Zinc Oxide is one of the most promising materials for the development of transparent optoelectronic devices. These expectations are based in the fact that Zinc is an earth abundant material, the exciton binding energy of ZnO is even high at room temperature and its energy gap is around 3.3 eV. On the other hand, it is well known that an internal electric field can generate a reduction of the effective gap energy, the Franz-Keldysh effect. For the first time, we have shown that this effect can be enhanced when a tensile strain is generated in ZnO thin films. Since the piezoelectric coefficient of ZnO is very high, very small strain can produce electric fields that reduce the effective energy gap on these films. In this way, our work can provide a building block to the new field of piezo-phototronic.

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