Low Operating Voltage, Solution-Processed Transistors with High-k/Self-Assembled Monolayer Dielectrics

Abstract

Solution-processed thin-film transistors (TFTs) have a high potential to be the key components of future portable, battery-powered devices and circuits. TFTs can be realised in different form factors from standalone, discrete, low-cost sensors to flexible, large-area electronics. However, most of the recently demonstrated solution-processed TFTs typically operate at or above 5 V, which is still too high for many applications where operating voltage and power consumption are the main concerns. One approach to decrease the operating voltage of TFTs is to lower their threshold voltage (Vth) and subthreshold swing (SS) which can be fulfilled by increasing gate dielectric capacitance and improving the dielectric/semiconductor interface. The high capacitance dielectric can be achieved by utilising high dielectric constant (high-k) materials, thinning the gate dielectric layer, or doing both simultaneously. Tantalum pentoxide (Ta2O5) is a highly promising, high-k metal oxide dielectric (k ~ 26) which is used in capacitors, transistors and memory devices. Recently, it has been shown that employing thick Ta2O5 films (d > 100 nm) as a gate dielectric does not only improve the characteristics of TFTs but significantly lowers their operating voltage. However, due to its relatively medium bandgap (4.4 eV), Ta2O5-based TFTs suffer from high leakage currents, particularly when Ta2O5 thickness is reduced below 20 nanometres. Typically, Ta2O5 is deposited by atomic layer deposition (ALD) or RF/DC magnetron sputtering. These methods are time-consuming and increase the cost of device fabrication due to the need for high-vacuum conditions. In comparison, anodic oxidation (so-called anodisation) is a material deposition technique which is a simple, cost-efficient and straightforward method to grow high-quality metal oxide layers on the surface of metallic substrates in ambient conditions. In this thesis, ultra-thin (d ~ 7 nm), anodic Ta2O5 modified with n-octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) has been used to realise ultra-low voltage operation of p-channel DPP-based organic TFTs (OTFTs) and n-channel a-IGZO-based metal oxide TFTs (MOTFTs). Moreover, it is shown that OTS SAM reduces the gate leakage current and improves the TFT dielectric/semiconductor interface. First, the morphology and dielectric properties of the anodised Ta2O5 films with and without OTS SAM treatment have been analysed. Several anodisation voltages were utilised to optimise the thickness of Ta2O5 for each type of TFTs to guarantee their optimal operation. The results show that the poly(3,6-di(2-thien-5-yl)-2,5-di (2-octyldodecyl)-pyrrolo[3,4-c] pyrrole-1,4-dione) thieno [3,2-b] thiophene)-polymethyl methacrylate (DPPDTT-PMMA) TFTs gated with OTS-modified tantalum pentoxide anodised at 3 V (d ~ 7 nm) exhibit the best performance. The optimised devices operate at 1 V with saturation field-effect mobility larger than 0.2 cm2 V-1 s-1, threshold voltage -0.55 V, subthreshold swing 120 mV/dec, and current on/off ratio in excess of 5 * 103. As a result, the demonstrated DPPDTT-PMMA TFTs display a promising performance for applications in ultra-low voltage, organic electronics. On the other hand, it is shown that the best performing capacitors and a-IGZO TFTs are realised with 10 V anodised (22 ± 2 nm), OTS-treated Ta2O5 dielectric. The fabricated Ta2O5/OTS capacitors show good stability in the 100 Hz to 100 kHz and capacitance density in excess of 400 nF/cm2. The fabricated TFTs display relatively high field-effect mobilities (2.3 cm2 V-1 s-1), threshold voltages around 0.4 V, subthreshold swings below 90 mV/dec, and high current on/off ratios well in excess of 105. It is envisaged that this approach is a promising alternative to fabricate ultra-low voltage, inexpensive TFTs and TFT-arrays for low-cost sensors and low-end, disposable electronics.

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Aimin Song (Supervisor) & Leszek Majewski (Supervisor)