Germanium Photodiodes Research Articles

Transfer Print Integration of Waveguide-Coupled Germanium Photodiodes Onto Passive Silicon Photonic ICs

We demonstrate the integration of waveguide-coupled germanium photodiodes onto passive silicon waveguide circuits by means of transfer printing. This involves the release of the photodiodes from the silicon-on-insulator source wafer by underetching the buried oxide layer while protecting the back-end stack. Tethers were formed to keep the released photodiode coupons in place. Coupons were then transfer printed to a silicon photonic target wafer with an alignment accuracy better than ±1 μm. 0.66 A/W waveguide-referred photodiode responsivity at 1550 nm was obtained. High-speed measurements yielded open eye diagrams at 40 Gb/s.

Integrated SOI High-Order Phase-Shifted Bragg Grating for Microwave Photonics Signal Processing

An integrated high-order phase-shifted Bragg grating, comprising six quarter-wave sections between Bragg grating mirrors in a laterally-corrugated strip waveguide has been realized in silicon-on-insulator technology. A box-like transmission window is created within the 10-nm-wide grating reflection band, realizing a sharp bandpass optical filter with out-of-band rejection exceeding 40 dB and a steep roll-off of ∼300 dB/nm in the transition band. The sharp optical filter has been experimentally tested in microwave photonics (MWP) signal processing applications, namely spectral separation of an optical sideband comprising 1.25 Gb/s data from a 15-GHz-spaced carrier, and sideband suppression for dispersion compensation in a radio-over-fiber link. The results of the characterizations indicate negligible power penalty in terms of bit-error rate for the sideband separation and robust mitigation of dispersion-induced transmission impairment. The device has an ultrasmall footprint of ∼450 × 0.5 μm2, and can be monolithically integrated with germanium photodiodes or silicon modulators as well as other passive subsystems to implement advanced on-chip MWP signal processing functionalities.

Pulse Laser Sulfur-Hyperdoping of Germanium and High Quantum Efficiency Photodiodes

Germanium was successfully hyperdoped with sulfur by means of nanosecond pulse laser and SF6 gas. A photodiode was fabricated based on sulfur-hyperdoped germanium. Rectifying current-voltage characteristics were observed for the device. The sulfur-hyperdoped germanium photodiode showed an internal quantum efficiency close to 150 $\%$ from visible to near infrared and increased to 275 $\%$ for the ultraviolet region. The −3 dB bandwidth was measured to be 20 MHz. Sulfur was demonstrated to be an effective deep level dopant for realizing high-performance germanium photodiodes.

Optimizing photon-pair generation electronically using a p-i-n diode incorporated in a silicon microring resonator

Silicon photonic microchips may be useful for compact, inexpensive, room-temperature optically pumped photon-pair sources, which unlike conventional photon-pair generators based on crystals or optical fibers, can be manufactured using CMOS-compatible processes on silicon wafers. It has been shown that photon pairs can be created in simple structures such as microring resonators at a rate of a few hundred kilohertz using less than a milliwatt of optical pump power, based on the process of spontaneous four-wave mixing. To create a practical photon-pair source, however, also requires some way of monitoring the device and aligning the pump wavelength when the temperature varies, since silicon resonators are highly sensitive to temperature. In fact, monitoring photodiodes are standard components in classical laser diodes, but the incorporation of germanium or InGaAs photodiodes would raise the cost and fabrication complexity. Here, we present a simple and effective all-electronic technique for finding the optimum operating point for the microring used to generate photon pairs, based on measuring the reverse-biased current in a silicon p-i-n junction diode fabricated across the waveguide that constitutes the silicon microring. We show that by monitoring the current, and using it to tune the pump laser wavelength, the photon-pair generation properties of the microring can be preserved over a temperature range of more than 30 °C.

