pubs.acs.org/NanoLett
High-Performance Single Nanowire Tunnel Diodes Jesper Wallentin,*,† Johan M. Persson,‡ Jakob B. Wagner,‡ Lars Samuelson,† Knut Deppert,† and Magnus T. Borgstro¨m† †
Solid State Physics, Lund University, Box 118, S-221 00, Lund, Sweden, and ‡ Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark ABSTRACT We demonstrate single nanowire tunnel diodes with room temperature peak current densities of up to 329 A/cm2. Despite the large surface to volume ratio of the type-II InP-GaAs axial heterostructure nanowires, we measure peak to valley current ratios (PVCR) of up to 8.2 at room temperature and 27.6 at liquid helium temperature. These sub-100-nm-diameter structures are promising components for solar cells as well as electronic applications. KEYWORDS Nanowire, MOVPE, tunnel diode, esaki diode, solar cell, heterostructure
H
alf a century since its invention by Leo Esaki,1 the tunnel diode is receiving continued interest. Tunnel diodes are a key component of multijunction solar cells, connecting p-n junctions of different band gap materials. To optimize the harvesting of solar energy, multiple junctions with different band gaps can be combined to match the solar spectrum,2 but lattice matching requirements severely limit the materials available for devices based on thin film growth. The nanowire geometry allows for radial strain relaxation, permitting a much broader range of materials combinations.3,4 Thus, nanowires could potentially be used for high-performance solar cells produced on low-cost silicon substrates,5 and a dual homojunction photovoltaic device has already been demonstrated.6 We report on the growth and characterization of high-performance III-V single-nanowire tunnel diodes, a key element of future multijunction nanowire solar cells. Tunnel diodes have also found promising applications in low-power memory cells, so-called tunneling SRAMs (TSRAM),7 in latches monolithically integrated with a standard CMOS process,8 as well as in low-power tunnel FETs.9 For integration with mainstream electronics, devices should have diameters comparable to MOSFET gate lengths. We show in this paper that a bottom-up process based on epitaxial growth of nanowires can create sub-100-nm tunnel diodes with very good properties. This bottom-up approach is scalable to even smaller sizes, and it avoids the defects often induced in top-down processes based on etching. In order to increase the desired interband tunneling current, a heterostructure with a type-II band alignment can be used to reduce the tunneling barrier.10 Additionally, heterostructures or δ-doping layers forming quantum wells at one or both sides of the junction may be utilized, creating
a so-called resonant interband tunnel diode (RITD).11 We have taken advantage of the heterostructure design freedom in nanowires and created a type-II InP-GaAs axial heterostructure, a materials combination unsuitable for thin film growth due to the 3.8% lattice mismatch. InP and GaAs nanowires can be degenerately n-doped and p-doped, respectively, and this materials combination has been previously grown in nanowires.12 Although InP and GaAs have similar band gaps, making this particular combination unsuitable for a tandem solar cell, our design opens up use of the InAsxP1-x-GaAsyP1-y materials system which can be tuned throughout most of the solar spectrum. The nonoptimized tunnel diodes show peak current densities similar to, and in some devices far exceeding, those in state of the art solar cells. Samples were prepared for nanowire growth by depositing 80 nm Au particles with an aerosol technique,13 at a density of 0.2 µm-2, on InP (111)B substrates. The nanowires were grown in a low-pressure (100 mbar) metal organic vapor phase epitaxy (MOVPE) system with a total flow of 6 L/min. For InP growth, trimethylindium (TMI) and phosphine (PH3) were used as precursors, with constant molar fractions of χTMI ) 3.5 × 10-6 and χPH3 ) 6.25 × 10-3. Hydrogen sulfide (H2S) was added as n-type doping precursor,14 at a molar fraction of χH2S ) 7.1 × 10-6. For the GaAs section, trimethylgallium (TMG) and arsine (AsH3) were used at molar fractions of χTMG ) 5.2 × 10-6 and χAsH3 ) 5.5 × 10-4, using diethyl zinc (DEZn) as a p-dopant precursor,15 χDEZn ) 2.9 × 10-7. Hydrogen chloride (HCl) at a molar fraction of χHCl ) 8.3 × 10-6 was used to control the radial growth.16 To desorb any surface oxides, the samples were first annealed at 550 °C for 10 min under a PH3/H2 gas mixture. The reactor was then cooled to 420 °C, at which temperature growth was initiated by adding TMI. After a 15 s nucleation time, HCl and H2S were added, after which InP growth was continued for 10 min. GaAs growth was initiated
* To whom correspondence should be addressed,
[email protected]. Received for review: 11/26/2009 Published on Web: 02/17/2010 © 2010 American Chemical Society
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FIGURE 1. (A) SEM image of a nanowire as-grown on substrate, tilted 45°. Scale bar 0.5 µm. (B) SEM image of a tunnel diode device. Scale bar 1 µm. In both images the InP-GaAs heterojunction is indicated by an arrow.
