Ge Photodiodes Research Articles

Selective Epitaxy of Germanium on silicon for the fabrication of CMOS compatible short-wavelength infrared photodetectors

Here we present the selective epitaxial growth of Ge on Si using reduced pressure chemical vapor deposition on SiO2/Si solid masks realized on 200 mm Si wafers, aiming at manufacturing integrated visible/short-wavelength infrared photodetectors. By a suitable choice of the reactants and of the process conditions, we demonstrated highly selective and pattern-independent growth of Ge microstructure featuring high crystalline quality. The Ge “patches” show a distinct faceting, with a flat top (001) facet and low energy facets such as e.g. {113} and {103} at their sidewalls, independently on their size. Interdiffusion of Si in to the Ge microcrystals is limited to an extension of ∼20 nm from the heterointerface. The Ge patches resulted to be plastically relaxed with threading dislocation density values better or on par than those observed in continuous two-dimensional Ge/Si epilayer in the low 107 cm−2 range. A residual tensile strain was observed for patches with size >10 μm, due to elastic thermal strain accumulation, as confirmed by μ-Raman spectroscopy and μ-photoluminescence characterization. Polarization-dependent Raman mapping highlights the strain distribution associated to the tridimensional shape. On this material, Ge photodiodes were fabricated and characterized, showing promising optoelectronic performances.

Scalable hot carrier–assisted silicon photodetector array based on ultrathin gold film

Abstract Silicon (Si) offers cost-effective production and convenient on-chip integration for photodetection due to its well-established CMOS technology. However, the indirect bandgap of Si inherently limits its detection efficiency in the near-infrared (NIR) regime. Here, we propose a strategy to achieve high NIR photoresponse in Si by introducing a strong light-absorbing ultrathin gold (Au) film to generate hot carriers. Using a 4.6 nm thick-Au film deposited on Si, we achieved photoresponsivity of 1.6 mA/W at 1310 nm under zero-bias conditions, and rapid temporal responses of 7.5 and 8 μs for rise and fall times, respectively, comparable to germanium (Ge) photodiodes. By utilizing an ultrathin (<6 nm) Au film as the light-detecting layer and thicker (>100 nm) Au film as electrodes, we introduce a unique approach to design a photodiode array based on a single metal (Au) platform. Comparative analysis with a commercial beam profiler image validates the performance of our designed array. This work presents an efficient strategy for manufacturing cost-effective and scalable NIR photodetector arrays, which eliminates the need for additional insulator layers.

Fabrication and characterisation of the PiN Ge photodiode with poly-crystalline Si:P as n-type region

Fabrication and characterisation of the PiN Ge photodiode with poly-crystalline Si:P as n-type region

Ultracompact Microinterferometer-Based Fiber Bragg Grating Interrogator on a Silicon Chip

We report an interferometer-based multiplexed fiber Bragg grating (FBG) interrogator using silicon photonic technology. The photonic-integrated system includes the grating coupler, active and passive interferometers, interferometers, a 12-channel wavelength-division-multiplexing (WDM) filter, and Ge photodiodes, all integrated on a 6x8 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> silicon chip. The system also includes optical and electric interfaces to a printed board, which is connected to a real-time electronic board that actively performs the phase demodulation processing using a multitone mixing (MTM) technique. The device with active demodulation, which uses thermally-based phase shifters, features a noise figure of σ = 0.13 pm at a bandwidth of 700 Hz, which corresponds to a dynamic spectral resolution of 4.9 fm/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> . On the other hand, the passive version of the system, based on a 90º-hybrid coupler, features a noise figure of σ = 2.55 pm at a bandwidth of 10 kHz, also showing successful detection of a 42 kHz signal when setting the bandwidth to 50 kHz. These results demonstrate the advantage of integrated photonics, which allows the integration of several systems with different demodulation schemes in the same chip and guarantees easy scalability to a higher number of ports without increasing the dimensions or the cost.

