LEED patterns measured during deposition of Ag on Ge(111) at 540 °C. Time given is seconds of deposition, at a deposition rate of 0.2 ML/min. (a) Clean Ge(111) before deposition. At this temperature and electron energy, only the Ge(111) first-order spots are visible (four of the first-order spots are visible, with the remaining two off screen). (b) (3 × 1) Ag. A solid arrow points to one 1/3rd-order (3 × 1) diffraction spot. The Ge(111) first order spots are not visible at this energy. (c) (3 × 1) Ag +√3 Ag. The solid arrow points to one (3 × 1) diffraction spot, and the dotted arrow to a √3 spot. (d) √3 Ag. The completion of the first √3 layer and the beginning of multilayer island growth beyond 1 ML coverage were observed by LEEM (images not shown here), between the LEED images in (c) and (d).
LEEM images of deposition of Ag on Ge(111) at (a) 640 °C and (b) 660 °C. Although the deposition rate was not calibrated, it is the same for both temperatures. The contrast in the LEEM images taken at 0 s, before deposition, is due to defects in the clean Ge(111) substrate accumulated over many Ar+ ion sputtering (cleaning) cycles. (a) A (3 × 1) overlayer, bright contrast in the image measured at 12 s, completely covers the surface before t = 26 s. Following completion of the (3 × 1) layer, a √3 phase forms with very bright contrast evident in the 67 s image. The √3 phase completes by 120 s. Additional deposition up to 330 s (data not shown) does not cause a change in the LEEM image shown for 120 s deposition, indicating that additional Ag beyond 1ML does not stick. (b) A (3 × 1) overlayer (bright contrast in 12 s images) completes at t < 26 s of deposition, as seen for the T = 640 °C case. Additional deposition up to 540 s (data not shown) does not produce any change in the LEEM image, suggesting that no further Ag sticks to the surface. Identification of the surface phases was accomplished with LEED. The contrast and resolution in (b) are worse than in (a) because of poorer adjustment of the LEEM lenses.
LEED images of the phase transformation, [(4×4) + (3×1)]→(3×1), upon increasing sample temperature. (a) The surface was prepared by deposition of 0.3 ML Ag at 170 °C and later annealed to 560 °C. The LEED pattern shows superposition of the (4×4) (dotted arrow) and (3×1) (solid arrow) diffraction patterns. (b) (3×1) (solid arrow). When heating, the (4×4) spots disappear before the (3×1) spots, but they return as soon as the temperature drops again. One can go back and forth between “a” and “b” multiple times, with additional Ag desorbing each time, as seen by the gradual strengthening of the Ge (2×1) spots and weakening of Ag (3 × 1) and (4 × 4) spots.
Transformation of 0.1 ML Ag (3 × 1), deposited onto Ge(111) at 580 °C, to Ag [(4 × 4) + (3 × 1)] upon decreasing sample temperature. The field of view (FOV) in the LEEM images in (a) and (b) is 2.0 μm, masked from original images with 10 μm FOV. (a) Ag (3 × 1) (bright) on Ge (dark) after deposition at 580 °C. (b) Image measured 18 s later shows Ag (4 × 4) (bright) on Ge (dark) at 540 °C. The decrease in surface area of the Ag phase upon transformation from (3 × 1)→[(4 × 4) + (3 × 1)] reflects the slightly larger density of the (4 × 4) phase. The change in phase is indicated in LEEM images by the increased (4 × 4) Ag-Ge contrast compared to (3 × 1) Ag-Ge contrast, as well as the change in the LEED pattern, shown in (c) after the sample has cooled further. Defects in the image due to burns of the LEEM channel plate are indicated with dotted circles. (c) Cooling surface shown in (b) results in LEED pattern with sharper diffraction spots. The primary LEED pattern is (4 × 4), with some very faint (3 × 1) spots.
Article metrics loading...
Full text loading...