Nanoscopic study of ZnO films by electron beam induced current in the scanning tunneling microscope

Scanning tunnelling microscopy and spectroscopy have been used to characterize electrically active grain boundaries in pulsed laser deposited ZnO films with grain sizes in the range 216-500 nm. The p-type behaviour at the boundary is evidenced by local STM conductance spectra, which reveal boundaries with a n-p-i-p-n structure. in addition, local differential conductance measurements show higher surface band gap at the grain boundaries than in the grain interior, which is related to the space charge at the boundaries. The beam induced current mode of the STM has been used to image electrically active grain boundaries with a resolution of few nanometers in the different films. It is demonstrated that in some of the small grains the space charge region extends to the whole grain area so that no electrical barrier can be related to the grain boundary.