Limits to charge transport and electrical dopant activation in transparent conductive (Al,Ga):ZnO prepared by reactive pulsed magnetron sputtering

Degenerately doped ZnO is a highly promising material for applications as transparent electrode (TE) in a variety of modern opto-electronic applications. All of them have in common that the TE material should be highly conductive and transparent at the same time. However, both properties cannot be improved simultaneously due to the optical absorption caused by the free charge carriers. Therefore, a well accepted strategy of materials design is the improvement of the free electron mobility resulting in both decreased resisitivity and enhanced near-infrared transmittance. The present work discusses the limitations to the charge carrier mobility in Al and Ga doped ZnO thin films prepared by reactive magnetron sputtering.
The dominant scattering mechanisms are identified by comparison of experimental data to different charge transport models. A systematic study covering a wide range of dopant concentrations and deposition conditions allows to estimate a material limit for the minimum resisitivity of transparent conductive zinc oxide. It is shown that this limit may be reached by a proper choice of depositions conditions during reactive magnetron sputtering – demonstrating the potential of the method for practical applications. Further, it is shown that electron scattering caused by the incorporation of the Al and Ga dopant into the ZnO host lattice is one of the main limitations for the electron mobility.
Therefore, the effective dopant activation in ZnO is quantified by a combination of electrical, optical and ion-beam analysis characterization methods. Possible mechanisms leading to the deactivation of the dopant at high growth temperatures are discussed. It is demonstrated that Ga is a more efficient electron donor than Al, confirming theoretical predictions on the point defect formation energetics in ZnO.