Characteristic Temperature T0 Research Articles

Transport studies at the Mott transition of the two-dimensional organic metal κ-(BEDT-TTF)2Cu[N(CN)2]Br x Cl1- x

The two-dimensional organic conductor κ-(BEDT-TTF)2Cu[N(CN)2]Br x Cl1- x undergoes a transition from an insulator to a superconductor upon substituting Cl by Br. We have performed in- and out-of-plane electric-transport measurements on the alloyed series with x = 20%, 40%, 70%, 80%, 85%, and 90% as a function of temperature in order to explore the bandwidth-controlled phase transition between the Mott insulator and the Fermi-liquid. All crystals exhibit a similar semiconducting behavior of ρ(T) from room temperature down to 100 K. Below approximately 50 K, a metal-to-insulator transition is found for compounds with x < 70%. Out of this Mott insulating state, magnetic order develops below T N ≈ 25 K. The Br-rich samples cross a bad-metal regime before they become coherent metals and eventually superconducting at T c ≈ 12 K. For these systems the resistivity at T c ≤ T ≤ T 0 reveals a ρ(T) ∝ T 2 dependence associated with a strongly correlated Fermi-liquid, limited by some characteristic temperature T 0. The conclusions are corroborated by data from microwave, magnetic and optical experiments.

Investigation of temperature characteristics of modern InAsP/InGaAsP multi-quantum-well TJ-VCSELs for optical fibre communication

AbstractContinuous-wave (CW) performance of modern 1.3-μm InAsP/InGaAsP multi-quantum-well (MQW) tunnel-junction vertical-cavity surface-emitting diode lasers (TJ-VCSELs) is investigated using our comprehensive self-consistent simulation model to suggest their optimal design for room and elevated temperatures. For increasing ambient temperatures, an increase in the VCSEL threshold current has happened to be mostly associated with the Auger recombination. Nevertheless, the InAsP/InGaAsP VCSELs have been found to exhibit encouraging thermal behaviour with the quite high value of maximal operating temperature of 350 K. It has been found that 5-μm devices seem to be the most optimal ones because they demonstrate both the room temperature (RT) threshold current equal to only 0.55 mA and maximum operating temperature equal to as much as 345 K. For these devices, the characteristic temperature T0 is equal to 92 K for 290–305 K, 51 K for 310–325 K and 29 K for 330–345 K. Therefore, the InAsP/InGaAsP VCSELs have been found to offer very promising performance both at room and elevated temperatures as sources of the carrier 1.3-μm wave in the fibre optical communication using silica fibres.

Simple method to estimate cell wall extensibility coefficient by using only two cardinal numbers

In this short communication we consider the extensibility properties of the cell wall. This is accomplished by a heuristically motivated equation for the expanding volume of the cell. The experimentally determined characteristic time t0 and temperature T0 are the only numbers required for evaluating the effective yielding coefficient Ф(t, T) in the respective time and temperature domains.

High Characteristic Temperature InGaAsP/InP Tunnel Injection Multiple-Quantum-Well Lasers

We fabricate 1.5 μm InGaAsP/InP tunnel injection multiple-quantum-well (TI-MQW) Fabry—Perot (F-P) ridge lasers. The laser heterostructures, including an inner cladding layer and an InP tunnel barrier layer, are grown by metal-organic chemical-vapor deposition (MOCVD). Characteristic temperature T0 of 160 K at 20°C is obtained for 500-μm-long lasers. T0 is measured as high as 88 K in the temperature range of 15–75°C. Cavity length dependence of T0 is investigated.

High-performance quantum cascade lasers in the 7.3- to 7.8-μm wavelength band using strained active regions

We consider the use of strained quantum cascade (QC) laser designs in the 7.3- to 7.8-µm wavelength region to improve the cw operating characteristics of these lasers at room temperature. We compare the performance of a QC laser with a strain-balanced active region with that of two similarly designed QC lasers with lattice-matched active-region layers, and we show that the strain-balanced design significantly outperforms the unstrained-layer designs in terms of threshold-current density, power slope efficiency, and cw output power at room temperature. A epi-side-down-mounted, double-channel ridge-geometry laser that was fabricated from the strained design with a ridge width of 13 µm and a cavity length of 3 mm produced cw output power in excess of 500 mW at room temperature. The characteristic temperature T0 for the strained-layer design as determined from pulsed measurements between 25 and 80°C is 221.5 K, compared with a value of 194.0 K for one of the unstrained designs.

