Ilgu Yun

Effects of Thickness Variation on Properties of ZnO Thin Films Grown by Pulsed Laser Deposition

A series of ZnO films with various thicknesses were prepared on (0001) sapphire substrate by pulsed laser deposition (PLD). Scanning electron microscopy (SEM) and x-ray diffraction (XRD) analysis were utilized to investigate the effects of thickness variation on the surface morphology and the crystallinity. The electrical and optical properties of the films were also investigated as a function of the film thickness. It was found that the crystalline quality, electrical and optical properties of the films depended on the film thickness and were improved with increasing the film thickness. This is attributed to the fact that the films thinner than 4000 A are under the severe misfit strain, which decreases with increasing the film thickness further.

Formation of p-type ZnO film on InP substrate by phosphor doping

Abstract ZnO thin film was initially deposited on InP substrate by radio frequency (rf) magnetron sputtering and the diffusion process was performed using the closed ampoule technique where Zn3P2 was used as the dopant source. To verify the junction formation of ZnO thin films, the electrical properties were measured, and the effects of Zn3P2 diffusion on ZnO thin films were investigated. It is observed that the electrical property of the film is changed from n-type to p-type by dopant diffusion effect. Based on the results, it is confirmed that ZnO thin films can be a potential candidate for ultraviolet (UV) optical devices.

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.

Annealing effect on the structural and optical properties of ZnO thin film on InP

Abstract II–VI semiconducting ZnO thin films have been fabricated by pulsed laser deposition (PLD) process on indium phosphide (InP) (100) substrates. Thin films were annealed at various temperatures in order to study the annealing temperature dependence of the structural and optical properties of ZnO thin film grown on InP substrate. The structural and optical properties were characterized with X-ray diffraction (XRD) and photoluminescence (PL), respectively. In our study, we have found some defect levels from the PL spectra and derived the defect centers activation energy. According to XRD data, it could be thought that the films had some strains but relaxed by annealing processes.

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.

Effects of channel thickness variation on bias stress instability of InGaZnO thin-film transistors

Here, we report on the effects of channel (or active) layer thickness on the bias stress instability of InGaZnO (IGZO) thin-film transistors (TFTs). The investigation on variations of TFT characteristics under the electrical bias stress is very crucial for commercial applications. In this work, the initial electrical characteristics of the tested TFTs with different channel layer thicknesses (40, 50, and 60 nm) are performed. Various gate bias (VGS) stresses (10, 20, and 30 V) are then applied to the tested TFTs. For all VGS stresses with different channel layer thickness, the experimentally measured threshold voltage shift (DVth )a s a function of stress time is precisely modeled with stretched-exponential function. It is indicated that the DVth is generated by carrier trapping but not defect creation. It is also observed that the DVth shows incremental behavior as the channel layer thickness increases. Thus, it is verified that the increase of total trap states (NT) and free carriers resulted in the increase of DVth as the channel layer thickness increases. 2011 Elsevier Ltd. All rights reserved.

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.

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.

Zinc diffusion process investigation of InP-based test structures for high-speed avalanche photodiode fabrication

Abstract The characterization of zinc diffusion processes for three different test structures has been investigated. The comparison between the different diffusion process conditions for different test structures were explored. The zinc diffusion profiles, such as the diffusion depth and the zinc dopant concentration, were examined using secondary ion mass spectrometry with varying the amount of Zn3P2 source, the ampoule volume, and the diffusion time. It is observed that the diffusion profiles are severely impacted on the process parameters, such as the amount of Zn3P2 source, the ampoule volume, and the diffusion time, as well as material parameters, such as doping concentration of the diffusion layer. These results from the Zn diffusion process can be utilized for the high-speed InP/InGaAs avalanche photodiodes fabrication.