Infrared and microwave shielding of transparent Al-doped ZnO superlattice grown via atomic layer deposition

The system of atomic-layer Al-doped ZnO (AZO) superlattice is rather interesting in that it exhibits a “bipolar transparency,” as it is transparent in the visible and opaque in the infrared to microwave. Here, we report on our measurements of the infrared and microwave shielding properties of thin films (~150 nm) of an Al-doped ZnO superlattice grown via atomic layer deposition (ALD). These optically transparent conductive oxide thin films have large DC electrical conductivity (>50,000 S/m) which increases with Al doping. Their infrared optical properties are well described by a free-electron (Drude) model, which results from the very large carrier concentrations (>3 × 1020 cm−3) resulting from AlOx-heterolayer-doping. It is found that increasing Al concentrations lead to an increase in the relaxation energy which, however, is not strong enough to cause the plasma frequency to red-shift. Microwave shielding properties were investigated in the frequency range from 1 to 30 GHz, and shielding efficiencies as high as ~22 dB were observed, confirming that the free-electron picture extends into the microwave regime. The dynamic conductivity in the microwave range was found to correspond well to the measured DC values. Due to their high electrical conductivity and high microwave shielding efficiency in thin film format, these materials may be desirable for applications in transparent electronics, optically transparent EMI shielding coatings, and heat/microwave reflecting coatings for windows.