Supplementary Information Physical device modeling of carbon nanotube/GaAs photovoltaic cells Hong Li, Wan Khai Loke, Qing Zhang*, S. F. Yoon Microelectronics Center, School of Electrical and Electronics Engineering, Nanyang Technological University, S1-B2c-20, Singapore 639798 I. FABRICATION METHOD The schematic of fabrication process is shown in Fig. S1. An n-type GaAs substrate (doping level of 2 1018 cm-3) was used as the substrate. A 100-nm-thick Si3N4 layer was deposited onto the substrate with a plasma-enhanced chemical vapor deposition system at a deposition rate of 18 nm/min using a mixed flow gases of 100-sccm 10% silane gas (diluted by nitrigen gas), 20-sccm 99% ammonia gas and 800-sccm nitrogen gas. The deposition temperature and RF power were 300 ºC and 40 W, respectively. The substrate was soaked in diluted HCl solution (10% acid by volume) for 5 mins to etch away possible native Ga2O2 layer, and then the back-contact G (5 nm Ni/25 nm Ge/100 nm Au/20 nm Ni/200 nm Au) was deposited in an E-beam evaporator. Next, the devices were baked in a chamber, in which the temperature was increased from 200 to 400 °C and held at 400 °C for 1 *Author to whom correspondence should be addressed. Electronic address: eqzhang@ntu.edu.sg. 1 min in nitrogen ambient, to form an Ohmic contact to the n-type GaAs. The electrode pair S and D (5 nm Ti /40 nm Au) was patterned on the surface of Si3N4 using optical lithography and lift-off processes. A photoresist window was defined between S and D. Then, the Si3N4 layer in the window was etched away via reactive ion etching (RIE) process using 10-sccm carbon tetrafluoride, 5-sccm oxygen at room temperature. The pressure and RF power were 30 mTorr and 50 W, respectively. Finally, single-walled carbon nanotubes (SWNTs) were bridged between the S and D electrodes. Thus, the center segment of the SWNTs was in contact with the n-type GaAs substrate through a thin layer of Ga2O3 formed in the RIE process. FIG. S1. The flow chart of fabrication process. The blue, cyan, orange, wine and magenta colors delineate the GaAs, Si3N4, Au electrodes, photoresist and SWNTs, respectively. II. FITTING OF THE IDG-VDG CHARACTERISTICS AT LOW BIAS 2 Upon fitting the observed results at low FB with Equation 1, we obtained n and A listed in Table I. Figure S2 depicts a relation3 between n and temperature T, i.e., n(T ) 2.3 530 . Such a temperature T dependence could be attributed to the tunneling current component.3,4 In an ideal case, where the current is dominated by recombination of minority carriers injected into the bulk semiconductors, the ideality factor is 1.5 Recombination of carriers in the space charge region could result in an ideality factor around 2.6 While high ideality factors (n>>2) was attributed to deep-level-assisted tunneling.7 In our case, n ~ 4.1 at 300 K suggests the tunneling current through the SWNT/GaAs interband states. A is exponentially dependent on the temperature, because it is related to the portion of electrons thermally emitted from the GaAs to the SWNT.8 T (K) 50 100 125 175 225 250 275 300 n 13.4 7.4 6.6 5.343 4.778 4.36 4.418 4.2 A ( 10-15) 0.7 12 80 550 2400 3800 10000 19500 Table I: n and A obtained by fitting the observed curves with Equation 1 at different temperatures. 16 14 ideality factor, n fitting data 12 n 10 8 6 4 2 50 100 150 200 250 300 T (K) Fig. S2. Fitting the ideality factor n with n(T ) 2.3 530 . The open symbols are data from Table I. T 3 III. FITTING OF THE IDG-VDG CHARACTERISTICS AT LOW BIAS The heterojunction shows a PV effect under the illumination of a beam of white light (50 mW/cm2). According to Prince,9 the PV effect could be modeled by I DG ( I J I sc ) Rsh VDG , where IJ is the FB Rct Rsh heterojunction current described by Equation 1 (in manuscript), Isc is the measured short-circuit current, Rsh is a shunt resistor and Rct is the resistance at SWNT/Au contact. The fitting parameters are displayed in Table II. Rsh is in GΩ range, implying that the surface leakage current through adsorbents on SWNT is negligible. T (K) Rsh (GΩ) 50 9 125 2 200 1.3 300 1.1 Table II. Fitting parameters of the IDG-VDG characteristics with illumination. 4 References: 1D. Kang, N. Park, J-h. Ko, E. Bae, and W. Park, Nanotechnology 16, 1048 (2005). 2L. B. Ruppalt and J. W. Lyding, Nanotechnology 18, 215202 (2007). 3F. A. Padovani and G. G. Sumner, J. Appl. Phys. 36, 3744 (1965). 4F. A. Padovani, J. Appl. Phys. 37, 921 (1966). 5C-T, 6A. Sah, R. N. Noyce, and W. Shockley, Proc. IRE 45, 1228 (1957). Chitnis, A. Kumar, M. Shatalov, V. Adivarahan, A. Lunev, J. W. Yang, G. Simin, M. A. Khan, R. Gaska, and M. Shur, Appl. Phys. Lett. 77, 3800 (2000). 7J. M. Shah, Y-L. Li, Th. Gessmann, and E. F. Schubert, J. Appl. Phys. 94, 2627 (2003). 8R. 9M. L. Anderson, Solid State Electron. 5, 341 (1962). B. Prince, J. Appl. Phys. 26, 534 (1955). 5