  During the scheduled high luminosity upgrade of LHC, the world's largest
particle physics accelerator at CERN, the position sensitive silicon detectors
installed in the vertex and tracking part of the CMS experiment will face more
intense radiation environment than the present system was designed for. To
upgrade the tracker to required performance level, extensive measurements and
simulations studies have already been carried out. A defect model of Synopsys
Sentaurus TCAD simulation package for the bulk properties of proton irradiated
devices has been producing simulations closely matching with measurements of
silicon strip detectors. However, the model does not provide expected behavior
due to the fluence increased surface damage. The solution requires an approach
that does not affect the accurate bulk properties produced by the proton model,
but only adds to it the required radiation induced properties close to the
surface. These include the observed position dependency of the strip detector's
charge collection efficiency (CCE). In this paper a procedure to find a defect
model that reproduces the correct CCE loss, along with other surface properties
of a strip detector up to a fluence $1.5\times10^{15}$ 1 MeV n$_{\textrm{eq}}$
cm$^{-2}$, will be presented. When applied with CCE loss measurements at
different fluences, this method may provide means for the parametrization of
the accumulation of oxide charge at the SiO2/Si interface as a function of
dose.