High Quality Ge Growth on Si (111) and Si (110) by Using Reduced Pressure Chemical Vapor Deposition

Heteroepitaxial growth of Ge on Si has great interest for various optoelectronic applications such as Ge photodiodes(1). However 4.2% of lattice mismatch causes dislocation formation and island growth. High quality Ge(001) growth techniques are reported in ref.(2-4). Moreover, Ge(111) surface is also interesting because of higher carrier mobility(5). Furthermore, Ge(110) is preferred orientation of virtual substrates for epitaxial graphene growth(6). In the case of the Ge deposition on Si(111) and Si(110) substrates, it seems that the process conditions used for Ge growth on Si(001) are not suitable to realize high crystallinity and smooth surface (7). In this paper, we present a method of high quality and smooth Ge layer growth on Si(111) and Si(110), which is the same level as the Ge growth on Si(001).Epitaxial growth of Ge on Si(111) and Si(110) is carried out using a reduced pressure chemical vapor deposition system. After HF last clean, a wafer is baked at 1000°C and cooled down to 600°C in H2 and further to 300-550°C in N2 to form a hydrogen-free Si surface. Then a 100 nm thick Ge layer is deposited as a seed layer using GeH4 with N2 carrier gas. Afterward the wafer is heated up to 450-650°C in H2 and the main part of Ge is deposited using a H2-GeH4 gas mixture. For threading dislocation density (TDD) reduction, annealing at 800°C in H2 is performed for several times (cyclic annealing) by interrupting the Ge growth. Atomic-force microscopy (AFM) is used for surface roughness analysis. Scanning transmission electron microscopy (STEM) and X-ray diffraction (XRD) are used for structural characterization of the Ge layer. Secco defect etching combined with angle view scanning electron microscopy (SEM) or optical microscope is used for TDD evaluation.Figure 1(a,b) summarize the root mean square (RMS) roughness of Ge(111) and Ge(110) seed layers grown at 300-550°C before and after postannealing at 600-800°C. If the growth temperature is lower than 350°C for Ge(111) and 400°C for Ge(110), a significant increase of the surface roughness is observed after postannealing at 700°C and 800°C, respectively. For both crystal orientations, the lowest RMS roughness is observed by depositing at 450°C for as deposited and postannealed samples. The maintained RMS roughness even after postannealing at 800oC may be indicating good crystal quality even at as deposited condition.To confirm the influence of the growth temperature on the crystallinity, cross section TEM images of the Ge(111) and the Ge(110) seed layers deposited at 300°C and 450°C are shown in Fig. 2(a-d). In the case of Ge growth at 300°C (Fig. 2(a,b)), a very high density of stacking faults (SF) and high surface roughness are observed for both crystal orientations. In contrast, by depositing at 450°C (Fig. 2(c,d)), lower SF density in the Ge layer is observed compared to that at 300°C. By postannealing, an improvement of crystallinity is observed for the Ge seed layers deposited at 450°C. However, in the case of 300°C, the crystallinity cannot be improved by the postannealing, because a too high density of dislocations and SF may cause irregular Ge atom migration. As the result, surface roughening occurs.Figure 3(a,b) show AFM surface roughness images after 5 μm-thick Ge(111) and Ge(110) deposited with cyclic annealing at 800°C, respectively. Clear terraces of ~0.3 and ~0.2 nm, whose heights are close to those of Ge(111) bilayer and Ge(110) monolayer, are observed, respectively. RMS roughness of the Ge(111) and the Ge(110) are 0.51 and 0.35 nm, respectively. These RMS roughnesses are comparable to a level reported for Ge (001) in ref.(1).Figure 4 shows TDD of Ge(111) and Ge(110) surfaces as a function of the Ge thickness deposited with cyclic annealing on Si(111) and Si(110) substrates. For both orientations, TDD of ~4×108 cm-2 is obtained for 500 nm-thick samples. With increasing the Ge thickness, the TDD is reduced and levels below TDD of ~5×106 cm-2 are achieved for both Ge (111) and Ge(110) for 5 μm-thick Ge.These methods enable high quality virtual substrate fabrication not only for (001) surfaces but also for (111) and (110) orientation without a chemical mechanical polishing process. References Lischke et al. Nature Photonics15 (2021) 925Yamamoto et al. Solid-State Electron. 60 (2010) 2Yamamoto et al. Semicond. Sci. Technol. 33 (2018) 124007 M. Hartmann et al. J. Appl. Phys. 95 (2004) 5905 H. Lee et al. IEDM Tech. Digest (2009) 09-457J-H. Lee et al. Science 344 6181(2014) 286M. Hartmann et al. J. Cryst. Growth 310 (2008) 5287 Figure 1

