Ji Hun Choi

Solution-processed indium-free ZnO/SnO2 bilayer heterostructures as a low-temperature route to high-performance metal oxide thin-film transistors with excellent stabilities

The realization of high performance solution-processable metal oxide thin-film transistors (TFTs) with low annealing temperatures remains a challenge in the field of flexible and/or transparent electronics. Indium-based metal oxides are one of the most widely used materials as channel layers of metal oxide TFTs. However, the need for developing indium-free metal oxide materials has become urgent because of the high cost and limited supply of indium. Herein, we report high-performance solution-processed indium-free metal oxide TFTs prepared with low annealing temperatures by introducing ZnO/SnO2 bilayer heterostructures. After photo- and thermal annealing, ZnO/SnO2 bilayers form a unique nanostructure composed of three zones: Zn-only, Zn–Sn-mixed, and Sn-rich zones. The resulting ZnO/SnO2 TFTs exhibit outstanding mobility values as high as 15.4 cm2 V−1 s−1 with a low annealing temperature of 300 °C. These values are the highest yet measured among indium-free and solution-processed metal oxide TFTs prepared under similar annealing conditions. The ZnO/SnO2 TFTs also show remarkable outstanding operational stabilities under various external bias stresses. Their high performances and excellent stabilities can be attributed to the combinational effects of the highly conductive ultrathin Sn-rich channel and balanced carrier concentrations in the Zn–Sn-mixed region. We believe that our work provides a facile route to prepare inexpensive solution-processed electronic devices with earth-abundant materials such as backplane circuits for large-area and flexible displays.

Defects and Charge-Trapping Mechanisms of Double-Active-Layer In–Zn–O and Al–Sn–Zn–In–O Thin-Film Transistors

Active matrix organic light-emitting diodes (AMOLEDs) are considered to be a core component of next-generation display technology, which can be used for wearable and flexible devices. Reliable thin-film transistors (TFTs) with high mobility are required to drive AMOLEDs. Recently, amorphous oxide TFTs, due to their high mobility, have been considered as excellent substitutes for driving AMOLEDs. However, the device instabilities of high-mobility oxide TFTs have remained a key issue to be used in production. In this paper, we present the charge-trapping and device instability mechanisms of high-mobility oxide TFTs with double active layers, using In-Zn-O (IZO) and Al-doped Sn-Zn-In-O (ATZIO) with various interfacial IZO thicknesses (0-6 nm). To this end, we employed microsecond fast current-voltage (I-V), single-pulsed I-V, transient current, and discharge current analysis. These alternating-current device characterization methodologies enable the extraction of various trap parameters and defect densities as well as the understanding of dynamic charge transport in double-active-layer TFTs. The results show that the number of defect sites decreases with an increase in the interfacial IZO thickness. From these results, we conclude that the interfacial IZO layer plays a crucial role in minimizing charge trapping in ATZIO TFTs.

Switchable subwavelength plasmonic structures with phase-change materials for reflection-type active metasurfaces in the visible region

In this work, a switchable plasmonic structure is proposed for reflection-type spatial light modulation in the visible range with subwavelength resolution. This structure is based on a metallic grating in which each resonant cavity couples the incident light into a gap surface plasmon mode and then reflects the light modulated in the cavity. By incorporating an ultrathin layer of the phase-change material Ge2Sb2Te5 at the entrance of the cavity, the optical modulation characteristic of the structure can be switched between two modes. Numerical investigations are conducted to verify the proposed structure, with the focused analysis of two common types of binary modulations: amplitude-only and phase-only modulations.

Development of high-resolution active matrix spatial light modulator

Abstract. We developed a high-resolution active matrix spatial light modulator on a glass substrate. To integrate a switching device on the glass substrate, we designed a high-performance oxide thin-film transistor with a minimum channel length of 1  μm and a maximum processing temperature of 380°C. To drive a large number of data lines, we used multiple source drivers and data drivers as well. For an optical modulation, we optimized a liquid crystal of a high anisotropic refractive index of 0.25 with a cell gap of 2.5  μm, which was effectively operated until pixel pitch is 1.6  μm. Hologram was successfully reconstructed by fabricated SLM with 7-μm pixel pitch. For the other approach for a high-resolution spatial light modulator, we tested a phase change material of Ge2Sb2Te5 [GST]. The variation of refractive index between a polycrystalline phase and an amorphous phase of the GST film is used for a hologram reconstruction. By optimizing the underlying oxide thickness, we can show a color hologram without color filters.

