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

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

Automated summary

The study investigated the structural and morphological characteristics of Alq(3) thin films deposited at different rates using XRR and GISAXS. Slowing down the deposition rate increased the electron density, reduced defects and porosity, and decreased the diffusion depth of Co in Alq(3). This suggests that controlling the growth rate of Alq(3) can improve the performance of organic spin valve devices.