Versatile Germanium Photodiodes with 3dB Bandwidths from 110 GHz to 265 GHz

We demonstrate waveguide coupled germanium fin photodiodes for C- and O-band applications. By scaling the germanium (Ge) fin widths record 3dB-bandwidths reaching from 110GHz to 265GHz are achieved. The intrinsic, undoped germanium fins of the photodiodes are sandwiched in between two complementary in situ-doped silicon regions, circumventing ion-implantation into Ge. This reduces the impact of minority carrier diffusion, which is beneficial for the bandwidth [1,2].In silicon photonics, waveguide integrated germanium photodiodes are a key asset. However, until 2020 optoelectrical bandwidth of foundry manufactured germanium photodiodes remained substantially below the 100GHz benchmark, typical values ranging up to approximately 70GHz [3,4,5]. All of these devices rely on ion-implantation into silicon and/or germanium. Discrete devices in InP technology, on the other hand, achieved 170GHz bandwidth and 0.27A/W responsivity already several years ago [6]. Having such performance available in silicon technology has been a major motivation for our work.The most recent germanium fin photodiodes show a 3dB-bandwidths of 240GHz (with internal responsivity of 0.45A/W) and 265GHz (with internal responsivity of 0.3A/W) both at 1550nm wavelength and 1mA photocurrent at 2V reverse bias [2]. These devices have been fabricated in IHP’s BiCMOS pilot line on 200mm silicon-on-insulator (SOI) wafers. These novel photodiodes do not rely on ion-implantation, but instead the undoped germanium is contacted by in situ-doped silicon offshoot, thus minimizing minority carrier diffusion effects. This novel technology approach allowed us to match or even surpass the state-of-the-art of III-V devices. By scaling the width of the germanium region, different combinations of bandwidths and responsivities can be realized. Certainly, broader fins will yield higher responsivities but on the expense of 3dB-bandwidths.In this paper, we present for the first time the responsivity and bandwidth at 1310nm and 1550nm, as well as dark current and capacitance of the high-speed germanium fin photodiodes. Several aspects important for high performance photodiodes will be discussed:(1) Optoelectrical 3dB-bandwidths and responsivities for various germanium fin widths.(2) Improved power handling capability of the photodiode at 1310nm and 1550nm.(3) Temperature dependent electrical behaviour for photodiodes with various germanium fin widths.Figure 1 shows STEM cross-sections (cut perpendicular to the light-incidence direction) of the 70GHz Ge photodiode integrated at IHP EPIC platform (left) and a novel germanium-fin photodiode with 3dB-bandwidth of 265GHz (right). Two major changes, the transition from Ge-stripe to germanium fin and from ion-implantation to in situ-doped silicon offshoot, eventually lead to record 3dB-bandwidths.[1] Lischke, S. et al., "Ge photodiode with −3-dB OE bandwidth of 110 GHz for PIC and ePIC platforms", In Proc. 2020 IEEE International Electron Devices Meeting (IEDM) 7.3.1–7.3.4, 2020.[2] Lischke, S., et al., "Ultra-fast germanium photodiode with 3-dB bandwidth of 265 GHz", Nat. Photon. 15, 925–931, 2021.[3] Chen, H., et al., " −1-V bias 67-GHz bandwidth Si-contacted germanium waveguide p-i-n photodetector for optical links at 56 Gbps and beyond", Opt. Express 24, 4622–4631, 2016.[4] Boeuf, F., et al., "A silicon photonics technology for 400-Gbit/s applications", In Proc. 2019 IEEE International Electron Devices Meeting (IEDM) 33.1.1–33.1.4, 2019.[5] Lischke, S., et al., "High bandwidth, high responsivity waveguide-coupled germanium p-i-n photodiode", Optics express, vol. 23, no. 21, pp. 27213–27220, 2015.[6] Rouvalis, E., et al., "170-GHz uni-traveling carrier photodiodes for InP-based photonic integrated circuits", Opt. Express 20, 20090–20095, 2012. Figure 1

Silicon Photonics FMCW LiDAR Chip With a Slow-Light Grating Beam Scanner

Photonic crystal slow-light gratings fabricated using Si photonics enable high-speed, high-resolution, and wide field-of-view two-dimensional beam scanning via the thermo-optic effect. In this paper, we built a frequency-modulated continuous-wave light detection and ranging system on a chip by combining a beam scanner with Ge photodiodes for delay homodyne coherent detection. Emitting and scanning frequency-swept laser beam, point cloud images of 154 × 32 = 4928 points were obtained. The real-time operation and velocity imaging were also demonstrated. This device is expected to detect Lambertian targets over long distances in the 100-m class by reasonably reducing chip and optics losses and suppressing internal noise components.