A 40 Gb/s Monolithically Integrated Linear Photonic Receiver in a <formula formulatype="inline"><tex Notation="TeX">$0.25~\mu {\rm m}$</tex></formula> BiCMOS SiGe:C Technology

This letter presents the first 40 Gb/s monolithically integrated silicon photonics linear receiver (Rx) comprising a germanium photodiode (Ge-PD) and a linear transimpedance amplifier (TIA). Measured optical-electrical (O/E) 3 dB bandwidth (BW) of the Rx is 31 GHz. At 40 Gb/s, the Rx achieves a sensitivity of $-3~{\rm dBm}$ average optical input power with BER of $2.5\times 10^{-11}$ . It operates at $\lambda =1.55 ~\mu{\rm m}$ wavelength, uses 3.3 and 3.7 V power supplies, dissipates 275 mW of power, provides maximum differential output amplitude of $500 ~{\rm mV}_{\rm pp}$ , and occupies an area of $3.2~{\rm mm}^{2}$ . The presented receiver achieves the highest bit rate among the published work in monolithically integrated silicon photonics receivers.

1.3-<inline-formula> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>m Silicon Optical Switch Integrated With Germanium Photodiodes for Stable Operation

A carrier-injection-type silicon optical switch (OSW) monolithically integrated with germanium photodiodes (Ge-PDs) is proposed and operated at a wavelength of 1310 nm. The Ge-PDs used for monitoring photocurrent achieve stable switching control. A 4 × 4 silicon OSW, ideally a path-independent-loss type, was fabricated on a silicon-on-insulator substrate. Measured loss variations of all 16 paths were ~3 dB, and the switching response of the OSW was faster than 10 ns. Feedback control of injection current of the OSW was demonstrated. This result shows that the proposed closed-loop feedback control by monitoring photocurrents of Ge-PDs has the potential for stable operation of matrix OSWs.

Study and realisation of an original silicon-germanium light trap detector for large band radiometric measurements

An original accuracy light-trap photodetector has been developed at the Materials, Measurements and Applications laboratory of INSAT to serve as a transfer standard detector for optical power measurements in the spectral range from 400 nm to 1600 nm. This detector is a silicon and germanium photodiode-based device and consists of, respectively, two silicon (Hamamatsu) and two germanium (UDT) photodiodes in a light-trapping arrangement. The average reflection coefficient of our detector in the studied spectral range is equal to 0.0020 with a standard deviation of ∼4 × 10−4 (at 1σ level). The obtained average linearity coefficient over a wide dynamic range is equal to 1.001. This factor is acceptable compared with the uncertainty (about 1.1 × 10−3). The realised trap detector offers an average responsivity increase equal to almost two compared with the individual photodiodes. This trap detector was calibrated compared with, respectively, silicon and germanium photodiodes standards and provides a convenient standard with a relative standard uncertainty of 1.5 × 10−3 and 2.3 × 10−3 for, respectively, silicon and germanium photodiodes (at the 1σ level). This higher uncertainty is mainly due to standard photodiodes uncertainties, to the laser beam quality and to its power stability. In fact, the last two factors were not good enough in our case.

A germanium hybrid pixel detector with 55μm pixel size and 65,000 channels

Hybrid pixel semiconductor detectors provide high performance through a combination of direct detection, a relatively small pixel size, fast readout and sophisticated signal processing circuitry in each pixel. For X-ray detection above 20 keV, high-Z sensor layers rather than silicon are needed to achieve high quantum efficiency, but many high-Z materials such as GaAs and CdTe often suffer from poor material properties or nonuniformities. Germanium is available in large wafers of extremely high quality, making it an appealing option for high-performance hybrid pixel X-ray detectors, but suitable technologies for finely pixelating and bump-bonding germanium have not previously been available.A finely-pixelated germanium photodiode sensor with a 256 by 256 array of 55μm pixels has been produced. The sensor has an n-on-p structure, with 700μm thickness. Using a low-temperature indium bump process, this sensor has been bonded to the Medipix3RX photoncounting readout chip. Tests with the LAMBDA readout system have shown that the detector works successfully, with a high bond yield and higher image uniformity than comparable high-Z systems. During cooling, the system is functional around -80°C (with warmer temperatures resulting in excessive leakage current), with -100°C sufficient for good performance.

