**1. Introduction**

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This chapter covers recent advances in SiGe based detector technology, including device operation, fabrication processes, and various optoelectronic applications. Optical sensing technology is critical for defense and commercial applications including telecommunica‐ tions, which requires near-infrared (NIR) detection in the 1300-1550 nm wavelength range. [Here we consider the NIR wavelength band to span approximately 750-2000 nm; the upper portion of this band, e.g., 1400 nm and longer wavelengths, is sometimes elsewhere designated short-wave infrared (SWIR).] Although silicon (Si) photodetectors have been widely used to detect in the visible to short NIR wavelength regime, the relatively large Si band gap of 1.12 eV, corresponding to an absorption cutoff wavelength of ~1100 nm, hinders the application of Si photodetectors for longer wavelengths vital for medium-and longhaul optical fiber communications.

Group III-V compound semiconductors possess the advantages of high absorption efficiency, high carrier drift velocity, excellent noise characteristics, and mature design and fabrication technology for optical devices, and are commonly used in IR detection related devices [1]. InGaAs based IR photodetectors have been developed for NIR (up to ~1700 nm) applications, InSb for 3-5 μm applications, and HgCdTe for 1-3, 3-5 and 8-14 μm applications [2]; the spectral responses of these and various other IR detector material systems are shown in Figure 1. While it is possible to integrate III-V semiconductor materials on Si by wafer bonding or epitaxy [3], III-V based detectors normally require cooling (typically down to 77 K), and incorporating III-V materials into the prevalent silicon process is at the expense of high cost and increased

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complexity. In addition, there is the potential of introducing doping contaminants into the silicon, since III-V semiconductors act as dopants for Group IV materials [4].

**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.

Consequently, strained absorption layers composed of Ge/SiGe can provide much higher optical absorption and enhanced transport properties over the ~1300-1600 nm wavelength range than layers of pure Si, enabling SiGe based photodetectors with extended NIR capabil‐ ities. (Although potential drawbacks of Si-Ge integration exist including lattice mismatch between the materials and a relatively low thermal budget for Ge, the growth processes can be adjusted to compensate in each case.) While detectors based on Ge crystals have been used for NIR detection for many years, these have required cooling down to 77 K, making them expensive and limiting their use [10]. Detectors incorporating epitaxially grown Ge/SiGe on Si substrates can operate at room temperature (RT), thus offering substantially reduced cost and size, weight, and power (SWaP). Furthermore, SiGe photodetectors can be designed to exhibit low dark currents (nA range) and dark current densities comparable to those of large area Group III-V detectors, with accordingly high signal-to-noise ratios (SNRs) [11]. Conse‐ quently, SiGe based devices have become promising and practical candidates for many applications requiring detection of radiation at visible to NIR wavelengths.

Perhaps the most important advantage of SiGe based devices is that SiGe epitaxial growth processes are compatible with both front-and back-end silicon complementary metal-oxidesemiconductor (CMOS) fabrication technologies. Consequently, SiGe detector devices can be heterogeneously combined with CMOS circuitry using widely installed manufacturing infrastructure used for production of CMOS integrated circuits (ICs). In addition, SiGe photodetectors and Si CMOS receiver circuits can be simultaneously fabricated and then monolithically integrated [12]. Fabricated SiGe detectors can be incorporated directly with low noise Si readout integrated circuits (ROICs) to yield low-cost and highly uniform IR focal plane arrays (FPAs) to maximize the fill factor, as will be discussed in Section 7. This allows SiGe detector based imaging devices to be produced much less expensively and with less difficulty than those based on III-V detectors. An attractive feature of CMOS-compatible SiGe IR detectors/imagers is that they can be fabricated on large diameter (up to 450 mm) Si wafers [13], further decreasing costs and maximizing production output.
