GaSb Core–Shell

Letter pubs.acs.org/NanoLett

Tunable Esaki Effect in Catalyst-Free InAs/GaSb Core−Shell Nanowires M. Rocci,*,† F. Rossella,*,† U. P. Gomes,† V. Zannier,† F. Rossi,‡ D. Ercolani,† L. Sorba,† F. Beltram,† and S. Roddaro*,† †

NEST, Scuola Normale Superiore and Istituto Nanoscienze-CNR, Piazza S. Silvestro 12, I-56127 Pisa, Italy IMEM-CNR,Parco Area delle Scienze 37/A, I-43010 Parma, Italy

‡

S Supporting Information *

ABSTRACT: We demonstrate tunable bistability and a strong negative differential resistance in InAs/GaSb core−shell nanowire devices embedding a radial broken-gap heterojunction. Nanostructures have been grown using a catalyst-free synthesis on a Si substrate. Current−voltage characteristics display a top peak-to-valley ratio of 4.8 at 4.2 K and 2.2 at room temperature. The Esaki effect can be modulatedor even completely quenchedby field effect, by controlling the band bending profile along the azimuthal angle of the radial heterostructure. Hysteretic behavior is also observed in the presence of a suitable resistive load. Our results indicate that high-quality broken-gap devices can be obtained using Au-free growth.

T

In this context, catalyst-free growth techniques are particularly relevant since they allow the growth of nanostructures without using gold, which forms deep traps in all semiconductor systems of interest and has well-known detrimental effects on device performance.24,25 Importantly NWs can be directly grown on oxidized Si substrates.26 A number of GaSb/InAs heterostructures were demonstrated within this growth paradigm in recent years.21,23,27 In this work we demonstrate that high-quality broken-gap radial junctions can be obtained using this Au-free synthesis. We investigate charge transport in InAs/GaSb core−shell radial NW heterostructures and demonstrate the realization of a radial broken-gap junctions. Charge transport is studied as a function of the temperature and backgate voltage, using electrodes that selectively contact either the InAs core or the GaSb shell. A marked NDR is observed for contact configurations inducing radial charge transport. In particular, a bilateral NDR can be obtained for a shell-to-shell conduction geometry, i.e., in an architecture that does not necessarily require any advanced selective etching/contacting method. In all the measurement configurations, the NDR characteristics can be continuously modulated by field effect and we demonstrate a tunability of the peak-to-valley ratio (PVR) from a maximum of 4.8 at 4.2 K down to zero. The control of the magnitude of the Esaki effect is understood in terms of band-bending modulation which occurs at the radial interface between InAs and GaSb

he GaSb/InAs system provides a unique playground for the creation of low-strain heterostructures where the broken-gap band alignment between GaSb and InAs can lead to the coexistence of mobile electrons and holes in facing regions of the sample.1,2 Recently, devices featuring this peculiar band alignment gained a renewed interest since they can be adopted to implement tunnel field-effect transistors3−6 and due to the still controversial possibility to induce electronic phases characterized by a nontrivial topology at the interface between the two band-inverted materials.7−10 Another exciting possibility is the exploitation of Coulomb interaction between electrons and holes in the nanostructure,11 which is particularly interesting both for the fundamental investigation of bosonic exciton condensates12 and for exotic device applications exploiting interaction effects.13 To date, the GaSb/InAs material combination has been studied mostly with twodimensional epitaxy,14,15 but the realization of broken-gap heterostructured nanowires (NWs) offers interesting perspectives. Axial and hybrid axial−radial InGaAs−GaSb NW heterostructures were implemented using the vapor−liquid− solid mechanism16 (VLS), and the presence of a broken-gap alignment was demonstrated in transport experiments through the observation of a marked negative differential resistance5,17 (NDR). Radial junctions are particularly suited for the investigation of effects raising from the interaction of the ptype and n-type conductors. In fact, in this geometrical configuration, ambipolar conduction was demonstrated in NWs consisting of a GaSb p-type core covered by a InAs ntype shell,18−20 and the growth of nanostructures with an InAs core was demonstrated using various epitaxial techniques.21−23 © XXXX American Chemical Society

Received: October 11, 2016 Revised: November 22, 2016 Published: November 29, 2016 A

