Abstract
In this study the energy and mass dependences of the carrier removal cross section for high energy (0.25-2.0 MeV) light ion (H[+], D[+] and He[+]) irradiation of n-type GaAs are investigated. The materials used were of two types: (i) semi-insulating bulk GaAs and (ii) Vapour Phase Epitaxially (VPE) grown n-type GaAs on Semi-insulating substrate. The bulk material was implanted with Se[+], Zn[+] and Cd[+] ions at 390-400 keV and annealed (700° and 900°C) to create a thin (0.5 um) n- and p-type conductive layers, with carrier concentrations of 1 to 7x10[17] cm[-3]. The carrier concentrations for the Epitaxial materials were between 2.1x10[15] to 3.2x10[17] cm[-3]. The irradiations were carried out using the Van de Graaff accelerators at the University of Surrey and at AERE, Harwell. The ion doses were between 1x10[10] to 5x10[13] cm[-2]. The materials were irradiated at room temperature and at 8° off the normal to the surface. The measurement techniques used were "insitu" sheet conductivity and also Hall effect measurements. From the rates of change of the sheet conductivity and carrier concentration with the ion dose, values for the carrier removal rate (CRR) and carrier removal cross section (sigma[CR]) were calculated. These cross sections were compared to the elastic displacement cross section derived using Kinchin and Pease [116] theoretical model (sigma[KP]). It was found that the energy dependence of the cross section agrees with the theoretical prediction, but the mass dependence, at energies below 500 keV, was found to diverge from the theoretical mass dependence. Molecular ions (H[2+] and H[3+]) were found to dissociate upon impact with the surface of the target, with the resulting particles behaving similarly to Protons. The carrier removal rate was found to depend on the initial carrier concentration of the irradiated material, which is attributed to the movement of the Fermi level through the shallowest defect level reported (E1=E[c]-0.12 eV). From this dependence, values for defect introduction rates (DIR) for this level and for other deeper levels were estimated . The comparison of the DIR for different ions, showed that Deuterons and Protons followed the isotopic mass dependence, where as Helium differed. The surface layers of some samples were doped with Deuterons and it was demonstrated that the presence of the Deuterons has little effect upon the carrier removal cross section during 1.5 MeV Proton irradiation.