ARTICLE https://doi.org/10.1038/s43246-023-00351-w OPEN Hard superconducting gap in germanium Alberto Tosato1, Vukan Levajac1, Ji-Yin Wang1, Casper J. Boor1, Francesco Borsoi 1, Marc Botifoll 2, Carla N. Borja2, Sara Martí-Sánchez2, Jordi Arbiol 2,3, Amir Sammak4, Menno Veldhorst 1 & Giordano Scappucci 1 ✉ The co-integration of spin, superconducting, and topological systems is emerging as an exciting pathway for scalable and high-fidelity quantum information technology. High- mobility planar germanium is a front-runner semiconductor for building quantum processors with spin-qubits, but progress with hybrid superconductor-semiconductor devices is hindered 1234567890():,; by the difficulty in obtaining a superconducting hard gap, that is, a gap free of subgap states. Here, we address this challenge by developing a low-disorder, oxide-free interface between high-mobility planar germanium and a germanosilicide parent superconductor. This super- conducting contact is formed by the thermally-activated solid phase reaction between a metal, platinum, and the Ge/SiGe semiconductor heterostructure. Electrical characterization reveals near-unity transparency in Josephson junctions and, importantly, a hard induced superconducting gap in quantum point contacts. Furthermore, we demonstrate phase control of a Josephson junction and study transport in a gated two-dimensional superconductor- semiconductor array towards scalable architectures. These results expand the quantum technology toolbox in germanium and provide new avenues for exploring monolithic superconductor-semiconductor quantum circuits towards scalable quantum information processing. 1 QuTech and Kavli Institute of Nanoscience, Delft University of Technology, PO Box 5046, Delft 2600 GA, The Netherlands. 2 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, Barcelona 08193 Catalonia, Spain. 3 ICREA, Pg. Lluís Companys 23, Barcelona 08020 Catalonia, Spain. 4 QuTech and Netherlands Organisation for Applied Scientific Research (TNO), Delft, The Netherlands. ✉email: g.scappucci@tudelft.nl COMMUNICATIONS MATERIALS | (2023)4:23 | https://doi.org/10.1038/s43246-023-00351-w | www.nature.com/commsmat 1 ARTICLE COMMUNICATIONS MATERIALS | https://doi.org/10.1038/s43246-023-00351-w T he intimate coupling between superconductors and semi- use an undoped and compressively-strained Ge quantum well, conductors in hybrid devices is at the heart of exciting grown by chemical vapor deposition on a Si(001) wafer35 and pursuits, including topological qubits with Majorana zero separated from the surface by a SiGe barrier (Methods). This modes1,2, superconducting (Andreev) spin qubits3, and gate- heterostructure supports a two-dimensional hole gas (2DHG) tunable superconducting qubits4. Combining hybrid devices with with high mobility (~6 × 105 cm2/Vs), long transport scattering high-fidelity semiconductor spin qubits in a single material time τ (~30 ps), and long mean free path (~7 μm) (Supplementary platform may resolve key challenges for scalable quantum Fig. 1) and hosts high-performance spin-qubits20. Crucial for the information processing. In particular, quantum information reliable search of topological superconductivity36 and for scaling transfer between spin and topological qubits5–8 may enable a to large spin-qubit architectures37, the disorder in our buried Ge universal gate set for topological quantum computation and, quantum wells is characterised by an energy level broadening ℏ/ conversely, superconductors may be used to coherently couple 2τ of ~0.01 meV, which is more than one order of magnitude spin qubits at a distance via crossed Andreev reflection5,9 or smaller than in the other material systems exhibiting a hard topologically protected links10. superconducting gap (Supplementary Table 1). The use of epitaxial superconducting Al to induce a hard As shown by the schematics in Fig. 1a, we obtain PtSiGe superconducting gap in III-V semiconductors11,12 stimulated contacts to the quantum well by room-temperature evaporation great progress with hybrid devices, leading to experimental of a Pt supply layer, metal lift-off, and rapid thermal process at reports of topological superconductivity in planar Josephson 400 ∘C (Methods). This low-temperature process preserves the junctions13 and in electrostatically defined quasi-1D wires14, the structural integrity of the quantum well grown at 500 ∘C, whilst demonstration of Andreev spin qubits3, and the realization of a activating the solid phase reaction driving Pt into the hetero- minimal Kitaev chain in coupled quantum dots15. However, spin structure and Ge and Si into the Pt (Supplementary Fig. 3). As a qubits in III-V semiconductors suffer from the hyperfine inter- result, low-resistivity germanosilicide phases are formed38,39 