Blue Light-Emitting Diode Based on ZnO

Abstract

A near-band-edge bluish electroluminescence (EL) band centered at around 440 nm was observed from ZnO p – i – n homojunction diodes through a semi-transparent electrode deposited on the p -type ZnO top layer. The EL peak energy coincided with the photoluminescence peak energy of an equivalent p -type ZnO layer, indicating that the electron injection from the n -type layer to the p -type layer dominates the current, giving rise to the radiative recombination in the p -type layer. The imbalance in charge injection is considered to originate from the lower majority carrier concentration in the p -type layer, which is one or two orders of magnitude lower than that in the n -type one. The current-voltage characteristics showed the presence of series resistance of several hundreds ohms, corresponding to the current spread resistance within the bottom n -type ZnO. The employment of conducting ZnO substrates may solve the latter problem.