A Sputtered Silicon Oxide Electrolyte for High-Performance Thin-Film Transistors

Abstract

Low operating voltages have been long desired for thin-film transistors (TFTs). However, it is still challenging to realise 1-V operation by using conventional dielectrics due to their low gate capacitances and low breakdown voltages. Recently, electric double layers (EDLs) have been regarded as a promising candidate for low-power electronics due to their high capacitance. In this work, we present the first sputtered SiO2 solid-state electrolyte. In order to demonstrate EDL behaviour, a sputtered 200 nm-thick SiO2 electrolyte was incorporated into InGaZnO TFTs as the gate dielectric. The devices exhibited an operating voltage of 1 V, a threshold voltage of 0.06 V, a subthreshold swing of 83 mV dec−1 and an on/off ratio higher than 105. The specific capacitance was 0.45 µF cm−2 at 20 Hz, which is around 26 times higher than the value obtained from thermally oxidised SiO2 films with the same thickness. Analysis of the microstructure and mass density of the sputtered SiO2 films under different deposition conditions indicates that such high capacitance might be attributed to mobile protons donated by atmospheric water. The InGaZnO TFTs with the optimised SiO2 electrolyte also showed good air stability. This work provides a new pathway to the realisation of high-yield low-power electronics.

Oxide-Based Electric-Double-Layer Thin-Film Transistors on a Flexible Substrate

“…2(a), the IGZO has a similar microstructure to the SiO 2 , due to sequential deposition of IGZO on top of SiO 2 . As previously reported [21], despite the polycrystalline-like microstructure shown in the SEM image, the SiO 2 layer is amorphous with an inhomogeneous density distribution due to the porous microstructure. Similarly, the IGZO layer should also be amorphous, which is in agreement with the microstructure of most reported IGZO thin films that are deposited at room temperature [4, 5, 22].…”

“…A possible reason for this frequency dependent capacitance is the low ionic mobility of the mobile protons in the SiO 2 dielectric. As indicated in our previous study [21], some water molecules might be absorbed from the ambient air, forming three coordinate oxygen centers, Si-OH + -Si, in the SiO 2 electrolyte [18,23]. The external electric field will force the protons to jump from one hydroxyl group to the next due to the non-stable bonding between hydrogen and oxygen.…”

“…However, PECVD might have toxic, explosive and flammable processing gases and by-products. We recently used radiofrequency (RF) magnetron sputtering to develop a costeffective, industry compatible way to deposit a porous SiO 2 electrolyte that uses absorbed water molecules to generate mobile protons [20,21].…”

8.4: Invited Paper: Oxide devices for displays and low power electronics

“…Such a material can be deposited using industrial compatible techniques including sputtering and plasma-enhanced chemical-vapor deposition. Here a 200 nm thick SiO x electrolyte is RF sputtered with a loose structure, by tuning the deposition power and pressure [8] . SEM and TEM images of the sputtered SiO x electrolyte are shown in Figures 3(j) and 3(k).…”

“…Here, we review our recent work on a) high-performance oxidebased Schottky diodes with an ideality factor of 1.09, ultra-low noise, and operating speed >20 GHz on glass [2] and 2.45 GHz on flexible substrate [3] ; b) IGZO TFTs capable of reaching a benchmark speed of 1 GHz [4] , which are, to the best of our knowledge, the fastest oxide-based diodes and transistors to date; c) a few different methods to achieve IGZO TFTs capable of onevolt operations [5][6][7][8][9] ; d) CMOS-like oxide logic gates and functional circuits including inverters with a gain up to 150 [10,11] , NAND gate [12] , D-latch [13] , 51 stage ring oscillator [13] , complementary static random access memories [14] , and a one-bit full adder [13] , etc, by integrating SnO-based p-type TFTs with IGZO-based n-type TFTs; and finally e) novel oxide TFTs with a Schottky source contact that show no short channel effect, almost total immunity to negative bias illumination stress, and have a gain over two orders of magnitude higher than that of a typical silicon transistor [15] .…”