Himchan Oh

Photon-accelerated negative bias instability involving subgap states creation in amorphous In–Ga–Zn–O thin film transistor

We investigated the visible photon accelerated negative bias instability (NBI) in amorphous In–Ga–Zn–O (a-IGZO) thin film transistor (TFT). As reported in previous works, the rigid shift in transfer curves with insignificant changes in field-effect mobility and subthreshold swing was observed. On the other hand, there is substantial change in capacitance-voltage characteristics caused by created subgap states. The suggested nature of created states is the ionized oxygen vacancy (VO2+) by the combination of visible light and negative bias. The generated VO2+ states enhance the NBI under illumination as increased deep hole trapping centers. Furthermore, the photoexcitation of VO to stable VO2+ yields excess free carriers in conduction band. The increased carrier density also enhances the negative shift in turn-on voltage of a-IGZO TFT.

Transition of dominant instability mechanism depending on negative gate bias under illumination in amorphous In-Ga-Zn-O thin film transistor

The gate bias dependence on the negative bias instability under illumination was examined. As the gate bias got more negative, dominant mechanism was changed from simple charge trapping to that accompanied by generation of subgap states. Degree of threshold voltage shift was not monotonously dependent on the magnitude of negative gate bias. It is strongly related with the corresponding instability modes for different gate bias regimes. The transition of instability mechanism depends on how much the gate bias stabilizes ionized oxygen vacancy states.

Low-Temperature Processed Flexible In–Ga–Zn–O Thin-Film Transistors Exhibiting High Electrical Performance

In-Ga-Zn-O thin-film transistors processed at 150°C on laminated polyethylene naphthalate substrates exhibit ing high electrical performances such as a saturation mobility of 24.26 cm2/(V · s), a subthreshold slope of 140 mV/dec, a turn-on voltage Von of -0.41 V, and an on-off ratio of 1.8 × 109 were fabricated. Cool-off-type adhesive was adopted to easily detach the plastic substrate from the carrier holder. Devices also showed highly uniform characteristics with a variation of 0.09 V in turn-on voltage. Stability characteristics under the positive gate bias stress can be enhanced by increasing the annealing time at 150°C.

Enhanced bias illumination stability of oxide thin film transistor through insertion of ultrathin positive charge barrier into active material

A novel strategy to enhance the bias and illumination stress stability of oxide thin-film transistors (TFTs) is presented. The ultrathin positive charge barrier is employed to block the movement of photo-generated charges toward the interface between gate insulator and semiconductor under negative gate bias and illumination. This method can break through the limitation in stability enhancement caused by the inevitable oxygen vacancy and facilitates the fabrication of highly stable oxide TFTs at low process temperature.

Nonvolatile memory thin-film transistors using an organic ferroelectric gate insulator and an oxide semiconducting channel

Organic–inorganic hybrid-type nonvolatile memory thin-film transistors using an organic ferroelectric gate insulator and an oxide semiconducting active channel are a very promising solution to the memory devices having both features of low-cost and high-performance, which are embeddable into the next-generation flexible and transparent electronics. In this paper, we discuss some important issues for this proposed device, such as device structure design, process optimization and memory array integration. Promising feasible applications and remaining technology issues to solve were also discussed.

Double-layered passivation film structure of Al2O3/SiNx for high mobility oxide thin film transistors

The optimization of the passivation process for oxide thin film transistors with high carrier mobility was investigated. Hydrogen incorporation into oxide channels during the deposition of SiNx could degrade device stability and uniformity, especially for high-mobility devices. A novel double-layered passivation film structure composed of Al2O3/SiNx was proposed, in which thin and dense Al2O3 film prepared by atomic layer deposition was introduced underneath the SiNx layer. In-Ga-Zn-O TFT passivated with the proposed double-layered films showed no significant negative shift in turn-on voltage, even after passivation. The field-effect mobility and subthreshold swing were typically measured as 27.7 cm2 V−1 s−1 and 0.11 V/dec, respectively. Hydrogen doping was effectively protected by the introduction of Al2O3 as thin as 15 nm.

Vertical Channel ZnO Thin-Film Transistors Using an Atomic Layer Deposition Method

Vertical channel ZnO thin-film transistors (TFTs) were fabricated on glass and flexible substrates. Conformally deposited thin films prepared using atomic layer deposition were used for the active layer, gate insulator, and gate electrode. Owing to the very short channel (0.5 μm) and very thin (20 nm) gate insulator layer, the ON-current of the vertical channel ZnO TFT was 57 μA at the gate and drain voltages of 3 and 4 V, respectively. Vertical channel oxide TFTs may be promising for device applications with low power consumption.

Characterization of amorphous multilayered ZnO-SnO2 heterostructure thin films and their field effect electronic properties

Multilayered ZnO-SnO2 heterostructure thin films were produced using pulsed laser ablation of pie-shaped ZnO-SnO2 oxides target, and their structural and field effect electronic transport properties were investigated as a function of the thickness of the ZnO and SnO2 layers. The films have an amorphous multilayered heterostructure composed of the periodic stacking of the ZnO and SnO2 layers. The field effect electronic properties of amorphous multilayered ZnO-SnO2 heterostructure thin film transistors (TFTs) are highly dependent on the thickness of the ZnO and SnO2 layers. The highest electron mobility of 37 cm2/V s, a low subthreshold swing of a 0.19 V/decade, a threshold voltage of 0.13 V, and a high drain current on-to-off ratio of ∼1010 obtained for the amorphous multilayered ZnO(1.5 nm)-SnO2(1.5 nm) heterostructure TFTs. These results are presumed to be due to the unique electronic structure of an amorphous multilayered ZnO-SnO2 heterostructure film consisting of ZnO, SnO2, and ZnO-SnO2 interface layers.

Non-uniform sampling and wide range angular spectrum method

A novel method is proposed for simulating free space field propagation from a source plane to a destination plane that is applicable for both small and large propagation distances. The angular spectrum method (ASM) was widely used for simulating near field propagation, but it caused a numerical error when the propagation distance was large because of aliasing due to under sampling. Band limited ASM satisfied the Nyquist condition on sampling by limiting a bandwidth of a propagation field to avoid an aliasing error so that it could extend the applicable propagation distance of the ASM. However, the band limited ASM also made an error due to the decrease of an effective sampling number in a Fourier space when the propagation distance was large. In the proposed wide range ASM, we use a non-uniform sampling in a Fourier space to keep a constant effective sampling number even though the propagation distance is large. As a result, the wide range ASM can produce simulation results with high accuracy for both far and near field propagation. For non-paraxial wave propagation, we applied the wide range ASM to a shifted destination plane as well.

Unusual instability mode of transparent all oxide thin film transistor under dynamic bias condition

We report a degradation behavior of fully transparent oxide thin film transistor under dynamic bias stress which is the condition similar to actual pixel switching operation in active matrix display. After the stress test, drain current increased while the threshold voltage was almost unchanged. We found that shortening of effective channel length is leading cause of increase in drain current. Electrons activate the neutral donor defects by colliding with them during short gate-on period. These ionized donors are stabilized during the subsequent gate-off period due to electron depletion. This local increase in doping density reduces the channel length.