Phase transition between $(2\times1)$ and $c(8\times8)$ reconstructions observed on the Si(001) surface around $600^{\circ}$C

The Si(001) surface subjected to different treatments in ultrahigh vacuum molecular beam epitaxy chamber for SiO$_2$ film decomposition has been in situ investigated by reflected high energy electron diffraction (RHEED) and high resolution scanning tunnelling microscopy (STM). A transition between $(2\times1)$ and $(4\times4)$ RHEED patterns was observed. The $(4\times4)$ pattern arose at $T\lesssim 600^{\circ}$C during sample posttreatment cooling. The reconstruction was observed to be reversible. The $c(8\times8)$ structure was revealed by STM at room temperature on the same samples. The $(4\times 4)$ patterns have been evidenced to be a manifestation of the $c(8\times8)$ surface structure in RHEED. The phase transition appearance has been found to depend on thermal treatment conditions and sample cooling rate.