Threshold Voltage Engineering in Crystalline ZnO Thin-Film Transistors via Alkali Metal Incorporation

We report spray-pyrolyzed ZnO thin-film transistors (TFTs) doped with alkali metals (Li, Na, K, Rb, and Cs) to tune the threshold voltage (VTH) while preserving lattice strain. The varying atomic radii of alkali dopants modulate strain in the ZnO lattice, significantly impacting its electronic and structural properties. The optical band gap ranges from 3.16 to 3.25 eV, with shallow donor states enhancing near-band-edge (NBE) and UV emission around 400 nm. While the band gap remains nearly constant, slight shifts in band structure are observed due to strain effects. Alkali doping also influences photoluminescence, with larger ions increasing visible emissions through defect recombination, while Li doping improves crystallinity and boosts UV emission. X-ray diffraction (XRD) reveals variation in peak intensity, grain size (15.93–36.25 nm), and surface roughness (0.58–1.95 nm). X-ray photoelectron spectroscopy (XPS) of the O 1s peak shows an increase in M–O bonds (60.19% to 71.38%) and a decrease in oxygen-related defects (39.81% to 28.62%). These structural changes enhance electrical performance, with saturation mobility rising from 12.30 to 23.45 cm2/V·s, VTH shifting from 1.79 to 2.98 V, and subthreshold swing decreasing from 290 to 200 mV/dec. An atomic model is proposed to explain strain distribution, revealing that larger alkali ions induced greater strain and donor-like states, leading to a negative VTH shift. These results underscore the potential of alkali metal doping for optimizing ZnO TFTs in flexible and transparent electronic devices.