Induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures

Search for Majorana fermions renewed interest in semiconductor–superconductor interfaces, while a quest for higher-order non-Abelian excitations demands formation of superconducting contacts to materials with fractionalized excitations, such as a two-dimensional electron gas in a fractional quantum Hall regime. Here we report induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures and development of highly transparent semiconductor–superconductor ohmic contacts. Supercurrent with characteristic temperature dependence of a ballistic junction has been observed across 0.6 μm, a regime previously achieved only in point contacts but essential to the formation of well separated non-Abelian states. High critical fields (>16 T) in NbN contacts enables investigation of an interplay between superconductivity and strongly correlated states in a two-dimensional electron gas at high magnetic fields.

Proximity effects in GaAs quantum wells have been intensively investigated in the past and Andreev reflection has been observed by several groups [28][29][30][31] . Unlike in InAs, where Fermi level (E F ) at the surface resides in the conduction band, in GaAs E F is pinned in the middle of the gap, which results in a high Schottky barrier between a 2DEG and a superconductor and low transparency non-ohmic contacts. Heavy doping can move E F into the conduction band and, indeed, superconductivity has been induced in heavily doped bulk n þ þ GaAs 32 . In quantum wells, similar results were obtained by annealing indium contacts 33 ; however, the critical field of indium is B30 mT, well below the fields required to form quantum Hall effect (QHE) states.
In this article, we report the development of transparent superconducting ohmic contacts to high-mobility 2DEG in GaAs. The superconducting contact is type-II NbN with large critical field 416 T. Induced superconductivity is observed across 1.6 mm of a 2D gas at zero field. From temperature dependence of the critical current and analysis of Andreev reflection, we estimate contact transparency parameter Zu0.2. Induced superconductivity is observed in magnetic fields up to 0.2 T. At high magnetic fields, we observe deviations of longitudinal and Hall resistances from a similar data obtained with normal contacts, a clear indication of an interplay between superconductivity and QHE edge states.