DOI: 10.1021/acs.nanolett.6b04260 Nano Lett. XXXX, XXX, XXX−XXX

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(EDX), see Figure 1d and 1e. Elemental maps for Ga and In indicate a high homogeneity and sharp interface between the crystalline InAs core and epitaxial GaSb shell. EDX maps also reveal that the as-grown NWs exhibit a minor GaSb axialgrowth of about 100−150 nm: this part of the NW was not used in the devices and does not play any role in the experiment. NWs were transferred to a prepatterned p++Si/ SiO2 substrate by dropcasting. In order to selectively contact the InAs and GaSb regions of the NWs, we optimized a highly selective GaSb wet-etching protocols. Contacts were realized in two distinct e-beam lithography (EBL) steps. The first EBL process was performed in order to remove the GaSb shell and deposit the contact electrodes on top of the InAs core. Selective GaSb wet-etching process was achieved soaking the sample into an NaOH solution (1 M in water, leading to an etch rate of ≈5−10 nm/s) for 10 s to ensure a complete removal of the GaSb shell. Before the first Ni/Au metallization, the InAs cores were passivated using a standard ammonium polysulfide (NH4)2Sx solution in order to promote the formation of low-resistance ohmic contacts. The Ni/Au (10/140 nm) electrodes were deposited by thermal evaporation. The second EBL step allowed the creation of Ni/Au contacts to the GaSb shells. Prior to metal deposition, the contact area was passivated in a HCl:H2O 1:3 bath for 30 s in order to diminish the impact of the native oxide. The Ni/Au (10/140 nm) electrodes were thus evaporated using the same procedure used for the core contacts. The resulting device structure is shown in Figure 2a,b. Panel c displays a crosssectional sketch of the NW on top of the oxidized Si substrate and the resulting embedded radial broken-gap heterojunction. Charge transport measurements were performed from room temperature down to T = 4.2 K, in the dark and in the presence of a low-pressure He exchange gas. All experiments were performed using a two-wire configuration, using a DL1211 current preamplifier. Depending on the contacts used, various transport schemes can be implemented. Owing to the conductive properties of the InAs core, core−core (CC) current−voltage (IV) characteristics were found to be trivially linear, similarly to what observed in the absence of the shell. Differently, both the shell−shell (SS, contacts 2−3, top sketch in Figure 3a) and core−shell (CS, contacts 1−2, bottom sketch in Figure 3a) configurations lead to a non trivial behavior with a marked NDR which is visible from T = 4.2 K up to room temperature. The general transport features of the two latter configurations are compared in Figure 3b, where IV semilog

semiconductors as a consequence of the applied back-gate voltage. Devices were fabricated starting from catalyst-free InAs/ GaSb core−shell NWs grown by chemical beam epitaxy (CBE) on Si (111) substrates (see Figure 1a, see Methods for further

Figure 1. InAs/GaSb core−shell nanowires. (a) Scanning electron micrograph of as-grown InAs/GaSb nanowires (45° tilt). (b) Sketch of an individual InAs/GaSb core−shell nanowire, where the InAs (core) and GaSb (shell) are colored in green and red, respectively. The core and full nanowire diameters are labeled by d and D respectively, whereas t represents the shell thickness. (c) Scanning TEM image of two core−shell nanowires transferred onto a standard TEM grid. (d) EDX element map of the same InAs/GaSb nanowires as in part c. The presence of the gallium (red-dotted area) is confined at the surface (shell) whereas the In (green-dotted area) is present only in correspondence of the core of the nanowire. (e) Cross-section line profile corresponding to the yellow cut visible in part d showing clearly the presence of the indium peak only in correspondence of the nanowire core and the gallium peaks identifying the nanowire shell.

details). The core of the nanostructure consists of about a 2 μm-long core of nominally undoped InAs, with a diameter d = 60 nm. A GaSb shell with a thickness of t = 30 nm was grown on top of the side facets of the core (see sketch of Figure 1b). The structure and chemical composition of the nanostructures were studied by transmission electron microscopy (TEM), see Figure 1c and high-resolution images in the Supporting Information, and by energy dispersive X-ray spectroscopy

