Principal issues in producing new ultraviolet light emitters based on transparent semiconductor zinc oxide

In order to develop electroluminescent and laser devices based on the ultraviolet exciton emission of ZnO, it will be important to fabricate p–n junctions with adequate characteristics. Attempts to observe p-type conductivity have been to some extent successful, resulting in hole concentrations and mobilities, respectively up to 1019 cm−3 and 100 cm2/V s in thin layers of ZnO grown by conventional techniques. A new technique is also introduced by Tsukazaki et al. [Tsukazaki et al., Nat. Mater. 4 (2005) 42] to fabricate p-type ZnO reproducibility. Moreover, in order to produce quantum well lasers from an optically active ZnO layer similar to the previously introduced AlxGa1−xAs/GaAs hetero-structures, ZnO based hetero-structures using generally magnesium as a third component have been built-in and two-dimensional electronic confinement has been achieved. Nevertheless, stable and reproducible light emitting devices with sufficient light output and expected UV wavelength around 370 nm have yet been fabricated, neither from the p–n homo- and hetero-junctions nor from the double hetero-structures. Therefore, in order to produce ultraviolet light emitters based on ZnO, we outline this paper as a comprehensive study involving our and some other important recent results to underline the crucial key points concerned with the production of p-type materials, observation of electroluminescent devices, improvement of minority carrier lifetime and modification of the bandgap. It is aimed that the predicted interpretations and proposed models will draw an appropriate path towards fabrication of optoelectronic devices working at short wavelength ultraviolet range of the electromagnetic spectrum.