Nanomembrane-Based Mesoscopic Superconducting Hybrid

Aug 5, 2010 - Log In Register .... A new method for combining top-down and bottom-up approaches to ... metal−superconductor niobium-based Josephson ...
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Nanomembrane-Based Mesoscopic Superconducting Hybrid Junctions Dominic J. Thurmer,*,† Carlos Cesar Bof Bufon,*,† Christoph Deneke,† and Oliver G. Schmidt†,‡ †

Institute for Integrative Nanosciences, IFW Dresden, Helmholtzstrasse 20, D-01069 Dresden, Germany, and Material Systems for Nanoelectronics, Chemnitz University of Technology, Reichenhainer Strasse 70, 09107 Chemnitz, Germany ‡

ABSTRACT A new method for combining top-down and bottom-up approaches to create superconductor-normal metal-superconductor niobium-based Josephson junctions is presented. Using a rolled-up semiconductor nanomembrane as scaffolding, we are able to create mesoscopic gold filament proximity junctions. These are created by electromigration of gold filaments after inducing an electric field mediated breakdown in the semiconductor nanomembrane, which can generate nanometer sized structures merely using conventional optical lithography techniques. We find that the created point contact junctions exhibit large critical currents of a few milliamps at 4.2 K and an IcRn product placing their characteristic frequency in the terahertz region. These nanometer-sized filament devices can be further optimized and integrated on a chip for their use in superconductor hybrid electronics circuits. KEYWORDS Self-assembly, nanomembranes, Josephson junction, superconductivity

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through a semiconductor nanomembrane which forms the walls of the hybrid rolled-up microtube. The three-dimensional (3D) nanomembrane based Josephson junctions are prepared by the self-rolling of a strained semiconductor nanomembrane patterned with Au-coated superconducting contacts. After rolling, a voltage bias is applied to induce a breakdown field mediated electromigration of Au into the semiconductor nanomembrane region between the superconducting contacts, which is locally controlled by point contacts arising from the intrinsic roughness of the metal films. In this way it is possible to fabricate mesoscopically sized SNS Josephson junctions by standard photolithographic processes. Consequently, such point contact junctions can be integrated on chip and, due to the parallel nature of the lithography involved, even manufactured on a larger scale. This represents a clear advantage over the state-of-the-art, where more complicated/expensive fabrication methods such as electron beam lithography are commonly used.30-32 Furthermore, our results indicate that after the initial formation process, the devices remain stable over many temperature cycles and are mechanically robust, one of the main problems faced with most previously reported point contact Josephson junctions found in literature.33,34 Additionally, this provides a self-assembly method for fabricating multiple junctions on the same substrate in a reproducible manner. Alternative techniques such as transferred inorganic membranes35 or lift-off float-on (LOFO) techniques36 could be imagined as alternatives though both techniques show distinct drawbacks which would make integration on the scale as presented here very difficult: Transferred semiconductor membranes produce a highly parallel fabrication method, but it is not clear if the formed films will allow such point-like contacting. Conversely, the LOFO technique has

ombining modern self-assembly techniques with well-established top-down processing methods is clearly paving the way for more sophisticated device generations in the future.1,2 Nanomembranes, made of many different material classes, have been shown to provide the necessary framework for a diverse range of structures and devices incorporating wrinkling, buckling, folding, and rolling of thin films.3-5 In the past decade, an elegant symbiosis of bottom-up and top-down methods has emerged to fabricate hybrid layer systems incorporating the controlled release and rearrangement of inherently strained layers.6 Self-assembled rolled-up structures5-7 have become increasingly attractive in a number of fields including micro/nanofluidics,8,9 optics10,11 (including metamaterial optical fibers)12 Lab on a Chip applications,13 and micro- and nanoelectronics.14,15 The use of such structures for microelectronic applications has been driven by the versatility in contacting geometries and the abundance of material combinations that these devices allow. Similarly, recent progress in nanostructured superconducting electronic structures has been receiving increased attention.16 The advancement of such devices has been motivated by their use in quantum computation,17-19 high sensitivity radiation sensors,20-24 precision voltage standards,25-27 and superconducting spintronics28,29 to name a few. In this Letter, we report on the fabrication of Nb-Au-Nb superconductor-normal metal-superconductor (SNS) point contact junctions fabricated by the electromigration of Au

