Ge Photodiodes Research Articles

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

A Ge photodiode, directly coupled to a silicon nitride waveguide, showing more than 67 GHz bandwidth is demonstrated. This 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. 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 wave-guide coupled devices. We attribute this effect mainly to substrate leakage, being confirmed by simulations. We demonstrate that bulk-Si based diodes can be fabricated with high yield and low metrics tolerances.

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

Ge photodiodes were integrated on the 3 µm SOI platform, where the 3 µm thick Si waveguides offer ultra-low propagation losses (~0.1 dB/cm), ultra-dense integration (μm-scale bends), small polarization dependency (down-to-zero birefringence) and the ability to tolerate relatively high optical powers (>1W). A horizontal Ge PIN photodiode was monolithically integrated on the 3 µm SOI platform. It reached a 3 dB cutoff frequency of 40 GHz and 1.0 A/W responsivity at -1 V bias and 1.55 µm wavelength. Germanium was selectively grown into cavities in the 3 µm SOI layer and the Ge waveguide detector was patterned with the same hard mask with the Si waveguides to achieve waveguide self-alignment. The development of 40 GHz PDs was an important step in improving the feasibility of the 3 µm SOI platform in high-bandwidth applications.

Development of photodiode via the rapid melt growth (RMG) materials for energy conversion device

<span>Germanium (Ge) photodiodes were fabricated with the new RMG crucible materials that were established in this study. Results show that Ge large square patterns with size of 208 µm x 208 µm were unable to be achieved if ion implantation process was used in formation of photodiode. Delamination can be observed on all test samples during polycrystalline silicon (poly-Si) deposition at 620 <sup>o</sup>C. This result was in contrast to a previous intrinsic Ge test structure, where good formation of squares with size similar to that 208 µm x 208 µm had been successfully attained even with high annealing temperature above 938 <sup>o</sup>C. This indicates that doping through ion implantation has affected Ge film and caused delamination even at low temperature. However, good formations of Ge stripes were attained along with the ion implantation process in fabricating the photodiode. Results show that the sheet resistance of Ge stripe has significantly decreased compared to previous Ge resistors. The better resistance is due to the thicker (500 nm) Ge layer. In the case of Ge stripes with a p-i-n junction, only small fraction of test samples have shown a diode characteristic but with high leakage current. Results of I-V measurement show that a large fraction of test samples produced resistor characteristics. The high leakage current is believed due to the fact that the Ge films formed at this stage were polycrystalline in structure. Thus the grain size of Ge stripe has affected the performance. Slight changes on the I-V characteristic of single Ge can be observed when samples were tested under illumination. Therefore, it has potential to be used for future development of energy conversion device. </span>

4-channel 200 Gb/s WDM O-band silicon photonic transceiver sub-assembly.

We demonstrate a 200G capable WDM O-band optical transceiver comprising a 4-element array of Silicon Photonics ring modulators (RM) and Ge photodiodes (PD) co-packaged with a SiGe BiCMOS integrated driver and a SiGe transimpedance amplifier (TIA) chip. A 4×50 Gb/s data modulation experiment revealed an average extinction ratio (ER) of 3.17 dB, with the transmitter exhibiting a total energy efficiency of 2 pJ/bit. Data reception has been experimentally validated at 50 Gb/s per lane, achieving an interpolated 10E-12 bit error rate (BER) for an input optical modulation amplitude (OMA) of -9.5 dBm and a power efficiency of 2.2 pJ/bit, yielding a total power efficiency of 4.2 pJ/bit for the transceiver, including heater tuning requirements. This electro-optic subassembly provides the highest aggregate data-rate among O-band RM-based silicon photonic transceiver implementations, highlighting its potential for next generation WDM Ethernet transceivers.

