Abstract
Coupling a twodimensional (2D) semiconductor heterostructure to a superconductor opens new research and technology opportunities, including fundamental problems in mesoscopic superconductivity, scalable superconducting electronics, and new topological states of matter. One route towards topological matter is by coupling a 2D electron gas with strong spin–orbit interaction to an swave superconductor. Previous efforts along these lines have been adversely affected by interface disorder and unstable gating. Here we show measurements on a gateable InGaAs/InAs 2DEG with patterned epitaxial Al, yielding devices with atomically pristine interfaces between semiconductor and superconductor. Using surface gates to form a quantum point contact (QPC), we find a hard superconducting gap in the tunnelling regime. When the QPC is in the open regime, we observe a first conductance plateau at 4e^{2}/h, consistent with theory. The hardgap semiconductor–superconductor system demonstrated here is amenable to topdown processing and provides a new avenue towards lowdissipation electronics and topological quantum systems.
Introduction
Recent work on semiconductor nanowires has offered evidence for the existence of Majorana zero modes, a signature of topological superconductivity^{1,2,3}. A characteristic of the first studies in this area was significant subgap tunnelling conductance (a socalled soft gap), attributed to disorder at the semiconductor–superconductor (Sm–S) interface^{4,5}. In nanowires, the softgap problem was recently resolved by growing Al epitaxially on InAs nanowires, yielding greatly reduced subgap conductance^{6,7}. Studies of Sm–S systems based on topdown processed gateable twodimensional electron gases (2DEGs) coupled to superconductors have not explicitly addressed the softgap issue yet^{8,9}. However experiments on such systems have demonstrated other theoretical predictions, such as quantization of critical current^{9,10,11}, the retroreflection property of Andreev scattering^{12}, and spectroscopy of a gatedefined quantum dot with superconducting leads^{13,14}, which do not require a hard proximityinduced gap in the semiconductor.
The two main results we present in this paper are both consequences of the transparent epitaxial Sm–S interface and overcome the soft gap problem for 2D electron gases. The first is a doubling of the the lowest quantized conductance plateau, from 2e^{2}/h in the normal state to 4e^{2}/h in the superconducting state, as predicted theoretically^{15}. The second is a strong suppression of conductance for voltages smaller than the superconducting gap when the quantum point contact (QPC) is in the tunnelling regime—that is, the detection of a hard superconducting gap in a proximitized 2DEG. Conductance doubling arises from Andreev reflection transferring charge 2e into the superconductor^{16}. The hard gap reflects the absence of electronic states below the superconducting gap in the semiconductor. Using gate voltage to control the QPC, we measure conductance across the transition from weak tunnelling to the openchannel regime and find good (but not perfect) agreement with the theory of a normalQPCsuperconductor structure^{15}.
Results
Properties of the 2DEG and the superconducting Al film
The starting material is an undoped InAs/InGaAs heterostructure with epitaxial Al as a top layer, grown by molecular beam epitaxy^{17}. A crosssectional TEM showing a sharp epitaxial Sm–S interface is shown in Fig. 1a. In the devices reported here, the thickness of the InGaAs barrier was b=10 nm, and the Al film thickness was a=10 nm. A Hall ball fabricated on the same wafer with the Al removed (see Methods) gave density n=3 × 10^{12} cm^{−2} and mobility μ=10^{4} cm^{2} V^{−1} s^{−1}, yielding a mean free path l_{e}∼230 nm. In a similar wafer, weak antilocalization analysis gave a spin–orbit length l_{so}∼45 nm (ref. 17). The Al film has a critical temperature T_{c}=1.56 K, corresponding to a gap Δ_{0}=235 μeV, enhanced from the bulk value of Al, and consistent with other measurements on Al films of similar thickness^{18}. The inplane critical field of the Al film is B_{c}=1.65 T (ref. 17).
Quantized conductance doubling
A scanning electron micrograph of Device 1 is shown in Fig. 1b. The conductance of the QPC is tuned by negative voltages applied to the gates. The QPC is located ∼150 nm in front of the region where the Al film has not been removed. Figure 2 shows conductance traces for two lithographically similar QPCs. In the superconducting state, both devices show increased conductance at the plateau of the QPC and suppressed conductance below G∼0.8G_{0}, where G_{0}≡2e^{2}/h, relative to the normal state. This behaviour is the hallmark of Andreev reflection being the dominant conduction mechanism through the QPC^{15,19}. Raising the temperature above the critical temperature of the Al film, applying an outofplane magnetic field, or applying a bias larger than the gap, all bring the lowest plateau back to 2e^{2}/h (Fig. 2). The dip structure at the transition between conductance plateaus was also observed in a similar experiment on nanowires^{20}, and is presumably caused by mode mixing due to disorder, leading to a reduction in transparency of the already open first channel. A constant contact resistance R_{c}∼1 kΩ has been subtracted in each viewgraph, a value chosen to move the first plateau in the normal state to G_{0}.
