Synthesis and characterization of solution-processed oxide films formed using SiO2 nanoparticles and organoalkoxysiloxane precursors

Abstract

Since the early 1990s, remarkable progress has been made in the development of organic field-effect transistors (OFETs), which are of great interest from a technological standpoint. Because organic materials can be deposited at low temperatures, they are compatible with plastic substrates. An organic semiconductor and suitable gate dielectrics are required for high-performance OFETs, and polymeric dielectrics have been evaluated as replacements for widely used silicon dioxide (SiO2) dielectrics in these transistors. Also OFETs are targeting a wide range of potentially inexpensive applications, low-cost fabrication methods are of interest, including solution-processable polymer gate dielectrics that can be deposited by spin-coating, spray-coating, or printing, rather than vacuum deposition. It should be noted that advantage of solubility of polymers in the sense of inexpensive deposition methods could be a problem regarding possible intermixing and swelling in the preparation of multilayer stacks as given in all-polymer devices. For organic-based devices, the dielectric materials must also be compatible with organic semiconductors. Therefore, orthogonal solvents must be used to avoid swelling and dissolution of the underlying polymeric layer. Also to overcome this problem, PVP were mixed with poly(melamine-co-formaldehyde) as a cross-linker to reduce dielectric swelling. In particular, spin on glass (SOG) is a solution-processable material and has advantages of inherent network-forming properties and resistance to chemical solvents. However, the SOGs are prone to film-shrinkage, which can lead to the formation of cracks and an irregular surface morphology. SiO2 nanoparticles (NPs) can be used to overcome problems with shrinkage or to enhance the mechanical or thermal stability. To form pinhole-free and crack-free films using organically modified silicate materials, low-temperature processing is desirable to minimize the stresses. To increase the strength of films formed using SiO2 NPs, and provide uniform and dense coatings without sintering, organoalkoxysiloxanes have been used with the sol-gel process to form networks. In the first part, the process parameters were studied in the solution-processable oxide film deposition using SiO2 NPs and organoalkoxysiloxanes via the sol-gel method. The process parameters were optimized for forming crack-free uniform films. The synthesis conditions were characterized by varying the molar ratio of the reagents, the curing temperatures and the aging times of sol solutions. The higher MTMS content induced the formation of denser siloxane network and smooth and uniform surface. However, high MTMS content induced a precipitate in a sol solution during the sol-gel reaction. A lower MTMS content promoted the termination of the sol-gel reaction. And an insufficient condensation reaction between SiO2 NPs and MTMS, led the cracking of the oxide films. The aging times was related to the sol stability. The sol solution was transparent when aging for 1-3 d and became gelated or precipitated after 5 d. A precipitate was observed in the sol solution aged for 3 d at higher contents of MTMS. Condensation reactions were occurred when the as coated oxide films cured above 120°C. The oxide films cured at 150 and 180°C exhibited low RMS roughness and high contact angles. The process parameters were optimized for forming crack-free and uniform films as followed. The sol solutions were synthesized at molar ratio of SiO2 NPs to MTMS of 1:1, and then aged for 3 d. For characterization of the oxide films, 100-nm-thick oxide films were spin-coated using 5 wt.% solution at 4000 rpm. For condensation reaction, as coated oxide films were cured at 150°C for 1 h. In the second part, the effects of three different organic side groups in the organoalkoxysiloxane; i.e., methyl, phenyl and 3-glycidyloxypropyl on the oxide films were compared. The structure and surface morphology was investigated for the influence of organic side groups on the oxide films. The electrical characteristics of the oxide films were characterized for applicable to gate dielectrics. FT-IR spectra and particle size measurements were used to show that the precursor with methyl groups effectively formed a network with SiO2 NPs. The oxide films formed using precursors with methyl and phenyl groups exhibited uniform and smooth surfaces, with RMS surface roughness values of 1.05 and 1.16 nm, respectively. Some aggregation was observed on the surface of the oxide film formed using precursors with 3-glycidyloxypropyl groups. The dielectric properties of the oxide films were investigated, and we found no hysteresis and a leakage current density of 10-9?10-8 A/cm2 for the oxide films formed using precursors with methyl and phenyl groups. In the third part, the annealing effects on the oxide films prepared with precursors containing methyl, phenyl and 3-glycidyloxypropyl groups. The thermal property of the oxide films was influenced by the thermal stability of the organic side group. The structure, surface morphology and the electrical characteristics were investigated of the oxide films containing different organic side groups with annealing temperatures. The SiO2 NPs were enhanced the crack resistance of synthesized materials. The shrinkage with annealing temperature was considerably related with thermal stability of organic side group. The methyl group contained precursor effectively formed the Si?O?Si network with The SiO2 NPs and showed the thermally stable electrical characteristics. The nearly zero hysteresis was observed and dielectric strength was enhanced annealed up to 400°C. The phenyl and 3?glydixyloxypropy group contained dielectrics affected by thermal stability of organic side group. The hysteresis increased with decomposition of organic side group which induced the trap charges. The organic groups were rarely affected the dipole polarization of the dielectric, so the dielectrics were showed the typical low dielectric constant materials. To characterize the dielectric properties, the pentacene FETs were fabricated on the solution-processed oxide films. The organic side groups of organoalkoxysiloxane precursors effects on the surface morphology and crystallinity were investigated. The surface morphology of an overlaying pentacene layer was affected by the surface polarity of the dielectric film. Pentacene film exhibited a dendritic structure with a larger grain size when deposited on films formed using 3-glycidyloxypropyl precursors than deposited on MTMS or PTMS. Crystallinity of pentacene deposited onto the oxide films formed using precursors containing phenyl groups was enhanced despite of its relatively small grain size. The field effect mobilities and on/off current ratios of the pentacene FETs were 0.10 cm2 Vs?1 and 105, respectively.