Kwang-Seok Jeong

Investigation of zinc interstitial ions as the origin of anomalous stress-induced hump in amorphous indium gallium zinc oxide thin film transistors

In this paper, we investigated an anomalous hump in the bottom gate staggered amorphous indium-gallium zinc oxide thin-film transistors. During the positive gate bias stress, a positive threshold voltage shift is observed in transfer curve and an anomalous hump occurs as the stress time increases. The hump becomes more serious as the gate bias stress increases while it is not observed under the negative bias stress. From the simulation of a long range migration of zinc interstitial ions (Zni) and the measurement of the diode characteristics after the constant positive bias stress, the origin of the hump can be explained by the migration of the positively charged mobile Zni during the constant positive gate bias stress, which can be conformed by increasing the concentration of Zni from the result of the Auger ZnL3M4.5M4.5 spectra.

Crystal Quality Effect on Low-Frequency Noise in ZnO TFTs

The effect of ZnO active film quality on the low-frequency noise behavior in ZnO thin-film transistors has been investigated. The film crystalline is varied by differentiating the thickness and adding postannealing. To discriminate the origin of 1/f noise, the gate bias dependence of noise spectra is investigated. It is found that the number fluctuation noise model related with trapping/detrapping by traps near the interface becomes dominant as the crystal quality improves, which is also confirmed by another noise parameter, i.e., α Extracted αapp can also well explain the electrical characteristics.

Anomalous Stress-Induced Hump Effects in Amorphous Indium Gallium Zinc Oxide TFTs

In this paper, we investigated the anomalous hump in the bottom gate staggered a-IGZO TFTs. During the positive bias stress, a positive threshold voltage shift was observed in the transfer curve and an anomalous hump occurred as the stress time increased. The hump became more serious in higher gate bias stress while it was not observed under the negative bias stress. The analysis of constant gate bias stress indicated that the anomalous hump was influenced by the migration of positively charged mobile interstitial zinc ion towards the top side of the a-IGZO channel layer.

Analysis of stability improvement in ZnO thin film transistor with dual-gate structure under negative bias stress

In this study, we fabricated dual-gate zinc oxide thin film transistors (ZnO TFTs) without additional processes and analyzed their stability characteristics under a negative gate bias stress (NBS) by comparison with conventional bottom-gate structures. The dual-gate device shows superior electrical parameters, such as subthreshold swing (SS) and on/off current ratio. NBS of VGS = −20 V with VDS = 0 was applied, resulting in a negative threshold voltage (Vth) shift. After applying stress for 1000 s, the Vth shift is 0.60 V in a dual-gate ZnO TFT, while the Vth shift is 2.52 V in a bottom-gate ZnO TFT. The stress immunity of the dual-gate device is caused by the change in field distribution in the ZnO channel by adding another gate as the technology computer aided design (TCAD) simulation shows. Additionally, in flicker noise analysis, a lower noise level with a different mechanism is observed in the dual-gate structure. This can be explained by the top side of the ZnO film having a larger crystal and fewer grain boundaries than the bottom side, which is revealed by the enhanced SS and XRD results. Therefore, the improved stability of the dual-gate ZnO TFT is greatly related to the E-field cancellation effect and crystal quality of the ZnO film.

Effects of the Al2O3 interlayer in ZnO thin-film transistors fabricated via atomic layer deposition

In this paper, electrical analysis was carried out on ZnO thin-film transistors (TFTs) to investigate the effects of the Al2O3 interlayer between the ZnO active-layer and the SiO2 gate insulator. The Al2O3 interlayer was deposited via atomic layer deposition. From the electrical analysis, it was found that the Al2O3 interlayer improves the electrical properties, such as the subthreshold slope and on/off-current ratio. The stability degradation under positive bias stress, however, is severe in TFTs with an Al2O3 interlayer. According to the crystallographic view, the crystal structures in a ZnO film on Al2O3 are prone to align in the (002) direction compared with ZnO on SiO2, and can attract more oxygen ions because of the Zn polar, which degrades the stability of devices.

