P-TYPE ELECTRICAL-CONDUCTION IN TRANSPARENT THIN-FILMS OF CUALO2

Optically transparent oxides tend to be electrical insulators, by virt ue of their large electronic bandgap (greater than or equal to 3.1 eV) . The most notable exceptions are doped versions of the oxides In2O3, SnO2 and ZnO-all n-type (electron) conductors-which are widely used as the transparent electrodes in flat-panel displays(1,2). On the other hand, no transparent oxide exhibiting high p-type (hole) conductivity is known to exist, whereas such materials could open the way to a rang e of novel applications, For example, a combination of the two types o f transparent conductor in the form of a pn junction could lead to a ' functional' window that transmits visible light yet generates electric ity in response to the absorption of ultraviolet photons. Here we desc ribe a strategy for identifying oxide materials that should combine p- type conductivity with good optical transparency. We illustrate the po tential of this approach by reporting the properties of thin films of CuAlO2, a transparent oxide having room-temperature p-type conductivit y up to 1 S cm(-1). Although the conductivity of our candidate materia l is significantly lower than that observed for the best n-type conduc ting oxides, it is sufficient for some applications, and demonstrates that the development of transparent p-type conductors is not an insurm ountable goal.