Semiconductors Stick Through a Cool Marriage

Hank Hogan

The ubiquitous semiconductor silicon is electronically powerful, but optically weak. It is blind, for example, in the 1.3- to 1.6-µm region of the spectrum that is useful for telecommunications applications. Germanium, on the other hand, excels at near-IR detection but is a poor electronics platform. Now a team from University Roma Tre in Rome has developed a relatively low-temperature process that integrates germanium photodetectors and standard silicon-based electronics.

"The CMOS compatibility of the process we introduced to fabricate the detectors is the key innovation," said team member and professor of electronic engineering Gianlorenzo Masini. "Compatibility is mainly due to the low temperature of the process."

The researchers fabricated the electronic chips using a standard 2.0-µm CMOS process. They evaporated high-purity germanium in a vacuum and deposited it on the chips at 350 °C. This temperature is cool for CMOS processing, so the growth process did not disturb the silicon circuitry. The result was a polycrystalline germanium film, about 200 nm thick over the entire chip. Then they patterned the film and completed the device.

In a demonstration of the technique, they manufactured a linear array of eight 120 x 120-µm pixels, an analog CMOS multiplexer to select one of those pixels and an amplifier for the output signal. This integrated microsystem displayed a photoresponse up to 1.6 µm.

Since the proof-of-concept experiment, the research team has designed a linear array that features 64 pixels, analog-to-digital conversion and dark-current-suppression circuitry. The array should be finished and characterized by the end of the year, and a square array is in the design stage.

Applications

The devices are suitable for low-cost near-IR cameras in automotive or medical applications. And Masini said that the technique has already attracted commercial attention.

"The availability of a low-cost near-IR camera, as well as the possibility to integrate near-IR detectors in an optical communication transceiver based on silicon electronics, seems to be of interest to a number of companies."