High-performance Ge/Si electro-absorption optical modulator up to 85°C and its highly efficient photodetector operation.

We studied a high-speed Ge/Si electro-absorption optical modulator (EAM) evanescently coupled with a Si waveguide of a lateral p-n junction for a high-bandwidth optical interconnect over a wide range of temperatures from 25 °C to 85 °C. We demonstrated 56 Gbps high-speed operation at temperatures up to 85 °C. From the photoluminescence spectra, we confirmed that the bandgap energy dependence on temperature is relatively small, which is consistent with the shift in the operation wavelengths with increasing temperature for a Ge/Si EAM. We also demonstrated that the same device operates as a high-speed and high-efficiency Ge photodetector with the Franz-Keldysh (F-K) and avalanche-multiplication effects. These results demonstrate that the Ge/Si stacked structure is promising for both high-performance optical modulators and photodetectors integrated on Si platforms.

High-speed Ge/Si electro-absorption optical modulator in C-band operation wavelengths.

We studied a high-speed electro-absorption optical modulator (EAM) of a Ge layer evanescently coupled with a Si waveguide (Si WG) of a lateral pn junction for high-bandwidth optical interconnect. By decreasing the widths of selectively grown Ge layers below 1 µm, we demonstrated a high-speed modulation of 56 Gbps non-return-to-zero (NRZ) and 56 Gbaud pulse amplitude modulation 4 (PAM4) EAM operation in the C-band wavelengths, in contrast to the L-band wavelengths operations in previous studies on EAMs of pure Ge on Si. From the photoluminescence and Raman analyses, we confirmed an increase in the direct bandgap energy for such a submicron Ge/Si stack structure. The operation wavelength for the Ge/Si stack structure of a Ge/Si EAM was optimized by decreasing the device width below 1-µm and setting the post-growth anneal condition, which would contribute to relaxing the tensile-strain of a Ge layer on a Si WG and broadening the optical bandwidths for Franz-Keldysh (FK) effect with SiGe alloy formation.

Enhanced direct-gap light emission from Si-capped n+-Ge epitaxial layers on Si after post-growth rapid cyclic annealing: impact of non-radiative interface recombination toward Ge/Si double heterostructure lasers: erratum.

We report an error in Fig. 13(a) of our article [Opt. Express25, 21286 (2017)], and the correction is presented. Typographical errors in the text are also corrected.

Germanium vertically light-emitting micro-gears generating orbital angular momentum.

Germanium (Ge) is capturing researchers' interest as a possible optical gain medium implementable on complementary metal-oxide-semiconductor (CMOS) chips. Band-gap engineering techniques, relying mainly on tensile strain, are required to overcome the indirect band-gap nature of bulk Ge and promote electron injection into the direct-gap valley. We used Ge on silicon on insulator (Ge-on-SOI) wafers with a high-crystalline-quality Ge layer to fabricate Ge micro-gears on silicon (Si) pillars. Micro-gears are created by etching a periodic grating-like pattern on the circumference of a conventional micro-disk, resulting in a gear shape. Thermal built-in stresses within the SiO2 layers that encapsulate the micro-gears were used to impose tensile strain on Ge. Biaxial tensile strain values ranging from 0.3-0.5% are estimated based on Raman spectroscopy measurements and finite-element method (FEM) simulations. Multiple sharp-peak resonances within the Ge direct-gap were detected at room temperature by photo-luminescence (PL) measurements. By investigating the micro-gears spectrum using finite-difference time-domain (FDTD) simulations, we identified vertically emitted optical modes with non-zero orbital angular momentum (OAM). To our best knowledge, this is the first demonstration of OAM generation within a Ge light source.

Enhanced direct-gap light emission from Si-capped n+-Ge epitaxial layers on Si after post-growth rapid cyclic annealing: impact of non-radiative interface recombination toward Ge/Si double heterostructure lasers.

