Finite Element Analysis Of Thermal Characteristics in Continuous Wave Long Wavelength Surface Emitting Lasers (I): Dielectric Cavity Structures

We numerically analyzed the thermal characteristic in continuous wave (cw) GalnAsP/InP surface emitting lasers with dielectric multilayer mirrors by using finite element method. From the simulation of temperature distribution in cw devices, we found that MgO/Si multilayer mirror which has a high thermal conductivity is effective for heat sinking; the thermal resistance of device using MgO/Si mirror is nearly half of that using SiO₂/Si one, almost independently of their thicknesses. It was also indicated that the optimum design for other structural parameters, especially the thickness of active region, is important to effectively suppress the temperature increase. The minimum threshold current was estimated to be 3 mA under cw condition for the active region diameter of 4 μm and thickness of 0.5 μm.     

Enhancement of field emission of SnO2 nanowires film by exposure of hydrogen gas

The effect of hydrogen (H₂) gas exposure on the field emission properties of tin oxide (SnO₂) nanowires films synthesized by the carbon thermal reduction vapor transport method was investigated. The exposure of H₂ gas results in the reduction of the turn-on voltage for driving a current of 10 nA from 7.6 V/μm to 5.5 V/μm and the increase of the field current based on 10 V/μm from 0.47 μA to 2.1 μA. The Fowler–Nordheim plot obtained from the current–voltage data supports that the field emission enhancement of SNW film is attributed to the reduction of the work function by the H₂ exposure.     

Optically pumped lead chalcogenide infrared emitters on silicon substrates

Two low cost-infrared sources emitting above 4 μm wavelength are described: (i) Double heterostructure or quantum well EuSe/PbSe/Pb_1−xEu_xSe edge emitting lasers on silicon substrates show peak powers up to 200 mW and differential quantum efficiencies up to 20%. They operate up to 250 K when pumped with 870 nm laser diodes (with peak powers of 5.5 W). (ii) A “wavelength transformer”, a EuSe/PbSe/Pb_1−xEu_xSe active resonant cavity with epitaxial bottom and top mirror on a Si(1 1 1) substrate transforms the incoming 870 nm pump radiation into e.g. 4.2 μm wavelength. The device operates at room temperature, and the width and value of the emission wavelength can be tuned by design.     

Quantitative assessment of solid oxide electrochemical doping

Quantitative analysis of metal cation doping by solid oxide electrochemical doping (SOED) has been performed under galvanostatic doping conditions. A M–β″-Al₂O₃ (M=Ag, Na) microelectrode (contact radius: about 10 μm) was used as cation source to attain a homogeneous solid–solid contact between the β″-Al₂O₃ and doping target. In Ag doping into alkali borate glass, the measured dopant amount closely matched the theoretical value. High Faraday efficiencies of above 90% were obtained. This suggests that the dopant amount can be precisely controlled on a micromole scale by the electric charge during electrolysis. On the other hand, current efficiencies of Na doping into Bi₂Sr₂CaCu₂O_y (BSCCO) ceramics depended on the applied constant current. Efficiencies of above 80% were achieved at a constant current of 10 μA (1.6 A cm⁻²). The relatively low efficiencies were explained by the saturation of BSCCO grain boundaries with Na. By contrast, excess Na was detected on the anodic surface of ceramics at a constant current of 100 μA (16 A cm⁻²). In the present study, we demonstrate that SOED enables micromole-scale control over dopant amount.     

Effect of SiO2/Si3N4 dielectric distributed Bragg reflectors (DDBRs) for Alq3/NPB thin-film resonant cavity organic light emitting diodes

In this article, we report on the effect of SiO₂/Si₃N₄ dielectric distributed Bragg reflectors (DDBRs) for Alq₃/NPB thin-film resonant cavity organic light emitting diode (RCOLED) in increasing the light output intensity and reducing the linewidth of spontaneous emission spectrum. The optimum DDBR number is found as 3 pairs. The device performance will be bad by further increasing or decreasing the number of DDBR. As compared to the conventional Alq₃/NPB thin-film organic light emitting diode (OLED), the Alq₃/NPB thin-film RCOLED with 3-pair DDBRs has the superior electrical and optical characteristics including a forward voltage of 6 V, a current efficiency of 3.4 cd/A, a luminance of 2715 cd/m² under the injection current density of 1000 A/m², and a full width at half maximum (FWHM) of 12 nm for emission spectrum over the 5-9 V bias range. These results represent that the Alq₃/NPB thin-film OLED with DDBRs shows a potential as the light source for plastic optical fiber (POF) communication system.     