Group IV Heteroepitaxy for Advanced Electronic Devices Integrated in BiCMOS Technology

Epitaxy is a key process for group IV semiconductor device fabrication. For precisely-controlled fabrication of steep profiles for high performance devices, lowering the thermal budget of the epitaxy process is getting increasingly important. In this paper, we review group IV heteroepitaxy processes used at IHP and their application. In particular, we present SiGe HBT devices using low-temperature Si2H6 based epitaxy process and discuss potential advantages compared to standard SiH4 based process. Additionally, we show a growth technique for high-quality Ge photodiodes with several cycles of annealing by interrupting the Ge growth (cyclic annealing) followed by HCl etching. Furthermore, we discuss the device performance of SiGe/Si multi-quantum well (MQW) microbolometers and a technique to improve the quality of SiGe/Si MQW structures by introducing C delta layer at interface.

A 106-Gb/s PAM-4 Silicon Optical Receiver

We present a 106-Gb/s four-level pulse-amplitude modulation (PAM-4) silicon optical receiver consisting of a low-noise fully differential transimpedance amplifier (TIA) wirebonded to a high-speed silicon photonic Ge photodiode (PD). A low-noise operation is achieved through a fully differential TIA architecture, combined with active on-chip biasing for the PD. To the best of our knowledge, this is the first integrated low-power optical receiver to report (sub-)KP4-FEC bit error ratios up to 53 GBd PAM-4 without any digital signal processing or equalization to compensate the receiver. An optical modulation amplitude sensitivity of −5 dBm at 53 GBd is obtained, while consuming 160 mW (1.51 pJ/b at 53 GBd). The receiver achieves the best-reported sensitivities from 32- up to 53-GBd PAM-4, at the lowest reported power consumption.

90-Gb/s NRZ Optical Receiver in Silicon Using a Fully Differential Transimpedance Amplifier

We present the design and implementation of a 90 -Gb/s non-return-to-zero (NRZ) direct detection optical receiver that consists of a low-noise transimpedance amplifier (TIA), fabricated in a 55-nm SiGe BiCMOS technology, and a Ge photodiode integrated into a Silicon Photonic integrated circuit. Low-noise broadband operation is achieved using a fully differential TIA that provides the photodiode reverse bias. 50-, 56-, and 64-Gb/s NRZ operation is demonstrated at bit-error ratios (BERs) below $10^{-12}$ at sensitivities of $-$ 12.3-, $-$ 11.2-, and $-$ 11.1-dBm optical modulation amplitude (OMA) while consuming 128, 141, and 157 mW, respectively. Furthermore, a BER below $10^{-12}$ is achieved at 80 Gb/s with a sensitivity of $-$ 6.1 dBm (OMA) at 165 mW, and operation below KP4-FEC $({2.4 \times {10}^{-4}})$ is demonstrated up to 90 Gb/s with a sensitivity of $-$ 7.1 dBm (OMA) while consuming 222 mW. All BERs were measured in real time and without DSP or equalization to compensate the receiver. The presented receiver achieves the best reported sensitivities up to 90 Gb/s, at the second-lowest power consumption, and is the first reported integrated optical RX up to 90 Gb/s that is tested in real time without any equalization or DSP to compensate the receiver. The presented optical receiver is ideally suited for upcoming short-reach applications.

Silicon photonics traveling wave photodiode with integrated star coupler for high-linearity mm-wave applications.

Next-generation wireless communication will require increasingly faster data links. To achieve those higher data rates, the shift towards mmWave frequencies and smaller cell sizes will play a major role. Radio-over-Fiber has been proposed as a possible architecture to allow for this shift but is nowadays typically implemented digitally, as CPRI (Common Public Radio Interface). Centralization will be important to keep next-generation architectures cost-effective and therefore shared optical amplification at the central office could be preferable. Unfortunately, limited power handling capabilities of photodetectors still hinder the shift towards centralized optical amplification. Traveling wave photodetectors (TWPDs) have been devised to allow for high-linearity, high-speed opto-electronic conversion. In this paper, an architecture is discussed consisting of such a TWPD implemented on the iSiPP25G silicon photonics platform. A monolithically integrated star coupler is added in the design to provide compact power distribution while preserving the high linearity of the TWPD. The traveling wave structure (using 16 photodetectors) has a measured 3 dB bandwidth of 27.5 GHz and a fairly flat S21 up to 50 GHz (less than 1 dB extra loss). Furthermore, the output referred third-order intercept point at 28 GHz, is improved from -1.79 dBm for a single Ge photodiode to 20.98 dBm by adopting the traveling wave design.

