|
We demonstrate the use of an area selective zinc in-diffusion technique as a simple and efficient technique for the
fabrication of integrated photonic devices. In this work, the zinc in-diffusion process has a two fold application. It is well
known that the diffusion of zinc in InP follows an interstitial-substitutional diffusion mechanism. This provides a
concentration dependent diffusion profile, which allows us to control the sharpness of the diffusion front by controlling
the background doping concentration of the semiconductor wafer. By controlling the zinc depth combined with a sharp
diffusion front, the insertion losses of the devices can be minimized. In addition, this results in selective definition of p-n
junctions across the semiconductor wafer and therefore offers the potential for integration with electronic devices. Using
this technique an integrated 2x2 Mach-Zehnder modulator/switch was fabricated. The semiconductor wafer is based on
InGaAsP multiple quantum wells. To selectively define p-n regions for the contacts, we use a 200-nm thick silicon
nitride mask during the diffusion. The Mach-Zehnder structure is then patterned using photolithography and dry etching.
After a cyclotene planarization process, p-type contacts are deposited on top of the diffused regions by evaporation and
lift-off. Our experimental results demonstrate that on-chip losses on the order of 4-dB are obtained, which is
significantly lower compared to the use of isolation trenches. The device response as a modulator requires an additional
insertion loss of 3-dB for voltage controlled operation, with an extinction ratio better than 16 dB. In the case of electrical
current operation, better than 20 dB extinction ratio was obtained with only 8 mA.
|
