**9. Summary**

As was shown to be the case with bulk SiGe alloys, strain alters the intrinsic interatomic distances and thus affects the band gap energy, and also impacts the effective masses and mobility [85]. However, their reduced dimensionality and small size allow NSs to tolerate relatively large stress and strain without introducing significant dislocations or other defects that could undermine their electrical properties. Due to the nature of their geometry, NWs especially the core-shell variety—experience tensile stress due to bending in addition to that caused by lattice mismatch [86]. By applying an external tensile strain of around 2.8% to Sicore/Ge-shell NWs, a transformation from direct band gap to indirect one can likewise be

In addition to the enhancement of performance properties due to the QCE and strain, detectors based on one-dimensional Q-wires [i.e., nanowires (NWs)] offer potentially greater sensitivity primarily due to larger surface-to-volume ratios [90]. There is still progress to be made in this area, however, as Ge NW based photodetectors currently have significantly longer photocur‐ rent rise and decay kinetics and associated time constants than those based on bulk Ge/SiGe. As illustrated in Figure 29(a), the optical absorption of Si1-xGex NWs is largely affected by the material concentration, with the band gap (and thus SiGe NW based photodetector response) shifting to lower energies and longer wavelengths as x increases. As might be expected based on the previous discussion on QDs, a shift to lower energies was observed with increasing NW diameter for both Si and Ge NWs, again evidencing the potential for tuning the optical

properties of NS based photodetector devices by varying the constituent NS sizes.

(a) (b)

**Figure 29.** (a) Optical absorption spectra vs. band gap energy of Si1‐*<sup>x</sup>*Ge*<sup>x</sup>* NWs of five representative compositions (following the arrow, the spectrum corresponds to Ge, Si0.3Ge0.7, Si0.5Ge0.5, Si0.7Ge0.3, and Si NWs, respectively); the inset summarizes variation of optical band edges with the known values from bulk Si1‐*<sup>x</sup>*Ge*<sup>x</sup>* crystals [91]. (b) I‐V characteristics for amorphous Ge QD photodetector at different illumination powers; the inset shows a schematic

**Figure 29.** (a) Optical absorption spectra vs. band gap energy of Si1-xGex NWs of five representative compositions (follow‐ ing the arrow, the spectrum corresponds to Ge, Si0.3Ge0.7, Si0.5Ge0.5, Si0.7Ge0.3, and Si NWs, respectively); the inset summariz‐ es variation of optical band edges with the known values from bulk Si1-xGex crystals [91]. (b) I-V characteristics for amorphous Ge QD photodetector at different illumination powers; the inset shows a schematic of the device [92].

In the past few years, a number of detector devices comprising QDs, which exhibit quantum confinement in all three dimensions, have been developed. QD detectors offer the advantages of increased sensitivity to normally incident radiation as a result of breaking of the polarization selection rules, large photoelectric gain associated with a reduced capture probability of photoexcited carriers due to suppression of electron‐phonon scattering, and small thermal generation rate resulting from the zero‐dimensional character of the electronic spectrum that renders improved SNR [93]. Compared to Si QDs, Ge QDs have higher absorption coefficients due to localized defect states [92]. SiGe QD detectors have been reported that operate up to the LWIR regime; however, the responsivity of these devices is typically much greater at NIR wavelengths, i.e., below 2000 nm [93]. The response at NIR wavelengths of photodetectors comprising Ge QDs grown on SiGe has been attributed to interband transitions between electrons in Ge/SiGe layers and holes in the Ge QDs. Figure 29(b) shows the I‐V characteristics of a Ge QD photodetector exposed to different intensities of visible illumination. Ge QD based photodetectors have recently demonstrated peak responsivities as high as 4 A/W at ‐10 V bias and response times

In the past few years, a number of detector devices comprising QDs, which exhibit quantum confinement in all three dimensions, have been developed. QD detectors offer the advantages of increased sensitivity to normally incident radiation as a result of breaking of the polarization

This chapter has covered the operation, design, fabrication, and applications of SiGe based photodetector technology. A model to predict SiGe sensor performance over a wide range of light levels has been presented, which indicates that a low‐cost, small pixel, uncooled SiGe based detector will respond well to small amounts of illumination from a direct NIR source. The operation and relative performance characteristics of Ge based avalanche photodiodes (APDs), metal‐semiconductor‐metal (MSM) detectors, and *pin* detectors have been discussed. SiGe *pin* photodetectors offer performance advantages including high responsivities, high bandwidths, low bias voltage requirements, and low dark current compared to other types of SiGe detectors. The impact of detector dark current and techniques for reducing it in *pin* photodetector devices have been examined. The nature and impact of strain and stress on extending SiGe based detector response to longer NIR wavelengths

Installed infrastructure and heterogeneous integration can be leveraged to fabricate small feature CMOS‐ compatible SiGe based *pin* detector array devices exhibiting optimal properties for NIR detection. A common fabrication process for SiGe based *pin* photodetectors incorporating two‐step low/high temperature epitaxial growth of Ge/SiGe layers on Si substrates followed by a high temperature anneal and additional processing steps has been outlined, which was found to reduce threading dislocation density and thereby improve device quality. In addition, fabricated SiGe detectors can be directly integrated with low noise Si readout integrated circuits to yield low SWaP, low cost, and highly uniform IR focal plane arrays (FPAs) that can function as imaging devices. Various integrated SiGe based FPA imagers have been demonstrated that exhibit enhanced functionality and performance characteristics. Finally, the impact of the quantum confinement effect and strain on band gap in low

**8.2. Photodetectors based on SiGe nanowires and quantum dots**

352 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

achieved [89].

of the device [92].

down to ~40 ns [92].

**9.0 Summary**

were also discussed.

This chapter has covered the operation, design, fabrication, and applications of SiGe based photodetector technology. A model to predict SiGe sensor performance over a wide range of light levels has been presented, which indicates that a low-cost, small pixel, uncooled SiGe based detector will respond well to small amounts of illumination from a direct NIR source. The operation and relative performance characteristics of Ge based avalanche photodiodes (APDs), metal-semiconductor-metal (MSM) detectors, and *pin* detectors have been discussed. SiGe *pin* photodetectors offer performance advantages including high responsivities, high bandwidths, low bias voltage requirements, and low dark current compared to other types of SiGe detectors. The impact of detector dark current and techniques for reducing it in *pin* photodetector devices have been examined. The nature and impact of strain and stress on extending SiGe based detector response to longer NIR wavelengths were also discussed.

Installed infrastructure and heterogeneous integration can be leveraged to fabricate small feature CMOS-compatible SiGe based *pin* detector array devices exhibiting optimal properties for NIR detection. A common fabrication process for SiGe based *pin* photodetectors incorpo‐ rating two-step low/high temperature epitaxial growth of Ge/SiGe layers on Si substrates followed by a high temperature anneal and additional processing steps has been outlined, which was found to reduce threading dislocation density and thereby improve device quality. In addition, fabricated SiGe detectors can be directly integrated with low noise Si readout integrated circuits to yield low SWaP, low cost, and highly uniform IR focal plane arrays (FPAs) that can function as imaging devices. Various integrated SiGe based FPA imagers have been demonstrated that exhibit enhanced functionality and performance characteristics. Finally, the impact of the quantum confinement effect and strain on band gap in low dimensional nanostructures was analyzed, and the characteristics of photodetectors based on quantum dots and nanowires were discussed.
