Search

We aim to achieve wavelength division multiplexing with a circular defect in two‐dimensional photonic crystal (CirD) laser that has an orthogonal lattice waveguide (OLW). In a simulation, we calculate the output extraction efficiency to discover a structure in which stronger optical output can be extracted for the CirD laser with OLW. Consequently, adjusting the edge face position and applying a semicircular port can strengthen the energy extracted from the edge face. This appears to be caused by changing the positional relationship between the standing wave mode and the edge face. Furthermore, the OLW‐W1 structure proposed in this report can achieve higher output extraction efficiency than the conventional OLW. The transmittance is enhanced by changing the structure near the edge face because this improves the matching of light speed in different media. [ABSTRACT FROM AUTHOR]

A circular defect in 2D photonic crystal (CirD) lasers is developed, which has the potential to achieve a target communication density of 10 Pbps cm−2 for intrachip optical communications by vertical current injection. The AlGaOx cladding layer's oxidation width strongly influences the device's light confinement and electronic resistance. Herein, samples with different oxidation widths are fabricated and the relationship between threshold power and oxidation width through photoexcitation is revealed. The optimal range of the oxidation width is found to be 349–439 nm, which can ensure low electronic resistance and low threshold power. [ABSTRACT FROM AUTHOR]

Herein a circular defect in a 2D photonic crystal (CirD) laser for wavelength division multiplexing (WDM) optical communications is theoretically analyzed. By coupling CirD lasers to a waveguide, a huge communication capacity can be expected. For example, if high‐speed, 50 Gbps laser operations are successfully achieved, WDM devices with 20 CirD lasers could achieve a communication capacity of 1 Tbps. Based on finite‐difference time‐domain (FDTD) simulation results, in this study, both current‐output and high‐frequency characteristics are investigated by solving 2D rate equations, revealing that an excessively high‐quality factor in laser cavities reduces the relaxation‐oscillation frequency. When the photon lifetime is 2 ps, which corresponds to a quality factor of ≈3000, a small threshold current of 5 μA and a high relaxation‐oscillation frequency of 40 GHz can be simultaneously obtained. These results suggest the potential of developing a WDM device using CirD lasers. [ABSTRACT FROM AUTHOR]

Featured Application: The technique described in this paper is meaningful for achieving the wavelength division multiplexing devices in intra-chip communications. We describe a device for inter-chip or intra-chip optical communications that contains the Circular Defect in photonic crystal (CirD) lasers array driven by vertical current injection. In order to improve the conductivity of the structure while also preventing current leakage, we introduce the oblique deposition of electrodes on a photonic crystal pattern by using an electron beam evaporation apparatus. The performance of an electrode is investigated by a transmission line method, and the CirD structure is fabricated with the electrode. We analyze the voltage-current relationship and confirm the CirD structure's low resistance of under 1 kΩ. [ABSTRACT FROM AUTHOR]

Extremely high etch selectivity of GaAs over AlOx is mandatory to fabricate a current blocking trench (CBT) between adjacent lasers in a photonic crystal (PhC) laser array for intra-chip optical interconnections. In this paper, selective etching of GaAs on AlOx with PhC structure is reported for the first time by using inductively coupled plasma etching with Cl2/BCl3-containing gas mixture. The etched profile of the CBT on GaAs/AlOx heterostructure with PhC structure was significantly different from that without PhC structure. By optimizing the etching conditions, the etching of the CBT was stopped at AlOx, and extremely high etch selectivity of GaAs over AlOx was achieved. Thus, we successfully fabricated the CBT in the PhC laser array. The horizontal electrical resistance between adjacent laser cavities with a 0.4 μm or wider CBT reached the instrumentation-limit of 100 MΩ, confirming the electrical isolation effect of the CBT. [ABSTRACT FROM AUTHOR]

We study energy-band engineering with nitrogen delta (δ)-doping in GaAs-related quantum structures. A tight-binding calculation indicates that the band structure can be engineered by introducing the one-dimensional doping profile of nitrogen into GaAs. Using molecular beam epitaxy, we prepare δ-doped samples of AlGaAs/GaAs quantum wells and GaAs/δ-doped nitrogen superlattice structures at the growth temperature 560 °C. Photoluminescence obtained from the samples shows a clear redshift of the spectral peak positions dependent on the nitrogen coverage. The transition energies of the superlattice structures agree well with those obtained from photoreflectance, indicating the feasibility of band modification with a single or a multiple nitrogen δ-doped layer. [ABSTRACT FROM AUTHOR]

Focuses on the observation of resonant optical-phonon assisted tunneling in asymmetric double quantum wells. Information on the use of asymmetric double quantum wells for investigating the electron tunneling; Discussion on the Pl intensity ratio between the narrow and wide quantum wells; Examination of the electric field dependence of the intensity of the LO phonon stokes lines.

Presents a study that derived a model of impact ionization due to the primary hole in silicon for a full band Monte Carlo simulation. Band structure, density of states and impact-ionization model employed in the study; Results; Discussion.

