Local Ordering in GaN-Rich Ternary GaNP Alloys
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ABSTRACT Using micro-Raman scattering we have studied atomic distribution and the ordering effects in GaN-rich GaNP layers on (0001) sapphire substrates grown by electron cyclotron resonance molecular beam epitaxy (ECR-MBE). Raman spectrum from layers grown at higher temperatures (Ž_700 "C) shows coexistence of GaP-rich region with cubic symmetry and GaN-rich region with hexagonal symmetry. Sharp TO and LO phonon lines, indicative of {111 } ordering in the GaP region are observed. Increasing phosphorous (P >1.5%) in GaNP alloy leads to phase separation that is reflected in the suppression of GaN-like Raman modes. The phase-separated region shows an additional Raman line at 384 cm"1 between the TO and LO phonon of GaP due to strongly confined LO phonon in ordered {111 } (GaP)n(GaN)m nanometer size clusters. Decreasing growth temperature increases the phosphorous concentration in the GaNP layer and disorder activated acoustic modes appear in the Raman spectrum as a result of symmetry break down. When phosphorous concentration reaches 8.2 % in the layer grown at 570 "C, Raman spectrum shows broad amorphous like bands indicative of short-range ordering.
INTRODUCTION Recent progress in the epitaxial growth of alloys and heterostructures based on GaN semiconductor promises improvement in optoelectronic device performance. One of the features of these alloys is the occurrence of spontaneous long-range ordering under certain growth condition [1-3]. The band bowing parameter depends strongly upon the detailed atomic arrangement in the alloy lattice and it has been theoretically predicted that the bowing parameter of an ordered alloy is considerably larger than that for a random alloy [4]. Ternary alloy GaNP is attractive as it offers strong ordering effects as well as anomalously large band bowing parameter. However, presence of a wide miscibility gap in GaNP alloy, resulting from the differences in the lattice constants and lattice structure, poses a major problem in the growth of high quality epitaxial layers. Phase separation in GaNl.xPx occurs by spinodal decomposition during epitaxial growth at 750'C when phosphorous concentration reaches x > 0.015 [5-8]. Decomposition at the spinodal may take place in the form of clustering, phase separation, and ordering. The phWse separation takes place asymmetrically into a GaN-rich hexagonal GaNi.xP• phase and an ordered cubic GaP-rich GaPl.xNx phase. The nitrogen concentration in the ordered phase is x=0.0006. The photoluminescence (PL) emission line from the phase separated alloy shows significantly large red shift compared to nonphase separated GCaNP [7]. Further, the scanning tunneling microscopy images reveal closely spaced bright clusters of 3 nm size while I-V measurements at the bright clusters
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Mat. Res. Soc. Symp. Proc. Vol. 618 © 2000 Materials Research Society
show reduced onset voltage, and consequently reduction of band gap energy, compared to surrounding regions [7]. Low temperature PL spectrum from phase separated GaN-rich GaNP layers grown on sapphire subs
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