  Type Ia supernovae (SNe Ia) often show high-velocity absorption features
(HVFs) in their early phase spectra; however the origin of the HVFs is unknown.
We show that a near-Chandrasekhar-mass white dwarf (WD) develops a silicon-rich
layer on a carbon-oxygen (CO) core before it explodes as an SN Ia. We
calculated the nuclear yields in successive helium shell flashes for 1.0
$M_\odot$, 1.2 $M_\odot$, and 1.35 $M_\odot$ CO WDs accreting helium-rich
matter with several mass-accretion rates ranging from $1 \times
10^{-7}~M_\odot$ yr$^{-1}$ to $7.5 \times 10^{-7}~M_\odot$ yr$^{-1}$. For the
$1.35~M_\odot$ WD with the accretion rate of $1.6 \times 10^{-7}~M_\odot$
yr$^{-1}$, the surface layer developed as helium burning ash and consisted of
40% $^{24}$Mg, 33% $^{12}$C, 23% $^{28}$Si, and a few percent of $^{20}$Ne by
weight. For a higher mass accretion rate of $7.5 \times 10^{-7}~M_\odot$
yr$^{-1}$, the surface layer consisted of 58% $^{12}$C, 31% $^{24}$Mg, and
0.43% $^{28}$Si. For the $1.2~M_\odot$ WDs, silicon is produced only for lower
mass accretion rates (2% for $1.6 \times 10^{-7}~M_\odot$ yr$^{-1}$). No
substantial silicon ($< 0.07%$) is produced on the $1.0~M_\odot$ WD
independently of the mass-accretion rate. If the silicon-rich surface layer is
the origin of Si II HVFs, its characteristics are consistent with that of mass
increasing WDs. We also discuss possible Ca production on very massive WDs ($
\gtrsim 1.38~M_\odot$).
