ZnCdO/ZnO hetero- and quantum well structures for light-emitting applications

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 108 V/m in ZnCdO/ZnO single quantum well structures. Carrier injection by moderate optical excitation in the 10 kW/cm2 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/cm2 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.