Graduate Exam Abstract
July 13, 2010, 2:30
ERC Electronic Classroom (A210)
Characterization of scandium oxide thin films for use in interference coatings for high-power lasers operating in the near-infrared
Abstract: The work presented in this thesis aims to investigate scandium oxide (scandia), deposited using dual ion beam sputtering, as a high-index material to be used in interference coatings to be implemented in high-power lasers. Ion beam sputtered scandia coatings have the potential to allow for the power scaling of high-power lasers operating in the near-infrared. Ion beam sputtering is the technique currently used by many commercial companies to produce low-loss, high-damage-threshold coatings required by lasers operating with high fluences. Scandia is predicted to have a higher damage threshold than the current state-of-the-art high-index materials, like hafnia, because of its high bandgap.
The development of scandia, and other thin film materials, requires the reduction of defects in the material through modification of growth processes and post deposition treatment. Material defects give rise to absorption of laser light and laser induced damage initiation sites. The growth parameter investigated in this work is the oxygen partial pressure in the deposition chamber during the reactive sputtering process of a metal Sc target to form Sc2O3. The film properties are sensitive to the oxygen partial pressure. At 2 ?Torr oxygen partial pressure, the films are metallic and highly absorbing with an absorption, at ? = 1.064 ?m, of > 104 ppm. The absorption decreases to 10 ppm at 2 ?Torr oxygen partial pressure and at 38 ?Torr, the absorption increases to 35 ppm. This, along with the increase in absorption near the optical band edge, suggests an increase in shallow-type defect concentrations for increasing oxygen partial pressures. The observed defects contain unpaired electrons, as assessed by electron paramagnetic measurements, that have a paramagnetic absorption signal with principle g-values [gxx, gyy, gzz] = [2.018, 2.019, 2.058]. Generally, the concentration of the paramagnetic species measured increased with increasing oxygen partial pressure. These spin defects are possibly O2- interstitials in the deposited films. The increased density of these defects causes the absorption to increase near ? = 212 nm, which is the optical band edge of the Sc2O3 films. Furthermore, these defects contribute to an approximately 40% increase in the film stress observed in x-ray diffraction measurements and measurements of stress-induced fused silica substrate curvature.
Adviser: Carmen Menoni
Non-ECE Member: John Williams (Mechanical Engineering)
Member 3: Mario Marconi (Electrical Engineering)
Addional Members: N/A
Program of Study: