Effects of well number on temperature characteristics in 1.3-μmAlGaInAs-InP quantum-well lasers

Effects of well number on temperature characteristics have been investigated in 1.3-μm AlGaInAs-InP compressively strained multiple-quantum-well lasers. Well-number dependence of threshold currents (I_th), external quantum efficiencies (η_d ), characteristic temperatures of I_th and η_d arid maximum operation temperatures have been experimentally determined and analyzed. The characteristic temperature of the threshold current (T₀) and the maximum operation temperature (T_max ) were found to increase with increasing the number of quantum wells and a record high pulsed T_max of 220°C has been achieved in lasers with ten wells. In contrast, the characteristic temperature of the external efficiency (T_η) was found to decrease with increasing the number of wells. Because of this opposite well-number dependence of the T₀ and T_η, each of them alone is not necessarily a good measure to optimize the number of wells. Therefore, in this work, me also evaluated a power reduction at a constant current with increasing temperature, which depends on both T₀ and T_η and thus should be a more practical measure of the temperature characteristics, and discuss the optimum number of the quantum wells     

A novel material for long-wavelength lasers: InNAsP

We show that a novel material InNAsP grown on InP is superior for long-wavelength microdisk lasers (and so expected for edge-emitting lasers) because of its larger conduction band offset from the addition of a small amount of nitrogen (0.5%-1%). The maximum temperature of operation for an InNAsP-GaInAsP microdisk laser is 70°C, which is about 120°C higher than that of a similar laser fabricated from GaInAs-GaInAsP. The characteristic temperature T₀ of the former is 97 K, also higher than that of the latter     

The temperature dependence of 1.3- and 1.5-μm compressivelystrained InGaAs(P) MQW semiconductor lasers

We have studied experimentally and theoretically the spontaneous emission from 1.3- and 1.5-μm compressively strained InGaAsP multiple-quantum-well lasers in the temperature range 90-400 K to determine the variation of carrier density n with current I up to threshold. We find that the current contributing to spontaneous emission at threshold I_Rad is always well behaved and has a characteristic temperature T₀ (I_Rad)≈T, as predicted by simple theory. This implies that the carrier density at threshold is also proportional to temperature. Below a breakpoint temperature T_B, we find I α n^Z, where Z=2. And the total current at threshold I_th also has a characteristic temperature T₀ (I_th)≈T, showing that the current is dominated by radiative transitions right up to threshold. Above T_B, Z increases steadily to Z≈3 and T₀ (I_th) decreases to a value less than T/3. This behavior is explained in terms of the onset of Auger recombination above T_B; a conclusion supported by measurements of the pressure dependence of I_th. From our results, we estimate that, at 300 K, Auger recombination accounts for 50% of I_th in the 1.3-μm laser and 80% of I_th in the 1.5-μm laser. Measurements of the spontaneous emission and differential efficiency indicate that a combination of increased optical losses and carrier overflow into the barrier and separate confinement heterostructure regions may further degrade T₀ (I_th) above room temperature     

1.55-μm InP-lattice-matched VCSELs with AlGaAsSb-AlAsSb DBRs

We review the design, fabrication, and characterization of 1.55-μm lattice-matched vertical-cavity surface-emitting lasers, operating continuous wave up to 88°C. For one embodiment, the threshold current is 800 μA, the differential quantum efficiency is 23%, and the maximum output power is more than 1 mW at 20°C and 110 μW at 80°C. The basic structure consists of AlAsSb-AlGaAsSb mirrors, which provide both high reflectivity and an InP-lattice-matched structure. The quaternary mirrors have poor electrical and thermal conductivities, which can raise the device temperature. However, a double-intracavity-contacted structure along with thick n-type InP cladding layers circumvents these drawbacks and finally leads to an excellent performance. The measured voltage and thermal impedances are much lower for the intracavity-contacted device than an air-post structure in which current is injected through the Sb-based quaternary mirror. The structure utilizes an undercut aperture for current and optical confinement. The aperture reduces scattering loss at the etched mirror and contributes to high differential efficiency and low threshold current density     

