Excitons bound to nitrogen pairs in GaAs as seen by photoluminescence of high spectral and spatial resolution

High resolution photoluminescence (PL) spectroscopy was performed on high quality bulk GaAs, lightly doped with the nitrogen isoelectronic impurity. The shallowest nitrogen pair bound exciton center labeled as ${\mathrm{X}}_1$ revealed a total of six transitions. The photoluminescence lines from a small ensemble of nitrogen centers showed polarization dependent intensity. High spectral resolution PL spectroscopy was combined with confocal spectroscopy experiments performed on a $\mathrm{Ga}\mathrm{As}:\mathrm{N}∕\mathrm{Al}\mathrm{Ga}\mathrm{As}$ heterostructure. The high spatial resolution achieved by this technique enables us to localize and examine individual nitrogen bound excitons. Similar spectral structure and polarization dependence was observed for individual N-pair centers in GaAs. Both techniques support the $C_{2v}$ symmetry of such isoelectronic impurity centers. The comparison between the PL spectra from an ensemble of nitrogen pairs and individual centers demonstrate the ability of the single impurity technique to lift the orientational degeneracy.

Figure 1

Photoluminescence spectra of the GaAs and GaAs:N samples at $4.2\mathrm{K}$ showing the excitonic region. ${\mathrm{A}}^0\mathrm{X}$ and ${\mathrm{D}}^0\mathrm{X}$ label the neutral acceptor and donor bound excitons, respectively. The lower spectra obtained from the GaAs doped with nitrogen show the first transition of excitons bound to N pairs labeled in the literature as ${\mathrm{X}}_1$.

Figure 2

Ensemble photoluminescence spectra of the exciton bound to nitrogen pairs ${\mathrm{X}}_1$ at 4.2 and $1.8\mathrm{K}$. In (a), six transitions can be resolved, marked by the vertical black arrows. By decreasing the temperature from $4.2\text{to}1.8\mathrm{K}$, the higher energy transitions thermalize to the lower energy ones, indicating that all six transitions originate from the same center. This behavior can be better observed (b), where the excitation at $650\mathrm{nm}$ was provided by the dye laser. In order to improve the signal to noise ratio, the spectra in (b) were collected at a lower resolution; therefore, some of the spectral components have not been resolved.

Figure 3

(Color online) (a) Two-dimensional map of the polarization dependence of an ensemble of nitrogen pair bound excitons $\left({\mathrm{X}}_1\right)$ at $5\mathrm{K}$. The color PL intensity in arbitrary units increases from yellow (light gray) to blue (dark gray). The six vertical black arrows at 45°, 135° (four lines), and 45°, respectively, indicate the polarizations at which the transitions maximize. The number of the allowed transitions and their polarization behavior of the emitted light provide information about the symmetry of the center, i.e., the arrangement of the nitrogen atoms in the host lattice. (b) shows the spectra at the two crucial polarizations: 45° (black line) and 135° (red/gray line), with the vertical black arrows indicating the transitions. The inset in (b) shows the six allowed transitions corresponding to the $C_{2v}$ center symmetry.

Figure 4

(Color online) Two-dimensional photoluminescence map of the nitrogen pair bound exciton center at $5\mathrm{K}$. In a spacial area of $5\times 5\mu {\mathrm{m}}^2$, a single excitonic center has been identified. The polarization map of this center is shown in Fig. 5. The diffraction limited spatial resolution is $0.6\mu {\mathrm{m}}^2$, and the area was scanned in $0.2\mu \mathrm{m}$ steps. The color PL intensity in arbitrary units increases from yellow (light gray) to blue (dark gray).

Figure 5

(Color online) Two-dimensional map of the polarization dependence of a single nitrogen pair bound exciton $\left({\mathrm{X}}_1\right)$ photoluminescence transition at $5\mathrm{K}$. The center lies most likely in the plane parallel to the surface. The color PL intensity in arbitrary units increases from yellow (light gray) to blue (dark gray). The four vertical black arrows at 90°, 0°, 0°, and 90° indicate four polarizations at which the transitions maximize. The number of the allowed transitions and the polarization behavior of the emitted light are determined by the symmetry of the center, i.e., the arrangement of the nitrogen atoms in the host lattice.

Figure 6

(Color online) Two-dimensional map of the polarization dependence of a single nitrogen pair bound exciton $\left({\mathrm{X}}_1\right)$ photoluminescence transition at $5\mathrm{K}$. The center lies most likely in the plane perpendicular to the surface. The color PL intensity in arbitrary units increases from yellow (light gray) to blue (dark gray). The six vertical black arrows at 45°, $-45°$, 45°, $-45°$, 45°, and 45° indicate six polarizations at which the transitions maximize. The number of the allowed transitions and the polarization behavior of the emitted light are determined by the symmetry of the center, i.e., the arrangement of the nitrogen atoms in the host lattice.