We demonstrate two-dimensional photonic crystals of silicon carbide (SiC)—a wide bandgap semiconductor and one of the hardest materials—at near-infrared wavelengths. Although the refractive index of SiC is lower than that of a conventional semiconductor such as GaAs or Si, we show theoretically that a wide photonic bandgap, a broadband waveguide, and a high-quality nanocavity comparable to those of previous photonic crystals can be obtained in SiC photonic crystals. We also develop a process for fabricating SiC-based photonic crystals that experimentally show a photonic bandgap of 200 nm, a waveguide with a 40-nm bandwidth, and a nanocavity with a high quality factor of 4,500. This demonstration should stimulate further development of resilient and stable photonics at high power and high temperature analogous to SiC power electronics.
©2011 Optical Society of America
Two-dimensional (2-D) photonic crystals [1–10], which show arbitrary propagation and strong confinement of photons within a tiny space, can be used in various applications such as optical filters, switches, sensors, and cavity quantum electrodynamics. Given their mature fabrication processes and high refractive indices, mainly semiconductors such as silicon (Si) and gallium arsenide (GaAs) have been used to realize 2-D photonic crystals [6, 7] operated at telecommunication wavelengths. Recently, photonic crystals based on wide bandgap materials (GaN, GaP, ZnO, etc.) [8–10] have attracted interest because they provide not only the suppression of two-photon absorption at high input power, but also broadband operation including in the visible range. However, silicon carbide (SiC), which has a wide bandgap and is one of hardest materials, has not yet been employed in 2-D photonic crystals. SiC, which consists of 50% C atoms bonded covalently with 50% Si atoms, adopts various crystalline forms such as the 4H, 6H, and 3C structures; exhibits distinct and wide electronic bandgaps between 2.2 and 3.2 eV in magnitude; and possesses high mechanical strength, large thermal conductivity (kSiC = 490 Wm−1K−1), and a small thermo-optic coefficient (dnSiC/dT = 2.77 × 10−5 K−1) . By using the excellent physical properties of SiC in the field of photonic crystals, it is expected that nanophotonic devices resilient to high power and temperature will be realized, as in SiC power electronics . For example, because its thermal properties are superior to those of Si (kSi = 156 Wm−1K−1 and dnSi/dT = 1.86 × 10−4 K−1) , SiC should allow the realization of more stable photonic devices than Si during a local temperature fluctuation in reconfigurable optical systems. Although a few studies of SiC photonic crystals have been carried out very recently [14, 15], here we demonstrate SiC 2-D photonic crystals. First, we present designs for waveguides and nanocavities in SiC 2-D photonic crystals, and show that a broadband waveguide and high-quality cavity can be obtained in such crystals even if the refractive index of SiC (n = 2.5–2.7 ) is lower than that of Si or GaAs (n = 3.2–3.46). We then develop a process to fabricate SiC photonic crystals using SiC on an insulator (SiCOI) wafer. Finally, we experimentally assess the fundamental properties of the fabricated SiC photonic crystals at near infrared (NIR) wavelengths.
2. Design of SiC photonic crystals
First, we investigated the feasibility of SiC 2-D photonic crystals because the refractive index of SiC is lower than that of Si or GaAs. We calculated a photonic band diagram of the SiC 2-D photonic crystal slab structure using a three-dimensional (3-D) finite difference time domain (FDTD) method. A basic 2-D photonic crystal slab structure consists of a triangular pattern of air holes defined by the lattice constant (a), as shown in Fig. 1(a) . The radius of the air holes and the slab thickness were set to 0.29a and 0.6a, respectively. The refractive index of the SiC slab was assumed to be 2.5. The calculated band diagram for this structure is shown in Fig. 1(b). The dotted line is the so-called light line; modes above this line are leaky to air. It is apparent that a photonic bandgap (PBG) exists in the frequency region from 0.345 to 0.398 (c/a). The PBG corresponds to the NIR range of 1,380–1,590 nm when a = 550 nm. We next investigated a line-defect waveguide for delivering photons in the direction shown in Fig. 1(c). The calculated dispersion relation of this waveguide is shown in Fig. 1(d). As indicated by the solid-circle data points, the waveguide formed by a row of missing air holes (width W = = W1) has a bandwidth of 0.006 (c/a), which corresponds to 25 nm at the central wavelength of 1,550 nm. This is very narrow compared to the bandwidth in Si photonic crystal waveguides , because the SiC waveguide with a lower refractive index is significantly affected by the light-line effect. To obtain an SiC waveguide with a broader bandwidth, the dispersion of the waveguide can be controlled by adjusting the width of the waveguide. When W was set to 0.62W1, a broad bandwidth of 0.024 (c/a) was obtained. This corresponds to a bandwidth of 100 nm, which is comparable to that of previous photonic crystal waveguides [16, 17]. We also designed a point-defect cavity in the photonic crystal structure, as shown in
Figure 2(a) . The cavity consisted of three missing air holes (referred to as an L3 cavity) . Figure 2(b) shows the calculated electric field (Ey) distribution of the L3 cavity at the resonant frequency of 0.357 (c/a), which lies above the PBG region. It is apparent that the electric field is strongly confined in the cavity. The quality (Q) factor of the cavity was calculated to be 1,200 from the exponential decay of the electric field (or magnetic field) in the cavity mode. A modal volume of 0.54 (λ/n)3 was calculated according to the definition given in a previous report . In order to investigate the possibility of further increasing the Q factor of SiC nanocavities, we studied the optical characteristics of “air-hole shifted”  and “hetero-structured”  nanocavities, as shown in Figs. 2(c) and 2(d), respectively. The calculated Q factors of the nanocavities shown are 6,000 and 500,000. This implies that optical performance comparable to those of conventional photonic crystals can be achieved in SiC-based photonic structures.
