Reported performance specifications of NI
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 telecommunications, 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 long-haul 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 . 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 ; 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 , 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 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 .
Germanium (Ge) is a Group IV material as is silicon, and thus avoids the cross contamination issue . 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 . 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
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 capabilities. (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 . 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) . Consequently, 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-oxide-semiconductor (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 . 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 , further decreasing costs and maximizing production output.
2. Applications of SiGe detector technology
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-to-the-premises (FTTP) network architectures . 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 .
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 . Unlike their Si based counterparts, photodetectors 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 . 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 .
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 . In addition, RC delay and heat dissipation issues originating from metal interconnects on Si ICs have become increasingly problematic as feature sizes continue to shrink in accordance with Moore’s Law . Consequently, recent years have seen a rapid advancement in the adaption of Si based optical interconnects from rack-to-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 .
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 manufacturing in a separate facility combined with the costs associated with packaging and assembling . On the other hand, SiGe based photodetectors have been demonstrated that provide nearly all of the characteristics desirable for integrated optoelectronic receivers . 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 microelectronic devices.
2.3. Further commercial and military applications
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 thermography 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 . 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 .
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 , consist of an intermediate flash and, unless suppressed, a brighter secondary flash . 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 . 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 . Developing such miniature cameras based on SiGe integrated CMOS technology will require demonstrating small pixel and format NIR detector arrays that enable wide field-of-views. Producing a practical NIR imager will likewise involve further refining the thermal, mechanical, and optical analyses of encapsulation and optical materials to enable compatibility with NIR FPA manufacturing.
3. SiGe sensor performance modeling
3.1. Performance model overview
This section deals with modeling of SiGe NIR FPA imaging performance over a wide range of light levels that can occur for day-night operation . The model predicts detector dark currents, photocurrents, and readout and background noise associated with a novel small pixel, low-cost SiGe visible-NIR prototype camera. This type of imager, based on the ability to grow NIR-sensitive SiGe layers on silicon to form pixels utilizing existing high quality and low-cost semiconductor and electronic architectures, is intended to provide NIR night vision capability in addition to visible operation.
A fairly large matrix of variables, which include NIR background, pixel size, focal length,
3.2. Variable NIR background
The NIR background radiance between overcast dark night and full daylight varies by about eight orders of magnitude, spanning approximately 0.1 mlux to 25,000 lux. For daytime operation, spectral filtering, aperture reduction, and/or integration time reduction are required to prevent saturation of an FPA. The night radiance over the visible-NIR wavelength range spanning 400-1750 nm can also be quite varied: ~1.0×10-9 W/cm2 for overcast rural settings, ~1.5×10-9 W/cm2 for overcast urban conditions, ~1.2×10-8 W/cm2 for clear night sky rural conditions, and up to ~3.1×10-8 W/cm2 for clear moonlit night skies.
Since the primary source of illumination in the NIR regime is upper atmosphere airglow, the imaging performance NIR cameras typically degrades when used in dark night overcast conditions or under a thick canopy. Moonlight and light pollution that exist in more urban settings can also help to illuminate terrain, but such illumination occurs mostly at shorter wavelengths. This situation is shown in Figure 4 in which the lunar radiance is mainly significant in the 400-1200 nm region, and is particularly evident in Figure 4(c) which shows the effects of city light pollution (where the radiance level is derived from a Toronto, Canada based spectral measurement). These spectral radiances have been modeled in order to determine the electron noise level in NIR FPAs for a given FPA pixel size, spectral band, integration time, and set of optics. This provides the background limited performance (BLIP) conditions to which dark currents and the readout noise must be added.
Basic atmospheric transmittance and path radiance capabilities have been included in the model. The percent cloud cover, which attenuates the airglow and celestial sources as well as the specified solar scattering level, can be taken into account along with aerosol visibility (5 km or 23 km). The transmittance from scene to sensor assumes a horizontal path at the earth’s surface. The attenuation effects of the atmosphere on the images were computed, and subsequently a path radiance based on the ambient NIR background was reinserted into the images. This effectively compensated for the loss of scene brightness and contrast with attenuation and the overall increase in brightness due to path radiance. The images in Figure 5 illustrate the loss of contrast with increasing range.
3.3. Image quality metrics
Figure 6 displays images from a simulated camera to illustrate the effects of resolution and SNR on image quality and potential image identification. For 30 or 60 Hz image sequences, the eye can integrate some of the frames which allows for slightly better identification than is seen in these single images. Motion of an object over the field-of-view also aids in identification, since the eye can compensate for pixilation when viewing a moving object.
