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(Color online) (a) PL spectrum of nitrogen delta-doped GaAs at 5 K. The luminescence band, ranging from 1480 meV to 1510 meV, is due to the nitrogen luminescence centers. The PLE spectrum detected at 1488 meV, marked by an arrow, has a peak at 1512 meV, in addition to exciton absorption Eex. Inset: PL image taken with a 10-nm (18 meV) band-pass filter for the wavelength of 830 nm (1493 meV). (b) Corresponding spatial intensity profile on the CCD. Localized and delocalized states can be clearly distinguished by the presence of the spots. (c) Examples of μ-PL spectra. Vertical dashed lines represent the NN pair energies of NN F in Ref. 18 and the “1.4925 eV line” in Ref. 9.
(Color online) (a) Polarization resolved PL spectrum of a single center. The PL spectrum is strongly polarized along the  crystal axis and has only a single peak. Inset: Polar plot of the peak intensity. The angle dependence has been fitted by a cos2θ + b for a:b = 1:0.14, where θ is the angle between the polarization direction and  crystal axis. (b) PL spectrum of a single center below and above the saturation excitation power. The spectrum still has a single peak in the saturation regime. Inset: PL intensity vs. excitation power.
(Color online) (a) PL decay of a single center that has the shortest decay time, 650 ps. Also shown, in red, is the convoluted fitting curve. (b) PL decay rate vs. PL intensity in the saturation regime under cw excitation. (c) Distribution of coincidence counts for a single impurity center in nitrogen delta-doped GaAs.
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