Recently, oxide semiconductor materials have attracted much attention for their potential application in optoelectronic devices and transparent electronics. ZnO is one of the promising material because of its direct band gap (~ 3.3 eV) and large exciton binding energy (~60 meV). Although there have been many reports demonstrating ZnO based devices such as p-n junction ultraviolet-blue light emitting diodes and UV photodetectors, ZnO-based transparent thin film diodes has been less studied probably due to the difficulties in achieving p-type ZnO film.  Here, we report on the fabrication of a unipolar transparent thin film diode based on ZnO through a heterojunction implementation, without invoking to p-type materials.

Since unipolar diodes, which are one of majority carrier devices,  have a high switching speed, unipolar transparent thin film diodes can have many device applications in transparent electronics and nonvolatile resistive random access memories.   Our ZnO-based unipolar vertical diodes utilize a heterostructure of ZnO and Zn_1-xMg_xO (ZnMgO) film of graded-composition. ZnO and ZnMgO heterostructure films were deposited on indium-tin-oxide coated glass substrate by using ultrasonic spray pyrolysis. The diodes showed excellent rectifying characteristics with the rectification ratio exeeding 10³ in the range of applied volages between -3.0 and +3.0 V and turn-on voltage about 0.8-1.0 V.  Potential barriers produced in the diode structures and controllability of the diode characteristics will be discussed in terms of the graded band gap of ZnMgO films.

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