* To whom correspondence should be addressed, [email protected] and [email protected]. Received for review: 6/23/2010 Published on Web: 08/05/2010 © 2010 American Chemical Society

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FIGURE 2. (a) Gold film roughness measured by AFM used to create nanometer sized contacts, while (b) is a schematic of the as-rolled gold-InGaAs interface. (c) STEM image of a layer structure showing the high roughness of the Au/Nb/Au layer and the possible pointcontact locations (red arrows).

FIGURE 1. (a) A 3D schematic of the rolled up Josephson junction showing the planar device before rolling (left background) and a device after rolling (right foreground). The measurement geometry is schematically indicated by the constant voltage (CV) source connecting the inside and outside contact in the planar device (in yellow). (b) An optical microscope overview image of the device structure. (c) SEM of the exposed metal structure after etching away the semiconductor layer.

Furthermore, the image indicates an inhibition of tube formation in the surrounding areas (darker wrinkled areas) by selectively etching partially through the strained layer using a phosphoric acid solution (H3PO4:H202:H2O 1:10:500) which destroys the strain within the layer and consequently the rolling-up effect. In this way it is possible to form rolled up nanomembranes only in the desired regions, further illuminating the deterministic nature of our system. In Figure 1c a scanning electron microscopy (SEM) image of a device is shown. In this case, the semiconductor was removed using the same phosphoric acid solution to illustrate the way in which the outer Au/Nb/Au contact wraps around the inner contact. After rolling, the Au filaments are created between the Nb contacts through the semiconductor nanomembrane. This is done by linearly sweeping to a voltage level (>2.5 V) which induces a breakdown in the semiconductor and initiates Au migration into the region of the junction. A fundamental part of the fabrication process consists of tailoring the intrinsic roughness of the Au/Nb/Au trilayers to create metal-semiconductor nanocontacts. By controlling the sputtering conditions, an indented surface with hills that range up to 10 nm in height can be created in a reproducible manner (see Supporting Information for more details). An atomic force microscopy (AFM) image of the actual film roughness is shown in Figure 2a. From AFM analysis, we find a root mean square roughness of 0.9 nm between such distributed 10 nm hills. Consequently, such nanohills act as local nanometer sized point-contacts on the lower smooth semiconductor boundary after rolling as shown schematically in Figure 2b. Normally, rough surfaces are undesirable and commonly represent a problem on metal-semiconductor junctions.37,38 In this device however, we make use

been shown to make very delicate contacts to many different materials, but unfortunately a lack of parallelization is still evident. Furthermore, the layer thickness possible using our technique (membranes below 5 nm total thickness) is currently not achievable using either other method. Figure 1 shows details of the device fabrication process. In Figure 1a, a 3D schematic of the rolling-up process is depicted. The planar substrate (background of Figure 1a)) is patterned with an “inside” U-shaped and “outside” fingerlike contact. This nomenclature is chosen such that the “inside” contact is within the tube when the structure is rolled up. As shown in the foreground of Figure 1a, once the strained bilayer is released and rolling is initiated, the patterned film rolls up, and upon touching itself again, creates a structure where the semiconductor layer is sandwiched between two separated superconductor contacts (see Supporting Information for more details). Here, one side of the nanomembrane (inner) is contacted by the sputtering process directly, while the outer side is mechanically contacted to the metallic layer. While the first contacting process guarantees the full coverage of the patterned area on the nanomembrane, the mechanically formed contact will depend intimately on the film roughness. An applied current now has the possibility to flow radially through the semiconductor from the “inside” to the “outside” contact. Figure 1b is an optical microscope image of a rolled up junction. The image illustrates in detail the actual device geometry. The initial film rolled up from the position marked “starting edge” to the point where it was eventually taken out of the etching solution as a rolled up structure marked “tube”. © 2010 American Chemical Society

