temperatures [12][13][14]. Nevertheless, direct growth of semiconductor on metallic layers has suffered from problems such as unintentional incorporation of metallic elements into semiconductor [9,15,16]. Therefore, any superconducting material used as electrodes in semiconductor-superconductor hybrid structure needs to have Ohmic behavior, exhibit superconductivity at cryogenic temperature, and show limited or no diffusion into a semiconductor [9]. Titanium nitride-based superconductors, such as niobium titanium nitride (NbTiN) can fulfill the requirements because TiN and its alloys are known as diffusion barriers in semiconductor devices and NbTiN is a superconducting material of which properties are not heavily dependent on crystallite sizes [15,[17][18][19].
Here, we report a direct growth of crystalline silicongermanium (SiGe) nanowires on NbTiN thin films of which the critical temperature (T c ) is 6.1 K. Electrical characterization of the as-fabricated NbTiN/SiGe/NbTiN heterostructure suggests an Ohmic current-voltage (I-V ) characteristic behavior at cryogenic temperatures down to 1.8 K, in the range where we observe a superconducting transition. Abrupt changes of the sheet resistance of NbTiN/SiGe NWs/NbTiN heterostructure at 4 K indicates that the NbTiN behaves as superconducting electrodes. Structural characterization reveals that the shift of T c of the NbTiN thin film from 6.1 to 4 K was explained by the out-of-plane compressively strained NbTiN lattices.

Growth
NbTiN thin films were deposited on Si (100) substrates by magnetron co-sputtering. Ti and Nb targets (>99.995%) were sputtered under the flows of nitrogen (5 sccm) and argon (30 sccm). The chamber pressure was kept at 3 mTorr. The NbTiN thin film thickness was 108 nm after deposition for 6 min. The Au-catalyzed SiGe NWs were grown on the NbTiN thin films by low-pressure chemical vapor deposition (CVD). A 3 nm thick Au layer was deposited onto the NbTiN thin films by e-beam evaporation prior to loading the substrates into the CVD chamber. The precursors were silane diluted in hydrogen (50% SiH 4 in H 2 ) and germane diluted in hydrogen (30% GeH 4 in H 2 ). The flow ratio of the silane to the germane was fixed at 3. The reactor pressure was kept at 3 Torr. The substrate temperature was changed in the range of 250°C and 550°C to control compositions of Si and Ge. Details are described elsewhere [20].

Characterization
Microstructures and crystallinity of SiGe NWs, NbTiN, and SiGe NW/NbTiN were characterized by bright field transmission electron microscopy (BF-TEM), high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) using a FEI Tecnai TF30 TEM with 300 kV accelerating voltage. X-ray diffraction (XRD) θ-2θ scans and rocking curve measurements were performed by a Rigaku SmartLab II diffractometer. The instrument was equipped with a Cu-Kα radiation source and was operated at 40 kV and 44 mA. For rocking curve measurements, a Ge(022) monochromator was used. Temperature-dependent electrical characterization of NbTiN thin film and NbTiN/SiGe NWs/ NbTiN was performed using a Quantum Design Physical Property Measurement System (PPMS) with four-wire configuration in the temperature range between 1.8 K and 300 K.

Results
The crystallinity and superconducting characteristics of the NbTiN film were investigated by TEM and temperaturedependent I-V measurements as shown in figures 1(a) and (b). Figure 1(a) shows that the NbTiN thin film deposited on Si (100) substrate is polycrystalline. Superconductivity was confirmed by the vanishing electrical sheet resistance of the NbTiN thin film at 6.1 K. After confirmation of superconducting NbTiN thin film, a 3 nm thick Au layer was deposited onto the NbTiN thin film to provide nucleation sites of SiGe NWs. Figure 1(c) shows that the Au deposited on NbTiN thin film forms non-spherical islands. These nonspherical islands with lateral sizes of >10 nm indicate that surface diffusion of Au on NbTiN is facilitated as compared to that on TiN [21]. After annealing the 3 nm thick Au/ NbTiN thin film at 340°C, a typical growth temperature of Ge-rich SiGe NWs, the film shows noticeable agglomeration of Au on the NbTiN thin film due to the surface diffusion of Au. Such enhanced surface diffusion of Au implies that precise diameter control of metal-catalyzed semiconductor NWs on superconducting NbTiN requires careful calibration of Au nucleation by changing the deposition conditions [22,23].
