Energy distribution of positively and negatively charged plasma ions of a pulsed magnetron sputtering discharge in an argon/oxygen gas mixture and deposition of functional ZnO films

The energy distribution of negatively and positively charged ions in a magnetron discharge is investigated. A zinc cathode operated in an argon/oxygen gas mixture is employed. The magnetron is operated in the pulsed mode with repetition frequencies of 10–30 kHz. Positively charged atomic O+, Ar+, and Zn+ ions, molecular O2+, ArO+, Ar2+, ZnO+, and ZnO2+, and doubly charged Ar2+ ions and negatively charged O−, O2−, ZnO−, and ZnO2− ions are observed. Negatively charged ions either form inside the plasma volume by electron attachment reactions or are sputtered from the negatively biased cathode and as such receive high kinetic energies corresponding to the cathode potential. The intensity of the negatively charged O− strongly depends on the plasma parameters, in particular, the repetition rate. Zinc oxide films are deposited at room temperature at repetition frequencies of 10–30 keV. The deposited films are characterized by means of scanning electron microscopy, x-ray diffractometry, photothermal deflection spectroscopy, optical and infrared ellipsometry, Fourier-transform infrared and Raman spectroscopy, and photoluminescence. These films are highly oriented along the surface normal and are highly transparent in the visible spectral region. The extracted optical bandgap and Urbach energy are about 3.3 eV and 62 meV, respectively.