Investigation of Al_2O_3 barrier film properties made by atomic layer deposition onto fluorescent tris-(8-hydroxyquinoline) aluminium molecular films

摘要

Al_2O_3 films have been deposited at 85 ℃ by atomic layer deposition onto single 100 nm thick tris-(8-hydroxyquinoline) aluminium (AlQ_3) films made onto silicon wafers. It has been found that a thick ALD-deposited Al_2O_3 layer (＞ 11 nm) greatly prevents the photo-oxidation of AlQ_3 films when exposed to continuous UV irradiation (350 mW/cm~2). Thin Al_2O_3 thicknesses (＜11 nm) on the contrary yield lower barrier performances. Defects in the Al_2O_3 layer have been easily observed as non-fluorescent AlQ_3 singularities, or black spots, under UV light on the system Si/AlQ_3/Al_2O_3 stored into laboratory conditions (22 ℃/50% Relative Humidity (RH)) for long time scale (～2000 h). Accelerated aging conditions in a climatic chamber (85 ℃/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 SiO_2 onto the Si/AlQ_3/Al_2O_3 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 ℃/50% RH and 85 ℃/85% RH testing conditions. These observations highlight the moisture sensitivity of low temperature ALD-deposited Al_2O_3 films and confirm the general idea that a single Al_2O_3 ALD film performs as an ultra-high barrier but needs to be overprotected from water condensation by an additional moisture-stable layer.