Results
Heterostructures design. In conventional quantum well structures AlGaAs, barrier between 2DEG and the surface of the sample adds an extra 0.3 eV to the Schottky barrier when contacts are defused from the top. We alleviated these problems by growing an inverted heterojunction structures, where a 2DEG resides at the GaAs/AlGaAs interface but the AlGaAs barrier with modulation doping is placed below the 2DEG, see Fig. 1b, where band diagram was calculated using a self-consistent Poisson solver 34 (The program can be downloaded from http://www3.nd.edu/~gsnider/). Contacts are recessed into the top GaAs layer to bring the superconductor closer to the 2DEG. A thin layer of AuGe and NbN superconductor form low resistance ohmic contacts to the 2DEG after annealing. The inverted heterostructure increases the contact area of side contacts compared with quantum well structures by utilizing all GaAs layer above the heterointerface for carrier injection (130 nm in our inverted heterostructure versus 20-30 nm in typical quantum wells, see Supplementary Fig. 1 and Supplementary Note 1).
Induced superconductivity. We report induced superconductivity in two devices from different wafers. Sample A has long (70 mm) contacts separated by 1.6 mm of 2DEG, contacts to sample B are formed to the edge of a mesa with 0.6 mm separation, see Fig. 1a. Details of device fabrication are described in Methods. When cooled down to 4 K in the dark, both the samples show resistance in excess of 1 MO. After illumination with red light-emitting diode a 2DEG is formed and 2-terminal resistance drops to o500 O. As shown in Fig. 1d, sample resistance RB 3 À 4 gradually decreases on cooldown from 4 K to the base temperature and the superconductor-2DEG-superconductor (S-2DEG-S) junction becomes superconducting at T c B0.3 K.
Voltage-current V(I) characteristics for two S-2DEG-S junctions (between contacts 8 and 9 for sample A, and 3 and 4 for sample B) are shown in Fig. 2. Both the samples show zero-resistance state at small currents with abrupt switching into resistive state at critical currents I c ¼ 0.22 and 0.23 mA for samples A and B, respectively. We attribute hysteresis in V(I) characteristics to Joule heating in the normal state.
The most attractive property of a high-mobility 2DEG is large mean free path lcx 0 , with l ¼ 24 mm and the Bardeen-Cooper- p =m is the Fermi velocity, n is a 2D gas density, m is an effective mass and D ¼ 1.76k B T c ¼ 46 meV is the induced superconducting gap. Evolution of V(I) with T is shown in Fig. 3a. Experimentally obtained T-dependence of I c is best described by the Kulik-Omelyanchuk theory for ballistic junctions (Lo ol) (ref. 35), the blue curve in Fig. 3b. For comparison, we also plot I c (T) dependence for the dirty limit The reduction is consistent with the geometry of sample A, where a region of the 2DEG with induced superconductivity is shunted by a large region of a 2DEG in a normal state.
Transparency of a superconductor/2DEG interface. In onedimensional junctions, the induced gap D ¼ D 0 G G þ D0 depends on the broadening of Andreev levels within the semiconductor 38 where we introduce contacts transparencies D 1 and D 2 . We assume for simplicity that D 1 where 0oZoN is a interface barrier strength introduced in ref. 39, and Bagwell's effective channel length L eff ¼ L þ 2x 0 . Using NbN superconducting gap, This value is consistent with the fit of the I c versus T-dependence with D as a free parameter (Supplementary Fig. 2; Supplementary Note 2). Similar values of Z can be estimated from the analysis of the shape of dI/dV(V) characteristics at elevated temperatures, as shown in Fig. 3. At ToT 0 c , Andreev reflection at S-2DEG interfaces results in an excess current flowing through the junction for voltage biases within the superconducting gap D 0 /e and corresponding reduction of a differential resistance dV/dI by a factor of 2. In the presence of a tunnelling barrier, normal reflection competes with Andreev reflection and reduced excess current near zero bias, resulting in a peak in differential resistance. Within the Blonder-Tinkham-Klapwijk theory 39 , a flat dV/dI(V) within D 0 /e, observed in our experiments, is expected only for contacts with very high transparency Zo0.2. For larger Z40.2, a peak at low biases is expected (Supplementary Fig. 3, Supplementary Note 3). Several features of the experimental I(V) need to be mentioned.
First, we observe several sharp peaks in the resistance at high biases (around 2 and 4 mV for T ¼ 4 K). Similar sharp resonances has been observed previously 40 , where authors attributed their appearance to the formation of Fabry-Pérot resonances between superconducting contacts. In our devices, the superconducting region is shunted by a low resistance (o100 O) 2DEG, thus appearance of 410 kO resonances cannot be explained by resonant electron trapping between contacts. These resonances are also observed in I(V) characteristics of a single S-2DEG interface (measured in the S-2DEG-N configuration between contacts 3 and 6, see Supplementary Fig. 3). Differential resistance does not change substantially across resonances, ruling out transport through a localized state. We speculate that in the contacts where these resonances are observed superconductivity is carried out by quasi-one-dimensional channels, and jumps in I/V characteristics are due to flux trapping at high currents. This scenario is consistent with the observation that peaks shift to lower currents at higher fields, see Fig. 4. The second notable feature of our data is reduction of the zero-bias resistance by E2.6 times at low temperatures, while Andreev reflection limits the reduction to the factor of 2. We attribute this reduction to the multiple Andreev reflection between two closely spaced contacts, for contacts with larger separation (20 mm) multiple Andreev reflection is suppressed and the reduction of resistance by a factor of 2 is observed ( Supplementary Fig. 3).
Induced superconductivity in low magnetic fields. Finally, we present magnetic field dependence of induced superconductivity. The low-field data is shown in Fig. 4a,b, where black regions   ARTICLE correspond to zero-differential resistance. Induced superconductivity is suppressed at E0.2 T in both the samples. In sample A, a narrow region of a 2DEG with induced superconductivity is confined between large NbN superconducting leads with rigid phases. Perpendicular magnetic field twists the phase in the 2DEG resulting in Fraunhofer-like oscillations of the critical current. In this sample, although the 2DEG extends beyond the narrow region between the contacts and I c does not decrease to zero and abrupt jumps in I c reflect multiple flux jumps. The period of oscillations is B0.5 mT, which corresponds to an area of 4.1 mm 2 , much smaller than the area of the 2DEG between the contacts (E120 mm 2 ). This observation is consistent with the reduced I c RN product measured for this sample as discussed above. In sample B, contacts are fabricated along the edge of the mesa and 2D gas is not enclosed between the contacts. Consequently, I c is a smooth function of B.
Superconductivity and quantum Hall effect. Competition between superconductivity and chiral quantum Hall edge states is shown in Fig. 4c, where resistance is measured in a 3-terminal configuration over a wide range of magnetic fields. Simple Landauer-Büttiker model of edge states predicts zero resistance for negative and quantized Hall resistance for positive field direction for integer QHE and fractional QHE states, which is clearly seen in a sample with non-superconducting (AuGe) ohmic contacts (red curve). When a superconducting contact serves as a current injector (blue curve), integer n ¼ 1 and fractional n ¼ 2/3 and 3/5 states are well developed for Bo0, while the same states are not quantized at proper QHE values for B40. If we assume that current injection via a superconducting contact results in an extra voltage offset at the contact by V off ED ind /e, the measured voltage will be reduced by V off . The magenta bars for B40 indicate corrected resistance (V À V off )/I for V off ¼ 140 mV. While this offset may explain the measured values for fractional states, a twice smaller V off is needed to reconcile the resistance at n ¼ 1.
Note that NbN critical field B c 416 T. At low fields, states n ¼ 3, 4 and 5 have resistance minima for Bo0, indicating a partial equilibration of chiral edge currents with the superconducting contact, while resistance near n ¼ 2 has a maximum. Zero resistance at n ¼ 1 and large resistance at n ¼ 2 are in contrast to the theoretical prediction that n ¼ 2 state should be stronger coupled with a superconducting contact than n ¼ 1 (ref. 23).
Fabrications of superconducting contacts. Superconducting contacts were defined by standard electron beam lithography. First, a 120 nm-deep trench was created by wet etching. Next, samples were dipped into HCl:H 2 O (1:6) solution for 2 s and loaded into a thermal evaporation chamber, where Ti/AuGe (5/50 nm) was deposited. Finally, 70 nm of NbN was deposited by DC magnetron sputtering in Ar/N 2 (85/15%) plasma at a total pressure of 2 mTorr. Deposition conditions were optimized for producing high quality NbN films with T c ¼ 11 K and B c 415 T, see Supplementary Fig. 4, and with minimal strain 41 . After metallization, contacts were annealed at 500°C for 10 min in a forming gas (10% H 2 in Ar). Measurements were performed in a dilution refrigerator with the base temperature o30 mK, high-temperature data was obtained in a variable temperature 3 He system. Samples were illuminated with red light-emitting diode at 4 K to form a 2D gas, 2-terminal resistance drops from 41 MO before illumination to o500 O after illumination.  Bo0 (B40) induces clockwise (counterclockwise) chiral edge channels, note resistance scales difference for two field directions.