Monthly Archives: June 2014

Metallization selection and the performance of amorphous In-Zn-O thin film transistors

We report on the effect of two different source/drain metallizations (Ti, Mo) on the performance and stability of amorphous In-Zn-O (IZO) thin film transistors (TFTs). In the as-deposited state, stable Mo/IZO TFTs show low threshold voltage (VT) and clear drain current saturation behavior compared to Ti/IZO devices, despite both having similar channel conductivity. Low temperature annealing (200 °C) results in VT shifts in Ti/IZO TFTs (ΔVT ∼ −33 V) that are significantly larger than in Mo/IZO TFTs (ΔVT ∼ −14 V). These differences are attributed to the injection of additional carriers into the channel of Ti/IZO devices due to reaction at the unstable Ti/IZO interface.

Ultra-violet light enhanced super critical fluid treatment in In-Ga-Zn-O thin film transistor

A low-temperature ultra-violet (UV) light enhanced supercritical CO2 (SCCO2) fluid treatment is employed to improve the performance of In-Ga-Zn-O (IGZO) thin film transistor (TFT) device. In this study, amorphous IGZO film deposited by sputtering is investigated in SCCO2 ambient under different illumination conditions. After SCCO2 treatment with UV exposure, the mobility and subthreshold swing of the TFT can be significantly improved. A model is proposed to explain the mechanism, and the improvement is due to the reduction of dangling bonds at the grain boundary. With the help of UV, dangling bonds can be effectively passivated by OH chemical groups.

Low temperature thin film transistors with hollow cathode plasma-assisted atomic layer deposition based GaN channels

We report GaN thin film transistors (TFT) with a thermal budget below 250 °C. GaN thin films are grown at 200 °C by hollow cathode plasma-assisted atomic layer deposition (HCPA-ALD). HCPA-ALD-based GaN thin films are found to have a polycrystalline wurtzite structure with an average crystallite size of 9.3 nm. TFTs with bottom gate configuration are fabricated with HCPA-ALD grown GaN channel layers. Fabricated TFTs exhibit n-type field effect characteristics. N-channel GaN TFTs demonstrated on-to-off ratios (ION/IOFF) of 103 and sub-threshold swing of 3.3 V/decade. The entire TFT device fabrication process temperature is below 250 °C, which is the lowest process temperature reported for GaN based transistors, so far.

Nanomechanical switches based on metal-insulator-metal capacitors from a standard complementary-metal-oxide semiconductor technology

We report experimental demonstrations of contact-mode nano-electromechanical switches obtained using a capacitor module based on metal-insulator-metal configuration of a standard commercial complementary metal oxide semiconductor technology. The developed 2 terminals Titanium Nitride switches operate at low voltages (∼10 V) thanks to its small gap (27 nm), showing an excellent ION/IOFF ratio (104) and abrupt behavior (5 mV/decade, one decade of current change is achieved with a 5 mV voltage variation). A switch configuration is also presented where using two electrodes three different contact mode states can be obtained, adding functionalities to mechanical switches configurations.

Highly Transparent, High-Performance IGZO-TFTs Using the Selective Formation of IGZO Source and Drain Electrodes

In this letter, high-performance, highly transparent amorphous InGaZnO (IGZO) thin-film transistors (TFTs) with IGZO source/drain electrodes were fabricated. Rapid thermal annealing treatment effectively converted IGZO from a semiconductor into a conductor. Using a patterned ${rm SiO}_{2}$ capping layer, highly transparent IGZO-TFTs with selectively formed IGZO electrodes were fabricated on a glass substrate. The mobility of the fabricated IGZO-TFT was 8.3 ${rm cm}^{2}/{rm V}~{rm s}$ , ON/OFF ratio was $3.1times 10^{6}$ , and subthreshold swing was 0.44 V/decade. Thus, the proposed scheme provides a simple and practical method of fabricating high-performance, highly transparent IGZO TFTs.

Poly-Si active matrix organic light-emitting diode pixel circuit with compensation for threshold voltage and mobility variations

A new pixel circuit and driving method for large-area, high-luminance active matrix organic light-emitting diode (AMOLED) displays were studied to improve the image quality. The AMOLED display was designed with low-temperature poly-silicon thin film transistors (TFTs), which have good stability but poor uniformity. To overcome the uniformity problem, the proposed pixel circuit compensates for variations in the threshold voltage and the mobility of the driving TFT. The proposed pixel circuit can operate in two compensation modes for high-luminance operation; and the black data insertion was introduced to improve the characteristics of moving images. The pixel circuit was designed for an 11.6"WXGA top-emission AMOLED panel, and its non-uniformity was estimated to be < 4% with a mobility compensation time of 1 μs.

All-Printed Flexible Organic Transistors Enabled by Surface Tension-Guided Blade Coating

A combination of surface energy-guided blade coating and inkjet printing is used to fabricate an all-printed high performance, high yield, and low variability organic thin film transistor (OTFT) array on a plastic substrate. Functional inks and printing processes were optimized to yield self-assembled homogenous thin films in every layer of the OTFT stack. Specifically, we investigated the effect of capillary number, semiconductor ink composition (small molecule-polymer ratio), and additive high boiling point solvent concentrations on film fidelity, pattern design, device performance and yields.

High-performance HfO<sub>2</sub>/ZrO<sub>2</sub>/IGZO thin-film transistors deposited using atmospheric pressure plasma jet

An atmospheric pressure plasma jet (APPJ) deposition technology that is employed for the preparation of transparent oxide semiconductors film using an eco-friendly water-based metal salt solution as a precursor is presented. Through the use of APPJ indium–gallium–zinc-oxide (IGZO) film as the channel material and a high-k dielectric HfO2/ZrO2 gate stack, IGZO-based transparent thin-film transistors (TFTs) were fabricated and characterised. The HfO2/ZrO2/IGZO-TFTs by APPJ demonstrated excellent electrical characteristics, including a low Vth of 0.63 V, a small subthreshold swing of 0.37 V/dec, a high mobility of 40 cm2/V-s and a large Ion/Ioff ratio of 7 × 108.

Normally-off vertical-type mesa-gate GaN MOSFET

A vertical-type mesa-gate GaN metal–oxide semiconductor field-effect transistor (MOSFET) has been fabricated. The mesa-gate structure can be easily achieved by a single deep etch to the n+-GaN which is the drain of the device, whereas the trench-gate structure, the commonly used structure for the vertical-type MOSFETs, requires an additional etching process to define the gate region. The mesa-gate GaN MOSFET exhibited a normally-off operation with the threshold voltage of 3 V, a normalised drain current of ∼ 55 mA/mm and a high on/off current ratio of 108.