Figure 2. Device structure. (a) False-colored SEM image (60° tilt) of a typical InAs/GaSb device, deposited on a SiO2/p-Si substrate. (b) Top-view of one of the devices. The contact design allows to explore the transport properties of both the core−shell (by electrodes 1−2) and shell−shell (by electrodes 2−3) configuration, on the same device. (c) Schematic illustration device cross-section embedding a radial broken-gap InAs/GaSb junction. Sketch of the band alignment along the radial direction AB. B

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they were never observed in the SS one. Such a nonideal behavior is plausibly caused by the etching-induced damage in the region between the central contact and the InAs one, which is mostly relevant to transport in the CS configuration (see SEM picture in Figure 2b). Selected values of VBG are reported in the two panels of Figure 3b: in the top (bottom) panel measurements were performed in a range going from VBG = −30 V (−2 V) up to +30 V (+30 V). The specific explored gating range was chosen in order to yield the full quenching of the NDR in the device, according to a tuning mechanism discussed in the following section. The exact gate voltage required to quench the NDR was found to be devicedependent, in particular in the CS configuration. Our devices display a marked NDR up to room temperature and they were also often found to display a hysteretic behavior and an IV bistability. A typical temperature evolution of the device IV characteristics can be observed in Figure 4a, referring to a CS configuration. Upon lowering the temperature, the NDR evolves into a stronger and stronger hysteresis cycle, with a saturation below ≈50 K. A combined shift of the nonlinear features is obsvered in the VDS bias value. The current density values shown on the right axis were calculated assuming a radial

Figure 3. Core−shell and shell−shell transport configurations. (a) The device architecture allows a two-wire measurement of both the core− shell (electrodes 1−2) and shell−shell (electrodes 2−3) transport characteristics, as a function of the backgate voltage VG. The two different conctact cross sections are sketched on the right end of the panel. (b) Absolute value of the current (|IDS|) versus the drain-source voltage (VDS) of GaSb−GaSb (shell−shell, top panel) and InAs-GaSb (core−shell, bottom panel) at T = 4.2 K. The |IDS| versus VDS characteristics were measured at different backgate voltages ranging from −30 to +30 V in the top panel and from −2 to +30 V in the bottom panel.

curves are visible for the SS (top panel) and CS (bottom panel) contact configurations. Data were measured at T = 4.2 K for various values of the backgate voltage VBG. We note that all curves exhibit NDR for positive bias values VDS, but when the junctions are reverse biased NDR persists only in the SS configuration. Such a behavior stems from the presence of the radial broken-gap junction. In the CS configuration, radial transport must occur and the Esaki effect is expected for VDS > 0 owing to the overlap of the two bandgaps of InAs and GaSb.15 Differently, trivial conduction is expected for VDS < 0. In the SS configuration, the naive expectation would be to simply observe transport within the GaSb shell that is not expected to display dramatic deviations from a simple linear response. In practice, the experimental behavior of the devices is different and NDR is observed for both VDS signs. Indeed, the GaSb shell is significantly more resistive than the InAs core and devices in the SS configuration behave as two back-to-back radial Esaki diodes. Consistently, NDR is observed for both VDS polarities. We note that multiple NDRs could be often observed in the CS configuration, while

Figure 4. Temperature evolution and bistability. (a) The temperature evolution of the IV characteristics typically lead to the emergence of a bistability which, in some devices, is directly visible even at room temperature (see Supporting Information). The corresponding current density JDS calculated assuming a junction area ≈0.04 μm2, corresponding to a circular junction with an axial extension of 200 nm. (b) The device can be thought as a series of the broken-gap junction plus a parasitic load resistor RL due to the NW itself, as visible on the right plot showing the junction IV and the load line at bias VDS and load RL. Bistability is expected to emerge when RL exceed the absolute NDR value. The bias at which the bistability is observed also depends on the value of RL and the observed temperature evolution of the IV is consistent with an increase of RL at low temperatures. C

