Two-Dimensional Phase Diagram of Lead on Molybdenum (110) Substrate

Abstract

Surface structures constructed by metallic ultrathin layers on molybdenum (110) or tungsten (110) substrates have been investigated for many years with using reflection high-energy electron diffraction (RHEED), low-energy electron diffraction (LEED), auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and along with other effective techniques, for examples, see refs. 1–7. Historically, Tikhov et al. investigated the lead surface structures on molybdenum (110) substrate by using LEED, AES and TDS, however primarily restricted to room temperature (RT). After that, Jo et al. investigated the lead surface structures on molybdenum (110) substrate as the functions of both film thicknesses and substrate temperatures. At present, it is well-known that four kinds of surface structures appear in case of the lead-on-molybdenum (110) system. A two-dimensional phase diagram of this system has been developed based on the RHEED results. The change process of the surface structures from the (10 4) to (2 10) via (2 3) structures in the temperature range from RT to 873K is also discussed mainly based on the lattice coincidency. And more, the growth modes of the lead-onmolybdenum (110) system have been firstly confirmed by using RHEED specular beam intensity oscillations. All experiments were performed in an ultrahigh-vacuum (UHV) chamber with a base pressure of 1 10 10 Torr. A molybdenum single crystal with a (110) plane within a 0.05 deviation was prepared as a substrate. Details of the experimental procedure have been described elsewhere. To investigate both surface structures and growth modes in detail, the lead deposition thicknesses were changed from 0.1 to 60 monolayers (ML) in 0.1ML steps and the substrate temperatures were changed from RT to 1173K in 50K steps. Resultantly, four kinds of surface structures, as shown in Fig. 1, such as (a) the (10 4), (b) (2 3), (c) (2 10), and (d) (3 2) structures could be confirmed to appear in the leadon-molybdenum (110) system. The detailed discussion for RHEED analyses has already been performed in the previous paper. Each regions appearing each given structures are indicated in Fig. 2 as a two-dimensional phase diagram. The (10 4) structure appeared in the thickness range from 0.4 to 1.1ML and in the temperature range from RT to 1023K, and in the thickness range above 1.1ML and in the temperature range from 973 to 1023K, as indicated by [ Pb]. Unit vectors of the (10 4) structure are ja1j 1⁄4 3:50 A and jb1j 1⁄4 3:44 A, and the angle between them is 121 . The line coincidency can be recognized as follows; in every ninth lead-row and tenth molybdenum-row along the 1⁄2 110 Mo direction, and in every third lead-row and every fourth leadrow along the 1⁄2001 Mo direction. The atomic density is calculated as 9:6 10 atoms cm , corresponding to the 1.02ML. As with the (10 4) structure, the basic informations on the other surface structures are summarized as follows; (2 3) structure [ Pb]: the thickness range of 1.1– 1.3ML and the temperature range of RT to 923K, and the thickness range above 1.3ML and temperature range of 923 to 973K, unit lengths ja2j 1⁄4 3:15 A and jb2j 1⁄4 3:43 A, unit angle of 103 , and atomic density of 9:52 10/cm; the (2 10) structure [ Pb]: 1.3–1.5ML and RT to 923K, above 1.3ML and 873–923K, ja3j 1⁄4 3:15 A, jb3j 1⁄4 3:30 A, 105.8 , b1 a1