FIGURE 2. HRTEM of a nanowire heterojunction, indicated by an arrow, with InP on the left and GaAs on the right. Insets show Fourier transforms of the InP (left) and GaAs (right) sections aquired some distance away from the junction, showing that the InP section is wurtzite structure and the GaAs is of the zinc blende structure. Scale bars are 20 nm and 5 nm-1, respectively.
by simultaneously switching group III, group V, and dopant precursors, and after 10 min growth was interrupted and the sample was cooled down in a PH3/H2 gas mixture. The growth resulted in nanowires about 3.4 µm long, with roughly equally long InP and GaAs sections. A scanning electron microscopy (SEM) image of an as-grown nanowire is shown in Figure 1A. To be able to identify the InP section by optical microscopy, we intentionally grew the first 5 min of the InP section tapered by using a lower HCl molar fraction. As can be seen in Figure 1A, the intentional tapering renders the lower part of the InP slightly conical-shaped, which in an optical microscope creates a characteristic rainbow spectrum due to interference. Using this technique, we correctly oriented close to 100% of the contacts. For electrical measurements nanowires were mechanically transferred to a degenerately doped silicon substrate with a 100 nm thick oxide. The position of the nanowires was measured with an optical microscope, and after resist deposition contacts were created to selected nanowires by electron beam lithography (EBL) and metal lift off. The GaAs p-sections were etched with a HCl:H2O 1:1 solution for 10 s, followed by a 2 min surface passivation in a NH4Sx solution. A Pd/Zn/Pd (10/10/80 nm) metal combination was used, known to give ohmic contacts to bulk p-type GaAs.17 After another EBL step, the InP n-sections were etched with H3PO4:H2O 1:9 for 2 min and then with H2SO4:H2O 1:3 for 1 min and finally passivated with NH4Sx for 10 min. A Ti/Pd (15/85 nm) metal combination was used for the n-section contacts. One device is displayed in Figure 1B. Transmission electron microscopy (TEM) analysis was performed in an FEI Titan 80-300 ETEM. Samples were © 2010 American Chemical Society
prepared by gently scraping part of the InP substrate with a standard TEM lacey carbon Cu grid. High-resolution TEM (HRTEM) corrected for objective lens spherical aberration, together with diffractogram analysis, was used to analyze crystal structure and morphology, and high angle annular dark field scanning TEM (HAADF-STEM) together with energy dispersive X-ray spectroscopy (EDX) was used for chemical analysis. Figure 2 shows a lattice fringe TEM image of an InP-GaAs heterojunction. The InP nanowire segments are almost defect-free wurtzite crystals, similar to previous observations of H2S-doped InP nanowires.18 The GaAs segments, on the other hand, are pure zinc blende with twinning layers. Algra et al.19 showed that DEZn induces zinc blende growth in InP nanowires, and we speculate that a similar effect occurs for the GaAs segments studied here. EDX line scans, shown in Figure 3, indicate a quite sharp transition of the group V elements, i.e., from P to As, on the order of 20 nm, followed by about 10 nm of InAs. Thereafter the line scans consistently display a transition region of about 40 nm in length in which the group III composition gradually changes from In to Ga. Thus, the nanowire composition at the heterojunction is complex, changing from InP to almost pure InAs, then via InGaAs to pure GaAs. The EDX measurements are consistent with the bright region in the HAADF image, since InAs has higher Z-contrast than both InP and GaAs. Phosphorus and arsenic have low solubility in gold, and therefore they will probably not be stored in the metal particle. The axial junction sharpness for these elements is expected to 975
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FIGURE 4. Ideal energy band diagram of the nanowire heterostructure, assuming a materials composition as in Figure 3. We have assumed a free electron concentration of 3 × 1018 cm-3 in InP and the transition region and a free hole concentration of 1 × 1019 cm-3 in GaAs.