GaInNAs: a novel material for long-wavelength semiconductor lasers

GaInNAs was proposed and created in 1995 by the authors. It can be grown pseudomorphically on a GaAs substrate and is a light-emitting material having a bandgap energy suitable for long-wavelength laser diodes (1.3-1.55 μm and longer wavelengths). By combining GaInNAs with GaAs or other wide-gap materials that can be grown on a GaAs substrate, a type-I band lineup is achieved and, thus, very deep quantum wells can be fabricated, especially in the conduction band. Since the electron overflow from the wells to the barrier layers at high temperatures can he suppressed, the novel material of GaInNAs is very attractive to overcome the poor temperature characteristics of conventional long-wavelength laser diodes used for optical fiber communication systems. GaInNAs with excellent crystallinity was grown by gas-source molecular beam epitaxy in which a nitrogen radical was used as the nitrogen source. GaInNAs was applied in both edge-emitting and vertical-cavity surface-emitting lasers (VCSELs) in the long-wavelength range. In edge-emitting laser diodes, operation under room temperature continuous-wave (CW) conditions with record high temperature performance (T₀=126 K) was achieved. The optical and physical parameters, such as quantum efficiency and gain constant, are also systematically investigated to confirm the applicability of GaInNAs to laser diodes for optical fiber communications. In a VCSEL, successful lasing action was obtained under room-temperature (RT) CW conditions by photopumping with a low threshold pump intensity and a lasing wavelength of 1.22 μm     

A study of the temperature sensitivity of GaAs-(Al,Ga)As multiplequantum-well GRINSCH lasers

We have measured the temperature sensitivity, T₀, of GaAs-(Al,Ga)As, GRINSCH, multiple quantum-well (MQW) lasers with different numbers of quantum wells ranging from one to ten. Our data suggests that there is an optimum number of wells, namely five, where T ₀ is highest. Using a temperature-dependent model based on drift-diffusion equations, we have systematically analyzed the temperature sensitivity of a MQW GaAs-(Al, Ga)As laser. The T₀ versus well-number behavior observed experimentally is verified, and the important temperature-dependent factors are identified     

Room-temperature lasing operation of GaInNAs-GaAssingle-quantum-well laser diodes

We have succeeded in demonstrating continuous-wave (CW) operation of GaInNAs-GaAs single-quantum-well (SQW) laser diodes at room temperature (RT). The threshold current density was about 1.4 kA/cm², and the operating wavelength was approximately 1.18 μm for a broad-stripe geometry. Evenly spaced multiple longitudinal modes were clearly observed in the lasing spectrum. The full-angle-half-power far-field beam divergence measured parallel and perpendicular to the junction plane was 4.5° and 45°, respectively. A high characteristic temperature (T₀) of 126 K under CW operation and a small wavelength shift per ambient temperature change of 0.48 nm/°C under pulsed operation were obtained. These experimental results indicate the applicability of GaInNAs to long-wavelength laser diodes with excellent high-temperature performance     

Cleaved and etched facet nitride laser diodes

Room-temperature (RT) pulsed operation of blue (420 nm) nitride-based multiquantum-well laser diodes grown on a-plane and c-plane sapphire substrates has been demonstrated. Structures investigated include etched and cleaved facets as well as doped and undoped quantum wells. A combination of atmospheric and low-pressure metal organic chemical vapor deposition using a modified two-flow horizontal reactor was employed. Threshold current densities as low as 12.6 kA/cm² were observed for 10×1200 μm lasers with uncoated reactive ion etched facets on c-plane sapphire. Cleaved facet lasers were also demonstrated with similar performance on a-plane sapphire. Laser diodes tested under pulsed conditions operated up to 6 h at RT. Lasing was achieved up to 95°C and up to a 150-ns pulselength (RT). Threshold current increased with temperature with a characteristic temperature T₀ of 114 K     

Analysis of gain in determining T0 in 1.3 μmsemiconductor lasers

Rapid decrease of differential gain has been determined to dominate the temperature dependence of threshold current in 1.3-μm multiquantum well and bulk active lasers giving rise to low values of T ₀. Extensive experimental characterization of each type of device is described. Results are presented for the dependence of gain on chemical potential and carrier density as a function of temperature. The data indicate the important role of the temperature-insensitive, carrier density dependent chemical potential in determining differential gain. Modeling of the temperature dependence of threshold carrier density in MQW and bulk active lasers based on a detailed band theory calculation is described. The calculated value of T₀ depends on the structure of the active layer, e.g., multiquantum well versus bulk. However, the calculated values are substantially higher than measured     

An investigation into the temperature sensitivity of strained andunstrained multiple quantum-well, long wavelength lasers: new insightand methods of characterization