Uniformity measurements of large-area indium gallium arsenide and germanium photodetectors

The properties of six large area semiconductor photodetectors were investigated in the near infrared wavelength range. For potential use as transfer standard detectors in absolute spectral responsivity calibrations, the spatial uniformity and spectral responsivity of four InGaAs and two Ge photodiodes were characterized. Spatial uniformity measurements carried out at 1000 nm, 1550 nm, and 1650 nm show that photodiode spatial non-uniformity changes with wavelength for both InGaAs and Ge detectors. The photodiode characterization apparatus, results, and analysis are presented.

Enhanced Performance of Ge Photodiodes via Monolithic Antireflection Texturing and α-Ge Self-Passivation by Inverse Metal-Assisted Chemical Etching.

Surface antireflection micro and nanostructures, normally formed by conventional reactive ion etching, offer advantages in photovoltaic and optoelectronic applications, including wider spectral wavelength ranges and acceptance angles. One challenge in incorporating these structures into devices is that optimal optical properties do not always translate into electrical performance due to surface damage, which significantly increases surface recombination. Here, we present a simple approach for fabricating antireflection structures, with self-passivated amorphous Ge (α-Ge) surfaces, on single crystalline Ge (c-Ge) surface using the inverse metal-assisted chemical etching technology (I-MacEtch). Vertical Schottky Ge photodiodes fabricated with surface structures involving arrays of pyramids or periodic nano-indentations show clear improvements not only in responsivity, due to enhanced optical absorption, but also in dark current. The dark current reduction is attributed to the Schottky barrier height increase and self-passivation effect of the i-MacEtch induced α-Ge layer formed on top of the c-Ge surface. The results demonstrated in this work show that MacEtch can be a viable technology for advanced light trapping and surface engineering in Ge and other semiconductor based optoelectronic devices.

Gold-Nanoparticle-Coated Ge MIS Photodiodes

Ge photodiodes with ultra-high responsivities have been demonstrated with gold nanoparticle assistance. The responsivity can reach a value of 37.7 A/W, which corresponds to a gain of 76. Such a high responsivity originates from the amassment of photo-generated holes in Ge under the boundary of gold nanoparticles, which reduces the barrier for electron tunneling from metal to the hole-amassment Ge ring areas. The depletion layer thinning due to hole amassment is not only proved by band diagram simulation but also confirmed by capacitance measurement.

Spectral responsivity measurement of photovoltaic detectors by comparison with a pyroelectric detector on individual nano-second laser pulses

We demonstrate that the individual single pulse of a nano-second laser can be used to measure the spectral responsivity of photovoltaic detectors. With this new scheme, the relative spectral responsivity of a photodiode can be determined from the spectral reflectance of the surface of a pyroelectric detector. We also developed the adequate data acquisition procedure, which can eliminate systematic errors caused by the nonlinear pulse response. The method is experimentally demonstrated for Si and Ge photodiodes in a wide wavelength range from 420 nm to 1600 nm and verified by comparison with the method using a CW source.

Design considerations and performance analysis of dual photodetector system for reliable laser wavelength and power monitoring

In this paper, a simple and low-cost sensor system based on dual photodetector for reliable laser wavelength and power monitoring, is presented. Different spectral sensitivities of Ge and InGaAs photodiodes were used to obtain ratiometric measurement of wavelength. As a signal splitter, the 2×2 fiber-optic coupler is used in order to increase further the spectral sensitivity of the system. This system is investigated with tunable laser source in the 1525–1630nm range and has showed to successfully operate in the L-range of wavelengths with high linearity of R2=0.99942. Measurement error around ±0.2nm and repeatability of ±0.16nm are achieved. By measuring system stability, the system resolution of 0.017nm is estimated. Temperature dependence of the system is investigated and measured results proved to be consistent with theory of semiconductor behavior in the function of temperature. The sensor system dependence on changing power of tunable laser source is also investigated. In the range of 4–26mW of the source optical power, the proposed sensor system shows wavelength stability of ±0.1nm.

