Fabrication of Photonic Band Gap Crystals

Abstract

The past decade has witnessed intensive research efforts related to the design and fabrication of photonic crystals. These periodically structured dielectric materials can represent the optical analogue of semiconductor crystals, and provide a novel platform for the realization of integrated photonics. Despite intensive efforts, inexpensive fabrication techniques for large-scale three-dimensional photonic crystals of high enough quality, with photonic bandgaps at near-infrared frequencies, and built-in functional elements for telecommunication applications, have been elusive. Direct laser writing by multiphoton polymerization of a photoresist has emerged as a technique for the rapid, cheap and flexible fabrication of nanostructures for photonics. In 1999, so-called layer-by-layer or woodpile photonic crystals were fabricated with a fundamental stop band at 3.9 μm wavelength. In 2002, a corresponding 1.9 μm was achieved, but the important face-centred-cubic (f.c.c.) symmetry was abandoned. Importantly, fundamental stop bands or photonic bandgaps at telecommunication wavelengths have not been demonstrated. In this letter, we report the fabrication—through direct laser writing—and detailed characterization of high-quality large-scale f.c.c. layer-by-layer structures, with fundamental stop bands ranging from 1.3 to 1.7 μm.

Abstract

The optical properties of two-dimensional photonic crystals (PhCs) in anodic aluminum oxide (AAO) films obtained using a simple and low-cost pre-patterning procedure are described. The prepatterning of the initial Al film surface was carried out by imprinting with an optical diffraction grating; the anodization of the prepatterned sample led to the formation of a good quality, large-area PhC with a triangular lattice of air holes (lattice period a=0.48 µm, hole radius r=0.2 µm) in an AAO film. The optical transmission spectra of the sample were measured at visible wavelengths in the range of 0.4–1.0 µm for various incidence angles and linear polarizations of the probing light. The detailed analysis of the transmission data indicates a photonic band gap in the 0.9–1.0 µm wavelength range for light waves linearly polarized in the direction perpendicular to the axes of PhC pores.

Methodology

Collection of Materials

Silicon, Germanium, Gallium, Arsenide and Indium Phosphide will be purchased from DKL Laboratory supplies or other Laboratories that are available with these materials.

Optimization of the Mask Fabrication Sequence

To optimize the fabrication of three-dimensional structures such as photonic crystals, it is extremely important to accurately characterize the quality and the depth of the structures during all of the steps in the processing sequence. In submicron patterns, this inspection is most conveniently accomplished by using a scanning electron microscope. Figures 3~a! and 3~b! are electron micrographs taken during the etch mask fabrication after ion etching of the gold mask and after reactive ion etching of the silicon dioxide mask, respectively. From these scanning electron microscopy (SEM) micrographs, we observe that the high resolution of the original electron beam resist pattern does not deteriorate significantly during the pattern transfer into the thick silicon dioxide layer.

Pattern Transfer by CAIBE

High etch rate selectivities are usually obtained when the semiconductor reactivity of the etch is increased. When etching large structures, this is accomplished either by increasing the surface temperature or the reactive gas pressure. However, isotropic etching and mask undercut results from both of these cases, hollowing out the structure underneath the etch mask. The extent of the undercut is revealed by cleaving the semiconductor wafer through the photonic crystal. Therefore, the etching system is optimized to maximize the surface etch rate without undercutting the mask, and it is important to hold constant the temperature and the reactive gas pressure during the CAIBE process. 11 Cleaved cross-sectioning also allows the determination of the relative etch depths of the holes which define the three ^110& directions. Etching of the first etched hole removes more than 50% of the semiconductor material, and thus the second and third holes require correspondingly less etching time to define a crystal with a given depth. Careful calibration of the etching times of all three holes is required in order to obtain a uniform photonic crystal with a given depth.

Process Reliability and Limitations

When the CAIBE process is optimized, we can obtain high quality crystals. Here, we show photonic band-gap structures which have been defined three layers deep into a GaAs crystal, by etching a pattern of 400 nm holes spaced 520 nm from center to center. This crystal was defined in a microfabricated aperture for optical measurement, and subsequently cleaved to reveal the three-dimensional nature of the crystal. Since these photonic crystals exhibit 84% porosity, the etch depth of optimally etched structures can also be determined nondestructively by slightly tilting the surface of the patterned semiconductor, and observation of the number of repeating features underneath the surface in the SEM. This method can be used on a GaAs photonic crystal which was etched four lattice periods deep. This inspection also allows the measurement of the hole sizes, hole spacings, and porosities. A similar inspection of our smallest etched photonic band-gap crystal in GaAsP diode material reveals that the crystal is defined by 280-nm-diam holes with 350 nm center to center distance and a depth of three lattice periods. This crystal was optimized for a center wavelength in the reflectivity spectrum of 700 nm, which matches the luminescence peak of the GaAsP crystal.