Employing electroreflectance (ER) spectroscopy, we have studied optical transitions in porous Si (PSi) with a thickness of 100±50 nm made from crystalline Si (c-Si) substrates with different resistivities, 4–10 ωcm (p
-), 0.1–1 ωcm (p), and < 0.018 ωcm (p
+). The ER features observed at 1.1–2.8 eV in p
+ PSi are analyzed with a simple effective mass approximation (EMA) model for confined electron-hole (e-h) pairs in a spherical quantum dot as we have previously done for those observed at 1.2–3.1 eV in p
- PSi. From the ER analysis with the EMA model, the effective crystal size is estimated without destruction, and the kinetic energy and the Coulomb attraction energy of the confined e-h pairs are also deduced.
Furthermore, the ER features corresponding to the optical transitions at E
0') critical point (CP) are observed at 2.8–3.3 eV in p
+ PSi. With an increase in the crystal size, the transition energy of E
0') CP in p
+ PSi is decreased, while that in p
- and p PSi is unchanged from that of 3.4 eV found in c-Si including the p
c-Si. From the Raman results, strain- and disorder-induced spectral changes are found to be negligible, so that the high doping induced effect is the most acceptable mechanism for the red-shift in E
0') transitions of Si nanocrystals with the crystal size of 2–3 nm.