Simulation of the effects of defects in low temperature Ge buffer layer on dark current of Si-based Ge photodiodes**Project supported by the Key Project of Natural Science Foundation of China (No. 61534005), the National Science Foundation of China (No. 61474081), the National Basic Research Program of China (No. 2013CB632103), the Natural Science Foundation of Fujian Province (No. 2015D020), and the

The influence of defects in low temperature Ge layer on electrical characteristics of p-Ge/i-Ge/n-Si and n-Ge/i-Ge/p-Ge photodiodes (PDs) was studied. Due to a two-step growth method, there are high defect densities in low-temperature buffer Ge layer. It is shown that the defects in low-temperature Ge layer change the band diagrams and the distribution of electric field, leading to the increase of the total dark current for p-Ge/i-Ge/n-Si PDs, whereas these defects have no influence on the dark current for n-Ge/i-Ge/p-Ge PDs. As a complement, a three-dimensional simulation of the total current under illumination was also performed.

Waveguide-integrated vertical pin photodiodes of Ge fabricated on p+ and n+ Si-on-insulator layers

Vertical pin structures of Ge photodiodes (PDs) integrated with Si optical waveguides are fabricated by depositing Ge epitaxial layers on Si-on-insulator (SOI) layers, and the performances of n+-Ge/i-Ge/p+-SOI PDs are compared with those of p+-Ge/i-Ge/n+-SOI PDs. Both types of PDs show responsivities as high as 1.0 A/W at 1.55 µm, while the dark leakage current is different, which is consistent with previous reports on free-space PDs formed on bulk Si wafers. The dark current of the p+-Ge/i-Ge/n+-SOI PDs is higher by more than one order of magnitude. Taking into account the activation energies for dark current as well as the dependence on PD area, the dark current of the n+-Ge/i-Ge/p+-SOI PDs is dominated by the thermal generation of carriers via mid-gap defect levels in Ge, while for the p+-Ge/i-Ge/n+-SOI PDs, the dark current is ascribed to not only thermal generation but also other mechanisms such as locally formed conduction paths.