Synthesis, characterization and low field emission of CNx nanotubes

Aligned CN_x nanotubes were fabricated by pyrolyzing ethylenediamine on p-type Si(1 1 1) substrates using iron as the catalyst. Scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectrum (XPS) and Raman spectroscopy were used to characterize the CN_x nanotubes. The CN_x nanotubes with the average length of 20 μm and diameters in the range of 50–100 nm have the “bamboo-like” structure and worse crystalline order. The low-field emission measurements of the CN_x nanotubes indicated that 20 μA/cm² current densities were observed at an electric field of 1.4 V/μm and 1.280 mA/cm² were obtained at 2.54 V/μm. The CN_x nanotubes exhibit better field emission properties than the carbon nanotubes and the BCN nanotubes. The emission mechanism of CN_x nanotubes is also discussed.     

Si1−xGex/Si multi-quantum well phototransistor for near-infrared operation

For the first time in the literature, we report the monolithic integration of SiGe near-infrared phototransistor and planar hetero-junction bipolar transistor (HBT). The phototransistor is made with SiGe/Si multi-quantum well structure (MQW_PHT). At room temperature, the MQW_PHT reveals an optical responsivity of 1904 mW/A at 0.85 μm and 1.25 mW/A at 1.3 μm under the reversed bias of V_CE=1.5 V. For electrical DC and microwave performance, the SiGe HBT has a current gain of 160 and a cut-off frequency (f_T) of 25 GHz, respectively.     

Enhanced field emission from PbTiO3 nanodots prepared by phase separation approach

Uniformly distributed PbTiO₃ nanodots were successfully prepared by phase separation approach. A precursor sol film was first spin-coated on Si wafer and then spontaneously separated into two distinct phases owing to the Marangoni instability. PT nanodots with tailorable size and density were obtained after further heat treatment. X-ray diffraction analysis indicated that these nanodots showed a perovskite structure. An excellent room temperature field emission property of PbTiO₃ nanodots was observed: the minimum turn-on voltage was about 5.3 V/μm; while the emission current density reached about 270 μA cm⁻² at an applied field of about 9.25 V/μm.     

Tuning of long-wavelength emission in InxGa1−xAs quantum dot structures

This work explores the conditions to obtain the extension of the PL emission beyond 1.3 μm in InGaAs quantum dot (QD) structures growth by MOCVD. We found that, by controlling the In incorporation in the barrier embedding the QDs, the wavelength emission can be continuously tuned from 1.25 μm up to 1.4 μm at room temperature. However, the increase in the overall strain of the structures limits the possibility to increase the maximum gain in the QD active device, where an optical density as high as possible is required. By exploring the kinetics of QD surface reconstruction during the GaAs overgrowth, we are able to obtain, for the first time, emission beyond 1.3 μm from InGaAs QDs grown on GaAs matrix. The wavelength is tuned from 1.26 μm up to 1.33 μm and significant improvements in terms of line shape narrowing and room temperature efficiency are obtained. The temperature-dependent quenching of the emission efficiency is reduced down to a factor of 3, the best value ever reported for QD structures emitting at 1.3 μm.     

1.5 μm emission from InAs quantum dots with InGaAsSb strain-reducing layer grown on GaAs substrates

InGaAsSb strain-reducing layers (SRLs) are applied to cover InAs quantum dots (QDs) grown on GaAs substrates. The compressive strain induced in InAs QDs from the GaAs is reduced due to the tensile strain induced by the InGaAsSb SRL, because the lattice constant of InGaAsSb is closer to InAs lattice constant than that of GaAs, resulting in a significant red shift of photoluminescence peaks of the InAs QDs. The emission wavelength from InAs QDs can be controlled by changing the Sb composition of the InGaAsSb SRL. The 1.5 μm band emissions were achieved in the sample with an InGaAsSb SRL whose Sb compositions were above 0.3. The calculation of the electron and the hole wave functions using the transfer matrix method indicates that the electron and the hole were localized around InAs QDs and InGaAsSb SRL.     

