Al-doped zinc oxide films deposited by simultaneous rf and dc excitation of a magnetron plasma: Relationships between plasma parameters and structural and electrical film properties

A new technique of the simultaneous excitation of a magnetron sputtering discharge by rf and dc was used for the deposition of undoped ZnO- and Al-doped ZnO (ZnO:Al) films. By this technique, it was possible to change the ion-to-neutral ratio $j_i{\mathrm{/j}}_n$ on the substrates during the film growth by more than a factor of ten, which was revealed by plasma monitor and Langmuir probe measurements. While for a pure dc discharge the ions impinging onto a floating substrate have energies of about $E_i\text{≈17\hspace{0.17em}}\mathrm{eV},$ the rf discharge is characterized by Ar-ion energies of about 35 eV. Furthermore, the ion current density for the rf excitation is higher by a factor of about five, which is caused by the higher plasma density in front of the substrate. This leads to a much higher ion-to-neutral ratio $j_i{\mathrm{/j}}_n$ on the growing film in the case of the rf discharge, which strongly influences the structural and electrical properties of the ZnO(:Al) films. The rf-grown films exhibit about the three times lower specific resistances ${\text{(ρ≈6×10}}^{\text{−4}}\mathrm{\hspace{0.17em}\Omega \hspace{0.17em}}\mathrm{cm}\mathrm{),}$ due to lower mechanical stress, leading to higher charge carrier concentrations and mobilities. Undoped ZnO films exhibited the largest compressive stress values up to 2.8 GPa. The aluminium-doped films have a better (001) texture and larger grains ${\mathrm{(d}}_g\text{≈38\hspace{0.17em}}\mathrm{nm}\mathrm{),}$ which can be attributed to the beneficial role of Al as a surfactant. The better crystalline film quality of the ZnO:Al films is the reason for the much lower compressive stress of $\text{<0.5\hspace{0.17em}}\mathrm{GPa}$ in these layers.