Fig. 2: Universal model for RHEED diffracted-intensity oscillations during alternating-shutter growth.

a Model: intensity oscillations starting from a TiO₂-terminated surface. 3 panels showing the oscillation shape during one deposition cycle of Sr₁/Ti₁, Sr₂/Ti₂ and Sr₃/Ti₃. The oscillation of a full Sr₃/Ti₃ cycle (dashed red/blue lines) serves as the reference curve for Sr_x/Ti_x cycles (x < 3). A cycle of Sr₁/Ti₁ or Sr₂/Ti₂ will trace the corresponding fraction (solid lines) of the reference Sr₃/Ti₃ curves. During Sr deposition, an inflection point (maximum) occurs at ~1.3 Sr layers on the TiO₂ surface (b). During a Sr₁/Ti₁ cycle, the inflection point is not reached and the oscillation traces a triangular shape (up on Sr/down on Ti); a Sr₂/Ti₂ cycle traces a “shallow double-peak” shape; and a Sr₃/Ti₃ cycle traces the full “deep double-peak” shape. At the end of each cycle, the intensity returns to its starting value corresponding to TiO₂ termination. Colors: red = Sr shutter open; blue = Ti shutter open; black = all shutters closed. Fluxes are assumed constant. b Experiment: diffracted intensity during Sr_x/Ti_x cycles on TiO₂ termination. Diffracted intensity during a sequence of Sr_x/Ti_x cycles, with x = 1, 1.5, 2 and 2.5, showing triangular and double-peak oscillations. c Model: intensity oscillations starting from mixed termination. 3 panels showing oscillation shapes during a Sr₁/Ti₁ cycle, starting from different partial SrO coverages (0.3, 0.8 and 1.3 layers) on a TiO₂ surface. The observed intensity (solid lines) follows the universal curves for a Sr₃/Ti₃ cycle (dashed lines), starting at the point on the curve corresponding to the partial Sr coverage, and tracing an arc for one-third of the Sr₃ (Ti₃) shutter time. Consequently, a Sr₁/Ti₁ cycle starting from 0.3 SrO layers/TiO₂ shows an upward triangular shape (up on Sr/down on Ti); a cycle starting from 0.8 Sr layers/TiO₂ passes the inflection point and shows a symmetric “double-peak” shape; and a cycle starting from 1.3 Sr layers/TiO₂ shows an downward triangular shape (down on Sr/up on Ti). d Experiment: intensity oscillations for different mixed terminations. Diffracted intensities during 3 cycles of Sr₁/Ti₁ for different partial SrO coverage on TiO₂: starting from [A] TiO₂ termination, [B] 0.33 layer SrO, [C] 0.66 SrO, [D] 1.0 SrO and [E] 1.33 SrO. The original TiO₂ termination is recovered by depositing Ti in increments of 0.33 layers [D’]→[C’]→[B’]→[A’]. During the deposition of each partial 0.33 SrO layer, the intensity shows a step-like increase; and during each partial 0.33 TiO₂ layer, the intensity shows a step-like decrease. Note the similar shape of oscillations obtained on surfaces with similar net Sr coverage, e.g., [A] and [A’], [B] and [B’], etc. The oscillation shapes agree with those predicted by the model in (c) for similar net Sr coverage, i.e., triangular (starting Sr = 0 and 0.33), shallow double-peak (Sr = 0.6), deep double-peak (Sr = 1.0), and downward triangle (Sr = 1.33).