Ludovico Megalini

Nonpolar III-nitride vertical-cavity surface-emitting laser with a photoelectrochemically etched air-gap aperture

We demonstrate a III-nitride nonpolar vertical-cavity surface-emitting laser (VCSEL) with a photoelectrochemically (PEC) etched aperture. The PEC lateral undercut etch is used to selectively remove the multi-quantum well (MQW) region outside the aperture area, defined by an opaque metal mask. This PEC aperture (PECA) creates an air-gap in the passive area of the device, allowing one to achieve efficient electrical confinement within the aperture, while simultaneously achieving a large index contrast between core of the device (the MQW within the aperture) and the lateral cladding of the device (the air-gap formed by the PEC etch), leading to strong lateral confinement. Scanning electron microscopy and focused ion-beam analysis is used to investigate the precision of the PEC etch technique in defining the aperture. The fabricated single mode PECA VCSEL shows a threshold current density of ∼22u2009kA/cm2 (25u2009mA), with a peak output power of ∼180u2009μW, at an emission wavelength of 417u2009nm. The near-field emission p...

N-Polar Deep Recess MISHEMTs With Record 2.9 W/mm at 94 GHz

W-band power performance is reported on an N-polar GaN HEMT for the first time, resulting in a record output power density for any GaN device on a sapphire substrate. This result is achieved using an N-polar GaN deep recess MISHEMT structure grown by metal-organic chemical vapor deposition on the sapphire substrates. The key component in this device design is the addition of an in situ unintentionally doped GaN epitaxial passivation layer in the access regions of the transistor. This GaN layer functions both to control DC-to-RF dispersion as well as to increase the conductivity in the access regions of the HEMT. Devices with very low dispersion and a simultaneous fmax/ft combination of 276/149 GHz are demonstrated. Load pull measurements at 94 GHz give a peak power added efficiency (PAE) of 20% with an associated output power density of 1.73 W/mm at VDS = 8 V. A record 2.9-W/mm maximum output power density with an associated 15.5% PAE at VDS = 10 V is achieved despite the low thermal conductivity of the samples sapphire substrate.

1550-nm InGaAsP multi-quantum-well structures selectively grown on v-groove-patterned SOI substrates

We report direct growth of 1550-nm InGaAsP multi-quantum-well (MQW) structures in densely packed, smooth, highly crystalline, and millimeter-long InP nanoridges grown by metalorganic chemical vapor deposition on silicon-on-insulator (SOI) substrates. Aspect-ratio-trapping and selective area growth techniques were combined with a two-step growth process to obtain good material quality as revealed by photoluminescence, scanning electronic microscopy, and high-resolution X-ray diffraction characterization. Transmission electron microscopy images revealed sharp MQW/InP interfaces as well as thickness variation of the MQW layer. This was confirmed by atom probe tomography analysis, which also suggests homogenous incorporation of the various III-V elements of the MQW structure. This approach is suitable for the integration of InP-based nanoridges in the SOI platform for new classes of ultra-compact, low-power, nano-electronic, and photonic devices for future tele- and data-communications applications.

Continuous-wave operation of a

We demonstrated selective and controllable undercut etching of the InGaN/GaN multiple quantum well (MQW) active region of a laser diode (LD) structure by photoelectrochemical etching. This technique was used to fabricate current aperture edge-emitting blue laser diodes (CA-LDs), whose performance was compared with that of shallow-etched ridge LDs with a nominally identical epitaxial structure. The threshold current density, threshold voltage, peak output power, and series resistance for the CA-LD (shallow-etched LD) with a 2.5-µm-wide active region were 4.4 (8.1) kA/cm2, 6.1 (7.7) V, 96.5 (63.5) mW, and 4.7 (6.0) Ω under pulsed conditions and before facet coating, respectively.

(20\bar{2}\bar{1})

A 3D integrated hybrid silicon laser was realized for high-thermal performance by integrating silicon photonic (SiPh) chips and indium phosphide (InP) chips. The optical gain is provided by the InP chip with a total internal reflection mirror for surface emission. The surface grating couplers on the SiPh chip couples light into a silicon waveguide. The InP chips were flip-chip bonded P-side down to metal pads on the silicon chips. Two lasers are reported. For laser A, the InP chip was bonded on the top cladding oxide of the silicon waveguide. For laser B, the InP chip was bonded to the silicon substrate in an etched recess. Both lasers demonstrate milliwatt-level light coupled into the silicon waveguide. Laser B demonstrated three times better thermal performance with a measured thermal impedance of 6.2 °C/W.

