NANO LETTERS
Hybrid Devices from Single Wall Carbon Nanotubes Epitaxially Grown into a Semiconductor Heterostructure
2004 Vol. 4, No. 2 349-352
Ane Jensen,*,† Jonas Rahlf Hauptmann, Jesper Nygård, Janusz Sadowski,‡ and Poul Erik Lindelof Nano-Science Center, Niels Bohr Institute, UniVersity of Copenhagen, UniVersitetsparken 5, DK-2100 Copenhagen, Denmark Received November 10, 2003
ABSTRACT To take advantage of nanoscale molecular electronic components in semiconductor technology, there will be a desire to integrate new elements such as one-dimensional (1D) carbon nanotubes in conventional 2D or 3D semiconductor systems. We report on hybrid devices based on single wall carbon nanotubes encapsulated in epitaxially grown semiconductor heterostructures of GaAs/AlAs and (Ga,Mn)As below and above the carbon nanotube. In our devices the semiconductor serves as leads to the nanotubes, forming the first reported electronic hybrid nanotube-semiconductor devices.
We have successfully fabricated electronic devices from single wall carbon nanotubes (SWNTs) encapsulated in a semiconductor heterostructure, grown by molecular beam epitaxy (MBE). SWNTs are tubular molecules of carbon with unique electron transport properties. These tubes may be either metallic or semiconducting, determined by their chiral vector, which circumferences the tube. The electron transport in the tubes is one-dimensional (1D) and ballistic over a µm length scale.1 They have the capability of extraordinarily high current densities, up to about 109 A/cm2,2 even at room temperature. Given these unique electronic properties, many possible applications have been proposed for devices consisting of carbon nanotubes.3,4 A promising device is the tube field-effect transistor (FET), reported in 1998 from semiconducting SWNTs.5,6 High quality semiconductor material, grown by MBE, already enables semiconductor materials to be designed into molecular layer-by-layer nanostructures, to create 2D systems or band gap engineered devices.7 Epitaxially grown semiconductor materials are the foundation of advanced electronic and photonic devices, and thereby are of tremendous importance to the electronics and optoelectronics industry.8 The ability to merge conventional semiconductor techniques with nanoscale molecular electronics is likely to be essential for future integrated nanoelectronics. We find that epitaxial growth can indeed be compatible with carbon nanotube devices. By incorporating tubes in semiconductor materials, nanosize circuits with many * Corresponding author. E-mail:
[email protected] † Also at Department of Physics, DTU, Denmark. ‡ Also at MAX-lab, Lund University, Sweden. 10.1021/nl0350027 CCC: $27.50 Published on Web 01/10/2004
© 2004 American Chemical Society
potential applications may be attained. For instance, onedimensional tubes could be utilized as interconnects in conventional semiconductor devices, and tube FETs may be integrated in epitaxially grown semiconductor circuits. Carbon nanotubes have been interesting for basic research in 1D electron systems. For example Coulomb blockade (CB) has been detected at low temperatures,1 in addition measurements indicate the formation of a Luttinger liquid (LL) state in the 1D tubes.9-11 Since, 2D electron systems in GaAs based heterostructures have been important for fundamental mesoscopic physics so far,12 our work could possibly lead to new combinations of 1D SWNTs and 2D semiconductor systems that would be exciting for future basic investigations. We have fabricated integrated devices of SWNTs and MBE grown heterostructures on III-V GaAs substrates. To contact the nanotubes we use (Ga,Mn)As, which is a ferromagnetic semiconductor and has potential in the rapidly growing field of spintronics.13-15 We explore the new aspects provided by these structures, e.g., spin transport phenomena16 and magnetically contacted LL.17-19 These results will be published elsewhere.20 The tube-semiconductor devices were fabricated as schematically presented in Figure 1. First, the substrate was prepared by MBE, Figure 1a. The base was a heavily n-doped GaAs layer, serving as a back-gate. On top of this, a 100 period superlattice, which has proven to be an efficient insulating barrier,21 of 2 nm GaAs plus 2 nm AlAs ended by 20 nm GaAs was added. Finally, the wafer was capped by a layer of amorphous As, which is crucial for the later successful epitaxial overgrowth. Laser ablated SWNTs from
Figure 2. AFM images of tube-semiconductor devices. (a) A device with two (Ga,Mn)As electrodes (SC-NT-SC type). (b) A device with one metal (Cr/Au) and one (Ga,Mn)As electrodes (MNT-SC). (c) A schematic image of the SWNT encapsulated in the MBE grown semiconductor crystal.
