  Pre-growth preparation of the Si(001) clean surface is a key process for
Ge/Si UHV MBE. It usually includes pre-cleaning by liquid etchants, thermal
pretreatment in UHV and removal of SiO2 or a different protective coat prior to
MBE. Chemical pre-cleaning usually results in formation of the thin SiO2 film
or the surface passivated by H. Depending on chemical pretreatment different
purification processes are applied to prepare the clean surface in UHV. A
structure of the Si(001) surface which may affect MBE was studied by STM and
RHEED. Experiments were carried out in UHV using the Riber SSC2 instrument
coupled with GPI-300 STM. Si wafers were either etched beforehand in the
solutions of RCA or HF and HNO3 mixture or passivated in the HF solutions. The
(2x1) reconstruction was found to mainly arise in UHV after deoxidization
processes such as high-temperature annealing or SiO2 removal by a weak flux of
Si. The c(8x8) structure was also often observed by STM at room temperature and
found to occupy whole surface or a part of it depending on sample cooling rate
and chemical pretreatment. The c(8x8) structure observed by STM was evidenced
to correspond to (4x4) one in the RHEED patterns; the later replaced (2x1) when
the samples were cooled below 600C and turned out to be reversible with the
transition point of ~600C. A structural model of the c(8x8) surface and a
pathway of its formation in assumption of surface stress field self ordering
are proposed.
