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 sputteri ng discharge by rf and de 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)/j(n) on the substrates during th e film growth by more than a factor of ten, which was revealed by plas ma monitor and Langmuir probe measurements. While for a pure de discha rge the ions impinging onto a floating substrate have energies of abou t E-i approximate to 17 eV, the rf discharge is characterized by Ar-io n energies of about 35 eV. Furthermore, the ion current density for th e rf excitation is higher by a factor of about five, which is caused b y the higher plasma density in front of the substrate. This leads to a much higher ion-to-neutral ratio j(i)/j(n) on the growing film in the case of the rf discharge, which strongly influences the structural an d electrical properties of the ZnO(:Al) films. The rf-grown films exhi bit about the three times lower specific resistances (rho approximate to 6 X 10(-4) Ohm cm), due to lower mechanical stress, leading to high er charge carrier concentrations and mobilities. Undoped ZnO films exh ibited the largest compressive stress values up to 2.8 GPa. The alumin ium-doped films have a better (001) texture and larger grains (d(g) ap proximate to 38 nm), 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 <0.5 GPa in these layers. (C) 1998 American Institute of Physics.