Nobuhiko Susa

Large absolute and polarization-independent photonic band gaps for various lattice structures and rod shapes

Despite the considerable amount of research undertaken on various lattice structures, the photonic band gap (PBG) for a triangular lattice remains the largest both in the transverse magnetic (TM) and transverse electric (TE) modes. The PBG for a square lattice can be doubled by using square air holes rather than air cylinders. Reducing the symmetry was effective in terms of a honeycomb lattice in that the PBG can be increased 40% by deforming the lattice and using oval dielectric rods instead of cylindrical rods. The PBGs for all the examined structures increase monotonously as the refractive index is increased. The overlap PBG between the TM and TE gaps (polarization-independent PBG) is the largest for a triangular lattice of circular air rods. The overlap PBG for a hybrid square lattice of air rods is the next largest, and is twice as large as that for the well-known honeycomb lattice consisting of dielectric cylinders. When the refractive index of a dielectric material is increased to more than 3.50, t...

New InGaAs/InP avalanche photodiode structure for the 1-1.6 µm wavelength region

Low dark current and low multiplication noise properties for an In 0.53 Ga 0.47 As/InP avalanche photodiode are described. The diode is prepared with an In 0.53 Ga 0.47 As light absorption layer and an InP avalanche multiplication region. The lowest dark current density of 5.2 \times 10^{-4} A/cm2is obtained at 90 percent of a breakdown voltage. Multiplication noise power is proportional to the 2.7th power of the current multiplication factor. Impact ionization coefficient by holes is larger by 2-3 times than that by electrons in

Characteristics in InGaAs/InP avalanche photodiodes with separated absorption and multiplication regions

Improved characteristics of compound semiconductor avalanche photodiodes with separated absorption and multiplication regions (SAM) are discussed. Temperature dependences of dark current and breakdown voltage show that the tunneling current in the narrow energy gap layer can be suppressed in InGaAs/InP APDs with the SAM structure. Dark currents above punch-through voltages, at which the depletion layer reaches the InP-InGaAs heterointerface, are caused by the generation-recombination process in the InGaAs and at the heterointerface. Dark currents near breakdown depend on the n-layer thickness and are strongly affected by the electric field strength in the ternary layer. Tunneling currents are dominant in diodes with thin n-InP layers, while the generation-recombination processes in the InGaAs layers are dominant in those with a thick n-InP layer. The dark current was as low as7.8 \times 10^{4}A/cm2atM = 10when the interface electric field strength is reduced. A maximum multiplication factor of 60 was observed for the6 \times 10^{-7}A initial photocurrent. Rise time and full width at half maximum in a pulse response waveform were 100 and 136 ps, respectively, atM = 10.

Threshold gain and gain-enhancement due to distributed-feedback in two-dimensional photonic-crystal lasers

The threshold gain (gth) of a two-dimensional (2D) photonic crystal distributed-feedback (DFB) laser composed of 8×8 dielectric cylinders was one order of magnitude smaller than that of an 8-pair 1D DFB laser with the identical refractive indexes. In the 2D finite-width photonic crystal laser, gth using the 1st photonic band was smaller than that using the higher photonic band, contrary to the expectation from the flatness of the photonic band structures. This unexpected gth is probably due to the longer optical path caused by the reflection at the side boundary. Moreover, gth using the 1st photonic band was the smallest in the Γ–X direction of the square-lattice photonic crystal. The gain-enhancement using the 1st–3rd photonic bands were 10–30 in the 2D photonic crystal and that using the 3rd band was the largest. The gain-enhancement using the 1st photonic band of the 2D photonic crystal consisting of dielectric cylinders was larger than that of air cylinders.

Electric-field-induced refractive index changes in three-step asymmetric coupled quantum wells

A scheme for enhancing the refractive index change Δn in the wavelength region longer than that for an n=1 heavy‐hole exciton peak, where the absorption coefficient α is small, is proposed and three‐step GaAs/AlGaAs quantum wells (QWs) are numerically analyzed as an example. The basis of this scheme is that an electric‐field‐dependent oscillator strength can be controlled by designing a QW structure which yields either positive or negative absorption change, Δα at all wavelengths, resulting in an increase in the Δn in the longer wavelength region and a reduction in the operating electric field. The numerical analysis predicts that the electric field for the three‐step QW that yields a Δn of ∼10−2 is as small as 5–15 kV/cm which is about one order of magnitude smaller than that for a conventional square‐potential QW. In addition, the Δn and the figure‐of‐merit, which is defined as Δn/α, are larger than those for the conventional QW in the longer wavelength region.

