Surface segregation of Sn and Sb in the low index planes of Cu

[摘要] In this study, the segregation parameters for Sn and Sb in Cu were determined for the firsttime using novel experimental procedures. Sn was first evaporated onto the three lowindex planes of Cu(111), Cu(110) and Cu(100) and subsequently annealed at 920°C for44 days to form three binary alloys of the same Sn concentration. Experimentalquantitative work was done on each of the crystals by monitoring the surface segregationof Sn. Auger electron spectroscopy (AES) was used to monitor the changes inconcentration build up on the surface by heating the sample linearly with time (positivelinear temperature ramp, PLTR) from 450 to 900 K and immediately cooling it linearlywith time (negative linear temperature ramp, NLTR) from 900 to 650 K at constant rates.The usage of NLTR, adopted for the first time in segregation measurements, extended theequilibrium segregation region enabling a unique set of segregation parameters to beobtained.The experimental quantified data points were fitted using the modified Darken model.Two supportive models - the Fick integral and the Bragg- Williams equations - were usedto extract the starting segregation parameters for the modified Darken model thatdescribes surface segregation completely. The Fick integral was used to fit part of thekinetic section of the profile, yielding the pre-exponenrial factor and the activationenergy. The Bragg- Williams equations were then used to fit the equilibrium profilesyielding the segregation and interaction energies. For the first time, a quantified value forinteraction energy between Sn and Cu atoms through segregation measurements wasdetermined (ΩCuSn = 3.8 kJ/mol). The different Sn segregation behaviours in the three Cuorientations were explained by the different vacancy formation energies (that make up theactivation energies) for the different orientations. The profile of Sn in Cu(110) lay atlowest temperature which implies that Sn activation energy was lowest in Cu(110).Sb was evaporated onto the binary CuSn alloys and annealed for a further 44 daysresulting in Cu(111)SnSb and Cu(100)SnSb ternary alloys. Sn and Sb segregationmeasurements were done via AES. The modified Darken model was used to simulate Snand Sb segregation profiles, yielding all the segregation parameters. Guttman equationswere also used to simulate the equilibrium segregation region that was extended by theNLTR runs to yield the segregation and interaction energies. These segregation valuesobtained from the modified Darken model for ternary systems completely characterize thesegregation behaviours of Sn and Sb in Cu. For the ternary systems, it was found that Snwas the first to segregate to the surface due to its higher diffusion coefficient, whichcomes about mainly from a smaller activation energy (ESn(100)= 175 kJ/mol and ESb(100)186 kJ/mol). A repulsive interaction was found between Sn and Sb (ΩSnSb = - 5.3kJ/mol) and as a result of the higher segregation energy of Sb, Sn was displaced from thesurface by Sb. This sequential segregation was found in Cu(100) (∆GSb(100)= 84 kJ/mol;∆GSn(100)= 65 kJ/mol) and in Cu(111) (∆GSb(111) = 86 kJ/mol; ∆GSn(l1l) = 68 kJ/mol). Itwas also found that the profile of Sn in the ternary systems lay at lower temperatures duethe higher pre-exponential factor (DoSn(binary) = 9.2 x 10-4 m2/mol and DoSn(ternary) = 3.4 X10³ m2/mol) if compared to the binary systems.This study successfully and completely describes the segregation behaviour of Sn and Sbin the low index planes of Cu.

[发布机构] University of the Free State