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We report on the observation of biexciton luminescence from single isoelectronic traps formed by nitrogen–nitrogen pairs in nitrogen δ-doped GaAs. The biexciton luminescence intensity showed a quadratic dependence on the excitation power while the exciton luminescence intensity increased linearly with increasing excitation power. The biexciton binding energy was found to be 8 meV, which is considerably larger than that reported for single InAs quantum dots in GaAs. We have also found that both the biexciton and exciton emission lines show completely unpolarized and no fine-structure splitting. This is suitable for the application to polarization-entangled photon pairs.

We have studied the properties of photoluminescence (PL) from individual isoelectronic traps formed by nitrogen-nitrogen (NN) pairs in nitrogen atomic-layer doped (ALD) GaAs. Micro-PL measurements were performed to investigate the properties of single photons generated from individual isoelectronic traps. Twin PL peaks were observed from individual isoelectronic traps in nitrogen ALD GaAs (001). The PL transitions at longer and shorter wavelength sides were linearly polarized in the [110] and [1-10] directions, respectively. The peak splitting and polarization properties can be explained by some in-plane anisotropy most likely due to strain in host crystal. From individual isoelectronic traps in nitrogen ALD GaAs (111), a single PL peak with random polarization was observed, showing that the growth on (111) surface is an effective way to obtain unpolarized single photons. As for nitrogen ALD GaAs (110), different polarization properties were obtained depending on the atomic configuration of NN pairs. In addition, we have used AlGaAs layers to diminish the in-plane anisotropy and could successfully obtained single emission lines with unpolarized character. Introducing AlGaAs layers was also useful for improving the luminescence efficiency.

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