Near infrared detectors

The term "near infrared" or NIR is most commonly used with reference to a wavelength spectral range between 0.7 and 2.0um. This portion of infrared is of paramount importance in several applications, first among them optical communications with transmission windows located at 0.85, 1.3 and 1.55um, corresponding to GaAs-based laser emission and to two minima in attenuation for standard silica fibers, respectively. Moreover, wavelength division multiplexing for high capacity links encourages to using the whole interval between 1.3 and 1.6ƒÝm (S, C and L bands). Due to the growing demand for wideband internet and massive data transmission, applications have shifted from long-haul point-to-point connections to local networks down to subscribers, softening the specifications and opening entire new markets. Besides communications, NIR spectroscopy is employed in remote sensing of the environment, monitoring of industrial processes, biology and medicine. For example, water has absorption lines which allow to detect its content in the flora for fire prevention, various gas species exhibit NIR absorption bands useful for emission or toxicity analysis. In addition, NIR spectroscopy has been exploited for DNA sequencing, brain activity mapping and cancer detection. However, since optical fiber communications remains the main field driving research in NIR detectors, this chapter will focus on NIR detectors on silicon from the receiver standpoint.Among semiconductor materials, SiGe alloys have allowed to fabricate novel, highly performative electronic devices such as heterojunction bipolar and field-effect transistors ( ), opening new perspectives also in NIR optoelectronics by exploiting the band gap of germanium (0.66eV), much lower than of silicon (1.12eV) which is otherwise useless for detection at these wavelengths. Numerous NIR devices have been proposed, from emitters to waveguides, couplers, modulators and detectors . The centrosymmetry and the indirect nature of Si, Ge and SiGe pose important limitations to SiGe devices as compared to III-V's; nevertheless, exploiting a few structurally related modifications such as quantum confinement, acceptable performances have been foreseen, with the significant advantage of the compatibility with the unsurpassed silicon VLSI technology. The key of success of SiGe-based optoelectronics is the ability to conveniently compromise between material performance and monolithic integration. The fabrication of embedded optoelectronics and electronics in the same process does considerably reduce alignment and interconnection problems, offering improved reliability, yield, compactness and reduced parasitics. Conversely, monolithic integration with SiGe requires heteroepitaxy, which is known to be critical for lattice mismatched materials such as Si and Ge, with mismatch of about 4% . In the specific case of photodetectors, material quality requirements are less stringent than in other devices, thus encouraging towards the realization of innovative and competitive devices.This chapter is organized as follows. After a short introduction on photodetector operation and figures of merit in Section 2, Section 3 deals with SiGe technology for photodiodes, focusing on the quality of both strained and relaxed epilayers, their optical and electrical characteristics and their bearing in device performances. Design strategies and fabrication are discussed in Section 4 along with an overview of the most relevant approach