Effect of growth rate on quality of Alq₃ films and Co diffusion

The quality of organic semiconductor tris-(8-hydroxyquinoline)aluminum (Alq₃) thin films, deposited at the rate of 5.0 Å s⁻¹, 2.5 Å s⁻¹, and 1.0 Å s⁻¹ respectively, have been investigated using x-ray reflectivity (XRR) and grazing incidence small angle x-ray scattering (GISAXS) studies. While XRR provides information about structural details, GISAXS provides information about the morphology of the films. The film quality is very good with a surface roughness of only 8 Å. The electron density of the film increases from 0.39 e Å⁻³ to 0.43 e Å⁻³ due to reduction of growth rate from 5 Å s⁻¹ to 1 Å s⁻¹. The higher electron density (0.43 e Å⁻³) of the film deposited at the lower deposition rate (1 Å s⁻¹) indicates the presence of less defects and/or porosity in the film. Average separation between pores increases from 200 Å to 500 Å and simultaneously pore depth decreases from 300 Å to 120 Å due to reduction of growth rate from 5 Å s⁻¹ to 1 Å s⁻¹. The mixing between Alq₃ and Co at the Alq₃/Co interface and diffusion of Co into Alq₃ layer through Co/Alq₃ interface have been studied by combined XRR and grazing incidence x-ray standing wave (GIXSW) measurements for the three Alq₃/Co/Alq₃/W/Si multilayers in which Alq₃ films were deposited at the above mentioned three different rates. It is found that there is about 30 Å thick (∼ two monolayer of Alq₃) mixing region at the Alq₃/Co interface in three multilayer samples, which suggests that mixing at this interface is independent of the growth rate of Alq₃ films. The depth of diffusion of Co into Alq₃ through Co/Alq₃ interface reduces from 300 Å to 160 Å due to the reduction of deposition rate from 5 Å s⁻¹ to 1 Å s⁻¹. This study suggests that by controlling the growth rate of Alq₃, porosity as well as the diffusion of ferromagnetic Co in an organic semiconductor Alq₃ can be reduced, which plays an important role in the performance of organic spin valve devices. (paper)