Because of the 4% lattice mismatch between Ge and Si, threading dislocations (TDs) are generated in Ge epilayers on Si, deteriorating the performance of Ge devices on Si platform. We recently modeled the reduction of TD density in heteroepitaxial coalesced layers in terms of the bending of TDs induced by image forces at non-planar selective epitaxial growth (SEG) surfaces before the coalescence. The reduction of TD density was quantitatively verified for Ge layers on (001) Si with line-and-space SiO2 masks. In the present paper, detailed theoretical calculation and experimental results are presented. Numerical calculation shows that the image force is large enough to bend/move dislocations considering the Peierls stress or the mobility of TDs in Ge. Transmission electron microscope observations show that the TD bending is certainly induced. The TD density is lower above the SiO2 masks, as confirmed by the etch pit method. Such spatial distribution is well explained by the image-force-induced dislocation bending.
The high temperature sensitivity of silicon material limits the applications of silicon-based micro-ring resonators in integrated photonics. To realize a low but broadband temperature-dependence-wavelength-shift (TDWS) micro-ring resonator, designing a broadband athermal waveguide becomes a significant task. In this work, we propose a broadband athermal waveguide which shows a low effective thermos-optical coefficient (TOC) of ±1×10-6/K at 1400 nm to 1700 nm. The proposed waveguide shows low-loss performance of 0.01 dB/cm and stable broadband-athermal ability when it’s applied in micro-ring resonators, and the optical loss of micro-ring resonator with a radius of 100 μm using this waveguide is 0.02 dB/cm.
We have developed an innovated fabrication technology of Si, GaAs, and Ge nano-structures, i.e., we called defect-free neutral beam etching. The technology has been successfully applied to prototype the quantum nano-disks and nano-wires with ferritin based bio-templates. SEM observation verifies that the designed structures are prototyped. Photoluminescence measurements demonstrates high optical quality of nano-structures based on the technology.
The mid-Infrared wavelength range (2-20 µm), so-called fingerprint region, contains the very sharp vibrational and rotational resonances of many chemical and biological substances. Thereby, on-chip absorption-spectrometry-based sensors operating in the mid-Infrared (mid-IR) have the potential to perform high-precision, label-free, real-time detection of multiple target molecules within a single sensor, which makes them an ideal technology for the implementation of lab-on-a-chip devices.
Benefiting from the great development realized in the telecom field, silicon photonics is poised to deliver ultra-compact efficient and cost-effective devices fabricated at mass scale. In addition, Si is transparent up to 8 µm wavelength, making it an ideal material for the implementation of high-performance mid-IR photonic circuits. The silicon-on-insulator (SOI) technology, typically used in telecom applications, relies on silicon dioxide as bottom insulator. Unfortunately, silicon dioxide absorbs light beyond 3.6 µm, limiting the usability range of the SOI platform for the mid-IR. Silicon-on-sapphire (SOS) has been proposed as an alternative solution that extends the operability region up to 6 µm (sapphire absorption), while providing a high-index contrast. In this context, surface grating couplers have been proved as an efficient means of injecting and extracting light from mid-IR SOS circuits that obviate the need of cleaving sapphire. However, grating couplers typically have a reduced bandwidth, compared with facet coupling solutions such as inverse or sub-wavelength tapers. This feature limits their feasibility for absorption spectroscopy applications that may require monitoring wide wavelength ranges. Interestingly, sub-wavelength engineering can be used to substantially improve grating coupler bandwidth, as demonstrated in devices operating at telecom wavelengths.
Here, we report on the development of fiber-to-chip interconnects to ZrF4 optical fibers and integrated SOS circuits with 500 nm thick Si, operating around 3.8 µm wavelength. Results on facet coupling and sub-wavelength engineered grating coupler solutions in the mid-IR regime will be compared.
