Fe/Ge catalyzed carbon nanotube growth on HfO2 for nano-sensor applications

A carbon nanotube (CNT) growth process on HfO₂ is reported for the first time for application in nano-sensors. The process uses a combination of Ge nanoparticles and ferric nitrate dispersion and achieves an increase in CNT density from 0.15 to 6.2 mum length/mum² compared with the use of ferric nitrate dispersion alone. The growth process is validated by the fabrication of back-gate CNT field-effect transistors (CNTFETs) using Al source/drain (S/D) contacts and a H₂ anneal at 400degC. The transistors exhibit p-FET behavior with an I_on/I_off ratio of 10⁵ and a steep sub-threshold slope of 130 mV/dec. These results are rather surprising, as earlier research in the literature on CNTFETs with Al S/D electrodes showed n-FET behavior. The p-FET behavior is shown to be due to the H₂ anneal, which we ascribe to the smaller electron affinity of hydrogenised CNTs. Measurements of the temperature dependence of the drain current show low Schottky barrier height Al S/D contacts after a H₂ anneal, which tends to confirm this explanation.