Electroluminescence microscopy and spectroscopy of silicon nanocrystals in thin SiO2 layers

Stable continuously operable electroluminescent diodes have been fabricated by Si ⁺-ion implantation and annealing of thin SiO ₂ layers on a silicon substrate. The external quantum efficiency of the device is reaching 3×10 ⁻⁵ for the best diodes. Electroluminescence (EL) emission band is wide and centered at 800 nm. EL is due to radiative recombination of tunneling carriers in Si-nanocrystals(NC) with small contribution of oxide defects (peak at 650 nm). EL images reveal inhomogeneous emission structures on the micrometer scale. Mainly, a small number of bright spots with diffraction limited size (∼600 nm) with a homogeneous background are observed and their EL spectra measured using an imaging spectrometer with a liquid-nitrogen-cooled CCD camera. The bright EL spots originate from the efficiently excited emission of oxide defects and/or emission of a few (possibly single) Si-NC, most likely at places with locally increased current. The low efficiency is probably a consequence of current tunneling through optically inactive nanocrystals or defects in a very thin oxide layer.