Amorphous Transition-Metal-Oxides for Transparent Flexible Displays: Device Fabrication and Characterization
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This Ph.D. dissertation presents the development and demonstration of optically transparent back-channel etched flexible InGaZnO (IGZO) thin-film transistors (TFTs) using a conventional TFT process flow implemented at low-temperatures. The study includes the development of the transition metal oxide (TMO) channel layers by relating the materials properties, surface topography, and chemical composition of the channel layer to the process integration of IGZO TFTs. Investigation of the process parameters included process temperature and post processing thermal-anneal on the electronic properties of the semiconductor and the effect of the chemical composition of the gate-dielectric layer on the active channel layer of TMO TFTs. A bi-layer SiOx/SiNx gate dielectric was found to be an effective structure for high-performance IGZO-based TFTs compared to single dielectric layers. The SiOx capping layer within the dual dielectric structure was found to be an effective hydrogen (H) diffusion barrier, preventing H contamination into the overlying semiconducting IGZO layer during the IGZO deposition, minimizing the generation of H induce oxygen vacancy (Vo) formation in the active IGZO channel. A low-temperature, 150˚C plasma-enhanced chemical vapor deposition (PECVD) process was used to produce TFTs having field-effect mobility, μ, of 5.7 cm2/V.sec, sub-threshold swing, S.S., of 0.54 V/decade, and Ion/off > 106. The same dual-dielectric stack was also found to be an effective passivation layer for back-channel etched IGZO TFTs. A low-temperature approach employing a thin room-temperature-deposited e-beam SiOx barrier layer directly deposited onto the IGZO back-channel to prevent both the plasma damage and unintentional hydrogen (H) doping of the IGZO channel region. In order to complete the process integration for fully transparent flexible TFTs, the development of the dielectric layers of the TFT structure were augmented by an investigation of ohmic transparent contacts patterned using selective wet-chemical etching. A high-selectivity wet-etch patterning process was developed to take advantage of the etch-rate differences between polycrystalline Al-doped ZnO (AZO) and amorphous (IGZO) TMO thin-films. This patterning technique resulted in the fabrication of back-channel etched flexible transparent IGZO TFTs using a conventional TFT process flow implemented at low-temperatures. A selectivity of nearly 20 was found for dilute HCl solution in water for patterning AZO source/drain electrodes on IGZO channel layers. The resulting patterned electrodes had a low contact resistance of < 19 KΩ and high optical transparency of ~85%. The transparent back-channel etched flexible IGZO TFTs exhibited a μ of ~9.3 cm2/V.sec, VT of <5 V, and Ion/off ratio of ~107. Finally, the integration of the transparent semiconductor, dielectric, and conductive electrodes onto a flexible platform was demonstrated. Through a combination of the low-temperature processes developed in this work, the integration of transparent flexible TFTs onto polyethylene naphthalate (PEN) substrates was accomplished. The flexible IGZO TFTs had current-voltage (I-V) characteristics similar to their rigid counterparts. The fully encapsulated transparent devices had μ of ~6.7 cm2/V.sec, VT of ~1 V, and Ion/off ratio of >106. Electrical stability measurements of the flexible devices under tensile and compressive mechanical strain showed no appreciable change in the I-V characteristics during bending. The electrical characteristics under mechanical bending suggest that carrier transport is unaffected during mechanical strain due to the overlapping spherical s-orbitals in the IGZO conduction band. Testing under dc gate bias conditions, the electrical stability of the TFTs showed a positive VT shift of 3.8 V after 3600 s without any change in subthreshold-swing (S.S.). Pulsed-gate recovery measurements also showed rapid recovery of the drain current, both of which suggest that the dominant aging mechanisms is charge trapping in the back-channel etched transparent flexible IGZO TFTs.