Photocurrent derivative spectra of ZnCdSe-ZnSe double multi-quantum wells

Photocurrent (PC) spectra of ZnCdSe-ZnSe double multi-quantum wells are measured at different temperature. Its corresponding photocurrent derivative (PCD) spectra are obtained by computing, and the PCD spectra have greatly enhanced the sensitivity of the relative weak PC signals. The polarization dependence of the PC spectra shows that the transitions observed in the PC spectra are heavy-hole related, and the transition energy coincide well with the results obtained by envelope function approximation including strain. The temperature dependence of the photocurrent curves indicates that the thermal activation is the dominant transport mechanism of the carriers in our samples. The concept of saturation temperature region is introduced to explain why the PC spectra have different temperature dependence in the samples with different structure parameters. It is found to be very useful in designing photovoltaic devices.

Wavelength tunable distributed Bragg reflector laser integrated with electro-absorption modulator by a combined method of selective area growth and quantum well intermixing - art. no. 68240N

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.

Design and performance of monolithic integrated electro-absorption modulated distributed feedback laser - art. no. 67820Y

High performance InGaAsP/InGaAsP strained compensated multiple-quantum-well (MQW) electroabsorption modulators (EAM) monolithically integrated with a DFB laser diode have been designed and realized by ultra low metal-organic vapor phase epitaxy (MOVPE) based on a novel butt joint scheme. The optimization thickness of upper SCH layer for DFB and EAM was obtained of the proposed MQW structure of the EAM through numerical simulation and experiment. The device containing 250(mu m) DFB and 170(mu m) EAM shows good material quality and exhibits a threshold current of 17mA, an extinction ratio of higher than 30 dB and a very high modulation efficiency (12dB/V) from 0V to 1V. By adopting a high-mesa ridge waveguide and buried polyimide, the capacitance of the modulator is reduced to about 0.30 pF corresponding to a 3dB bandwidth more than 20GHz.

Widely Frequency-Tunable Optical Microwave Source Based on Amplified Feedback Laser

A monolithic integrated amplified feedback semiconductor laser is demonstrated as an optical microwave source. The optical microwave frequency is continuously tunable in the range of 19.87-26.3 GHz with extinction ratio above 6 dB, 3-dB linewidth about 3MHz.

An Improved Selective Area Growth Method in Fabrication of Electroabsorption Modulated Laser

An improved selective area growth (SAG) method is proposed to better the fabrication and performance of the Electroabsorption modulated laser The typical threshold current of the EML is 18mA, and the output power is 5.6mW at EAM facet.

Anomalous temperature-dependent bimodal size evolution of InAs quantum dots on vicinal GaAs(100) substrates

Temperature-dependent bimodal size evolution of InAs quantum dots on vicinal GaAs(100) substrates grown by metalorganic chemical vapor deposition (MOCVD) is studied. An abnormal trend of the evolution on temperature is observed. With the increase of the growth temperature, while the density of the large dots decreases continually, that of the small dots first grows larger when temperature was below 520 degrees C, and then there is a sudden decrease at 535 degrees C. Photoluminescence (PL) studies show that QDs on vicinal substrates have a narrower PL line width, a longer emission wavelength and a larger PL intensity.

Photovoltaic effect of tin disulfide with nanocrystalline/amorphous blended phases

Tin disulfide (SnS2) nanocrystalline/amorphous blended phases were synthesized by mild chemical reaction. Both X-ray diffraction and transmission electron microscopy measurements demonstrate that the as-synthesized particles presented very small size, with a diameter of only a few nanometers. The photoluminescence (PL) spectrum suggests efficient splitting of photo-generated excitons in poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and SnS2 hybrid films. Organic/inorganic hybrid solar cells comprising MDMO-PPV and SnS2 were prepared, giving photovoltage, photocurrent, fill factor and efficiency values of 0.702 V, 0.549 mA/cm(2), 0.385 and 0.148%, respectively, which suggests that this phase-blended inorganic semiconductor can also serve as a promising solar energy material. (C) 2009 Elsevier Ltd. All rights reserved.

Optical properties and electrical bistability of CdS nanoparticles synthesized in dodecanethiol

We reported the synthesis of CdS semiconductor nanoparticles using a simple one-pot reaction by thermolysis of cadmium acetylacetonate in dodecanethiol. Optical measurements of the as-obtained CdS nanoparticles revealed that their optical properties were closely related to surface effects. Based upon the cocktail of poly (N-vinylcarbazole) (PVK) and CdS nanoparticles, a bistable device was fabricated by a simple solution processing technique. Such a device exhibited a remarkable electrical bistability, which was attributed to the electric field-assisted charge transfer between PVK and the CdS nanoparticles capped by dodecaethiol. The conduction mechanism changed from an injection-controlled current to a bulk-controlled one during switching from OFF-state to ON-state.

Substrate temperature dependence of ZnTe epilayers grown on GaAs(001) by molecular beam epitaxy

ZnTe thin films have been grown on GaAs(0 0 1) substrates at different temperatures with constant Zn and Te beam equivalent pressures (BEPs) by molecular beam epitaxy (MBE). In situ reflection high-energy electron diffraction (RHEED) observation indicates that two-dimensional (2D) growth mode can be established after around one-minute three-dimensional (3D) nucleation by increasing the substrate temperature to 340 degrees C. We found that Zn desorption from the ZnTe surface is much greater than that of Te at higher temperatures, and estimated the Zn sticking coefficient by the evolution of growth rate. The Zn sticking coefficient decreases from 0.93 to 0.58 as the temperature is elevated from 320 to 400 degrees C. The ZnTe epilayer grown at 360 degrees C displays the narrowest full-width at half-maximum (FWHM) of 660 arcsec from (0 0 4) reflection in double-crystal X-ray rocking curve (DCXRC) measurements. The surface morphology of ZnTe epilayers is strongly dependent on the substrate temperature, and the root-mean-square (RMS) roughness diminishes drastically with the increase in temperature.

