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

Abstract

Al2O3 films have been deposited at 85°C by atomic layer deposition onto single 100nm thick tris-(8-hydroxyquinoline) aluminium (AlQ3) films made onto silicon wafers. It has been found that a thick ALD-deposited Al2O3 layer (>11nm) greatly prevents the photo-oxidation of AlQ3 films when exposed to continuous UV irradiation (350mW/cm2). Thin Al2O3 thicknesses (<11nm) 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 (~2000h). Accelerated aging conditions in a climatic chamber (85°C/85% RH) also allow faster visualization of the same defects (168h). The black spot density grows upon time and the black spot density occurrence rates have been calculated to be 0.024h−1·cm−2 and 0.243h−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 0.004h−1·cm−2 and 0.04h−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.