Figure 1. Fabrication of tube-semiconductor devices. (a) The MBE grown substrate of n-doped GaAs with the superlattice barrier and amorphous As. (b) Nanotubes are dispersed on the As surface. (c) The substrate with tubes is reloaded in the MBE chamber. Here the amorphous As is evaporated, leaving the nanotubes on the clean GaAs crystal surface. (d) The tubes are overgrown by (Ga,Mn)As. (e) A trench is etched in the (Ga,Mn)As, leaving a nanotube connecting two islands of (Ga,Mn)As. (f) In a different device, a metal (Cr/Au) lead is evaporated to contact one end of a tube.
Rice University were deposited on the surface of the amorphous As, by applying a suspension of the tubes in dichloroethane while spinning the substrate, Figure 1b. After tube deposition, the sample was loaded into the MBE chamber dedicated to (Ga,Mn)As growth. To overgrow the tubes, the amorphous As was removed by desorption at a temperature of about 400 °C and the GaAs surface was kept in As flux at 500-600 °C for several minutes. This process leaves the nanotubes on the clean and molecular smooth GaAs crystal surface. Subsequently, the sample was overgrown epitaxially by the ferromagnetic semiconductor Ga0.95Mn0.05As at 250 °C. The epitaxial growth of singly crystalline, smooth (Ga,Mn)As layer was confirmed by twodimensional reflection-high energy electron diffraction (RHEED) patterns. Details of the MBE growth were reported elsewhere.22 (Ga,Mn)As films of thicknesses from 20 to 50 nm have been prepared. The (Ga,Mn)As was capped by 3 nm GaAs to prevent oxidation. To optimize the magnetic properties of the semiconductor, annealing of the (Ga,Mn)As film was performed, by keeping the sample in the MBE system at a temperature 20 °C higher than the 350
growth temperature for 0.5 to 1.5 h after the growth. The result is SWNTs encapsulated in the semiconductor, beneath the epitaxial (Ga,Mn)As film, see Figure 1d. Lithographic techniques were used to fabricate devices from the grown structures. First, UV-lithography and wet etching (using H3PO4/H2O2/H2O (1:1:38)) was employed to create mesas of (Ga,Mn)As with the encapsulated tubes. By use of electron-beam lithography, approximately 1 µm wide and 5-10 µm long trenches were etched in the (Ga,Mn)As, with the wet etchant, leaving nanotubes as connectors between separated islands of (Ga,Mn)As, Figure 1e. The trenches were etched about 20 nm deeper than the (Ga,Mn)As film, to prevent electrical short cut of the tube devices by the semiconductor base. Another type of device has been made. Here, metallic leads (5 nm Cr and 20 nm Au) defined by e-beam lithography and lift-off technique were applied next to an edge of the (Ga,Mn)As mesa. In this manner reference devices consisting of tubes contacted by one metal lead and one semiconducting lead were fabricated, Figure 1f. The tube-semiconductor devices were imaged by atomic force microscopy (AFM), as presented in Figure 2. The surface roughness of the GaAs compound semiconductor structures was around 0.5-1 nm. The tubes were visible in the trenches between the leads, Figure 2a. The height of the tubes above the bottom of the trench was typically a few nm. From this we conclude that the tubes bend in order to reach the bottom of the trenches. In Figure 2a, where the tube is covered at both ends by the epitaxially grown (Ga,Mn)As, no structure of the underlying tube is visible on the surface of the contacts. This indicates that the nanotube has been successfully incorporated in the epitaxially Nano Lett., Vol. 4, No. 2, 2004
Figure 4. (a) Conductance G vs the gate voltage Vg at 4 K for the SC-NT-SC device. (b) G vs Vg for M-NT-SC device at 300 mK. (c) Gray scale plot of the differential conductance as a function of bias voltage V and Vg (high dI/dV ) dark). Solid lines indicate a Coulomb blockade diamond structure. Figure 3. (a) The resistivity of the 40 nm (Ga,Mn)As film versus temperature. (b) A typical room temperature I-V curve of the tube device SC-NT-SC (solid line) and a trench in the (Ga,Mn)As without a tube SC-SC (dashed line). (c) The conductance versus temperature for the two devices: SC-NT-SC (solid line) and M-NT-SC (dashed line). (d) The temperature dependence for SC-NT-SC on a log-log scale. The line is a power law fit. (e) T ) 10 K, the differential conductance for SC-NT-SC as a function of the applied bias voltage on a log-log scale, together with a power-law fit (line).
