Radiation damage and surface modification of GaAs(0 0 1) by MeV C<SUP>+</SUP> and C<SUB>2</SUB><SUP>+</SUP> co-implantation with Ga<SUP>2+</SUP>

Radiation damage and Surface modifications of semi-insulating GaAs(0 0 1) substrates upon 1 MeV C<SUP>+</SUP> and 2 MeV C<SUB>2</SUB><SUP>+</SUP> coimplantation with Ga<SUP>2+</SUP> have been studied by X-ray reflectometry, combined Rutherford backscattering/channeling and transmission electron microscopy. Two disordered layers - one near-surface and another deeper - are formed. Additionally the electron density of the near-surface region has been found to be lower compared to bulk GaAs. This has been attributed to vacancy-enrichment in this region. The disorder caused by C<SUB>2</SUB> implantation is higher compared to C. Assuming that the near-surface disorder is partially caused by electronic excitation in the semi-insulating substrate, a possible reason for this difference is the coherent dynamic response of the electrons in the target due to vicinage of C atoms in the C<SUB>2</SUB> cluster. In the deeper layer, overlap of damage cascades might be responsible for the higher damage caused in the cluster implantation