Phase transition between (2 x 1) and c(8 x 8) reconstructions observed on the Si(001) surface around 600C

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 x 1) and (4 x 4) RHEED patterns was observed. The (4 x 4) pattern arose at T <~600C during sample posttreatment cooling. The reconstruction was observed to be reversible. The c(8 x 8) structure was revealed by STM at room temperature on the same samples. The (4 x 4) patterns have been evidenced to be a manifestation of the c(8 x 8) surface structure in RHEED. The phase transition appearance has been found to depend on thermal treatment conditions and sample cooling rate.