(Invited) SiGe BiCMOS for Optoelectronics

Silicon-based electronic-photonic integrated circuit (ePIC) technology enables a high degree of integration of optoelectronic sub-systems for optical communications. There are basically two approaches for merging photonics with electronics, hybrid assembly or monolithic integration. In hybrid integration approaches, electronic-only chips are combined with photonic-only chips after their separate fabrication [1]. Advantage is here that a mutual interference of photonic and electronic device fabrication can easily be avoided which is a more difficult task for most monolithic integration approaches. On the other hand, monolithic integration of photonic devices such as detectors and modulators in the frontend-of-line (FEOL) of a Si-based integrated circuit technology allows for shortest possible interconnects between photonics and electronics from which high-speed performance of electronic-photonic integrated circuits can greatly benefit. Moreover, monolithic electronic-photonic integration leads to shrinking form-factors and reduced assembly. Monolithic integration of Si photonics has been pursued for some time, chiefly on base of CMOS technologies [2, 3]. Looking beyond 100G systems, baud-rates exceeding 50GBd are needed for 400G transceivers, requiring very fast driving electronics. High-speed transistors are therefore a prerequisite for high-performance ePIC technology. Already today, SiGe bipolar or BiCMOS technologies are frequently deployed for broadband applications in high-speed discrete photonics. A key device figure of merit is the product of transit frequency and breakdown voltage (fT × BV) in which state-of-the-art SiGe heterojunction bipolar transistors (HBTs) outperform scaled MOS transistors. In result, SiGe HBTs are often preferred over CMOS for high-speed analog circuits. Although waveguide-based optics leads to lower form-factors compared to discrete solutions, there are limits to downsizing of integrated photonic devices because of the wavelength of light and the available index contrast. This puts constraints on the reduction of voltage swing required to drive photonic devices such as modulators. In practice, the required voltage swing remains comparable to discrete solutions. On the other hand, complex real-time mixed signal systems requiring large digital cores are typically implemented in highly scaled CMOS technologies that provide only a limited output voltage swing. Integrating Si photonics with SiGe HBT technology is therefore an excellent match for high-speed A/D or D/A interfaces. SiGe-HBT-based ePIC technology enables direct driving of integrated photonic devices by high-speed digital electronics such as advanced signal processing. Here, we will give an overview about IHP’s work in ePIC technology development under use of different SiGe BiCMOS baseline processes. Main focus is on “Photonic BiCMOS”, a new monolithic ePIC technology which combines high-performance BiCMOS technology with high-speed photonic devices for electronic-photonic sub-modules for next generation communication networks. We will describe the main features of this technology, including an overview of offered photonic and electronic devices. Examples of demonstrator circuits fabricated in a first generation of the new technology, comprising 200GHz SiGe HBTs and 30GHz Ge photodiodes, are also presented. Among these circuits are a 40Gbps monolithically integrated linear receiver [4] and a monolithically integrated segmented driver and Mach-Zehnder modulator with 13 dB extinction ratio at 28 Gbps [5]. We will also present our work to integrate a new Ge photodiode with 70GHz bandwidth and to improve the bipolar transistor performance to enable the fabrication of integrated receivers and transmitters with strongly increased optical line rates [6, 7]. In a second part of the talk we will demonstrate the potential of SiGe BiCMOS as the ‘electronics supplier’ for the hybrid integration approach, using IHP’s 0.13µm BiCMOS process SG13S offering HBTs with 240/320GHz fT/fmax. [1] F. Boeuf et al., “A multi-wavelength 3D-compatible silicon photonics platform on 300mm SOI wafers for 25Gb/s Applications,” in 2013 IEEE International Electron Devices Meeting (IEDM), pp. 353-356 [2] S. Assefa et al., “A 90nm CMOS integrated nano-photonics technology for 25Gbps WDM optical communications applications,” in 2012 IEEE International Electron Devices Meeting (IEDM), pp. 809-811 [3] R. Meade et al., “Integration of silicon photonics in bulk CMOS,” 2014 Symposium on VLSI Technology Digest of Technical Papers, DOI:10.1109/VLSIT.2014.6894427 [4] A. Awny et al., “A 40Gb/s monolithically integrated linear photonic receiver in a 0.25μm BiCMOS SiGe:C Technology,” IEEE Microwave and Wireless Components Letters, 25 (2015), 7, 469-471 [5] P. Rito et al., “A Monolithically Integrated Segmented Driver and Modulator in 0.25 μm SiGe:C BiCMOS with 13 dB Extinction Ratio at 28 Gb/s,” in press [6] D. Knoll et al., “High-performance photonic BiCMOS Process for the fabrication of high-bandwidth electronic-photonic integrated circuits,” in 2015 IEEE International Electron Devices Meeting (IEDM), pp. 402-405 [7] S. Lischke et al., “Photonic BiCMOS technology – Enabler for Si-based, monolithically integrated transceivers towards 400 G,” in press

Asymmetrically contacted germanium photodiode using a metal-interlayer-semiconductor-metal structure for extremely large dark current suppression.