InGaAs/InP separated absorption and multiplication regions avalanche photodiode using liquid- and vapor-phase epitaxies

Heterostructure planar InGaAs/InP avalanche photodiodes, which consist of a vapor-phase epitaxial InP avalanche multiplying layer and a liquid-phase epitaxial In 0.53 Ga 0.47 As optical absorption layer, were fabricated. Dark current, multiplication, spectral response, and pulse response characteristics are reported. Diodes were prepared by InGaAs liquid-phase epitaxy on an InP substrate, followed by InP vapor-phase epitaxy. The vapor-phase epitaxy was adopted in the InP growth to avoid ternary layer melting encountered in the liquid-phase process. Cd diffusion was carried out in the InP layer to form a p-n junction. A uniform multiplication factor of 5.5 was observed without a guard ring. The quantum efficiency was 70 percent in the 1-1.6 \mu m wavelength region without antireflection coating. Dark current density was as low as 1.5 \times 10^{-4} A/cm2at 90 percent of breakdown voltage. A fast rise time of 100 ps was observed.

Improvement in electroabsorption and the effects of parameter variations in the three-step asymmetric coupled quantum well

Exciton binding energies, radiuses, emission energies, and oscillator strengths in a three‐step asymmetric coupled quantum well (QW) are numerically analyzed as a function of electric field. This analysis predicts that the change in the absorption coefficient for an n=1 heavy‐hole exciton can be made larger and on‐state transmission loss can be lowered by reducing oscillator strength at zero bias and increasing it at operating voltages. The Stark shifts also can be increased, thus allowing lower operating voltages. Estimation of influences of alloy composition and thickness variations on electroabsorption reveals that the thickness of epitaxial layers must be controlled to within one monoatomic layer and that variations in alloy composition must be controlled to within ±4%. An analysis of exciton peak shifts caused by one‐monolayer change in the QW layer thicknesses predicts that interface roughness broadening may be small when the electric field is increased, in contrast to the case of conventional squar...

Properties of GaAs/Al0.53Ga0.47As Avalanche Photodiode with Superlattice Fabricated by Molecular Beam Epitaxy

A GaAs/AlGaAs APD with superlattice, exhibiting low dark current and large photocurrent multiplication characteristics, has been successfully fabricated by MBE. Dark currents were as low as 2.1×10-6 A/cm2 for a multiplication factor of 30. A study of the influence of oval defects on APD characteristics showed that no one-to-one correlation was found between oval defects and fatal diode deterioration. The ionization rate ratio, β/α, estimated from multiplication noise measurement, was less than 0.6–0.8. MBE is a useful technique for fabricating high performance APDs.

Carrier Density Profiles in Zn- and Cd-Diffused InP

This letter reports on studies of Zn and Cd diffusion in (100) n-InP single crystals, carried out using Zn3P2 and Cd3P2 sources. When n-InP background carrier density ND is lower than 1016 cm-3, two diffusion fronts are observed and a p+-n--n junction is formed for Zn and Cd diffusion. A p+-n one-sided abrupt junction is formed when ND is 1016–1017 cm-3. The carrier density profiles were observed through EBIC measurement, van der Pauw method, C-V measurement using a Schottky barrier contact, and diffusion velocity dependence on n-InP background carrier density.

Plasma Enhanced CVD Si3N4 Film Applied to InP Avalanche Photodiodes

Si3N4 films fabricated by plasma CVD process were examined for suitability as a preferential diffusion mask and antireflection coating for InP avalanche photodiodes. Planar type InP APDs with low leakage currents were obtained by applying the Si3N4 mask in Cd preferential diffusion at 480°C. The Si3N4 film on InP with an optimized thickness showed surface reflection reduction to as low as 4%.