Novel optical features in Cd⁺ ion-implanted LEC-grown GaAs

High-energy Cd⁺ ions were implanted into undoped GaAs grown by the liquid encapsulated Czochralski (LEC) method for a wide Cd concentration, [Cd] between 1 × 10¹⁶ and 3 × 10²¹ cm^-3. Raman scattering spectra indicate that for [Cd] lower than 1.7 × 10¹⁸ cm^-3, the damage induced by high-energy ion-implantation can be eliminated by high-temperature annealing. For [Cd] higher than 3 × 10¹⁸ cm^-3, the intensity of the LO-phonon mode gradually reduces and its width significantly broadens with increasing [Cd]. For [Cd] = 1 × 10²¹ and 3 × 10²¹cm^-3, a forbidden mode appears on the lower-frequency side of the LO-phonon mode. The results of Rutherford backscattering spectrometry (RBS), and Hall-effect measurements suggest that a solid solution of Cd in GaAs is formed in this high-concentration region. 2 K photoluminescence (PL) spectra indicate that four emissions related with acceptor-acceptor pairs, [g-g], [g-g]₂, [g-g]₃, and [g-g]_β are formed in the near band-edge region even by using LEC-GaAs as a substrate. In the energy region far below the band-edge, many emissions associated with vacancies and complexes with doped Cd atoms are produced specifically for high [Cd].