Anomalous Schottky Barriers and Contact Band-to-Band Tunneling in Carbon Nanotube Transistors

Abstract

Devices incorporating nanoscale materials, particularly carbon nanotubes (CNTs), offer exceptional electrical performance. Absent, however, is an experimentally backed model explaining contact-metal work function, device layout, and environment effects. To fill the void, this report introduces a surface-inversion channel model based on low temperature and electrical measurements of a distinct single-walled semiconducting CNT contacted by Hf, Cr, Ti, and Pd electrodes. Anomalous barrier heights and metal-contact dependent band-to-band tunneling phenomena are utilized to show that, dependent upon contact work function and gate field, transport occurs either directly between the metal and CNT channel or indirectly via injection of carriers from the metal-covered CNT region to the CNT channel. The model is consistent with previously contradictory experimental results, and the methodology is simple enough to apply in other contact-dominant systems.

Schottky barriers in carbon nanotube-metal contacts

“…101 The two other studies that have reported measurements with different metals on CNTs with identical diameters show a decrease in Schottky barrier height with increasing work function. 67,84 However, the slope of this decrease is less than what is expected from the Schottky-Mott description (Eq. (2)) of a barrier unaffected by Fermi level pinning (dashed line in Figure 16(a)).…”

“…Ideally, a combination of activation energy, capacitive 80 and optical 87 measurement techniques should be used to study the Schottky barriers on different metals deposited on a single CNT. 84 In combination with Kelvin probe microscopy 105 to measure the work function of the different metals on the device instead of relying on tabulated values for ideal surfaces, such a study would yield a wealth of important information on the physics of Schottky barrier formation in nanoscale contacts and the influence of interface states on the Schottky barrier height.…”

“…80 Due to these difficulties the activation energy method is the most commonly used to measure Schottky barrier heights in CNT-metal contacts. 34,36,[81][82][83][84][85][86] Even though scanning photocurrent measurements have been used to image the depletion region in CNT-metal contacts, quantitative estimates of the Schottky barrier heights using the photoelectric effect are still lacking. 87 …”

“…To circumvent this problem Perello et al have fabricated CNTFETs with Hf, Cr, Ti and Pd electrodes on the same CNTs. 84 Since the diameter is identical along the CNT, this approach allows for a comparison between different metal work functions. The authors used temperature dependent measurements to extract the activation energy at different gate voltages and extrapolate to zero gate voltage (Figure 14) to exclude any effect from barrier lowering due to tunneling or image force lowering at higher gate voltages and obtain a good estimate of the Schottky barrier height.…”

“…97 To summarise the experimental results, the Schottky barrier heights from the activation energy studies 34,36,81,83,84,86 where the CNT diameter was determined are plotted as a function of the metal work function 98 in Figure 16. In addition, results from Chen et al who extracted the Schottky barrier height from the on-current for three different metals are also included.…”