Enhanced direct-gap light emission is reported for Si-capped n+-Ge layers on Si after post-growth rapid cyclic annealing (RCA), and impact of non-radiative recombination (NRR) at the Ge/Si interface is discussed toward Ge/Si double heterostructure (DH) lasers. P-doped n+-Ge layer (1 × 1019 cm-3, 400 nm) is grown on Si by ultra-high vacuum chemical vapor deposition, followed by a growth of Si capping layer (5 nm) to form a Si/Ge/Si DH structure. Post-growth RCA to eliminate defects in Ge is performed in N2 at temperatures between 900°C and 780°C, where the annealing time is minimized to be 5 s in each RCA cycle to prevent an out-diffusion of P dopants from the Ge surface. Direct-gap photoluminescence (PL) intensity at 1.6 µm increases with the RCA cycles up to 40, although the threading dislocation density in Ge is not reduced after 3 cycles in the present condition. The PL enhancement is ascribed to the suppression of NRR at the Ge/Si interface, where an intermixed SiGe alloy is formed. For Ge/Si DH lasers, NRR at the Ge/Si interface is found to have a significant impact on the threshold current density Jth. In order to achieve Jth on the order of 1 kA/cm2, similar to III-V lasers, the interface recombination velocity S is required below 103 cm/s in spite of S as large as 105 cm/s at the ordinary defect-rich Ge/Si interface.

Enhanced light emission from improved homogeneity in biaxially suspended Germanium membranes from curvature optimization.

A silicon compatible light source is crucial to develop a fully monolithic silicon photonics platform. Strain engineering in suspended Germanium membranes has offered a potential route for such a light source. However, biaxial structures have suffered from poor optical properties due to unfavorable strain distributions. Using a novel geometric approach and finite element modelling (FEM) structures with improved strain homogeneity were designed and fabricated. Micro-Raman (µ-Raman) spectroscopy was used to determine central strain values. Micro-photoluminescence (µ-PL) was used to study the effects of the strain profiles on light emission; we report a PL enhancement of up to 3x by optimizing curvature at a strain value of 0.5% biaxial strain. This geometric approach offers opportunity for enhancing the light emission in Germanium towards developing a practical on chip light source.

A new material platform of Si photonics for implementing architecture of dense wavelength division multiplexing on Si bulk wafer.

A new materials group to implement dense wavelength division multiplexing (DWDM) in Si photonics is proposed. A large thermo-optic (TO) coefficient of Si malfunctions multiplexer/demultiplexer (MUX/DEMUX) on a chip under thermal fluctuation, and thus DWDM implementation, has been one of the most challenging targets in Si photonics. The present study specifies an optical materials group for DWDM by a systematic survey of their TO coefficients and refractive indices. The group is classified as mid-index contrast optics (MiDex) materials, and non-stoichiometric silicon nitride (SiNx) is chosen to demonstrate its significant thermal stability. The TO coefficient of non-stoichiometric SiNx is precisely measured in the temperature range 24-76 °C using the SiNx rings prepared by two methods: chemical vapor deposition (CVD) and physical vapor deposition (PVD). The CVD-SiNx ring reveals nearly the same TO coefficient reported for stoichiometric CVD-Si3N4, while the value for the PVD-SiNx ring is slightly higher. Both SiNx rings lock their resonance frequencies within 100 GHz in this temperature range. Since CVD-SiNx needs a high temperature annealing to reduce N-H bond absorption, it is concluded that PVD-SiNx is suited as a MiDex material introduced in the CMOS back-end-of-line. Further stabilization is required, considering the crosstalk between two channels; a 'silicone' polymer is employed to compensate for the temperature fluctuation using its negative TO coefficient, called athermalization. This demonstrates that the resonance of these SiNx rings is locked within 50 GHz at the same temperature range in the wavelength range 1460-1620 nm (the so-called S, C, and L bands in optical fiber communication networks). A further survey on the MiDex materials strongly suggests that Al2O3, Ga2O3 Ta2O5, HfO2 and their alloys should provide even more stable platforms for DWDM implementation in MiDex photonics. It is discussed that the MiDex photonics will find various applications such as medical and environmental sensing and in-vehicle data-communication.

High-performance silicon photonics technology for telecommunications applications.

