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Room temperature operation of UV LED is realized for the first time using a hetero p-n composed of transparent conductive oxides, p-SrCu 20 2 and n-ZnO. Ni/

SrCu 20 2/ZnO/ITO multi-layered film was epitaxially grown on an extremely flat YSZ (I11) surface by a PLD. The grown films were processed by a conventional photolithography, followed by reactive ion etching to fabricate p-n junction diode. The resultant device exhibited rectifying I-V characteristics inherent to p-n junction whose turn-on voltage was about 1.5V. A relatively sharp electro-luminescence band centered at 382nm was generated when applying the forward bias voltage larger than the turn-on voltage of 3V. The red shift in the EL peak was noticed from that of photo-luminescence (377nm), which was most likely due to the difference in the excited state density between the emission processes. The EL band is attributed to transition in ZnO, probably to that associated with electron-hole plasma. The photo-voltage was also generated when the p-n junction was irradiated with UV light of which energy coincided with both exciton and band-to-band transitions in ZnO. INTRODUCTION ZnO has been known to emit visible light efficiently when excited with either electron beams or UV light. Utilizing this feature together with n-type conductivity, it has been commercialized as green phosphor for vacuum fluorescence devices and CRTs. The material is also promising for UV light emitting phosphor, as it has a direct band structure with an energy gap of 3.3eV. Additionally, binding energy of exciton in ZnO is about 60meV, sufficient enough to be thermally stabilized even at room temperature. 283 Mat. Res. Soc. Symp. Proc. Vol. 623 © 2000 Materials Research Society

Laser operation excited with electron beams or UV light has been reported so far. 2-5 However, no UV emission associated with p-n junction has been realized to date. Primary reason for that is its difficulty to prepare p-type conductive ZnO, although several attempts including laser doping or co-doping 6 have been made. Thus, ELs of green or UV light has been obtained only from MIS diode structure using SiO as insulator. ' Our approach to realizing ZnO based light emitting diode is rather unique in a sense that TCOs exhibiting p-type conductivity other than p-ZnO was employed as a partner of n-ZnO to form hetero p-n junction, in place of developing homo p-n junction of ZnO. Among several p-TCOs, 8-11 which we have developed recently, p-SrCu 20 2 was selected in this study, primary because it can be deposited as low as 350TC. The low temperature deposition process makes it possible to minimize chemical reaction at the SrCu 20 2 / ZnO interface. Further, SrCu 20 2 could be hetero-epitaxially grown on ZnO, as the lattice mismatch between 5 lattices of ZnO (0001) and 6 lattices of SrCu 20 2 (112) is less than 0.9%. That is, so called "domain matching epitaxy 12,, is expected to work in this combination. In general, epitaxial grown is possibly to result in the formation of high quality, optically active p-