Integration of InP-based photonic devices by zinc in-diffusion

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.