We theoretically investigate the effect of a defect at the interface between a conductor and reservoirs in an atomic-scale device. Since fabrication of atomic-scale contacts is a complex task, there could be defects at the interface between the conductor and reservoirs. Such defective contacts will make it difficult to measure currents properly. In this paper, we calculate current-voltage characteristics in two-dimensional devices with a defective connection to reservoirs by using the nonequilibrium Green’s function method. Results show that the magnitude of resistance change depends on the amplitude of quantized wave functions at the position of the defect. [ABSTRACT FROM AUTHOR]

We investigate theoretically cavity–waveguide coupling in 2-D photonic crystals. In using photonic crystal cavities in a device such as an optical circuit, the cavities must be coupled to a waveguide. For optimum performance, careful design of the device structure is necessary. In order to find out the rule for the design, we evaluate coupling strength by carrying out finite-difference time-domain (FDTD) calculations with systematic variation of structural parameters. Results are interpreted in terms of the density of the states of waveguide bands and symmetry of the cavity and waveguide modes. We found that the selection rule based on parity of the modes breaks down because of linewidth broadening, which arises from transition in finite time. [ABSTRACT FROM PUBLISHER]

The authors carry out δ-doping at the middle of AlGaAs/GaAs quantum wells employing molecular beam epitaxy and varying the nitrogen coverage up to 0.5 monolayers. Transmission electron micrography and x ray diffraction indicate the introduction of a nitrogen δ-doped layer with precisely controlled position and nitrogen coverage. Photoluminescence spectra obtained for the samples show clear redshift of spectral peak positions depending on the amount of nitrogen, suggesting the band structure is modified by the δ-doping. The growth can be carried out at a substrate temperature of 560 °C. The growth temperature, which is high compared with that of standard dilute nitride compounds, could suppress the formation of growth-induced defects, resulting in the weak effect of post-growth thermal annealing on the characteristics of room-temperature photoluminescence. [ABSTRACT FROM AUTHOR]

Abstract: The degradation of magnetic tunnel junctions (MTJs) with barrier was experimentally investigated. Constant voltage stress (CVS) measurement was carried out to monitor the time evolution of the conductance and tunneling magnetoresistance (TMR) of MTJs. The gradual increase of the stress-induced leakage current (SILC) was observed prior to the breakdown, following a power law function of stress time with an exponent of about 0.2–0.4, which is similar to the case of the ultrathin gate oxide films in MOSFETs. The measured TMR for SILC suggests that the spin-dependent current component would be involved in the early stage of degradation, while spin-independent conduction becomes dominant before the breakdown resulting in a decrease of TMR. [Copyright &y& Elsevier]

We investigate the formation process of eigenstates in electromagnetic fields. Electronic eigenfunctions in crystalline solids in the electromagnetic fields are localized wavepackets associated with the discrete energy levels. The properties of such states (e.g. Landau levels in magnetic fields and Stark-ladders in electric fields) have been well revealed. However, it is obscure how transformation from the extended Bloch functions to the localized states occurs. By using the path-integral theory, we describe the transformation process of wave functions into quantized eigenstates. It is shown that self-interference, which may be interpreted as an afterimage of electronic motion, plays an important role in the transformation. [Copyright &y& Elsevier]

We report the calculation of gate leakage currents through the ultra-thin gate oxides (2.6–3.4 nm) in MOSFETs. We simulate J- V characteristics for the direct tunneling of valence electrons and inversion layer holes, which are measured using a charge separation technique. A two-band model is employed to express the complex band structure of the gate oxide, and its validity is discussed by calculating the complex band structure of β-cristobalite based on the second nearest neighbor sp3 s* tight-binding scheme. [ABSTRACT FROM AUTHOR]

We describe the design of photonic crystal circular defect (CirD) lasers to construct a compact optical module with a wavelength division multiplexing function for the application of inter-chip or intra-chip optical interconnects. Subsequently, we investigated the characteristics of CirD lasers including the quality factor of the cavity, the lasing threshold, and the modulation speed with a three-dimensional finite-difference time-domain method and two-dimensional rate equations. Finally, we demonstrated the single mode lasing and wavelength tuning behaviors of the CirD lasers using optical pumping technology under room-temperature continuous-wave conditions. [ABSTRACT FROM AUTHOR]

To clarify the role of cladding layers in heat transfer in a laser diode fabricated using a photonic crystal, we theoretically investigate heat transfer in a semiconductor slab with a two-dimensional photonic crystal consisting of an array of air holes. For a photonic crystal laser, temperature increase may be a serious problem since air holes can cause poor heat transfer even if a large Q-factor reduces threshold current. We carry out simulations of heat transfer in a structure with AlOx layers that are naturally generated by oxidizing conventional AlAs cladding layers. Temperature changes in structures with and without AlOx cladding layers show that the AlOx cladding layers suppress temperature rise down to about 1/3 of that in the structure without AlOx cladding layers. [ABSTRACT FROM AUTHOR]

