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850-nm Photodetector Can Be Integrated into Waveguide

Photonics Spectra
Dec 2007
The photodetector is fabricated on the sloping sidewall of a mesa structure.

Breck Hitz

It’s no secret that electronics technology will not keep pace with Moore’s Law for much longer. Most experts look to photonics technologies to replace electronics and to allow the rapid growth to continue unabated. Although all-optical computers eventually may become practical, in the short term, the processing of information still will be done electrically, though the communication between — and even within — chips increasingly will depend on photonics.

But for that to happen, photonics must undergo an integration of many functions into a small package, similar to the process that occurred in electronics decades ago. Although it has been relatively straightforward to integrate passive components — waveguides, multiplexers and so forth — active components such as lasers, modulators and detectors have proved more difficult.

A feature in this issue reports impressive results with integrating photodetectors into a monolithic package (see “Photonic Large-Scale Integration: The Future of Photonics,” p. 74). A different approach recently was investigated by Zhihua Li and his colleagues at the Chinese Academy of Sciences in Beijing. Whereas the former approach uses a novel germanium-on-silicon detector to function in the communications bands at 1.5 μm, the scientists in Beijing addressed the 850-nm spectral region with a GaAs/AlGaAs detector.

PRwaveguide_fig1.jpg

Figure 1. The conventional approach to photodetection in today’s microphotonic devices is to reflect light out of the device into a separate detector (a). As an alternative, the scientists fabricated a p-i-n junction on a sloping surface that was part of the monolithic device (b). Reprinted with permission of Optics Letters.


Today’s approach to photodetectors in microphotonic devices typically uses a mirror to couple light out of the device and into the detector (Figure 1a). Although this works, the cost of fabrication and the difficulties of alignment make it impractical for high-density, parallel optical interconnects. As an alternative, Li’s group developed what it believes is the first operational photodetector on a three-dimensional sloping surface (Figure 1b).

To fabricate the device, the investigators defined a deep mesa structure on a GaAs wafer by anisotropic liquid etching. Then, on the mesa slope, they grew the p-i-n structure using either molecular beam epitaxy or metallorganic chemical vapor deposition, followed by postprocess anode and cathode metal contact formation.

The electrical characteristics of the initial devices were inferior to those of typical p-i-n photodetectors. The responsivity, for example, was about 0.13 A/W. The scientists expect that fairly simple design improvements, such as electropositioning of the photoresist to define the anode contact lithography, and optimizing the growth of the p-i-n structure on the sloping surface, will result in significant improvements.

Optics Letters, Oct. 15, 2007, pp. 2906-2908.


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