pubs.acs.org/NanoLett
InAs/GaSb Heterostructure Nanowires for Tunnel Field-Effect Transistors B. Mattias Borg,*,† Kimberly A. Dick,†,‡ Bahram Ganjipour,† Mats-Erik Pistol,† Lars-Erik Wernersson,†,¶ and Claes Thelander† †
Division of Solid State Physics, Lund University, Box 118, SE-221 00, Lund, Sweden, ‡ Polymer and Materials Chemistry, Lund University, Box 124, SE-221 00, Lund, Sweden, and ¶ Electrical and Information Technology, Lund University, Box 118, SE-221 00, Lund, Sweden ABSTRACT InAs/GaSb nanowire heterostructures with thin GaInAs inserts were grown by MOVPE and characterized by electrical measurements and transmission electron microscopy. Down-scaling of the insert thickness was limited because of an observed sensitivity of GaSb nanowire growth to the presence of In. By employing growth interrupts in between the InAs and GaInAs growth steps it was possible to reach an insert thickness down to 25 nm. Two-terminal devices show a diode behavior, where temperaturedependent measurements indicate a heterostructure barrier height of 0.5 eV, which is identified as the valence band offset between the InAs and GaSb. Three-terminal transistor structures with a top-gate positioned at the heterointerface show clear indications of band-to-band tunnelling. KEYWORDS Nanowire, heterostructure, InAs, GaSb, TFET, tunneling
T
barrier in this junction, in order to suppress the parasitic thermionic current that competes with the tunneling current. This scheme has been successfully used in zero-bias InAs/AlSb/ GaSb tunnel diodes for terahertz detection.9,10 One dimensional (1D) nanowires allow for optimal electrostatic coupling in structures with a wrap-gate11 that may be formed directly at the tunnel junction. Additionally, 1D nanowires may be crucial for obtaining low off-state currents in TFETs, because of a suppression of the density of states and a reduced carrier scattering.7 A reduced effective screening in 1D nanowires is also believed to increase the tunnel probability, allowing for a higher on-current.12 Thus, the realization of InAs/GaSb nanowire heterostructures is of high interest. From a growth point of view, however, this heterostructure is challenging due to a change of both anion and cation atoms at the heterojunction.13 In particular, anion switching in Au-nucleated nanowire growth is known to be nontrivial due to the high solubility of the anion species in Au and can lead to nanowire kinking14,15 or a compositional grading at the interface.16,17 In this work, InAs/GaSb nanowire heterostructures are realized with a thin barrier insert nominally consisting of GaAs in between the InAs and GaSb segments. Conceptually, this barrier insert is beneficial as it can suppress thermionic currents. The direct growth of GaSb on InAs nanowires is also investigated and the fundamental challenges are discussed. Structural and electrical characterization of InAs/ GaSb nanowire heterostructures with GaInAs inserts are presented, together with calculations of the strain-dependent band structure. The nanowires were grown in a commercial metalorganic vapor phase epitaxy system. Before growth, Au aerosol nanoparticles with 40 nm diameter were deposited on epi-
unnel field-effect transistors (TFETs) have emerged as one of the top candidates for switching applications in future low-power electronic circuits. The benefit of these devices is the possibility to obtain a subthreshold swing below what is achievable with conventional transistors (80% GaSb yield is shown in Table 1. Using a high Tpause allowed for growing thin inserts while maintaining a high GaSb yield. However, the thinnest inserts, 25 nm, were here obtained using Tpause ) 300 °C with a remarkably high yield >90% (Supporting Information). Nanowires from the sample with a growth interrupt at 300 °C are shown in Figure 1A, and were investigated in detail with HRTEM and XEDS. An XEDS line scan across the heterojunction is shown in Figure 2A. The In and Ga content of this insert has a close to linear grading from InAs to almost pure GaAs, while the GaSb segment contains roughly 5 at. © 2010 American Chemical Society
FIGURE 1. (A) SEM image at 30° tilt of InAs/GaSb nanowires with 25 nm insert, nominally consisting of GaAs, in between the InAs and GaSb segments. The GaSb segment on the top has a larger diameter than the other segments. (B) Illustration of the growth sequence for InAs/GaSb nanowires, including a growth interrupt and GaAs barrier segment. The two types of growth interrupts are indicated: annealing at Tpause (black line) and cool-down to 300 °C (gray line). TABLE 1. Influence of Tpause on the Minimum Insert Thickness while Maintaining >80% GaSb Yield Tpause (°C)
Min. tinsert with >80% yield (nm)
300 450 460 490
25 375 70 45
% In. Since the In content is uniform throughout the whole GaSb nanowire, the In most likely comes from decomposition of the InAs substrate during the GaSb growth step. It should also be noted that the Sb L-R line scan exhibits a false Sb signal in the InAs segment, originating from an overlap between the Sb L-R peak and other In L-peaks. Quantitative measurements using several Sb-related peaks show no indication of Sb inside the InAs nanowires. A continuous strain field is visible in HRTEM images at the InAs/GaInAs interface, indicating that the graded interface is defect-free (Figure 2C,D). On the other hand, the 4081
DOI: 10.1021/nl102145h | Nano Lett. 2010, 10, 4080-–4085
FIGURE 2. (A) STEM image of the optimized InAs/insert/GaSb heterostructure with XEDS revealing that the barrier insert is graded Ga1-xInxAs with x ) 1.0 near the InAs, and x ≈ 0 near the GaSb segment. There is also a significant 5% In background in the GaSb segment. (B) Quantitative point XEDS analysis of the seed particle composition after growth. (C,D) HRTEM images of the heterointerface with FFTs of various parts of the image, showing that the InAs and GaInAs segment consist of a mixture of zinc blende and wurtzite, while the Ga(In)Sb is defect-free zinc blende.
