InP Platform Research Articles

Conductance slope and curvature coefficient of InGaAs/GaAsSb heterojunctions at varying band alignments and its implication on digital and analog applications

We assess InGaAs/GaAsSb heterojunctions at varying band alignments for applications in both tunnel field effect transistors (TFETs) as well as for nonlinear analog components such as millimeter wave detectors. We use conductance slope measurements as a fundamental figure of merit, as it is not affected by the three-terminal parasitics of subthreshold-slope in a TFET and represents the ideal subthreshold slope intrinsic to the junction in the absence of three-terminal parasitics. We prove that conductance slope/subthreshold slope is not equivalent to curvature coefficient, indicating that it is actually easier to exceed the thermal limit of curvature for analog applications than it is to exceed the subthreshold slope limit for digital applications. In addition, we show that no published heterojunction that exceeds the curvature limit would be capable of exceeding the subthreshold slope limit. We experimentally demonstrate the formation of epitaxial InGaAs/GaAsSb heterojunctions at varying band alignments accomplished using lattice-mismatched epitaxy with graded buffers. We show a dependence of conductance slope on material quality, adding further proof that “steepness” is limited by materials defects and inhomogeneity. We demonstrate that the conductance slope does not depend on temperature for type-II band alignment, adding further proof that TFETs, which show strong temperature dependencies, are dominated by thermal parasitics. Finally, we develop and demonstrate an InGaAs/GaAsSb heterojunction system integrated on an InP platform with a record 76 mV/decade conductance slope and 43 V−1 curvature coefficient near zero-bias, with the capability of up to 60 V−1 curvature coefficient.

High-Speed InP-Integrated Pre-Amplified Demodulator for WDM-DPSK Signals

A compact demodulator for wavelength division multiplexed (WDM)-differential phase shift keying signals monolithically integrated in an InP platform with a semiconductor optical pre-amplifier is reported. The photonic integrated circuit, made by an semiconductor optical amplifier and an ad hoc arrayed waveguide grating (AWG), is designed for the simultaneous demodulation of a WDM comb in a single circuit thanks to narrow filtering. The demodulation of 56-Gb/s channels on the eight, 100-GHz-spaced, AWG outputs is experimentally demonstrated.

Design of an ultra-short coupler in an asymmetric twin-waveguide structure using transformation optics.

An integrated vertical coupler that transfers light from the lower passive waveguide to the upper active waveguide in an asymmetric twin-waveguide integration structure (in InGaAsP/InP material system) is designed using transformation optics (TO). The length of the coupler is as short as 3μm, which is two orders of magnitude shorter than that of traditional tapered couplers. According to three-dimensional full-wave simulations, the designed optimized coupler has a high coupling efficiency of 94.9%, and a low reflection at the wavelength of 1.55μm. Subsequently, quasi-conformal mapping is employed to reduce the material complexity and to make it possible to realize the coupler by purely using an isotropic dielectric material. Applying TO to integrated photonic devices may motivate new applications, and improve integration density on the InP platform.

(Invited) Monolithic Integration of InP Based Structures on Silicon for Optical Interconnects

Monolithic integration of InP based structures on Si for optical interconnects is presented. Different strategies are demonstrated to achieve requisite InP platform on Si. In the first strategy, defect free isolated areas of epitaxially and laterally overgrown InP are obtained on Si and the InGaAsP based quantum wells directly grown on these templates have shown high material quality with uniform interfaces. In the second strategy, selective area growth is exploited to achieve InP nano pyramids on Si which can be used for the growth of quantum dot structures. In the third and the final strategy, a method is presented to achieve direct interface between InP and Si using corrugated epitaxial lateral overgrowth.