Monolithically integrated Ge lasers on Si have long been one of the biggest challenges for electronic and photonic integration on Si Complementary Metal Oxide Semiconductor (CMOS) platform. The “last one mile” is to reduce the threshold current of the electrically pumped Ge-on-Si laser. We have studied the growth of heavily doped n type (n+) Ge and analyzed its photoluminescence (PL) characteristics of Ge with a Si cap and thermal oxide layers. It is found that the PL intensity of n+ Ge was significantly reduced by the cap and etching off the cap showed a ~100% recovery to the intensity of n+ Ge without the cap. Thermally oxidized n+ Ge, on the other hand, showed a ~50% increase in the PL intensity of uncapped n+ Ge. These finding indicated that capping of n+ Ge introduces non-radiative recombination centers due to defects (dislocations) to reduce the PL intensity, while oxidation passivates surface defects remained even on uncapped n+ Ge. Considering these, we have designed and fabricated an electrically pumped n+ Ge light emitting diode with no Si cap layer but oxidation. A broad luminescence of Ge at 1500-1700 nm has been demonstrated but yet lasing not observed.
The bandgap tuning of sub-micron wide Germanium (Ge) waveguides by selective epitaxial growth (SEG)
method with a SiO2 mask has been demonstrated. SEG-grown Ge waveguides on Si substrate are designed to show
various compressive strain depending on the growth parameters, such as the width and thickness of Ge waveguides and
SiO2 masks. X-Ray Diffraction (XRD) verifies that -0.25% (compressive) strain is induced in a 0.6μm-wide Ge
waveguide with SiO2 mask of 20μm width and 1.0μm thickness. The strained Ge waveguide should show the absorption
edge wavelength of ~1.55μm. Furthermore, compressive strain can be tuned between -0.03% and -0.25% by changing
the lateral structure of the device, which correspond to the absorption edge wavelength of 1.548~1.568μm. It means that
only one epitaxial growth with specific lateral design of the electro-absorption modulator can modulate light in the
We experimentally and theoretically investigate GeSi-based photonics for future on-chip optical interconnect on bulk Silicon substrates with dense wavelength division multiplexing (WDM) system. We experimentally show that Ge-rich Si1-xGex can be used as both a passive low loss waveguide and a substrate to facilitate low-temperature epitaxial growth of Ge-based active devices working at low optical loss wavelength of Ge-rich Si1-xGex waveguides. We also theoretically discussed the possibilities to realize a compact passive component based on Ge-rich Si1-xGex material system on bulk Si wafer. From simulation the system based on Ge-rich Si1-xGex waveguide and the Si1-yGey (y < x) lower cladding layer is good enough to ensure compactness of important on-chip photonic components including passive waveguide and GeSi-based array waveguide grating (AWG). The small refractive index contrast between Ge-rich Si1-xGex waveguide and the Si1-yGey lower cladding layer potentially avoid the polarization dependent loss and detrimental fabrication tolerance of WDM system. Our studies show that GeSi-based photonics could uniquely provide both passive and active functionalities for dense WDM system.
Silicon (Si) photonic wire waveguides provide a compact photonic platform on which passive, dynamic, and active photonic devices can be integrated. This paper describe the demonstrations of several kinds of integrated photonic circuits. The platform consists of Si wire, silicon-rich Si dioxide (SiOx) and Si oxinitride (SiON) waveguides for passive devices and a Si rib waveguide with a p-i-n structure and a germanium (Ge) device formed on Si slab for active devices. One of the key technologies for the photonic integration platform is low temperature fabrication because a back-end process with high temperature may damage active and electronic devices. To overcome this problem, we have developed electron cyclotron resonance chemical vapor deposition as a low-temperature deposition technique. Another key technology is polarization manipulation for reducing polarization dependence. A polarization diversity circuit is fabricated by applying Si wire and SiON integration. The polarization-dependent loss of the diversity circuit is less than 1 dB. Moreover we have developed several kinds of integrated circuit including passive, dynamic and active devices. Ge photodiodes are monolithically integrated with an SiOx-arrayed waveguide grating (AWG). We have confirmed that the operation speed of the integrated Ge photodiode is over 22 Gbps for all 16 channels. Variable optical attenuators (VOAs) fabricated on the Si p-i-n rib waveguides and an AWG based on the SiOx waveguide are integrated successfully. The total size of 16-ch-AWG-VOAs is 15 8 mm2. The device has already been made polarization independent. Furthermore electronic circuits are successfully mounted on the integrated photonic device by using flip-chip bonding.