By way of a brief review of Si photonics technology, we show that significant improvements in device performance are necessary for practical telecommunications applications. In order to improve device performance in Si photonics, we have developed a Si-Ge-silica monolithic integration platform, on which compact Si-Ge-based modulators/detectors and silica-based high-performance wavelength filters are monolithically integrated. The platform features low-temperature silica film deposition, which cannot damage Si-Ge-based active devices. Using this platform, we have developed various integrated photonic devices for broadband telecommunications applications.

Strain induced bandgap and refractive index variation of silicon.

We present a study of the influence of high strain on the bandgap and the refractive index of silicon. The results of photoluminescence show that with the strain applied, the silicon bandgap can be adjusted to 0.84 eV and the refractive index of silicon increases significantly. 1.4% change of refractive index of silicon was observed. The strain-induced bandgap shrinkage and absorption coefficient change of silicon are considered as the main cause of the significant refractive index change. The present work indicates that the application of strain is promising to control the refractive index of silicon in devices so that applications such as compensation of thermal effect in optical devices can be achieved.

Demonstration of optical computing logics based on binary decision diagram.

Optical circuits are low power consumption and fast speed alternatives for the current information processing based on transistor circuits. However, because of no transistor function available in optics, the architecture for optical computing should be chosen that optics prefers. One of which is Binary Decision Diagram (BDD), where signal is processed by sending an optical signal from the root through a serial of switching nodes to the leaf (terminal). Speed of optical computing is limited by either transmission time of optical signals from the root to the leaf or switching time of a node. We have designed and experimentally demonstrated 1-bit and 2-bit adders based on the BDD architecture. The switching nodes are silicon ring resonators with a modulation depth of 10 dB and the states are changed by the plasma dispersion effect. The quality, Q of the rings designed is 1500, which allows fast transmission of signal, e.g., 1.3 ps calculated by a photon escaping time. A total processing time is thus analyzed to be ~9 ps for a 2-bit adder and would scales linearly with the number of bit. It is two orders of magnitude faster than the conventional CMOS circuitry, ~ns scale of delay. The presented results show the potential of fast speed optical computing circuits.

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.

Laterally coupled silicon-germanium modulator for passive waveguide systems.

We model a laterally coupled Franz-Keldysh add-drop ring modulator designed to overcome the C-band indirect absorption of silicon-germanium. Although our concept is based on loss-sensitive interferometry, it utilizes the same highly absorptive germanium-rich compositions geared toward complementary metal-oxide semiconductor (CMOS) photodetectors and electroabsorption modulators. The proposed device can be integrated with passive waveguide networks in which the carrier plasma modulation mechanism is ineffective. In addition, unlike previous silicon-germanium modulator schemes, complex butt-coupling between the passive transport and the active silicon-germanium waveguides is not required. Instead, the optical mode remains guided within the transport waveguide, minimizing transition losses.

Chiral germanium micro-gears for tuning orbital angular momentum.

Group IV light sources with vertical emission and non-zero orbital-angular momentum (OAM) promise to unlock many novel applications. In this report, we demonstrate cylindrically symmetrical germanium micro-gear cavities, fabricated by etching a grating around the circumference of standard micro-disks, with periods ranging from 14 to 22. Photoluminescence (PL) measurements were done to identify the confined whispering-gallery modes (WGM). Finite-difference time-domain (FDTD) simulations were conducted to map the resonant modes to their modal profiles and characteristics. Vertical emission of WGMs with non-zero OAM was demonstrated, with a clear dependence of the OAM order ([Formula: see text]) on the WGM azimuthal order and the number of micro-gear grating periods. As the chirality, or the direction of rotation, is not controlled in a symmetrical cavity, we propose introducing staircase or triangular-shaped gear periods resulting in an asymmetry. By choosing the diameter, number of periods, and the asymmetrical direction of the gear-teeth, it is possible to generate OAM signals with certain wavelength, OAM order and chirality.

Emission wavelength tuning by mechanical stressing of GaAs/Ge/Si microbeams.

We propose an approach for tuning a gain spectrum of semiconductor lasers under temperature fluctuations, where the heat-induced effect is dynamically compensated using a mechanical stressing. By stressing GaAs/Ge/Si microbeams, emission wavelength tuning is experimentally demonstrated for the overlying GaAs layers as a proof-of-concept, and the results are followed by theoretical calculations. It is discussed that this approach is effective to cancel the gain spectrum shift and will be indispensable to the integration of light sources toward WDM systems on a chip.