The Future of Silicon Photonics: Not So Fast? Insights From 100G Ethernet LAN Transceivers

While many articles have touted Si photonics' potential to bring the bandwidth and power-efficiency benefits of photonics to mainstream semiconductor applications, rigorous economic analysis has been lacking. This paper leverages extensive data from the major electronic and photonic semiconductor manufacturers to model the competitiveness of two Si photonic designs against InP-based alternatives for a 1310 nm, 100 gigabit ethernet LAN transceiver. Our results suggest that silicon photonics may struggle finding low-volume opportunities for early-stage market adoption for the very reasons that silicon photonics is attractive-the existing capital-intensive infrastructure in Si-CMOS. Contrary to popular belief, we demonstrate that InP platforms can, depending on the yields achieved in each technology, have equal to or lower production costs than silicon for all expected production volumes. Silicon photonics does hold great potential to be cost competitive in markets with annual sales volumes above 900 000, including servers, computing, and mobile devices.

Ultracompact Multimode Interference Coupler Designed by Parallel Particle Swarm Optimization With Parallel Finite-Difference Time-Domain

An ultra-compact multimode interference (MMI) coupler with a length ~10 ?m for dense photonic integration is designed by parallel particle swarm optimization (PSO) with parallel finite-difference time-domain (FDTD). The MMI coupler uses a strongly-guiding deep-etched InP platform and the modal analysis of this strongly-guiding deep-etched InP waveguide gives the cross section of the single-mode nanophotonic waveguide, the geometry of the multimode interference region, and the etched-depth requirement. An efficient global optimizer combining parallel PSO algorithm and parallel FDTD is developed, which speedups the optimization and enables automatic nanophotonic design. Two 2 × 2 multimode interference couplers with a respective coupling ratio of 50%:50% and 90%:10% are designed as numerical examples using the global optimizer developed and the optimization result shows a ultra-compact multimode interference coupler with a length of ~10 ?m can be achieved with a low excess loss, which is numerically verified by a parallel three-dimensional FDTD.

Wavelength conversion using semiconductor optical amplifiers in differential Mach-Zehnder interferometer with tunable input coupler

A monolithically-integrated wavelength converter on an InP platform with a semiconductor optical amplifier in each arm of a differential Mach-Zehnder interferometer is presented. The data input has a tunable coupler that adjusts the splitting ratio of the optical power sent to each arm. Wavelength conversion at 40 Gbit/s is demonstrated and a conversion penalty of <2 dB is found when less optical power is sent to the delayed arm.

Optical-CDMA in InP

This paper describes the InP platforms for photonic integration and the development on these platforms of an optical code division multiple access (O-CDMA) system for local area networks. We demonstrate three building blocks of this system: an optical pulse source, an encoder/decoder pair, and a threshold detector. The optical pulse source consists of an integrated colliding pulse-mode laser with nearly transform-limited 10 Gb/s pulses and optical injection locking to an external clock for synchronization. The encoder/decoder pair is based on arrayed waveguide gratings. Bit-error-rate measurements involving six users at 10 Gb/s showed error-free transmission, while O-CDMA codes were calibrated using frequency resolved optical gating. For threshold detection after the decoder, we compared two Mach--Zehnder interferometer (MZI)-based optical thresholding schemes and present results on a new type of electroabsorber-based MZI.

Process-based cost modeling of photonics manufacture: the cost competitiveness of monolithic integration of a 1550-nm DFB laser and an electroabsorptive modulator on an InP platform

The monolithic integration of components holds promise to increase network functionality and reduce packaging expense. Integration also drives down yield due to manufacturing complexity and the compounding of failures across devices. Consensus is lacking on the economically preferred extent of integration. Previous studies on the cost feasibility of integration have used high-level estimation methods. This study instead focuses on accurate-to-industry detail, basing a process-based cost model of device manufacture on data collected from 20 firms across the optoelectronics supply chain. The model presented allows for the definition of process organization, including testing, as well as processing conditions, operational characteristics, and level of automation at each step. This study focuses on the cost implications of integration of a 1550-nm DFB laser with an electroabsorptive modulator on an InP platform. Results show the monolithically integrated design to be more cost competitive over discrete component options regardless of production scale. Dominant cost drivers are packaging, testing, and assembly. Leveraging the technical detail underlying model projections, component alignment, bonding, and metal-organic chemical vapor deposition (MOCVD) are identified as processes where technical improvements are most critical to lowering costs. Such results should encourage exploration of the cost advantages of further integration and focus cost-driven technology development

