Hollow cathode plasma-assisted atomic layer deposition of crystalline AlN, GaN and AlxGa1−xN thin films at low temperatures
The authors report on the use of hollow cathode plasma for low-temperature plasma-assisted atomic layer deposition (PA-ALD) of crystalline AlN, GaN and AlxGa1−xN thin films with low impurity concentrations. Depositions were carried out at 200 °C using trimethylmetal precursors and NH3 or N2/H2 plasma. X-ray photoelectron spectroscopy showed the presence of 2.5–3 at.% O in AlN and 1.5–1.7 at.% O in GaN films deposited using NH3 and N2/H2 plasma, respectively. No C impurities were detected within the films. Secondary ion mass spectroscopy analyses performed on the films deposited using NH3 plasma revealed the presence of O, C (both <1 at.%), and H impurities. GIXRD patterns indicated polycrystalline thin films with wurtzite crystal structure. Hollow cathode PA-ALD parameters were optimized for AlN and GaN thin films using N2/H2 plasma. Trimethylmetal and N2/H2 saturation curves evidenced the self-limiting growth of AlN and GaN at 200 °C. AlN exhibited linear growth with a growth per cycle (GPC) of ∼1.0 Å. For GaN, the GPC decreased with the increasing number of deposition cycles, indicating substrate-enhanced growth. The GPC calculated from a 900-cycle GaN deposition was 0.22 Å. Ellipsometric spectra of the samples were modeled using the Cauchy dispersion function, from which the refractive indices of 59.2 nm thick AlN and 20.1 nm thick GaN thin films were determined to be 1.94 and 2.17 at 632 nm, respectively. Spectral transmission measurements of AlN, GaN and AlxGa1−xN thin films grown on double side polished sapphire substrates revealed near-ideal visible transparency with minimal absorption. Optical band edge values of the AlxGa1−xN films shifted to lower wavelengths with the increasing Al content, indicating the tunability of band edge values with the alloy composition.