of the wire, indicating that any radial growth is below the EDX detection limit of 2%. A total of 26 devices were electrically characterized at room temperature, of which 10 devices were measured at seven different temperatures using a liquid helium tank and a thermocouple. Measurements of the InP and GaAs sections separately showed ohmic contacts and metallic behavior, i.e., no significant effect on I-V characteristics when sweeping the back gate. The total device resistance including contacts was 10-20 kΩ for the InP part and 5-10 kΩ for the GaAs part, but the measurements failed to quantify the doping levels in the nanowires. Using a simple Drude model, assuming previously reported nanowire mobilities of 150 cm2 V-1 s-1 for InP22 and 90 cm2 V-1 s-1 for GaAs,15 we estimate the carrier concentrations to be n ≈ 3 × 1018 cm-3 in InP and p ≈ 1 × 1019 cm-3 in GaAs. As the small-signal resistance of the tunnel diodes was at least 2 MΩ, the contact resistance has been ignored. A schematic ideal band diagram based on the materials composition measured by EDX and the carrier concentrations as above is shown in Figure 4. The carrier concentration in the transition region was assumed to be equal to the InP section. The band diagram was created using a Poisson solver23 with band offsets as reported by Pistol and Pryor.24 The transition region creates a well below the InP conduction band edge, which enhances tunneling and gives a band structure resembling that of an RITD. InAs has a lower electron effective mass than InP and GaAs, which further promotes tunneling. The current-voltage characteristics of a typical device, at selected temperatures, are shown in Figure 5. The measurements display a clear tunnel diode behavior with the characteristic negative differential resistance (NDR) region. The current in tunnel diodes can be divided into three components: (i) interband tunneling, electron tunneling directly from the n-side conduction band to the p-side valence band dominates the current at small biases (with increasing bias the overlap of the densities of states de-
FIGURE 3. (A) HAADF-STEM image of a nanowire heterojunction, scale bar 20 nm, and EDX line scans of (B) P and As and (C) In and Ga. The bright contrast in region II in (A) is consistent with the high Z-density of InAs, although additional diffraction contrast is visible as well.
be determined by memory effects in the reactor.20 The group III elements on the other hand, alloy with the gold in the particle. We assume that as the TMI and TMG sources are switched, some indium will still be supplied for nanowire growth from the particle and possibly by diffusion from the surfaces of the nanowire and the substrate. At the same time, there will be some delay before the metal alloy has reached its steady-state composition of gallium. The dopant levels were below the detection limits for chemical analysis with EDX. We expect most of the transition region to be n-type, since it is difficult to achieve p-type doping in InAs nanowires due to surface Fermi level pinning in the conduction band.21 Zinc is known to diffuse rapidly in III-V semiconductors, but the device performance discussed below indicates a sharp doping profile. The HRTEM analysis consistently revealed some crystal defects in the transition region, while no such defects were found in the InP and GaAs sections. Note that the transition region increases the strain, as compared to a pure InP-GaAs heterojunction, since InAs has a larger lattice constant (6.06 Å) than both InP (5.87 Å) and GaAs (5.65 Å). No Ga or As was detected in the InP part © 2010 American Chemical Society
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Our measurements showed a large distribution in the peak current between devices, from 0.1 to 21.4 nA, as well as valley currents, from 0.05 to 12.6 nA. The peak and valley currents were strongly correlated, thus limiting the range of measured peak-to-valley current ratios (PVCR) to between 1 and 8.2. As the direct tunneling and excess currents are both exponentially dependent on the effective barrier thickness,26 the correlation indicates that the large spreads in valley current and peak current are due to a variation in the effective barrier thickness between the different nanowires. Interband tunneling is traditionally seen as a direct tunneling mechanism, but recent publications indicate that the peak current is in fact dominated by a resonant tunneling through defects such as oxygen.27 Since the excess current also relies on defects, another (not mutually excluding) explanation for the large variations in peak and valley currents is therefore a variation in defect density. In contrast to the variations in peak current, the peak voltage was quite stable between devices, showing a variation of about 10%. The peak voltage was also stable as the temperature was changed. These results are not surprising since the peak voltage is mainly determined by the positions of the Fermi levels at the p-side and the n-side far away from the junction, and it is insensitive to the effective barrier thickness and defect density.28 The stability of the peak voltage as a function of current density confirms that the I-V characteristics are not significantly affected by the contact resistance, as any serial resistance would result in a shift in the peak voltage. For use in multijunction solar cells, a high peak current density and a low small-signal resistance are the key performance indicators. The typical peak current density of 15 A/cm2 in our devices is similar to tunnel diodes of state of the art solar cells,29 while the highest value we measured was 329 A/cm2 at room temperature and 386 A/cm2 at 4.5 K. The small-signal resistance was inversely proportional to the peak current density, and we measured typical smallsignal resistances of about 50 MΩ and a best value of 2.4 MΩ. The reverse-biased region may be utilized to get a high current density for electronic applications such as tunnel FETs. We observed strong interband tunneling and very little temperature dependence in this region, as shown in Figure 5. One typical device was purposively exposed to high reverse bias to find the breakdown current density, which was measured to 106 A/cm2 before the device was destroyed. A lowering of the temperature decreased the valley current, as shown in Figure 5. Since the peak currents increased slightly, despite the increasing band gaps, the PVCR for this device increased from 2.6 at room temperature to 23.7 at liquid helium temperature. The highest PVCR was 8.2 at room temperature and 27.6 at liquid helium temperature, for the same device. Nanowires have a large surface to volume ratio, and surfaces are associated with
FIGURE 5. (A) Absolute current versus bias for a nanowire device, at selected temperatures. (B) Current versus temperature for the same device, at selected biases: peak current (0.1 V), excess current at onset (0.5 V) and at high bias (0.6 V). The lines are guides for the eye.