and actions with the nuclear spin bath16 that severely deteriorate their under these process conditions the obtained PtSiGe films are quantum coherence17 and challenges their integration with superconducting with a Tc ≈ 0.5 K and an in-plane critical field of hybrid devices. Bc∥ ≈ 400 mT (Supplementary Fig. 2). Finally, we use patterned On the other hand, spin qubits with quantum dots in Ge18–21 electrostatic gates, insulated by dielectric films in between, to can achieve long quantum coherence due to the suppressed accumulate charge carriers in the quantum well and to shape the hyperfine interaction22 and the possibility of isotopic purification electrostatic confinement potential of the hybrid superconductor- into a nuclear spin-free material23. Thanks to the light effective semiconductor devices (Methods). This approach to hybrid mass24 and high mobility exceeding one million cm2/Vs25, holes devices is different compared to the conventional process with in planar Ge/SiGe heterostructures have advanced semi- 1D nanowires, where an epitaxial superconductor proximitizes conductors spin qubits to the universal operation on a 2 × 2 qubit the semiconductor region underneath. Because we do not array26, and the shared control of a 16 semiconductor quantum perform any etch during the nanofabrication of hybrid devices, dot crossbar array27. Moreover, the ability of holes to make the low-disorder landscape that determines the 2DHG high contacts with low Schottky barrier heights to metals28, including mobility is likely to be preserved when further dimensional superconductors, makes Ge a promising candidate for hybrid confinement is achieved by means of electrostatic gates. By devices. Initial work used superconducting Al to contact Ge either contrast, for processes where etching of the superconductor is via thermal diffusion29–31 or by deposition on the sidewalls of required, the fabrication of hybrid devices yields to mobility etched mesas32,33. However, the key demonstration of a super- degradation40. conducting gap in Ge free of subgap quasiparticle states is lacking, The morphological, structural, and chemical properties of the challenged by the difficulty of contacting uniformly a buried hybrid devices are inferred by aberration corrected high-angle quantum well (QW) with a superconductor, whilst maintaining annular dark-field scanning transmission electron microscopy the low disorder at the superconductor-semiconductor interface (HAADF-STEM) and electron energy-loss spectroscopy (EELS). and in the semiconductor channel. Figure 1b shows a HAADF-STEM image of a cross-section of a Here we address these challenges and demonstrate a hard superconductor-normal-superconductor quantum point contact superconducting gap in Ge. We contact the quantum well with a (SNS-QPC) taken off-center to visualise the two gate layers superconducting germanosilicide (PtSiGe), similar to the silici- (Fig. 2a shows a top view of the device). We observe a uniform dation process used by the microelectronics industry for low quantum well of high-crystalline quality, with sharp interfaces to resistance contacts34. The superconductor is formed uniformly the adjacent SiGe and absence of extended defects. As a result of within the heterostructure and reaches the buried quantum well the annealing, Pt diffuses predominantly vertically through the via a controlled thermally-activated solid phase reaction between SiGe spacer reaching the quantum well. The sharp lateral the metal (Pt) and the semiconductor stack (Ge/SiGe). This interfaces between the two PtSiGe contacts and the QW in process is simple, robust, and does not require specialised vacuum between set the length of the channel populated by holes via the conditions or etching because the superconductor-semiconductor top-gates. The PtSiGe film presents poly-crystalline domains with interface is buried into the pure semiconducting heterostructure a crystal size up to 50 × 50 nm and orthorhombic phase (PBNM, and consequently remains pristine. This represents a conceptually space group number 62)41. This is inferred from the power different approach compared to the subtractive nanofabrication spectra or fast Fourier transforms (FFTs) taken from the two processes commonly used for hybrid devices, since our additive PtSiGe domains interfacing with the QW from the left contact, process does not deteriorate the active area of the semiconductor. shown in Fig. 1c along with a schematic view of the unit cell of As a result, we demonstrate a suite of reproducible Ge hybrid such phase. More detailed studies by atomic-resolution plane devices with low disorder and excellent superconducting HAADF-STEM are required to assess the junction uniformity in properties. the direction parallel to the junction