Next, Si x Ge 1−x (0.15 x 0.8) NWs were grown on the superconducting NbTiN thin film. Figures 2(a)-(d) shows low magnification scanning electron microscopy (SEM) images of the SiGe NWs with different compositions controlled by changing the growth temperature (T g ). In the range of the various compositions of SiGe NWs, a dense forest of NWs was grown on the NbTiN thin film. Composition analyses of Si and Ge were performed by TEM energy dispersive x-ray (EDX) spectroscopy. The length and diameter of SiGe NWs increase with rising T g , which is consistent with previous observations of SiGe NWs growth [20,24]. The same Si-Ge compositions have been observed for the SiGe NWs grown on NbTiN thin films and on Si substrates at the same T g . Therefore, instead of substrate effect, the compositions of SiGe NWs are mainly governed by the partial pressure of silane and germane. A representative high-resolution (HR) TEM image and its fast Fourier transform (FFT) from the reciprocal space of Si 0.15 Ge 0.85 NWs indicate that SiGe NWs are single crystalline as shown in figures 2(e), (f). There is no noticeable difference in crystallinity and composition of SiGe NWs on different substrates, i.e. NbTiN thin film and Si.
The electrical properties of SiGe NWs/NbTiN were characterized using temperature-dependent I-V measurements by fabricating a two-terminal architecture NbTiN/ SiGe NWs/NbTiN. Figure 3(a) shows a schematic of fabrication of NbTiN/SiGe NWs/NbTiN. After the growth of SiGe NWs on NbTiN thin film, a 50 nm thick insulating hafnium oxide (HfO 2 ) layer was conformally deposited onto the surfaces of SiGe NWs as well as the bottom NbTiN thin film by atomic layer deposition. The HfO 2 /SiGe NWs/ NbTiN was coated with polymethyl(methaacrylate) (PMMA, Microchem ® 495K A3), which covered the bottom NbTiN thin film layer and lower segment of SiGe NWs. Next, the HfO 2 layer of the exposed ∼1 μm-long top segment of HfO 2 /SiGe NWs was etched by a 10% hydrofluoric acid solution. The protective PMMA layer was then removed by immersion in acetone. On the surface of exposed SiGe NWs, a 100 nm thick NbTiN layer was deposited to form the top NbTiN layer. The Au-SiGe alloy droplets at the tips of SiGe NWs provided small contact area between SiGe NWs and the alloy droplets. The top NbTiN layer deposited onto the sidewalls of the SiGe NWs has significantly larger contact area between the NbTiN and the SiGe NWs. The NbTiN/ SiGe interface was considered as the main current path. After fabrication of NbTiN/SiGe NWs/NbTiN structure, silver paint was applied to make for contacts for electric transport measurements. Figure 3(b) shows the temperature-dependent I-V curves of NbTiN/SiGe NWs/NbTiN structure. The linear I-V characteristics observed in the range of 1.8 K and 300 K indicate that NbTiN serves as an Ohmic contact for SiGe NWs, showing similar contact behavior for Ge/Si NW heterostructure [25,26]. According to the temperature-dependent resistance (R-T) of NbTiN/SiGe NWs/NbTiN as depicted in figure 3(c), the resistance increases as temperature decreases from 300 to 5 K, which is typical for semiconducting materials. Therefore, the R-T is governed by the SiGe NWs. At 4 K, the resistance exhibits an abrupt decrease and maintains its low value down to 1.8 K. The abrupt change of the resistances originates from the transition of NbTiN from the normal metallic to the superconducting phase. It is also noted that the linear increase in the resistance from 300 to 70 K does not fit to an exponential function, which implies that the Fermi level of the top and bottom NbTiN electrodes are pinned near the edge of the valence band of the SiGe NWs. The undoped SiGe nanostructures are natively p-type due to surface states [27][28][29].
Further analysis of the R-T curve in an Arrhenius plot as shown in figure 3(d) indicates that there are different carrier transport mechanisms represented by the activation energy (E a ). The temperature-dependent resistance can be expressed by the following form: where R is the resistance, R o is the pre-exponential factor, k B is the Boltzmann constant, and E a is the activation energy. At all temperatures in the measurement, the E a extracted by fitting over a narrow temperature window was less than 5 meV, which is significantly lower than the band gap of SiGe and the thermal energy of 70 K. The small activation energy suggests that carrier hopping or quantum mechanical tunneling could be the governing transport mechanism in NbTiN/SiGe NWs/NbTiN instead of thermal activation processes of dopants or electronically active point defect, which usually requires an activation energy of >20 meV. We also note that there is no temperature range where the Arrhenius plot reveals linear behavior over an extended temperature range, indicating that there is no well-defined activation energy. Interestingly, figure 3(e) shows a noticeable feature of the R-T curve of NbTiN/SiGe NWs/NbTiN. The superconducting transition temperature (T c ), originally measured as 6.1 K in NbTiN thin film, was lowered to 4 K in the NbTiN/SiGe NWs/NbTiN. Because there was no proximity induced superconductivity in SiGe NWs surrounded by NbTiN electrodes, the transition temperature indicates that the growth of SiGe NWs contributes to the lowered T c of NbTiN layer.
