Piezoreflectance measurements were carried out on InP samples with different doping concentrations near the absorption edge in the temperature range between 20 and 300 K. We show that the large features often found below the bandgap in piezomodulated spectra of InP samples are related not only to the impurity states but also to the back-surface reflection effects. Their proximity to the band edge depends on temperature, the impurity species and the geometry of the species in which the multiple reflections occur. At the band edge, the Burstein-Moss shift for the heavily S-doped InP and a pronounced excitonic effect for the Fe-doped semi-insulating InP substrates are observed. In addition, the temperature dependence of the energy position and broadening parameter of the excitonic and direct-bandgap (E 0 ) transition features are also evaluated. The expression E 0 (T ) = 1.465 − 0.045{1 + 2/[exp(237/T (K)) − 1]} eV is proposed for the temperature dependence of the semi-insulating InP bandgap over the temperature range 20-300 K.

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