Bandgap control using strained beam structures for Si photonic devices.

We have demonstrated that bandgap energy of Si can be controlled by micro-mechanically structured Si beams (250 nm thick, 3 µm wide, and 15 µm long) elastically deformed by an external force. Microscopic photoluminescence spectroscopy reveals that downward bending of the beam by 3 µm reveals a red shift in the peak from ~1100 nm up to ~1300 nm. It is found from calculations based on deformation potentials and finite element method that tensile strain as large as ~1.5% is generated in the top surface of the deformed beam and responsible for the red shift of the peak. The presented result should be a proof of concept to cancel wavelength fluctuation unavoidably occurring on uncooled LSIs in terms of stress application, and thereby an enabler of wavelength division multiplexing implementation on a chip. The applications of other beam materials such as Ge and GaAs are discussed.

Monolithic integration and synchronous operation of germanium photodetectors and silicon variable optical attenuators.

We demonstrate the monolithic integration of germanium (Ge) p-i-n photodetector (PDs) with silicon (Si) variable optical attenuator (VOAs) based on submicrometer Si rib waveguide. A PD is connected to a VOA along the waveguide via a tap coupler. The PDs exhibit low dark current of ~60 nA and large responsivity of ~0.8 A/W at the reverse bias of 1 V at room temperature. These characteristics are uniform over the chip scale. The PDs generate photocurrents precisely with respect to DC optical power attenuated by the VOAs. Two devices work synchronously for modulated optical signals as well. 3-dB cut-off frequency of the VOA is ~100 MHz, while that of the PD is ~1 GHz. The synchronous response speed is limited by the VOA response speed. This is the first demonstration, to the best of our knowledge, of monolithic integration of Ge PDs with high-carrier-injection-based optical modulation devices based on Si.

Theoretical Calculation and Experimental Verification for Dislocation Reduction in Germanium Epitaxial Layers with Semicylindrical Voids on Silicon.

Reduction of threading dislocation density (TDD) in epitaxial germanium (Ge) on silicon (Si) has been one of the most important challenges for the realization of monolithically integrated photonics circuits. The present paper describes methods of theoretical calculation and experimental verification of a novel model for the reduction of TDD. The method of theoretical calculation describes the bending of threading dislocations (TDs) based on the interaction of TDs and non-planar growth surfaces of selective epitaxial growth (SEG) in terms of dislocation image force. The calculation reveals that the presence of voids on SiO2 masks help to reduce TDD. Experimental verification is described by germanium (Ge) SEG, using an ultra-high vacuum chemical vapor deposition method and TD observations of the grown Ge via etching and cross-sectional transmission electron microscope (TEM). It is strongly suggested that the TDD reduction would be due to the presence of semicylindrical voids over the SiO2 SEG masks and growth temperature. For experimental verification, epitaxial Ge layers with semicylindrical voids are formed as the result of SEG of Ge layers and their coalescence. The experimentally obtained TDDs reproduce the calculated TDDs based on the theoretical model. Cross-sectional TEM observations reveal that both the termination and generation of TDs occur at semicylindrical voids. Plan-view TEM observations reveal a unique behavior of TDs in Ge with semicylindrical voids (i.e., TDs are bent to be parallel to the SEG masks and the Si substrate).

Enhanced direct bandgap emission in germanium by micromechanical strain engineering.

We propose a new class of optoelectronic devices in which the optical properties of the active material is enhanced by strain generated from micromechanical structures. As a concrete example, we modeled the emission efficiency of strained germanium supported by a cantilever-like platform. Our simulations indicate that net optical gain is obtainable even in indirect germanium under a substrate biaxial tensile strain of about 1.75% with an electron-hole injection concentration of 9 x 10(18) cm(-3) while direct bandgap germanium becomes available at a strain of 2%. A large wavelength tuning span of 300 nm in the mid-IR range also opens up the possibility of a tunable on-chip germanium biomedical light source.