Structural and optical characterization of low-temperature ALD crystalline AlN

• Extensive structural and optical characterization has been performed on crystalline aluminum nitride films deposited by atomic layer deposition at 250oC using nitrogen 5% hydrogen plasma and trimethylaluminum precursors.
• The films are smooth (0.7 nm average roughness), have X-ray reflectivity density of 2.94 gcm-3 and the wurtzite crystal structure with a (100) preferred orientation.
• PEALD AlN has oxygen impurities, but low carbon concentration. XPS and FTIR analysis revealed that oxygen atoms are exclusively bonded to aluminum with no Al-O-N bonds detected.
• The refractive index and optical bandgap of ALD AlN increases with thickness, with the index reaching a plateau of 1.96 at 632 nm wavelength for film thickness >30 nm while the band gap plateaued at 6.04 eV for film thickness >15 nm.

A plasma enhanced atomic layer deposition (PEALD) process has been used to deposit crystalline AlN thin films at 250 °C using nitrogen 5% hydrogen plasma and trimethylaluminum precursors. Films grown on single crystal silicon and sapphire substrates are crystalline with strong (100) preferred orientation. Mass density measured by x-ray reflectivity (XRR) was 2.94 g cm−3, compared to 3.25 g cm−3 for the bulk materials. Photoelectron and infrared studies were used to investigate the elemental analysis and the exact bonding environment of the constituents. The optical band gap, measured using spectroscopic ellipsometry, is 6.04 eV, and the refractive index, measured at 632 nm wavelength, is 1.96. The optical properties were thickness-dependent below 30 nm for refractive index and below 15 nm for band gap.