Ga doping of ZnO allows for generation of free electrons up to concentrations of about 10<sup>21</sup> cm<sup>-3</sup> without significant deterioration of the crystal structure. In this way, a metallic dielectric function is formed with a negative-to-positive crossover of the real part tunable from mid infrared up to telecommunication wavelengths. The losses are at least one order of magnitude lower than for traditional metals. We demonstrate surface plasmon polaritons with dispersion relations that can be engineered in a unique way by utilizing epitaxial multi-layer structures of different doping level.
Molecular-beam epitaxial growth far from thermal equilibrium allows us to overcome the standard solubility limit
and to alloy ZnO with CdO in strict wurtzite phase up to mole fractions of several 10%. In this way, a band-gap
range extending from 3.3 eV down to 2.3 eV can be covered. Strong improvement of the crystalline quality
indicated by a rocking curve width of only 45 arc sec is achieved when growing the ternary on ZnO substrates.
Despite very low growth temperatures (~150 °C), layer-by-layer growth indicated and controlled by RHEED
oscillations is accomplished. This enables us the fabrication of atomically smooth heterointerfaces and well-defined quantum well structures exhibiting prominent band-gap related light emission in the whole composition
range. Post-growth annealing increases the radiative efficiency up to two orders of magnitude and demonstrates
thermal stability of the structures with respect to phase separation even up to temperatures of about 500°C.
Low-energy shifts of the photoluminescence features reaching the order of 1 eV as well as a dramatic increase of
the lifetime from the sub-ns to the 100-μs time-scale uncover the presence of huge polarization-induced electric
fields of some 10<sup>8</sup> V/m in ZnCdO/ZnO single quantum well structures. Carrier injection by moderate optical
excitation in the 10 kW/cm<sup>2</sup> screens these fields and recovers practically the bare quantum-confined energy
transitions. On appropriately designed structures, laser action from the UV down to the green wavelength
range is observed under optical pumping. The threshold at low temperature is only 60 kW/cm<sup>2</sup> and increases
only moderately up to room temperatures. All these findings make ZnO-based heterostructures a promising
alternative to group-III-nitrides for opto-electronic applications in the short-wavelength range.