3.4. Maximizing the signal
The available signal is determined based on the FPA integration time, quantum efficiency (QE), optics
where the noise level consists of background, read, and dark current noise. The signal as measured by collected electrons
where the transmittance
3.5. Minimizing noise
The NIR camera noise is a combination of the background noise, readout noise (typically consisting of a few electrons to tens of electrons), and dark current noise. In designing the optics and setting the parameters of the FPA, the dark current and readout noise must be maintained below the background noise. The
The readout noise
Likewise, the dark current noise may be expressed as
If the dark current, which is a function of the bias voltage, is further reduced by decreasing the negative bias, uniformity and responsivity may be degraded.
In addition to these basic temporal noises, NIR cameras exhibit spatial noise. Although calibrations usually reduce the spatial noise to levels below that of the temporal noise, spatial noise varieties such as random pattern noise, fixed row and column noise, temporal row and column noise, and frame fluctuation noise all can be observed in the images. The model incorporates all the noise types described in a three-dimensional (3D) noise model, the concept of which is illustrated in Figure 7.
The background SNR formula for specific dependencies is expressed as
while the dark current SNR is given by the formula
The sensor NEI condition is
Typical NEIs are in the 8×108 to 5×109 photon/s-cm2 range and vary with integration time and
The SNR can also be improved by spatially binning pixels, but this is at the expense of sacrificing spatial resolution. This SNR improvement is generally proportional to the square root of the number of binned pixels. Thus, implementing 2×2 binning improves the SNR by a factor of 2. Adding these two phenomena to the previously derived SNR gives:
This expression shows the improvement in SNR with the square root of the product of summed pixels and summed frames for the temporal noise part, and demonstrates the ineffectiveness of summing toward improving the SNR of spatial noise dominated imagery.
3.6. Predicting NEI and SNR
NEI vs. operating temperature for pixel sizes of 30, 20, 10 and 5 µm is shown plotted in Figure 8 using a diffusion expression derived from low dark current density data. For these simulations, the following parameters were employed: integration time of 33 ms, gain of unity, read noise of 10 electrons rms, dark current residual nonuniformity calculated for a temperature delta of 0.1 K, optics with
Minimizing dark current in SiGe detectors, especially for those with smaller pixels, is a driving requirement. Figure 9 shows the SNR vs. dark current density for 7.5 and 12 µm pixels as a function of dark current density. The level lines are the readout and background SNRs and the slanted lines are the dark current SNRs. The background SNR is the best attainable SNR. The intersection of the lines thus signifies the dark current level where the dark current SNR is equal to the readout or background SNR. In Figure 9(a), the yellow lines signify the background or BLIP SNRs for clear skies with no moonlight for the two pixel sizes, with the upper yellow line showing the SNR for 12 µm pixels and 0.89 moonlight conditions. In Figure 9(b), the readout noise SNR (red squares) has been added for both large and small pixels (based on 10 noise electrons per integration).
Dark current appears to be the performance limiting factor for small pixel NIR FPAs operating at RT. The performance can be improved by compensating for the dark currents using lookup tables, though nonuniformity due to uncompensated variance in dark current over the FPA must also be characterized. While utilizing lookup tables should smooth out most of the nonuniformity, there will be residual nonuniformity as a result of the FPA pixels’ dark current to temperature difference ratios at RT in combination with the temperature increment used in the lookup tables. The dark current residual nonuniformity must be kept below the average dark current noise level to preserve the performance, as illustrated in Figure 9.
3.7. SiGe imager performance based on modeling results
Miniature SiGe detector based FPAs that can be incorporated into handheld cameras or inserted into smartphones require
Predictions based on the modeling that has been detailed in this section are summarized as follows: Imaging under rural night sky conditions becomes challenging for small pixel, small optics designs, and dark currents can significantly impact performance in an uncooled NIR camera. A small NIR camera will respond well to minimal amounts of illumination from a direct NIR source, such as one imaged in indoor or shorter-range outdoor environments. In addition, the performance limitations of small uncooled NIR cameras are not found to be problematic for live fire detection and identification applications. Overall, these findings indicate that low-cost, small pixel, uncooled detectors based on growth of SiGe on Si are potentially advantageous for imaging in indoor or low light level outdoor environments.