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Figure 5. Field-effect gating of the Esaki diode. (a) Back-gating has a different impact on the junction band profile depending on the specific facet and strongly affects the overall transport characteristics. In addition, the gate modulates the conductivity of the InAs and GaSb portions of the NW and the values of resistances RL and Rleak indicated in the equivalent circuit schematic (see main text for further details). (b) Current versus drainvoltage characteristics of the SS configuration as a function of the backgate voltage VBG at T = 4.2 K. The thick curves correspond to VBG = −30 (blue), −15 (dark yellow), 0 (black), and +30 (red) V, whereas the thin (gray) curves correspond to the intermediate values, one every 3 V of backgate step. (c) The corresponding peak-to-valley ratio indicates that the Esaki junction be continuously modulated from 1 (no NDR) up to 4.8 by sweeping VBG from −30 to +30 V.

addition, field effect affects the conductivity of the InAs (ntype) and GaSb (p-type) portion of the device: the first one mostly contributes to the load resistor RL, the second one to a direct GaSb conduction path with resistance Rleak (see Figure 5a). Figure 5b reports a set of IV curves as a function of the backgate voltage VBG in the range −30/+30 V, for a SS configuration. A strong negative VBG can disrupt the NDR, as a consequence of the mechanism sketched in panel a and of the increased transport in the p-type GaSb region. The overall shift of the nonlinear features to large voltages is also indicative of an overall increase of the value of RL, plausibly caused by an increase in the resistance of the n-type InAs region. These trends are also confirmed by the behavior of the current IDS versus VBG for large bias voltages (VDS > 0.4 V): in this range of the IV the current has an ambipolar reaction to the gate, confirming the parallel tuning of conductances in the InAs and GaSb regions. For positive VBG the PVR is typically found to slightly increase and shift further to lower bias values. This behavior is interpreted as caused by the reduction of Rleak, or even the depletion of the GaSb at the bottom of the NW, and by the further increase in conductivity of the InAs core and thus reduction of RL. Experimental data in Figure 5 demonstrate that the magnitude of the PVR can be strongly modulated by fieldeffect. In particular, when VBG = −30 V the junction virtually displays no NDR while a top PVR of 4.8 is observed for VBG = +30 V, for T = 4.2 K (see Figure 5c). In conclusion, we demonstrated the successful fabrication of radial broken-gap junctions in InAs/GaSb core−shell NWs grown by catalyst-free CBE. Closely spaced electron and hole systems in NWs provide a workbench for the investigation of interactions in low dimensions12,13 and of their impact on heat and charge transport in NWs.20,28,29 The creation of a radial broken-gap band alignment was demonstrated through the observation of unilateral or bilateral NDR as a function of the device bias in the core−shell or shell−shell configurations. In addition, we demonstrated that Esaki effect can be tuned, and even quenched, by field-effect. This modulation is understood in terms of the tuning of the local band alignment of the InAs/ GaSb interface along the radial heterojunction.

junction with a length of 200 nm, corresponding to the contact size of ≈0.04 μm2. The mechanism giving rise to this behavior is sketched in Figure 4b: in general, the emergence of bistability in a device featuring an NDR is caused by the presence of a large-enough load resistor RL in series with the junction, which in our case can be expected due to parasitic resistance of the NW itself. In the plot on the right side, the red line represents the IV curve of the bare junction as a function of its voltage drop VJ. In the presence of a bias VDS and a resistive load RL, the current IDS can be obtained using the resistive load line in blue. When RL exceedsin absolute valuethe NDR slope, one of the three resulting solutions for IDS becomes unstable (open red circle) and the device develops two stable configurations (full red circles). The overall evolution of Figure 4a can be well first of all understood in terms of an increase of RL at low temperatures: this is in fact expected to lead to an increasingly strong bistability and to a shift of the features to larger values of VDS, due to voltage partitioning between the junction and RL. In addition, the presence of thermally activated transport (thermoionic emission and/or surface conduction) can be expected to lead to larger current values at large temperatures. A top PVR = 2.2 was measured at room temperature (see also Supporting Information). The device parameters are consistent with those recently reported in Auseeded NWs,5 considering the approximation on the estimate of the junction size in our architecture. Our data suggest that the radial Esaki junction features a fairly uniform band alignment along the azimuthal angle of the NW. Indeed any strong built-in disorder or device asymmetry would lead to NDR conditions locally occurring at different values of the externally applied bias V DS . A strong nonuniformity in the junction can be artificially induced by exploiting the substrate gate, which is highly asymmetric with respect to the NW center. The key idea is depicted in Figure 5a: the gate bias can induce quite different band alignments at the various NW facets (the two extreme cases of the top and bottom facets are sketched in panel a) for a give core−shell bias. In particular, the gate can be expected to affect the carrier density at the bottom of the NW since this region is closer to the gate and less affected by the contact electrode screening. In D