Process Temperature Dependence of Al2O3Film Deposited by Thermal ALD as a Passivation Layer for c-Si Solar Cells

This paper presents a study of the process temperature dependence of Al₂O₃ film grown by thermal atomic layer deposition (ALD) as a passivation layer in the crystalline Si (c-Si) solar cells. The deposition rate of Al₂O₃ film maintained almost the same until 250 ℃, but decreased from 300 ℃. Al₂O₃ film deposited at 250 ℃ was found to have the highest negative fixed oxide charge density (Qf) due to its O-rich condition and low hydroxyl group (-OH) density. After post-metallization annealing (PMA), Al₂O₃ film deposited at 250 ℃ had the lowest slow and fast interface trap density. Actually, Al₂O₃ film deposited at 250 ℃ showed the best passivation effects, that is, the highest excess carrier lifetime (τPCD) and lowest surface recombination velocity (Seff) than other conditions. Therefore, Al₂O₃ film deposited at 250 ℃ exhibited excellent chemical and field-effect passivation properties for p-type c-Si solar cells.

Blistering Induced Degradation of Thermal Stability Al2O3Passivation Layer in Crystal Si Solar Cells

Different kinds of post-deposition annealing (PDA) by a rapid thermal process (RTP) are used to enhance the field-effect passivation of Al2O3 film in crystal Si solar cells. To characterize the effects of PDA on Al2O3 and the interface, metalinsulator semiconductor (MIS) devices were fabricated. The effects of PDA were characterized as functions of RTP temperature from 400~700 °C and RTP time from 30~120 s. A high temperature PDA can retard the passivation of thin Al2O3 film in c-Si solar cells. PDA by RTP at 400 °C results in better passivation than a PDA at 400 °C in forming gas (H2 4% in N2) for 30 minutes. A high thermal budget causes blistering on Al2O3 film, which degrades its thermal stability and effective lifetime. It is related to the film structure, deposition temperature, thickness of the film, and annealing temperature. RTP shows the possibility of being applied to the PDA of Al2O3 film. Optimal PDA conditions should be studied for specific Al2O3 films, considering blistering.

Novel silicon surface passivation by Al2O3/ZnO/Al2O3 films deposited by thermal atomic layer deposition

In this paper, a novel Al2O3/ZnO/Al2O3 stack is proposed as the silicon passivation layer for c-Si solar cell application. Recently, the Al2O3 film has been proved to be effective for passivating the p-type c-Si surface by forming the negative fixed oxide charge. It is confirmed by this experiment that the amount of negative fixed oxide charge can be controlled by inserting a ZnO interlayer (IL), which is explained by acceptor-like defect (VZn, Oi, and OZn) formation determined by the room-temperature photoluminescence (RTPL) analysis. The effect of ZnO IL is investigated using Al2O3 bottom layers of various thicknesses by electrical and physical analyses. The effective lifetime measurement shows that the electronic recombination losses at the silicon surface are reduced effectively by optimizing the Al2O3/ZnO/Al2O3 stack.

An analysis on applicability of Ti-doped ZnO films as the channel layer of TFTs

In this paper, Ti-doped ZnO TFTs on SiO2/Si substrates by simultaneous RF sputter of Zn and DC magnetron sputter of Ti are successfully fabricated. With undoped ZnO TFTs, as-grown Ti-doped ZnO are compared with post-annealed Ti-doped ZnO TFTs in the furnace at O2 atmosphere of 300 °C. As the annealing time increases, the electrical characteristics such as sub-threshold slop (SS) and on/off current ratio of Ti-doped ZnO TFT become better. In order to find out the reason of performance improvement, the optical analysis is carried out. The data of XRD and AFM indicate that grain size and RMS (root mean square) roughness increase in accordance with annealing time, and the potential barrier and work function of Ti-doped ZnO is smaller than that of undoped ZnO, which indicates that the performance improvement by post-annealing in O2 atmosphere is due to a crystalline reformation in Ti-doped ZnO films.

Enhanced Photo Current in n-ZnO/p-Si Diode Via Embedded Ag Nanoparticles for the Solar Cell Application

In this study, an n-ZnO/p-Si heterojunction diode with embedded Ag nanoparticles was fabricated to investigate the possible improvement of light trapping via the surface plasmon resonance effect for solar cell applications. The Ag nanoparticles were fabricated by the physical sputtering method. The acquired current-voltage curves and optical absorption spectra demonstrated that the application of Ag nanoparticles in the n-ZnO/p-Si interface increased the photo current, particularly in specific wavelength regions. The results indicate that the enhancement of the photo current was caused by the surface plasmon resonance effect generated by the Ag nanoparticles. In addition, minority carrier lifetime measurements showed that the recombination losses caused by the Ag nanoparticles were negligible. These results suggest that the embedding of Ag nanoparticles is a powerful method to improve the performance of n-ZnO/p-Si heterojunction solar cells.