  The foreseen upgrade of the LHC to its high luminosity phase (HL-LHC), will
maximize the physics potential of the facility. The upgrade is expected to
increase the instantaneous luminosity by a factor of 5 and deliver an
integrated luminosity of 3000 fb-1 after 10 years of operation. As a result of
the corresponding increase in radiation and pileup, the electromagnetic
calorimetry in the CMS endcaps will sustain maximum integrated doses of 1.5 MGy
and neutron fluences above 1e16 neq/cm2, necessitating their replacement for
HL-LHC operation.
  The CMS collaboration has decided to replace the existing endcap
electromagnetic and hadronic calorimeters by a High Granularity Calorimeter
(HGCAL) that will provide unprecedented information on electromagnetic and
hadronic showers in the very high pileup of the HL-LHC. In order to employ Si
detectors in HGCAL and to address the challenges brought by the intense
radiation environment, an extensive R&D program has been initiated, comprising
production of prototype sensors of various types, sizes and thicknesses, their
qualification before and after irradiation to the expected levels, and
accompanying simulation studies.
  The ongoing investigation presented here includes measurements of
current-voltage and capacitance-voltage characteristics, along with predicted
charge collection efficiences of the sensors irradiated to levels expected for
the HGCAL at HL-LHC. The status of the study and the first results of the
performance of neutron irradiated Si detectors, as well as their comparison
with numerical simulations, are presented.
