Chang Eun Kim

Analysis of Bias Stress Instability in Amorphous InGaZnO Thin-Film Transistors

In this paper, we report an analysis of electrical bias stress instability in amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs). Understanding the variations of TFT characteristics under an electrical bias stress is important for commercial goals. In this experiment, the positive gate bias is initially applied to the tested a-IGZO TFTs, and subsequently, the negative gate bias is applied to the TFTs. For comparison with the subsequently negative-gate-bias-applied TFTs, another experiment is performed by directly applying the negative gate bias to the tested TFTs. For the positive gate bias stress, a positive shift in the threshold voltage (Vth) with no apparent change in the subthreshold swing (SSUB) is observed. On the other hand, when the negative gate bias is subsequently applied, the TFTs exhibit higher mobility with no significant change in SSUB, whereas the shift of the Vth is much smaller than that in the positive gate bias stress case. These phenomena are most likely induced by positively charged donor-like subgap density of states and the detrapping of trapped interface charge during the positive gate bias stress. The proposed mechanism was verified by device simulation. Thus, the proposed model can explain the instability for both positive and negative bias stresses in a-IGZO TFTs.

Modeling and optimization of the growth rate for ZnO thin films using neural networks and genetic algorithms

The process modeling for the growth rate in pulsed laser deposition (PLD)-grown ZnO thin films was investigated using neural networks (NNets) based on the back-propagation (BP) algorithm and the process recipes was optimized via genetic algorithms (GAs). Two input factors were examined with respect to the growth rate as the response factor. D-optimal experimental design technique was performed and the growth rate was characterized by NNets based on the BP algorithm. GAs was then used to search the desired recipes for the desired growth rate on the process. The statistical analysis for those results was then used to verify the fitness of the nonlinear process model. Based on the results, this modeling methodology can explain the characteristics of the thin film growth mechanism varying with process conditions.

Density-of-States Modeling of Solution-Processed InGaZnO Thin-Film Transistors

The effects of Ga composition on the performance of InGaZnO (IGZO) thin-film transistors (TFTs) prepared by a sol-gel method are investigated, and the density of states (DOS) is characterized by the device modeling. The TFT mode is changed from a depletion type to an enhancement type, and the extracted DOS parameters are reduced with the increase of Ga contents. The extracted DOS distribution has a higher peak value than that of an IGZO TFT prepared by physical vapor deposition.

Mobility enhancement in amorphous InGaZnO thin-film transistors by Ar plasma treatment

The effects of Ar plasma treatment on the back-channel of amorphous InGaZnO (a-IGZO) thin-film transistors are investigated. A decrease in metallic ion-oxygen bonding in the Ar plasma-treated a-IGZO channel layer was observed by X-ray photoelectron spectroscopy (XPS) depth profile analysis. An increase in the channel charge carrier concentration is estimated from the increased oxygen vacancy atomic ratio using XPS curve decomposition analysis. The plasma-treated area of the a-IGZO back-channel is varied with a photoresist screening layer with a varied open window length (Lp). From the Lp-dependent channel resistance analysis, a carrier concentration-dependent field-effect mobility enhancement is observed.The effects of Ar plasma treatment on the back-channel of amorphous InGaZnO (a-IGZO) thin-film transistors are investigated. A decrease in metallic ion-oxygen bonding in the Ar plasma-treated a-IGZO channel layer was observed by X-ray photoelectron spectroscopy (XPS) depth profile analysis. An increase in the channel charge carrier concentration is estimated from the increased oxygen vacancy atomic ratio using XPS curve decomposition analysis. The plasma-treated area of the a-IGZO back-channel is varied with a photoresist screening layer with a varied open window length (Lp). From the Lp-dependent channel resistance analysis, a carrier concentration-dependent field-effect mobility enhancement is observed.

The effects of cation-substitution on the ferroelectric properties of sol-gel derived PZT thin film for FRAM application

Abstract Cation-substituted PZT thin films are more desirable for ferroelectric memory applications due to their lower coercive field, higher resisitivity and lower dielectric memory-aging rate compared to PZT thin films. The resulting effect of cation-substitution can be explained on the basis of defect chemistry in a perovskite lattice. In this study, La and Nb were chosen as substituents and their effects on fatigue behavior and leakage properties of PZT-base thin films were investigated. La- or Nb-substituted PZT thin films were deposited by the sol-gel processing method on Pt electrode. The Zr/Ti ratio was fixed as 40/60 with the tetragonal perovskite phase. The excess Pb added onto the starting precursor was fixed to 15 wt.%. Each sol-gel process condition and heating process were optimized based upon its thermal analysis result. The surface microstructure, crystallinity, ferroelectric properties, and leakage characteristics were investigated. It is shown through C–V and I–V characteristics that both A-site (La) and B- site (Nb) substituent play a role in development of electrical properties. However, cation substitution induces lattice site defect to maintain a charge neutrality. In case of Nb, Pb-deficient pyrochlore phase was formed due to A-site vacancies (Pb vacancies). PNZT films showed the degradation of ferroelectric properties.