A new method for increasing the laser damage threshold of metal mirrors

In order to increase the damage threshold of metal mirrors we propose to create a special structure on the surface of the mirrors (“photonic surface”). This structure must have the period about λ/2 and will suppress propagation of surface plasmons with the frequency ω₀=2πc/λ along the surface. This structure will also slightly increase the heat removal from the mirror’s surface by the excitation of the thermostimulated plasmon emission from the surface. The heat removal from the surface is estimated and possible implementation of this approach for use with CO₂-lasers (λ=10.6 μm) and Nd-YAG-lasers (λ=1.06 μm) is analyzed.     

Characteristics of InAlGaAs/InAlAs Superlattice Avalanche Photodiodes for Ultra-low Optical Power Detection in the Near Infrared

Static characteristics of two different structured InAlGaAs/InAlAs superlattice avalanche photodiodes (SLAPDs) cooled by liquid nitrogen were evaluated at a wavelength of 1.5 μm. The dark current of the SLAPD having a thick superlattice layer of 0.504 μm was 5 × 10⁻¹³ A. This was successively reduced by four orders of magnitude compared to that of the thin layer SLAPD of 0.231 μm at a breakdown voltage of around 20 V. The thickened layer was effective in suppressing tunneling dark current. An output current of 1.7×l0⁻¹² A at a bias voltage of 15 V was measured for an optical input with a wavelength of 1.5 μm and a signal power of 1 × 10⁻¹² W. This showed a sharp distinction from the dark current.     

Synthesis and field emission of patterned ZnO nanorods

A simple and self-catalytic method has been developed for synthesizing finely patterned ZnO nanorods on ITO-glass substrates under a low temperature of 500 °C. The patterned ZnO nanorod arrays, a unit area is of 400 × 100 μm², are synthesized via vapor phase transport method. The surface morphology and composition of the as-synthesized ZnO nanorods are characterized by means of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The mechanism of formation of ZnO nanorods is also discussed. The measurement of field emission (FE) reveals that the as-synthesized ZnO nanorods arrays have a turn-on field of 3.3 V/μm at the current density of 0.1 μA/cm² and a low threshold field of 6.2 V/μm at the current density of 1 mA/cm². So this approach must have a potential application of fabricating micropatterned oxide thin films used in FE-based flat panel displays.     

High temperature (T>300 K) light emitting diodes for 8–12 μm spectral range

Contrary to conventional light emitting diodes for visible and very near infrared utilizing interband (ω>E_g) luminescence, the longer infrared emitting diodes (LIREDs) we describe here utilize intraband (ω<E_g) electron transitions and emit beyond the fundamental absorption range of the material used. Made of indirect band gap semiconductors (like Ge, Si) and, therefore, free from the Auger recombination impact, LIREDs efficiently operate at higher temperatures (T>300 K) in the longer wavelength atmospheric window (8–12 μm). Electrically modulated power emitted is comparable to that for black body sources whereas shorter rise–fall times are due to recombination processes (200 μs) and not dependent on pixel thermal mass and thermal conduction. LIREDs can be made of different semiconductor structures provided the controllable modulation of free carrier concentration in the device base is achieved. The main parameters of Ge based LIREDs under injection mode are reported.     

Synthesis and field emission of diamond-like carbon nanorods on TiO2/Ti nanotube arrays

Highly ordered TiO₂/Ti nanotube arrays were fabricated by anodic oxidation method in 0.5 wt% HF. Using prepared TiO₂/Ti nanotube arrays deposited Ni nanoparticles as substrate, high quality diamond-like carbon nanorods (DLCNRs) were synthesized by a conventional method of chemical vapor deposition at 750 °C in nitrogen atmosphere. DLCNRs were analyzed by filed emission scanning electron microscopy and Raman spectrometer. It is very interesting that DLCNRs possess pagoda shape with the length of 3–10 μm. Raman spectra show two strong peaks about 1332 cm⁻¹ and 1598 cm⁻¹, indicating the formation of diamond-like carbon. The field emission measurements suggest that DLCNRs/TiO₂/Ti has excellent field emission properties, a low turn-on field about 3.0 V/μm, no evident decay at 3.4 mA/cm² in 480 min.     

