MOCVD Ge-on-GaAs and Its p-Type Doping via Incorporating Ga Atoms

Abstract

Ge and Ga-doped Ge thin films have been epitaxially grown on GaAs (001) substrates by metalorganic chemical vapor deposition (MOCVD). Structural studies revealed that the temperature of the substrate played an important role in controlling the Ga-incorporations as well as affecting the surface morphologies. Dot, dot-in-hole, and hole structures have been observed by changing the growth temperatures under, otherwise, the same MOCVD conditions for p-type Ge. These surface features are associated with segregation, migration, and coalescence of Ga-rich droplets on the growing surface, especially when Ga-doping is introduced. Mode softening of the longitudinal optical (LO) phonon of Ge has also been observed. This mode softening is dominated by the effect of free-carrier concentrations more than the effect of doping-induced crystal disordering and lattice strain. We have also observed atomic interdiffusions across the Ge/GaAs interface by employing spatially resolved Raman scattering and high-resolution X-ray diffraction (HRXRD), measuring from the cross-section of the Ge/GaAs heterostructure. Such atoms intermixing cause great challenges in controlling the doping concentrations in the Ge epilayer, they could also give rise to an intermediate layer in between the Ge epilayer and the GaAs substrate. Detailed morphological and structural properties of these Ge-on-GaAs heterostructures have been discussed in this paper, which could have important consequence when optimizing the Ge-on-GaAs growth by MOCVD.