Date of Award
Doctor of Philosophy
Materials Science and Engineering
Philip D. Rack
Thomas T. Meek, Carl J. McHargue, David C. Joy
Rare-earth and transition metal-doped yttrium aluminum garnet (Y3A15O12 or YAG) phosphors have been widely used because of very stable chemical, mechanical properties of YAG and subsequent high efficiency of doped YAG phosphors. However, to synthesize pure YAG materials by different approaches such as solid-state reaction, sol-gel, spray pyrolysis, co-precipitation, and combustion method. But nearly all of these techniques require high temperature (>1600C) and prolonged processing time.
In this work, rf magnetron reactive sputtering was used to synthesize YAG thin films at relatively low temperature and short processing period. An appropriate post heat treatment, 10 hours at 1000C, was conducted to crystallize the films. A combination of 80W for yttrium target, 130W for aluminum target, and a total pressure of 3 mTorr gas mixtures with 25 sccm argon and 1.4 sccm oxygen flow rate produced a Y3A15O12 stoichiometric thin film which was verified by energy dispersive x-ray spectroscopy (EDS). The x-ray diffraction (XRD) patterns of the annealed YAG films showed only YAG peaks with a dominant peak of YAG (420) at a 2Ɵ of 33.33°.
Gadolinium-doped YAG thin films and chromium-doped YAG thin films were synthesized in a combinatorial fashion. Gadolinium-doped YAG thin films emit at 312 nm due to 8S7/2- 6P7/2 intrashell 4f-4f transition which is in ultraviolet (UV) region. YAG:Cr thin films have red emission at 687 nm due to 4A2- 4E2 transition. The combinatorial thin film sputtering technique rapidly determined the gadolinium and chromium concentration that yielded the optimum luminescence intensity. It was determined that a gadolinium concentration of ~5.5 at% resulted in a maximum determined that a gadolinium concentration of ~5.5 at% resulted in a maximum Cathodoluminescence (CL) intensity. Similarly, a chromium concentration of ~0.69 at% contributed to the optimum Photoluminescence (PL) intensity of YAG:Cr films. The concentration quenching phenomena were discussed.
Based on the determined sputtering conditions that resulted in the optimum luminescence Gd and Cr concentrations, a 23 full factorial design of experiments (DOE) was conducted to investigate the effects that substrate temperature, substrate bias, and oxygen flow rate have on the CL properties and the crystallinity of YAG:Gd and YAG:Cr thin films, respectively. DOE results showed that increasing O2 flow rate decreases both CL efficiency or PL intensity and has a negligible effect on XRD intensity, and substrate bias enhances CL efficiency or PL intensity but decreases XRD intensity. The effects of the parameters were thoroughly interpreted by correlating the composition and morphology of the films. The results of the DOEs were analyzed by statistical methods. Single factor studies for each of the variables, substrate temperature, substrate bias, and oxygen flow rate, verified the DOE results. The combination of low O2 flow rate and high substrate bias was necessary in the optimum process condition which leads to the highest CL efficiency or PL intensity of YAG:Gd and YAG:Cr thin films.
The effect of total pressure of the Ar/O2 gas mixture on CL efficiency and XRD intensity was also investigated.
The PL temperature dependent behaviors of YAG:Gd and YAG:Cr thin films were studied in a temperature range of 15-298 K using a Specs spectrometer equipped with a photocounting detection system and a double grating monochrometer. The thermal quenching phenomenon was observed on both YAG:Gd and YAG:Cr films. It can be due to a significant increase in the non-radiative recombination at temperatures above a certain temperature, in our case, ~110K. An activation energy of 24.7 meV and an A value of 2.08 were estimated for YAG:Gd films. The non-radiative activation energy is suspected to be due to electron-phonon coupling
Similarly, the PL temperature dependent behavior of YAG:Cr film was investigated. The observed sharp zero-phonon R-line emission of YAG:Cr film is believed to be due to 4E2-4A2 transition, instead of 4T2-4A2 transition. A slight red-shift of the sharp zero-phonon R-line emission was observed with increasing temperature. The total integrated PL emission intensity was found to rapidly decrease at temperatures above 110K. An activation energy of 25.2 meV and an A value of 1.44 were obtained which may be also attributed to electron-phonon coupling.
Deng, Yuepeng, "Thin Film Sputtering of Gadolinium and Chromium-doped Yttrium Aluminum Garnet. " PhD diss., University of Tennessee, 2005.