Low- and high-energy irradiations of InGaAs/GaAs heterostructures: a comparison

Abstract

The effects of ion irradiation on InGaAs/GaAs heterostructures have been studied using Raman spectroscopy. The structures consist of molecular beam epitaxy grown InGaAs layers on GaAs(0 0 1), having layer thicknesses of 12, 36, 60 and 96 nm. For irradiation, 3 MeV Ag2+ and 150 MeV Ag12+ have been used at 1×1015 and 1×1013 ions/cm2 fluences, respectively. We refer to 3 and 150 MeV irradiations as low- and high-energy irradiations, respectively. The low-energy irradiation damaged the lattice heavily and no phonon modes were observed in the as-irradiated samples. The samples were then subjected to rapid thermal annealing (RTA) to recover the phonon modes. Though the phonon modes were observed after RTA, the GaAs-type TO mode is more intense than the LO mode indicating residual defects. The TO mode position is found to be the same in all the annealed samples, whereas the LO phonon mode positions do not follow any trend in the low-energy irradiated samples. In contrast, no post-annealing was performed in high-energy irradiated samples. An intense GaAs-type LO mode was observed comparable with the as-grown ones, indicating no loss of crystalline quality. The TO mode intensity was found to decrease after irradiation. After high-energy irradiation, the GaAs-type longitudinal optical (LO) mode blueshifted in thin samples and redshifted in thick samples when compared with as-grown samples. Raman spectra of thin samples indicate a compressive strain developing in the substrate, close to the interface upon irradiation. This along with inter-diffusion is expected to be the reason for the observed modifications. The variations in FWHM of the Raman modes are discussed in detail. The results are discussed invoking the penetration depth of the probe radiation (λ=514.5 nm) in InGaAs. We have demonstrated that the high-energy irradiation is useful to modify the material property without the loss of crystalline quality and does not need post-annealing. Atomic force microscopy has also been employed for surface analysis.