4. Operation and performance of SiGe photodetectors
A photodetector may be basically defined as a device that converts an optical signal (photons) into an electrical one (electrons). There exist three primary classes of semiconductor based photodetectors: avalanche photodiodes (APDs), metal-semiconductor-metal (MSM) detectors, and p-i-n (
4.1. Avalanche Photodiodes (APDs)
APDs, which are commonly employed in high bitrate optical communication systems, achieve high built-in gain through avalanche multiplication, and require high bias voltages (~20 V/µm) to achieve desired ionization rates and provide detection of low power signals with high sensitivity . Under sufficiently high external bias, the electrical field in an APD’s depletion region causes photogenerated electrons from the absorption layer to undergo a series of impact ionization processes. This enables a single absorbed incoming photon to generate a large number of electron/hole pairs (EHPs), which effectively amplifies the photocurrent and improves the sensitivity, providing a QE potentially greater than unity. SiGe APDs typically have separate absorption-charge-multiplication (SACM) structures (see Figure 10), in which light is absorbed in an intrinsic Ge film and electrons are multiplied in an intrinsic Si film; such structures allow optimization of both QE and multiplication gain .
The most important performance metrics for APDs are ionization ratio (which should be minimized), internal electric field distribution, excess noise factor, gain-bandwidth product, and sensitivity . The device structure of a basic APD is similar to that of a
4.2. Metal-Semiconductor-Metal (MSM) photodetectors
MSM photodetectors comprise two back-to-back Schottky contacts and feature a closely spaced interdigitated metal electrode configuration on top of an active light absorption semiconductor layer . The material, physical, and electrical properties of MSM devices are depicted in Figure 11(a), (b), and (c), respectively. MSM detectors are photoconductive devices not functional under zero bias, and require sufficient external bias for the semiconductor layer to become fully depleted. The Schottky junctions present in MSM detectors exhibit rectified current-voltage (I-V) characteristics as do
Advantages of MSM detectors include low capacitance and consequent low RC delay, which enables high-speed operation. Detection bandwidths for SiGe based MSM devices are comparatively high, making them suitable for fast optical fiber communications. In addition, since MSM detectors are inherently planar and require only a single photolithography step, they are relatively easy to fabricate, boosting their potential for practical integration. However, the external QE and effective responsivity in MSM devices are generally lower than those in
In addition, high dark current associated with SiGe based MSM devices, primarily as a result of hole injection over the Schottky barrier , is a significant problem that raises the noise floor and increases standby power consumption . This dark current may include current associated with thermally generated electron-hole pairs and carrier injection over the Schottky barriers, since SiGe MSM detectors typically have poor Schottky contacts with Ge . While techniques to suppress dark current in MSM devices, such as dopant segregation and utilizing an intermediate layer of amorphous Ge and SiC, have suppressed dark current in detection devices significantly , MSM detectors generally still exhibit higher levels of dark current than comparable SiGe based
As their name may suggest,
The intrinsic/depletion region thickness, which is normally made substantially larger than that of the
There are two main classifications of SiGe based
4.4. Reported performance of SiGe
Due to their comparative ease of fabrication, performance advantages, and prevalence, the focus throughout the remainder of this chapter centers primarily on SiGe based
Compared to NI detectors, WC SiGe
In comparison to SiGe based MSM devices, SiGe
|Responsivity @ 1.55 µm||0.8-1.2 A/W||0.53-0.75 A/W||0.10-0.14 A/W|
|Dark Current Density||85-100 mA/cm2||0.6-2.0 A/cm2||650-1000 A/cm2|
|Dark Current||0.011-0.020 µA||4-10 µA||90-4000 µA|
|Bandwidth||36.5-40.0 GHz||10-25 GHz||1.0-4.3 GHz|
Bandwidths of Ge/SiGe
|Responsivity @ 1.55 µm||0.80 A/W||0.40 A/W||0.17 A/W|
|Dark Current||~10 µA||~50 µA||~100 µA|
|Bandwidth||~35 GHz||~12 GHz||~5 GHz|
|Gain-bandwidth Product||350 GHz||105 GHz||50 GHz|
Reported dark currents and dark current densities in SiGe
5. Design objectives of SiGe pin photodetectors
5.1. Si1-xGex photodetector design parameters
