Phase separation and superlattice formation by spontaneous vertical composition modulation in GaAs _{1− x} N _x /GaAs

Abstract

A phase separation with a periodic spontaneous vertical composition modulation in GaAs 0.97 N 0.03 /GaAs epilayers is observed by transmission electron microscopy. At growth temperatures of 530 -550 °C a spatial period of 10 -15 nm is found. Unstable growth near the solubility limit is expected to occur at topographical defects such as surface steps which may induce a strong local variation in the N adatoms concentration. Consequently the solid content is affected.1 Introduction Growth of homogeneous phase for InGaAsN and GaNAs alloys is technologically important as both compounds are key materials for future optical devices from 1 eV to 3 eV energy range [1]. The N-solubility in the GaAs crystal lattice, resp. As in GaN, is low due to the 60% size mismatch between As and N atomic radii. Both internal strain and growth conditions do affect the phase diagram of group-III nitrides. Spontaneous superstructures have been seen previously in epitaxial films including lateral and vertical composition modulation (VCM) in InGaN, AlGaN leading to a two phase system [2 -4]. We show here some experimental results on phase separation in low nitrogen content GaAsN/GaAs epitaxial layers where a spontaneous superlattice is formed by vertical composition modulation in the N concentration. The growth temperature and the growth rate were varied. We first address the question of solubility. Different mechanisms leading to the VCM are discussed. Added to a limited solubility, it appears that the substrate misorientation, high vapor phase supersaturation and high desorption rate of N are key parameters.