Silicon Photonics: Fundamentals and Devices

8.9 Photodetectors Using Heteroepitaxy

Heteroepitaxy in silicon is used to fabricate silicon photodetectors for an extended range of wavelengths and to increase the absorption of physically thin silicon layers. Owing to material properties and existing technologies, the heteroepitaxy in silicon usually considers germanium as the second material. The SiGe structures provide photodetection of infrared light for the widely used telecom wavelengths of 1.3 and 1.5 µm, for which the silicon itself is transparent and unsuitable due to negligible light absorption.

8.9.1 Si and SiGe Multiple Quantum Wells

The alternation (or undulation) of several nm-thin layers of silicon and germanium creates a quantum well (QW) superlatice with lower band gap energy (well) where Ge is present, which is separated by Si layers of higher band gap energy (barriers or spacers). When the stack of the multiple quantum well (MQW) structure is in a depleted region, for example near the collector of a bipolar transistor, as illustrated in Figure 8.21 with an example from [49, 50], then the photogenerated charges are normally confined in the Ge quantum wells, but do not freely propagate through MQW, since there are energy barriers that the charges in the quantum well need to overcome in order to reach the “continuum” of the silicon-separating layers. In this way, Ge QWs provide photogeneration for IR, while Si separators prevent large dark currents.

Figure 8.21 SiGe HBT with MQW in the depleted region neighboring ...