and whether this would impact Majorana experiments. The analysis of EELS elemental concentration profiles across the Ge QW → PtSiGe heterointer- Results face (Supplementary Fig. 4) reveals that the threefold PtSiGe Material properties. Our approach to superconductor- stoichiometry is Ge-rich, with relative composition in the range semiconductor hybrid devices in Ge is illustrated in Fig. 1a. We between Pt0.1Si0.2Ge0.7 and Pt0.1Si0.05Ge0.85 depending locally on 2 COMMUNICATIONS MATERIALS | (2023)4:23 | https://doi.org/10.1038/s43246-023-00351-w | www.nature.com/commsmat COMMUNICATIONS MATERIALS | https://doi.org/10.1038/s43246-023-00351-w ARTICLE Fig. 1 Material properties of superconductor-semiconductor Ge devices. a Schematics of the fabrication process for a superconductor-normal- superconductor quantum point contact (SNS-QPC). First, platinum is deposited on the heterostructure, then thermal annealing at 400 ∘C drives Pt in the heterostructure to form PtSiGe, finally two gate layers are deposited, insulated by Al2O3. b False-color high angle annular dark field scanning transmission electron microscopy (HAADF STEM) image of a cross-section of a SNS-QPC. The PtSiGe contacts are violet, the Ti/Pd constriction gate (CG) operated in depletion mode is yellow, the Ti/Pd accumulation gate (AG), used to populate the quantum well, is green. A scanning electron microscopy top view image of this device is shown in Fig. 2. c Atomic resolution HAADF STEM image of the Ge/PtSiGe interface along with the indexed fast Fourier transforms (FFTs) of the two regions (black squares) within the PtSiGe contacts and a schematics of the PtSiGe orthorhombic unit cell. The corresponding ternary lattice parameters T = aT, bT, cT that define the dimensions of the unit cell can be calculated, in a first approximation, by Vegard’s law: T PtSi1x Gex ¼ x BPtGe þ ð1  xÞ BPtSi where B = aB, bB, cB are the lattice parameters of the binary compounds PtSi and PtGe, and x is the relative content of Ge with respect to Si. d Electron energy-loss spectroscopy (EELS) composition maps showing the Pt, Ge, Si and O signals for the central area of the TEM lamella of panel (b), the scale-bar indicates 50 nm. The PtGeSi stoichiometry is extracted by quantitative EELS analysis and reported in Supplementary Fig. 4. Fig. 2 Highly-transparent Josephson junctions. a False-color scanning electron microscope image of the SNS device. The PtSiGe contacts are violet, the constriction gates (CG) are yellow and the accumulation gate (AG) is green. The channel length between the two superconducting leads is 70 nm and the channel width between the constriction gates is 40 nm. The two constriction gates are separate by design but always shorted together during measurements. b Color map of the voltage drop across the junction V vs source-drain current ISD and constriction-gate voltage VCG at zero magnetic field along with normal-state conductance GN trace vs VCG. GN is calculated as the conductance average where the voltage drop across the device is in the range [500, 650] mV or -[650, 500] mV, that is much higher than the estimated superconducting gap. c Color map of G in units of 2e2/h vs the source-drain voltage VSD and VCG. Bottom panel shows line-cuts of conductance at VCG = [−1.25,−1.4,−1.49]V, red lines are the fit with the coherent scattering model from which transparency τ is extracted. Right inset shows the evolution of the transparency, as extracted from the fitting of conductance curves to the coherent scattering model (Methods), with the constriction gate VCG. d Color map of G vs T and VSD (top panel), and vs B∥ and VSD (bottom panel), where B∥ is the in-plane magnetic field in the direction of transport and T the temperature. The color scale in panel (d) has been saturated to better infer the low conductance limit. The source-drain bias is applied between the PtSiGe contacts, and the voltage drop across the junction is measured with a standard 4-terminal setup. The accumulation voltage for measurements in (b, c and d) was set to −4.5V, where the 2DHG is expected to reach saturation density (see Supplementary Fig. 1) of ≃6 × 1011 cm−2 35. Measurement presented in (b, c) and in panel (d (bottom)), are performed at 15 mK, corresponding to an electron temperature of ~25 mK. the analysed grain. The EELS compositional maps in Fig. 1d show high-purity quantum well and a pristine superconductor- the elemental distribution of Ge, Si, Pt, Al, and O, at the key semiconductor interface. regions of the device. We observe Pt well confined to the two contacts areas, which also appear Ge-rich. Crucially, O is detected Highly transparent Josephson junction. We perform low- only in the Al2O3 dielectric layer below the gates, pointing to a frequency four-terminal current