grown material. Figure 2c presents schematically how the SWNT appears in the single-crystal heterostructure. Contrary, in Figure 2b one tube end is covered by a polycrystalline metal electrode of the same thickness as the (Ga,Mn)As layer, and made by thermal evaporation. Here the surface bears an imprint of the tube beneath it. Similar images of a nanotube under metal leads are often reported in the literature.23 The transport properties of bulk (Ga,Mn)As films were investigated. The resistivity as a function of the temperature for a 40 nm thick film is presented in Figure 3a. The resistivity of the film shows a local maximum, which is typical for such films.14 The maximum in the resistivity, here 70 K, provides an estimate of the Curie temperature of the magnetic semiconductor. The transport measurements of the tube-semiconductor devices were two-terminal dc measurements, as is shown schematically in Figure 1e. The bias voltage V is applied to the source, and the current I is measured from drain to ground. On the third terminal, the gate, a voltage Vg is applied. We have focused on measurements from two devices: (i) A SWNT contacted by two semiconducting leads of (Ga,Mn)As, denoted by SC-NT-SC and (ii) A SWNT with a metal (Cr/Au) source contact and (Ga,Mn)As drain, denoted M-NT-SC. These devices are chosen to represent results from a collection of 20 measured devices. The I-V characteristics of the SC-NT-SC device displayed in Figure 3b (solid) is nearly linear at room temperature with a conductance G ≈ 0.2e2/h In the same plot is shown the I-V measured on another device with no tube between the two contacts (dashed curve). Here the current is negligible (below 1 nA with 0.5 V bias) compared to device SC-NT-SC, proving that leak currents through the substrate can be neglected and that the measured transport Nano Lett., Vol. 4, No. 2, 2004
in device SC-NT-SC indeed occurs through the contacted nanotube. The present tube devices displayed no gate voltage dependence of G at room temperature (not shown), i.e., these incorporated nanotubes are metallic.1,24 The temperature dependence of G is shown in Figure 3c for the two devices SC-NT-SC and M-NT-SC. A decrease in conductance with decreasing temperature is found in all our investigated devices. The behavior of SC-NTSC is consistent with a power law, G ∝ TR with R ≈ 0.6, as presented in Figure 3d. We find similar power law-like behavior in the majority of our devices, with the scaling coefficients 0.5 e R e 1. The nonlinear I-V characteristics for SC-NT-SC, as shown in Figure 3e, can be fitted to a similar power law of dI/dV ∝ VR with R ≈ 0.7. Power lawlike characteristics have previously been found in nanotube devices.11,24,25 Here it has been attributed to the suppression of tunneling into an LL in the tubes due to strong e-e interactions in the 1D electron system.9,10 The power R is determined by the strength of the e-e interactions and the screening provided by the environment. Although the environment of our nanotubes embedded in a GaAs crystal (r ≈ 12) is different from typical devices prepared on SiO2 substrates (r ≈ 4) we find surprisingly that the scaling coefficients are comparable to such devices, see, e.g., ref 11. At low temperatures clear oscillations appeared in G vs Vg as shown in Figure 4a (SC-NT-SC at 4 K) and Figure 4b (M-NT-SC at 0.3 K). These are signs of CB,26,27 showing that a quantum dot is formed in the nanotube segment confined by the source and drain contacts.1,28,29 The CB behavior furthermore indicates that tunnel barriers are formed between the semiconductor as well as the metal contacts and the nanotube. In Figure 4c a plot of the differential conductance against V and Vg is shown for device M-NT-SC. Dashed lines indicate a CB diamond pattern from which we can deduce the electrostatic charging energy U ≈ 1.5 meV. The asymmetric diamond structure is common in this type of devices. The left sloping lines represent transition where the highest unoccupied level of the nanotube is aligned with the source, i.e., the metal contact. These diamond edges have the highest conductance (darkest) and the smallest absolute 351