While some early attempts to develop SiGe IR detectors concentrated on potential LWIR applications [62,63], in this chapter we focus solely the development of devices for applications involving detection in the NIR band (up to ~1700 nm). The most straightforward method by which to adjust the cutoff wavelength of a SiGe photodetector in order to tune its range of response is to modify the Si1-xGex alloy composition. Si and Ge have the same type of crystallographic structure and the materials can thus be alloyed with varying Ge concentrations. For Si1-xGex alloys, the lattice constant does not exactly follow Vegard’s law. The relative change of the lattice constant is given by :
The concentration of Ge in a layer of Si1-xGex may be accurately measured using characterization techniques such as X-ray diffraction (XRD). As the Ge concentration is increased, the band gap of the material is reduced, and therefore the cutoff wavelength of a detector will increase (extending its operational wavelength range) assuming all other factors remain constant. However, from a practical device fabrication standpoint, depositing pure Ge or SiGe with very high Ge concentration entails certain technical challenges; for instance, as predicted by Equation (11), a higher Ge concentration of Si1-xGex grown on Si results in a larger lattice mismatch between the materials. This can lead to Stranski-Krastanov growth in which islands form to relieve the misfit strain, which in turn leads to rougher surfaces . However, as will be discussed in the following section, the incorporation of even small amounts of tensile strain can be utilized to extend the operating range of a SiGe photodetector having an absorption layer of a given Ge concentration further into the NIR regime. In addition, modification of parameters such as the doping concentration and growth temperature can be undertaken to further fine-tune the spectral response of a device. Thus, there are multiple factors that more or less influence the operational wavelength range of a SiGe based detector. These must be properly balanced in the process of designing and developing a detector device that exhibits required and optimal performance characteristics for a given application(s).
5.2. Incorporating strain to improve NIR detection
Strains and consequent stresses normally arise during epitaxial growth of thin films on substrates of different compositions and/or crystal structures. Internal strains and stresses can result from a mismatch in the lattice constants of the individual layers, which is illustrated in Figure 14. If the lattice mismatch between two materials is less than ~9%, the initial layers of film will grow pseudomorphically, i.e., the films strain elastically in order to maintain the same interatomic spacing. As the film grows thicker, the increasing strain will create a series of misfit dislocations separated by regions of relatively good fit.
Since the lattice constant of Ge exceeds of that of Si by 4.18%, very thin Si1-xGex (
However, the difference in thermal expansion coefficients between the layers can also play a significant role in the development of strain following epitaxial growth. Since Ge has a larger thermal expansion coefficient than Si, when the temperature cools to RT after growth the consequent reduction in the lattice constant of a deposited Ge/SiGe layer will be suppressed by the Si substrate . This results in the generation of residual tensile strain in the Ge/SiGe layer normally within the range of 0.15-0.30% [9,66]. The changes in band gap energy and absorption that occur with the introduction of strain are depicted in Figure 15(a) and (b), respectively.
The presence of this biaxial tensile stress in Ge causes the valence subbands to split, where the top of the valence band comprises the light hole band. The light hole band energy increases and consequently both the direct and indirect gaps shrink, with the direct gap shrinking more rapidly. Thus, with the increase of tensile strain, Ge transforms from an indirect gap material towards a direct gap material. This stress-induced shift in valence subbands is depicted in Figure 16(a).
Upon application of tensile strain, e.g., of 0.2%, the direct band gap of Ge reduces from 0.80 eV for unstrained material to ~0.77 eV, which effectively increases the corresponding cutoff wavelength from 1550 to 1610 nm [15,65]. As shown in Figure 16(b), this provides greater sensitivity for sensor operation at NIR wavelengths of 1600 nm and above due to the higher absorption coefficient (~5X) and recombination rates of the strained Ge over this range . This extended operational range is very useful for telecommunications, since strained layer SiGe based sensors can operate over most or all of the L band spanning 1560-1620 nm, as well as for other applications requiring detection of longer wavelengths in the NIR regime.