3. Fabrication of SiC 2-D photonic crystals
The SiC phonic crystals were fabricated on thin SiC-on-insulator (SiCOI) wafers, which were specially prepared using the smart-cut technique . The wafers consisted of an SiC (6H crystalline structure) surface layer with a thickness of 180 nm above an SiO2 layer (680 nm), on top of an Si substrate (300 μm) that was used for handling. The fabrication process for SiC nanophotonic structures is summarized in Fig. 3 .
Aluminum (Al) layers with a thickness of 30 nm were evaporated onto the SiCOI wafers as hard masks (Fig. 3(b)). The photonic crystal patterns were first defined in the electron beam resist using an electron beam writer (Figs. 3(c) and 3(d)). The lattice constants of the photonic crystals were set to be 525–600 nm. The resist patterns were transferred to the aluminum mask using inductively coupled plasma (ICP) etching with Cl2 gas (Fig. 3(e)), after which the mask was transferred to the SiC layer using ICP etching under the following conditions: ICP/RF powers of 600 W/15 W, a temperature of 25°C, and an etch rate of 215 nm/min (Fig. 3(f)). Finally, the aluminum mask was removed using a hydrochloric acid solution (Fig. 3(g)). The SiO2 layer (680 nm) was also removed in order to form air-bridge structures using a hydrogen fluoride (HF) solution (Fig. 3(h)). According to our calculations, it was found that optical loss occurs to the Si substrate because of the short air gap (680 nm), as part (>620 nm) of the Si substrate is etched using an alkali solution. The final air gap is over 1.3 μm, which is sufficiently distant to ignore optical loss to the substrate. Scanning electron micrograph (SEM) images of a fabricated SiC 2-D photonic crystal before using the HF solution are shown in Fig. 4 . The air holes were etched uniformly and relatively vertically (~82°).
4. Measured results and discussion
In order to investigate the optical properties of the fabricated SiC photonic crystals, light from a continuous waveguide laser (λ = 1,270–1,620 nm) was coupled to the cleaved facet of individual samples. The transmission of the samples and emission from the cavities were observed (or measured) using an NIR camera (or photoreceiver). Figure 5 shows the measured spectra of the transmission of SiC photonic crystal samples without a waveguide or cavity. As seen in the figure, the transmission of the sample with a = 525 nm shows a dip around 1300 nm and increases in intensity for wavelengths beyond 1,300 nm. Furthermore, the region of the dip is shifted to longer wavelengths as the lattice constant increases. The dip region corresponds to the PBG because light cannot transmit through the photonic crystals, whereas the high-transmission region corresponds to the slab mode outside the PBG. A maximal PBG of 200 nm is obtained in the sample with a = 600 nm, which is in good agreement with the calculations considering the fabricated structure.
Next, we investigated the transmission characteristics of a waveguide (a = 575 nm), as shown in Fig. 6 . Comparing the spectrum with the PBG of Fig. 4 (a = 575 nm), we can observe a high transmission range from 1,360 to 1,400 nm within the PBG (1,270–1,410 nm) and another high transmission range from 1,430 to 1,620 nm corresponding to the slab mode region outside the PBG. The narrow bandwidth of the photonic crystal waveguide is mainly due to the thin SiC slab (t = 0.31a) according to our FDTD calculations. By using a thicker slab (t = 0.6a), it is expected that a wider bandwidth comparable to that of high-index Si photonic crystal waveguides [16, 17] may be realized.
Figure 7 shows the measured drop spectrum of the L3 cavity (a = 575 nm). A resonant wavelength of 1373 nm exists within the PBG region. The Q factor of the cavity is estimated as 550 by a Lorentzian fit to the spectrum. As seen in the near-field pattern of the resonance, light is confined to the cavity and emitted to free space. In order to increase the Q factor of SiC nanocavities further, we fabricated “air-hole shifted” and “hetero-structured” nanocavities. Figure 8(a) shows the measured Q factor of the air-hole shifted cavities (a = 525 nm). The maximal Q factor is obtained as ~2,000 when s = 0.2a. As seen in Fig. 8(b), the Q factor of a hetero-structured nanocavity is approximately 4,500. However, the experimental Q factor of the hetero-structured cavity is lower than that of the simulation (Q factor ~78,000), even when considering only the two factors of actual slab thickness (t = ~0.31a) and tilted air-hole angle (~82°). We are currently investigating the reasons for this discrepancy, which may be due to additional optical loss resulting from other imperfections in air-hole position or size, surface roughness, quality of SiC material, etc. If the quality of the SiC itself and the fabrication process of the photonic crystal are improved, we expect that ultrahigh Q factors comparable to state-of-the art photonic crystal nanocavities may be achieved in SiC photonic crystals that retain the excellent physical properties of SiC.
In summary, we developed SiC 2-D photonic crystals at NIR wavelengths. It was found theoretically that optical performance comparable to conventional semiconductor-based photonic crystals could be obtained in SiC photonic crystals. Furthermore, we fabricated SiC 2-D photonic crystals and experimentally demonstrated a PBG of 200 nm, a waveguide bandwidth of 40 nm, and nanocavities with high Q factors of 550–4,500 in the NIR range. These results should stimulate further development of resilient and stable photonics at high power and high temperature analogous to SiC power electronics.
This work was supported by the Japan Society for the Promotion of Science (JSPS) through its “Funding Program for World-Leading Innovation R&D on Science and Technology (FIRST Program),” a Grant-in-Aid from the MEXT Japan, and the WCU program (R32-2008-000-10204-0) of the National Research Foundation of Korea (NRF).
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