Photoluminescence polarization properties of single GaN nanowires containing ${\text{Al}}_x{\text{Ga}}_{1-x}\text{N}/\text{GaN}$ quantum discs

The polarization anisotropy of single GaN nanowires containing ${\text{Al}}_x{\text{Ga}}_{1-x}\text{N}/\text{GaN}$ multiquantum disc (MQDisc) structures is characterized by polarization-resolved microphotoluminescence $\left(\mu \text{PL}\right)$. Single nanowires exhibit at $T=4.2\text{K}$ two main luminescence contributions: one is peaked at $E=3.45–3.48\text{eV}$ related to near-band-edge GaN bulk excitonic transitions and is polarized parallel to the nanowire axis ($\mathbf{\pi }$ polarization) at moderate excitation-power density; the other, lying at higher energy, is related to excitonic transitions confined in the MQDisc and is polarized perpendicularly to the nanowire axis ($\mathbf{\sigma }$ polarization). The results are interpreted in terms of the selection rules for excitonic transitions in wurtzite semiconductor crystals and of the polarization anisotropy arising from the elongated nanowire shape. Finally, the analysis of photoluminescence at $T=300\text{K}$ shows that the thermal population of light-hole states in the MQDisc produces a blueshift of the PL peak when polarization is rotated from $\mathbf{\sigma }$ to $\mathbf{\pi }$.

Figure 1

(Color online) (a) Schematic representation of the analyzed nanowire samples A1, A2, and B. GaN and AlGaN regions are depicted in dark and light gray, respectively. (b) TEM image of one nanowire from sample A1, showing the multiquantum-disc region. (c) High-resolution TEM image of a nanowire from sample A2, showing the detail of atomic planes in the multiquantum-disc region.

Figure 2

(Color online) PL spectra at $\mathbf{\pi }$ (blue dotted lines) and $\mathbf{\sigma }$ polarization (red solid lines) collected from different individual nanowires belonging to the three samples described in Table . First row refers to sample A1, second row refers to sample A2, and third row to sample B. All spectra were collected at temperature $T=4.2\text{K}$.

Figure 3

(Color online) (a) Micro-PL $\left(T=4.2\text{K}\right)$ spectra from a single nanowire from sample A2 taken in $\mathbf{\pi }$ (blue dotted line) and in $\mathbf{\sigma }$ polarization (red solid line). (b) Normalized polar diagram illustrating the variation in the peak amplitudes according to the polarizer angle.

Figure 4

(Color online) Statistics of the polarization ratio $P^{\text{e}}$ for the different nanowire samples. Red-patterned and blue columns refer to the polarization degree of MQD and NBE luminescences, respectively. The dotted lines indicate the average values for the polarization degree of MQD and NBE for each sample. First plot refers to sample A1, second plot to sample A2, and third plot to sample B.

Figure 5

(Color online) (a) $\mathbf{\pi }$-polarized (blue dotted line) and $\mathbf{\sigma }$-polarized (red solid line) micro-PL spectra $\left(T=4.2\text{K}\right)$ from a single nanowire from sample A2 at different excitation powers. The NBE peak is expanded and vertically shifted for clarity. (b) The dependence of the polarization ratios of NBE and MQDisc emissions on the excitation power.

Figure 6

(Color online) (a) Room temperature micro-PL spectra from a nanowire bundle from sample A2 taken in $\mathbf{\pi }$ (blue dotted line) and in $\mathbf{\sigma }$ polarization (red solid line). (b) Normalized polar diagram illustrating the variation in the peak amplitudes according to the polarizer angle.