Characterization of the Metal-Semiconductor and Metal-Insulator-Semiconductor Junctions between Single-walled Carbon Nanotube Films and Si Substrates

Ashkan Behnam, Nischal Arkali Radhakrishna, and Ant Ural Electrical and Computer Engineering, University of Florida, Gainesville, Florida 32611 USA

Email: abehnam@ufl.edu Phone: +1(352)328-7645 Fax: +1(352)392-8381 Single-walled carbon nanotube (CNT) film is a transparent, conductive, and flexible material that exhibits

uniform physical and electronic properties1. Several promising optoelectronic and photovoltaic device applications of these films have recently been demonstrated2. However, in previous works, the properties of the junction between the CNT film and the semiconductor substrate (typically Si) have not been properly characterized2. Here, we analyze the interface and transport properties of the junction between the CNT film and Si substrates by fabricating metal-semiconductor (MS) and metal-insulator-semiconductor (MIS) structures, where the CNT film acts as the metal and Si is the semiconductor. Our results help to better understand the electrical properties of the CNT film-Si contacts and to improve the design of optoelectronic and photovoltaic devices which use CNT films as transparent conductive electrodes. Device fabrication begins by preparing CNT films using a vacuum filtration approach1,3 (Fig. 1a) and opening windows in SiO2 layers on 1015-1016 cm-3 doped n- and p-type Si substrates (Figs. 1c and 1d). For MIS structures, a thin oxide layer is then thermally grown on the exposed Si areas (Fig. 1d2). Next, the CNT films are deposited over both MS and MIS samples (Figs. 1d) and then patterned by O2 plasma etching3 to form individual devices (Figs. 1e). Finally, metallic rings are deposited on the films for electrical probing (Figs. 1e). For comparison, control samples in which CNT film is replaced with a Ti/Au layer (10/90 nm) have also been fabricated and characterized. An optical image of a CNT film-Si MS structure is shown in Fig. 1b.

Figure 2 shows the rectifying behavior of the current-voltage (I-V) characteristics for a CNT film - p-type Si (CNT-pSi) MS structure. At high temperatures (T), thermionic emission (TE) is most likely the dominant transport mechanism for both CNT-pSi and CNT film - n-type Si (CNT-nSi) devices due to the strong T dependence of I (see inset of Fig. 3), the rectifying behavior, and the slope of the I-V curves in the forward bias (inset of Fig.2). At lower temperatures (< 180 K), however, the weak dependence of I on T (inset of Fig. 3) suggests that tunneling becomes dominant. The barrier heights (Φ0) extracted in the TE regime from the slope of the Richardson plots (Fig. 3) are 0.41 ± 0.01 eV and 0.63 ± 0.03 eV for CNT-pSi and CNT-nSi devices, respectively. The inset of Fig. 4 shows the lowering of the extracted barrier heights due to image force as a function of reverse bias, from which the ideality factor (nif) and zero-bias image force lowering (δΦ0) are extracted as 1.01 ± 0.006 and 0.024 ± 0.007 eV for CNT-pSi and 1.025 ± 0.008 and 0.016 ± 0.0025 eV for CNT-nSi devices, respectively4. The extracted values of Φ0 and δΦ0 sum up to a value close to the bandgap of Si. From these results, the location of the Fermi level in the CNT film with respect to the vacuum level (when modeled as a metallic material in contact with Si) can be estimated to be about 4.6 - 4.7 eV. The current levels observed, however, are 3 to 4 orders of magnitude lower than the values expected from the TE theory, which can be related to the one dimensional structure of the tubes and the porousness of the CNT film (which reduces the effective MS contact area), as well as the presence of a very thin native oxide (due to the air-exposed Si surface after the film deposition). The photoresponse of both CNT film and control MS structures are shown in Fig. 4. When the CNT film device is reverse-biased, it can be used as a photodetector at high bias with a large photo-to-dark current ratio of 8.22×105 and a responsivity of 2.21 A/W (at 4 V bias) measured at a wavelength of 632 nm. These values are significantly larger than the highest values obtained for the control device (2.745×104 and 0.187 A/W at 0.5 V) due to the strong barrier lowering of the CNT film structure under illumination. Figure 5 shows that the I-V characteristics of CNT-pSi MIS structures are exponential with slopes significantly smaller than those of the MS I-Vs, an implication of tunneling transport5. The slopes of the E-field renormalized currents (inset of Fig. 5) confirm that for oxide thicknesses of 10 nm or above, Fowler-Nordheim tunneling is the sole transport mechanism, while for the 5 nm oxide, direct tunneling plays a significant role as well5. Capacitance-voltage (C-V) characteristics of MIS structures (Fig. 6) agree with the analytical expectations based on the oxide thickness and Si doping density. Estimated Si/SiO2 interface charge density from these characteristics is ~ 5×1011 cm-2. The larger value of the CNT film Fermi level (5.0 - 5.2 eV) that is estimated from a plot of the C-V flat-band voltage (Vfb) vs. oxide thickness (Tox) (inset of Fig. 6)6 compared to the one extracted from the MS structures (4.6 - 4.7 eV) suggests that the Fermi level is at least partially pinned when nanotubes come in touch with the Si substrate.

