A Study of Electrical Resistance in Carbon Nanotube–Insulator–Metal Diode Arrays for Optical Rectenna

Vertical tunnel diode arrays made from multiwall carbon nanotubes (MWCNTs) have shown recent promise for developing a practical optical rectenna, which is a device to convert electromagnetic waves at optical frequencies to direct current. Realizing an optical rectenna requires an antenna to be coupled to a diode that operates on the order of PHz (switching speed on the order of fs). Previously, we have demonstrated an optical rectenna device by engineering MWCNT-insulator-metal (MWCNT-I-M) tunnel diodes at the tips of vertically aligned MWCNT arrays, which act collectively as the antenna. However, the high electrical resistance of the MWCNT-I-M diode must be reduced to enable improved impedance matching between diode and antenna, which limited the rectified power in our prior work. Here, we address this issue of impedance mismatch through a series of experiments designed to elucidate contributions to the total electrical resistance of the device. Different combinations of metals, and metal and insulator thicknesses were tested for reduced contact resistance, while maintaining a working diode. Another development toward reducing resistance was to open MWCNT tips using oxygen plasma, which exposed multiple walls for bonding rather than just outer wall of closed tip CNTs. These developments were combined to reduce zero-bias resistance of MWCNT-I-M diode arrays to as low as 100 Ω ·cm
²
, which is 75 times lower than in our previous report.