slope, indicating that the metal source contact has the most efficient capacitative coupling to the tube.26,27 We find that the CB peaks are spaced by around ∆V GaAs g ) 10-20 mV in our devices with a 400 nm GaAs/AlAs dielectric between tube and gate. In previous nanotube devices on Si substrates with SiO2 gate oxide barriers of 2 similar thickness, the typical peak spacing is ∆V SiO ) 50g 100 mV.23,24,29 For any quantum dot we have ∆Vg ≈ e/Cg, where Cg is the gate capacitance which we expect to scale with the dielectric constant, Cg ∝ r. The smaller peak spacings in our devices reflect the stronger gate coupling due to the about three times higher dielectric constant for the GaAs/AlAs superlattice compared to the dielectric constant for SiO2. In all previously reported nanotube transport experiments the tubes were contacted by evaporated polycrystalline metal electrodes. We have successfully incorporated single-wall carbon nanotubes in semiconductor heterostructures by epitaxial overgrowth. We find that the semiconductor material can provide electrical contacts to the nanotubes. In our devices we observe CB at low temperature as well as indications of LL behavior, confirming that the electron transport is dominated by the properties of the embedded nanotubes. Further work is needed in order to explore routes for improving the nanotube-semiconductor contacts. We have utilized a ferromagnetic semiconductor for these experiments. In a subsequent publication we will describe in detail the effect of the spin valve properties of the devices. In this context we note that new aspects of Luttinger liquids in nanotubes may be accessed at temperatures below Tc where our contacts are magnetic.17-19 Generally, the novel fabrication technique combining MBE and nanotube devices may enable new nanoscale electronic and photonic devices, based on hybrids of epitaxially grown semiconductor structures and nanotubes. Future work may investigate the compatibility of nanotube devices and other material systems and growth methods such as Si MOVPE, which is important for industrial semiconductor manufacturing. Acknowledgment. We thank Brian Skov Sørensen, Søren Erfurt Andresen, and Jørn Bindslev Hansen for fruitful discussions, and Kasper Grove-Rasmussen for assisting with the AFM. The MBE growth was carried out in the MBE
352
systems located at MAX-lab, Lund University and III-V nanolab, operated jointly by the NBIfAPG, University of Copenhagen, and Research Center COM, the Technical University of Denmark. This work is supported by NEDO spintronics and the Danish Research Councils (STVF and SNF). References (1) Carbon Nanotubes. Synthesis, Structure, Properties, and Applications. In Topics in Applied Physics; Dresselhaus, M. S., Dresselhaus, G., Avouris, Ph., Eds.; Springer-Verlag: Berlin, 2001; Vol. 80. (2) Yao, Z.; Kane, C. L.; Dekker, C. Phys. ReV. Lett. 2000, 84, 2941. (3) McEuen, P. L.; Fuhrer, M.; Park, H. IEEE Trans. Nanotech. 2002, 1, 78. (4) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (5) Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris, Ph. Appl. Phys. Lett. 1998, 73, 2447. (6) Tans, S.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49. (7) Weisbuch, C.; Vinter, B. Quantum Semiconductor Structures, Fundamentals and Applications; Academic Press Inc.: San Diego, 1991. (8) Davies, J. The Physics of Low-dimensional Semiconductors: An Introduction, Cambridge University Press: Cambridge, 1998. (9) Kane, C. L.; Balents, L.; Fischer, M. P. A. Phys. ReV. Lett. 1997, 79, 5086. (10) Egger, R.; Gogolin, A. O. Phys. ReV. Lett. 1997, 79, 5087. (11) Bockrath, M. et al. Nature 1999, 397, 598. (12) Stormer, H. L. et al. Phys. ReV. Lett. 1983, 50, 1953. (13) Wolf, S. A. et al. Science 2001, 294, 1488. (14) Ohno, H. Science 1998, 281, 951. (15) Ohno, H. J. Magn. Magn. Mater. 1999, 200, 110. (16) Tsukagoshi, K.; Alphenaar, B. W. Nature 1999, 401, 572. (17) Balents, L.; Egger, R. Phys. ReV. Lett. 2000, 85, 3464. (18) Si, Q. Phys. ReV. Lett. 1998, 81, 3191. (19) Mehrez, H.; Taylor, J.; Guo, H.; Wang, J.; Roland, C. Phys. ReV. Lett. 2000, 84, 2682. (20) Jensen, A. et al., to be published. (21) Kawaharazuka, A.; Saku, T.; Hirayama, Y.; Horikoshi, Y. J. Appl. Phys. 2000, 87, 952. (22) Sadowski, J. et al. J. Vac. Sci. Technol. B 2000, 18, 1697. (23) Cobden, D. H.; Nygård, J. Phys. ReV. Lett. 2002, 89, 046803. (24) Nygård, J.; Cobden, D. H.; Bockrath, M.; McEuen, P. L.; Lindelof, P. E. Appl. Phys. A 1999, 69, 297. (25) Yao, Z.; Postma, H. W. C.; Balents, L.; Dekker, C. Nature 1999, 402, 273. (26) Kouwenhoven, L. P.; Scho¨n, G. Mesoscopic Electron Transport, Sohn, L. L., Kouwenhoven, L. P., Scho¨n, G., Eds.; Kluwer: Dordrecht, 1997; pp 105-214. (27) Van Houten, H.; Beenakker, C. W. J.; Staring, A. A. M. Single Charge Tunneling; Grabert, H., Devoret, M. H., Eds.; Plenum Press: New York, 1992; pp 167-216. (28) Tans, S. J. et al. Nature 1997, 386, 474. (29) Bockrath, M. et al. Science 1997, 275, 1922.
NL0350027
Nano Lett., Vol. 4, No. 2, 2004