and voltage bias measurements COMMUNICATIONS MATERIALS | (2023)4:23 | https://doi.org/10.1038/s43246-023-00351-w | www.nature.com/commsmat 3 ARTICLE COMMUNICATIONS MATERIALS | https://doi.org/10.1038/s43246-023-00351-w (Methods) on the SNS-QPC device shown in Fig. 2a to infer the direction Bc⊥ = 0.1 T (Supplementary Fig. 6). This in-plane vs out- properties of the superconductor-semiconductor interface. of-plane anisotropy is expected given the thin-film nature of the Accumulation (AG, in green) and constriction (CG, in yellow) PtSiGe superconductor45. gates control transport within the 70 nm long channel between the two PtSiGe leads. We apply a large negative voltage to the Hard induced superconducting gap. To gain insights into the accumulation gate to populate the quantum well with holes, and quality of the Ge/PtSiGe junction we characterise transport we then control the effective width of the channel by applying a through the normal-superconductor quantum point contact (NS- more positive voltage to the constriction gates, thus depleting the QPC) device shown in Fig. 3a. Importantly, the methodology underlying quantum well. based on spectroscopy of NS devices alleviates the ambiguity of The current bias measurements (Fig. 2b) reveal a tunable measuring the amount of quasiparticle states inside the gap with supercurrent with a plateau when the constriction gate voltage SNS junctions31. On the left side of the QPC there is a PtSiGe VCG is in the range ≈[−1.75, −1.50] V. This is the same range superconducting lead and on the right side a normal lead con- where we observe the first conductance plateau in the normal- sisting of a 2DHG accumulated in the Ge QW. With the accu- state conductance GN (Fig. 2b, right inset), indicating that the mulation gate (AG) set at large negative voltages to populate the switching current (Isw) plateau observed in the color plot stems QW we apply a more positive voltage to the constriction gates form the supercurrent discretization due to the discrete number (CG), creating a tunable barrier between the superconducting and of modes in the QPC30,42. Supercurrent discretization up to the the normal region. In Fig. 3b we progressively decrease the barrier third conductance plateau is shown in Supplementary Fig. 5 (data height (decreasing VCG) going from the tunneling regime, where are for a different SNS-QPC device with identical design to the conductance is strongly suppressed, to a more open regime where one presented here). The discretization of the supercurrent at conductance approaches the single conductance quantum G0. zero magnetic field, indicates that the quality of the 2DHG is Line-cuts of the conductance color map are presented in the preserved also upon the formation of the superconducting bottom panel of Fig. 3c. In the tunneling regime, we observe a contacts. We use the switching current as a lower bound for hard induced superconducting gap, characterised by a two orders the critical current and we estimate an IswRN product of 51 μV, of magnitude suppression of the in-gap conductance to the showing an improvement as compared to previous results normal-state conductance, and the arising of coherence peaks at obtained with pure Al contacts in Ge QWs30,32,33, despite the e∣VSD∣ ≈ Δ* = 70 μeV. Figure 3b also shows that the induced Al Tc is higher than the PtSiGe Tc. The measured IswRN product is superconducting gap varies with the constriction gate voltage. ~0.5 times the theoretical IcRN product calculated for a ballistic This observation brings confidence that we are measuring the short junction using the Ambegaokar-Baratoff formula πΔ*/ induced superconducting gap rather than the parent gap52. A 2e = 110 μV with Ic being the critical current, Δ* the induced possible explanation is that, upon increasing the density in the superconducting gap and e the electron charge43. This discre- semiconductor nearby the junction, the coupling to the parent pancy has been observed in previous works30,44 and is consistent superconductor might vary, as also observed in other hybrid with a premature switching due to thermal activation45. nanostructures53. By operating the device in voltage-bias configuration and The evolution of the gap as a function of in-plane magnetic stepping the constriction gates, we observe in the conductance field (B∥) shown in Fig. 3c confirms that the gap remains hard for color plot the typical signature of multiple Andreev reflections finite magnetic fields up to 0.25 T, ultimately vanishing at (MARs) (Fig. 2c). When the applied voltage bias corresponds to B∥ ≈ 0.37 T. The magnetic field evolution of the gap in all three an integer fraction of 2Δ*, with Δ* being the induced super- directions matches the behaviour observed in the SNS-QPC conducting gap, we observe