80Gb/s multi-level BPSK experiment with an InP-monolithic source based on prefixed optical phase switching

Monolithic photonic integrated circuits (PIC) transmitters using the prefixed optical phase switching concept for BPSK modulation format have been shown promising at 1.55 μm band. These devices could also be crucial for short reach connections and access networks. With this aim, we are studying basic quantum well designs for a laser and an electro‐absorption modulator switch to be integrated by selective area growth into PICs at 1.3 μm. Photocurrent measurements and band offset modeling have been performed to determine the MQW stack well‐fitted for this application. Broad area laser measurements have also been checked on these structures to verify the material lasing properties. A 6 nm thick well with low barrier seems to be the best trade‐off between absorption and shift for 1.3 μm EAM and it also gives good lasing properties.

Gallium Nitride (GaN) possesses a wide band gap and excellent transport properties, making it a highly promising material for high-power and high-frequency applications. Microwave power amplification for commercial and military purposes has already benefited from GaN-based high electron mobility transistors (HEMTs). However, currently available GaN optoelectronic and electronic devices use planar junctions and heterostructures prepared by epitaxy, typically via metalorganic chemical vapor deposition (MOCVD). To enable more sophisticated device configurations, in-plane lateral junctions are necessary. The most effective method for achieving this is through selective area etching (SAE) followed by selective area growth (SAG). In this talk, we will discuss the progress made at Yale in the selective-area etching, growth, and doping of GaN.Selective-area etching of GaN is the most challenging and critical step in this process. Instead of using conventional ICP etching, we have employed tertiarybutylchloride (TBCl) as an in-situ chemical etchant of GaN in MOCVD to create smooth trenches and remove plasma etching-generated damages on the surface. With electrical and material characterizations, we have confirmed that TBCl etching is a low-damage and clean etching process. We will also describe our recent efforts to bypass the constraint of dielectric masking during in-situ etching and perform maskless selective area etching (SAE) to create low-defect lateral p-n junctions for GaN power electronics. Additionally, we will discuss non-planar selective-area growth and doping, including the principles of growth evolution, non-uniform Mg distribution, and parasitic impurities.This work is supported by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. DOE, under DE-AR0000871 as part of the PNDIODES program managed by Dr. Isik Kizilyalli.

The goal of the present work was to perform mid-infrared characterization of two dimensional photonic crystal slabs fabricated in silicon with laser interference lithography. A two-dimensional (2D) silicon photonic crystal (PhC), which is designed to provide a modified dispersion for photon energies of less than half of the electronic band gap of silicon, and which has been fabricated by a novel modification of laser interference lithography (LIL), is studied by angular dependent infrared reflectivity and transmission measurements. The existence of resonance features is experimentally demonstrated as observed in the polarized reflectivity and transmission spectra, and which arises from resonant coupling of the incident infrared radiation to photonic modes. The measured photonic crystal resonances are used to derive the quality factors of the probed photonic modes via fits to the Fano-type line shapes found. The quality factor of the corresponding photonic modes is also theoretically calculated. The obtained theoretical values, and comparison with the experimentally obtained quality factors, provide the first information on the LIL fabrication-inherent quality of the crystals. In the named experiments, the LIL fabricated crystals are investigated with low intensity probe beams in the near to mid-infrared range, such that an optically nonlinear response cannot be detected. To provide also information on the nonlinear response, we present initial results also at high intensities. For this, we have studied the light induced change in the optical phase upon reflection from the photonic crystal using a Mach-Zehnder setup. The technique involves measuring the time dependent reflection of a pulsed probe beam on the 2D PhC sample due to the excitation from an additional, pulsed drive beam. With this technique we have realized what we believe to be the first experimental observation of optical switching of the reflection phase from a guided resonance of a 2D PhC slab. In conclusion, the goals that have been achieved are the development of a novel fabrication process for high-index 2D photonic crystals, the optically linear characterization of the properties of the fabricated crystal using specular reflectivity and normal incidence transmission experiments, and the first interferometric characterization of the optically nonlinear response of photonic crystal in terms of an optical switching of the reflection phase from a photonic resonance.

