The phase transition between c(4 x 2) and p(2 x 2) structures on heavily doped n-type Si(001) substrates at low temperatures under observation by the scanning tunneling microscopy is theoretically investigated. The model potential of the dimer system on the Si(001) surface is improved with an additional term which depends on temperature and the surface electric field. We show that the torque applied by the additional surface electric field introduced by the scanning tunneling microscopy causes the transformation of the c(4 x 2) phase, that is realized in the absence of the observation, to the p(2 x 2) phase on the Si(001) substrates. At higher temperatures, the c(4 x 2) phase is recoverd by the reduction of the additional surface electric field. The broad phase transition between the c(4 x 2) and p(2 x 2) phases is reproduced by the Monte Carlo simulations on a system with the type-C defects. We show that the broad temperature range of the phase transition is induced by the type-C defects.
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