Stability of gallium-, indium-, and tin-covered tungsten surfaces and formation of In-W and Sn-W alloys

It has been reported that the tin coverage on a tungsten tip increases the stability of the tip surface at an elevated temperature under an applied external field. In the present work the existence of the covering atoms on the heated tungsten tips was examined by measuring the variation in the field emission current with the reduction of the coverage of adatoms due to the gradual field evaporation. The result indicates that a substantial amount of the adatoms remain on the heated surface and no noticeable amount of the unknown entity contributes to the increased surface stability. The observed field strength required to deform a tin-covered tungsten tip was found to be two to three times higher than gallium-covered, indium-covered, or bare tungsten tips. As a heated field emission cathode the stable tin-covered tip emitted a high current corresponding to a current density of well above 1 x 106 A/cm2, which is an order of magnitude higher than that obtained for a bare tungsten tip. Occasionally, the covered tip heated above 900 degreeC exhibited an entirely new cubic structure which would indicate the formation of the presently unknown ordered Sn-W and In-W alloys