The magnitude of the temperature rate of change of the threshold current density (J_th) is examined with respect to J_th , for a variety of unstrained and strained, long wavelength multiple quantum-well (MQW) lasers. A strong correlation is found between these parameters, and a new relationship describing the J_th -T relationship for these lasers is arrived at in terms of two new essentially temperature and length independent constants. A third constant, T_max, also appears which estimates the theoretical maximum operating temperature of the laser. It is proposed that these constants may prove to be more useful in characterizing the temperature sensitivity of semiconductor lasers than the conventional parameters T ₀ and I₀ which exhibit both a length and temperature dependence. Furthermore, an expression is found which relates the magnitude of T_max to adjustable device structural and material parameters, such as: the cavity length, L; facet reflectivity, R; transparency current density, J_tr; and, the modal gain coefficient, β. It is revealed that a close examination of this relationship may provide new insight into the physics of semiconductor lasers as well as a means for optimizing device design to obtain a high maximum operating temperature in order to eliminate the need for thermoelectric coolers in device packaging. Finally, the measured T_max, versus L characteristics of six different strained and unstrained MQW laser structures are presented     

Breakdown behavior of SF6 gas-insulated systems at lowtemperature

The controversy surrounding low-temperature SF₆ breakdown is addressed in detail. Earlier relevant studies are reviewed, some of the existing data is analyzed in a new light, and further theoretical considerations are presented. These discussions served to outline an experimental approach aimed at confirming or invalidating breakdown invariance at subnormal temperatures. Low-temperature dc breakdown of an SF₆ gas-insulated system was investigated experimentally for temperatures ranging from -50 to 24°C, using associated pressure values that had been selected carefully to avoid phase transition of the gas-insulating medium. The context allowed experimentation under both uniform and nonuniform field conditions; the nonuniformity was due to the active role of the cathode-gas interface at high fields. The experiment was conducted for molecular densities ranging from 2.596 to 16.43×10¹⁹ cm^-3 (equivalent to pressures of 105 and 624 kPa, respectively, at 24°C) and for gap lengths starting at 0.5 mm and extending to 7 mm. Data sets show consistency, low statistical scattering, and high reproducibility. Data analysis led to several major conclusions. At constant density, the breakdown of the SF₆-insulated system is temperature dependent, which is responsible for a decrease in the electric strength, by ~10%. This decrease occurs for uniform field conditions, the effect being small if not negligible for nonuniform field conditions, and is noted to appear at a threshold temperature (-25 to -30°C), take a constant value, and be fairly independent of density     

1.3-μm InAsP n-type modulation-doped MQW lasers grown bygas-source molecular beam epitaxy

The effect of n-type modulation doping as well as growth temperature on the threshold current density of 1.3-μm InAsP strained multiple-quantum-well (MQW) lasers grown by gas-source molecular beam epitaxy (GSMBE) was investigated for the first time. We have obtained threshold current density as low as 250 A/cm² for 1200-μm long devices. The threshold current density per well for infinite cavity length J_th/N_w∞ of 57 A/cm² was obtained for the optimum n-doping density (N_D=1×10¹⁸ cm^-3) and the optimum growth temperature (515°C for InP and 455°C for the SCH-MQW region), which is about 30% reduction as compared with that of undoped MQW lasers. A very low continuous-wave threshold current of 0.9 mA have been obtained at room temperature, which is the lowest ever reported for long-wavelength lasers using n-type modulation doping, and the lowest results grown by all kinds of MBE in the long-wavelength region. The differential gain was estimated by the measurement of relative intensity noise. No significant reduction of differential gain was observed for n-type MD-MQW lasers as compared with undoped MQW lasers. The carrier lifetime was also reduced by about 33% by using n-type MD-MQW lasers. Both reduction of the threshold current and the carrier lifetime lead to the reduction of the turn-on delay time by about 30%. The 1.3-μm InAsP strained MQW lasers using n-type modulation doping with very low power consumption and small turn-on delay is very attractive for laser array application in high-density parallel optical interconnection systems     

Continuous-Wave (CW) operation of GaInP-AlGaInP visiblecompressively strained multiple quantum-wire (CS-WQWR) lasers

GaInP-AlGaInP compressively strained multiple quantum-wire layers were fabricated by the in situ strain induced lateral layer ordering process, during gas source molecular beam epitaxial (GS-MBE) growth. The effect of compositional modulation was described in terms of PL spectra, and TEM images for GaInP-AlGaInP MQWR lasers with 18 period (GaP)_1.5-(InP)_1.5 SPBS active layers. Based on transmission electron microscopy (TEM) images, the size of quantum-wire width was estimated, and the size fluctuation of quantum wires were discussed. Quantum-wire effect was discussed in terms of anisotropic lasing characteristics and EL polarization, which were reflected by an anisotropic oscillation strength in quantum wires and the comparison with GaInP-AlGaInP compressively strained quantum-film lasers was examined in terms of threshold current density. The condition under which quantum wires were formed by strained induced lateral layer ordering process was discussed in terms of anisotropic behaviors of lasing characteristics, such as threshold current density and lasing wavelength for GaInP-AlGaInP MQWR lasers with (GaP)_m/(InP) _mSPBS active layers. The lowest obtained J_th value was 278 A/cm² under the room temperature (r.t.) pulsed condition. The first CW operation of GaInP-AlGaInp quantum-wire laser was described. Threshold current was 294 A/cm² and CW operation up to 70°C was obtained     