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semiconductor behavior (region A) (also shown in the comparison in Figure 3c). In region B, a drastic increase in current is observed which we attribute to an irreversible breakdown of the semiconductor material (see Supporting Information for more details). During this process we consider that Au filaments connect the inside and outside contacts and create a pathway through the semiconducting nanomembrane. Afterward, we find a linear Ohmic behavior in region C which is an irreversible and stable state. We find a reproducibility of this phenomena for roughly 70% of the devices on a single sample (see Supporting Information for more details). A similar breakdown and transition to an Ohmic behavior is also observed for Nb-semiconductor-Nb junctions (not shown) indicating that the breakdown phenomenon is ascribed to properties of the semiconductor. We find however, that if no Au is used in the layer structure, the resistance in region C is always larger than for junctions created with Au. This observation indicates that the composition of the intermediate region after the transition from region A to region C is strongly dependent on the nature of the metal present at the nanomembrane interface. Further fundamental differences between the two cases are clearly observed at low temperatures and will be discussed in more detail below. Lastly, we find that, if the doping concentration of the semiconductor nanomembrane increases, the voltagemediated breakdown of the semiconductor is suppressed (region A′) due to the drastically lower voltage drop across the semiconductor. Figure 3c compares the junction formation current-voltage characteristics for various sample structures. Here, the red circular trace is once more the rolled up structure containing Au as shown in Figure 3b. The graph indicates that for low applied bias, the rolled up Josephson junction (3D) initially exhibits similar characteristics to a planar (not rolled up) structure (2D). For the (2D) case we also find that it was not possible to create an Au pathway induced by breakdown of the semiconductor due to the large distance between the contacts (3 µm at the closest point). After breakdown, the rolled up structure (3D) assumes a lower resistance state, which is clearly Ohmic rather than showing a nonlinear behavior such as the rolled up doped sample (n+ 3D) (also shown in Figure 3b). In an equivalent structure having both sides of the GaAs/InGaAs nanomembrane contacted using deposition methods, we were not able to create gold filaments within the semiconductor (3D Direct) due to the lower current concentration in this case arising from the uniform contact on both sides of the nanomembrane (see Supporting Information for more details). Finally, we have also prepared a similar planar structure using 50 nm of ALD grown Al2O3 replacing the semiconductor nanomembrane. We found it was not possible to create the same type of junction with this equivalent configuration. In the voltage range measured we were unable to induce a breakdown of the oxide material between the separated contacts, as shown in the figure. The rise in the Al2O3 trace is merely attributed to a leakage current through the semi-

FIGURE 3. (a) Measurement geometry in which the nanofilaments are fabricated. (b) The Au filament fabrication process is shown in more detail through selected I-V curves, revealing the semiconductor behavior (A), the breakdown (B), and the Ohmic region (C). A comparison with a similar, highly doped sample (A′) is also shown. (c) Further details of the fabrication process, showing the comparison between a planar (2D), rolled (3D), highly doped rolled (n+ 3D), directly contacted (3D Direct), and planar Al2O3 reference sample.

of such self-assembled nucleation processes to give rise to metal-semiconductor nanojunctions by allowing the atomically flat semiconductor nanomembrane to contact the Au hills during the rolling procedure. Hence, once the rolled structure is under a voltage bias, the current can only flow through the point contacts, leading to an Au electromigration strictly in the regions where the semiconductor is touching the Au hills. Figure 2c is a scanning transmission electron microscopy (STEM) image of a cross section taken from a rolled up sample structure. The image further highlights the roughness of the sputtered metal films used. The red arrows indicate areas where the films are touching and where filaments could possibly form. Figure 3a schematically illustrates the way the structures are connected in order to create a breakdown path across the rolled up semiconductor nanomembrane. The image indicates a schematic current pathway radially through the structure, flowing through the created Au filament junction between the Nb leads. In Figure 3b the current injected through the point contacts and flowing across the semiconductor nanomembrane is plotted as a function of the applied voltage to form the SNS junction: the voltage is increased linearly (region A) until the breakdown point (region B) is reached after which it is swept back down (region C). Initially, the nanomembrane shows a typical nonlinear © 2010 American Chemical Society

