Satoshi Urakawa

Thermal analysis of amorphous oxide thin-film transistor degraded by combination of joule heating and hot carrier effect

Stability is the most crucial issue in the fabrication of oxide thin-film transistors (TFTs) for next-generation displays. We have investigated the thermal distribution of an InSnZnO TFT under various gate and drain voltages by using an infrared imaging system. An asymmetrical thermal distribution was observed at a local drain region in a TFT depending on bias stress. These phenomena were decelerated or accelerated with stress time. We discussed the degradation mechanism by analyzing the electrical properties and thermal distribution. We concluded that the degradation phenomena are caused by a combination of Joule heating and the hot carrier effect.

Analysis of electronic structure of amorphous InGaZnO/SiO2 interface by angle-resolved X-ray photoelectron spectroscopy

The electronic structures of amorphous indium gallium zinc oxide (a-IGZO) on a SiO2 layers before and after annealing were observed by constant final state X-ray photoelectron spectroscopy (CFS-XPS) and X-ray adsorption near-edge structure spectroscopy (XANES). From the results of angle-resolved CFS-XPS, the change in the electronic state was clearly observed in the a-IGZO bulk rather than in the a-IGZO/SiO2 interface. This suggests that the electronic structures of the a-IGZO bulk strongly affected the thin-film transistor characteristics. The results of XANES indicated an increase in the number of tail states upon atmospheric annealing (AT). We consider that the increase in the number of tail states decreased the channel mobility of AT samples.

Effect of contact material on amorphous InGaZnO thin-film transistor characteristics

Amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) having several metals, namely Ag, Ti, and Mo, as the source and drain electrodes were characterized. TFTs with Ti and Mo electrodes showed drain current–gate voltage characteristics without fluctuation. However, TFTs with Ag electrodes indicated a low noisy on-state current at a large channel length under a low drain–source voltage condition. The source and drain resistances [Rs/d (Ω)] of the TFTs with each of the three metals were calculated from the IDS–VGS characteristics. The Rs/d values of the Ag, Ti, and Mo samples reached 4 × 104, 2 × 104, and 1 × 104 Ω, respectively. This implies that a spatial potential barrier exists at the a-IGZO/Ag interface and that the resistance of the potential barrier changes with the application of gate voltage.

Thermal analysis for observing conductive filaments in amorphous InGaZnO thin film resistive switching memory

Local heat produced by an electrical path inside the memory was detected and imaged by the method “Thermal Analysis.” It turned out that the visualized heat spots were conductive filaments (CFs) formed between interlayers of Pt/amorphous InGaZnO (a-IGZO). By using the thermal analysis, the location of CFs and their surface temperature was detected. This method indicated that there was a lot of emitted heat when the memory cell was switched off. It is thought to be accumulated heat causing disruption of the CFs. With great range of measurement, it was found that some memory cells drive with a single CF and others drive with multiple CFs. For the formation of CFs, it is assumed that there are CFs formation sites such as oxygen-related defects, roughness of the layer of a-IGZO, and so on. This method “Thermal analysis” can contribute to detection of the CFs location, the number of CFs, and thermal activity inside the memory devices.

Self-heating induced instability of oxide thin film transistors under dynamic stress

Degradation caused by Joule heating of transparent amorphous oxide semiconductor thin-film transistors (TFTs) is an important issue for display technology. Deep understanding of the mechanism of self-heating degradation generated by driving pulse voltage will pave the way for the development of highly reliable flexible displays. In this work, by using a pseudo interval measurement method, we examined the relationship of the highest and the lowest heating temperature in pulse 1 cycle and frequency. These self-heating converged to a constant temperature under pulse voltage applied at 1 kHz. Moreover, the long-term reliability under positive-bias stress voltage at 1 kHz of low converged temperature condition was improved relative to that of the stress voltage at 10 Hz of dynamic temperature change condition. We discussed the degradation mechanism of oxide TFTs generated by pulse voltage, and clarified that the degradation was accelerated by thermionic emission which occurred at low frequency.

Analysis of self-heating phenomenon in oxide thin-film transistors under pulsed bias voltage

Degradation by Joule heating of amorphous metal oxide thin-film transistors is one of the important issues for realizing next-generation displays. To clarify the self-heating degradation mechanism, it is indispensable to analyze the detailed temperature change of TFTs. In this study, we proposed a technique to suppress deterioration caused by self-heating in terms of driving methods, and investigated relation on the temperature change in the degradation mechanism caused by self-heating.

Analysis of heating phenomenon in oxide thin-film transistor under pulse voltage stress

Degradation by Joule heating of the thin-film transistors (TFTs) is one of the important issues for realizing next-generation displays. We have investigated the thermal distribution on the channel region of transparent amorphous oxide semiconductor TFT in pulse operation using an infrared imaging system. We also discussed the relationship between the self-heating temperature and the degradation.