Yttrium and Scandium in Solution-processed Oxide Electronic Materials.

Abstract

Large area electronics are critical for many novel applications such as smart windows, wearable electronics and Internet of Things. Among candidate materials, metal oxides have relatively good performance and stability and can be deposited by low-cost solution processes.

This thesis explores the roles of rare-earth elements yttrium and scandium in solution-processed metal oxide thin films including semiconducting scandium- or yttrium-doped ZTO, conducting scandium- or yttrium-doped zinc oxide, and insulating yttrium-scandium oxide. Yttrium and scandium can act as oxygen getters and stabilizers, and the use of higher-order alloys can improve film thermal stability and electrical performance.

First, thin film transistors (TFTs) are used to characterize undoped ZTO films as a baseline. The device performance of solution-processed ZTO TFTs depends on ink Zn to Sn ratio and annealing temperature, optimized to be 7:3 and 480⁰C, respectively. The optimized ZTO has a shallow donor energy level of 7meV and a steep exponential subgap band tail with a percolation energy of 3meV. Sputtered Mo forms an excellent ohmic contact to solution-processed ZTO with a width-normalized contact resistance of 8.7Ω•cm and a transfer length of 0.34μm, making the technology suitable for future sub-micron channel length devices. Yttrium enhances performance of ZTO TFTs at low concentrations (<1at%), but compromises device DC performance at high concentrations (>3at%). High yttrium concentrations slightly improve TFT negative bias illumination stress stability by reducing oxygen vacancy-related defects.

Second, the introduction of scandium or yttrium in solution-processed ZnO decreases the conductivity by three orders of magnitude, which is ascribed to formation of insulating structures along grain boundaries. Scandium or yttrium also make the resistivity of ZnO more thickness-dependent than undoped ZnO after forming gas anneal, by causing surface depletion and grain disruptions in the film.

Third, solution-processed (YxSc1-x)2O3 insulating alloys have comparable dielectric performance to vacuum deposited (YxSc1-x)2O3, with high breakdown field > 4MV/cm, low leakage current and low dielectric frequency dispersion. Even after 900°C anneals induce crystallization, the alloys maintain a high breakdown field.

The yttrium- and scandium- doped solution-processed oxides developed here form a complete suite of electronic materials suitable for fabrication of future large-area electronic devices.