Germanium avalanche receiver for low power interconnects

Recent advances in silicon photonics have aided the development of on-chip communications. Power consumption, however, remains an issue in almost all integrated devices. Here, we report a 10 Gbit per second waveguide avalanche germanium photodiode under low reverse bias. The avalanche photodiode scheme requires only simple technological steps that are fully compatible with complementary metal oxide semiconductor processes and do not need nanometre accuracy and/or complex epitaxial growth schemes. An intrinsic gain higher than 20 was demonstrated under a bias voltage as low as -7 V. The Q-factor relating to the signal-to-noise ratio at 10 Gbit per second was maintained over 20 dB without the use of a trans-impedance amplifier for an input optical power lower than -26 dBm thanks to an aggressive shrinkage of the germanium multiplication region. A maximum gain over 140 was also obtained for optical powers below -35 dBm. These results pave the way for low-power-consumption on-chip communication applications.

Contributions of Franz–Keldysh and Avalanche Effects to Responsivity of a Germanium Waveguide Photodiode in the $\hbox{L}$-Band

When driven with a high-voltage reverse bias of 15 V, a germanium photodiode with a silicon waveguide exhibits responsivity of over 1.14 A/W in the entire C-and L-bands. The high responsivity in the L-band is due to Franz-Keldysh (F-K) and avalanche effects. We prove by numerical calculation that the high responsivity under high bias driving is due to both effects. For accuracy in the calculation, we consider a change in the thickness of depletion layer in the germanium mesa with bias voltage. Calculation results, considering the contributions of both effects, show good agreement with measurement results. Considering the contributions of both effects to minimum detection limit, we discuss optimization for dark current and the electric field applied to Ge.

Irradiation of new optoelectronic components for HL-LHC data transmission links

Candidate optoelectronic components for use in future data-transmission links at the High-Luminosity Large Hadron Collider (HL-LHC) were irradiated with 20 MeV neutrons at the University Cyclotron in Louvain-La-Neuve, Belgium and 24 GeV protons at the CERN PS irradiation facility. The results from this test for multi-channel transmitters, Germanium photodiodes, and Silicon photonics modulators are presented here.

High-performance waveguide-integrated germanium PIN photodiodes for optical communication applications [Invited

This paper reports on high-performance waveguide-integrated germanium photodiodes for optical communications applications. 200 mm wafers and production tools were used to fabricate the devices. Yields over 97% were obtained for three different compact photodiodes (10×10 μm and intrinsic region width of 0.5, 0.7, and 1 μm) within the same batch of three wafers. Those photodiodes exhibit low dark currents under reverse bias with median values of 74, 62, and 61 nA for intrinsic widths of 0.5, 0.7, and 1 μm, respectively, over a full wafer. Responsivities up to 0.78 A/W at 1550 nm and zero bias were measured. Zero bias operation is possible for 25 and 40 Gbps with receiver sensitivity estimated to −13.9 and −12.3 dBm, respectively.

Metal-optic cavity for a high efficiency sub-fF Germanium photodiode on a Silicon waveguide

We propose two designs of nanoscale sub-fF germanium photodiodes which are efficiently integrated with silicon waveguides. The metal-optic cavities are simulated with the finite difference time domain method and optimized using critical coupling concepts. One design is for a metal semiconductor metal photodiode with <200 aF capacitance, 39% external quantum efficiency, and 0.588 (λ/n)³ cavity volume at 1.5 µm wavelength. The second design is for a vertical p-i-n photodiode with <100 aF capacitance, 51% external quantum efficiency, and 0.804 (λ/n)³ cavity volume. Both designs make use of CMOS compatible materials germanium and aluminum metal for potential future monolithic integration with silicon photonics.

High-Bandwidth and High-Responsivity Top-Illuminated Germanium Photodiodes for Optical Interconnection

In this paper, we report efficient high-speed top-illuminated p-i-n photodiodes with high responsivity fabricated from germanium (Ge) films grown directly on silicon-on-insulator substrates. The devices were characterized with respect to their dark current, responsivity, and 3-dB bandwidth (BW) in the near infrared. For a 20-μm-diameter device at room temperature, the dark current densities were approximately 38.3 mA/cm2 at -1 V. The responsivity (\mmb R) at 1.55 μm was 0.30 A/W, corresponding to a quantum efficiency of 24%. The 3-dB BW of the detector with 20-μm diameter is as high as 23.3 GHz.