InGaN laser diode with a photoelectrochemically etched current aperture

We demonstrate the selective and controllable undercut etching of the InGaN/GaN multiple quantum well active regions of nonpolar and semipolar laser diode (LD) structures by photoelectrochemical (PEC) etching without external bias. The lateral etch rate ranged from ~20 nm/min to ~1.2 µm/min. Metal masks were used to define the undercut and to improve the PEC etch resolution by reducing the scattered light in the system, which contributes to degradation of the lateral etch resolution, as suggested by ray tracing simulations. This resulted in a light-exposed-area: masked-area etch selectivity of .

High-Thermal Performance 3D Hybrid Silicon Lasers

We employ a simple two-step growth technique to grow large-area 1550-nm laser structures by direct hetero-epitaxy of III–V compounds on patterned exact-oriented (001) silicon (Si) substrates by metal organic chemical vapor deposition. Densely-packed, highly uniform, flat and millimeter-long indium phosphide (InP) nanowires were grown from Si v-grooves separated by silicon dioxide (SiO2) stripes with various widths and pitches. Following removal of the SiO2 patterns, the InP nanowires were coalesced and, subsequently, 1550-nm laser structures were grown in a single overgrowth without performing any polishing for planarization. X-ray diffraction, photoluminescence, atomic force microscopy and transmission electron microscopy analyses were used to characterize the epitaxial material. PIN diodes were fabricated and diode-rectifying behavior was observed.

Selective and controllable lateral photoelectrochemical etching of nonpolar and semipolar InGaN/GaN multiple quantum well active regions

A novel 3D hybrid integration platform combines group III-V materials and silicon photonics to yield high-performance lasers is presented. This platform is based on flip-chip bonding and vertical optical coupling integration. In this work, indium phosphide (InP) devices with monolithic vertical total internal reflection turning mirrors were bonded to active silicon photonic circuits containing vertical grating couplers. Greater than 2 mW of optical power was coupled into a silicon waveguide from an InP laser. The InP devices can also be bonded directly to the silicon substrate, providing an efficient path for heat dissipation owing to the higher thermal conductance of silicon compared to InP. Lasers realized with this technique demonstrated a thermal impedance as low as 6.2°C/W, allowing for high efficiency and operation at high temperature. InP reflective semiconductor optical amplifiers were also integrated with 3D hybrid integration to form integrated external cavity lasers. These lasers demonstrated a wavelength tuning range of 30 nm, relative intensity noise lower than -135 dB/Hz and laser linewidth of 1.5 MHz. This platform is promising for integration of InP lasers and photonic integrated circuits on silicon photonics.

Large-Area Direct Hetero-Epitaxial Growth of 1550-nm InGaAsP Multi-Quantum-Well Structures on Patterned Exact-Oriented (001) Silicon Substrates by Metal Organic Chemical Vapor Deposition

We report on the use of InGaAsP strain-compensated superlattices (SC-SLs) as a technique to reduce the defect density of Indium Phosphide (InP) grown on silicon (InP-on-Si) by Metal Organic Chemical Vapor Deposition (MOCVD). Initially, a 2 μm thick gallium arsenide (GaAs) layer was grown with very high uniformity on exact oriented (001) 300 mm Si wafers; which had been patterned in 90 nm V-grooved trenches separated by silicon dioxide (SiO2) stripes and oriented along the [110] direction. Undercut at the Si/SiO2 interface was used to reduce the propagation of defects into the III–V layers. Following wafer dicing; 2.6 μm of indium phosphide (InP) was grown on such GaAs-on-Si templates. InGaAsP SC-SLs and thermal annealing were used to achieve a high-quality and smooth InP pseudo-substrate with a reduced defect density. Both the GaAs-on-Si and the subsequently grown InP layers were characterized using a variety of techniques including X-ray diffraction (XRD); atomic force microscopy (AFM); transmission electron microscopy (TEM); and electron channeling contrast imaging (ECCI); which indicate high-quality of the epitaxial films. The threading dislocation density and RMS surface roughness of the final InP layer were 5 × 108/cm2 and 1.2 nm; respectively and 7.8 × 107/cm2 and 10.8 nm for the GaAs-on-Si layer.