Time- resolved photoluminescence studies of Al0.72Ga0.28N films with incommensurate chemical ordering

摘要

An important topic in the understanding of AlGaN alloys for deep ultraviolet emitters is the role of carrier localization on radiative lifetime and emission efficiency. Electron and hole wavefunction overlap, and therefore radiative lifetime and recombination efficiency, can be strongly influenced by the degree and nature of chemical ordering in the AlGaN alloy, with spatially indirect, type II-like transitions predicted to occur between electrons in ordered alloy regions and holes in random alloy regions for Al contents in the 40% to 90% range [1]. In this paper, we study the photoluminescence (PL) lifetimes in AlGaN alloys with incommensurate chemical ordering. AlGaN films were grown by RF plasma-assisted MBE under different kinetic conditions. The compositions in all films were maintained at a constant value of 72 % AlN mole fraction by using the same Al flux, while the kinetics of growth were controlled by varying the Ga flux and thus the III / V flux ratio during deposition of the various films, with some of the films grown under Ga-rich conditions and others under N-rich conditions. The structure of these films was previously investigated by XRD using synchrotron radiation and by SED-TEM [2]. These spectra show that the characteristic (0002) diffraction is identical in all samples, from which the composition of the films was inferred. However, in addition to the (0002) diffractions all samples show additional superlattice peaks indicative of atomic ordering. Specifically, the analysis of the diffraction data indicates that the films grown under N-rich conditions have a superlattice structure consistent with 4 MLs periodicity, while those grown under III /V ~1 or III /V > 1 (Ga-rich conditions) have a superlattice structure with a repeat distance that has no clear relationship with the crystal lattice (incommensurate ordering). This type of chemical ordering was attributed to a change of the growth mode from vapor phase to liquid phase epitaxy a-ns the Ga flux increases [3]. The latter growth mode leads to compositional inhomogeneities and thus potential fluctuations in the band structure of the films.