Improved electrical characteristics of porous germanium photodiode obtained by phosphorus ion implantation

Germanium–based photodetectors are the standard technology used for radiometric measurement in the IR spectral range. Shallow PN+ junctions were obtained by using ion implantation in p–type germanium. N–type doping was achieved by phosphorus implantation at a dose of 1 × 1015 cm−2 doses and an energy of 150 keV followed by annealing at 600°C for 2 h. In order for the new detector to be used as a standard for radiometric measurement, a porous layer is introduced on the active surface. After metallisation by thermal evaporation of indium (In) on the front side and gold (Au) on the back side, the new porous germanium–based photodetector is essentially done. The electrical behaviour of this porous germanium photodiode is characterised by current–voltage measurements that are compared to the I–V characteristic of classical germanium photodiodes. The results of this study show a huge increase of the photocurrent, a significant increase of the shunt resistance and low series resistance. Effects of low shunt resistance are also discussed. AFM showed that the porous layer contributes to trap the incident optical radiation and to reduce the reflection coefficient fluctuations of the front face of the photodiode.

Monolithic integration of a silica AWG and Ge photodiodes on Si photonic platform for one-chip WDM receiver

On the silicon (Si) photonic platform, we monolithically integrated a silica-based arrayed-waveguide grating (AWG) and germanium (Ge) photodiodes (PDs) using low-temperature fabrication technology. We confirmed demultiplexing by the AWG, optical-electrical signal conversion by Ge PDs, and high-speed signal detection at all channels. In addition, we mounted a multichannel transimpedance amplifier/limiting amplifier (TIA/LA) circuit on the fabricated AWG-PD device using flip-chip bonding technology. The results show the promising potential of our Si photonic platform as a photonics-electronics convergence.

Near infrared image sensor with integrated germanium photodiodes.

A near-infrared image sensor with monolithically integrated Ge photodiodes is demonstrated. The technology for the integration of the Ge photodiodes into the CMOS process is outlined, and the measurement results of test-diodes and the full imager are discussed in detail. The heterojunction-photodiodes show a quantum efficiency of about 30% up to a wavelength of 1500 nm. A tensile strain of 0.17% was measured in the epitaxial Ge layer, which is in good agreement with the optically measured direct bandgap absorption edge of 1580 nm. The image sensor can be operated at room temperature or with moderate cooling.

Wafer Level High-Speed Germanium Photodiode Array Integration

High-density germanium photodiode arrays are integrated on top of a dummy CMOS 200-mm Silicon wafer. Using a conventional available semiconductor fabrication line, the target specifications are reached with a yield exceeding 99% for several thousands of tested photodiodes with respect to bandwidth, responsivity, and dark current. A very low dark current density in the range of 7 mA/cm2 is obtained. A median bandwidth above 9 GHz is reached with a large 30-μm diameter photodiode.

Study and characterization of porous germanium for radiometric measurements

AbstractThe aim of this article is to study and realize a new detector based on a porous germanium (pGe) photodiode to be used as a standard for radiometric measurement in the wavelength region between 800 nm and 1700 nm. We present the development and characterization of a porous structure realized on a single‐crystal substrate of p‐type germanium (Ga doped) and of crystallographic orientation (100). The obtained structure allows, on the one hand, to trap the incident radiation, and on the other hand, to minimize the fluctuations of the front‐face reflection coefficient of the photodiode. The first studies thus made show that it is possible to optimize, respectively, the electrical current density and the electrochemical operation time necessary for obtaining exploitable porous structures. The obtained results show that for 50 mA/cm2 and 5 min as operational parameters, we obtain a textured aspect of the porous samples that present a pyramidal form. The reflectivity study of the front surface shows a constant value of around 38% in a spectral range between 800 nm and 1700 nm approximately. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

21-GHz-Bandwidth Germanium-on-Silicon Photodiode Using Thin SiGe Buffer Layers

Backside-illuminated germanium photodiodes fabricated on silicon substrate with two Si xGe1-x buffer layers are reported. At 1.3 mum, the responsivity was 0.62 A/W for reverse bias greater than 0.1 V. The 3-dB bandwidth was 21.5 GHz at 10-V reverse bias, achieving a bandwidth-efficiency product of 12.6 GHz