Piezoelectric photoacoustic spectra of silicon at liquid helium temperature

Photoacoustic (PA) measurements of p- and n-Si single crystals are carried out at liquid helium temperature (4.2 K). Two peaks are observed around 1.2 eV in both samples. They vanish when the temperature increases to 20 K. The energies of the peaks agree with thresholds of exciton absorption with the emission of momentum-conserving phonons. However, the lower energy peak shifts to the higher energy side with the decrease of the intensity of the incident light. We also observe an annealing behavior of PA spectra of p-Si at 4.2 K. Fine structures of the PA spectra below the band gap were observed. This indicates that the PA measurements at very low temperature may be a useful tool to investigate the electronic transitions