5.3. Reducing dark current
The growth of Ge on Si can be characterized as Stranski-Krastanov growth, an example of which is shown in Figure 17(a). For film thicknesses below the critical thickness, a 2D wetting layer is formed, beyond which a transition to 3D islanding growth mode occurs to relieve the built-in strain in the Ge layers . Defects and threading dislocations arising during Stranski-Krastanov growth typically form recombination centers. At RT, dark current in
Since dark current can be particularly high in SiGe based photodetectors, a major research thrust has been to reduce the dark current to the greatest extent possible in order to enhance sensitivity and boost overall device performance. (It is noted that in SiGe
Various approaches have been proposed to further reduce the dark current in SiGe detectors by several orders of magnitude, including superlattice structures , incorporation of quantum dots , use of buried junctions , and graded compositional layer designs . Dark current generally scales with device area, so reducing the overall size of SiGe detector devices is one means of limiting leakage current for a given photodetector design. For the fabrication of SiGe
6. Fabrication of SiGe
6.1. SiGe detector growth methods
Epitaxial growth of Si/SiGe using gas precursors has been utilized for the past three decades . Selective growth of Ge/SiGe epitaxial films, using mask layers such as SiO2 and Si3N4, generally requires the formation of vertical sidewalls [usually by reactive ion etching (RIE)] to minimize faceting and enhance trench filling . An early method for growing Ge on Si, first proposed by Luryi
In recent years the most prevalent and useful method to deposit Ge/SiGe layers to form functional
6.2. Two-step growth process overview
The two-step growth process commonly used for fabricating NI
The epitaxial growth in this two-step process is usually performed using a variant of the chemical vapor deposition (CVD) method. The most commonly employed variant is ultrahigh vacuum CVD (UHV-CVD) [53,75], in which the operating pressures are high enough to control oxygen background contamination levels. However, SiGe based devices have also been grown using low-pressure CVD (LP-CVD)  more broadly utilized by industry , low-energy plasma-enhanced CVD (LEPE-CVD) , reduced pressure CVD (RP-CVD) , and rapid thermal CVD (RT-CVD) . These CVD based methods enable high control of layer and multi-layer thickness and suitability for future large wafer-scale fabrication. The two-step process is likewise compatible with the molecular beam epitaxy (MBE) method, which has been employed in fewer but still a considerable number of instances [37,45,70,76]. Primary advantages of MBE are allowance of lower thermal budgets  and tight control over doping profiles .
6.3. LT growth
In the first LT (slower growth rate) step of this low/high temperature growth process, fully planar homoepitaxial deposition of a thin Ge/SiGe seed or buffer layer on a Si wafer is performed to ensure smooth surface morphology and to avoid islanding of the film . Si wafers with (100) orientation are associated with lower leakage currents than (001) oriented wafers . The Ge seed layer is deposited on the surface of the substrate, which is often highly doped to facilitate the future requirement of low resistance ohmic contacts. This seed layer is designed to prevent strain release from undesirable 3D island growth, reduce surface roughness, and enhance the migration of threading dislocations (Figure 20) to decrease their proliferation. Buffer/seed layer thicknesses in the range of 30-75 nm are most optimal to withstand the temperature ramp and homoepitaxially grow Ge films with smooth surface morphologies  with reduced threading dislocation densities ; for layers less than 30 nm thick, islanded surfaces have a tendency to form . The first ~0.7 nm (i.e., below the critical thickness) of the buffer layer will be strained due to the 4.18% lattice mismatch between it and the underlying Si substrate, after which a progressive strain relaxation takes place, and a fully strain-relaxed Ge epilayer is produced for growth beyond a few additional nanometers . Therefore, this layer, assuming it is of sufficient thickness, is initially predominately relaxed.
The seed layer growth temperature influences adatom processes on the surface, crystalline growth, surface morphology, abruptness of doping transitions, and relaxation processes . Temperatures employed for seed layer deposition are predominately in the 300-400°C range, and usually from 320-360°C . Depositing seed layers at temperatures below 300°C can lead to crystallographic defect formation, while temperatures above 400°C have been found to produce surface roughening due to increased surface mobility of Ge . At such relatively low growth temperatures, the low surface diffusivity of Ge kinetically suppresses undesired islanding that can otherwise result .
6.4. HT growth
In the subsequent HT step of the growth process, a layer of intrinsic Ge or SiGe serving as the
6.5. HT anneal
Following the LT/HT growth steps, HT
Cyclic annealing for up to 10 cycles compared to a single cycle was found in multiple cases to further reduce the dislocation density by a significant degree . On the other hand, a single anneal cycle can result in lower boron diffusion out from the p+SiGe layer while still maintaining acceptably low dislocation density . High and low cyclic annealing durations are most commonly 10 min; however, a single anneal lasting up to 2 h has been found to be equally effective in certain cases . As the anneal time increases, Ge/Si interdiffusion can become an issue and limit tensile strain . An alternate approach involves a hydrogen ambient, by which annealing at ~800ºC for 30 min can effectively reduce surface roughness and threading dislocation density attributed to enhanced atomic mobility from the annealing .
