Co + -ion implanted sapphire-annealing in oxidizing atmosphere
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I. INTRODUCTION Ion implantation techniques are finding increasing application as a means of modifying near-surface properties of materials in an as-implanted state as well as in states annealed after ion implantation.1 Implanted atoms are found to exist in a variety of forms on the surface or in the surface layer when ion-implanted crystalline oxides are annealed at high temperature.1"9 In nonoxidizing atmospheres they deposit as metals2'3'6'9 or solid solutions,9 while in oxidizing atmospheres they usually deposit as oxides2'4'5*7 in the form of precipitates or solid solutions. Precipitates are generally deposited as islands3'7"9 with an epitaxial relation to a host crystal. 3 ' 7 ' 9 In most of the preceding work, however, the ion dose was at most 2 x 1017 ions/cm2. It is generally thought that ion implantation can form a modified surface of a given pattern using a relevant mask. High dose ion implantation and subsequent annealing are expected to result in a layered deposition of precipitates. CoSi2 has been reported to be formed as a homogeneous layer buried beneath the silicon surface when the silicon crystal was implanted with Co+ ions to a dose of 3 X 1017/cm2 and annealed at 1273 K.10 It is interesting to study whether a layer deposition of a precipitate occurs when a crystalline oxide is implanted with a high dose of ions and then annealed at high temperature. In this work, we studied which precipitate was formed and how it deposited when single crystal alumina was implanted with Co+ ions to a dose as high as 5 x 1017/cm2 and then annealed at temperatures up to 1473 K in static air. II. EXPERIMENTAL A c-cut single crystal alumina (sapphire) plate was polished to a mirror finish and then heated at 1773 K in air for 5 h to anneal surface damages induced by polishing. The size of the sapphire plates was 5 mm x 25 mm x 1 mm. One of the major surfaces was implanted with 400 keV Co + ions, but a small area was shielded from J. Mater. Res., Vol. 4, No. 3, May/Jun 1989
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the ion beam by a copper plate. The ion doses were 3 X 1017/cm2 and 5 X 1017/cm2 and the current density was about 10 /uA/cm2. The specimen temperature was not controlled during the ion implantation. Annealing was carried out for the implanted specimen in a tube furnace sequentially at 1273, 1373, and 1473 K and each annealing time was 2 h. Accuracy of the annealing temperature was ±20°. It has been reported that amorphous alumina transforms to y-alumina at about 1200 K and y-alumina to a-alumina at about 1450 K.12 Before and after annealing the surface state was examined by Rutherford backscattering (RBS) and x-ray diffraction (XRD) methods. For the specimen annealed at 1473 K, the surface morphology was also studied by a surface profilometer and a scanning electron microscope (SEM), and the optical absorption spectrum was measured by a UV-VS spectrophotometer. III. RESULTS AND DISCUSSION A. Chemical phase of precipitate The color of sapphire implanted with Co+ ions was dark brown in the as-i
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