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literature (roughly 8 - 9 K) is thought to be due to the high sputter pressure used, the correlated impurities, and the relative thickness (50 nm) of the films.39 Furthermore, experimental conditions in the sputtering machine appear to cause slight Tc variations between successive depositions. We estimate the resistivity of the Nb film to be roughly 2-3 times larger than the bulk value at room temperature.40 This fact is further supported by the higher resistance and lower transition temperature of the device fabricated without Au layers surrounding the Nb leads (open triangles): In addition to diffusing into the semiconductor upon breakdown, the samples containing Au generally display higher Tc and are thought to protect the Nb from oxidation when exposed to air. Here we find a transition of the Nb leads at 4.6 K and no further transition of the intermediate region between the superconducting leads. This further supports the assumption that there is a Au diffusion into the intermediate region during the breakdown of the semiconductor nanomembrane. The third trace (open diamonds) indicates that in the highly doped semiconductor only the transition of the leads to a superconducting state can be observed while no further transition of the intermediate region is noticed either. Again, the Nb leads were covered with an Au layer giving a somewhat higher Tc of 5.8 K. Clearly, the superconducting proximity effect is only observable in samples where Au was employed in the layer structure. Additionally, we find that the Au filaments exhibit a dependence on the measurement current as shown in Figure 4b. Here, the measurements were carried out by ranging the current from 100 µA to 1 mA. For current values above 1 mA, no transition of the filament to the superconducting state was observed in the temperature range measured. The inset of Figure 3b shows the typical I-V sweep for the measured samples: a clear hysteresis can be seen which is attributed to electron heating within the normal region,32 also a factor correlated with the current dependence of the transition. Extracting the normal state resistance of the junction (∼30 Ω) from Figure 3b, and taking into account the increase in resistivity (F) of materials on the nanometer scale,41 we estimate the filament diameter to be roughly in the tens of nanometers range making them clearly of mesoscopic dimensions. The microscopic size of such junctions is critical for reducing the junction capacitance (correlated to the junction area), while the total capacitance is further reduced by the inherent cross type geometry of the structures which reduces the parasitic capacitance created by overlapping leads.42 Taking the IcRn product for these devices, we find values of ∼10 mV for many devices (for the device shown in Figure 3 a value of 15 mV is calculated). This value is substantially larger than other devices fabricated using niobium-normal metal-niobium junctions, usually having IcRn products in the range of hundreds of microvolts up to a few millivolts.43,44 This can be partially explained by two distinct factors present

FIGURE 4. (a) Normalized resistance vs temperature for different device structures measured. Inset: Difference in I-V traces indicating the critical current Ic above and below the critical temperature Tc. (b) Current dependence of the Au filament created. Inset: Typical I-V trace used to measure the current dependence.

conductor substrate around the edges of the contact pads. This result further suggests that the creation of such a mesoscopic gold-filament-based superconducting Josephson junctions of this length is only possibly using semi-isolating semiconductor nanomembranes. Further differences between the various samples were investigated at low temperatures. In Figure 4a the normalized resistance vs temperature is shown for three distinct material combinations. The initial resistance at 10 K varies between 263, 392, and 398 Ω for the junctions with gold, without gold, and highly doped, respectively. All traces are furthermore normalized to their Tc in order to reduce confusion. For junctions created by an electric field breakdown of the GaAs including an Au layer (closed circles), we observe a transition of the Nb leads to the superconducting state at 5.5 K. Furthermore, a second transition can be seen at 4.5 K which is ascribed to the Au filament being driven into a superconducting state due to the proximity effect.30-32 The lower transition temperature of the Nb film compared to © 2010 American Chemical Society

