Layer disordering and aluminum-gallium interchange in aluminum-gallium arsenide - gallium-arsenide quantum well heterostructures

Abstract

In the experiments described here, Al$\sb{\rm x}$Ga$\sb{\rm 1-x}$As-GaAs superlattice and quantum well heterostructure (QWH) crystals have been used as test vehicles to study Al-Ga interdiffusion. The data demonstrate that Al-Ga interchange is strongly influenced by the interdependence of the crystal surface-ambient interaction and the Fermi-level effect. We have investigated the crystal surface-ambient interaction by varying both the surface encapsulation condition (e.g., SiO$\sb2$-cap, Si$\sb3$N$\sb4$-cap) and the anneal ambient (As-rich, Ga-rich). The Fermi-level effect has been examined for QWH crystals doped with either donor or acceptor impurities during crystal growth and annealed, and for crystals converted to n-type conductivity by high-temperature Si diffusion or by Si$\sp+$ ion implantation and annealing.

The data show that Al-Ga interchange is enhanced for n-type samples annealed under As-rich conditions, and for p-type samples annealed under Ga-rich conditions. These trends suggest that acceptor native defects (V$\sb{\rm III}$) and donor native defects (I$\sb{\rm III}$, V$\sb{\rm As}$) are responsible for Al-Ga interdiffusion in n-type and p-type samples, respectively. By varying the anneal As$\sb4$ over-pressure we have demonstrated that the degree of Al-Ga interchange does not increase monotonically for n-type samples as expected for a simple Column III vacancy controlled process. In addition, we show that the activation energy for Al-Ga interdiffusion (E$\sb{\rm Al-Ga}$) is reduced by $\sb\sim$2 eV for n-type samples as compared to nominally undoped samples. These results indicate that E$\sb{\rm Al-Ga}$ can be used to label the various Al-Ga interdiffusion regimes and, thereby, provide for more accurate identification of the native defect species involved in the interchange process. Furthermore, by employing three single-well QWH crystals that differ only in the location of the QW relative to the crystal surface, we demonstrate that the Al-Ga interchange mechanism is depth-dependent because of the re-equilibration of native defect concentrations at the crystal free surface. Finally, we report on Si$\sp+$ ion implantation experiments that demonstrate enhanced Si$\sp+$-IILD for very low implant doses, hence minimizing the effects of implant damage.