Toward Super Temporal Resolution by Suppression of Mixing Effects of Electrons

The theoretical temporal resolution limit of silicon (Si) image sensors is 11.1 ps. The super temporal resolution (STR) is defined as a frame interval less than this limit. The temporal resolution of burst image sensors is significantly affected by “mixing” of signal electrons traveling from different starting positions. For example, the penetration depths of photons incident to a surface of a photodiode (PD) disperse exponentially, resulting in mixing of the photoelectrons from different depths at the other end, which causes a distribution of the arrival times. The temporal resolution is proportional to the standard deviation of the arrival time. Toward STR imaging, this article proposes measures to suppress various mixing phenomena during the travel of the signal electrons: 1) a germanium (Ge) PD for visible light to practically eliminate the effect of vertical mixing caused by the distribution of the penetration depth; 2) an inverted pyramid PD to efficiently suppress the horizontal mixing, keeping a 100% fill factor; 3) a standard column PD with negative potential on the vertical walls to squeeze the trajectory bundle of electrons falling on the guide gate on the front side; and 4) a resistive guide gate to linearize the potential profile to maximize the horizontal drift velocity to minimize the mixing together with the squeezed electron bundle.

A new 3-dB bandwidth record of Ge photodiode on Si

A new 3-dB bandwidth record of Ge photodiode on Si

Single-Event Transients in a Commercially Available, Integrated Germanium Photodiode for Silicon Photonic Systems

Ion-induced SETs were captured from a germanium photodiode (PD) in an integrated silicon photonics technology platform. Statistics from 400 ion-strike events were extracted for various optical powers and LETs incident on the PD. The results show that the transient peak decreases with increasing optical power. TCAD simulation results indicate that photo-generated carriers decrease the built-in electric field of the PD, thereby decreasing the transient peak. The photo-generated carriers also increase the probability of Auger recombination, which further reduces the transient peak under illuminated conditions, but this effect is small and does not contribute significantly to the transient peak reduction. Finally, a short case study with an ON–OFF keying modulation scheme is presented to show how these transients have the potential to corrupt bits in realistic communication systems. Furthermore, it is also shown that hardening techniques can be utilized to mitigate bit errors in communication systems that use integrated Ge PDs.

Si photonic crystal optical antenna serial array and frequency-modulated continuous-wave light detection and ranging action

Photonic crystal waveguide slow-light grating emits a free-space optical beam and steers it widely by changing the optical wavelength or waveguide refractive index. In the reverse process, returned light is coupled into the device again. We have proposed to use this optical transmission and reception antenna as a beam scanner for light detection and ranging (LiDAR). Ideally, a large-aperture antenna can narrow the transmission beam and enhance the reception efficiency. Actually, however, the transmission and reception performance is not scalable owing to waveguide loss even though the waveguide is simply lengthened. A serial array configuration in which the waveguide is divided into multiple antennas is effective for mitigating this problem. In this study, we fabricated such a device using Si photonics technology and obtained a small beam divergence of 0.02° at a telecom wavelength. Then, we observed the ranging operation by adding an optical setup of frequency-modulated continuous-wave (FMCW) LiDAR and confirmed that the divided antenna device improved the reception intensity by 12 dB. Moreover, we fabricated a FMCW LiDAR chip in which the serial array antennas were integrated in parallel with switch trees and Ge photodiodes and obtained point cloud images by two-dimensional beam scanning.

On-chip optical interconnection using integrated germanium light emitters and photodetectors.

Germanium (Ge) is an attractive material for monolithic light sources and photodetectors, but it is not easy to integrate Ge light sources and photodetectors because their optimum device structures differ. In this study, we developed a monolithically integrated Ge light emitting diode (LED) that enables current injection at high density and a Ge photodiode (PD) having low dark current, and we fabricated an on-chip optical interconnection system consisting of the Ge LED, Ge PD, and Si waveguide. We investigated the properties of the fabricated Ge LED and PD and demonstrated on-chip optical interconnection.

