**2. Applications of SiGe detector technology**

### **2.1. Telecommunications**

complexity. In addition, there is the potential of introducing doping contaminants into the

**Figure 1.** Spectral response characteristics of various IR detectors of different materials/technologies. Detectivity (verti‐ cal axis) is a measure of signal-to-noise ratio (SNR) of an imager normalized for its pixel area and noise bandwidth [5].

Germanium (Ge) is a Group IV material as is silicon, and thus avoids the cross contamination issue [6]. Ge, which can now be produced with extremely high purities, has an absorption spectrum similar to that of InGaAs, and can be alloyed with Si to improve the mobility and/or velocity of mobile carriers [7]. Ge forms a covalent bond with Si, and a number of SiGe based alloys involving the addition of hydrogen or oxygen are known. Amorphous alloys such as *a-*SixGeyHz are characterized by the absence of long-range order, but often possess a considerable degree of short-range order which is referred to as chemical ordering. A useful table listing Si and Ge bond and defect energies is found in Ref. [8]. One property of Ge that is of particular interest is the nature of its band gap. Though Ge like Si is predominately an indirect band gap (*E*g=0.66 eV) material, it has a direct band gap of 0.80 eV that is only 140 meV above its indirect band gap [9]. This enables Ge to transform from an indirect gap material towards a direct gap material through the incorporation of tensile strain, as will be discussed

in more detail in Section 5.2.

silicon, since III-V semiconductors act as dopants for Group IV materials [4].

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

The relatively recently realized capability of growing Ge epitaxially on Si has enabled the incorporation of Ge in an expanded variety of detector applications. A primary application for SiGe NIR detectors involves optical telecommunications networks. Due to fundamental physical advantages over copper as well as improved bandwidth, power dissipation, cost, and noise immunity, fiber optic based communications have been utilized to enhance available bandwidths for services such as internet, cable television, and telephone, e.g., using fiber-tothe-premises (FTTP) network architectures [14]. By replacing electrical wires with optical fibers, data rates can be enhanced from 10 Mb/s up to the order of 10 Gb/s with much lower power budgets [15].

Monolithic integration of optics with Si electronics is a primary means to realize low-cost and high performance interconnections, and Ge is a promising material to bridge low-cost electronics with the advantages of optics [12]. Unlike their Si based counterparts, photodetec‐ tors with tensile strained Ge/SiGe layers can provide high optical absorption over the entire C band (1530-1565 nm) and most of the L band (1565-1625 nm). The L band is commonly utilized by dense wavelength division multiplexing (DWDM) systems, and it has been determined that expanding the detection limit from 1605 nm to 1620 nm can enable 30 additional channels for long-haul optical telecommunications [16]. The performance of SiGe based photodetectors operating at extended NIR wavelengths is now comparable to or in some cases exceeds the performance of InGaAs based devices that have traditionally been used in telecommunications networks [10].

### **2.2. Optical interconnects**

Conventional copper interconnects become bandwidth limited above 10 GHz due to frequency-dependent losses such as skin effects and dielectric losses from printed circuit board substrate materials [18]. In addition, RC delay and heat dissipation issues originat‐ ing from metal interconnects on Si ICs have become increasingly problematic as feature sizes continue to shrink in accordance with Moore's Law [15]. Consequently, recent years have seen a rapid advancement in the adaption of Si based optical interconnects from rackto-rack and board-to board to chip-to-chip as well as to on-chip applications. The latter two applications require a large number of high-speed, low-cost photodetectors densely integrated with Si electronics [12].

**Figure 2.** The compatibility of SiGe technology with standard CMOS processing makes new types of optoelectronic ICs possible. Shown here is a new IC technology from IBM designated *CMOS Integrated Silicon Nanophotonics* [17].

While compound semiconductor devices offer high performance due to their excellent light emission and absorption properties, the process of integrating them in optical interconnects is generally very complicated, as well as costly due to the overhead associated with manufac‐ turing in a separate facility combined with the costs associated with packaging and assembling [19]. On the other hand, SiGe based photodetectors have been demonstrated that provide nearly all of the characteristics desirable for integrated optoelectronic receivers [20]. SiGe detectors offer high speeds (10 Gb/s and greater), high sensitivity, a broad detection spectrum, and the potential for monolithic integration with IC CMOS fabrication technology as will be discussed in Section 7.2. Thus, SiGe technology holds much promise for optical interconnects in next generation ICs (Figure 2) to overcome bottlenecks inherent in conventional microelec‐ tronic devices.