GaInAs/Ga(In)Sb interface has a less uniform strain field, indicating the presence of misfit dislocations. In Figure 2D, a HRTEM image is shown together with the fast Fourier transforms (FFTs) from parts of the image. The InAs segment is of wurtzite crystal structure, while the GaSb is zinc blende without stacking faults. The crystal structure of the Ga1-xInxAs barrier is a mixture of wurtzite, and zinc blende. The change of lattice parameter in the Ga1-xInxAs segment is visible in the FFT as a smearing of the reciprocal space points along the growth direction. To understand the connection between the GaSb yield and the thickness of the inserts, the direct growth of GaSb on InAs was investigated. In Figure 3A, a SEM image is shown of a sample where InAs nanowires were first grown, followed by 20 min of GaSb growth. Similar to nanowires with too-thin GaAs inserts, these nanowires have a uniform diameter, and no GaSb is found by SEM inspection on the top of the nanowires. Instead, STEM/XEDS inspection reveals an unintentionally grown 5-10 nm long Ga1-xInxAs segment with x > 0.3, beneath the seed particle (Figure 3C). The growth temperature and precursor molar fraction for the GaSb growth were extensively varied during the course of these experiments, but regardless of the growth parameters it was not possible to grow GaSb directly on InAs nanowires. An explanation for the lack of GaSb growth directly on top of InAs nanowires is found by considering the composition of the seed particle. Measurements of the seed particle composition after InAs growth show that the ex situ composition of the seed particle is normally Au/In with 30-40% In (Figure 3B). For the enlarged and faceted seed particles © 2010 American Chemical Society
FIGURE 3. (A) SEM image taken at 30° tilt of InAs nanowires followed by 20 min GaSb growth. The insert clearly shows the faceted particle on top of these nanowires. (B,C) STEM images and XEDS point analysis of the seed particle (B) after InAs growth and (C) after InAs/ GaSb growth. The arrow indicates the start of an unintentionally grown Ga1-xInxAs segment formed during the GaSb growth step. (D) Concentrations of Au, In, and Ga relative to the Au concentration in the seed particle during the InAs growth step and GaSb growth step. The dashed line indicates the time for introduction of Sb into the reactor.
on the InAs nanowires after GaSb growth, we observed a similar In concentration (40%), while the Ga concentration is only 20% (Figure 3C). We believe that the relatively high In concentration after InAs growth prevents Ga from entering the seed particle. The results of several groups suggest that the thermodynamic affinity of In in Au may be higher 4082
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FIGURE 4. (A) Calculated band structure of the InAs/GaSb nanowire heterostructure, including the effect of strain on the valence bands (red and blue), and Γ conduction band (black). (B) I-V characteristics of two-terminal InAs/insert/GaSb nanowire diodes at 200, 245, and 294 K, where the applied voltage was on the GaSb side. (C) Activation energy, EA, as acquired from Arrhenius plots, as function of applied bias. The red line shows an extrapolation to zero bias to acquire the barrier height.