Investigation of the Possibility to Use Ge p-i-n Photodiodes in Infrared SPR Sensors

Considered in this work are the possibility and advantages of applying a Ge photodiode to measure the surface plasmon resonance (SPR) in the near infrared range (IR). The investigations were performed using the prototype of IR SPR refractometer that operates with two fixed wavelengths (1310 and 1552 nm) for excitation of surface plasmons in a thin gold film in the Kretschmann optical scheme. The obtained results of experiments enabled us to draw the conclusion that application of the Ge photodiode in the SPR sensor at the wavelength 1.5 µm allows increasing the sensitivity and response magnitude as well as widening the dynamic range of this sensor device.

Optoelectronic chromatic dispersion and wavelength monitoring in a photodiode.

The optoelectronic process of light absorption and current formation in photodiodes is shown to be a significant source of optoelectronic chromatic dispersion (OED). Simple design rules are developed for fabricating a photodiode-based dispersion device that possesses large, small, zero, and either positive or negative OED. The OED parameter is proportional to a spectrally-dependent absorption term α-1dα/dλ . Silicon-based devices are predicted to display significant OED throughout the near IR, while Ge and InGaAs have high OED in the C- and L-bands and 1650 nm region, respectively. The OED of a commercial Ge PN photodiode is measured to be 3460 ps/nm at 1560 nm wavelength with 500 kHz modulation, demonstrating 8pm spectral resolution with the phase-shift technique. Temperature-tuning of the OED in the Ge photodiode is also demonstrated. The ubiquitous photodiode is a tunable OED device, with applications in high-resolution optical spectroscopy and optical sensing.

Gourd-shaped hole array germanium (Ge)-on-insulator photodiodes with improved responsivity and specific detectivity at 1,550 nm.

Gourd-shaped hole array germanium (Ge) vertical p-i-n photodiodes were designed and demonstrated on a germanium-on-insulator (GOI) substrate with the excellent responsivity of 0.74 A/W and specific detectivity of 3.1 × 1010 cm·Hz1/2/W. It is calculated that the gourd-shaped hole design provides a higher optical absorption compared to a cylinder-shaped hole design. As a result, the external quantum efficiency for the gourd-shaped hole array photodetector was enhanced by ∼2.5× at 1,550 nm, comparing with hole-free array photodetectors. In addition, the extracted specific detectivity is superior to that of commercial bulk Ge photodiodes. The 3-dB bandwidth for the hole array photodetectors is improved by ∼10% due to a lower device capacitance. This work paves the way for low-cost and high-performance CMOS compatible photodetectors for Si-based photonic-integrated circuits.

80 GHz germanium waveguide photodiode enabled by parasitic parameter engineering

A high-speed germanium (Ge) waveguide photodiode (PD) is one of the key components of an integrated silicon photonics platform for large-capacity data communication applications, but the parasitic parameters limit the increase of its bandwidth. Several studies have been reported to reduce parasitic parameters, at the cost of compromising other performances. Here, we propose and investigate a bandwidth-boosting technique by comprehensively engineering the parasitic parameters. Experimentally, a bandwidth up to 80 GHz is realized for vertical positive-intrinsic-negative (PIN) Ge PDs without decreasing the responsivity and dark current, indicating that parasitic parameter engineering is a promising method to promote high-speed performance of Ge PDs.

Low-power and high-detectivity Ge photodiodes by in-situ heavy As doping during Ge-on-Si seed layer growth.

Germanium (Ge)-based photodetectors have become one of the mainstream components in photonic-integrated circuits (PICs). Many emerging PIC applications require the photodetectors to have high detectivity and low power consumption. Herein, we demonstrate high-detectivity Ge vertical p-i-n photodiodes on an in-situ heavily arsenic (As)-doped Ge-on-Si platform. The As doping was incorporated during the initial Ge-on-Si seed layer growth. The grown film exhibits an insignificant up-diffusion of the As dopants. The design results in a ∼45× reduction on the dark current and consequently a ∼5× enhancement on the specific detectivity (D*) at low reverse bias. The improvements are mainly attributed to the improved epi-Ge crystal quality and the narrowing of the device junction depletion width. Furthermore, a significant deviation on the AsH3 flow finds a negligible effect on the D* enhancement. This unconventional but low-cost approach provides an alternative solution for future high-detectivity and low-power photodiodes in PICs. This method can be extended to the use of other n-type dopants (e.g., phosphorus (P) and antimony (Sb)) as well as to the design of other types of photodiodes (e.g., waveguide-integrated).