Efficiency limiting processes in 1.55 μm InAs/InP-based quantum dots lasers

The threshold current density, Jth, and its radiative component, Jrad, in 1.55 μm InAs/InP (100) quantum dot lasers are measured as a function of temperature and hydrostatic pressure. We find that Jrad is relatively temperature insensitive. However, Jth increases significantly with temperature leading to a characteristic temperature T0=72 K over the range 220–290 K. Nonradiative recombination accounts for up to 94% of Jth at T=293 K. Jth decreases with increasing pressure by 35% over 8 kbar causing an increase in T0 from 72 to 88 K. The results indicate that nonradiative Auger recombination determines temperature behavior of these devices and T0 value.

Temperature dependence of the key electro-optical characteristics for midinfrared emitting quantum cascade lasers

The equations for threshold-current density Jth, differential quantum efficiency ηd, and maximum wallplug efficiency ηwp,max for quantum-cascade lasers (QCLs) are modified for electron leakage and backfilling. A thermal-excitation model of “hot” injected electrons from the upper laser state to upper active-region states is used to calculate leakage currents. The calculated characteristic temperature T0 for Jth is found to agree well with experiment for both conventional and deep-well (DW) QCLs. For conventional QCLs ηwp,max is found to be strongly temperature dependent; explaining experimental data. At 300 K for optimized DW-QCLs, front-facet, continuous-wave ηwp,max values &amp;gt;20% are projected.

MBE growth and characterization of 5-μm quantum-cascade lasers

Quantum-cascade lasers, which are obtained by molecular-beam epitaxy, emit at wavelengths of 5.0 μm at 77 K and 5.2 μm at 300 K and are based on a design with four quantum wells in the active region with vertical transitions and strain-compensated superlattices with high-efficiency injection and a short lifetime of the ground state are fabricated. The typical thresholds for lasing at 300 K were in the range 4–10 kA/cm2. The maximum emission power was as high as ∼1 W, the maximum lasing temperature was ∼450 K, and the maximum characteristic temperature T 0 ≈ 200 K. The use of a modified process of postgrowth treatment made it possible to reproducibly obtain high-quality devices.

High Characteristic Temperature 1.3 μm InAs/GaAs Quantum-Dot Lasers Grown by Molecular Beam Epitaxy

We report the molecular beam epitaxy growth of 1.3 μm InAs/GaAs quantum-dot (QD) lasers with high characteristic temperature T0. The active region of the lasers consists of five-layer InAs QDs with p-type modulation doping. Devices with a stripe width of 4 μm and a cavity length of 1200 μm are fabricated and tested in the pulsed regime under different temperatures. It is found that To of the QD lasers is as high as 532K in the temperature range from 10°C to 60°C. In addition, the aging test for the lasers under continuous wave operation at 100°C for 72 h shows almost no degradation, indicating the high crystal quality of the devices.

High-power diode lasers (λ = 1.7–1.8 µm) based on asymmetric quantum-well separate-confinement InGaAsP/InP heterostructures

Advantages of the concept of high-powered semiconductor nanoheterostructure lasers for the spectral range 1700–1800 nm, grown by MOCVD in the InGaAsP/InP solid solution system, have been experimentally demonstrated. It has been found that using an expanded waveguide enables reduction to 2 cm−1 of the internal optical loss in quantum-well asymmetric separate-confinement double InGaAsP/InP heterostructures emitting at a wavelength of 1.76 µm. The heterostructures developed have been used to create multimode lasers with a room-temperature CW output power of 2.5 W in an aperture of 100 µm. It is shown that use of highly stressed quantum-well InGaAs layers as the active region makes it possible to obtain characteristic temperatures T0 = 50–60 K.

AlGaN-based laser diodes for the short-wavelength ultraviolet region

We have demonstrated the room-temperature operation of GaN/AlGaN and indium-free AlGaN multiple-quantum-well (MQW) laser diodes under the pulsed-current mode. We have successfully grown low-dislocation-density AlGaN films with AlN mole fractions of 20 and 30% on sapphire substrates using the hetero-facet-controlled epitaxial lateral overgrowth (hetero-FACELO) method. GaN/AlGaN and AlGaN MQW laser diodes have been fabricated on the low-dislocation-density Al0.2Ga0.8N and Al0.3Ga0.7N films, respectively. The GaN/AlGaN MQW laser diodes lased at a peak wavelength ranging between 359.6 and 354.4 nm. A threshold current density of 8 kA cm−2, an output power as high as 80 mW and a differential external quantum efficiency (DEQE) of 17.4% have been achieved. The AlGaN MQW laser diodes lased at a peak wavelength down to 336.0 nm far beyond the GaN band gap. For the GaN/AlGaN MQW laser diodes, the modal gain coefficient and the optical internal loss are estimated to be 4.7±0.6 cm kA−1 and 10.6±2.7 cm−1, respectively. We have observed that the characteristic temperature T0 ranges from 132 to 89 K and DEQE shows an almost stable tendency with increase of temperature. A temperature coefficient of 0.049 nm K−1 is also found for the GaN/AlGaN MQW laser diode. The results for the AlGaN-based laser diodes grown on high-quality AlGaN films presented here will be essential for the future development of laser diodes emitting much shorter wavelengths.

