BAND GAP Scanning electron microscopy image shows edge of self-assembled silicon crystal that exhibits photonic band gap.

The thin-film crystals consist of silicon lattices with three-dimensional arrays of uniformly spherical air pockets. The photonic band gap of the crystals blocks transmission of photons at 1.3 µm, a technologically important optical wavelength for fiber-optic telecommunications and the development of all-optical integrated circuits.

The work was carried out by scientists Yurii A. Vlasov and David J. Norris at NEC Research Institute, Princeton, N.J., and professor of electrical engineering James C. Sturm and graduate student Xiang-Zheng Bo at Princeton University [Nature, 414, 289 (2001)].

In the first part of the process, the silicon wafer is placed vertically in a vial containing a colloidal suspension of silica microspheres in ethanol. As the ethanol evaporates, a thin layer of spheres self-assembles on the wafer in an ordered arrangement. Such assemblies are known as synthetic opals because they form in much the same way as gemstone opals do.

The spaces between the silica microspheres are then filled with silicon by means of a commercially available low-pressure chemical vapor deposition furnace and silane (SiH₄) gas. The microspheres, which act as a template for the photonic crystal, are removed by wet etching, leaving the air-filled silicon lattice, known as an inverted opal, attached to the silicon chip.

"Such structures exhibit, for the first time in a naturally assembled photonic crystal, optical properties that are consistent with a photonic band gap," Norris tells C&EN. "In addition, since they are planar and integrated into a silicon wafer, it is easy to imagine how they could be used in a photonic device. Previous approaches succeeded in obtaining these materials, but they have either been too expensive or resulted in a material that was impractical for producing useful photonic devices."

The crystals synthesized by Norris and colleagues contain negligible amounts of undesir-able intrinsic defects. To be of any use, however, the crystals will need to be doped with extrinsic--intentionally placed--defects. For example, line defects in the crystal could act as waveguides that connect photonic devices in all-optical microchips.

The Princeton researchers showed that extrinsic defects can be introduced by adding trace amounts of silica microspheres with a different diameter to the original colloidal suspension. They also demonstrated that the photonic crystal film can be patterned on the silicon substrate using standard photolithography and ion-etching techniques.

"This is important if silicon-based photonic components and existing silicon microelectronics are to be integrated onto a single chip, leading to advanced optoelectronic devices that will speed up the next generation of telecommunications and computers," remarks John D. Joannopoulos, professor of physics at Massachusetts Institute of Technology, in the same issue of Nature (page 257).

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