Suppressive influence of steps on a phase transition of the Si(001) surface

We have studied an order-disorder phase transition of a vicinal Si(001) surface from a $2\times 1$ to a $c(4\mathrm{}\times 2)$ structure using low-temperature scanning tunneling microscopy (STM). The order parameter for the $c(4\mathrm{}\times 2)$ structure is derived from the Fourier transform of the atomically resolved STM images in the 65–300 K temperature range. The order parameter on the $A$ terrace bounded with an $S_A$ step is found to reach only 0.7, even at 65 K, while the $c(4\mathrm{}\times 2)$ structure is almost complete on the $B$ terrace bounded with an $S_B$ step. In the STM images at 65 K, dimers at the lower side of the $S_A$ step still appear symmetric, resulting in an order-parameter reduction of the $B$ terrace.