Magnesium zinc oxide high voltage thin film transistors
Creators: name: Hong, Wen-Chiang
Publisher: name: No Publisher Supplied
Year: 2017

description: Energy is one of the most important topics in the 21st century, and solar energy has been a leading technology in the search to replace fossil-fuel energy as a sustainable and clean energy source. In order to provide an energy-efficient, less expensive, and reliable energy source, the PV system on glass (PV SOG) is emerging as an attractive concept. It integrates solar cells, solar inverters, and controller circuits on a single glass substrate.           This dissertation focuses on development of the novel oxide-based high voltage thin film transistor (HVTFT) on glass technology, which is one of the core devices for solar inverter of the PV-SOG. Currently, the inverter counts for more than 10% of the total cost of an entire PV system. The solar inverter will be the major challenge of PV-SOG because the conventional solar inverters are bulky and could not be directly built on glass substrates.  In particular, its key device, high voltage transistor, is not only pricy but also requires high process temperature which is incompatible with glass substrates.           In comparison of several semiconductor materials, such as polycrystalline silicon, amorphous silicon, SiC and GaN, ZnO based materials have several promising features suitable for HVTFT on glass technology, including wide bandgap, high thermal conductivity, high mobility, and low deposition temperature. However, the thin film transistor (TFT) made up of the pure ZnO generally suffers from poor stability and reliability due to high defect density in the material. Because energy source is a basic unit of the infrastructure, it’s critical for a solar energy system to have a long lifespan. The first important issue of this dissertation research was to improve the TFT stability by adding a small amount of Mg into ZnO to form the ternary oxide, MgXZn1-XO (MZO, X&lt;0.03) as the TFT channel. The density of oxygen vacancies in MZO was reduced so that after negative bias stress (NBS) the threshold voltage shift of MZO TFT was 30% smaller in comparison with the shift of ZnO TFT counterpart.           Based on the solid foundation of stable MZO TFTs, MZO high voltage TFT (MZO HVTFT) on glass technology was designed and developed. To eliminate the electrical field crowding around the corners of the conventional TFT with a rectangular channel, a symmetric circular-shape transistor was adapted. From the simulation result, the peak electrical field is reduced by 50% in the symmetric circular structure than in the conventional rectangular structure. However, the MZO HVTFT with the circular configuration only showed a blocking voltage of 92V. To further enhance the device performances, especially the blocking voltage, we developed a modified MZO (m-MZO) HVTFT, which had an ultrathin MZO transition layer (MZO-TL) using the in-situ modulation doping in the channel-dielectric interface. The comprehensive characterizations using X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) were conducted to study depth profiles of elements across the channel-gate dielectric interface.  It was proved that this interface engineering effectively suppressed the interdiffusion of Zn and Si between the channel and dielectric layers, resulted in the reduction of the interface states and the oxide trapped charges. The combination of the interface engineering with the symmetric device design significantly increased the blocking voltage of the m-MZO HVTFT on glass. As a result, the regular m-MZO HVTFT (channel length=10µm) has on/off ratio of 3.5×10^10 and blocking voltage of 305V, which is suitable for the regular AC 110V power system. The  m-MZO HVTFT with a channel length=25µm has on/off ratio of 3.3×10^9 and blocking voltage of 609V which is suitable for the regular AC 220V power system.           Finally, in order to expand the HVTFT technology from glass to the flexible substrate, the ZnO-based HVTFTs on plastic substrate were explored. By adopting low temperature even room temperature process, such as sputtering and atomic layer deposition, the flexible HVTFT consisting of the ZnO based channel with Al2O3 dielectric layer showed an on/off current ratio of 109 and blocking voltage of 92V.           The MZO HVTFT technology opens opportunities for cost-effective and highly efficient power management systems for many applications. The HVTFTs on glass will serve for the inverter in novel Building-integrated photovoltaics (BIPV) and smart glass while the flexible HVTFT is promising for the emerging self-powered wearable systems.