Threading Dislocation Reduction of Ge by Introducing a SiGe/Ge Superlattice

Ge growth on Si is essential e.g. for photonics device integration (e.g. Ge photodiode) into Si based CMOS platform (1). However, due to 4.2% of lattice mismatch between Ge and Si, misfit dislocations (MD) and threading dislocations (TD) are introduced for strain relaxation after exceeding critical thickness. In order to realize low-darkcurrent Ge photodiode, improvement of crystallinity is important. High quality Ge growth techniques by combining various annealing techniques (at 800~850oC) are reported (2-3). However, because of low melting point of Ge (937oC), the Ge growth process is typically integrated at the last part of front-end process. Therefore, these annealing processes are affecting device parameters of existing devices. In previous paper, we reported Ge/SiGe superlattice (SL) fabrication and found improvement of TD density (TDD) by increasing Si composition of SiGe in the SL (4). Here we report TDD improvement of Ge by introducing the SL without annealing.Ge growth with SiGe/Ge SL is carried out using reduced pressure chemical vapor deposition system. Si(100) wafers are used. After an HF last cleaning, the wafer is loaded into reactor and baked at 1000oC and cooled down to 600oC in H2 and further cooled down to 350oC in N2. After that 100 nm-thick seed Ge is deposited using N2-GeH4 gas. Then the wafer is heated up to 500oC in H2 and Ge/SiGe superlattice is fabricated using H2-GeH4 and H2-SiH4-GeH4 gases, respectively. Furthermore, 2.8 µm thick Ge is deposited at 550oC using H2-GeH4 gas.Thickness is measured by cross section SEM and spectroscopic ellipsometry. TDD is determined by Secco defect etching technique. Strain and relaxation of Ge and SiGe layers are measured by XRD reciprocal space mapping. Distribution of the MD and the TD are measured by cross section TEM.In the case of direct Ge growth on Si (Fig. 1a), MDs are formed at interface between Ge and Si. TDs are spreading into the Ge layer. Some of the TDs are long and extending to surface. These MD and TD formations are due to the lattice mismatch between Ge and Si. On the other hand, in the case of Ge sample grown on ×20 Si0.3Ge0.7 / Ge SL (Fig. 1b), most of MDs are formed at interface between 100 nm-thick Ge buffer and Si substrate. Additionally high density of TDs are observed in the Ge buffer and the SL layer. In the SL layer, lower TDD is observed in the upper part compared to that in the deeper part. At the interface between upper 2.8 µm thick Ge and the SL layer, MD and high density of TDs toward lateral direction is observed indicating relaxation. TDD in the 2.8 µm thick Ge layer seems to be lower compared to that in Fig. 1a).In the case of direct Ge growth on Si, TDD of ~7.5×108 cm-2 is obtained (Fig. 2). The TDD is comparable to previously reported results (2). By depositing 2.8 µm thick Ge on Si0.2Ge0.8 SL, TDD is decreased to ~5.4×108 cm-2. With decreasing Ge concentration of SiGe layer of the SL, decrease of TDD is observed until 40%. However, by replacing SiGe layers to mono-layers of Si, increased TDD is observed, which might be due to too high strain at interface between Si and SiGe.Figure 3 shows TDD of 2.8 µm thick Ge deposited on Si0.2Ge0.8/Ge SL with different Si0.2Ge0.8 and Ge thickness combination without changing periodicity of the SL. Lower TDD is observed when the 2.8 µm thick Ge is deposited on the SL with higher Si0.2Ge0.8 thickness. Possible reason is higher degree of relaxation of the SL layer.In Fig. 4, TDD of 2.8 µm thick Ge deposited on SLs of various lattice parameters are shown. By reducing lateral lattice parameter of the SiGe/Ge SL by increasing SiGe thickness or Si concentration, TDD decreases. The reduction of the TDD is fitting to one line for both cases. These results indicate that higher plastic relaxation of the SL occurs on the Ge buffer due to increased compressive strain of SiGe in the SL, and the TDD reduction of the Ge on the SL may be caused by similar mechanism of reverse graded buffer. By introducing SiGe/Ge SL, multiple effect of the reverse graded buffer may be expected. Reference (1) S. Lischke et al. Proc. IEEE Bipolar/BiCMOS Circuits and Technology Meeting (BCTM) , (2014) 29(2) Y. Yamamoto et al Solid-State Electron. 60 (2011) 2(3) Y. Yamamoto et al. Semicond. Sci. Technol. 33 (2018) 124007(4) Y. Yamamoto et al. Jpn. J. Appl. Phys. 59, SGGK10 (2020) Figure 1