InZnO/AlSnZnInO Bilayer Oxide Thin-Film Transistors With High Mobility and High Uniformity

In this letter, high-performance InZnO/AlSnZnInO (IZO/ATZIO) bilayer thin-film transistors (TFTs) with an inverted staggered back channel etch structure are presented. The channel width and the length were both 6 μm, which is small enough to be adapted to a high-resolution display backplane. High field-effect mobility (μFE) over 60 cm2/Vs was obtained from the structure with 8-nm IZO channel insertion between the gate dielectric and the ATZIO layer. The device shows good controllability in light of TFT operation. The subthreshold slope, turn-ON voltage (VON), and ON/OFF ratio were 0.16 V/decade, -1.52 V, and 5×109, respectively.

Development of spatial light modulator with ultra fine pixel pitch for electronic holography (Conference Presentation)

SLM (Spatial Light Modulator) with ultra fine pixel pitch (circa 1 micron meter) has been thought as a big issue to realize electronic hologram with wide viewing angle. Two types of approach are proposed for accomplish SLM panel with 1 micron meter pitch pixel. SLM with LC light modulator controlled by TFT based backplane constructed on glass substrate is proposed according to the methods of scaling down flat-panel display technology. Introduction of sub micron meter patterning processes, the SLM panel with 3 micron meter pitch pixel was successfully developed for the first time. The SLM with 2 inch diagonal length had the resolution of 16K by 2K. Hologram with depth was reconstructed with manufactured SLM. Oxide semiconductor TFTs of 1 micron meter channel length with high performance have been developed for the SLM. Technical issues to accomplish 1 micron meter pitch pixel will be discussed. PCM (phase change material) has been used for memory devices and information recording devices. In former case, information is recorded and read using electrical signal. For latter case, information is recorded and read using light (laser) signal. We propose a SLM with PCM such that information is recorded using electrical signal and read using light signal. The light modulation of PCM pattern recorded by pulsed laser was successfully demonstrated using reconstruction of hologram images. Operation of arrayed pixels with PCM pattern driven by Si MOSFET is under development. Technical challenges for SLM with PCM will be discussed.

Rewritable full-color computer-generated holograms based on color-selective diffractive optical components including phase-change materials

We propose rewritable full-color computer-generated holograms (CGHs) based on color-selective diffraction using the diffractive optical component with the resonant characteristic. The structure includes an ultrathin layer of phase-change material Ge2Sb2Te5 (GST) on which a spatial binary pattern of amorphous and crystalline states can be recorded. The CGH patterns can be easily erased and rewritten by the pulsed ultraviolet laser writing technique owing to the thermally reconfigurable characteristic of GST. We experimentally demonstrate that the fabricated CGH, having a fine pixel pitch of 2 μm and a size of 32.8 × 32.8 mm2, reconstructs the three-dimensional holographic images. In addition, the feasibility of the rewritable property is verified by erasing and rewriting part of the CGH.

Light illumination effect in AIZTO/IZO dual-channel TFTs

In oxide thin-film transistors (TFTs), light illumination effect is a big concern due to its operating condition. Light illumination can change many electrical properties in oxide TFTs such as mobility and threshold voltage (Vth). In many researches, Oxygen vacancy is suspected as a main cause of the changes by light illumination. Recently, the back channel formation by field-induced macroscopic barrier model is reported under light illumination. This is also related to Oxygen vacancy. In this letter, we investigate the gradual changes in DC and CV characteristics depending on the dual-channel thicknesses. For this purpose, we use the aluminum-doped indium zinc tin oxide (AIZTO)/indium zinc oxide (IZO) dual-channel TFTs. The main goal of this paper is to find the main cause of the changes by light illumination in various dual-channel thicknesses.