Char characteristics and particulate matter formation during Chinese bituminous coal combustion

The characteristics of char particles and their effects on the emission of particulate matter (PM) from the combustion of a Chinese bituminous coal were studied in a laboratory-scale drop tube furnace. The raw coal was pulverized and divided into three sizes, <63, 63–100, and 100–200 μm. These coal samples were subjected to pyrolysis in N₂ and combusted in 20 and 50% O₂ at 1373, 1523, and 1673 K, respectively. Char samples were obtained by glass fiber filters with a pore size of 0.3 μm, and combustion-derived PM was size-segregated by a low pressure impactor (LPI) into different sizes ranging from 10.0 to 0.3 μm. The characteristics of char particles, including particle size distribution, surface area, pore size distribution, swelling behavior and morphology property, were studied. The results show that, coal particle size and pyrolysis temperature have significant influence on the char characteristics. The swelling ratios of char samples increase with temperature increasing from 1373 to 1523 K, then decrease when the temperature further increases to 1623 K. At the same temperature, the swelling ratios of the three size fractions are markedly different. The finer the particle size, the higher the swelling ratio. The decrease of swelling ratio at high temperature is mainly attributed to the high heating rate, but char fragmentation at high temperature may also account for the decrease of swelling ratio. The supermicron particles (1–10 μm) are primarily spherical, and most of them have smooth surfaces. Decreasing coal particle size and increasing the oxygen concentration lead to more supermicron-sized PM formation. The influence of combustion temperature on supermicron-sized PM emission greatly depends on the oxygen concentration.     

Low energy cluster beam deposited BN films as the cascade for field emission

The atomic deposited BN films with the thickness of nanometers (ABN) were prepared by radio frequency magnetron sputtering method and the nanostructured BN films (CBN) were prepared by Low Energy Cluster Beam Deposition. UV-Vis Absorption measurement proves the band gap of 4.27 eV and field emission of the BN films were carried out. F-N plots of all the samples give a good fitting and demonstrate the F-N tunneling of the emission process. The emission of ABN begins at the electric field of 14.6 V/μm while that of CBN starts at 5.10 V/μm. Emission current density of 1 mA/cm² for ABN needs the field of 20 V/μm while that of CBN needs only 12.1 V/μm. The cluster-deposited BN on n-type Silicon substrate proves a good performance in terms of the lower gauge voltage, more emission sites and higher electron intensity and seems a promising substitute for the cascade of field emission.     

Investigation of the total conductivity of heterolasers with a strip contact with reverse bias

The electrical properties of DGS lasers with a strip contact on the basis of Al_xGa_1–xAs with reverse bias, including the breakdown domain, are investigated experimentally. It is shown that the dependence of the barrier capacitance on the voltage is described well by the relationship C_b = C_o/(1– V/V_k)^1/m, where C_o = (50–110) pF and m = 2.0–2.7. The capacitance for zero bias C_o depends weakly on the frequency in the range 660 kHz–60 MHz. The active conductivity for V = 0 increases by approximately an order as the frequency changes from 15–60 MHz. The dependence of the active and reactive components of the total laser conductivity on the current is investigated at fixed frequencies in the range mentioned. The weak dependence of the reactive component on the current is noted in the breakdown regime associated with stabilization of the reverse voltage on the laser. Possible physical mechanisms responsible for the kind of characteristics obtained are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 52–56, November, 1983.     

Laser diode spectroscopy of H2O at 2.63 μm for atmospheric applications

The 2.7 μm spectral range is highly suitable for the in situ monitoring of atmospheric H₂O using compact balloonborne laser diode spectrometers. Water vapour spectroscopic parameters of the 2₀₂   1₀₁ and the 4₁₃   4₁₄ transitions of the ν₃ band are revisited in this spectral region using a new distributed-feedback InGaAsSb laser diode emitting at 2.63 μm. Accurate line strengths are provided which are well adapted for the in situ probing of the middle atmosphere. Our measurements are thoroughly compared to an existing molecular database, laboratory measurements and ab-initio calculations. A laser hygrometer was developed for operation from small stratospheric balloons using this new laser diode technology, with emission at 2.6 μm. The realized sensor is described and results from a recent test-flight are reported. PACS 07.57.Ty; 92.60 Jq     

Spectroscopy of non-equilibrium electrons in quantum Hall conductors

Spectroscopy of local cyclotron emission from the hot spots is carried out on a GaAs/AlGaAs heterostructure two-dimensional electron gas system at B=6 T (ν=2.5) by applying a terahertz scanning microscope. The spectra of CE at the current entry and exit corners (hot spots) are remarkably broadened towards lower frequencies with increasing I up to 300 μA, indicating substantial relevance of non-equilibrium electrons generated in higher-level LLs; in terms of effective electron temperature, T_E reaching as high as 300 K is suggested while T_E=25–30 K on an average in the surrounding region (within a distance of 50 μm) about the hot point.