Field-effect transistors based on solution processed zinc tin oxide

• Main Author: Sykora, Reinhold Benedikt
• Format: Others
• Language: en
• Published: 2018
• Online Access: https://tuprints.ulb.tu-darmstadt.de/7209/1/Dissertation_Sykora.pdf; Sykora, Reinhold Benedikt <http://tuprints.ulb.tu-darmstadt.de/view/person/Sykora=3AReinhold_Benedikt=3A=3A.html> (2018): Field-effect transistors based on solution processed zinc tin oxide.Darmstadt, Technische Universität, [Ph.D. Thesis]

Most of the transistors are based on inorganic semiconductors such as the ubiquitous silicon (Si). Nevertheless they have some problematic issues. First, their crystalline quality must be very high to guarantee good device properties. Second, in order to achieve the first point costly and demanding processing methods are necessary. For certain application like radio-frequency identification (RFID) tags these processing cost are too high. On the other hand zinc tin oxide (ZTO) as an amorphous oxide semiconductor (AOS) can be processed out of solution and ensures good device characteristics even in the amorphous phase. In contrast to the widely studied indium gallium zinc oxide (IGZO) it does not contain the costly indium. In this thesis a precursor route for ZTO was developed that does not use the common but toxic 2-methoxyethanol as a solvent. It applied ethanol (EtOH) and ethylene glycol (EG) as an optional co-solvent. By applying this precursor solution thin ZTO films could be processed either by spin coating or ink-jet printing. Investigations revealed their smoothness, and predominant amorphous phase with small embedded zinc oxide (ZnO) nanocrystallites. The obtained transparent films exhibited an n-doped character. By building bottom-gate transistors using these ZTO films it could be shown that a Zn:Sn ratio of 7:3 produces the best transistor results, the replacement of 40% of EtOH by EG could improve the thin film transistor (TFT) performance, and the stabilizer hydrochloric acid (HCl) led to the creation of electronic trap states. A multiple layer approach led to the improvements of all transistor parameters. The highest ever reported mobility of up to 7.8 cm^2V^-1s^-1 for an ink-jet printed bottom-gate ZTO transistor was obtained by processing eight layers. It could also be shown that a transistor composed of more than four layers had better resistance against positive bias stress (PBS) and water (H2O) or oxygen (O2) in the atmosphere. While H2O acts as an electron donor in the ZTO system, O2 leads to electron accumulation at the oxide surface. Devices composed of multiple layers showed acceptable performance even if annealed at a reduced temperature of 350 °C compared to the standard setting of 500 °C. The reason for the multiple-layer enhancements was identified as an improved surface coverage and film quality. A setup for time dependent transistor measurements was realized and the first results revealed, that the fastest switching of the ZTO transistors took place in the linear regime. The highest switching frequency of a transistor was measured to be 1.33 MHz. The investigated on-switching behavior can be described by two time constants. It is very likely that the small time constant represents the transport of the injected charge carriers while the long time constant is a measure for the transport of trapped charge carriers. Functional hybrid top-gate transistors using ZTO and the organic dielectric materials poly(methyl methacrylate) (PMMA), polystyrene (PS), and a benzyl azide containing copolymer (PAZ) could be processed. It was shown that a minimal thickness of 490 nm of the organic dielectric PMMA was necessary to ensure good device performance and small leakage currents. The best performance could be achieved if new electrode masks were applied in order to reduce the overlap between the source/drain and gate electrode. The best working printed ZTO top-gate transistor using these masks and a PMMA layer showed a mobility of up to 4.4 cm^2V^-1s^-1. To the best of the author’s knowledge this is the highest reported mobility of an ink-jet printed ZTO/PMMA top-gate transistor. The top-gate transistors using PS and PAZ as dielectrics exhibited inferior device performance. This result could be explained by a better adhesion of the polar PMMA to the quite hydrophilic ZTO surface. The results presented in this thesis could contribute to realizing easily processable, cost efficient, environmental friendly, printable, and good performing TFTs.