High-density integration of carbon nanotubes by chemical self-assembly Hongsik Park, Ali Afzali, Shu-Jen Han, George S. Tulevski, Aaron D. Franklin, Jerry Tersoff, James B. Hannon and Wilfried Haensch 1. X-ray photoelectron spectroscopy measurement on the monolayer 2. Effect of dialysis on the nanotube deposition 3. The number of tubes per transistor channel estimated from images of fabricated nanotube transistors 4. Estimation of the yield of single-tube devices from numerical simulation 5. Yield of electrically connected devices and semiconducting devices 6. Effect of the thermal treatment on the device performance 7. Procedure of the measurement and hysteresis curves 8. Underestimated on/off ratio of the drain currents in the semi-automated measurement NATURE NANOTECHNOLOGY www.nature.com/naturenanotechnology 1
1. X-ray photoelectron spectroscopy measurement on the monolayer X-ray photoelectron spectroscopy (XPS) measurements were performed in order to verify that the ionexchange reaction illustrated in Fig. 1a takes place. A Spectrum recorded from a 10 nm HfO 2 exposed to NMPI is shown in blue in Fig. S1. The red plot shows the spectrum from a nominally identical sample after exposure to 1% solution of SDS in water for one hour, followed by rinsing with water. Before exposure to SDS, the iodine 3d peaks are clearly visible at binding energies of 620 and 632 ev. These spectral features suggest that NMPI is present at the HfO 2 surface. After exposure to the SDS solution and rinsing, the iodine core levels are absent, while the broad nitrogen feature remains. The absence of iodine suggests that the ion-exchange reaction has occurred. Figure S1. XPS spectra recorded from an NMPI coated HfO 2 surface before and after exposure to a surfactant solution, which correspond to iodine. 2. Effect of dialysis on the nanotube deposition In this work, purified single-walled carbon nanotubes were dispersed in 1% w/v solution of SDS in deionized (DI) water after purification and enrichment of semiconducting nanotubes. Since the placement method is based on the coulombic bonding between the negatively charged surfactant and the positively charged monolayer, as illustrated in Fig. 1a, it is essential to remove excess surfactants in the solution via dialysis. Figure S2(a) shows high-density nanotubes selectively deposited on an HfO 2 region from a dialyzed nanotube solution. Figure S2(b) shows the same NMPI-coated substrate, which was placed in a nanotube solution without removing excess surfactant by dialysis. This non-dialyzed solution yields no nanotube binding, presumably due to the surface sites being bound to free surfactant as expected by the suggested mechanism (Fig. 1a) and XPS result (Fig. S1). 2 NATURE NANOTECHNOLOGY www.nature.com/naturenanotechnology
SUPPLEMENTARY INFORMATION Figure S2. (a) High-density nanotubes selectively deposited on a HfO 2 area. (b) No nanotubes were deposited when a nanotube solution containing a large excess of surfactants was used. 3. The number of tubes per transistor channel estimated from images of fabricated nanotube transistors To estimate the number of nanotubes per trench, we fabricated transistors on on HfO 2 trenches of various widths. Two different concentrations of solution were used for nanotube deposition. Figure S3(a) shows SEM images of the channel regions of two devices, one with a single nanotube and the other with two nanotubes spanning the source and drain electrodes. The number of nanotubes in the channel regions was readily counted from the images. The average number of nanotubes per channel was dependent on various factors, including the trench dimension and the concentration of nanotubes in the solution. As expected, denser nanotube solutions or wider trenches resulted in a larger number of nanotubes per channel. Placement statistics from three representative experiments with different nanotube solutions and trench dimension are shown in Fig. S3(b). Analysis of chips B and C show that vast majority of the connected devices have the channels connected by one (B: 56%, C: 78%) or two nanotubes (B: 36%, C: 18%). Figure S3. (a) The channel regions of two devices, the left with a single nanotube and the right with two nanotubes spanning the source and drain electrodes (100 nm channel length). (b) The distribution of the number of tubes per channel estimated from device images. NATURE NANOTECHNOLOGY www.nature.com/naturenanotechnology 3
4. Estimation of the yield of single-tube devices from numerical simulation One benefit of the self-assembly approach we use is an increased yield of single-tube devices compared to that expected for a purely random deposition. A likely explanation for this enhancement is that nanotubes are repelled from placement sites ( trenches ) that are already occupied. Assuming this is the case, a very simple model can be used to gauge the magnitude of this effect. Consider the placement of M nanotubes from solution into N trenches on the surface. If the deposition is random i.e. all N trenches are equally probably for each nanotube then the fraction of trenches containing k nanotubes, P k, is given by the binomial distribution (for M, N >> 1): M k Pk! q (1 q k!