  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.
