Nonlinear strain effects in ion-implanted GaAs

Abstract

The nonlinear production of strain in (100) GaAs by room-temperature ion implantation has been studied. Ions of Ne, Si, and Te were used, with energies of 300, 300, and 500 keV, respectively. Doses ranged up to those required for amorphization. Strains were monitored by x-ray double-crystal diffractometry. Rocking curves were recorded about the (400) Bragg condition and detailed depth profiles of strain perpendicular to the sample surface, ε⊥(x), found by fitting the rocking curves with a kinematic model. These were compared with calculated profiles of the density of energy deposited in nuclear interactions, ρE(x). Rocking curves were also recorded about the (422) Bragg condition for selected samples, to monitor strain in the direction parallel with their surfaces. At low doses, ε⊥(x) is a linear function of ρE(x). At doses sufficient to create strains exceeding about 0.3%, strong nonlinearities are evident and strain profiles depart significantly from the ρE(x) curves. For the Ne and Si implantations, the profiles tend to saturate at 0.4%–0.5% over a depth of ∼4000 Å. At higher doses a narrow (∼2000 Å), sharply peaked region develops, with strains up to 1.5%. At still higher doses this region becomes amorphous. The Te-implanted samples do not experience appreciable saturation; rather a sharply peaked profile develops, and grows with dose to amorphicity. Curves of ε⊥ vs ρE were extracted by comparison of ε⊥(x) and ρE(x) profiles. These demonstrated that for each ion species ε⊥ is a unique function of ρE at all depths. Although this function has the same general form for all three implantations, the curves differ from species to species. Above ε⊥=0.3%, ε⊥ increases sublinearly with ρE for all three implanted ions. For Ne and Si, ε⊥ becomes almost constant at 0.4%, beginning at ρE∼0.15 eV/Å3. The strain ε⊥ starts increasing again with ρE at about 0.7 eV/Å3 for Ne and 0.3 eV/Å3 for Si, until the GaAs goes amorphous. The curve for Te shows only a slight inflection at ε⊥∼0.3%, continuing to increase with ρE to amorphicity. Parallel strains in the Si-implanted samples were not more than 0.02% at all values of ρE.