Photoacoustic spectra of Si wafers at liquid helium temperature

Photoacoustic (PA) measurements of p- and n-type Si single crystals are carried out at liquid helium temperature (4.2 K). Two small peaks around 1.2 eV are observed in both samples. The energy of the higher-energy peak at 1.21 eV agrees with the thresholds of free exciton absorption with the emission of TO momentum-conserving phonons. The lower-energy peak shifts to the higher energy side with the decrease of the excitation light intensity. The possibility of the formation of an electron-hole drop for the lower energy peak will be discussed to explain the experimental results.