Unprecedented high-temperature electrical insulation of reactively sputtered AlN thin films

Abstract

This study investigates the morphological and electrical properties of hexagonal aluminium nitride (h-AlN) films synthesized through different reactive sputtering techniques at low growth temperatures (270 °C) – specifically direct and pulsed current magnetron sputtering, and high-power impulse magnetron sputtering (HiPIMS). A 35 % reactive gas threshold, marking the onset of poisoning, is crucial for achieving desirable dielectric properties and single-crystalline h-AlN films across all sputtering techniques. The deposition technique significantly affects film morphology and orientation, with HiPIMS yielding the most (002) textured and dense films. Impedance spectroscopy correlated film morphology with electrical properties. HiPIMS AlN showed enhanced crystallinity and reached 8.7 × 10⁵ Ω·m at 700 °C, surpassing literature data. Variable Energy Electron Loss Spectroscopy (VEELS) combined with transmission electron microscopy revealed bandgaps of 6.9 eV for the DCMS AlN, 6.7 eV for HiPIMS, and 6.4 eV for PMS. X-ray photoelectron spectroscopy indicated oxygen impurities up to 4 at.% in all AlN films, highlighting the difficulties in growing purest h-AlN insulators, as oxygen impurities act as a dopant introducing other charge carriers. Despite this relatively high impurity level, reactive HiPIMS AlN films reach unprecedented high resistivity even at 700 °C, demonstrating their capabilities as insulating thin film material.