Electrical and Structural Properties of Heavily Ge-Doped GaAs Grown by Molecular-Beam Epitaxy

Abstract

The properties of GaAs highly doped with Ge grown by molecular-beam epitaxy (MBE) and migration-enhanced epitaxy (MEE) have been studied. We report here that, although Ge doped GaAs films grown by conventional MBE at temperatures lower than 450 °C tend to become semi-insulating materials, a conductive Ge-doped GaAs film with an electron concentration as high as 2×1019 cm-3 can be grown even at 300 °C by MEE. In MBE grown samples, a transition from n-type conductivity to p-type conductivity is observed when the Ge concentration of the films exceeds 1×1020 cm-3, whereas in MEE grown layers at 300 °C only n-type conductivity is observed regardless of the Ge concentration. The lattice constants of these MEE-grown samples are much larger than those of MBE-grown samples with equivalent Ge concentrations. These results indicate that Ge incorporation mechanism is quite different in MEE at low temperatures from that at high temperatures. The large lattice constant of as-grown MEE-grown layers suggests that a substantial fraction of Ge atoms occupy interstitial sites and act as donors. Postgrowth annealing results in marked reductions in lattice constant and electron concentration in MEE samples. The resulting lattice constants are even smaller than those of the n- and p-type GaAs films grown by MBE at high temperatures. This observed result can be explained in terms of (GaAs)1-x(Ge2)x alloy formation in MEE samples during annealing.