Driven by fabrication cost reduction and device performance improvement, the Silicon semiconductor industry continues its never-ending pursuit of new approaches for fabricating integrated circuits. In this context, the monolithic integration of III-V semiconductors epitaxially grown on Si substrate have been attracting much attention as building blocks for next-generation electronics and photonics due to their potential intrinsic properties. Direct heteroepitaxy of III-V compound semiconductors on Si has traditionally represented a formidable challenge, due to the high material mismatch (lattice parameter, polarity, thermal coefficient, ...) between Si and III-V semiconductors generating high density of defect density during the epitaxial process. To overcome this, selective area growth of III-Vs in a pre-patterned Si substrate by Selective Area Metal-Organic Vapor Phase Epitaxy allow the possibility to obtain high quality and reduced defect density III-V based active layers onto standard Si(001) substrate. Using this approach we integrate III-V materials monolithically on Si while focusing on both ultimate trench widths (W < 20 nm) scaling as well as relaxed trench dimension in function of the targeted application. We report here on the heteroepitaxy of InP buffer on initiated from a <111> V-groove Si surface and discuss the unique relaxation mechanism of such compound with respect to Si substrate. We derived a fundamental understanding and theoretical modeling of the growth mechanisms in STI trenches as well as the determining role of the InP nucleation layer. The subsequent growth approach of InGaAs active layer, used for both nFET or Laser applications, is further studied. Finally, III-V based electronic and photonic devices realized using selective area growth technique are presented and benchmarked to the current state of the art.

Surface selective growth of GaInAsP heterostructures by metalorganic MBE

We present a versatile method for in situ lateral growth control of optically efficient quantum structures. The method is based on molecular-beam epitaxy through an epitaxial shadow mask. Lateral control is achieved by selective area growth of short period superlattices. We demonstrate how the method can be applied to both lattice-matched and unmatched material systems. In the former, selective growth is employed in the formation of quantum wires. In the latter, this growth concept was also successfully applied to control the self-assembly of quantum dots within selected areas. The excellent quality of the quantum structures is demonstrated by their bright cathodoluminescence at room temperature.

Selective area growth of GaInNAs/GaAs by MOVPE

Successful selective area growth of GaAs on Si following low-temperature plasma cleaning of the Si surface is carried out by ECR plasma-excited MBE. Both cleaning and selective area growth are carried out at 630° C. GaAs selectively grows only in Si windows patterned in the SiN mask. Neither deposition on the SiN mask nor anomalous growth at the mask edge is observed. GaAs thickness is independent of the SiN mask width, which implies that the plasma-enhanced desorption of migrating atoms from the SiN mask is essential in this selective area growth. Furthermore, results for the helium-diluted arsine imply that the plasma-enhanced desorption is mainly caused by physical bombardment.

Wavelength tunable electro-absorption modulated distributed Bragg reflector lasers (TEMLs) are promising light source in dense wavelength division multiplexing (DWDM) optical fiber communication system due to high modulation speed, small chirp, low drive voltage, compactness and fast wavelength tuning ability. Thus, increased the transmission capacity, the functionality and the flexibility are provided. Materials with bandgap difference as large as 250nm have been integrated on the same wafer by a combined technique of selective area growth (SAG) and quantum well intermixing (QWI), which supplies a flexible and controllable platform for the need of photonic integrated circuits (PIC). A TEML has been fabricated by this technique for the first time. The component has superior characteristics as following: threshold current of 37mA, output power of 3.5mW at 100mA injection and 0V modulator bias voltage, extinction ratio of more than 20 dB with modulator reverse voltage from 0V to 2V when coupled into a single mode fiber, and wavelength tuning range of 4.4nm covering 6 100-GHz WDM channels. A clearly open eye diagram is observed when the integrated EAM is driven with a 10-Gb/s electrical NRZ signal. A good transmission characteristic is exhibited with power penalties less than 2.2 dB at a bit error ratio (BER) of 10^-10 after 44.4 km standard fiber transmission.

The emerging fields of silicon (Si) photonic micro–electromechanical systems (MEMS) and optomechanics enable a wide range of novel high-performance photonic devices with ultra-low power consumption, such as integrated optical MEMS phase shifters, tunable couplers, switches, and optomechanical resonators. In contrast to conventional SiO 2 -clad Si photonics, photonic MEMS and optomechanics have suspended and movable parts that need to be protected from environmental influence and contamination during operation. Wafer-level hermetic sealing can be a cost-efficient solution, but Si photonic MEMS that are hermetically sealed inside cavities with optical and electrical feedthroughs have not been demonstrated to date, to our knowledge. Here, we demonstrate wafer-level vacuum sealing of Si photonic MEMS inside cavities with ultra-thin caps featuring optical and electrical feedthroughs that connect the photonic MEMS on the inside to optical grating couplers and electrical bond pads on the outside. We used Si photonic MEMS devices built on foundry wafers from the iSiPP50G Si photonics platform of IMEC, Belgium. Vacuum confinement inside the sealed cavities was confirmed by an observed increase of the cutoff frequency of the electro-mechanical response of the encapsulated photonic MEMS phase shifters, due to reduction of air damping. The sealing caps are extremely thin, have a small footprint, and are compatible with subsequent flip-chip bonding onto interposers or printed circuit boards. Thus, our approach for sealing of integrated Si photonic MEMS clears a significant hurdle for their application in high-performance Si photonic circuits.