Infrared microscopy studies on high-power InGaAs-GaAs-InGaP laserswith Ga2O3 facet coatings

InGaAs-GaAs separate confinement, heterostructure single quantum-well (SCH-SQW) lasers (λ=0.98 μm) with lattice-matched InGaP cladding layers, using a new Ga₂O₃ low reflectivity (LR) front-facet coating, are reported. The CW peak power density (17 MW/cm²) of 6 μm×750 μm ridge-waveguide lasers is limited by thermal rollover, and repeated cycling beyond thermal rollover produced no change in operating characteristics. The high-power temperature distribution along the active stripe has been measured by high-resolution infrared (3-5 μm) imaging microscopy. The temperature profile acquired for a very high optical power density P_D=11 MW/cm³ was found to be uniform along the inner active laser stripe, and revealed a local temperature increase at the LR front facet ΔT_f of only 9 K above the average stripe temperature ΔT_s=24 K. An excellent front-facet interface recombination velocity <10⁵ cm/s has been inferred from the measured low local temperature rise in the front facet     

Effects of O2 rapid thermal annealing on themicrostructural properties and reliability of RF-sputteredTa2O5 films

The microstructural properties and reliability of sputtered Ta₂O₅ films treated by various temperatures of rapid thermal annealing (RTA) in O₂ atmosphere have been systematically investigated. Analytical results revealed that whenever the RTA temperature was >650°C, the noncrystallinity of as-grown Ta₂O₅ film would be effectively improved from an amorphous phase to the β-Ta₂O₅ phase. Leakage current measurement indicated that leakage current decreases with increasing annealing temperature in a low RTA temperature range (⩽650°C) and, contrarily, increases with increasing annealing temperature in a high RTA temperature range (650 to 950°C). The former result was asserted in that reducing pinholes and oxygen vacancies played key factors. However, the latter result arose due to significant Si diffusion into the Ta₂O₅ film, causing a leaky transition layer distributed along the grain boundary to form the leakage path. Finally, the time-dependent dielectric-breakdown experiments revealed that 950°C O₂ RTA treated Ta₂ O₅ film possessed the superior crystallinity, creating less interfacial hole trap states at the junction of Ta₂O₅/Si and exhibiting the best long-term reliability     

Loss and recovery of hydrophobicity, surface energies, diffusioncoefficients and activation energy of nylon

The loss of hydrophobicity of nylon 6/6 caused by immersion in saline water for up to 336 h at different conductivities (0.005 to 100 mS/cm) and different temperatures (0 to 98°C) and its subsequent recovery in air (during 4500 h) have been investigated. The hydrophobicity is determined by measuring the static contact angle &thetas; between the tangent to a droplet of distilled water and the horizontal surface. The changes in the surface roughness and in the weight of the specimens were determined and correlated with the changes in the contact angle. It has been found that &thetas; decreased with increasing conductivity and increasing temperature of the saline solution. After removal from the solution, the higher the conductivity and temperature, the longer it took for &thetas; to recover in air. &thetas; decreased from 70° to 54° after nylon was subjected for 521 h to a uniform field of 15 kV_dc/cm in air. The surface free energy of nylon was determined as a function of time of immersion, the conductivity and temperature of the solution and during the recovery in air. The surface energies calculated for the virgin specimen are in good agreement with the literature. The diffusion coefficient of water into nylon increased from 0.23×10^-12 m²/s at 23°C to 7.4×10^-12 m²/s at 75°C. The activation energy was determined to be 59.4±2.2 kJ/mol. For unaged nylon the surface energies were determined at 23°C to be γ_S=44.7 mJ/m², γ_SD=29.3 mJ/m², γ_SH=15.4 mJ/m², W_SL =97.7 mJ/m² and γ_SL=19.8 mJ/m²      