Hard superconducting gap
By further depleting the electron gas in the constriction, the device is operated as a tunnel probe of the local density of states in the InAs 2DEG. This technique has been applied to studying subgap properties of semiconductor nanowires coupled to superconductors^{1,2,3,6,21,22}. In Fig. 3a, the QPC voltage is decreased to gradually transition from the onechannel regime, where the zero bias conductance is 4e^{2}/h, to the tunnelling regime, where conductance is strongly suppressed for V_{sd}<190 μV. From these measurements, the gap in the density of states of the InAs due to the proximity to the Al is estimated to be Δ*∼190 μeV (measured peaktopeak). The value of Δ* is similar, but not identical, to the gap in the Al film as estimated from T_{c}, as discussed above.
In the case of perfect Andreev reflection from the superconductor/semiconductor interface, the conductance of one channel through a constriction proximal to the interface is given by
where G_{ns} is the conductance when the film is superconducting, and G_{nn} is the conductance in the normal state^{15}. In Fig. 3c, the prediction in equation (1) with no free parameters (green line) and experimental data are shown. Here, G_{nn} is the average conductance for V_{sd}>0.8 mV, justified by the equality of applying a bias and raising the temperature above T_{c}, as shown in Fig. 2a. Equation (1) is consistent with the data over two orders of magnitude in G_{ns}, indicating that the zero bias conductance up to 4e^{2}/h is well described by the prediction of perfect Andreev reflection of a single QPC mode. Equation (1) represents the only quantitative theory of the relation between subgap conductance and normal state conductance (that is, the hard gap) of which we are aware, and the agreement between equation (1) and the experiment in Fig. 3c leads to the designation of a hard gap in this superconductor–2DEG system. However, the systematic deviation between data and prediction in Fig. 3c for G_{ns}<10^{−2} × 2e^{2}/h could be a manifestation of a small remnant nonzero normal scattering probability.
The shapes of the conductance curves at eV_{sd}≲Δ* in the tunnelling regime (red line in Fig. 3b) are smeared relative to the conventional Bardeen–Cooper–Schrieffer (BCS) density of states of a superconductor. This could be due to broadening of the BCS coherence peaks in the disordered superconducting film formed in the 2DEG under the Al^{23}, a weak coupling between Al and 2DEG^{5} or the layout of the tunnel probe relative to the proximitized 2DEG^{24,25,26}.
Temperature dependence of the density of states
The temperature dependence of the conductance in the Andreev QPC is different in the onechannel and in the tunnel regime (Fig. 4). The onechannel regime (Fig. 4a,b) has a pronounced kink at T=T_{c}, presumably associated with the sudden onset of Andreev enhanced subgap conductance. In contrast, the temperature dependence in the tunnel regime (Fig. 4c,d) is smeared close to T_{c} due to thermally excited quasiparticles.
The temperature dependence is simulated (insets in Fig. 4) by calculating where f is the Fermi function that accounts for thermal broadening. The conductance is calculated by combining scattering matrices of electrons and holes in the normal region and Andreev reflection at the superconductor interface (details of the simulation are given in Methods). The scattering matrices are calculated using the numerical package Kwant^{27}, and the simulation are performed using the device geometry from the micrograph in Fig. 1b. The temperature dependence of the gap is modeled with , and the Andreev reflection amplitude is taken from ref. 15. The simulation shows good quantitative agreement with the data.
Magnetic field dependence of the density of states
To drive a superconductor/semiconductor device into a topological regime, one requirement is gμ_{B}B>Δ*, while the native superconductor retains its gap. Figure 5 shows the inplane magnetic field dependence of Δ*, from which an approximate critical field mT is extracted. A rough estimate of the gfactor can be inferred by assuming the critical results from Zeeman energy surpassing the induced superconducting gap, that is , which yields g∼10, similar to the gfactor in bulk InAs. In Fig. 5d, the zerobias conductance is shown for the two different inplane directions, and the slight direction dependence of could be due to an anisotropic gfactor in the InAs crystal lattice. The induced gap in the 2DEG disappears at inplane magnetic fields significantly smaller than the critical field of the Al film itself. The 2DEG has a strong spin–orbit interaction (l_{so}∼45 nm), which, taken together with the intimate coupling to the superconductor, makes this material system a feasible candidate to realize topological superconducting devices. By using topdown fabrication techniques and the electrostatic gating demonstrated here, effective onedimensional systems can be produced, in which an inplane magnetic field can close the induced superconducting gap to reach a topological phase.
In conclusion, we observe quantization doubling through a QPC proximal to a superconductor/semiconductor interface, confirming a longstanding theoretical prediction^{15}. Operated as a gatetunable tunnel probe of the local density of states, the QPC shows a hard superconducting gap induced in the 2DEG. The magnetic field dependence of the induced gap compares favourably with the critical field of the superconducting film, opening possibilities to pursue topological states of matter in onedimensional structures fabricated from epitaxial Al/2D InAs material.
Methods
Fabrication and measurement setup
Ohmic contacts to the InAs electron gas are formed directly by the epitaxial Al. Mesa structures are patterned by standard III–V chemical etching techniques. The aluminium is etched using commercial Transene Aluminum Etch D. Subsequent to the selective Al etch, an insulating 40 nm Al_{2}O_{3} layer is deposited using atomic layer deposition and metallic gates (5 nm Ti/50 nm Au) are evaporated onto the device. The measurements were performed in a dilution refrigerator with a basemixing chamber temperature T_{mc}∼30 mK, using fourterminal lockin techniques and DC measurements.
