Joonwoo Kim

Bright, wind-driven white mechanoluminescence from zinc sulphide microparticles embedded in a polydimethylsiloxane elastomer

A variety of mechanoluminescent (ML) materials have recently reinvigorated studies of luminescence activated by mechanical stress, but few practical applications have been demonstrated due to the destructive nature of the process. To overcome these shortcomings, elastico-mechanoluminescent (elastico-ML) materials, which generate luminescence under elastic deformation, have been suggested with a view to their use in practical devices. However, the weak brightness and limited white colour expression of these materials must be resolved before they can be employed in practical applications. Here, we report a wind-driven ML device that produces significant brightness and emits warm/neutral/cool white light over a range of colour temperatures from zinc sulphide (ZnS) microscopic particles embedded in a polydimethylsiloxane (PDMS) composite. Harnessing wind-activated mechanoluminescent devices in practical displays or lighting systems could pave the way to new environmentally friendly lights, which reduce energy waste and promote sustainability.

High performance, transparent a-IGZO TFTs on a flexible thin glass substrate

We investigated electrical properties of transparent amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) with amorphous indium zinc oxide (a-IZO) transparent electrodes on a flexble thin glass substrate. The TFTs show a high field-effect mobility, a good subthreshold slope and a high on/off ratio owing to the high temperature thermal annealing process which cannot be applied to typical transparent polymer-based flexible substrates. Bias stress instability tests applying tensile stress concurrently with the bending radius of up to 40 mm indicated that mechanically and electrically stable a-IGZO TFTs can be fabricated on the transparent thin glass substrate.

Electrical characterization of a-InGaZnO thin-film transistors with Cu source/drain electrodes

We analyzed the effects of Cu source/drain (S/D) electrodes on the performance of a-InGaZnO (a-IGZO) thin-film transistors (TFTs). Owing to the Cu migration, the parasitic resistance was as low as 10 Ω cm with small current transfer length. Based on the transfer characteristics, we found that VDS dependent Cu migration creates donor-like deep and tail states in the sub-bandgap region. The feasibility of Cu S/D electrodes for a-IGZO TFTs using inverter circuits indicates that fabrication of high performance circuits is possible by controlling the Cu electro-migration.

Oxygen Dispersive Diffusion Induced Bias Stress Instability in Thin Active Layer Amorphous In–Ga–Zn–O Thin-Film Transistors

We studied the bias stress instability of amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) by varying the active layer thickness (t) from 6 to 100 nm. We found that the stretched exponential relationship between the threshold voltage shift and the stress time can be explained by oxygen dispersive diffusion which is absorbed near the back channel region during an oxygen annealing process in the active layer. For an a-IGZO TFT with t=6 nm, direct exposure of the channel layer to the ambient oxygen greatly increases the bias stress instability and induces hump like characteristics, indicating that the creation of acceptor-like states is the dominant mechanism of the instability of a-IGZO TFTs with a thin active layer.

Scaling behaviour of a-IGZO TFTs with transparent a-IZO source/drain electrodes

We analysed the scaling behaviour of amorphous indium gallium zinc oxide thin-film transistors (a-IGZO TFTs) with amorphous indium zinc oxide (a-IZO) transparent source/drain (S/D) electrodes. Due to the sputtering damage of the back-channel region during the a-IZO deposition process, the output characteristics show early saturation behaviour and the field-effect mobility in the saturation region is severely decreased in comparison with that in the linear region, especially when the channel length is decreased. Based on the transmission line method, we found that a long gate overlap distance is required due to the long current transfer length. Therefore, optimizing the parasitic resistance is required for the scaling down of a-IGZO TFTs with transparent a-IZO S/D electrodes.

Intrinsic parameter extraction of a-InGaZnO thin-film transistors by a gated-four-probe method

We analyzed the intrinsic electrical characteristics of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) using a gated-four-probe method. Based on the back channel potential, the extraction of intrinsic field-effect mobility (μFEi) and parasitic resistance in source (Rs) and drain (Rd) electrodes was performed especially for low VGS and VDS conditions. The resulting μFEi showed typical VGS dependency of amorphous semiconductor TFTs. However, Rs and Rd showed that there can be non-uniformity in source/drain parasitic resistance, which indicates that a separate analysis of the parameters of each electrode is essential for further improvement of the performance of a-IGZO TFTs.

Analysis of temperature-dependent electrical characteristics in amorphous In-Ga-Zn-O thin-film transistors using gated-four-probe measurements

We analyzed the temperature-dependent electrical characteristics of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) using a gated-four-probe method (GFP) with temperatures ranging from 93 to 373 K. The intrinsic field-effect mobility and source/drain parasitic resistance were separately extracted using the GFP method. We found that temperature-dependent transfer characteristics originated from the temperature-dependent intrinsic field-effect mobility of the a-IGZO TFTs. The parasitic resistance was also correlated with the intrinsic-field effect mobility, which decreases as the intrinsic field-effect mobility increases, indicating that access parasitic resistance originated from bulk regions rather than metal/semiconductor junction barrier is a key factor to determine the parasitic resistance of a-IGZO TFTs.

Novel Gated-Multiprobe Method for Measuring a Back Electrode Effect in Amorphous Oxide-Based Thin-Film Transistors

In this paper, we investigated the variations in electrical characteristics of amorphous indium-gallium-zinc-oxide thin-film transistors using a gated-multiprobe method when additional probe electrodes are on the back-channel region. We found that the resistance of the probe region is much smaller than that of the nonprobe region, which can be modeled by a series connection of transistors and resistors indicating that the probe region is independent of VGS and induces a decrease in effective channel length. We also performed technology computer aided design (TCAD) simulations and found that the effective channel length decreases and drain current increases, which is consistent with the experiments.

Physical characterization of amorphous In-Ga-Zn-O thin-film transistors with direct-contact asymmetric graphene electrode

High performance a-IGZO thin-film transistors (TFTs) are fabricated using an asymmetric graphene drain electrode structure. A-IGZO TFTs (channel length = 3 μm) were successfully demonstrated with a saturation field-effect mobility of 6.6 cm2/Vs without additional processes between the graphene and a-IGZO layer. The graphene/a-IGZO junction exhibits Schottky characteristics and the contact property is affected not only by the Schottky barrier but also by the parasitic resistance from the depletion region under the graphene electrode. Therefore, to utilize the graphene layer as S/D electrodes for a-IGZO TFTs, an asymmetric electrode is essential, which can be easily applied to the conventional pixel electrode structure.

Screen-Printed Cu Source/Drain Electrodes for a-InGaZnO Thin-Film Transistors

We report screen-printed copper source/drain electrodes for a-InGaZnO (a-IGZO) thin-film transistors (TFTs). The best electrical characteristics of the a-IGZO TFTs were a field-effect mobility of 2.06 cm2/Vs, a threshold voltage of 3.40 V, an on/off current ratio of 6.0 × 103A/A, and a subthreshold swing of 7.02 V/decade. Resulting TFT performances indicate that blocking the inter-diffusion of Cu and impurities is a key factor to fabricate low leakage current and high performance a-IGZO TFTs with printed Cu S/D electrodes.