Tunneling two-level systems in amorphous aluminum oxide films

The properties of amorphous insulating oxides exhibit low-energy excitations at low temperatures, which are dominated by tunneling two-level systems (TLS). These low-energy excitations are not found in crystalline solids, and understanding the nature of TLS in amorphous materials is important for their use in various applications, such as quantum computing and high-precision metrology optics. Aluminum oxide glass films were grown by both magnetron sputtering and electron-beam physical vapor deposition techniques at substrate temperatures up to 700°C. Through mechanical loss measurements, we have determined the tunneling strength of TLS. The internal friction of amorphous films typically forms a plateau between 1 K and 20 K, and the value of this plateau is proportional to the film's TLS density. The plateau for most amorphous materials falls within a universal range, with amorphous silicon grown at elevated substrate temperatures being a notable exception that falls below this range and thus has a significantly lower TLS density. For both sets of alumina films, increasing the substrate temperature during deposition reduces the internal friction plateau, but not below the universal range. This suggests that the tunneling strength of aluminum oxide films grown by physical vapor deposition at elevated temperatures is found within the universal range, like most other amorphous solids.