Imaging of the Schottky Barriers and Charge Depletion in Carbon Nanotube Transistors

Abstract

The photovoltage produced by local illumination at the Schottky contacts of carbon nanotube field-effect transistors varies substantially with gate voltage. This is particularly pronounced in ambipolar nanotube transistors where the photovoltage switches sign as the device changes from p-type to n-type. The detailed transition through the insulating state can be recorded by mapping the open-circuit photovoltage as a function of excitation position. These photovoltage images show that the band-bending length can grow to many microns when the device is depleted. In our palladium-contacted devices, the Schottky barrier for electrons is much higher than that for holes, explaining the higher p-type current in the transistor. The depletion width is 1.5 μm near the n-type threshold and smaller than our resolution of 400 nm near the p-type threshold. Internal photoemission from the metal contact to the carbon nanotube and thermally assisted tunneling through the Schottky barrier are observed in addition to the photocurrent that is generated inside the carbon nanotube.

Inkjet Printing of MoS2

“…Although this photoresponsivity is much lower than that of photodetectors based on individual MoS2 flakes, [6,9,39] it is comparable to those based on individual carbon nanotubes [40] or graphene. [41][42][43] In particular, it is also comparable to that of MoS2 nanosheet films fabricated by the Langmuir-Blodggett (LB) method, [16] implying that our inkjet printing processes do not induce obvious performance degradation.…”

Schottky barriers in carbon nanotube-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 …”

“…Therefore, the depletion regions in CNT-metal contacts have been imaged using photocurrent and photovoltage measurements 87 as well as by scanned gate microscopy. 109 In the scanned gate microscopy experiments, it was observed that a biased AFM tip scanned over a CNTFET has a stronger gating effect when the tip is in the vicinity of one of the contacts implying that a barrier is present there.…”

Identifying signatures of photothermal current in a double-gated semiconducting nanotube

“…These properties make SWNTs an ideal platform for exploring photocurrent generation with SPCM. In early single-walled nanotubes SPCM work the interpretation of photocurrent was mostly based on photovoltaic mechanisms [6,7,8,9,11].The importance of photothermal effects has been suggested in the context of measurements of bulk SWNT films [12,13,14] and very recently, SPCM work on graphene and individual metallic SWNTs has emphasized the importance of photothermal mechanisms in materials with no or small bandgaps [22,26].The question of the role of photothermal mechanisms in larger bandgap semiconducting nanotubes has been studied very recently in double-gated [26] and single-gated [27] suspended carbon nanotube devices.These two studies report contradictory results, leaving the understanding of fundamental mechanisms underlying photocurrent generation in semiconducting nanotubes unclear.Here we report on the study of a suspended semiconducting nanotube device where we show that both photovoltaic and photothermal mechanisms compete in the generation of photocurrent. In particular, we find that the dominant or non-dominant character of one or the other processes is a function of the doping profile and that the magnitude of each contribution is strongly influenced by the band alignment with the metal contacts through the resulting contact resistance.…”