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[(a)–(d)] Schematic of the process flow for MSM photodetectors with CNT film electrodes along the dashed line shown in part (e): (a) SiN isolation layer deposited on a GaAs substrate, (b) CNT film prepared by vacuum filtration deposited on the substrate after opening the active windows in the SiN layer, (c) CNT film patterned into interdigitated electrode fingers by photolithography and ICP etching, and (d) Cr/Pd metal contacts patterned on the nanotube film contact pads using photolithography, e-beam evaporation, and subsequent lift-off. (e) Optical microscope image of the finished MSM photodetector, showing the various device dimensions. (f) AFM image showing the area between two CNT film electrode fingers of the MSM device of part (e).
(a) Dark current (log scale) vs applied voltage measured at six different temperatures between 230 and 340 K for a CNT film–GaAs MSM device with and . The inset shows the dark characteristics of this device at room temperature (294 K) in linear scale. The data clearly exhibit the characteristic curves of two back-to-back Schottky diodes making up the MSM photodetector, demonstrating that the CNT film indeed makes a Schottky contact to GaAs. (b) The Richardson plot of vs at voltage bias in the temperature range of 280–340 K for three CNT film–GaAs MSM devices of identical active area but different finger widths and spacings , as labeled in the figure. The dashed lines are linear best fits to the experimental data, from the slope of which Schottky barrier heights of , 0.54, and 0.54 eV are extracted for devices with , 20, and , respectively. The inset shows log current vs for the MSM device with in a wider temperature window (from 150 to 340 K) at bias. The current starts to saturate at temperatures lower than , which suggests that tunneling, which depends weakly on temperature, begins to dominate the transport across the CNT film–GaAs junction at lower temperatures.
(a) The schematic and (b) measured dark current vs applied voltage for a CNT film–GaAs MSM device without the interdigitated fingers between the electrode pads but with asymmetric CNT film contact areas. The source contact area is and the drain contact area is . The voltage is applied to the smaller drain contact and the larger source contact is grounded, as shown in (a). The band diagrams illustrating electron thermionic emission over the Schottky barrier for (c) positive and (d) negative voltage biases.
(a) Dark current vs applied voltage at room temperature (294 K) for CNT film–GaAs MSM devices with and but with spacing ranging from 10 to , as labeled in the figure. (b) Dark current vs applied voltage for CNT film–GaAs MSM devices with but with ranging from 15 to , as labeled in the figure. (c) MEDICI simulation of the cross-sectional current density distribution in the GaAs substrate between two CNT film electrode fingers at bias calculated using the value of the barrier height extracted from the measurements. Darker colors correspond to higher current density. (d) Dark current vs applied voltage for CNT film–GaAs MSM devices with and but with ranging from 200 to , as labeled in the figure.
Measured dark and photocurrent (log scale) vs applied voltage for MSM photodetectors with CNT film and Cr/Pd metal electrodes, as labeled in the figure. For both devices, and . The dark current of the CNT film MSM device is significantly lower than that of the Cr/Pd control device, while the photocurrents are comparable, particularly at high applied bias voltages.
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