We investigate the effect of micro-fabrication process damage on the photoluminescence characteristics of laser structures containing a GaInNAs and GaInAs quantum wells. By varying the size of the fabricated island structure, its impact on the photoluminescence intensity is studied. The GaInAs sample shows a strong decrease in its intensity with the reduction of the island size. In contrast, a slight increase is observed from the GaInNAs sample. This indicates the negligible impact of surface recombination on the GaInNAs sample, as well as the optical confinement within the micrometer-size island structure. The results suggest the feasibility of a GaInNAs gain medium for application to microfabricated optical devices. [ABSTRACT FROM AUTHOR]

Using molecular beam epitaxy, we fabricate a superlattice structure having periodically arranged δ-doped nitrogen within GaAs. X-ray diffraction indicates the formation of regularly arranged 0.1 ML δ-doped nitrogen having a certain dispersion every 3.7 nm, multilayered up to 10 periods. Optical transition energies obtained from photoreflectance reflect the number of superlattice periods. That suggests the formation of minibands in the short-period GaAsN/GaAs quantum wells, as well as the dispersed distribution of the nitrogen about ∼7 ML as also predicted from transmission electron microscopy. [ABSTRACT FROM AUTHOR]

We theoretically investigate the effects of carriers supplied from a dopant in AlAs cladding layers of a current-driven photonic crystal laser diode. The high doping concentration of cladding layers reduces the resistance of the diode, which is advantageous for practical operation. Carriers, however, bring about light loss and degradation of characteristics. It is thus important to investigate the effect of carriers so as to determine the most suitable doping profile of the cladding layers. For some typical photonic crystal cavity resonators, we carry out light wave simulations and investigate changes in laser characteristics, such as threshold current, electric power consumption, and maximum operation frequency. [ABSTRACT FROM AUTHOR]

We investigate the function of a circularly arranged submicron-scale optical cavity having a GaInNAs gain medium introduced into a two-dimensional photonic crystal slab. GaInNAs gain shows negligible optical degradations due to process damage, probably because of the small surface recombination velocity of the material. The preserved optical property realizes the observation of spectral peaks related to the cavity mode at wavelengths close to 1.3 µm. Moreover, the cavity shows fine tunability of its characteristics via the deliberate adjustment of its structure. [ABSTRACT FROM AUTHOR]

Current injection into a photonic crystal cavity is an important task in material science and technology. A photonic crystal built in a two-dimensional semiconductor slab is a promising candidate for novel optical devices since it has excellent light confinement capability. However, the presence of insulating cladding layers to reduce light loss makes current injection difficult, and hence practical active devices have not yet been developed. In this study, we numerically investigate the effect of selectively oxidized AlAs layers designed so as to make carrier injection possible. Results of finite-difference time-domain (FDTD) calculations show that both carrier injection with sufficiently low resistivity and light confinement are feasible for the structure with the oxidized AlAs/AlOx cladding layers. [ABSTRACT FROM AUTHOR]

Dilute nitrides are novel III–V semiconductors that have been developed for photonic and electronic devices. Although they have been investigated, some of their characteristics have not been established yet. In this study, the static dielectric constant of dilute nitrides was experimentally estimated from the exciton binding energy. The results imply that nitrogen incorporation in III–V semiconductors decreases both the static dielectric constant and bandgap. This behavior is opposite to that of conventional semiconductors. [ABSTRACT FROM AUTHOR]

The facet surface of a Ge nanocrystal prepared on Si(111) was investigated by scanning tunneling microscopy and spectroscopy (STM/STS) using a metal-coated carbon nanotube tip. Because of the small radius and high aspect ratio of the tip, we observed a steep facet surface in detail. It was revealed that the surface formed a Ge(105) 1× 2 structure. STS spectra obtained on the facet surface showed density of states (DOS) originating from the surface and bulk states, which shifted depending on the height. First-principles calculation revealed that the shift arose from the intermixing of Si with Ge. [ABSTRACT FROM AUTHOR]

A surface-potential-based compact model for polycrystalline silicon (poly-Si) thin-film transistors (TFTs) was developed, accounting for the effects of both deep and tail states across the band gap. The model describes the drain current in all regions of operation using the unified equation without the use of threshold voltage as an input parameter. Calculations using the drain current model produce results that are in good agreement with the measured current–voltage characteristics of poly-Si TFTs. [ABSTRACT FROM AUTHOR]

We study the effects of annealing on (Ga 0.64 ,In 0.36 ) (N 0.045 ,As 0.955 ) using hard X-ray photoelectron spectroscopy and X-ray absorption fine structure measurements. We observed surface oxidation and termination of the N-As bond defects caused by the annealing process. Specifically, we observed a characteristic chemical shift towards lower binding energies in the photoelectron spectra related to In. This phenomenon appears to be caused by the atomic arrangement, which produces increased In-N bond configurations within the matrix, as indicated by the X-ray absorption fine structure measurements. The reduction in the binding energies of group-III In, which occurs concomitantly with the atomic rearrangements of the matrix, causes the differences in the electronic properties of the system before and after annealing.