Polycrystalline Germanium on Silicon for Near Infrared Detectors

The diffusion of optical communications is still hindered by the high cost of transceivers operating in the near infrared. Such systems are currently realized by hybrid integration of sources, modulators, detectors and control/processing electronics. The development of a Silicon-compatible technology for the fabrication of sources and detectors is expected to dramatically reduce transceivers costs through a more mature technology (compared to III-V semiconductors) and monolithic integration (possibly including amplifying and biasing electronics) on a single optoelectronics integrated circuit (OEIC). Germanium-on-Silicon detectors have been introduced for the wavelengths of interest (1.3 and 1.6 μm), with characteristics comparable with those of commercial III-V (InGaAs, InGaAsP) photodiodes [1]. In spite of the remarkable performance of detectors fabricated by epitaxial Ge [2,3], the required high temperatures and aggressive cleaning processes hinder a seamless integration of these devices with standard Silicon IC. Among the approaches intended to increase such compatibility, we have recently proposed and demonstrated the use of polycrystalline Ge. Poly-Ge can be deposited at a low (250-300°C) substrate temperature (without substrate cleaning) by thermal evaporation. The films exhibit absorption spectra similar to those of monocrystalline Ge but, due to the inferior material quality, mobility and lifetimes are reduced. Nevertheless, polyGe-on-Si heterojunction photodiodes have been reported with a near infrared (NIR) responsivity extending to 1.6 μm and ranging from 16mA/W (at 1.3 μm) to 5mA/W (at 1.55 μm)[4]. Moreover, the full compatibility of the polyGe-on-Si process has been demonstrated by integrating linear-arrays of NIR photodiodes on a custom-prepared Si CMOS integrated circuit [5]. In this Communicaion we discuss advantages and drawbacks of the poly-Ge approach and report on the most recent advances in polyGe based near infrared photodetection. We will present both discrete photodiodes with increased responsivity (up to 50mA/W at 1.3μm) operating at 2.5Gbit/s and twodimensional arrays of detectors integrated with silicon CMOS electronics. Fig.1: Eye diagram obtained at 2.5Gbit/s with a PRBS at 1.54μm, without a transimpedance amplifier.