**8. Optoelectronic properties of Si/Ge based nanostructures**

#### **8.1. Quantum confinement and strain in Si/Ge nanostructures**

of phonons [86].

**Figure 26.** (a) Photograph of optoelectronic chip featuring 2D Ge photodetector array, showing zoomedin images of a portion of the array; (b) chip architecture, comprising readout electronics; and (c)

**Figure 26.** (a) Photograph of optoelectronic chip featuring 2D Ge photodetector array, showing zoomed-in image of a portion of the array; (b) chip architecture, comprising readout electronics; and (c) photograph of individual pixel [6].

(c)

(c)

In 2010 Vu *et al*. [75] developed arrays of both vertical and lateral *pin* photodetectors that were integrated into electronic‐photonic FPAs. Layers of metal were employed to connect the detector electrodes to transimpedance amplifiers and CMOS circuits, and light signals were then coupled from waveguides or inserted directly into the side of the Ge intrinsic layer via optical fibers. A responsivity of 1.11 A/W at 1550 nm, bandwidth of 15 GHz, and dark current density on the order of 100 nA/cm2 were achieved for the NI photodetectors. The integrated chips were produced in a standard CMOS foundry, where the fabrication processes was optimized for

In 2010 Vu *et al*. [75] developed arrays of both vertical and lateral *pin* photodetectors that were integrated into electronic-photonic FPAs. Layers of metal were employed to connect the detector electrodes to transimpedance amplifiers and CMOS circuits, and light signals were then coupled from waveguides or inserted directly into the side of the Ge intrinsic layer via optical fibers. A responsivity of 1.11 A/W at 1550 nm, bandwidth of 15 GHz, and dark current

**Figure 26.** (a) Photograph of optoelectronic chip featuring 2D Ge photodetector array, showing zoomedin images of a portion of the array; (b) chip architecture, comprising readout electronics; and (c)

In 2010 Vu *et al*. [75] developed arrays of both vertical and lateral *pin* photodetectors that were integrated into electronic‐photonic FPAs. Layers of metal were employed to connect the detector electrodes to transimpedance amplifiers and CMOS circuits, and light signals were then coupled from waveguides or inserted directly into the side of the Ge intrinsic layer via optical fibers. A responsivity of 1.11 A/W at 1550 nm, bandwidth of 15 GHz, and dark current density on the order of 100 nA/cm2 were achieved for the NI photodetectors. The integrated chips were produced in a standard CMOS foundry, where the fabrication processes was optimized for

chips were produced in a standard CMOS foundry, where the fabrication processes was

were achieved for the NI photodetectors. The integrated

**Figure 27.** (a) Schematic cross-section, and (b) top view, of a linear photodetector array consisting of 16

(b)

(a)

(a)

In 2014, Chong *et al.* [84] reported a parallel system of 16 element *pin* photodetector arrays, shown in Figure 27, for parallel optical interconnect applications. The detectors comprised Ge absorption layers epitaxially grown on a SOI substrate by UHV‐CVD using the two‐step LT/HT growth process, and incorporated a plasma etched double mesa vertical structure to reduce parasitic capacitance. The array featured responsivities of 0.38 and 0.23 A/W at 1310 and 1550 nm, respectively, with a very low dark current density of ~5 mA/cm2 with no applied bias

**Figure 27.** (a) Schematic cross-section, and (b) top view, of a linear photodetector array consisting of 16

(b)

In 2014, Chong *et al.* [84] reported a parallel system of 16 element *pin* photodetector arrays, shown in Figure 27, for parallel optical interconnect applications. The detectors comprised Ge absorption layers epitaxially grown on a SOI substrate by UHV‐CVD using the two‐step LT/HT growth process, and incorporated a plasma etched double mesa vertical structure to reduce parasitic capacitance. The array featured responsivities of 0.38 and 0.23 A/W at 1310 and 1550 nm, respectively, with a very low dark current density of ~5 mA/cm2 with no applied bias

**Figure 27.** (a) Schematic cross-section, and (b) top view, of a linear photodetector array consisting of 16 detectors [84].

