Zinc oxide (ZnO) is a wide-bandgap II-VI semiconductor with a direct bandgap of ~3.37 eV and a large exciton binding energy of ~60 meV — high enough to support excitonic emission at room temperature, in contrast to GaN (~25 meV). It crystallises preferentially in the hexagonal wurtzite structure under standard deposition conditions. Key parameters: electron mobility ~200 cm²/V·s in bulk single crystals (thin-film values typically 10–50 cm²/V·s), piezoelectric coefficient e₃₃ ~1 C/m², and birefringence at UV wavelengths.
ZnO sits at the intersection of four application classes. First, as a transparent conducting oxide (TCO): aluminium-doped ZnO (AZO) and gallium-doped ZnO (GZO) are lower-cost, indium-free substitutes for ITO in thin-film solar cells, displays, and touch panels. Second, in UV and visible optoelectronics: ZnO UV LEDs and laser diodes exploit the wide bandgap, though p-type doping of ZnO has remained notoriously difficult, limiting practical LED structures. Third, as a piezoelectric material: ZnO thin films are among the easiest piezoelectric layers to sputter-deposit on arbitrary substrates, making them standard in MEMS resonators, RF filters, and energy-harvesting devices. Fourth, in power and sensing electronics: ZnO TFTs are the workhorse channel material in amorphous-oxide TFT displays (IGZO incorporates ZnO).
The overlap with Gallium Oxide (Ga2O3) (Ga₂O₃, bandgap ~4.8 eV) is worth tracking: Ga₂O₃ has displaced ZnO as the preferred wide-bandgap candidate for next-generation power transistors where breakdown voltage per unit area is the metric. ZnO’s comparative advantage is cost, native-substrate availability, and the excitonic UV emission that Ga₂O₃ does not replicate.
Frontier
See frontmatter frontier: block.