Small currents hold big clues to understanding nanoscale diodes

While pn-junctions are a crucial component of modern electronics and optoelectronics, researchers building pn-junctions from nanoscale materials were confounded by the different and varied behavior of nanoscale pn-junctions as compared to traditional pn-junctions. A team of Oregon State University physicists offer a coherent explanation for these discrepancies, reported in the Journal of Applied Physics.

Experiments using suspended carbon nanotube (CNT) diodes revealed some regimes where the source-drain current (I_sd) is limited by the pn-junction, but in other regimes I_sd is limited by the Schottky barriers where metal and semiconductor contact. By analyzing temperature dependent measurements, the researchers were able to determine both p-type and n-type Schottky barrier heights (SBHs). They grew ultra-clean suspended carbon nanotubes over pre-made electrode structures. Then they used electrical characterization and scanning photocurrent microscopy to identify electrode pairs connected by individual CNTs. According to the authors, improving the measurement resolution and carefully examining the very small currents flowing through the diode when it was just barely turned on helped to understand the whole system. By varying the Schottky barrier heights the researchers established the effect of Schottky contacts on the nanoscale diode’s overall properties.

According to coauthor Ethan Minot, once the impact of the Schottky barriers was understood, it became clear that one environmental parameter, the metal work function, simply and elegantly explained the varied behavior of the nanoscale diodes. With this new understanding, his team was able to determine the key parameters needed to accurately model the diodes. Minot anticipates that nanoscale pn-junctions can be used as photodiodes to maximize the efficiency of converting light to electrical current. Several aspects of the model developed are relevant for nanomaterial-based diode devices with potential solar cell, photodetector and LED applications.

Source: “A nanoscale pn junction in series with tunable Schottky barriers,” by Lee Aspitarte, Daniel R. McCulley, and Ethan D. Minot, Journal of Applied Physics (2017). The article can be accessed at https://doi.org/10.1063/1.4994194 .