(Invited) Directly Silicon Nitride Waveguide Coupled Ge Photodiode for Non-SOI PIC and Epic Platforms

We demonstrate a Ge photodiode that is directly coupled to a silicon nitride waveguide and shows more than 67 GHz bandwidth. This device paves the way for utterly new SiN waveguide platform based applications. By light feeding through SiN waveguides, the new photodiode can also be a key enabler for a bulk-Si based, monolithically integrated electronic-photonic integrated circuit platform.Silicon nitride (SiN) waveguides (WGs) outperform silicon-on-insulator (SOI) based waveguides in various aspects: they offer an enhanced optical bandwidth, allowing for low loss waveguides in the classic communication bands (C- and O- bands), widely addressed by SOI based photonics, as well as in the visible wavelength range, were Si WGs have already become opaque. Moreover, due to the absence of two-photon absorption in SiN, the optical power handling is superior compared to Si WGs [1–3]. Various platforms with outstanding optical loss performance have been demonstrated over the last years and are meanwhile offered for commercial purposes, as summarized in [1]. However, despite all these advantages, so far, commercially offered SiN based photonic integrated circuit (PIC) platforms neither provide monolithically integrated high speed modulators, nor fast SiN WG coupled detectors. In this work we focus on the detector side of a high-speed SiN PIC platform.Communication applications for upcoming 400 Gbps standards require 56 Gbd symbol rates, which makes photodiodes (PDs) with more than 40 GHz opto-electrical (OE) bandwidth indispensable. Previous works with focus on Ge PDs in SiN platforms have either shown devices directly SiN WG fed or indirectly coupled via multi-layers (SiN on SOI WGs) where the Ge PDs are actually still located on SOI. However, the former ones showed only inferior bandwidth of less than 10 GHz [4] while the latter ones provide at least a bandwidth of about 30 GHz [5]. Here, we demonstrate for the first time a bulk-Si based, directly SiN WG coupled Ge PD with very high OE -3 dB bandwidth of more than 67 GHz which opens new perspectives for opto-electronic platform technologies:(1) The combination of truly high-speed PDs with SiN waveguides could be the start for a novel active SiN platform e.g. aiming at integrated coherent receivers at wavelengths far below the O-band.(2) The demonstrated devices are in principle compatible for co-integration with IHP’s photonic BiCMOS process [6], which opens various opportunities for novel sensing or spectroscopic applications as well.(3) This work could pave the way towards a non-SOI based technology as an alternative to established SOI based PIC or electronic-photonic integrated circuit (ePIC) platforms, e.g. IHP’s photonic BiCMOS technology. Circuit fabrication on pure bulk-Si wafers would significantly relieve process complexity, time and thus costs.We show that the new devices, fabricated on bulk-Si, provide the same bandwidths as Si waveguide coupled SOI based reference Ge photodiodes. However, their O-band responsivity is 0.3 A/W, which is about three times lower compared to the SOI waveguide coupled devices. We attribute this effect to substrate losses and few specific layout features but see some potential for improvement by design and technological optimizations. We further demonstrate that the diodes can be fabricated with high yield and low metrics tolerances. Figure 1 shows SEM cross-sections (perpendicular to the light incidence direction) of Ge photodiodes fabricated on SOI waveguide (left) as well as on bulk-Si (right).[1] P. Muñoz et al., “Silicon Nitride Photonic Integration Platforms for Visible, Near-Infrared and Mid-Infrared Applications,” Sensors, vol. 17, no. 9, 2017.[2] A. Rahim et al., “Expanding the Silicon Photonics Portfolio With Silicon Nitride Photonic Integrated Circuits,” J. Lightwave Technol., vol. 35, no. 4, pp. 639–649, 2017.[3] K. Ikeda et al, “Thermal and Kerr nonlinear properties of plasma-deposited silicon nitride/ silicon dioxide waveguides,” Optics express, vol. 16, no. 17, pp. 12987–12994, 2008.[4] D. Ahn et al., “High performance, waveguide integrated Ge photodetectors,” Optics express, vol. 15, no. 7, pp. 3916–3921, 2007.[5] W. D. Sacher et al., “Monolithically Integrated Multilayer Silicon Nitride-on-Silicon Waveguide Platforms for 3-D Photonic Circuits and Devices,” Proc. IEEE, vol. 106, no. 12, pp. 2232–2245, 2018.[6] D. Knoll et al. “(Invited) SiGe BiCMOS for Optoelectronics,” ECS Transactions, vol. 75, no. 8, pp. 121–139, 2016. Figure 1