Thermodynamic and transport properties of SmS under high pressure

We measured transport and thermodynamic properties of the valence-fluctuating phase of SmS up to 8.5 kbar, and found a bump structure in the temperature dependence of the electrical resistivity and a Schottky-type anomaly in the temperature dependence of the thermal expansion coefficient at a characteristic temperature T0. We also observed that the absolute value of the Hall constant rapidly increases below T0. From these results, we argue that the steep rise of the electrical resistivity below T0 is inherent to golden SmS and can be ascribed to the decrease in the carrier concentration possibly due to the pseudo gap formation.

Microscopic design of GaInNAs quantum well laser diodes on ternary substrates for high-speed and high-temperature operations

Material properties of highly strained GaInNAs quantum wells grown on GaInAs or quasi-GaInAs substrates are investigated by using microscopic theory together with a band structure calculation based on ten-band k⋅p theory specially formulated for highly strained materials. It is shown that the material gain of GaInNAs quantum wells is reduced by incorporating N into a well layer although the strain in the well layer becomes small. The reduction can be compensated by properly choosing barrier materials. The performance of laser diodes, such as characteristic temperatures T0 and differential gains, is also investigated, and the present results show that very high T0(≃140K) and differential gain with moderate strain (≃1.6%) can be achieved by carefully designing quantum well structures, indicating the applicability of these lasers for high-temperature and high-speed operation.

High‐performance UV emitter grown on high‐crystalline‐quality AlGaN underlying layer

AbstractAl0.25Ga0.75N films were grown on a grooved‐Al0.25Ga0.75N/ AlN/sapphire template by MOVPE. The dislocation density on the grooved areas was as low as 1 × 108 cm–2. We fabricated a UVA light‐emitting diode grown on such an AlGaN underlying layer exhibiting an output power of 12 mW at a DC current of 50 mA with a peak emission wavelength of 345 nm, which corresponds to an external quantum efficiency of 6.7%. This efficiency is the highest reported to date in this wavelength region. We also fabricated a 358 nm UVA laser diode (LD) using a GaN/AlGaN MQW active layer grown on an AlGaN underlying layer. This UV LD exhibits a threshold current of 73 mA and a corresponding current density of 3.8 kA/cm2 at 7 °C. The characteristic temperature T0 was 174 K in the temperature range of 7–27 °C. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Experimental characterization of high-speed 155 μm buried heterostructure InGaAsP/InGaAlAs quantum-well lasers

Detailed experimental characterization is performed for 1550 nm semi-insulating regrown buried heterostructure Fabry-Perot (FP) lasers having 20 InGaAsP/InGaAlAs strain-balanced quantum wells (QWs) in the active region. Light-current-voltage performance, electrical impedance, small-signal response below and above threshold, amplified spontaneous emission spectrum below threshold and relative intensity noise spectrum are measured. Different laser parameters such as external differential quantum efficiency ηd, background optical loss αi, K-factor, D-factor, characteristic temperature T0, differential gain dg/dn, gain-compression factor ϵ, carrier density versus current, differential carrier lifetime τd, optical gain spectrum below threshold, and chirp parameter α are extracted from these measurements. The FP lasers exhibited a high T0(78-86.5°C) and very high-resonance frequency (23.7 GHz). The results indicate that appropriately designed lasers having a large number of InGaAsP well/InGaAlAs barrier QWs with shallow valence-band discontinuity can be useful for uncooled high-speed direct-modulated laser applications.

Analysis of thermal performance of InGaP/InGaAlP quantum wells for high-power red laser diodes

The effect of quantum well (QW) strain on the thermal performance of InGaP/InGaAlP lasers emitting at around 635 nm is investigated theoretically. It is found that compressively-strained QWs offer superior thermal performance due to an increased barrier height for electrons, leading to a reduced electron leakage. The theoretical predictions are supported by experimental data from red-emitting laser diodes with an improved characteristic temperature T 0.