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(3) Borg, B. M.; Dick, K. A.; Ganjipour, B.; et al. InAs/GaSb heterostructure nanowires for tunnel field-effect transistors. Nano Lett. 2010, 10 (10), 4080−4085. (4) Lu, H.; Seabaugh, A. Tunnel field-effect transistors: state-of-theart. IEEE J. Electron Devices Soc. 2014, 2 (4), 44−49. (5) Dey, A. W.; Svensson, J.; Ek, M.; et al. Combining axial and radial nanowire heterostructures: radial Esaki diodes and tunnel field-effect transistors. Nano Lett. 2013, 13 (12), 5919−5924. (6) Zhou, G.; Li, R.; Vasen, T. Novel gate-recessed vertical InAs/ GaSb TFETs with record high I ON of 180 μA/μm at V DS= 0.5 V. Electron Devices Meeting(IEDM), 2012 IEEE International 2012, 32− 36. (7) Pribiag, V. S.; Beukman, A. J.; Qu, F.; et al. Edge-mode superconductivity in a two-dimensional topological insulator. Nat. Nanotechnol. 2015, 10 (7), 593−597. (8) Du, L.; Knez, I.; Sullivan, G.; et al. Robust helical edge transport in gated InAs/GaSb bilayers. Phys. Rev. Lett. 2015, 114 (9), 096802. (9) Qu, F.; Beukman, A. J.; Nadj-Perge, S.; et al. Electric and magnetic tuning between the trivial and topological phases in InAs/ GaSb double quantum wells. Phys. Rev. Lett. 2015, 115 (3), 036803. (10) Knez, I.; Du, R.-R.; Sullivan, G. Evidence for helical edge modes in inverted InAs/GaSb quantum wells. Phys. Rev. Lett. 2011, 107 (13), 136603. (11) Ganjipour, B.; Leijnse, M.; Samuelson, L.; et al. Transport studies of electron-hole and spin-orbit interaction in GaSb/InAsSb core-shell nanowire quantum dots. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91 (16), 161301. (12) Abergel, D. Excitonic condensation in spatially separated onedimensional systems. Appl. Phys. Lett. 2015, 106 (21), 213103. (13) Wu, K.; Rademaker, L.; Zaanen, J. Bilayer excitons in twodimensional nanostructures for greatly enhanced thermoelectric efficiency. Phys. Rev. Appl. 2014, 2 (5), 054013. (14) Sai-Halasz, G.; Esaki, L.; Harrison, W. InAs-GaSb superlattice energy structure and its semiconductor-semimetal transition. Phys. Rev. B: Condens. Matter Mater. Phys. 1978, 18 (6), 2812. (15) Longenbach, K.; Luo, L.; Wang, W. Resonant interband tunneling in InAs/GaSb/AlSb/InAs and GaSb/InAs/AlSb/GaSb heterostructures. Appl. Phys. Lett. 1990, 57 (15), 1554−1556. (16) Ek, M.; Borg, B. M.; Dey, A. W.; et al. Formation of the axial heterojunction in GaSb/InAs (Sb) nanowires with high crystal quality. Cryst. Growth Des. 2011, 11 (10), 4588−4593. (17) Ganjipour, B.; Dey, A. W.; Borg, B. M.; et al. High current density Esaki tunnel diodes based on GaSb-InAsSb heterostructure nanowires. Nano Lett. 2011, 11 (10), 4222−4226. (18) Ganjipour, B.; Ek, M.; Borg, B. M.; et al. Carrier control and transport modulation in GaSb/InAsSb core/shell nanowires. Appl. Phys. Lett. 2012, 101 (10), 103501. (19) Ganjipour, B.; Sepehri, S.; Dey, A. W.; et al. Electrical properties of GaSb/InAsSb core/shell nanowires. Nanotechnology 2014, 25 (42), 425201. (20) Gluschke, J. G.; Leijnse, M.; Ganjipour, B.; et al. Characterization of Ambipolar GaSb/InAs core-shell nanowires by thermovoltage measurements. ACS Nano 2015, 9 (7), 7033−7040. (21) Ji, X.; Yang, X.; Du, W.; et al. InAs/GaSb core-shell nanowires grown on Si substrates by metal-organic chemical vapor deposition. Nanotechnology 2016, 27 (27), 275601. (22) Namazi, L.; Nilsson, M.; Lehmann, S.; et al. Selective GaSb radial growth on crystal phase engineered InAs nanowires. Nanoscale 2015, 7 (23), 10472−10481. (23) Rieger, T.; Grützmacher, D.; Lepsa, M. Misfit dislocation free InAs/GaSb core-shell nanowires grown by molecular beam epitaxy. Nanoscale 2015, 7 (1), 356−364. (24) Noor Mohammad, S. Why self-catalyzed nanowires are most suitable for large-scale hierarchical integrated designs of nanowire nanoelectronics. J. Appl. Phys. 2011, 110 (8), 084310. (25) Mandl, B.; Stangl, J.; Mårtensson, T.; et al. Au-free epitaxial growth of InAs nanowires. Nano Lett. 2006, 6 (8), 1817−1821.

