Fabrication and characterization of ultraviolet-emitting diodes composed of transparent p-n heterojunction, p-SrCu2O2 and n-ZnO

An ultraviolet light-emitting diode (UV-LED) was realized using a $\mathrm{p-n}$ heterojunction composed of the transparent oxide semiconductors $\mathrm{p-}{\mathrm{SrCu}}_{\mathrm{2}}{\mathrm{O}}_2$ and $\mathrm{n-}\mathrm{ZnO}.$ A ${\mathrm{Ni/SrCu}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}\mathrm{/ZnO/ITO}$ multilayered film was epitaxially grown on an extremely flat YSZ (111) surface by a pulsed-laser deposition technique. ${\mathrm{SrCu}}_{\mathrm{2}}{\mathrm{O}}_2$ (112) was preferentially grown on ZnO (0001) at 350°C, while the preferential plane was changed into the (100) when the temperature was increased to 600 °C. The grown films were processed by conventional photolithography followed by reactive ion etching to fabricate heterojunction diodes. The resulting devices exhibited rectifying I-V characteristics inherent to $\mathrm{p-n}$ junctions. A relatively sharp electroluminescence band centered at 382 nm, attributed to transitions associated with exciton-exciton collision or electron-hole plasma in ZnO, was generated by applying a forward bias voltage greater than the turn-on voltage of 3 V. UV-LED performance characteristics such as threshold current and conversion efficiency improved with higher ${\mathrm{SrCu}}_{\mathrm{2}}{\mathrm{O}}_2$ deposition temperatures. On the other hand, increased laser power density at 600 °C during deposition raised the incidence of insulating layer formation between the p and n layers, probably due to migration of ${\mathrm{K}}^{+}$ ions doped as an acceptor impurity. The resulting $\mathrm{p-i-n}$ diode emits broad luminescence centered at 500 nm for forward voltage greater than 14 V.