Silicon photonic wire waveguides, featuring very strong optical confinement and compatibility with silicon electronics,
provide a compact photonic platform on which passive, dynamic, and active photonic devices can be integrated. We have
already developed a low-loss waveguide platform and integrated various photonic devices. For passive devices, we have
developed polarization-independent wavelength filters using a monolithically integrated polarization diversity circuit, in
which waveguide-based polarization manipulation devices are implemented. The polarization-dependent loss of a ring
resonator wavelength filter with polarization diversity is less than 1 dB. For dynamic devices, we have developed
compact carrier-injection-type variable optical attenuators (VOAs). The length of the device is less than one millimeter,
and the response time is nanosecond order. The device has already been made polarization independent. We have
recently monolithically integrated these fast VOAs with low-dark-current germanium photodiodes and achieved
synchronized operation of these devices. For nonlinear devices, a free-carrier extraction structure using a PIN junction
implemented in the waveguide can increase the efficiency of nonlinear functions. For example, in a wavelength
conversion based on the Four-wave-mixing effect, the conversion efficiency can be increased by 6 dB.
This paper has three sections on computation revolutions in the information age. In the first section key examples of
revolutions in the early history, i.e., how to overcome heat penalty arisen from high power consumption in silicon
electronics will be presented to bring to light in the ever evolving nature of the computation. In the second section the
optical interconnection in terms of silicon photonics will be discussed as the way in which computation have
fundamentally altered the previous trend of power consumption. The final section will focus on a paradigm shift in
computation, i.e., an architecture of logic implementation, binary decision diagram, to replace to current computation
system without fighting transistor logic circuitry.
The present paper describes Si microphotonics and its current status of electronics and photonics convergence on Si platform based on monolithic integration using CMOS (Complementary Metal Oxide Semiconductor) technologies. The Si CMOS platform is advantageous over III-V semiconductor based platform because of a short time-lag between basic research and commercialization in terms of the standardized materials and processes. To implement photonic devices on the Si CMOS platform, it is important to reduce materials diversity in current photonics devices. Low loss SiNx waveguides with sharp bends, high performance strained Ge photodetectors for C+L band, and demultiplexer/multiplexer for WDM (wavelength division multiplexing) have been successfully implemented on the Si CMOS platform. The current targets are cost-effective OADMs (optical add-drop multiplexers) for optical communication and optical clocking for Si LSIs beyond Cu-low k technologies.
Air trench structure for reduced-size bends in low (Δn=0.01-0.1) and medium (Δn=0.1-0.3) index contrast waveguides is proposed. Local high index contrast at bends is achieved by introducing air trenches. An air trench bend consists of cladding tapers to avoid junction loss, providing adiabatic mode shaping between low and high index contrast regions. Drastic reduction in effective bend radius is achieved. We present FDTD simulations of bends in representative silica index contrasts, fabrication scheme and waveguide loss measurement results using Fabry-Perot loss measurement technique. We employed CMOS compatible processes to realize air trench bends and T-splitters to achieve low production cost and high yield. A simple, compact waveguide and T-splitter are fabricated and evaluated. The loss measurement results show that losses are consistent with theoretical simulations. By using air trench waveguides, other applications such as BioMEMS (e.g. Evanescent-field sensor) or EDWA can be realized.
Optical communications networks must be terminated by receiver circuitry capable of converting an optical circuit to an electrical one. While current III-V technology is capable of delivering high performance, it is costly and difficult to integrate with low-cost Si based technologies. In order to overcome these barriers, we are pursuing a Si-compatible technology for integrated photodetectors. Ge, monolithically integrated with Si, offers a low-cost, high-performance materials system for photodetector integration with existing Si technology. In this paper we discuss the performance requirements and figures of merit for integrated photodetectors. We then discuss the materials issues associated with the integration of Ge on Si and show that high quality Ge films can be grown directly on Si, despite the 4% lattice mismatch. By cyclic annealing after growth, the dislocation density can be reduced to 2.3x107 cm-2, and diodes fabricated on these films show a responsivity of 300 mA/W at 1300 nm without an AR coating. Finally, we discuss the integration of waveguides with photodetectors and propose an integration scheme we believe will be capable of delivering high-performance integrated photoreceivers on a Si platform.
