Development and characterisation of an integrated 1D-2D hybrid field-effect transistor for force-sensing applications

Abstract

Transistors, as the basic components in modern electronics, become more functional to meet the requirement of the rapid growth of robotics and human-machine interface. More advanced nanomaterials have been developed and integrated on the transistor to achieve complex functionalities and higher performance. This thesis introduces a novel transistor as a force-sensing unit, consisting of one-dimensional (1D) piezoelectric zinc oxide (ZnO) nanorods (NRs) as the gate-control and two-dimensional (2D) material as the transistor channel. When a mechanical force is applied to ZnO NRs, the piezoelectric potential can modulate the current flowing in the 2D channel. The development of this “1D-2D” transistor includes the study of materials, microfabrication, and devices.

Firstly, ZnO NRs with a diameter ranging from ~45 nm to ~500 nm have been synthesised hydrothermally on Si. The influence of the seed layer, growth time, and precursors concentration on ZnO NR’s morphology has been investigated experimentally. Then, different patterning methods of ZnO NRs have been discussed. A novel simplified method for the selective growth of ZnO NRs has been introduced based on the absence of the ZnO seed layer on the photoresist. The influence of patterned geometry on the growth of ZnO NRs has been investigated.

Then, the piezoelectric gate-control of ZnO NRs has been studied by integrating ZnO NRs on a metal-oxide-semiconductor (MOS) capacitor (ZnO-MOS device). The open-circuited voltage output has been observed to range from ~5 mV to ~100 mV at 32 N force input. The voltage output of the ZnO-MOS device is found to be dependent on the type of seed layers and the patterned geometry.

Next, the 2D tungsten diselenide (WSe2) has been selected as the channel material of the transistor. The fabrication and the electrical characterisation of the 2D WSe2 field-effect transistor have been presented. The WSe2 channel can be n-type doped, p-type doped or ambipolar. The maximum field-effect mobility of fabricated devices with titanium(Ti) contact has been calculated to be 1.25 cm2/Vs.

Lastly, the integration of 1D ZnO NRs and 2D WSe2 transistor has been achieved. The applied mechanical force on piezoelectric NRs can induce a drain-source current change (ΔIds) on the WSe2 channel. The different doping types of WSe2 channel have been found to lead to different directions of ΔIds. For the fabricated devices, the pressure from calibration weight of 2 gram(g) has been observed to result in ~20% Ids increasing for the device with a p-type doped WSe2 channel, and up to 10% Ids decreasing for the device with an n-type doped WSe2 and a Ti intermediate layer.

The overall 1D-2D transistor could be a candidate as the unit in future adaptive force-mapping systems, and could be useful for applications in future human-machine interfaces and smart sensing systems.