In Situ Hard X-ray Photoelectron Spectroscopy of Space Charge Layer in a ZnO-Based All-Solid-State Electric Double-Layer Transistor

A space charge layer (SCL) at the (0001) oriented ZnO thin-film/lithium-ion conducting glass ceramics interface was investigated using in situ hard X-ray photoelectron spectroscopy (HAXPES) and Mott-Schottky analysis to understand the working mechanism of an all-solid-state electric double- layer transistor (EDLT). The EDLT showed a drain current decrement on application of a negative gate voltage, corresponding to electron depletion in ZnO. The SCL generation and steep band bending in ZnO were successfully observed through the drastic broadening of Zn 2p HAXPES spectra under reverse voltage conditions. The voltage dependence of SCL capacitance, analyzed on the basis of a Mott- Schottky plot, indicated a very thin SCL with a thickness of approximately 1-2 nm. The results indicated that a strong electric field, on average larger than 1 MV/cm, is generated in the ZnO thin film when a large reverse voltage is applied.