differential conductance dI/dV peaks (Supplementary Fig. 7). (dips) in the tunneling (open) regime46,47. We measure MARs up Finally, Fig. 3d reports the conductance traces in tunneling to the 5th order, suggesting that the coherence length ξN in the Ge regime for all the six measured devices (an overview of the QW is a few times larger than the junction length L, and setting a geometries of these devices and the respective measurements are lower bound to the phase coherence length in the QW available in the Supplementary Fig. 8, the conductance maps for lψ > 5L = 350 nm. These observations are consistent with the all these devices are shown in Supplementary Fig. 9). For all findings of ref. 33 where a similar Ge/SiGe heterostructure is used. devices we observe suppression of conductance equal or larger Fitting the differential conductance with the coherent scattering than two orders of magnitude. At a quantitative level, the model described in ref. 48 (and used in refs. 44,47,49) reveals single conductance traces of Fig. 3d are well fitted by the BTK theory54 channel transport with gate tunable transparency up to 96%. Such (Supplementary Fig. 9) consistent with a hard induced super- a high transparency confirms the high quality interface between conducting gap free of subgap states11,44. This finding is the the PtSiGe and the Ge QW. From the MARs fit we estimate an signature of a robust process that yields a reproducible high- induced superconducting gap Δ* = 70.6 ± 0.9 μeV, which is about quality superconductor-semiconductor interface, overcoming a half compared to the Δ* = 129 μeV14 and 150 μeV50 for recent long-standing challenge for hybrid superconductor- InAs-Al devices reporting topological superconductivity. semiconductor quantum devices in Ge. Further, we characterise the evolution of the induced super- conducting gap with temperature and magnetic field. After setting the device in tunneling regime, where sharp coherence peaks are Superconducting quantum interference devices. We use the expected at e∣VSD∣ = 2Δ* (Fig. 2d), we observe the induced superconducting quantum interference device (SQUID) in Fig. 4a superconducting gap closing with increasing temperature and to demonstrate phase control across a Josephson junction, an magnetic field. By fitting the temperature dependence of the important ingredient for achieving topological states at low coherence peaks with the empirical formula from ref. 51 we obtain magnetic field50,55–57. The device is composed of two Josephson a critical temperature of 0.5 K. The peak close to zero bias field-effect transistors (JoFETs) with a width of 2 μm and 1 μm emerging at T > 0.2 K can be explained in terms of thermally- for JoFET1 and JoFET2 respectively, and equal length of 70 nm. activated quasiparticle current49. The in-plane magnetic field in The critical current of the junctions Ic1 and Ic2 can be tuned the transport direction quenches the superconductivity at Bc∥ = independently by applying the accumulation gate voltages VAG1 0.37 T. The same critical field is found for the in-plane direction and VAG2 to the corresponding gates. We investigate the oscil- perpendicular to the transport direction while for the out of plane lations of the SQUID switching current as a function of the 4 COMMUNICATIONS MATERIALS | (2023)4:23 | https://doi.org/10.1038/s43246-023-00351-w | www.nature.com/commsmat COMMUNICATIONS MATERIALS | https://doi.org/10.1038/s43246-023-00351-w ARTICLE Fig. 3 Hard induced superconducting gap. a False-color SEM image of the normal-superconductor quantum point contact device (NS-QPC). The PtSiGe contact is violet, the constriction gate (CG) are yellow and the accumulation gate (AG) is green. The two constriction gates are separate by design but always shorted together during measurements. b Color map of conductance G vs the source-drain voltage VSD and constriction gate VCG, along with line cuts in log-scale of G at the constriction gate voltages VCG = [−733, −710, −695] mV marked by the colored segment in the color-plot. c Color map of G in units of 2e2/h vs the in-plane magnetic field B∥ perpendicular to the transport direction and constriction gate VCG, along with line cuts in log-scale of G at the field strength B∥ = [0.01, 0.1, 0.2, 0.3] T marked by the colored segment in the color-plot. d Conductance traces normalised to the above-gap conductance (G/GN) vs VSD in tunneling regime for 6 different NS-QPC devices D1−D6 processed in the same fabrication run, device D1 is the one reported in Fig. 3 a–c, in the remaining devices the constriction gates separation varies (specifications of these devices are provided in Supplementary Fig. 8). (CPR) of the second junction. This is demonstrated in Fig. 4b where the shown SQUID oscillations are well