Quasi-one-dimensional hopping conductivity of the spin-ladder CaCu2O3 single crystals: Influence of the cation and oxygen nonstoichiometry

The resistivity ρ(T) of the spin-ladder compound CaCu2O3 measured along the Cu–O–Cu leg (j∥b) exhibits a strongly activated character. It increases from ∼104 to ∼109 Ω m if T decreases from 350 to 100 K. The charge transfer above T∼200 K is governed by a quasi-one-dimensional (1D) nearest-neighbor hopping (NNH) conductivity mechanism characterized by the law ρ(T)∼exp(Ea/kT). Below 200 K a novel quasi-1D variable-range hopping (VRH) conductivity law ρ(T)∼exp[(T0/T)3/4] is observed, predicted recently by Fogler, Teber, and Shklovskii [Phys. Rev. B 69, 035413 (2004)]. The NNH activation energy Ea and the VRH characteristic temperature T0 exhibit high sensitivity to the cation (Ca, Cu) content, decreasing by 2.3–2.5 times and by 3.0–3.2 times, respectively, when the composition of Ca is changed from 0.854 to 0.786–0.798 and the composition of Cu from 2.039 to 2.159–2.163. The behavior of Ea and T0 can be attributed to a corresponding variation of the concentration of intrinsic defects associated with Cu vacancies. On the other hand, no direct dependence of Ea and T0 to the excess oxygen concentration is observed.

Temperature sensitivity (T) of tensile-strained GaAsP/(AlGa)As double-barrier separate confinement heterostructure laser diodes for 800 nm band

The temperature sensitivity of the double-barrier separate confinement heterostructure (DBSCH) laser diodes (LDs), intended for high-power, low vertical beam divergence emission, was shown to be somewhat lower than that of the large optical cavity (LOC) devices of similar beam divergences. For the lowest beam divergences, it still remains considerably high, resulting in low characteristic temperatures T0 not exceeding 80 K in the LDs of the vertical beam divergences below 15°. In this work, the decrease in the T0 values of such tensile-strained GaAsP/(AlGa)As DBSCH LDs has been found primarily due to an increased, thermally activated occupation of the heavy-hole subband and, to a minor degree, occupation of the second quantum level, both giving rise to transitions that do not contribute to the optical gain. This is caused by the higher and more temperature-dependent at-threshold band filling for devices of reduced confinement factor (Γ) aimed at achieving low beam divergence. This phenomenon seems to be common for all the tensile-strained LDs and acts in addition to other mechanisms that decrease the T0, such as the carrier escape from a quantum well to surrounding layers. However, in DBSCH LDs no carrier escape has been detected, presumably because of their thin waveguide layers. This can explain the somewhat higher T0 values of these devices when compared to their LOC counterparts.

A phenomenological model of galaxy clusters

We present a simple model to describe the dark matter density, the gas density, and the gas temperature profiles of galaxy clusters. Analytical expressions for these quantities are given in terms of only five free parameters with a clear physical meaning: the mass M of the dark matter halo (or the characteristic temperature T0), the characteristic scale radius a, the cooling radius in units of a (0 < α < 1), the central temperature in units of T0 (0 < t < 1), and the asymptotic baryon fraction in units of the cosmic value (f � 1). It is shown that our model is able to reproduce the three-dimensional density and temperature profiles inferred from X-ray observations of real clusters within a 20 per cent accuracy over most of the radial range. Some possible applications are briefly discussed.

Analysis of anticipated performance of 650-nm GaInP/AlGaInP quantum-well GaAs-based VCSELs at elevated temperatures

AbstractThe possibility of application of the 650-nm oxide-confined GaInP/AlGaInP quantum-well vertical-cavity surface-emitting diode lasers (VCSELs) at elevated temperatures as sources of the carrier 650-nm wave in the fibre optical communication using POFs has been investigated with the aid of the comprehensive self-consistent model. An increase in the VCSEL threshold current at higher temperatures has been found to be mostly associated with both the carrier leakage from the valley of the Ga0.43In0.57P quantum-well material to the X-valley of the (Al0.67Ga0.33)0.52In0.48P barriers and the band-to-band absorption within the Ga0.52In0.48P layer of the band-gap comparable with the energy of emitted radiation. Nevertheless, the AlGaInP VCSELs exhibit encouraging thermal behaviour with the characteristic temperature T0 equal to as much as 134 K for the active-region temperatures up to 357 K. For the 5-μm devices, the maximal achievable output has been determined to decrease from a quite high value of 1.0 mW for 293 K to 0.6 mW for 320 K and to still high 0.33 mW for 340 K. However, an efficient operation of the above VCSEL at elevated temperatures requires still some structure modifications leading to a reduction of both the above effects, the electron leakage from the valley and the band-to-band absorption within GaInP layers.