Investigation of charge transport properties in carbon-based transistors by charge modulation spectroscopy

Over the past years, semiconducting single wall carbon nanotubes (SWCNTs) materials have been attracting a lot of attentions in different research and application areas thanks to its outstanding electrical conductivity, mechanical flexibility, and biocompatible properties etc. Recently highly purified SWCNTs materials have been achieved by selective polymer wrapping method which is capable for the high-performance field effect devices. The exact species and composition of SWCNTs depend on the selected polymer and process. The obtained SWCNTs networks can be classified into mono-chirality and multi-chirality. Both SWCNTs networks can be easily processed by solution-based techniques such as spin-coating and printing to fabricate Field Effect Transistor (FET) devices. Such SWCNTs FETs show high on/off ratios on the order of 10^5, the reported mobility is around 10 cm^2 · V^-1 · S^-1 for mono-chirality and 2cm^2 · V^-1 · S^-1 for multi-chirality and balanced electron/hole behavior. However, further optimization is still in need to be further adopted in industrial applications. Therefore, a better understanding of charge transport in the SWCNTs networks would benefit. In this work, the Charge Modulation Spectroscopy (CMS) technique and a more advanced Charge Modulation Microscopy (CMM) techniques have been used to investigate the charge transport in mono- and multi-chirality SWCNTs networks based FETs. The CMS technique detects mobile charge induced optical transmission derivation while excluding trapped carrier information. The CMS spectra provide insight to the charge involved energetic transition and the measured intensity directly correlates to the carrier density. However, the measurement with CMS technique is done for the entire chip while the CMM technique can focus on a much smaller area, less than 1µm2, of the device channel. With the PZT stage integrated in the CMM setup, a CMM map, at a given wavelength, can be obtained by scanning a small area, e.g. 10µm × 10µm, of the device channel and taking measurements of local CMS signal pixel by pixel (pixel refers to the smaller area the CMM setup focuses on one at a time). When low offset potential is applied to gate (source and drain are grounded), the obtained CMM map can be correlated to charge carriers density distribution. For mono-chirality SWCNTs networks, the focus is on the impact of network density and degree of alignment order on charge transport. The experimental results show the charge carrier mobility increases along with network density up till a certain level, and in random network, the degree of alignment order is very low that no obvious contribution towards charge transport has been observed. While for multi-chirality SWCNTs networks, the focus is on the impact of SWCNTs species composition on charge transport. The experimental data show the mismatch of energy bandgap between different SWCNTs species has a negative impact on charge transport. Furthermore, the CMS technique is extended to charge transport investigation in Electrolyte-Gated Organic FETs (EGOFET). In case of this work, the target device is Water-Gated OFET with TEG-PNDIT2 used as semiconductor. Though the work done on EGOFET is still preliminary, we hope it could still shine some light in the area of EGOFET in terms of measurement and experiment methodology.