Committee

• Prof. Eric Vogel – MSE/EE (Advisor)
• Dr. Brent Wagner – GTRI (Advisor)
• Prof. Nazanin Basirri-Gharb – ME/MSE
• Prof. Mark Losego - MSE
• Prof. Asif Khan - ECE

Abstract

The numerous crystalline phases of indium selenide have attracted significant interest for their exceptional electronic, optical, ferroelectric, and antiferroelectric properties. These superlative properties have renewed research interest in thin-film synthesis of these materials for applications such as neuromorphic and quantum computing, spintronics, photovoltaics, photodetectors, and thin-film transistors.

While the many phases of indium selenide give rise to a wide range of properties, the polymorphic nature of indium selenide complicates the development of large-area, thin-film synthesis of these materials for adoption on an industrial scale. Therefore, understanding of how various synthesis parameters affect the phase purity and defect density of films is crucial to the continued development of thin-film synthesis of these materials, so that they may be harnessed for advanced electronics.

In this thesis, synthesis and physical characterization of In2Se3 films, characterization of the effect of cooling rate on the β→α phase transition, and electrical characterization β-, γ-, and κ-In2Se3 thin-film transistors is demonstrated.

The effects of substrate temperature, substrate choice, and Se/In flux ratio during molecular beam epitaxy synthesis on the phase composition, surface morphology and stoichiometry of indium selenide films was investigated. Higher substrate temperature combined with higher Se/In ratio promoted formation of β-In2Se3 over γ and/or κ-In2Se3. In2Se3 flake lateral dimensions were observed to increase as substrate temperature increased on all substrates, and the largest lateral dimensions were observed in films synthesized on HOPG at 973 K.  No evidence of α-In2Se3 was observed in the Raman spectra of all the films.

In2Se3 powders and films were cooled at controlled rates through the β→α phase transition temperature, ranging from 1200 K/hr to 12 K/hr. Some evidence of β to α-In2Se3 was observed in in-situ XRD, but it was independent of cooling rate.

A method of quantifying phase fraction and stoichiometry of indium selenide films via Raman and X-ray photoelectron spectroscopies was developed. Se 3d XPS data of single-phase β-In2Se3­, γ-In2Se3, InSe, κ-In2Se3 and selenium films were obtained and the distinct Se 3d binding energies of each of the films enabled their differentiation via XPS. Raman spectra of each film was also measured on each film and the results correlated with those from XPS. STEM and XRD characterization of the κ-In2Se3 film support the existence of this phase, distinct from γ-In2Se3. The accuracy of Wagner, Scofield, Thermo-modified Scofield, and Crist libraries of relative sensitivity factors for XPS were compared and the most accurate quantification of the selenium-indium stoichiometry of the films was achieved by utilizing an indium sensitivity factor that is 13-times (Crist) larger than the selenium sensitivity factor.

Finally, thin-film transistors with β-, γ-, and κ-In2Se3­ channels were fabricated and tested. Raman spectroscopy through optically transparent gate stacks revealed no β→α phase transition upon applying a vertical field of 4 x 10-2 V/nm. Transfer characteristics (VGS sweep -3 V to 3 V to -3V, VDS = 2 V) at 300K and 78 K revealed hysteresis in all devices at 300 K, but disappearance of the hysteresis in γ-In2Se3 at 78 K , and persistence of the hysteresis in β-In2Se3 and κ-In2Se3 device at 78 K. The disappearance of hysteresis at 78 K in γ-In2Se3­ suggests that the room temperature hysteresis on this device arose from charge trapping rather than ferroelectricity. Hysteresis in all devices occurred in a clockwise direction, which could be expected for ferroelectricity in an FeSFET with the device dimensions used in this study. Stepwise increase in the VGS sweep range was performed on all devices, from ±0.5 V to ±3V in 0.5 V increments, and the resultant increase in hysteresis width was gradual, similar to what is expected of either charge trapping, or thermally activated ferroelectric domain motion.

Together, these results advance understanding of links between thin-film processing parameters and film structure on multiple length scales, provide a method for characterizing phase fraction in films, and contribute to the evolving understanding of the origin of hysteresis in In2Se3 electronic devices.