Obliquely deposited tris(8-hydroxyquinoline) aluminium (Alq3) biaxial thin films with negative in-plane birefringence

Tilted columnar biaxial thin films were fabricated by thermally evaporating tris(8-hydroxyquinoline) aluminium (Alq(3)) at oblique angles of incidence a >= 65 degrees measured from the substrate normal. Variable angle spectroscopic ellipsometry and Bruggeman effective medium approximations were used to characterize the structure and the optical anisotropy of the films. The structural characteristics were also examined using scanning electron microscopy, revealing a lack of column broadening and a predominantly circular column cross-section. The films also exhibited larger column tilt angles than those typical of inorganic tilted columnar films. These structural properties gave rise to optical anisotropy leading to negative values of in-plane birefringence of the films that increased in magnitude with deposition angle. The observed negative in-plane birefringence contrasts with the typical behaviour of inorganic biaxial columnar thin films which exhibit positive values of in-plane birefringence, with peak values obtained for films grown at angles of 60 degrees < alpha < 70 degrees. The maximum in-plane birefringence obtained with Alq(3) films was -0.073 +/- 0.002 and was observed for films grown at alpha = 85 degrees.