fitted by the relation: I c;SQUID ¼ I c1 ðB? A1 Þ þ I c2 ðB? A2 Þ sinð2πB? ASQUID  LI c1 ðB? A1 Þ=Φ0 Þ where Ic1,2(BA1,2) are the Fraunhofer depen- dencies of the critical current obtained from fitting the Fraun- hofer pattern of each junction (Supplementary Fig. 10), A1,2 are the junction areas, B⊥ is the out-of-plane magnetic field and Φ0 the flux quantum. From the fit of the data in Fig. 4b (red dashed- line) we extract the effective SQUID loop area ASQUID = 8.9 μm2 (comparable to the 10 μm2 SQUID geometric area) and the self-inductance L = 1.65 pH. In order to confirm for the self- inductance effects, we also fit SQUID oscillations for the opposite direction of the current bias (blue dashed-line) and we get similar values for the effective loop area and self-inductance. Fig. 4 Phase control of a Josephson junction in a SQUID. a False-color SEM image of the two JoFET SQUID device. The JoFETs have a channel length of 70 nm and a channel width of 1 μm and 2 μm respectively and can Scalable junctions. As a first step towards monolithic be independently controlled by gates AG1 and AG2. The geometric loop superconductor-semiconductor quantum circuits in two dimen- area of the SQUID is of 10 μm2, calculated assuming a rectangle with sides sions, we fabricate and study transport in a macroscopic hybrid positioned in the center of the PtSiGe loop cross-section. b Color-plot of device comprising a large array of 510 PtSiGe islands (Fig. 5a) voltage drop (V) across the SQUID vs current (I) and out-of-plane magnetic and a global top gate. Each pair of neighbouring islands forms a field (B⊥). Arrows represent the direction of the current (I) sweep. With the Josephson junction whose transparency can be tuned by the gate voltages set at VAG1 = − 3.5 V and VAG2 = − 1.65 V the global accumulation gate. The top panel of Fig. 5b shows a cur- superconducting phase drops mainly over the second junction. Upon rent bias measurement of the junctions array resistance. As the sweeping the out-of-plane magnetic field B⊥ we observe oscillations of the accumulation gate becomes more negative, all the junctions are switching current. Red and blue dashed lines are the fit of the evolution of proximitized and a supercurrent flows through the device. the critical current with magnetic field. The magnetic field is applied in the Remarkably, as the source-drain current approaches the junctions out of plane direction as depicted in panel (a). critical current the whole array simultaneously switches from superconducting to resistive regime, as shown from the sharp out-of-plane-magnetic field penetrating the SQUID loop. resistance step (Fig. 5b top). Namely, we set VAG1 and VAG2, such that both arms support With this device we also study the evolution of the switching supercurrent and Ic1 ≫ Ic2. This condition provides that the first current in a small perpendicular magnetic field. In the bottom junction is used as a reference junction and that the phase drop panel of Fig. 5b we observe Fraunhofer-like interference, along on it is flux independent, while the phase drop over the second with the fingerprint of flux commensurability effects associated junction is therefore modulated by the external flux through the with the periodicity of the array. At integer numbers of flux loop. This allows the measurement of the current-phase-relation quantum per unit area of the periodic array f = B⊥/B0, where COMMUNICATIONS MATERIALS | (2023)4:23 | https://doi.org/10.1038/s43246-023-00351-w | www.nature.com/commsmat 5 ARTICLE COMMUNICATIONS MATERIALS | https://doi.org/10.1038/s43246-023-00351-w Fig. 5 A gated 2D superconductor-semiconductor array. a 3D and top view schematics of an array of 51 × 10 PtSiGe islands. The inset shows an atomic force microscopy image of the PtSiGe islands of the array. The PtSiGe islands are 930 × 930 nm wide and the separation between neighbouring islands is of 70 nm. b Top panel shows a color map of sheet resistance (RS) vs accumulation gate voltage VG and source-drain current ISD. Bottom panels shows a color map of sheet resistance vs out of plane magnetic field B and source-drain current ISD. The measurement is taken at gate voltage VG = − 1.99 V, where we expect carriers in the quantum well to approach a saturation density value of about 6 × 1011 cm−2 and have a mean free path (~7 μm) much longer than the separation between between neighbouring islands. Black arrows denote the magnetic field corresponding to one flux quantum Φ0 per unit cell of the array. Red arrows correspond to one-half flux per unit cell. c Sheet resistance as a function of temperature for gate voltages ranging from −2 V to −1.55 V. Yellow curves correspond to small negative gates, and purple curves to large negative gates. B0 = Φ0/A with A the junction