Nanoimprint lithography based selective area growth of indium phosphide nanopillar arrays on non-single-crystal templates

All-fiber integrated phase shifters and optical switches have important applications in photonic devices, such as optical controlling, optical fiber sensing, and signal processing. In this Letter, for the first time to the best of our knowledge, we integrated the photothermal effect of a nanomaterial based on gold nanorods (GNRs) and a microfiber interferometer to realize a compact all-optical fiber phase shifter. GNRs surrounding the microfiber were excited by near-infrared light via the evanescent interaction, subsequently releasing the heat through the photothermal effect. Then, the refractive index around the microfiber was varied to shift the interference dips in a reversible manner. Experimentally, a spectral shift efficiency of 0.16nm/mW near the wavelength of 1550nm was obtained using an excitation laser at the wavelength of 808nm. The device also provided an all-optical switching with the modulation depth of 76.4%. The proposed GNR-based all-fiber device can provide high potentials in all-optical signal control applications.

Low-temperature (LT) selective area growth (SAG) of degenerately silicon (Si) doped GaN by metal-organic-chemical vapor deposition (MOCVD) technique yielded a record-high charge carrier concentration (ns) of 2–2.2 × 1020 cm−3 with a mobility of 116–119 cm2/V s, utilizing triethylgallium and silane (SiH4) as gallium and Si precursors, respectively, and trimethylindium as a surfactant. While SiH4-doped GaN typically yields lower carrier concentrations (≤1020 cm−3) than disilane (Si2H6), this work demonstrates that defect and dislocation suppression enables degenerately doped GaN with ns exceeding 1020 cm−3 using the widely adopted SiH4 precursor. The use of an indium surfactant enhanced both ns and mobility by suppressing defect formation, enhancing 2D-step flow deposition mode compared to 3D island deposition mode in highly Si-doped GaN, enabling ns exceeding 1020 cm−3. SAG of LT-n++-GaN contact layer using SiH4 dopant in an AlGaN/GaN high-electron mobility transistor (HEMT) yielded an ultra-low sheet resistance (RSh) of 22 Ω/□ and a surface roughness of 0.33 nm, resulting in a record low Ohmic contact resistance of 0.065 Ω mm, along with a metal/n++-GaN contact resistance of 0.023 Ω mm, two-dimensional electron-gas-to-n++-GaN contact resistance of 0.025 Ω mm, and an access resistance (Rn+) of 0.017 Ω mm. A scaled AlGaN/GaN HEMT showed ultra-low on resistance (RON) of 0.36 Ω mm and a superior RF performance with current gain cutoff frequency (fT) and maximum oscillation frequency (fmax) of 160 and 195 GHz, respectively. Advancements in LT selective area MOCVD growth of n++-GaN using SiH4 enable regrown contacts in ultra-scaled AlGaN/GaN HEMTs, supporting commercialization for mm-wave applications.

ABSTRACTElectron-cyclotron-resonance plasma-excited molecular beam epitaxy (ECR-MBE) is a new technique for GaAs growth. This paper describes surface cleaning of GaAs and Si substrates at fairly low temperatures using hydrogen plasma, low temperature growth of GaAs on both substrates, and selective area growth of GaAs on both substrates partially covered with a silicon nitride (SiN) mask. The ability to clean and grow at low temperatures-assumed to be a benefit of using energetic particles—should permit us to grow layers on processed GaAs and/or Si substrates. The electrical properties of grown layers are also described. Selective area growth has been successfully carried out with no deposit on the mask or irregular growth at the mask edge. The desorption process introduced by impinging ions is found to play an important role in the selective area growth.

Planar avalanche photodiodes with edge breakdown suppression using a novel selective area growth based process