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in our system. First, the short normal metal channel length of 50 nm leads to an inherently high Ic (Ic ∝ L-1). Second, the properties of the Au filaments present in the junction, namely, their small size and larger resistivity (F), lead to higher Rn. Calculating the Thouless energy ETh ) pD/L2, and taking the diffusion constant35 D to be 68 cm2/s and L ) 50 nm, we find ETh ) 1.78 meV, which is the same order of magnitude as the energy gap (∆ ) 1.76kbTc ) 0.83 meV). Clearly, these junctions are in the intermediate region between the short and long junction limit (ETh ≈ ∆).34 Continuing, we see that the IcRn product lies between a long and short junction limit also, where IcRn ) 2.07∆ ) 1.71 meV and IcRn ) 10.82ETh ) 19.26 meV for the short and long junctions, respectively. A clearer theoretical understanding and analysis of this high IcRn product is however still needed. The high Ic of these structures would make them less sensitive to thermal and current noise, which is ideal for precision voltage standard applications. By decreasing the layer thickness of the semiconductor membrane, the resistance Rn could be reduced in order to make the junctions more applicable to standard precision voltage standard frequencies of 70 GHz (fc ) IcRn/φ0).41 The high IcRn product, however, indicates larger constant voltage Shapiro steps and places the characteristic frequency in the terahertz region (7.26 THz for the device shown in Figure 3), which has become an interesting region bridging the electronic and optical electromagnetic spectrum. Terahertz radiation penetrates the body in a similar way as X-rays, for example, but does not harm the body in ways that X-ray imaging is know to do. The emission and detection of radiation are key factors, both of which can be covered by superconducting Josephson junctions independently. Ideally, such terahertz sources and detectors could be coupled to similar rolled up technology such as metamaterial fiber optics systems22 or rolled up ring resonators20,21 for instance. The parallel nature of the fabrication process would allow arrays of radiation detectors to be processed on a chip creating imaging sensors much like CCDs. Furthermore, the microtube ring resonator structures could be used to more efficiently couple the incoming radiation into the junction area or even facilitate directional resolution given by the asymmetry created by the tubular structure. In conclusion, we have shown that rolled-up hybrid superconductor/semiconductor nanomembranes are suitable for creating mesoscopic SNS Josephson junctions. As opposed to conventional methods, ours does not require electron beam lithography but still manages to create Josephson junctions with mesoscopic dimensions and good quality interfaces due to the breakdown field and electromigration technique relying on the intrinsic roughness of metal films. Furthermore, the thickness of the semiconductor template used for rolling can be varied over a wide range, allowing the length of the Josephson junction to be tuned on the nanometer scale with high © 2010 American Chemical Society

precision. Finally, an integration of side gates in the semiconductor material could allow for lateral tuning of the electronic states needed for higher level control of Josephson junctions.17-19 For experimental purposes, the device structures shown here were made larger than necessary; however there are no technological problems with compacting the system an order of magnitude or more. Additionally, an up-scaling to multiple devices on a large substrate comes at no cost due to the parallel fabrication process associated with optical lithography techniques (here, single chips of 18 devices on 5 × 5 mm were fabricated for testing). All these aspects make these SNS junctions formed by rolling-up strained nanomembranes interesting candidates for superconductor electronic devices such as programmable voltage standards, quantum computation, and radiation sensors. Acknowledgment. The authors acknowledge Daniel Grimm, Ronny Engelhard, Elliot J. Smith, Martin Bauer, Paola Atkinson, Jens Ingolf Mo¨nch, Joachim Schumann, and Vladimir Fomin for their help and fruitful discussions. This work was partially supported by Nanett. Note Added after ASAP Publication. References 5 and 7 were modified in the version of this paper published ASAP August 5, 2010. The correct version published on August 11, 2010. Supporting Information Available. Experimental section and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES (1) (2) (3) (4) (5) (6)

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