Relationship Between Structure and Luminescent Properties of Epitaxial Grown Y 2 O 3 :Eu Thin Films on LaAlO 3 Substrate
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INTRODUCTION Doped Y20 3 (YO) thin films are of major interest for electroluminescence device applications. 1-12 A detailed microscopic understanding of the effect of specific defects on luminescence and their correlation with growth conditions is highly desirable for maximizing luminescent efficiency. Two major sources of defects can be identified; first, threading dislocations nucleated at the film/substrate interface, and second, voids distributed throughout the active film thickness. In this paper, we therefore first review a recent atomic-resolution study of the YO/LaA1O 3 interface structure,13 then demonstrate the relationship between pore structure and the luminescent properties. Using Z-contrast scanning transmission electron microscopy (STEM)"', we directly observe the nanometer-scale cathodoluminescence (CL) of the film, and show that the reduction in luminescent efficiency is due to a "dead layer" around each pore caused by strong recombination of electron-hole pairs at the internal pore surfaces. Eu-activated YO thin films were deposited by laser ablation15 ' 7 on (001) LaAlO 3 (LAO) substrates. For details see ref.18. Rocking curve measurements indicate a full width half maximum (FWHM) of 0.10. Cross-sectional slices were obtained by cutting the LAO along the [100] or [010] directions (using pseudocubic indexing) and then gluing face to face in the usual way. Both plan-view and cross-section specimens were prepared for transmission electron microscopy (TEM) and/or STEM observations by mechanical grinding, polishing, and dimpling, followed by Ar-ion milling. TEM bright field images and electron diffraction patterns were recorded in a Philips EM400 electron microscope operated at 100kV. High-resolution Z-contrast imaging was conducted in a VG HB603U STEM at 300kV,' 4 19 while the cathodoluminescent (CL) imaging was carried out in a VG HB501 STEM at 100 kV. The CL emission was collected by a lens system and detected by a photomultiplier, as shown in the schematic of Fig. 1.
203 Mat. Res. Soc. Symp. Proc. Vol. 589 © 2001 Materials Research Society
L'ens Sc~anning
'rb
.0 SObjective
.01
Lens LaAIO3 Image at 300 kV: ZAI=I 3, ZLa= 57
Annular Detector I - Z2
"0 La
*K4-1,
o Al
0.189 nm
Fig. 1 Schematic of the Z-contrast scanning transmission electron microscope (STEM) imaging together with the cathodoluminescent imaging system FILM/SUBSTRATE INTERFACE STRUCTURE It is well known that Y20 3 has a C-type rare-earth sesquioxide structure, closely related to the fluorite structure with a cell parameter a=1.060 nm and space group Th7(1a3). 2 °23 In the fluorite lattice, each cation is surrounded by eight anions located at the comer of a cube. The Ctype structure is derived by removing one-quarter of oxygen atoms and slightly rearranging the remaining ones.' For 75% of the cations the vacancies lie at the ends of a face diagonal, while for the other 25% they lie at the ends of a body diagonal. Therefore, each yttrium atom is surrounded by only six oxygen neighbors forming two different types of distorted
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