Cathodoluminescence Study of Gadolinium–Doped Yttrium Oxide Thin Films Deposited By Radio–Frequency Magnetron Sputtering

A multi–layer gadolinium–doped yttrium oxide thin film was deposited in a combinatorial fashion on a Si (001) substrate using radio–frequency magnetron sputtering. Alternating layers of Y2O3 and Gd were deposited for a total of 9 layers. The film was homogenized in composition by an 850 C 12 hour thermal treatment producing a range of composition from Y1.96Gd0.04O3 – Y1.54Gd0.46O3 in a single film. Ultraviolet emission with a peak wavelength at 314-315nm was observed from the gadolinium S7/2 P7/2 transition via cathodoluminescence (CL) excitation. The CL–induced intensity was found to be maximum at 9 at% Gd (Y1.82Gd0.18O3) and an optimized electron probe of Vacc = 13 keV was determined for the 1 μm thick thin film. Gadolinium activator saturation was observed for sample current density greater than 0.03 μA/cm. Nonradiative decay via thermal pathways is suspected for the observed activator saturation. Introduction Miniaturized ultraviolet (250 – 350 nm) emitting solid–state sources are required as components for proposed device structures such as non-line-of-sight communication transceivers and receivers and bio-particle detection units. Previous work on rare–earth doped yttrium oxide materials have shown emission from the blue to red range of the electromagnetic spectrum from trivalent Pr, Sm, Eu, Tb, Dy, Er, Ho, and Tm [1–2]. We have recently determined that gadolinium–doped yttrium oxide powder exhibits intense emission at ~ 314 – 315 nm. In this paper we will report work on gadolinium–doped yttrium oxide (also yttria and yttria sesquioxide) thin films that are being explored as a potential material for thin film solid-state UV device applications. Background and Experimental Gadolinium–doped, yttrium oxide thin films have been prepared by combinatorial sputtering using a radio–frequency magnetron sputtering system (AJA international, ATC 2000 – V). The magnets supporting the sputter targets were operated in an unbalanced mode to maximize the deposition rate. The magnetron system used has a “sputter–up” configuration, where the substrate surface is located above two sputter targets that are tilted and facing the substrate. The sputter guns are spaced 180 apart in the chamber in reference to the center of the substrate. Hence, the resulting film deposited from this configuration will have a maximum composition gradient along a line on the substrate surface. Gadolinium foil (0.024 in thick, Alfa Aesar) and a yttrium metal target (0.250 in. thick, Kurt J. Lesker), both 99.9% pure, were used as sputtering targets. During deposition, the gadolinium target was operated at 100 W forward power, and the yttrium target was operated at 200 W. The combinatorial film was deposited in a multi–layer fashion; 9 alternating layers, Y2O3 – [Gd – Y2O3]4 were deposited onto Si (001). The Si wafer was 4 in. in diameter, research grade, boron doped, and with 14 – 22 Ω–cm resistivity. The total pressure during sputtering was Mat. Res. Soc. Symp. Proc. Vol. 764 © 2003 Materials Research Society C7.12.1