Evidence for selective delocalization of N-pair states in dilute GaAs 1¿x N x

We report high-pressure photoluminescence (PL) experiments (to $P=62\mathrm{kbar}$ at 9 K) on ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}/\mathrm{GaAs}$ quantum wells (QWs) having N compositions $(x=0.0025,0.004)$ in the dilute regime where the ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ 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 ${\mathrm{NN}}_{3}$ replica) appear strongly in the $x=0.0025$ sample at energies below the QW absorption edge for $Pg~29\mathrm{kbar},$ but are completely absent in the $x=0.004$ sample\char22{}an effect that has not been seen previously in ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ 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 ${\mathrm{NN}}_{3}$ 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 $0.0025l~xl~0.004,$ while bound states and other resonant states closer to the CB edge remain unaffected, offers an important test for band-structure calculations in ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ 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 ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ dilute alloys that use a full-hybridization approach to treat the incorporation of N-pair (cluster) states.