6.6. Subsequent fabrication steps
Following selective two-step LT/HT growth and annealing, additional processing steps are required in the development of a practical Ge/SiGe
Following deposition of the polysilicon layer, an activation anneal can be performed, which serves to out-diffuse dopant atoms from the polysilicon layer into the underlying Ge/SiGe to form a vertical
7. Practical integration SiGe detectors for imaging arrays
7.1. IR FPA and ROIC technology
Because of the compatibility of Ge growth methods with standard silicon based CMOS processes, photodetectors developed through selective epitaxial growth of Ge/SiGe can be heterogeneously integrated with CMOS circuitry using manufacturing infrastructure already widely installed for the production of BiCMOS and CMOS integrated circuits. In addition, unlike charge-coupled device (CCD) based imagers that require specialized and relatively complicated processing techniques, CMOS based imagers can be built on fabrication lines designed for commercial microprocessors. This has enabled the resolution of CMOS imagers to continue to increase rapidly due to the ongoing transition to finer lithographies as predicted by Moore’s Law. This in turn has led to higher circuit density and levels of integration, better image quality, lower voltages, and lower overall system costs for CMOS devices in comparison with traditional CCD based solutions .
Readout integrated circuits (ROICs) enable a FPA to be fully functional by accumulating photocurrent from each pixel to provide parallel signal processing circuitry for readout. ROIC functions include pixel deselecting, antiblooming on each pixel, subframe imaging, and output preamplification . In monolithically integrated ROICs, both detection of light and signal readout (multiplexing) is performed in the detector material in the spacing between the pixels rather than in an external readout circuit . Advantages of this approach include reduced number of processing steps, increased yields, and reduced costs. Another common architecture for IR FPAs uses a hybrid based approach, in which the individual pixels are directly connected with readout electronics providing for multiplexing . Some benefits of this method are the potential for near 100% fill factor, increased signal processing area, and the ability to optimize the detector and multiplexer independently.
ROICs comprise input cells or unit cells, which in the case of hybrid FPAs consist of the areas located directly under each pixel that are connected to the pixels through indium bumps that bond the aligned FPA and ROIC together . This procedure allows multiplexing the signals from thousands of pixels onto a few output lines. FPAs can utilize either frontside illumination, where photons pass through the ROIC, or backside illumination, where photons pass through a transparent detector array substrate; the latter is often the most advantageous since ROICs typically have areas of metallization and other opaque regions that effectively reduce optical area of the structure . ROICs are processed in standard commercial foundries, and can be custom designed to feature any type of circuit that will fit in the unit cells, though this space is often quite limited. Microlenses deposited above each pixel arrays concentrate incoming light into photosensitive regions, and thus offer a means of further improving sensitivity for devices having relatively low fill factors. A typical indium bonded hybrid architecture FPA utilizing a microlens array is depicted in Figure 22.
7.2. Integration of SiGe technology in CMOS processes
The progressing technological development of low dark current SiGe detector arrays has made possible the fabrication of high density large format SiGe NIR FPAs. The frontside illumination process flow shown sequentially in Figure 23 was developed by DRS Technologies and provides various potential steps for the processes for fabrication and integration of FPA pixels with SOI wafers . In this process, the SOI wafers have a thin, high quality Si layer on top, and a buried oxide layer below. The detector
The first monolithic integration of Ge NIR photodetectors in a CMOS process that produced multichannel, high-speed optical receivers was reported by Masini
In 2010, Ang
7.3. Development of SiGe detector arrays for imaging
IR FPAs have traditionally been based on conventional materials utilized for IR detection including HgCdTe, InSb, InGaAs, and VOx . SiGe FPAs for NIR detection are relatively new to the scene. SiGe based FPAs with associated ROICs can leverage low-voltage, deeply scaled, nanometer class IC processes that enable high yield of low-power, high-component density designs with large dynamic, on-chip digital image processing (for SWaP-efficient sensor designs) and high-speed readouts. A common objective is to produce large format NIR FPAs that are very compact.
In 2010 Vu
In 2014, Chong
8. Optoelectronic properties of Si/Ge based nanostructures
8.1. Quantum confinement and strain in Si/Ge nanostructures
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
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 . 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 . By applying an external tensile strain of around 2.8% to Si-core/Ge-shell NWs, a transformation from direct band gap to indirect one can likewise be achieved .
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 . 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-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.
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 . Compared to Si QDs, Ge QDs have higher absorption coefficients due to localized defect states . 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 . 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 .
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
Installed infrastructure and heterogeneous integration can be leveraged to fabricate small feature CMOS-compatible SiGe based
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