A local vibrational mode investigation of p -type Si-doped GaAs

Infrared absorption (IR) and Raman scattering measurements have been made of the localized vibration modes (LVM) due to defects incorporating silicon impurities in p-type Si-doped GaAs grown by liquid phase epitaxy (LPE) on (001) planes and by molecular beam epitaxy (MBE) on (111)A and (311)A planes. Analysis of a closely compensated LPE sample indicated that an existing calibration factor for the SiAs LVM (399 cm−1) relating the integrated absorption coefficient (IA) to the concentration [SiAs] should be increased by 40%, so that IA=1 cm−2 corresponds to [SiAs]=7×1016 cm−3. The SiAs LVM appeared as a Fano dip in the hole absorption continuum at ∼395 cm−1 in the highly doped p-type material, some 4 cm−1 lower in frequency than its normal position in compensated GaAs. Electron irradiation of samples led to the progressive removal of the Fano dip and a shift with the emergence of the expected SiAs LVM absorption line at 399 cm−1. In MBE samples the irradiation also generated SiGa donors, but the site switching was not detected in LPE material. By contrast, Raman spectra of as-grown p-type samples exhibited a symmetrical peak at 395 cm−1, which also shifted towards 399 cm−1 as the free carriers were removed. MBE (111)A GaAs:Si compensated by SnGa donors revealed the SiAs LVM at its normal position. After hydrogenation of MBE and LPE samples, only stretch modes due to H-SiAs were observed. Passivated MBE GaAs (111)A codoped with Si and Be showed stretch modes due to both shallow acceptors. It was thereby concluded that only one type of acceptor (SiAs) was present in p-type Si-doped GaAs, contrary to previous proposals. There was no evidence for the presence of SiAs pairs or larger clusters.