MOCVD of the Blue Electro-luminescent Phosphor CaGa 2 S 4 :Ce from a Liquid Reagent Delivery System
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deposition equipment is attractive from a manufacturing standpoint because of its ease of integration to a manufacturing process. This paper covers the MOCVD of CaGa2S4 :Ce for use as a blue phosphor in flat panel applications. Previously, this group has reported the deposition of CaGa 2S4:Ce using a traditional reagent bubbling techniques.4 However, the use of this method is not optimal because of problems which are present with regards to the reproducibility of the experimental conditions. These problems are the result of several limitations such as changes in the surface area of the solid source and changes in the reagent chemistry from spending time at elevated temperatures. Also, from the prospective of understanding of the chemical processes with the deposition reactor, the traditional delivery method using the organometallic reagents has difficulties because the vapor pressures of these compounds have not been well defined. It is hoped that the liquid delivery system would eliminate these problems by providing reliable reagent delivery conditions from reagents which would be kept at room temperature. EXPERIMENT The deposition of CaGa 2S4 :Ce was done in a horizontal, warm wall reactor with a modified impinging jet injector. Heating was done by a quartz lamp which was focused on the backside of the substrate through a quartz window. Deposition was performed onto glass substrates coated with indium tin oxide, aluminum titanium oxide, and a thin zinc sulfide nucleation layer; the layered substrate allowed for color and brightness measurements to be made on the as-deposited material. All samples were deposited at 582 0 C and 5 torr;-the reagent flow rates, H 2S and Ar flow rates, and deposition time were varied in the experiments. The principal characterization technique for these samples was x-ray fluorescence (XRF) and electroluminescence (EL). The deposition thickness for all of the samples were maintained at approximately 5000A to allow for EL measurements. The reagent flows were composed of C-a(tmhd) 2, Ga(tmhd) 3 , and Ce(tmhd) 4, where the tmhd refers to 2,2,6,6-tetramethyl-3,5-heptanedionate. Gas flows of Ar were used as the carrier gas, and H2 S was used as the sulfur source. The delivery of the tmhd reagents was done using two methods. The first method was by saturating a carrier gas with the vapor from the reagent. The control of the flow rate was by the flow rate of the carrier gas and by the temperature of the bubbler. This setup introduced a large number of variables since each source's temperature and carrier flow rate had to independently controlled. The second method for reagent delivery was by the use of a commercially available liquid delivery system (LDS). The three source system (LDS-300B) was purchased from Advanced Technology Materials, Inc. For this system, the three sources contained Ca(tmhd) 2, Ga(thmd) 3, and Ca(tmhd) 2+Ce(tmhd) 4. The mixture of Ca(tmhd) 2 and Ce(tmhd) 4 in the last source was done to improve the resolution of the Ce doping in the deposition. The liquid sources consi
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