Optics & Photonics News

Initial experiments

The initial work on developing a hybrid silicon laser began in the summer of 2004 supported by Jag Shah at DARPA (the Defense Advance Research Projects Agency) and Intel. Hyundai Park explored the wafer bonding of InP active structures to silicon based on the bonding knowledge base that had been developed by the Bowers’ group in the 1990s. The hybrid silicon laser was actually the third type of hybrid device that we had created. Previous devices led to the first long-wavelength VCSEL made of InGa AsP active regions bonded to Ga As mirrors as well as avalanche photodiodes made of InGa As bonded onto silicon.

Early on, our team found the thermal expansion mismatch between InP and silicon to be the key hurdle for transferring InP epitaxial films to silicon, since conventional direct wafer bonding techniques were done at high temperatures (650° C). Large cracks would form in the transferred materials, rendering them useless for large devices like photonic integrated circuits. After many experiments, the team achieved the high-film-qual-ity transfer using a low-temperature (300° C) plasma-activated approach. This change together with a vertical outgassing channel design developed by Di Liang eventually led to the bonding of 150-mm-in-diameter InP and silicon wafers.

Alexander Fang, Hyundai Park and Richard Jones attempted to make optically pumped lasers using these wafer bonded III-V layers on SOI waveguides. Early attempts failed, however, because it was difficult to create high-quality laser facets through the bonded interface; cleaving techniques yielded stepped facets due to the incorrect orientation of the silicon and the InP layers.

100 mm
Silicon evanescent
photodetectors
Silicon evanescent
photodetectors
Dice and
polish line
DFB-Silicon
evanescent lasers

10 dB/div

1,560 1,600 1,640

50 dB
(Top) Photo of an array of three distributed feedback (DFB)
lasers and the power monitors. (Bottom) DFB spectra.

Stronger bonding and better polishing techniques eventually led to optically pumped hybrid silicon lasers.

Now that the team had demonstrated optically pumped lasers, we set our sights on the Holy Grail of silicon photonics: an electrically pumped laser on silicon. This structure provided unique challenges that had not been seen in typical in-plane semiconductor lasers, since current couldn’t be passed through the bonded interface. Instead, current below the quantum wells had to flow from the center of the waveguide and out laterally to the contacts. In June 2006, the team successfully demonstrated an electrically pumped hybrid silicon laser that lased continuous wave.

Subsequent advances

Since our team demonstrated an electrically pumped hybrid laser, other researchers have validated the hybrid silicon design as a promising device platform—and the pace of work in this area has rapidly accelerated. Yariv’s group at Caltech has demonstrated a variety of high confinement structures, for example. Bowers’ group and Arai’s group at the Tokyo Institute of Technology demonstrated distributed feedback (DFB) lasers. These DFB lasers were important because they removed the need for polished facets for reflectors and enabled the single wavelength operation capability that is needed for multiple wavelength applications such as for DWDM systems.