Process estimation and optimized recipes of ZnO: Ga thin film characteristics for transparent electrode applications

Ga-doped zinc oxide (ZnO:Ga) thin films were prepared on glass substrate by magnetron sputtering at room temperature (RT) and thermally annealed in hydrogen atmosphere for 1h. The effects of film thickness and annealing temperature on sheet resistance, transmittance and figure of merit of ZnO:Ga thin films were analyzed and modeled using the artificial neural networks (NNets). The NNet models presented the good prediction on sheet resistance, transmittance and figure of merit of ZnO:Ga thin films and it was found that the electrical and optical properties of ZnO:Ga thin films were enhanced by thermal annealing. After NNet models were verified, genetic algorithm (GA) was used to search the optimized recipe for the desired figure of merit of ZnO:Ga thin films. The methodology allows us to estimate the optimal process condition with a small number of experiments.

Effects of nitrogen doping on device characteristics of InSnO thin film transistor

The effects of nitrogen doping on the performance of InSnO thin film transistor were investigated. When the nitrogen was doped in the InSnO channel, device characteristics such as turn-on voltage, subthreshold swing, field effect mobility, and on/off current ratio were enhanced. The N2 insertion in the deposition process decreased the density of the interface trap states and enhanced the crystallinity of the InSnO channel layer. These results indicate that device characteristics can be improved by nitrogen doping in the deposition process.

Modeling and optimization of ITO/Al/ITO multilayer films characteristics using neural network and genetic algorithm

Highlights? We investigated ITO/Al/ITO multilayer films characteristics. ? Variations of Al film thickness and annealing temperature. ? NNet models well represent the characteristics of ITO/Al/ITO multilayer films. ? GA is used to find optimum process condition for maximum figure of merit. In this paper, ITO/Al/ITO multilayer films are fabricated with the variations of Al film thickness and annealing temperature. The effects of Al film thickness and annealing temperature on sheet resistance, optical transmittance, and the figure of merit are analyzed in the aid of the artificial neural network (NNet) models. In order to verify the fitness of NNet model, the root mean square error (RMSE) of training and testing data are calculated. The NNet models well represent the measured sheet resistance, optical transmittance, and the figure of merit. After NNet model is established, genetic algorithm (GA) is used to find the optimum process condition for the ITO/Al/ITO multilayer films to obtain maximum figure of merit in the design space.

Characterization of Al 2 O 3 films grown by electron beam evaporator on Si substrates

We report the characterization of aluminum oxide (Al<inf>2</inf>O<inf>3</inf>) films which are grown on Si substrates by electron beam evaporator. This paper focuses on the characteristic variation of Al2O3 films depending on the different annealing techniques, such as post-deposition annealing and post-metallization annealing. The capacitance-voltage (C-V) curves indicate a negative charge and interface trap charge density between the Al<inf>2</inf>O<inf>3</inf> film and Si interface. The current-voltage (I-V) curves show a leakage current. The x-ray diffraction (XRD) patterns show the crystallinity of Al<inf>2</inf>O<inf>3</inf> films. Based on the results, the annealing effect is important condition to increase negative fixed charge in the Al<inf>2</inf>O<inf>3</inf> films.

Electrical characterization and conduction mechanism of high-k Ti 1−x Si x O 2 gate dielectrics

Ti<inf>1−x</inf>Si<inf>x</inf>O<inf>2</inf> dielectric thin films were prepared by co-sputtering deposition at room temperature. Electrical properties of high-k Ti<inf>1−x</inf>Si<inf>x</inf>O<inf>2</inf> dielectric thin film were characterized and the leakage current mechanism was analyzed. As the TiO<inf>2</inf> power increases, the dielectric constant is increased from 14 to 43 and the dominant leakage current mechanism is changed from Schottky emission to Poole-Frenkel emission.