A review of silicon-based wafer bonding processes,an approach to realize the monolithic integration of Si-CMOS and III-V-on-Si wafers

The heterogeneous integration of III-V devices with Si-CMOS on a common Si platform has shown great promise in the new generations of electrical and optical systems for novel applications,such as HEMT or LED with integrated control circuitry.For heterogeneous integration,direct wafer bonding(DWB)techniques can overcome the materials and thermal mismatch issues by directly bonding dissimilar materials systems and device structures together.In addition,DWB can perform at wafer-level,which eases the requirements for integration alignment and increases the scalability for volume production.In this paper,a brief review of the different bonding technologies is discussed.After that,three main DWB techniques of single-,double-and multi-bonding are presented with the demonstrations of various heterogeneous integration applications.Meanwhile,the integration challenges,such as micro-defects,surface roughness and bonding yield are discussed in detail.

On-chip light sources for silicon photonics

Serving as the electrical to optical converter,the on-chip silicon light source is an indispensable component of silicon photonic technologies and has long been pursued.Here,we briefly review the history and recent progress of a few promising contenders for on-chip light sources in terms of operating wavelength,pump condition,power consumption,and fabrication process.Additionally,the performance of each contender is also assessed with respect to thermal stability,which is a crucial parameter to consider in complex optoelectronic integrated circuits(OEICs)and optical interconnections.Currently,III-V-based silicon(Si)lasers formed via bonding techniques demonstrate the best performance and display the best opportunity for commercial usage in the near future.However,in the long term,direct hetero-epitaxial growth of III–V materials on Si seems more promising for low-cost,high-yield fabrication.The demonstration of high-performance quantum dot(QD)lasers monolithically grown on Si strongly forecasts its feasibility and enormous potential for on-chip lasers.The superior temperature-insensitive characteristics of the QD laser promote this design in large-scale high-density OEICs.The Germanium(Ge)-on-Si laser is also competitive for large-scale monolithic integration in the future.Compared with a III-V-based Si laser,the biggest potential advantage of a Ge-on-Si laser lies in its material and processing compatibility with Si technology.Additionally,the versatility of Ge facilitates photon emission,modulation,and detection simultaneously with a simple process complexity and low cost.

Graphene-based optical phase modulation of waveguide transverse electric modes

In this paper we report TE-mode phase modulation obtained by inducing a capacitive charge on graphene layers embedded in the core of a waveguide.There is a biasing regime in which graphene absorption is negligible but large index variations can be achieved with a voltage–length product as small as V_(π)L_(π)≃0.07 V cm for straight waveguides and V_(π)L_(π)≃0.0024 V cm for 12μm radius microring resonators.This phase modulation device uniquely enables a small signal amplitude<1 V with a micrometer-sized footprint for compatibility with CMOS circuit integration.Examples of phase-induced changes are computed for straight waveguides and for microring resonators,showing the possibility of implementing several optoelectronic functionalities as modulators,tunable filters,and switches.

Single-drive high-speed lumped depletion-type modulators toward 10 fJ/bit energy consumption

Reduction of modulator energy consumption to 10 fJ∕bit is essential for the sustainable development of communication systems.Lumped modulators might be a viable solution if instructed by a complete theory system.Here,we present a complete analytical electro-optic response theory,energy consumption analysis,and eye diagrams on absolute scales for lumped modulators.Consequently the speed limitation is understood and alleviated by single-drive configuration,and comprehensive knowledge into the energy dependence on structural parameters significantly reduces energy consumption.The results show that silicon modulation energy as low as 80.8 and 21.5 fJ∕bit can be achieved at 28 Gbd under 50 and 10 Ω impedance drivers,respectively.A 50 Gbd modulation is also shown to be possible.The analytical models can be extended to lumped modulators on other material platforms and offer a promising solution to the current challenges of modulation energy reduction.

