ALD Al-doped ZnO thin film as semiconductor and piezoelectric material: Characterization

This chapter covers the physical characterization of various aluminum-doped ZnO films that were synthesized. At high level, it includes electrical characterization followed by mechanical characterization. The electrical characterization includes Hall measurements to assess the mobility of the films, resistance measurements using special microfabricated patterned structures and carrier concentration measurements. The impact of Al doping on the ZnO thin films shows a very strong electrical signal, which manifested itself in the measured values of these parameters that are mentioned above. The results indicate the ability to tune various electrical parameters of the ZnO films through Al doping and growth temperature. Through response surface modeling, a sweet spot is identified where resistivity, mobility, and carrier concentration can be optimized to target values. While the mechanical characterization includes the piezoeffect characterization along with stress and strain analysis. This includes the comparison of different dielectrics films vis-vis ZnO films, followed by an assessment of 1D versus 2D piezoelectric structures including the wurtzite ZnO thin film. This includes the explanation of why these films have the highest piezoelectric coefficient in their class of materials and the role played by c-axis alignment in interpreting these observations. This establishes the criticality of assessing these quantities to come up with the right understanding and explanation for any observations seen in new class of thin films where the method of synthesis or doping is changed. Finally, the assessment of stress and strain in these film systems is presented with the role played by the substrate and the direction of the bending of the thin film. The experimental results include the design and fabrication of a “curved” stage that is used to induce the strains, and compute the associated stress generated, in quantifiable fashion to model the thin-film behavior.