Methods. Catalyst-free InAs NWs were grown on Si(111) substrates by employing the growth procedure described in detail in ref 26. The growth protocol of the InAs core consists of a 5 min nucleation step at 420 ± 10 °C, with tert-butylarsine (TBAs) and trimethylindium (TMIn) precursor fluxes at pressures 3.00 and 0.30 Torr. This step was followed by a 10 min ramp to the growth temperature of 520 ± 10 °C in TBAs flux at a pressure of 3.00 Torr. Growth was continued for 105 min with TBAs and TMIn (pressure 3.00 and 0.20 Torr, respectively). The GaSb shell was grown for 45 min at the same substrate temperature, using triethylgallium (TEGa) at a pressure of 0.70 Torr and trimethylantimony (TMSb) at a pressure of 0.60 Torr. The morphology of the NWs was investigated using a scanning electron microscope (SEM), while the chemical composition and the thickness of the GaSb shell were derived from energy-dispersive X-ray spectroscopy (EDXS) measurements in a transmission electron microscope (TEM) JEOL 2200FS. The EBL standard processes were performed as follows. A 280 nm-thick layer of commercial poly(methyl methacrylate) AR-P 679.04 (Allresist GmbH) was spun onto the substrate and baked at 180 °C for 90 s. The PMMA was exposed at 20 kV with 40 pA of beam current and the development process was carried out dipping the sample in AR 600-56 (AllResist GmbH) developer for 60 s. Afterward, the sample was rinsed in pure 2-propanol for 30 s and blown dry with nitrogen. A standard descum process was used to remove organic residues remaining after the chemical development.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b04260. HRTEM images of the InAs/GaSb interface and roomtemperature bistability (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

F. Rossella: 0000-0002-0601-4927 F. Rossi: 0000-0003-1773-2542 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank J. Eroms for useful discussions. This work was supported by the Scuola Normale Superiore, and by the CNR through the bilateral CNR-RFBR Project 20152017. F.R. and M.R. acknowledge the support by the MIUR through the programs “FIRB - Futuro in Ricerca 2013” and Project “UltraNano” (Grant No. RBFR13NEA4).

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REFERENCES

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Nano Letters (26) Gomes, U.; Ercolani, D.; Sibirev, N.; et al. Catalyst-free growth of InAs nanowires on Si (111) by CBE. Nanotechnology 2015, 26 (41), 415604. (27) Wang, X.; Du, W.; Yang, X.; et al. Self-catalyzed growth mechanism of InAs nanowires and growth of InAs/GaSb heterostructured nanowires on Si substrates. J. Cryst. Growth 2015, 426, 287−292. (28) Roddaro, S.; Ercolani, D.; Safeen, M. A.; et al. Giant thermovoltage in single InAs nanowire field-effect transistors. Nano Lett. 2013, 13 (8), 3638−3642. (29) Wu, P. M.; Gooth, J.; Zianni, X.; et al. Large thermoelectric power factor enhancement observed in InAs nanowires. Nano Lett. 2013, 13 (9), 4080−4086.

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