Ultra-flat dispersion in an integrated waveguide with five and six zero-dispersion wavelengths for mid-infrared photonics

We propose a new type of dispersion flattening technology, which can generate an ultra-flat group velocity dispersion profile with five and six zero-dispersion wavelengths(ZDWs). The dispersion value varies from-0.15 to 0.35 ps/(nm·km) from 4 to 8 μm, which to the best of our knowledge is the flattest one reported so far, and the dispersion flatness is improved by more than an order of magnitude. We explain the principle of producing six ZDWs. Mode distribution in this waveguide is made stable over a wide bandwidth. General guidelines to systematically control the dispersion value, sign, and slope are provided, and one can achieve the desired dispersion by properly adjusting the structural parameters. Fabrication tolerance of this waveguide is also examined.

Broadband athermal waveguides and resonators for datacom and telecom applications

The high-temperature sensitivity of the silicon material index limits the applications of silicon-based micro-ring resonators in integrated photonics. To realize a low but broadband temperature-dependent-wavelength-shift microring resonator, designing a broadband athermal waveguide becomes a significant task. In this work,we propose a broadband athermal waveguide that shows a low effective thermo-optical coefficient of1 × 10^(-6)∕K from 1400 to 1700 nm. The proposed waveguide shows a low-loss performance and stable broadband athermal property when it is applied to ring resonators, and the bending loss of ring resonators with a radius of >30 μm is 0.02 dB/cm.

High-efficiency normal-incidence vertical p-i-n photodetectors on a germanium-on-insulator platform

In this paper, normal incidence vertical p-i-n photodetectors on a germanium-on-insulator(GOI) platform were demonstrated. The vertical p-i-n structure was realized by ion-implanting boron and arsenic at the bottom and top of the Ge layer, respectively, during the GOI fabrication. Abrupt doping profiles were verified in the transferred high-quality Ge layer. The photodetectors exhibit a dark current density of ~47 mA∕cm^2 at-1 V and an optical responsivity of 0.39 A/W at 1550 nm, which are improved compared with state-of-the-art demonstrated GOI photodetectors. An internal quantum efficiency of ~97% indicates excellent carrier collection efficiency of the device. The photodetectors with mesa diameter of 60 μm exhibit a 3 dB bandwidth of ~1 GHz, which agrees well with theoretical calculations. The bandwidth is expected to improve to ~32 GHz with mesa diameter of 10 μm. This work could be similarly extended to GOI platforms with other intermediate layers and potentially enrich the functional diversity of GOI for near-infrared sensing and communication integrated with Ge CMOS and mid-infrared photonics.

Robust cavity soliton formation with hybrid dispersion

Microresonator-based Kerr frequency combs have attracted a great deal of attention in recent years, in which mode locking of the generated combs is associated with bright or dark cavity soliton formation. In this paper, we show a unique that, different from soliton propagation along a waveguide, cavity solitons can be robustly formed under dispersion profile with four zero-dispersion wavelengths. More importantly, such a dispersion profile exhibits much smaller overall dispersion, thus making it possible to greatly reduce the pump power by five to six times.

Efficient above-band-gap light emission in germanium

We report an above-band-gap radiative transition in the photoluminescence spectra of single crystalline Ge in the temperature range of 20-296 K. The temperature-independence of the peak position at -0.74 eV is remarkably different from the behavior of direct and indirect gap transitions in Ge. This transition is observed in n-type, p-type, and intrinsic single crystal Ge alike, and its intensity decreases with the increase of temperature with a small activation energy of 56 meV. Some aspects of the transition are analogous to III-V semiconductors with dilute nitrogen doping, which suggests that the origin could be related to an isoelectronic defect.

