The segregation of indium from polycrystalline copper crystals
Madito, Moshawe Jack
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In this study the surface segregation of In from a dilute Cu(In) alloy to the surface was investigated. The Cu(In) alloy was prepared by diffusion doping. A thin layer of In was deposited onto the backside of a polycrystalline Cu crystal. The In layer was covered with a thin Cu layer to prevent the In from melting and evaporating from the Cu crystal. The Cu crystal was annealed in an Ar gas atmosphere. During annealing the temperature was increased stepwise from 150 to 900 to prevent the In layer from melting. Finally the Cu crystal was annealed at 900 for 456 hours. As a result, the polycrystalline Cu crystal was successfully doped with an In concentration of 0.059 at%. Using Auger electron spectroscopy (AES) coupled with an Ar+ ion gun (for sur- face sputter cleaning) and a programmed crystal heater inside the ultra high vacuum (UHV) chamber, the segregation profiles where recorded by monitoring the surface condition of In and S on the Cu crystal during linear heating and constant temperatures heating. The constant temperature segregation measurements were performed in the tem- perature range 460 to 580 . The segregation profiles showed that both In and S segregates from the Cu crystal. In segregates to the surface at a much higher rate than S. In reached a maximum surface coverage of 25 % for the temperatures 460 ,490 and 520 and 16 % for temperatures 550 and 580. Both the In and S segregation profiles were fitted with semi-infinite solution of Fick’s equation and the segregation parameters (Q and D0) were obtained for In as Q = 191.9 kJ.mol−1, D0 = 1.1 × 10−5 m2.s−1 and for S as Q = 201.1 kJ.mol−1, D0 = 4.4 × 10−2 m2.s−1. The linear temperature measurements were carried out from the temperature range 100 to 800 at heating rates of 0.025; 0.05; 0.1 and 0.2.s−1. All seg- regation profiles showed In segregation to the surface that reached a surface maxi- mum coverage of 23−25 %. The segregation of In was accompanied by a slow seg- regating S that replaced the In completely from the surface. The surface concen- tration of In reached a maximum coverage of 29−32 %. The replacement of In by S is due to a large difference between the segregation energies of In and S with S hav- ing the more negative segregation energy. The segregation profiles were fitted with the modified semi-infinity model of Fick to obtain the segregation parameters (pre- exponential factor (D0) and activation energy (Q)). The segregation profiles were also fitted with the Guttmann model to obtain the segregation energies (G) and interaction energies ( ) for In and S segregation in Cu. Finally, using segregation parameters obtained from these fits (Fick and Guttmann) the segregation profiles were fitted with modified Darken model to yield the segregation parameters for In and S segregation in Cu crystal. The segregation parameters for In that were ob- tained were Q = 184.1 kJ.mol−1, D0 = 1.6 × 10−5 m2.s−1,G = −60.4 kJ.mol−1 and In-Cu = 3.0 kJ.mol−1. The segregation parameters for S that were obtained were Q = 212.4 kJ.mol−1, D0 = 9.1 × 10−3 m2.s−1,G = −120.0 kJ.mol−1, S-Cu = 23.0 kJ.mol−1. The interaction energy for In and S was determined as In-S = −4.0 kJ.mol−1. The segregation parameters (Q and D0) obtained in this study for In segregation from a Cu crystal compare well with those reported in literature for In tracer diffusion in a Cu(In) bulk system. The segregation parameters (Q and D0) ob- tained in this study for S compare well with those reported in literature for both S segregation from a Cu crystal and S tracer diffusion in a Cu(S) bulk system.
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