area and Φ0 the flux quanta, we hybrid superconductor-semiconductor devices30–33. We were observe switching current peaks at ± 1f, 2f, 3f, 4f and 5f, denoted able to observe a hard gap with 100% yield across all the six by a black arrow in the plot. We also notice this effect at fractional measured devices, pointing to a robust and reproducible fabri- values of f, most notably at f/2 (red arrow). Flux commensur- cation process. Next to this central result, we further demonstrate ability effects, due to the pinning and interference of vortices in phase control across a Josephson junction and take advantage of Josephson junctions arrays, have been previously reported58,59. the planar geometry to scale these devices in 2D arrays. The observation of simultaneous switching of super-current and While we focused on the poly-crystalline superconducting of the Fraunhofer pattern with flux commensurability effects, PtSiGe compound, we anticipate two strategies to further increase suggests that all islands effective areas are similar and that the the size of the induced superconducting gap, which sets a relevant supercurrent through the various junctions is comparable, meaning energy scale for hybrid devices. Firstly, following the approach in that all junctions respond synchronously to the applied gate voltage. ref. 33 a superconducting layer with a larger gap, such as Al or Nb, This is further supported by the observation of sharp switching of may be deposited on top of the superconducting PtSiGe. Sec- super-current and the Fraunhofer pattern of a 1D array of ondly, other ternary superconducting germanosilicides with a superconducting islands presented in Supplementary Fig. 11. higher critical temperature may be explored, starting from the Finally we present in Fig. 5c the sheet resistance as a function deposition and thermal anneal of other platinoid metals such as of temperature for different gate voltages. As the gate voltage Rh and Ir60. becomes more negative, the coupling between neighbouring Based on our findings, we foresee the following use cases for superconducting islands increases and the system transitions superconductor-semiconductor hybrids in high mobility planar from an insulating to a superconducting regime. At low gate Ge. Although a hard gap is necessary but not sufficient on its own voltage the resistance increases with decreasing temperature for achieving a topologically protected system, this work positions (yellow curves) indicating the insulating state, while at high gates planar Ge as a promising platform to explore Majorana bound the resistance drops to zero (purple curves) owing to the global states in phased-biased Josephson junctions13,61,62. Calculations superconducting state. At intermediate gate voltages (−1.95V ≤ with experimentally realistic material parameters57 show that VG ≤ −1.93V, orange curves) there is a transition where the accessing the topological phase is feasible by careful design of Ge resistance shows a weak temperature dependence. It will be planar Josephson junctions geometries that relaxes magnetic field interesting to study this regime in detail, in light of the recent and spin-orbit constrains. More advanced future experiments claims of an anomalous metallic state between the super- should build on our current results to fully assess the readiness of conducting and the insulating phases59. Ge for Majorana bound states experiments, such as increasing the induced superconducting gap and measuring it by non local Discussion spectroscopy in multi-terminal devices and demonstrate the two- In conclusion, we have developed superconducting germanosili- electron charging effect in hybrid Ge/PtSiGe islands, a pre- cides for contacting Ge quantum wells, which has resulted in requisite for their use in topological quantum computation. excellent superconducting properties imparted to the high- Crucially, the realization of a hard superconducting gap posi- mobility 2DHG. We induced a hard superconducting gap in tions planar Ge as a unique material platform to pursue the Ge, a large advancement compared to previous work on Ge coherent coupling of high fidelity spin qubits using crossed 6 COMMUNICATIONS MATERIALS | (2023)4:23 | https://doi.org/10.1038/s43246-023-00351-w | www.nature.com/commsmat COMMUNICATIONS MATERIALS | https://doi.org/10.1038/s43246-023-00351-w ARTICLE Andreev reflection to enable two-qubit gates over micrometer for the fitting parameters Δ, Ni, τi with i ∈ 1, . . . , M. The fitting is performed for distances5,9. Remote coupling of spin qubits in Ge may also be increasing M, provided that all Ni and τi are nonzero. We note that we assume a coherent 1D system. When the MAR contribution is significant, this assumption achieved by coupling spin qubits via