An overview is presented of the current state-of-the-art in silicon nanophotonic ring resonators. Basic theory of ring resonators is discussed, and applied to the peculiarities of submicron silicon photonic wire waveguides: the small dimensions and tight bend radii, sensitivity to perturbations and the boundary conditions of the fabrication processes. Theory is compared to quantitative measurements. Finally, several of the more promising applications of silicon ring resonators are discussed: filters and optical delay lines, label-free biosensors, and active rings for efficient modulators and even light sources.

/pdf/silicon-microring-resonators-3fzr4lpxbh.pdf

Silicon microring resonators

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

An electrically pumped light source on silicon is a key element needed for photonic integrated circuits on silicon. Here we report an electrically pumped AlGaInAs-silicon evanescent laser architecture where the laser cavity is defined solely by the silicon waveguide and needs no critical alignment to the III-V active material during fabrication via wafer bonding. This laser runs continuous-wave (c.w.) with a threshold of 65 mA, a maximum output power of 1.8 mW with a differential quantum efficiency of 12.7 % and a maximum operating temperature of 40 degrees C. This approach allows for 100's of lasers to be fabricated in one bonding step, making it suitable for high volume, low-cost, integration. By varying the silicon waveguide dimensions and the composition of the III-V layer, this architecture can be extended to fabricate other active devices on silicon such as optical amplifiers, modulators and photo-detectors.

Electrically pumped hybrid AlGaInAs-silicon evanescent laser

The narrowest feature on present-day integrated circuits is the gate oxide—the thin dielectric layer that forms the basis of field-effect device structures. Silicon dioxide is the dielectric of choice and, if present miniaturization trends continue, the projected oxide thickness by 2012 will be less than one nanometre, or about five silicon atoms across1. At least two of those five atoms will be at the silicon–oxide interfaces, and so will have very different electrical and optical properties from the desired bulk oxide, while constituting a significant fraction of the dielectric layer. Here we use electron-energy-loss spectroscopy in a scanning transmission electron microscope to measure the chemical composition and electronic structure, at the atomic scale, across gate oxides as thin as one nanometre. We are able to resolve the interfacial states that result from the spillover of the silicon conduction-band wavefunctions into the oxide. The spatial extent of these states places a fundamental limit of 0.7 nm (four silicon atoms across) on the thinnest usable silicon dioxide gate dielectric. And for present-day oxide growth techniques, interface roughness will raise this limit to 1.2 nm.

The electronic structure at the atomic scale of ultrathin gate oxides

Characterization of amorphous and nanocrystalline carbon films

The bonding of Si atoms at the SiO2/Si interface is determined via high-resolution core level spectroscopy with synchrotron radiation. For oxides grown in pure O2, the SiO2/Si interface is found to contain Si atoms in intermediate oxidation states with a density of 1.5 ± 0.5 × 1015 cm−2. From the density and distribution of intermediate oxidation states, models of the interface structure are obtained. The interface is not abrupt, as evidenced by the non-ideal distribution of intermediate oxidation states and their high density (about 2 monolayers of Si). The finite width of the interface is explained by the bond density mismatch between SiO2 and Si. Annealing in H2 is found to influence the electrical parameters by removing the Pb centers that pin the Fermi level. The distribution of intermediate oxidation states is not affected.

Microscopic structure of the SiO2/Si interface.

The chemical composition of thin native oxide layers grown on GaAs and InAs by ultraviolet (UV)/ozone and thermal oxidation is investigated using x-ray-photoelectron spectroscopy. Core-level binding energies, core-level intensities, and valence-band spectra are compared with data for bulk crystalline binary or ternary As, In, and Ga oxides. The chemical compositions, which vary strongly from GaAs to InAs and from thermal to UV oxidation, appear to be controlled by both thermodynamic and kinetic factors. Only for GaAs thermal oxidation are the products predicted at thermodynamic equilibrium obtained. In all cases the native oxides can be described as single phase nonstoichiometric compounds and not as a macroscopic mixture of stoichiometric binary oxides.

Oxides on GaAs and InAs surfaces: An x-ray-photoelectron-spectroscopy study of reference compounds and thin oxide layers.

The chemical composition of surface oxides grown on InP is investigated using x‐ray photoelectron spectroscopy. Native oxides prepared by various chemical treatments, anodic oxides, and thermal oxides are examined. Core level energies and intensities and valence band spectra are compared with data for standard compounds. In a first step we show that, depending on the preparation procedure, oxides can be classified into three groups which have properties similar to crystalline In(OH)3, InPO4, and In(PO3)3, respectively. The large variations observed in atomic compositions and the shape of valence band spectra suggest that the oxides could be amorphous nonstoichiometric phases. A description based on a mixture of well defined compounds (e.g., In2O3+P2O5 or In2O3+InPO4) seems to be inadequate. Chemical and anodic phosphorus‐rich oxides are identified as Inx(PO3)y polyphosphates.

On the nature of oxides on InP surfaces

We show the role played by the buffer surface morphology and by alloying effects on the size, shape and lateral distribution of InAs nanostructures grown on InP(001) substrates by molecular beam epitaxy. Three buffers, viz., In0.53Ga0.47As, In0.52Al0.48As, and InP lattice matched on InP have been studied. Differences in nanostructure morphology and in carrier confinement have been evaluated by atomic force microscopy and by low-temperature photoluminescence measurements, respectively. Alongside the classical relaxation mode through two-dimensional/three-dimensional surface morphology change, a chemical relaxation mode has to be introduced as a competitive mode of relaxation of strained layers. This chemical relaxation mode, due to alloying between the InAs deposit and the buffer, is thought to be responsible for most of the observed differences in the InAs nanostructure properties.

/pdf/role-of-buffer-surface-morphology-and-alloying-effects-on-35vq8zvajj.pdf

Role of buffer surface morphology and alloying effects on the properties of InAs nanostructures grown on InP(001)

Room temperature pulsed laser operation of 2D photonic crystal microcavities around 1.55 /spl mu/m is reported. Such devices are based on thin III/V heterostructures transferred onto silicon and include an InGaAs/InP multiquantum well (MQW) active layer.

G. Hollinger

Papers

Microscopic structure of the SiO2/Si interface.

Oxides on GaAs and InAs surfaces: An x-ray-photoelectron-spectroscopy study of reference compounds and thin oxide layers.

On the nature of oxides on InP surfaces

Role of buffer surface morphology and alloying effects on the properties of InAs nanostructures grown on InP(001)

InP 2D photonic crystal microlasers on silicon wafer: room temperature operation at 1.55 [micro sign]m