Evidence for selective delocalization of N-pair states in dilute GaAs_1-xN_x

We report high-pressure photoluminescence (PL) experiments (to P=62kbar at 9 K) on GaAs_1-xN_x/GaAs quantum wells (QWs) having N compositions (x=0.0025,0.004) in the dilute regime where the GaAs_1-xN_x alloy conduction band (CB) evolves rapidly via delocalization of N-pair (cluster) states. Under increasing applied pressure, we observe low-energy broadening of the emission spectra, an increase in the Stokes shift of PL peaks relative to the QW absorption edge, and several new N-pair PL features that derive from CB-resonant states at 1 atm. Two of the latter features (assigned to NN₃ replica) appear strongly in the x=0.0025 sample at energies below the QW absorption edge for P>~29kbar, but are completely absent in the x=0.004 sample—an effect that has not been seen previously in GaAs_1-xN_x alloys to our best knowledge. The trends for broadening and increase in Stokes shift under pressure are accounted for using a model of the recombination kinetics that considers competing fluctuation and N-pair states. The absence of the NN₃ features in the x=0.004 sample provides evidence that N-pair states incorporate into the CB continuum via an energy- and/or state-selective delocalization process. The observed selectivity in the narrow composition range 0.0025<~x<~0.004, while bound states and other resonant states closer to the CB edge remain unaffected, offers an important test for band-structure calculations in GaAs_1-xN_x dilute alloys. Selective delocalization of resonant N-pair states is difficult to explain within an impurity-band model, but it is qualitatively consistent with recent theoretical studies of CB formation in GaAs_1-xN_x dilute alloys that use a full-hybridization approach to treat the incorporation of N-pair (cluster) states.