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Optical properties of GaN Photonic Crystal Membrane Nanocavities

Y.-S. Choi, C. Meier, R. Sharma, K. Hennessy, E. Haberer, Y. Gao, S. Nakamura, and E. L. Hu Departments of ECE and Materials, University of California, Santa Barbara, USA

cys@engineering.ucsb.edu

Abstract L7 photonic-crystal membrane nanocavities with Q factors of 600 have been realized based on the GaN/InGaN material system. Theoretically, a L7 nanocavity promises high Q factors as high as

4105× .

Photonic crystals (PC’s)[1] based on GaN[2] are promising structures for highly efficient nanophotonic devices in the short wavelengths from the ultraviolet to the green. However, the relatively low index of refraction ( 6.2≅n ) compared with those in GaAs and InP systems makes it difficult to achieve good optical confinement in submicron-scale PC devices. Recently, the bandgap selective photoelectro- chemical (PEC) etching technique, with the clever design of an InGaN sacrificial layer, has been successfully introduced to create free- standing structures that can confine photons[3-4] as well as electrons[5]. This has led to the realization of high-Q microdisk lasers[4], air-gap distributed Bragg reflectors[6], and two-dimensional PC membrane nano- cavities[7]. In this paper, we have investigated the L7 PC membrane nanocavity, which consists of seven missing holes in the K−Γ direction of the triangular-lattice PC structure. We have investigated the tuning behavior of the lowest energy mode in the L7 cavity by varying the radius of the air hole ( r ) and the slab thickness ( t ) using the three-dimensional finite-difference time-domain (FDTD) method. The FDTD space was aaa 31218 ×× , in which a is the lattice constant. Figure 1(a) shows the tuning behavior of the mode for different sets of ( r , t ). Assuming a lattice constant of 180 nm, the resonance wavelength was computed as the normalized frequency λ/a . The mode shifts to the blue by 30 nm with the increase of air hole radius from 30.0/ =ar to 37.0/ =ar while it shifts to the red by about 20nm with the increase of thickness from 50.0/ =at to 70.0/ =at . Next, the Q factor was examined. With the thickness fixed at 6.0/ =at , the air hole radius was varied from 30.0/ =ar to 37.0/ =ar . As shown in Figure 1(b), the Q factor is maximized

to about 4105× at 32.0/ =ar . For the different air hole sizes, Q factors of 410~ are maintained. The mode volume is about 1.27 × ( n/λ )3, a truly nano- scale dimension considering the emission wavelength (~ 480 nm) of the GaN/InGaN active material. (a) (b)

Figure1. (a) Tuning behavior of the lowest energy mode in the L7 cavity as a function of the air hole radius r and the membrane thickness t . The lattice constant is denoted by a . (b) The change of the Q factor with varying r . The thickness was fixed at t =0.6 a . The inset shows the electric field intensity distribution of the lowest energy mode.

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Experimentally, the starting material is a 2 mµ -thick GaN template grown on the [0001] sapphire substrate by metalorganic chemical vapor deposition. The structure incorporates an InGaN/InGaN superlattice sacrificial layer, and a GaN/InGaN active membrane layer. The sacrificial layer consists of 3 pairs of 20 nm In0.08GaN0.92 / 20 nm In0.04Ga0.96N superlattices. The membrane layer consists of a 44 nm unintentionally doped (uid) GaN top cladding, InGaN multiple quantum wells, a 14 nm uid GaN bottom cladding, and a 30 nm Al0.15Ga0.85N insulating layer. The total thickness of the membrane is about 120 nm. The center wavelength of the quantum-well emission was designed to be located from 460 nm to 510 nm. During the fabrication, we use electron-beam lithography to define the pattern in ZEP resist. The pattern is transferred to a SiO2 mask layer by reactive ion etching with CHF3 and then to the GaN/InGaN active membrane layer by reactive ion etching with Cl2. After the formation of a Pt contact around the PC pattern, we perform bandgap-selective PEC etching to undercut the sacrificial layer. Figure 2 (a) shows a typical image of the fabricated L7 nanocavities. The lattice constant is about 180 nm and the radius of the air hole is about 60 nm. Pits in the surface originate from the growth. We investigated the optical properties of the fabricated samples using room-temperature photoluminescence (PL) measurement. The excitation source was a continuous-wave HeCd laser with a peak wavelength of 325 nm. The excitation spot was about 3 mµ in diameter, and the incident power was about 5 mW. Figure 2(b) shows a PL spectrum of L7 nanocavity with a = 180 nm and r = 60 nm. The resonance mode at 488 nm exhibits a Q factor of about 600. These initial measurements are extremely promising, as there is considerable room for optimization of the smoothness of the etched membrane, the quality and dimensional control of the PC structure, and of the material itself. In summary, we have investigated the optical properties of GaN PC membrane nanocavities. Theoretically, a L7 cavity promises a Q factor of about 5 × 104, and a mode volume of about 1.27× ( n/λ )3. Experimentally, we observed Q factors ranging from 300 to 600. We believe that the GaN-based high-Q PC nanocavities will lead to the realization of low-threshold PC lasers, and single photon sources at the blue wavelengths.

(a) (b) Figure2. (a) Scanning electron microscope image of an L7 photonic-crystal membrane nanocavity with a = 180 nm and r = 60 nm. (b) Room- temperature photoluminescence spectrum. References 1) E. Yablonovitch, Phys. Rev. Lett. 58 2059 (1987). 2) Edited by S. Nakamura and S. F. Chichibu, Introduction to Nitride Semiconductor Blue Lasers and Light Emitting Diodes, Taylor & Francis, (2000). 3) E. D. Haberer, R. Sharma, C. Meier, A. R. Stonas, S. Nakamura, S. P. DenBaars, and E. L. Hu, Appl. Phys. Lett. 85 5179 (2004) 4) E. D. Haberer, R. Sharma, A. R. Stonas, S. Nakamura, S. P. DenBaars, and E. L. Hu, Appl. Phys. Lett. 85 762 (2004) 5) Y. Gao, I. Ben-Yaacov, U. K. Mishra, and E. L. Hu, J. Appl. Phys. Lett. 96, 6925 (2004). 6) R. Sharma, E. D. Haberer, C. Meier, E. L. Hu, and S. Nakamura, submitted to Appl. Phys. Lett. 7) C. Meier, K. Hennessy, E. D. Haberer, R. Sharma, Y. S. Choi, K. McGroddy, S. Nakamura, S. Keller, S. P. DenBaars, and E. L. Hu, to be published.

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