creases, which leads to NDR); (ii) excess current, electron tunneling via midgap states dominates as the interband tunneling is inhibited; (iii) thermal diffusion current, the tunnel diode may function as a normal forward-biased p-n junction at high bias, unless the excess current dominates. The NDR region is often observed as a discontinuity in I-V measurements of tunnel diodes,25 but the relatively high resistance of our small devices results in a continuous curve in both forward and reverse sweep directions. © 2010 American Chemical Society
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Acknowledgment. This work was performed within the Nanometer Structure Consortium at Lund University and supported by the Swedish Energy Agency, the Swedish Research Council, the Swedish Foundation for Strategic Research, and by the EU program AMON-RA (214814). This report is based on a project which was funded by E.ON AG as part of the E.ON International Research Initiative. Responsibility for the content of this publication lies with the authors. The authors would like to thank Claes Thelander and Mats-Erik Pistol for fruitful discussions, as well as Henrik Nilsson and Kristian Nilsson for assistance with measurements.
states that could contribute to the excess current. The low excess currents and high PVCR measured here show that the effect of nanowire surfaces on electron transport is limited. As any surface-mediated excess currents are not expected to correlate with the interband tunneling currents, the observed strong correlation between peak currents and excess currents also indicate a limited contribution from the surface. The temperature dependence of the current at three biases is shown in Figure 5B. The excess current is clearly divided into two parts, the onset and the high-bias current. The onset around 0.5 V increases sharply with an exponential dependence on bias, and the current in this small region is proportional to exp(V/kT) for all temperatures. At higher bias the excess current is exponentially dependent on the bias V, and the slope is independent of temperature. This demonstrates that the high-bias excess current is not a thermal diffusion current, something which has been previously observed in homojunction tunnel diodes.26 Note that the type II band alignment in our devices increases the barrier for thermal diffusion current. The current is proportional to exp(V/c), where the constant c is stable versus temperature at around 55 meV for this device. All devices displayed values in the range 50-70 meV. Although the slope of the high-bias excess current is temperature independent, the current at a fixed bias (in Figure 5B represented by 0.6 V) is exponentially dependent on temperature. The mechanism of the excess current requires more detailed studies to be well understood.
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(9) (10) (11) (12)
At liquid helium temperature, the excess current displayed structure in many devices. Around the valley, small plateaus and NDR regions were often observed, and each device showed a unique profile. Since our large band gap devices are so small, individual defects and states could contribute measurably to the excess current. Although any detailed investigation of these states is beyond the scope of this Letter, this demonstrates the potential for using our design for electron transport physics and materials science.
(13) (14)
(15) (16)
In conclusion, we have demonstrated single nanowire tunnel diodes with high performance. Our nonoptimized devices display peak current densities of up to 329 A/cm2, and we intend to exploit this design for nanowire-based multijunction solar cells in the InAsP-GaAsP materials system. Despite the large surface to volume ratio of these devices, we have measured PVCR as high as 8.2 at room temperature, and our design allows scaling to significantly smaller diameters. Together with the very high current densities measured in the reverse direction, these are promising results for low-power electronic applications. The lowtemperature measurements indicate that nanowire-based tunnel diodes could also be used for electron transport physics investigations. © 2010 American Chemical Society
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