than for Ga in Au.16,17 Very little is known about the Au-Ga-In ternary phase diagram,19 but from experimental observations it can be assumed that there is a tendency toward In-rich alloys, rather than Ga-rich. This assumption is supported by the experimentally measured composition of seed particles on GaInAs nanowires,20 where the particle was found to contain only Au and In after growth, regardless of the Ga content of the nanowire. For the nanowires in this work, assuming that the amount of Au in the particle is constant during the whole growth run, the absolute In content has actually increased by roughly 50% during the GaSb growth step (Figure 3C). This is remarkable, considering that the seed particle has only been exposed to Ga and Sb precursors during this time. The increase in In concentration could be caused by the introduction of Sb into the reactor, which has previously been shown to increase the In content of seed particles during the growth of InSb nanowires.21,22 To this end, Sb could either act as a surfactant, or simply change the favorable equilibrium composition of the Au/In/Ga/Sb alloy toward a more In-rich compound. In a previous work, we have shown that GaSb nanowire growth occurs along either the AuGa-GaSb or the AuGa2-GaSb pseudobinary tie-lines in the Au-Ga-Sb ternary system.16 This indicates that a high Ga concentration is required to enable GaSb nanowire growth. The seed particles on the nanowires with successful GaSb growth (Figure 2B) consist of roughly 20% In, 35-40% Ga, and 40-45% Au. This is a further indication that the Ga/Au-ratio of the seed particle composition needs to be close to 1 or above for GaSb nanowires to grow. We thus propose that GaSb nanowire growth on InAs nanowires is inhibited because of a too-low Ga concentration in the seed particle. This is attributed to the significant presence of In in the seed particle. The effect might also be further pronounced by the increased In content upon Sb introduction into the reactor. In contrast to GaSb growth, GaAs nanowire growth is possible despite the presence of In, because GaAs formation does not require a high Ga concentration in the seed particle. Thus, during the GaAs insert growth step, the In collected in © 2010 American Chemical Society
the seed particle is incorporated into the insert, resulting in a purification of the seed particle. Assuming that the inserts are thick enough to sufficiently reduce the In concentration, GaSb nanowires can be grown. By employing growth interrupts between the InAs and GaAs growth steps, the In concentration prior to GaAs growth was further reduced, thus enabling GaSb growth on InAs/GaInAs stems with very thin GaInAs segments. The use of a high AsH3 flow during the growth interrupts may explain why new In does not alloy with the particle as the temperature is ramped back to 450 °C, prior to the GaAs growth step. Having obtained control and understanding of the GaSb growth on InAs nanowires, we now turn to electrical characterization of the heterostructure. The conduction band of InAs lies below the valence bands of GaSb, forming a broken type-II interface which enables band-to-band current transport across the heterointerface.23 However, in the devices studied here, the graded Ga1-xInxAs barrier is assumed to block this tunnel current at low bias conditions. The band structure of the nanowires was modeled in two steps. First, the strain tensor elements were computed using linear elasticity theory. The strain energy minimization was performed on a 120 × 120 × 120 cubic grid. The local band edges were then computed using an eight band model with the strain tensor elements as input24 with parameters from Vurgaftman et al.25 Figure 4A shows the local band edges through the center of the wire. The graded Ga1-xInxAs segment was modeled as five segments with x ) 1, 0.70, 0.4, 0.20, and 0.02, respectively. Because of the large difference in lattice constant between the Ga1-xInxAs barrier and the InAs and Ga(In)Sb, the band structure at the Ga1-xInxAs/ Ga(In)Sb heterojunction is strongly modified, compared with the unstrained band structure. As is seen in Figure 4A, the model shows that the graded Ga1-xInxAs segment forms a shallow barrier with a band gap that approaches 0.9 eV near the Ga1-xInxAs/Ga(In)Sb interface. The model also suggests that there is no additional barrier for hole transport imposed by the strained Ga1-xInxAs insert, giving a valence band barrier height of 0.51 eV. This is because the valence bands are split (red and blue) by the large strain, lifting up the 4083
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effective valence band edge near the Ga1-xInxAs/Ga(In)Sb interface. Numerically solving the Poisson equation26 without the inclusion of strain, gives a higher value (0.77 eV) of the valence band barrier height. InAs/Ga(In)Sb nanowires with 25 nm graded Ga1-xInxAs inserts were broken off onto Si/SiO2 substrates and processed into diodes and transistors. Contacts to the GaSb sections were fabricated by electron beam lithography (EBL), followed by O2 plasma, a 1:1 HCl/H2O etch for 30s, evaporation of 25 nm Ni and 75 nm Au, and lift-off in acetone. The same procedure was repeated for the contacts to the InAs, however, using 1:9 (NH4)2Sx/H2O at 40 °C for 90s as contact etch before metal evaporation. InAs nanowires show n-type conductivity, which is explained by Fermi-level pinning in the conduction band at the surface and unintentional carbon doping.27 The unintentionally doped GaSb segments show p-type conductivity28 with a resistivity for bare GaSb