Measurement details
The data in Fig. 3 is measured in a DC setup, incrementing the voltage in steps of size 3 μV. The data are smoothed over 10 steps and the derivative is calculated numerically to obtain the differential conductance. A constant contact resistance R_{c}=800 Ω is subtracted from the data, moving the conductance at V_{g}=−8.2 V for V_{sd}>0.8 mV to 2e^{2}/h. The fourterminal resistance of the device is R_{d}=400 Ω with V_{g}=0 V. The difference between R_{c} and R_{d} is most likely dominated by the change of resistivity near the gated region, when the gate is turned on, as well as the distance from the voltage probe to the QPC region. The voltage probes are located ∼15 μm away from the QPC and the gates overlap the mesa over an area ∼1.6 μm^{2}. The normal state conductance is calculated as the average of G(V_{sd}) for V_{sd} in the range (±0.8 mV, ±1 mV). The analysis is largely unaffected by changing the averaging window for values V_{sd}>0.6 mV. The cuts in Fig. 3b are taken by averaging over a 12 mV (30 mV) window in V_{g} for the onechannel (tunnelling) regime. Finally, each datapoint in Fig. 3c is calculated as the average over a 10 mV range in V_{g}.
Model for numerical simulations
We calculate the conductance of the junction in two steps. First, we determine the scattering properties of the normal region which we assume is a 1.1 μm wide channel of length L, where we have taken dimensions from SEM in Fig. 1b. It is described by the spinless Hamiltonian,
We model the QPC as two rectangular gates located at X=400 nm, with the width 2W, separated by the length 2S and located at the distance d above the 2DEG (see Supplementary Fig. 3 for illustration of W and S). We calculate the potential generated by the QPC electrodes, V_{QPC}(x, y), for the gate voltage V_{g} as follows^{28}
where
and . The potential landscape of the simulation is shown in Supplementary Fig. 3.
We include disorder^{29} by adding a random onsite energy V_{d}(x, y) distributed uniformly between −U_{d}/2 and U_{d}/2 where
Due to limitation of the computational mesh resolution we exclude the disorder from the vicinity of the QPC and take U_{d}0 only for x>700 nm.
We calculate the scattering matrix of the normal part of the junction for a particle at the energy ɛ as
using Kwant package^{27} and discretizing the Hamiltonian in equation (2) on a mesh with the spacing Δx=Δy=3 nm. The quantities r(ɛ) and t(ɛ) denote reflection and transmission submatrices for a timereversal symmetric system. In the second step, we combine the scattering matrices calculated for ɛ and −ɛ (that correspond to electron and hole, respectively) with the matrix that accounts for the Andreev reflection at the superconductor interface
where
The latter equation describes the Andreev reflection amplitude^{15} including the temperaturedependent pairing potential . Finally, we calculate the conductance according to
where f stands for the Fermi function
and where . N is the number of modes in the normal lead. The quasielectron and quasihole reflection matrices are given by:
Additionally, the normalstate conductance is given by . Results of the simulations are shown in Supplementary Figs 3–5.
Data availability
All data presented in the main paper and supplement, as well as code used to generate simulations are available from the authors upon request.
Additional information
How to cite this article: Kjaergaard, M. et al. Quantized conductance doubling and hard gap in a twodimensional semiconductor–superconductor heterostructure. Nat. Commun. 7:12841 doi: 10.1038/ncomms12841 (2016).
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Acknowledgements
Research support by Microsoft Project Q, the Danish National Research Foundation. C.M.M. acknowledges support from the Villum Foundation. F.N. acknowledges support from a Marie Curie Fellowship (no. 659653). M.P.N. acknowledges support from ERC Synergy Grant. A.A. is supported by an ERC Starting Grant. M.W. and A.A. are supported by the Foundation for Fundamental Research on Matter (FOM) and the Netherlands Organization for Scientific Research (NWO/OCW) as part of the Frontiers of Nanoscience program. We are indebted to S. Kraemer for the TEM analysis, performed at the UCSB MRL Shared Experimental Facilities (NSF DMR 1121053), a member of the NSFfunded Materials Research Facilities Network.
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M.K., F.N., H.J.S. and C.M. conceived the experiment. M.K., F.N. and H.J.S. designed, fabricated and measured the devices and wrote the manuscript, with comments from all other authors. J.A.F. and K.F. provided input on interpretations. M.P.N., M.W and A.R.A. developed theory and code for the simulations. The wafer was grown by J.S. and C.J.P.
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Supplementary Information
Supplementary Figures 15, Supplementary Notes 12 and Supplementary References (PDF 1175 kb)
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Kjaergaard, M., Nichele, F., Suominen, H. et al. Quantized conductance doubling and hard gap in a twodimensional semiconductor–superconductor heterostructure. Nat Commun 7, 12841 (2016). https://doi.org/10.1038/ncomms12841
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DOI: https://doi.org/10.1038/ncomms12841
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