**8.0 Optoelectronic Properties of Si/Ge Based Nanostructures**

**8.1 Quantum Confinement and Strain in Si/Ge Nanostructures**

**8.0 Optoelectronic Properties of Si/Ge Based Nanostructures**

**8.1 Quantum Confinement and Strain in Si/Ge Nanostructures**

photograph of individual pixel [6].

(b)

density on the order of 100 nA/cm2

photograph of individual pixel [6].

optimized for manufacturability.

(b)

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

manufacturability.

manufacturability.

(a)

(a)

detectors [84].

and a bandwidth of up to 8 GHz.

detectors [84].

and a bandwidth of up to 8 GHz.

A growing number of optoelectronic devices including photodetectors are being developed that employ low-dimensional nanostructures (NSs), particularly quantum wires (Q-wires) and quantum dots (QDs), to enhance their performance. NSs offer unique optical and electronic properties resulting from the quantum confinement of electrons and holes. Such quantum confinement in NSs, which is directly affected by their dimensions, has a substantial impact on band gap energy. The quantum confinement effect (QCE) causes the band gap of crystalline Si and Ge Q-wires and QDs to increase as their diameters are reduced according to the relation *Eg* ~ 1/*Ra* , where *Eg* is the band gap and *a* falls between 1 and 2 [85]. Figure 28(a) and (b) show a comparison of band gap energy reported by various groups as a function of QD diameter in crystalline Si and Ge QD structures, respectively, where it can be seen that the band gap energies of the Si QDs follow a much more uniform and predictable pattern than those of their Ge counterparts. As a result of larger exciton energy, the QCE effect is stronger in Ge than in Si, and the electronic properties Ge QDs can thus be more easily modulated by the QCE than can those of Si QDs [85,88]. In addition, making the length scale in NSs small enough produces uncertainty in the momentum **k** vectors that consequently allows the **k** selection rules to be broken, causing the band gap to change from indirect toward direct that allows electron-hole recombination to take place without the need of phonons [86]. A growing number of optoelectronic devices including photodetectors are being developed that employ low‐ dimensional nanostructures (NSs), particularly quantum wires (Q‐wires) and quantum dots (QDs), to enhance their performance. NSs offer unique optical and electronic properties resulting from the quantum confinement of electrons and holes. Such quantum confinement in NSs, which is directly affected by their dimensions, has a substantial impact on band gap energy. The quantum confinement effect (QCE) causes the band gap of crystalline Si and Ge Q‐wires and QDs to increase as their diameters are reduced according to the relation *Eg* ~ 1/*Ra* , where *Eg* is the band gap and *a* falls between 1 and 2 [85]. Figure 28(a) and (b) show a comparison of band gap energy reported by various groups as a function of QD diameter in crystalline Si and Ge QD structures, respectively, where it can be seen that the band gap energies of the Si QDs follow a much more uniform and predictable pattern than those of their Ge counterparts. As a result of larger exciton energy, the QCE effect is stronger in Ge than in Si, and the electronic properties Ge QDs can thus be more easily modulated by the QCE than can those of Si QDs [85,88]. In addition, making the length scale in NSs small enough produces uncertainty in the momentum **k** vectors that consequently allows the **k** selection election rules to be broken, causing the band gap to change from indirect toward direct that allows electron‐hole recombination to take place without the need

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 Si‐core/Ge‐shell NWs, a

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 photocurrent 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‐*<sup>x</sup>*Ge*<sup>x</sup>* 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

**Figure 28.** Comparison of band gap energy as a function of QD diameter for **Figure 28.** Comparison of band gap energy as a function of QD diameter for (a) Si and (b) Ge [87].

transformation from direct band gap to indirect one can likewise be achieved [89].

**8.2 Photodetectors Based on SiGe Nanowires and Quantum Dots**

(a) Si and (b) Ge [87].

constituent NS sizes.

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 achieved [89].

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

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.

**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 **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].

amorphous Ge QD photodetector at different illumination powers; the inset shows a schematic

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

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

down to ~40 ns [92].

**9.0 Summary**

were also discussed.

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 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 down to ~40 ns [92].
