Investigation of Al2O3 barrier film properties made by atomic layer deposition onto fluorescent tris-(8-hydroxyquinoline) aluminium molecular films

• Thin Al2O3 films have been deposited by atomic layer deposition onto organic films.
• Defects in the Al2O3 barrier layer can be observed as black spots under UV light excitation.
• Occurrence of black spots is enhanced for device stored in hot/wet conditions.

Al2O3 films have been deposited at 85 °C by atomic layer deposition onto single 100 nm thick tris-(8-hydroxyquinoline) aluminium (AlQ3) films made onto silicon wafers. It has been found that a thick ALD-deposited Al2O3 layer (> 11 nm) greatly prevents the photo-oxidation of AlQ3 films when exposed to continuous UV irradiation (350 mW/cm2). Thin Al2O3 thicknesses (< 11 nm) on the contrary yield lower barrier performances. Defects in the Al2O3 layer have been easily observed as non-fluorescent AlQ3 singularities, or black spots, under UV light on the system Si/AlQ3/Al2O3 stored into laboratory conditions (22 °C/50% Relative Humidity (RH)) for long time scale (~ 2000 h). Accelerated aging conditions in a climatic chamber (85 °C/85% RH) also allow faster visualization of the same defects (168 h). The black spot density grows upon time and the black spot density occurrence rates have been calculated to be 0.024 h− 1·cm− 2 and 0.243 h− 1·cm− 2 respectively for the two testing conditions. A detailed investigation of these defects did show that they cannot be ascribed to the presence of a detectable particle. In that sense they are presumably the consequence of the existence of nanometre-scaled defects which cannot be detected onto fresh samples. Interestingly, an additional overcoating of ebeam-deposited SiO2 onto the Si/AlQ3/Al2O3 sample helps to decrease drastically the black spot density occurrence rates down to 0.004 h− 1·cm− 2 and 0.04 h− 1·cm− 2 respectively for 22 °C/50% RH and 85 °C/85% RH testing conditions. These observations highlight the moisture sensitivity of low temperature ALD-deposited Al2O3 films and confirm the general idea that a single Al2O3 ALD film performs as an ultra-high barrier but needs to be overprotected from water condensation by an additional moisture-stable layer.