Optimization of VI/II pressure ratio in ZnTe growth on GaAs(001) by molecular beam epitaxy

ZnTe epilayers were grown on GaAs(0 0 1) substrates by molecular beam epitaxy (MBE) at different VI/II beam equivalent pressure (BEP) ratios (R-VI/II) in a wide range of 0.96-11 with constant Zn flux. Based on in situ reflection high-energy electron diffraction (RHEED) observation, two-dimensional (2D) growth mode can be formed by increasing the R-VI/II to 2.8. The Te/Zn pressure ratios lower than 4.0 correspond to Zn-rich growth state, while the ratios over 6.4 correspond to Te-rich one. The Zn sticking coefficient at various VI/II ratios are derived by the growth rate measurement. The ZnTe epilayer grown at a R-VI/II of 6.4 displays the narrowest full-width at half-maximum (FWHM) of double-crystal X-ray rocking curve (DCXRC) for (0 0 4) reflection. Atomic force microscopy (AFM) characterization shows that the grain size enlarges drastically with the R-VI/II. The surface root-mean-square (RMS) roughness decreases firstly, attains a minimum of 1.14 nm at a R-VI/II of 4.0 and then increases at higher ratios. It is suggested that the most suitable R-VI/II be controlled between 4.0 and 6.4 in order to grow high-quality ZnTe epitaxial thin films.

One-pot synthesis and self-assembly of colloidal copper(I) sulfide nanocrystals

A simple one-pot method is developed to prepare size-and shape-controlled copper(I) sulfide (Cu2S) nanocrystals by thermolysis of a mixed solution of copper acetylacetonate, dodecanethiol and oleylamine at a relatively high temperature. The crystal structure, chemical composition and morphology of the as-obtained products are characterized by powder x-ray diffraction (PXRD), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The morphology and size of the Cu2S nanocrystals can be easily controlled by adjusting the reaction parameters. The Cu2S nanocrystals evolve from spherical to disk-like with increasing reaction temperature. The spherical Cu2S nanocrystals have a high tendency to self-assemble into close-packed superlattice structures. The shape of the Cu2S nanodisks changes from cylinder to hexagonal prism with prolonged reaction time, accompanied by the diameter and thickness increasing. More interestingly, the nanodisks are inclined to self-assemble into face-to-face stacking chains with different lengths and orientations. This one-pot approach may extend to synthesis of other metal sulfide nanocrystals with different shapes and sizes.

A Selective-Area Metal Bonding InGaAsP-Si Laser

A 1.55-mu m hybrid InGaAsP-Si laser was fabricated by the selective-area metal bonding method. Two Si blocking stripes, each with an excess-metals accommodated space, were used to separate the optical coupling area and the metal bonding areas. In such a structure, the air gap between the InGaAsP structure and Si waveguide has been reduced to be negligible. The laser operates with a threshold current density of 1.7 kA/cm(2) and a slope efficiency of 0.05 W/A under pulsed-wave operation. Room-temperature continuous lasing with a maximum output power of 0.45 mW is realized.

Self-consistent analysis of AlSb/InAs high electron mobility transistor structures

The influences of channel layer width, spacer layer width, and delta-doping density on the electron density and its distribution in the AlSb/InAs high electron mobility transistors (HEMTs) have been studied based on the self-consistent calculation of the Schrodinger and Poisson equations with both the strain and nonparabolicity effects being taken into account. The results show that, having little influence on the total two dimensional electron gas (2DEG) concentration in the channel, the HEMT's channel layer width has some influence on the electron mobility, with a channel as narrow as 100-130 angstrom being more beneficial. For the AlSb/InAs HEMT with a Te delta-doped layer, the 2DEG concentration as high as 9.1 X 10(12) cm(-2) can be achieved in the channel by enhancing the delta-doping concentration without the occurrence of the parallel conduction. When utilizing a Si delta-doped InAs layer as the electron-supplying layer of the AlSb/InAs HEMT, the effect of the InAs donor layer thickness is studied on the 2DEG concentration. To obtain a higher 2DEG concentration in the channel, it is necessary to use an InAs donor layer as thin as 4 monolayer. To test the validity of our calculation, we have compared our theoretical results (2DEG concentration and its distribution in different sub-bands of the channel) with the experimental ones done by other groups and show that our theoretical calculation is consistent with the experimental results.

cryptanalysis of the end-to-end security protocol for mobile communications with end-user identification/authentication

IEEE Computer Society

A novel ultra-narrow transmission-band fiber Bragg grating and its application in a single-longitudinal-mode fiber laser with improved efficiency

A novel design approach to ultra-narrow transmission-band fiber Bragg grating (FBG) is proposed and demonstrated for the first time. The new grating consists of multiple identical distributed-Bragg reflector (DBR) cavities and a it-phase-shifted gap, and hence, the proposed laser is constructed by the cascade of these identical DBR fiber lasers. By manufacturing the proposed grating in a piece of Er-Yb codoped fiber, a single-wavelength single-longitudinal-mode (SLM) fiber laser with improved efficiency is demonstrated experimentally. The experimental results show that the pump-to-signal conversion efficiency of the proposed laser is improved by a factor of two in comparison with the optimized distributed-feedback (DFB) fiber lasers. (c) 2007 Elsevier B.V. All rights reserved.