The present paper describes an emerging field, "Si microphotonics", and its application to on-chip interconnection beyond semiconductor roadmap. Current Si-LSIs have been facing three fundamental limits associated with metal interconnection; i.e., slow clock speed, multilayer interconnection for high density interconnects, and high power consumption. These limits are induced by "slow" signal messengers, electrons. There is no solution beyond the Cu and low k technology but optical interconnection. To implement optical clock distribution on a chip, one challenge is sharp bending of waveguides. High-index contrast optics has shown their significant potentials. Right angle bends have been proto-typed whose area is less than 1 tm2. Ge directly grown on Si wafers shows an excellent characteristics as photodetectors for 1.3 and 1 .55 tm. A high-density interconnection needs wavelength division multiplexing (WDM). Ultrasmall multiplexer/demultiplexer (DEMUX/DEMUX) has been achieved on a chip based on micro-ring resonators (1O tm). Minimization ofpower consumption is of importance when light sources are implemented on a chip. Microcavity resonators based on photonic crystal concepts should be a unique solution.
Sharp bends in low index contrast waveguides using tapered air trenches are proposed. To minimize cladding-trench junction loss, cladding tapers are designed to provide adiabatic mode shaping between low and high index contrast regions. Drastic reduction in effective bend radius is predicted. We present 2D FDTD/EIM simulations of bends in representative silica index contrasts. Substrate loss in air trenches of finite depth is investigated, and the required trench depth, given an acceptable substrate loss, is calculated. Fabrication steps are described.
The front-illuminated CCD devices on board the Chandra X-ray observatory have been damaged by proton beam irradiation during radiation belt passage. The scattered ions such as protons created the traps in the buried n-channel of the CCD. The effect of proton radiation induced defects in Si is summarized. The generation and evolution of the irradiation defects is studied and its relationship with CCD performance is discussed. The methods for enhancement of dissociation of defects by biasing and/or light illumination are proposed to recover the performance of CCD.
We report the fabrication of fast heterojunction Ge/Si photodetectors which, to the best of our knowledge, exhibit the highest near infrared responsivity at normal incidence reported to date. Such performances are related to the quality of the epitaxial Ge film grown by a two-step UHV-CVD process followed by cyclic thermal annealing. We have measured a fast (FWHM equals 850 ps at 1.3 micrometers ) and efficient (R equals 0.55 A/W at 1.3 micrometers and 0.25 A/W at 1.55 micrometers ) photoresponse. Our technology makes these devices suitable for integration with other electronic and optoelectronic components on Si chips. In the paper we discuss processing technology, material quality, device fabrication and performance measurements.
Static, field-mounted and time-resolved spectroscopic measurements were carried out to compare the electronic structures between AlGaN/GaN binary and GaN/InGaN ternary single quantum wells (SQWs). The internal field exits across the quantum well (QW) naturally induces quantum-confined Stark effects, namely the redshift of the QW resonance energy and separation of electron-hole wavefunction overlap. Thus AlGaN/GaN SQWs exhibited a weak luminescence peak due to the presence of nonradiative channels. However, optical absorption and degenerate pump-probe measurements revealed that excitonic character still remains for the thin QWs having the well width nearly the same as the bulk free exciton Bohr radius even under high electric field as high as 0.73 MV/cm. A slightly In-alloyed InGaN SQW exhibited bright luminescence peak in spite of the pronounced effective bandgap inhomogeneity in the QW, which was confirmed by the point excitation and monochromatic cathodoluminescence mapping methods to have the lateral potential interval smaller than 40 nm. Therefore the light emitting area of the potential minima has the size defined as 'quantum-disk'. Carriers generated in the InGaN QWEs are effectively localized in these regions to form localized QW excitons exhibiting highly efficient spontaneous emissions.