InGaAs-GaAs quantum-dot lasers

Quantum-dot (QD) lasers provide superior lasing characteristics compared to quantum-well (QW) and QW wire lasers due to their delta like density of states. Record threshold current densities of 40 A·cm ^-2 at 77 K and of 62 A·cm^-2 at 300 K are obtained while a characteristic temperature of 385 K is maintained up to 300 K. The internal quantum efficiency approaches values of ~80 %. Currently, operating QD lasers show broad-gain spectra with full-width at half-maximum (FWHM) up to ~50 meV, ultrahigh material gain of ~10⁵ cm^-1, differential gain of ~10^-13 cm² and strong nonlinear gain effects with a gain compression coefficient of ~10^-16 cm³. The modulation bandwidth is limited by nonlinear gain effects but can be increased by careful choice of the energy difference between QD and barrier states. The linewidth enhancement factor is ~0.5. The InGaAs-GaAs QD emission can be tuned between 0.95 μm and 1.37 μm at 300 K     

Highly strained GaInAs-GaAs quantum-well vertical-cavitysurface-emitting laser on GaAs (311)B substrate for stable polarizationoperation

We have realized high-quality GaInAs-GaAs quantum wells (QWs) with high strain of over 2% on GaAs (311)B substrate for a polarization controlled vertical-cavity surface-emitting laser (VCSEL). By increasing the In composition in GaInAs, the optical anisotropy in photoluminescence (PL) intensity was increased. The anisotropy of 50% was obtained at 1.15 μm emission wavelength. We have demonstrated edge-emitting lasers and VCSELs emitting at over 1.1 μm on GaAs (311)B substrate for the first time. The 1.15-μm edge-emitting laser showed a characteristic temperature of 210 K and the threshold current density of 410 A/cm². The threshold current and lasing wavelength of VCSELs are 0.9 mA and 1.12 μm, respectively. The orthogonal polarization suppression ratio was 25 dB and CW operation up to 170°C without a heat sink was achieved     

Recent Advances on InAs/InP Quantum Dash Based Semiconductor Lasers and Optical Amplifiers Operating at 1.55 μm

This paper summarizes recent advances on InAs/InP quantum dash (QD) materials for lasers and amplifiers, and QD device performance with particular interest in optical communication. We investigate both InAs/InP dashes in a barrier and dashes in a well (DWELL) heterostructures operating at 1.5 mum. These two types of QDs can provide high gain and low losses. Continuous-wave (CW) room-temperature lasing operation on ground state of cavity length as short as 200 mum has been achieved, demonstrating the high modal gain of the active core. A threshold current density as low as 110 A/cm² per QD layer has been obtained for infinite-length DWELL laser. An optimized DWELL structure allows achieving of a T₀ larger than 100 K for broad-area (BA) lasers, and of 80 K for single-transverse-mode lasers in the temperature range between 25degC and 85degC. Buried ridge stripe (BRS)-type single-mode distributed feedback (DFB) lasers are also demonstrated for the first time, exhibiting a side-mode suppression ratio (SMSR) as high as 45 dB. Such DFB lasers allow the first floor-free 10-Gb/s direct modulation for back-to-back and transmission over 16-km standard optical fiber. In addition, novel results are given on gain, noise, and four-wave mixing of QD-based semiconductor optical amplifiers. Furthermore, we demonstrate that QD Fabry-Perot (FP) lasers, owing to the small confinement factor and the three-dimensional (3-D) quantification of electronic energy levels, exhibit a beating linewidth as narrow as 15 kHz. Such an extremely narrow linewidth, compared to their QW or bulk counterparts, leads to the excellent phase noise and time-jitter characteristics when QD lasers are actively mode-locked. These advances constitute a new step toward the application of QD lasers and amplifiers to the field of optical fiber communications     

NOx removal process using pulsed discharge plasma

Nonequilibrium plasma can be used to promote chemical reactions that reduce the emission of gaseous pollutants, such as NO_x, produced by coal-burning power plants or by diesel engines. Laboratory experiments were carried out to study the decrease of NO_x in simulated flue gases (initial concentration of NO: 200-800 ppm, O₂ : 10%, N₂-balance gas) by means of a pulsed discharge plasma generated in a cylinder type reactor (outer electrode: 20-mm diameter). A rotating spark gap provided square wave high-voltages up to 25-kV, at a frequency of 250 Hz, to corona electrodes of 0.1-, 3.3-, and 6.4-mm diameter. The tests were performed at various temperatures (ambient to 220°C) and constant residence time (0.6 s). The removal performance depended on the size of the discharge electrode and was better at room temperature. The addition of C₂H₄ significantly enhanced the removal performance, concentration of NO_x decreased from 800 ppm to 300 ppm in the discharge. The by-products of this process were analyzed using infrared spectroscopy. No traces of toxic gases could be detected