Self-aligned via and trench for metal contact in III-V semiconductor devices

A semiconductor processing method for the formation of self-aligned via and trench structures in III-V semiconductor devices (in particular, on InP platform) is presented, together with fabrication results. As a template for such self-aligned via and trench formations in a surrounding polymer layer on a semiconductor device, we make use of a sacrificial layer that consists of either a SiO2 dielectric hard mask layer deposited on the device layers or a sacrificial semiconductor layer grown on top of the device epitaxial layers (e.g., InP on an InGaAs etch stop), both laid down on the device layers before patterning the device geometry. During the semiconductor device etching, the sacrificial layer is kept as a part of the patterned structures and is, therefore, perfectly self-aligned. By selectively removing the sacrificial layer surrounded by the polymer that is etched back within the thickness of the sacrificial layer, an opening such as a via and a trench is formed perfectly self-aligned on the device top area in the place of the sacrificial layer. This process yields a pristine semiconductor surface for metal contacts and fully utilizes the contact area available on the device top, no matter how small the device area is. This approach thus provides as low an Ohmic contact resistance as possible upon filling the via and the trench with metal deposition. The additional use of a thin Si3N4 protecting layer surrounding the device sidewalls improves the robustness of the process without any undesired impact on the device electrical passivation (or on the optical mode characteristics if the device also includes a waveguide). This method offers metal contacts scalable to the device size, being limited only by the feasible device size itself. This method is also applicable to the fabrication of other III-V based integrated devices.

Fabrication of Moisture-Desensitized Freestanding Structures from InP for Photonic-MEMS Applications

Compound semiconductor micromechanical structures offer enormous potential for convergence of photonics and microelectromechanical systems (MEMS). This manuscript lays out the fundamental fabrication process principles and postfabrication processing of micromechanical structures of photonic MEMS based on InP platform. For prevention of adhesive failures in these microstructures from “in-use” stiction, hydrophobicity of microstructure surfaces is a benign property. Unlike Si which has a hydrophyllic surface, InP is fortunately relatively hydrophobic, as confirmed by contact angle measurements. However, this hydrophobicity in InP is lost in the course of processing, as confirmed by contact angle measurements and detection of hydroxyl functional groups on surface by investigations using X-ray photoelectron spectroscopy. Vapor phase surface treatment with hexamethyldisilazane helps to restore the surface hydrophobicity of InP, as evidenced in post-treatment elevated contact angles. Additionally, the microstructures have been observed to show out-of-plane deformation, which has been characterized with white light interferometry. The stress gradient has been calculated to be from bending in cantilever arrays. The source of this stress gradient in InP layers was investigated through depth profiling using secondary ion mass spectrometry, which revealed arsenic gradient through the thickness of InP layers.

Ultrahigh-speed InP/InGaAs DHPTs for OEMMICs

This paper presents an ultrahigh-speed InP/InGaAs double-heterostructure phototransistor (DHPT) with a record optical gain cutoff frequency of 82 GHz. This excellent performance originates from the double-heterostructure's compatibility with high-performance double-heterostructure bipolar transistor (DHBT) and a new self-aligned process. To demonstrate the excellent performance of the DHPT, two kinds of optoelectronic MMICs (OEMMICs) were designed and fabricated. One is a 40-GHz-band DHPT/DHBT photoreceiver that shows the DHPT's ability to be simultaneously integrated with a high-performance DHBT. The 40 GHz operation frequency is also the highest reported for monolithically integrated HPT/HBT photoreceivers. The other is a direct optical injection-locked oscillator that can extract an electrical clock signal from optical data streams. The OEMMICs are promising for compact and low-power-consumption optical receivers on an InP platform for millimeter-wave photonics and ultrahigh-speed optical communication systems.