Enhanced Red Emission of Eu,O-Codoped $\mathrm{Ga}\mathrm{N}$ Embedded in a Photonic Crystal Nanocavity with Hexagonal Air Holes

In this work, we fabricate a nitride-based photonic crystal (PhC) nanocavity with an embedded active layer for unexplored red regions using simplified selective wet etching of an ${\mathrm{Al}}_{0.82}{\mathrm{In}}_{0.18}\mathrm{N}$ sacrificial layer. Eu,O-codoped $\mathrm{Ga}\mathrm{N}$ ($\mathrm{Ga}\mathrm{N}$:$\mathrm{Eu}$,$\mathrm{O}$) is embedded in L7-type two-dimensional-PhC (2D-PhC) nanocavities with hexagonal holes. Room-temperature photoluminescence (PL) shows $\mathrm{Eu}$ emission coupled to the cavity modes at wavelengths around 620 nm. The quality factor of the fundamental mode in an L7 nanocavity is 5400, which is comparable to those of state-of-the-art 2D-PhC nanocavities with embedded nitride-based active layers for the blue and ultraviolet regions. We show that the Purcell effect due to the small mode volumes of the 2D-PhC nanocavities and enhanced light extraction lead to a 54-fold increase in the integrated PL intensity per unit area with respect to the typical $\mathrm{Ga}\mathrm{N}$:$\mathrm{Eu}$,$\mathrm{O}$ emission.

Figure 1

(a) Schematic image of the structural design of a $\mathrm{Ga}\mathrm{N}$:$\mathrm{Eu}$,$\mathrm{O}$-based 2D PhC with an L7-type nanocavity comprising a triangular lattice pattern of air holes; ud-GaN, undoped $\mathrm{Ga}\mathrm{N}$. (b) Photonic band structure of a $\mathrm{Ga}\mathrm{N}$ 2D-PhC slab with a triangular lattice pattern of hexagonal air holes with $r/a=0.29$ for TE polarization.

Figure 2

Results of FDTD simulations of the designed L7-type 2D-PhC nanocavity with $r/a=0.32$ assuming TE-mode luminescence. (a) First two cavity modes in the L7 cavity, where the fundamental mode ($M1$) and second mode ($M2$) have $Q$-factors of approximately 28 000 and approximately 1800, respectively. (b,c) Calculated distributions of the electric field intensity for (b) $M1$ mode and (c) $M2$ mode.

Figure 3

(a) SEM image of a fabricated L7-type PhC nanocavity with hexagonal air holes. The periodicity and hole radius are 212.5 and 82 nm, respectively. (b) Relative standard deviation of the hexagonal-hole radii as a function of the average radius.

Figure 4

Room-temperature CL intensity maps of L7 nanocavities with $r/a$ values of (a) 0.27 and (b) 0.15 acquired at a wavelength of 622 nm. The area enclosed by broken lines indicates the PhC patterned area.

Figure 5

(a) PL spectra of L7-type 2D-PhC nanocavities with $r/a$ values of 0.27 and 0.32 under $80.8{\mathrm{mW/cm}}^2$ excitation. The black dotted curve is a 2.1-times-enlarged PL spectrum of a nonpatterned $\mathrm{Ga}\mathrm{N}$:$\mathrm{Eu}$,$\mathrm{O}$ film area. (b) PL intensity ratio ($I_{\mathrm{PL}}/I_{\mathrm{film}}$) obtained from an L7 nanocavity with $r/a=0.32$ under $16.2{\mathrm{mW/cm}}^2$ excitation. The solid lines are fitted Lorentzian curves for the two cavity peaks, and the estimated $Q$-factors are 5400 for the $M1$ mode and 1800 for the $M2$ mode.

Figure 6

Excitation-power dependence of the wavelength-integrated PL intensities for a nanocavity with $r/a=0.32$. (a) Off-resonant PL intensity of PhC ($I_{\mathrm{cav}}^{\mathrm{off}}+I_{\mathrm{Ph}\mathrm{C}}^{\mathrm{out}}$) and that of a nonpatterned $\mathrm{Ga}\mathrm{N}$:$\mathrm{Eu}$,$\mathrm{O}$ film ($I_{\mathrm{film}}$). (b) Resonant PL intensity of the $M1$ mode ($I_{\mathrm{cav}}^{M1}$).

Figure 7

(a) Enhancement of integrated PL intensity per unit area, expressed as $E_{\mathrm{cav}}^{M1}$, $E_{\mathrm{cav}}^{M2}$, and $E_{\mathrm{Ph}\mathrm{C}}^{\mathrm{off}}$ in Eqs. (4) and (5), as functions of the excitation power density. (b) Resonant-wavelength shift of the $M1$ mode in an L7 cavity with $r/a=0.32$ as a function of the excitation power density.

Figure 8

Enlarged PL spectrum of nonpatterned $\mathrm{Ga}\mathrm{N}$:$\mathrm{Eu}$,$\mathrm{O}$ film area.

Figure 9

Excitation-power dependence of the wavelength-integrated PL intensities for two different cavity modes and their sum.

Figure 10

Excitation-power dependence of the integrated PL intensity for the $M1$ mode in an L7 cavity with $r/a=0.32$ under cw and pulsed excitation conditions.

Figure 11

Resonant-wavelength shifts of the $M1$ mode in an L7 cavity with $r/a=0.32$ as a function of the excitation power density under cw and pulsed excitation conditions.