In this study, we proposed germanium (Ge) metal-interlayer-semiconductor-metal (MISM) photodiodes (PD), with an anode of a metal-interlayer-semiconductor (MIS) contact and a cathode of a metal-semiconductor (MS) contact, to efficiently suppress the dark current of Ge PD. We selected titanium dioxide (TiO2) as an interlayer material for the MIS contact, due to its large valence band offset and negative conduction band offset to Ge. We significantly suppress the dark current of Ge PD by introducing the MISM structure with a TiO2 interlayer, as this enhances the hole Schottky barrier height, and thus acts as a large barrier for holes. In addition, it collects photo-generated carriers without degradation, due to its negative conduction band offset to Ge. This reduces the dark current of Ge MISM PDs by ×8000 for 7-nm-thick TiO2 interlayer, while its photo current is still comparable to that of Ge metal-semiconductor-metal (MSM) PDs. Furthermore, the proposed Ge PD shows ×6,600 improvement of the normalized photo-to-dark-current ratio (NPDR) at a wavelength of 1.55 μm. The proposed Ge MISM PD shows considerable promise for low power and high sensitivity Ge-based optoelectronic applications.

Monitoring of the laser wavelength in modern fiber-optic communication systems using dual photodetectors

Fiber-optic communication systems enable very fast access to large amounts of data for all users of the information and communication systems. Monitoring and control of such fiber-optic systems is of crucial importance for the very large data rate traffic over the internet. DWDM systems have become a standard in order to maximize utilization of existing resources of the fiber-optic systems. These systems use large number of laser light sources with adjacent wavelengths and therefore constant monitoring and control of all lasers in the system is of vital importance. There are different methods for the determination of the lasers wavelength such as optical spectrometers with a monochromator or Fabry-Perot interferometers. In this paper a simple and cost effective solution for laser wavelength monitoring in DWDM L-band with two different photodetectors is presented. Different sensitivities of Ge and InGaAs photodiodes in the L-band were used for implementation of a simple and accurate sensing system for the monochromatic light source wavelength measurement and monitoring. By illuminating simultaneously both photodiodes with the help of the fiber-optic coupler and corresponding signal processing, a wavelength dependent signal was obtained. While the distance between two adjacent channels in DWDM systems is around 0.8 nm, measurement accuracy achieved using proposed method was around ±0.4 nm. Compared to channel spacing in DWDM systems, acquired accuracy is considered sufficient for monitoring of the laser wavelength.

High-Performance Ge p-n Photodiode Achieved With Preannealing and Excimer Laser Annealing

A germanium (Ge) n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> /p shallow junction photodiode fabricated by a combination of low-temperature preannealing and excimer laser annealing of the phosphorus-implanted p-type Ge is demonstrated. The Ge photodiode shows a high responsivity of 0.48 A/W at the 1.55-μm wavelength and an extremely low leakage current density of 1 pA/μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at room temperature, over two orders of magnitude lower than that of diodes formed by the rapid thermal annealing process. In addition, the reverse leakage current is close to the theoretical limitation of Ge diodes dominated by the ideal carrier diffusion model. The well-controlled dopant profile of the Ge shallow junction should be beneficial for improving the performance of the diodes, which may also be used in making the source and drain of scaled Ge nMOSFET.

Performance analysis of a photonic integrated interferometer circuit based on silicon-on-insulator

Small footprint photonic integrated interferometer based on silicon-on-insulator platform is reported. A passive interferometer comprising small footprint $$4\times 4$$ and $$2\times 2$$ multi-mode interference (MMI) couplers is designed and tested. The $$4\times 4 \,(2\times 2)$$ MMIs feature 0.16 dB (0.06 dB) excess loss, 0.22 dB (0.09 dB) power imbalance, and 1.7 $$^{\circ }$$ (0.12 $$^{\circ }$$ ) phase error. An efficient use of the small design area is accomplished by positioning the input and output ports of the passive interferometer along the same side of the chip with a separation of $$127\,\upmu \hbox {m}$$ between each other (pitch size). A passive-active interferometer terminated with Ge photodiodes has also been designed and tested. Sinusoidal fitting of the phase differences between output port number 1 and all other output ports are estimated to be under 2 $$^{\circ }$$ margin, i.e., $$90^{\circ }\pm 1^{\circ },180^{\circ }\pm 1^{\circ }$$ and $$270^{\circ }\pm 1^{\circ }$$ .