  2D bismuth oxyselenide (Bi2O2Se) with high electron mobility shows great
potential for nanoelectronics. Although in-plane properties of Bi2O2Se have
been widely studied, its out-ofplane electrical transport behavior remains
elusive, despite its importance in fabricating devices with new functionality
and high integration density. Here, we study the out-of-plane electrical
properties of 2D Bi2O2Se at nanoscale by conductive atomic force microscope. We
find that hillocks with tunable heights and sizes are formed on Bi2O2Se after
applying vertical electrical field. Intriguingly, such hillocks are conductive
in vertical direction, resulting in a previously unknown out-of-plane
resistance switching in thick Bi2O2Se flakes while ohmic conductive
characteristic in thin ones. Furthermore, we observe the transformation from
bipolar to stable unipolar conduction in thick Bi2O2Se flake possessing such
hillocks, suggesting its potential to function as a selector in vertical
devices. Our work reveals unique out-of-plane transport behavior of 2D Bi2O2Se,
providing the basis for fabricating vertical devices based on this emerging 2D
material.