There are various factors inducing the T c shift of NbTiN, such as changes in stoichiometry, lattice parameters, crystal structures, strain, and alloying [18,19,30,31]. Growth of Aucatalyzed SiGe NWs on polycrystalline NbTiN could facilitate the interdiffusion of atoms (e.g. Si, Ge, Au) since the grain boundaries serve as channels for atomic exchange. However, there was no obvious evidence of interdiffusion along the grain boundaries of NbTiN. From the cross-sectional TEM (X-TEM) images of SiGe NWs/NbTiN shown in figures 4(a), (b), the NbTiN film exhibits sharp contrast with its surface marked by the yellow dashed lines, atomic arrangements at the interface from real space (figure 4(c)) and reciprocal space ( figure 4(d)) indicate a high-quality interface. Based on the FFT displayed in figure 4(d), four diffraction were picked (red circled) and the retrieved inverse FFT (iFFT) suggest a desired matching relationship between the SiGe and NbTiN lattice planes. Moreover, alloying of Nb and Ti with Si, Ge, and Au are not observed in TEM images, which is also confirmed by energy-dispersive x-ray spectroscopy (EDX) data as will be discussed later. This is due to the fact that typical alloying temperature to form titanium silicides or germanides is higher than 500°C [32], which is not feasible for NbTiN. Excluding factors such as alloying and noticeable change of crystal structure in NbTiN, the main factor for T c shift could be correlated with the changes in lattice parameters or strain in NbTiN.
XRD scans from a large scale provide information to analyze strain and lattice parameters. Figure 5(a) shows XRD θ-2θ scans of NbTiN thin film and SiGe NWs/NbTiN. The (111) and (200) peaks of NbTiN have been detected in both NbTiN and SiGe NWs/NbTiN. Growth of SiGe NWs at 340°C did not induce any obvious increase of NbTiN diffraction peak intensities. The effect of postannealing on structural properties of NbTiN has been previously observed for growth temperatures above 600°C [30]. The absence of enhanced XRD intensity by SiGe NWs growth is consistent with no observable alloying and crystal structure change. Upon growth of NWs, we observe new peaks which can be assigned to (220) and (311) of Si 0.15 Ge 0.85 . These peak positions of NbTiN and SiGe correspond to a lattice parameter of 4.084 Å for NbTiN, and 5.55 Å for Si 0.15 Ge 0.85 , respectively. While the lattice spacing of Si 0.15 Ge 0.85 lies in the expected range, the parameter for NbTiN is smaller as compared to reported values, indicating a compressive strain along the out-of-plane direction (c-axis), which could be affected by growth temperature as well as the polycrystalline grain intersections [18,19]. From the peak intensity ratio, we could infer that the interfacial matching could be more desired for the dominated (200) NbTiN and (110) SiGe orientations, which minimizes the overall strain energy with a 3.87% d-spacing mismatch at the SiGe/NbTiN interface. Further, detailed XRD scans of NbTiN(111) and (200) peaks before and after SiGe NWs, are shown in figures 5(b), (c). As a comparison of peak shift, the NbTiN (111) peak shows more obvious shift to higher angle after NW growth, which also suggests a lattice compression. According to previous reports, T c of NbTiN becomes lower as the lattice parameter decreases [33]. Without any noticeable changes of the full width half maximums of NbTiN (111) and (200) diffraction peaks, we believe that the growth of SiGe NWs at 340°C induced no change in grain sizes of NbTiN thin film.
Finally, the elemental distribution was characterized using EDX point scan. According to the high-angle annular dark field (HAADF) scanning TEM (STEM) image shown in figure 5(d), EDX signals were collected at two different positions, i.e. on the film surface (red) where SiGe NWs show a slightly brighter contrast, and in the NbTiN film region (yellow). From the energy spectra (figures 5(e), (f)) displayed at both positions and corresponding quantified elemental concentration with respect to Si (K), Ge (K), Ti (K) and Nb (K), we could conclude that there is very limited interdiffusion between the film and SiGe NW. Specifically, signal from Position 1 is dominated by Si (16.25 wt.%) and Ge (83.68 wt.%) while tracing Ti content (<1%) can be negligible. At Position 2, most signals come from Ti (35.53 wt.%) and Nb (63.92 wt.%), while the weak peak from Si (<1.5%) can be considered as background noise or collection error. Considering the general refractory nature of NbTiN, the coupling between NbTiN thin film and SiGe maintains desired crystallinity and sharp interface without chemical interactions upon growth at elevated temperature.

Conclusion
We conclude that the coupling between Si 0.15 Ge 0.85 and NbTiN thin film enables tunable superconductivity behavior which could be further enhanced by improving the growth quality of the nitride layer. Our fabricated NbTiN/SiGe NW/ NbTiN heterostructure is a novel device architecture, that has promise for device applications such as quantum computing. Meanwhile, NbTiN is a robust superconducting material that prevents potential chemical interactions at the interface that could deteriorate the device performance. We correlated the observed downward shift of T c to the out-of-plane compressive strain affected by the NW growth at elevated temperature. We also found evidence that a desirable strained lattice matching between d (200) of NbTiN and d (110) of SiGe can be realized at some grain intersections. Further studies such as identifying the interfacial coupling at the SiGe/NbTiN interface could be interesting for controlling the superconductivity property.