Structural characterization of AlN films synthesized by pulsed laser deposition

We obtained AlN thin films by pulsed laser deposition (PLD) from a polycrystalline AlN target using a pulsed KrF* excimer laser source (248 nm, 25 ns, intensity of ∼4 × 108 W/cm2, repetition rate 3 Hz, 10 J/cm2 laser fluence). The target-Si substrate distance was 5 cm. Films were grown either in vacuum (10−4 Pa residual pressure) or in nitrogen at a dynamic pressure of 0.1 and 10 Pa, using a total of 20,000 subsequent pulses. The films structure was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and spectral ellipsometry (SE). Our TEM and XRD studies showed a strong dependence of the film structure on the nitrogen content in the ambient gas. The films deposited in vacuum exhibited a high quality polycrystalline structure with a hexagonal phase. The crystallite growth proceeds along the c-axis, perpendicular to the substrate surface, resulting in a columnar and strongly textured structure. The films grown at low nitrogen pressure (0.1 Pa) were amorphous as seen by TEM and XRD, but SE data analysis revealed ∼1.7 vol.% crystallites embedded in the amorphous AlN matrix. Increasing the nitrogen pressure to 10 Pa promotes the formation of cubic (≤10 nm) crystallites as seen by TEM but their density was still low to be detected by XRD. SE data analysis confirmed the results obtained from the TEM and XRD observations.

Research highlights▶ We obtained AlN thin films by PLD in low-pressure N2 from an AlN target with a pulsed KrF* excimer laser. ▶ TEM and XRD studies showed a strong dependence of the film structure on the N2 content. ▶ The films grown at low nitrogen pressure (0.1 Pa) were prevalently amorphous. ▶ Increasing N2 pressure to 10 Pa promotes the growth of cubic (≤ 10 nm) crystallites.