(Invited) Monolithic Ge Integration on the 3 μm SOI Platform for 40 GHz Photodiodes

Silicon photonics offers the possibility to integrate a large number of photonic components on a small Si chip in a similar fashion as microelectronic circuits have been realized for many decades. This enables low power consumption, small footprint and low-cost mass production. Application areas for Si photonics reach from optical communication and computing to optical sensing and imaging. Monolithic germanium PIN detectors are one of the key components in photonic integrated circuits (PICs) and convert light into electrical signals. Germanium is the preferred material for photodetectors in silicon photonics as it absorbs light in the datacom and telecom wavelength range (1.2-1.7 μm) due to its small bandgap. Germanium is also tolerated in the IC-clean manufacturing lines, unlike many other potential semiconductor materials. Electron and hole mobilities in germanium are high, enabling high-bandwidth detectors. Challenges in Ge integration on silicon-on-insulator (SOI) wafers include unintentional oxidization and etching in many of the standard silicon technology processing steps, fast diffusion of n-type dopants in Ge and relatively large lattice mismatch between Ge and Si.High refractive index contrast between silicon (3.5) and silicon dioxide (1.5) is the key to achieve ultra-dense photonics integration. Most of the silicon photonics research is carried out using submicron-thick SOI waveguides and latest microelectronics fabrication technologies that involve (and require) very small wafer topography. In this paper, we present Ge photodiode integration on a 3 µm SOI platform with 3 µm thick Si waveguides. Those offer ultra-low propagation losses (~0.1 dB/cm), ultra-dense integration (μm-scale bends), small polarization dependency (down-to-zero birefringence) and ability to tolerate relatively high optical powers (>1W). One major limitation on the 3 µm SOI platform has been the lack of as fast modulators and photodetectors as those integrated on sub-µm SOI platforms. In this work, we monolithically integrated a fast horizontal PIN photodiode on the 3 µm SOI platform. The 3 dB cutoff frequency was 40 GHz and the responsivity was 1.0 A/W at -1 V bias and 1.55 µm wavelength. This high bandwidth indicates that the detector is limited by the transit time of the carriers over the i-region, rather than the junction capacitance. The electric field in the i-region at -1 V is sufficient to keep the carrier drift speed close to the maximum carriers velocity in Ge.The design of the 40 GHz PD included analytical calculations and numerical simulations. Physical dimensions of the detector were chosen so that they are compatible with the 3 µm SOI process flow and meet the bandwidth and responsivity targets. Germanium was selectively grown into cavities in the 3 µm SOI layer, leading to very low amount of stress-induced crystal defects. The Ge waveguide detector and the Si waveguides were patterned with a common hard mask to achieve self-alignment between them. The n- and p-contacts were directly made into Ge using Ti/Al metallization. The vertical sidewalls of the Ge detector were implanted in order to create a horizontal PIN structure. Due to high confinement of light into the 3 µm thick SOI and Ge waveguides, the 3 µm thick and 1 µm wide Ge detector could be made as short as 9 µm. This led to small junction capacitances. The electrical output pulse shape was not distorted by the slow diffusion current of electrons and holes because the incoming light does not enter the doped regions. The development of 40 GHz PDs was an important step in improving the feasibility of the 3 µm SOI platform in high-bandwidth applications. Figure 1