nanowires in vacuum of roughly 1 Ωcm, about 100 times higher than for typical InAs nanowires. The I-V characteristics of two-terminal devices exhibit an exponential behavior (Figure 4B), which is typical for thermally activated transport across a p-n junction. I-V measurements were performed at temperatures between 200 and 294 K, and Arrhenius plots were made for each measured bias point. From the slope in these diagrams, the activation energy for the current transport was estimated and is shown in Figure 4C as a function of applied bias (V). The activation energy, EA, decreases linearly with increasing V, as expected. Extrapolation of the data to V ) 0 V provides an estimation of the barrier height of 0.5 eV for transport across this junction, which is in agreement with the valence band offset obtained from the band structure modeling. This indicates that the dominant transport mechanism in these two-terminal devices is thermionic emission of holes from the GaSb to the InAs. The data further supports the argument that strain plays an important role for the band structure of these heterostructures. Three-terminal transistor structures based on nanowires from the same growth were also fabricated and characterized. Source contacts to the GaSb sections were fabricated first, followed by deposition of a thin HfO2 gate oxide onto the nanowires, where the drain sections of the InAs segments were left uncovered. This was achieved by EBL exposure, atomic layer deposition of 10 nm HfO2 at 100 °C,29 and lift-off.30 Finally, gate and drain contacts were fabricated simultaneously using the same conditions as described above. As the heterojunction is visible in SEM due to the clear change in diameter, (Figure 5A) it was possible to very accurately ( 0.
the three-terminal devices, a forward bias of the p-n junction is achieved by applying negative drain voltage (VD) to the InAs segment. Figure 5D shows that for positive gate voltages (VG) the asymmetry of the I-V is increased. In contrast, for a sufficiently large negative VG ) -0.5 V, an abrupt transition is observed at which point the junction shows almost linear I-V in the range -100 mV < VD < 100 mV. This is the expected behavior of a device where band-toband tunneling is possible between the InAs conduction band and the GaSb valence band. This transport channel was previously blocked when the GaSb valence band edge was aligned within the band gap of the GaInAs segment. The I-V also appears to be more noisy, which could be due to a sensitivity of the tunneling current to discrete impurity levels in the heterojunction. For positive VD, (Figure 5D,E) the transport shows the expected gate dependence. A higher VD is here required for onset of transport as VG is made more positive, again in agreement with the expected behavior for band-to-band tunneling. The observed on-current saturation may imply that the transport also in this case is mediated by discrete levels in the heterojunction. The maximum current density for positive VD, normalized to the gate width in this device, is 0.02 µA/µm, which is very similar to values reported for tunnel FETs based on Si nanowire p-i-n junctions.2 We believe that the on-state performance of the device may be improved substantially by increasing the hole concentration in the GaSb segment, which was not intentionally doped in 4084
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this study. A reduction of the barrier thickness would also allow for tunneling at lower bias- and gate-voltages, and at the same time decrease the resistance of the tunnel junction. Moreover, we note that to achieve a low inverse subthreshold swing it is critical to control the density of interface states at the nanowire/high-k dielectric interface. This will be increasingly important as the diameter of the devices is scaled down. Although the first reports on capacitance measurements on III-V nanowires have been published,31 further optimization of the high-k deposition procedure is required to improve the off-state performance, here limited to a subthreshold swing of 350 mV/dec at VD ) 100 mV. In conclusion, Au-nucleated InAs/GaSb nanowire heterostructures with GaInAs inserts were realized by MOVPE. The minimum thickness of the inserts was limited by the observed sensitivity of the GaSb nanowire growth to the presence of In. The heterostructure was improved using growth interrupts between the InAs and GaAs insert growth steps, enabling growth of InAs/GaSb heterostructures with only 25 nm thick graded GaInAs inserts. The activation energy for transport in this heterostructure was measured to be 0.5 eV, in good agreement with a model of the valence band offset that includes strain. Three terminal transistor devices exhibited normal p-n diode properties at zero or positive VG. In contrast, a linear current is observed at low bias with VG ) -0.5 V, indicating band-to-band tunnelling. This is promising for future development of high-performance heterostructure-based TFETs.
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Acknowledgment. This work was conducted with financial support from the Swedish Research Council (VR), the Swedish Foundation for Strategic Research (SSF), The Nanometer Consortium at Lund University (nmC@LU), and the Knut and Alice Wallenberg Foundation (KAW).
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Supporting Information Available. Detailed data on the growth interrupt experiments. This material is available free of charge via the Internet at http://pubs.acs.org.
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