[1] Z. Wu, et al., Science 305, 1273 (2004); Z. Li, et al., Langmuir 24, 2655 (2008). [2] J. Wei, et al., Nano Lett. 7, 2317 (2007); Y. Jia, et al., Adv. Mater. 20, 4594 (2008); Z. Li, et al., ACS Nano 3, 1407 (2009); P-L. Ong, et al., Appl. Phys. Lett. 96, 033106 (2010).

[3] A. Behnam, et al., Appl. Phys. Lett. 89, 093107 (2006). [4] S. M. Sze, et al., J. Appl. Phys. 35, 2534 (1964). [5] M. Lenzlinger, et al., J. Appl. Phys. 40, 278 (1969); Y-C. Yeo, et al., IEEE Trans Electron Dev. 50, 1027 (2003). [6] T.W. Hickmott, et al., J. Appl. Phys. 52, 3464 (1981).

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Figure 6. C-V characteristics of four CNT-pSi MIS structures with different oxide thicknesses, but the same area of 2002 μm2. The inset shows the flat-band voltage extracted from the C-V curves as a function of oxide thickness.

Figure 2. I-V characteristics for a CNT-nSi MS structure with an area of 8002 μm2 at various temperatures in the range 180 - 340 K. The inset depicts the I-V characteristics for the same device in the small forward bias regime.

Figure 4. Reverse bias dark and photo currents for CNT-nSi and control MS structures (both with an area of 5002 μm2). Photocurrent was measured with a 632 nm HeNe laser with a power of ~ 6.5 mW and 830 μm2 spot size. The inset shows barrier height vs. reverse bias voltage for the CNT-nSi devices in Fig. 3.

Figure 5. I-V characteristics of one CNT-pSi MS structure and three CNT-pSi MIS structures with various oxide thicknesses, all with an area of 4002 μm2. The inset compares I/E2 vs. 1/E for the three MIS structures.

Figure 3. Plot of log (I/T2) vs. 1000/T at 0.5 V for two CNT-nSi (with areas of 5002 μm2 and 8002 μm2) and two CNT-pSi (with areas of 4002 μm2 and 8002 μm2) MS devices. The inset shows I vs. 1000/T at 0.5 V for CNT-nSi and CNT-pSi MS structures with areas of 8002

μm2 in a wider temperature range of 77 – 340 K.

Figure 1. (a) AFM image of a 50 nm thick CNT film. (b) Optical microscope image (top view) of a CNT film-Si MS structure. (c)-(e) Cross-sectional schematic of the fabrication process for MS [(c)-(e1)] and MIS [(c)-(e2)] structures: (d1) & (d2) Oxide is etched in the active region (brown circle in (b)) and CNT film is deposited. In (d2) a thin thermal oxide is grown before CNT film deposition. (e1)&(e2) CNT film is patterned and a Cr/Pd metal contact (10/90 nm) is evaporated on the film (the yellow ring in (b)).

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