Planar Ge photodetectors on Si substrates for Si/Ge-based optical receivers
Operation of photodetectors at a wavelength of 1.3 µm has extensive application in the rapidly growing field of optical transmission systems. As optical networks spread deeper into the consumer market, it will become important to have low-cost, manufacturable optical components that can be integrated on a chip with other electrical components. Enhanced performance of many of these systems can be achieved by monolithically integrating the discrete optical devices in existing Si integrated circuits (ICs). The use of Ge is advantageous in terms of lower cost of fabrication and compatibility with Si integrated circuit technology. The high electron mobility and high optical absorption coefficient at 1.3 µm make Ge attractive for some telecommunication applications. In addition, Ge is promising for other applications such as microwave and millimeterwave photonic systems that require high photocurrent and high linearity. To this end, interdigitated Ge PIN photodetectors were fabricated on Si substrate using 10-µm-thick graded SiGe buffer layers. Their operation at 1.3 µm was successfully demonstrated. A 3-dB bandwidth of 3.8 GHz was obtained at low bias of -5 V and the external quantum efficiency at 1.3 µm was 49 % without external bias. The SiGe buffer layers effectively relieved strain and resulted in high quality Ge epitaxial layers with a low threading dislocation density of ~ 105 cm -2 and smooth surface morphology. A more practical approach was to directly deposit thin epitaxial layers of Ge on Si substrate. The challenge to this approach was to accommodate the lattice mismatch of 4 % without significant degradation in the material quality. Our approach to overcome island formation was to grow the Ge layers at low temperature. Metal-Ge-metal photodetectors were fabricated on a Ge epitaxial layer directly grown on Si (100) substrate. Amorphous Ge was used to increase the Schottky barrier height, which resulted in a reduction of the dark current by more than two orders of magnitude.