superconducting quantum leads to an overestimation of the sharpness and amplitude of the peaks. dots5,6, potentially offering a topological protection10. Coupling Nonetheless, overall we find a good agreement between the data and the model. on an even longer distance may be obtained via superconducting resonators63. In such a scenario, a capacitive interaction may suffice, but connecting the resonator to a superconducting ohmic, Data availability such as PtSiGe, could result in a larger lever arm and therefore All data included in this work are openly available at 4TU research data repository boost the coupling, while a direct tunnel coupling would give https://doi.org/10.4121/19940174. All other relevant data are available from the further directions to explore. The ability to couple qubits over corresponding author upon reasonable request. different length scales is highly relevant and a critical component in network-based quantum computing37. Received: 17 January 2023; Accepted: 24 March 2023; Furthermore, the demonstration of a hard gap in Ge motivates the investigation of alternative spin qubits systems, such as Andreev spin qubits (ASQ)64,65, that may be coupled with gatemons66 or superconductors67. Similar to semiconductor spin qubits, the use of isotopically purified Ge23 may overcome the References strong decoherence from the nuclear environment currently 1. Lutchyn, R. M. et al. Majorana zero modes in superconductor-semiconductor limiting progress with ASQs in III-V materials3,66. heterostructures. Nat. Rev. Mater. 3, 52–68 (2018). All together, these findings represent a major step in the Ge 2. Flensberg, K., von Oppen, F. & Stern, A. Engineered platforms for topological superconductivity and Majorana zero modes. Nat. Rev. Mater. 6, 944–958 quantum information route, aiming to co-integrate spin, super- (2021). conducting, and topological systems for scalable and high-fidelity 3. Hays, M. et al. 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A.S. grew the Ge/SiGe heterostructures. A.T. fabricated the devices. C.N.B., M.B, S.M., 47. Kjaergaard, M. et al. Transparent semiconductor-superconductor interface J.A. performed transmission electron microscopy characterisation. A.T. with C.B. mea- and induced gap in an epitaxial heterostructure Josephson junction. Phys. Rev. sured Josephson junction devices, SNS-QPCs, and 2D arrays in wet dilution refrigerators, Appl. 7, 034029 (2017). analysed the data and performed numerical simulation of the MARs processes. V. L. and 48. Averin, D. & Bardas, A. ac Josephson effect in a single quantum channel. Phys. A.T. measured SNS-QPC, NS-QPC devices and SQUIDs in dry dilution refrigerators and Rev. Letters 75, 1831–1834 (1995). analyzed the data with the supervision of J.W. and input from F.B. and M.V. A.T wrote 49. Borsoi, F. et al. Single shot fabrication of semiconducting-superconducting the manuscript with input from all authors. G.S. conceived and supervised the project. nanowire devices. Adv. Funct. Mater. 31, 2102388 (2021). 50. Fornieri, A. et al. Evidence of topological superconductivity in planar Josephson junctions. Nature 569, 89–92 (2019). Competing interests 51. Nilsson, H. A., Samuelsson, P., Caroff, P. & Xu, H. Q. Supercurrent and The authors declare no competing interests. multiple andreev reflections in an InSb nanowire Josephson junction. Nano Lett 12, 233 (2012). 52. van Loo, N. et al. Electrostatic control of the proximity effect in the bulk of Additional information Supplementary information The online version contains supplementary material semiconductor-superconductor hybrids. Preprint at https://doi.org/10.48550/ available at https://doi.org/10.1038/s43246-023-00351-w. arXiv.2211.06709 (2022). 53. De Moor, M. W. A. et al. Electric field tunable superconductor-semiconductor Correspondence and requests for materials should be addressed to Giordano Scappucci. coupling in Majorana nanowires. New J. Phys. 20, 103049 (2018). 54. Blonder, G. E., Tinkham, M. & Klapwijk, T. M. Transition from metallic to Peer review information Communications Materials thanks the anonymous reviewers tunneling regimes in superconducting microconstrictions: excess current, for their contribution to the peer review of this work. Primary Handling Editor: Aldo charge imbalance, and supercurrent conversion. Phys. Rev. B 25, 4515–4532 Isidori. Peer reviewer reports are available. (1982). 55. Pientka, F. et al. Topological superconductivity in a planar Josephson junction. Reprints and permission information is available at http://www.nature.com/reprints Phys. Rev. X 7, 1–17 (2017). 56. Ren, H. et al. Topological superconductivity in a phase-controlled Josephson Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in junction. 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