( M n)! ) M k, (S1) where q = 1/N. The values of p k are a strong function of the average density (M/N), as shown by the dashed lines in Fig. S4(a). Note that P k exhibits a maximum when (M/N) = k, and that in particular, p 1 has a maximum value of ~0.37 which occurs when (M/N) = 1. However, if the deposition is not random, the distributions can be significantly different. This is shown by computing p k using the following placement algorithm: pick one of the N trenches at random. If the trench is unoccupied, place a nanotube in the trench. If the trench is occupied, place a nanotube in the trench with a probability, p, where 0 < p 1. Repeat this procedure until M nanotubes have been placed in the N trenches. If p = 1, the binomial distribution is recovered. Repulsion between nanotubes implies p < 1. The solid lines in Fig. S4(a) show the values of p k as a function of (M/N) assuming p = 1/3. For (M/N) ~ 1 it is clear that p 1 for this model is significantly higher than that of the random model, and that the increase of p 1 coincides with a decrease in p 0 and p 2. That is, the nanotubes tend to spread out over the available empty trenches before a significant number of multi-tube trenches appear. In fact, the value p = 1/3 reproduces the data for chip C [shown in Fig. S3(b)] rather well, assuming an average density close to 1. This means that nanotube adhesion in an empty trench is three times more likely than adhesion in an occupied trench. 4 NATURE NANOTECHNOLOGY www.nature.com/naturenanotechnology
SUPPLEMENTARY INFORMATION Figure S4. (a) Plot of the fraction of 0, 1, 2 and 3 nanotube device yield as a function of average nanotube density. Solid curves are for a model in which adhesion in an empty trench is three times larger than that in an empty trench. Dashed lines correspond to purely random deposition. Symbols are the measured data for Chip C. (b) Yield of 0, 1, 2, and 3 nanotube devices for Chip C. The total number of devices measured was 128. 5. Yield of electrically connected devices and semiconducting devices The yields of electrically connected devices and the percentage of semiconducting devices among connected devices were evaluated by measuring the electrical properties of more than 3,000 nanotube transistors. The transistors were fabricated on nanotubes placed on trenches of 100, 150, or 200 nm width and 1 μm length. We obtained the connection yield larger than 90% from devices with the 150-nm-wide and 200-nm-wide trenches as shown in Fig. S5. The percentage of semiconducting devices decreases with reduced trench width, which indicates an increased average number of nanotubes per channel. Figure S5. The yield of electrically connected devices (blue bar) evaluated by measuring 3,300 devices and the percentage of semiconducting devices among connected devices (red bar). 6. Effect of the thermal treatment on the device performance NATURE NANOTECHNOLOGY www.nature.com/naturenanotechnology 5
To remedy the adverse effect of the monolayer on the electrical properties of the devices, we annealed the samples at 450 C under Ar/H 2 for five minutes before electrode formation. To verify this effect, we fabricated two samples including about 60 semiconducting devices. One sample was fabricated by the standard process and the other sample was fabricated without the annealing process. As shown in Fig. S6, the average on-current and the inverse subthreshold slope were significantly improved by the annealing process. Figure S6. Plots of drain current versus gate voltage from nanotube transistors fabricated (a) with annealing and (b) without annealing process after nanotube deposition. The on-current (I on ) was measured at V G = V T 1.5V and V DS = 0.5V. The threshold voltage (V T ) was defined as the gate voltage corresponding to a drain current of 3 na. 7. Procedure of the measurement and hysteresis curves When probes touched down on the pads, all probes/gates were grounded. To screen out disconnected devices, V DS = -0.5V and V G = -2.5V were applied to a measured devices and I DS was monitored. If the absolute value of the drain current was larger than 10 na, a full curve of V G I DS was measured by sweeping the gate voltage from -2.5 V to 2.5 V with a 0.1V step. If the value was smaller than 10 na, the semi-automated probe setup skipped to the next device. While measuring a device by two probes of the probe array, other probes were left electrically floating. Figure S7 shows representative hysteresis curves from the fabricated transistors. Further studies are required to reduce the hysteresis by appropriate passivation layers and optimization of the quality of dielectric materials. 6 NATURE NANOTECHNOLOGY www.nature.com/naturenanotechnology
SUPPLEMENTARY INFORMATION Figure S7. Representative hysteresis curves of nanotube transistors fabricated on a 10 nm HfO 2 substrate without a passivation layer. The gate voltage was swept from 2V to +2V and back to 2 V. 8. Underestimated on/off ratio of the drain currents in the semi-automated measurement In this work, the large sets of devices were measured in a semi-automated probe station. Due to the high level of noise offsets (~ 0.1 na) in the back-gating connection, the off-state currents in the semiautomated measurement were limited by the offset; therefore, the ratio of on/off currents of most semiconducting devices shown in Fig. 3b,c were underestimated. Figure S8 shows the plots of the drain current versus gate voltage of seven semiconducting devices measured in (a) the semi-automated probe station and (b) a manual probe station (b) with an identical semiconductor parameter analyzer (Agilent B1500). Figure S8. Drain currents versus gate voltages of seven semiconducting nanotube transistors measured in (a) the semi-automated probe station and (b) a manual probe station. NATURE NANOTECHNOLOGY www.nature.com/naturenanotechnology 7