### **2.3. Further commercial and military applications**

Monolithic integration of optics with Si electronics is a primary means to realize low-cost and high performance interconnections, and Ge is a promising material to bridge low-cost electronics with the advantages of optics [12]. Unlike their Si based counterparts, photodetec‐ tors with tensile strained Ge/SiGe layers can provide high optical absorption over the entire C band (1530-1565 nm) and most of the L band (1565-1625 nm). The L band is commonly utilized by dense wavelength division multiplexing (DWDM) systems, and it has been determined that expanding the detection limit from 1605 nm to 1620 nm can enable 30 additional channels for long-haul optical telecommunications [16]. The performance of SiGe based photodetectors operating at extended NIR wavelengths is now comparable to or in some cases exceeds the performance of InGaAs based devices that have traditionally been used in telecommunications

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

Conventional copper interconnects become bandwidth limited above 10 GHz due to frequency-dependent losses such as skin effects and dielectric losses from printed circuit board substrate materials [18]. In addition, RC delay and heat dissipation issues originat‐ ing from metal interconnects on Si ICs have become increasingly problematic as feature sizes continue to shrink in accordance with Moore's Law [15]. Consequently, recent years have seen a rapid advancement in the adaption of Si based optical interconnects from rackto-rack and board-to board to chip-to-chip as well as to on-chip applications. The latter two applications require a large number of high-speed, low-cost photodetectors densely

**Figure 2.** The compatibility of SiGe technology with standard CMOS processing makes new types of optoelectronic ICs possible. Shown here is a new IC technology from IBM designated *CMOS Integrated Silicon Nanophotonics* [17].

networks [10].

**2.2. Optical interconnects**

integrated with Si electronics [12].

The detection of visible-NIR radiation offered by SiGe based sensors and imaging devices operating at RT make them useful for a variety of additional industrial, scientific, and medical applications. Applications requiring low-cost NIR capable sensors include medical thermog‐ raphy for cancer and tumor detection during diagnosis and surgery, machine vision for industrial process monitoring, sorting of agricultural products, biological imaging techniques such as spectral-domain optical coherence tomography, and imaging for border surveillance and law enforcement [21]. SiGe based NIR sensors/imagers also provide a low-cost solution for a wide range of military applications. These military applications include, but are not limited to, day-night vision, soldier robotics, plume chemical spectra analysis, biochemical threat detection, and night vision for occupied and autonomous vehicles [13].

An additional military application of particular significance is hostile mortar fire detection and muzzle flash (Figure 3). Muzzle flashes, which approximate a blackbody spectrum from 800 K to 1200 K [22], consist of an intermediate flash and, unless suppressed, a brighter secondary flash [23]. Such incendiary events produce large amounts of energy in the NIR spectral region. The ability to image flashes from hostile fire events combined with target detection capability [e.g., by using spectral tags (chemical additives) for identification of friendly fire] provides a vital function in the battlefield that can be key to saving the lives of soldiers as well as making good strategic decisions such as knowing when and where to attack [24]. The realization of small and low-cost SiGe devices that can detect hostile fire sources therefore has the potential to greatly benefit our armed forces.

Another commercial application in view involves very small form factor SiGe based visible-NIR cameras. Since imaging has become a core feature to most mobile phone users and manufacturers, the industry puts much effort into related performance improvements and optimization of camera manufacturing methods. Wafer-level packaging of CMOS image sensors and wafer-level optics provide a cost-effective means of potentially equipping future generations of camera smartphones with visible-NIR imaging capability with smaller form factors [26]. Developing such miniature cameras based on SiGe integrated CMOS technology will require demonstrating small pixel and format NIR detector arrays that enable wide fieldof-views. Producing a practical NIR imager will likewise involve further refining the thermal,

**Figure 3.** SiGe technology is associated with a number of military applications involving NIR sensitivity, including muzzle flash detection [25].

mechanical, and optical analyses of encapsulation and optical materials to enable compatibility with NIR FPA manufacturing.
