Mg Implantation and Characterization of Sapphire Surfaces
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Mg IMPLANTATION AND CHARACTERIZATION OF SAPPHIRE SURFACES
by A.P.Jardine, S.Mitra Mukhopadhyay and J.M.Blakely Dept. of Material Science, Bard Hall Cornell University, Ithaca NY 14853 Abstract Mg ion-implantation of A1203 wafers followed by air annealing at high temperatures was investigated as a way to provide bulk doped samples for studies of Mg surface segregation and its effect on surface mass transport. SIMS was used to analyse Mg concentrations in implanted and unimplanted near-surface regions and in the bulk. It was found that the concentration of Mg decreases dramatically with depth from both surfaces of an annealed wafer and is also observed in the bulk. These observations are attributed to bulk diffusion of the Mg combined with equilibrium surface segregation. Surface mass transport associated with the (1120) surface of an A12 0 3 single crystal wafer doped and equilibrated by such a method was studied by the grating decay method; the Mg doped sample showed a decay rate in air higher than that in the undoped wafer by a factor of - 2.4 at 1500°C. Introduction It has long been known that the addition of small amounts (250 ppm) of MgO allows sintering of alumina (A1203) powders to near-theoretical density by the eradication of pores. For pores to remain attached to moving grain-boundaries and so be filled by Al and 0 atoms diffusing via fast grain boundary diffusion, Heuer [1] postulated that the pore mobility must be large compared with grain boundary mobilities. The high pore mobility could in turn be caused by the equilibrium segregation of Mg to pore surfaces where it may increase the surface mass transport rate. Baik et al [2] have indeed observed Mg segregation to a (0001) surface between 1300'C and 1500'C. Its effect on surface mobilities can be measured by the grating decay method [3] where the exponential decay in the amplitude of a periodic profile etched into the surface is related to surface and volume ionic mobilities. Mg Implantation and Equilibration 2
Large (>lcm ) Mg-doped single crystal sapphire wafers were required for the diffusion studies. Preoriented and polished 1" diameter, 250/Am thick single crystal sapphire wafers were obtained from Adolf Meller Inc and Crystal Systems Inc. Chemical analysis of the impurities in as-received wafers showed Mg levels of 0.2 to 0.5 ppm and a total impurity content of < 100 ppm. The wafers were first cleaned by rinsing in acetone and then methanol before being loaded into the implanter. The vacuum in the diffusion pumped implanter was typically 106 Torr. Most of the carbon contamination from the pumps was removed by liquid N 2 traps.
Mat. Res. Soc. Symp. Proc. Vol. 74. c 1987 Materials Research Society
366
The Mg beam was generated from a metallic Mg compacted powder, heated beyond its melting point and reacted with 2 -- 3 x 10-5 torr of H2 carrier gas; the resulting mixture of 2 gases is ionized to form a plasma. With an implant time of - 40 mins, 1016 ions/cm were 40 ppm of Mg. This is to an average bulk concentration of 20 to implanted correspo
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