Direct bandgap photoluminescence from n-type indirect GaInP alloys

This work studies Te doping effects on the direct bandgap photoluminescence(PL) of indirect Ga_(x)In_(1-x)P alloys(0.72 ≤ x ≤ 0.74). The temperature-dependent PL shows that the energy difference between direct Γ valley and indirect X valleys is reduced due to the bandgap narrowing(BGN) effect, and the direct band transition gradually dominates the PL spectra as temperature increases. Carrier thermalization has been observed for Te-doped Ga_(x)In_(1-x)P samples, as integrated PL intensity increases with increasing temperature from 175 to 300 K. The activation energy for carrier thermalization is reduced as doping concentration increases. Both BGN effect and carrier thermalization contribute to the carrier injection into the Γ valley. As a result, the direct band transition is enhanced in the Te-doped indirect Ga_(x)In_(1-x)P alloys. Therefore, the PL intensity of the Ga_(0.74)In_(0.26) P sample with active doping concentration of 9 × 10^(17)cm^(-3)is increased by five times compared with that of a nominally undoped sample. It is also found that the PL intensity is degraded significantly when the doping concentration is increased to 5 × 10^(18)cm^(-3). From cross-section transmission electron microscopy,no large dopant clusters or other extended defects were found contributing to this degradation.

High-performance AlGaInP light-emitting diodes integrated on silicon through a superior quality germanium-on-insulator

High-performance GaInP/AlGaInP multi-quantum well light-emitting diodes(LEDs) grown on a low threading dislocation density(TDD) germanium-on-insulator(GOI) substrate have been demonstrated. The low TDD of the GOI substrate is realized through Ge epitaxial growth, wafer bonding, and layer transfer processes on 200 mm wafers. With O_2 annealing, the TDD of the GOI substrate can be reduced to ~1.2 × 10~6 cm^(-2). LEDs fabricated on this GOI substrate exhibit record-high optical output power of 1.3 mW at a 670 nm peak wavelength under 280 mA current injection. This output power level is at least 2 times higher compared to other reports of similar devices on a silicon(Si) substrate without degrading the electrical performance. These results demonstrate great promise for the monolithic integration of visible-band optical sources with Si-based electronic circuitry and realization of high-density RGB(red, green, and blue) micro-LED arrays with control circuitry.

PIC-integrable,uniformly tensile-strained Ge-on-insulator photodiodes enabled by recessed SiN_(x) stressor

Mechanical strain engineering has been promising for many integrated photonic applications. However,for the engineering of a material electronic bandgap,a trade-off exists between the strain uniformity and the integration compatibility with photonic-integrated circuits (PICs). Herein,we adopted a straightforward recess-type design of a silicon nitride (SiNx) stressor to achieve a uniform strain with enhanced magnitude in the material of interest on PICs. Normal-incidence,uniformly 0.56% tensile strained germanium (Ge)-on-insulator (GOI) metal-semiconductor-metal photodiodes were demonstrated,using the recessed stressor with 750 MPa tensile stress. The device exhibits a responsivity of 1.840.15 A∕W at 1550 nm. The extracted Ge absorption coefficient is enhanced by -3.2×to 8340 cm^(-1) at 1612 nm and is superior to that of In_(0.53)Ga_(0.47)As up to 1630 nm limited by the measurement spectrum. Compared with the nonrecess strained device,additional absorption coefficient improvement of 10%–20% in the C-band and 40%–60% in the L-band was observed. This work facilitates the recessstrained GOI photodiodes for free-space PIC applications and paves the way for various (e.g.,Ge,GeSn or III-V based) uniformly strained photonic devices on PICs.

Introduction for the Group-IV Photonics feature

Silicon-based photonics technology,which has the DNA of silicon electronics technology,promises to provide a compact photonic integration platform with high integration density,mass-producibility,and excellent cost performance.This technology has been used to develop various photonic devices based on silicon,such as waveguides,filters,and modulators.In addition,germanium photodetectors have been built on a silicon-based photonic platform.These photonic devices have already been monolithically integrated,and a group-IV photonic platform based on silicon and germanium,or standard group-IV materials,has been established.Moreover,photonics–electronics convergence is now being pursued based on this platform.

Robust generation of frequency combs in a microresonator with strong and narrowband loss

Octave-spanning frequency comb generation in microresonators is promising, but strong spectral losses caused by material absorption and mode coupling between two polarizations or mode families can be detrimental. We examine the impact of the spectral loss and propose robust comb generation with a loss of even 300 dB/cm.Cavity nonlinear dynamics show that a phase change associated with spectral losses can facilitate phase matching and Kerr comb generation. Given this unique capability, we propose a novel architecture of on-chip spectroscopy systems.