DIRECT DETERMINATION OF IMPACT-PARAMETER-DEPENDENT STOPPING POWERS FOR MILLION-ELECTRON-VOLT HE IONS PENETRATING ER-DOPED GAAS

We have directly determined the impact-parameter-dependent stopping po wers for 2.0- and 2.5-MeV He ions passing through GaAs single crystals . The points reside in the preparation of the single-crystal sample wi th a dopant of a heavy element located in some definite interstitial s ite at definite depth and in synthesis of a Monte Carlo program to sim ulate accurately the ion trajectories. Er-doped homoepitaxial GaAs lay ers grown by molecular-beam epitaxy and by metal-organic chemical-vapo r deposition were used for this purpose, As previously reported, fine single-crystal clusters of ErAs are formed in the GaAs host and Er tak es the position exactly equivalent to the tetrahedral interstitial sit e. The present Monte Carlo simulation has revealed the fact that some definite impact-parameter region dominates the backscattering Er peak position and this region shifts continuously by tilting the incident b eam axis slightly from a major crystal axis. The results obtained are compared with the Oen-Robinson [Nucl. Instrum. Methods 132, 647 (1976) ] model and other theoretical predictions and dearly show that inner-s hell excitations and ionizations contribute significantly to the stopp ing power even for large impact parameters.