Threshold Carrier Concentration Research Articles

Temperature-Dependent Conduction and Photoresponse in Few-Layer ReS2.

The electrical behavior and the photoresponse of rhenium disulfide field-effect transistors (FETs) have been widely studied; however, only a few works have investigated the photocurrent as a function of temperature. In this paper, we perform the electrical characterization of few-layer ReS2-based FETs with Cr-Au contacts over a wide temperature range. We exploit the temperature-dependent transfer and output characteristics to estimate the effective Schottky barrier at the Cr-Au/ReS2 interface and to investigate the temperature behavior of parameters, such as the threshold voltage, carrier concentration, mobility, and subthreshold swing. Through time-resolved photocurrent measurements, we show that the photocurrent increases with temperature and exhibits a linear dependence on the incident light power at both low and room temperatures and a longer rise/decay time at higher temperatures. We surmise that the photocurrent is affected by the photobolometric effect and light-induced desorption of adsorbates which are facilitated by the high temperature and the low pressure.

Charge transport in a polar metal

The fate of electric dipoles inside a Fermi sea is an old issue, yet poorly explored. Sr{}_{1-x}Ca{}_{x}TiO{}_{3} hosts a robust but dilute ferroelectricity in a narrow (0.0018 < x < 0.02) window of substitution. This insulator becomes metallic by removal of a tiny fraction of its oxygen atoms. Here, we present a detailed study of low-temperature charge transport in Sr{}_{1-x}Ca{}_{x}TiO{}_{3-delta }, documenting the evolution of resistivity with increasing carrier concentration (n). Below a threshold carrier concentration, {n}^{* }(x), the polar structural-phase transition has a clear signature in resistivity and Ca substitution significantly reduces the 2 K mobility at a given carrier density. For three different Ca concentrations, we find that the phase transition fades away when one mobile electron is introduced for about 7.9pm 0.6 dipoles. This threshold corresponds to the expected peak in anti-ferroelectric coupling mediated by a diplolar counterpart of Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction. Our results imply that the transition is driven by dipole–dipole interaction, even in presence of a dilute Fermi sea. Charge transport for n < {n}^{* }(x) shows a non-monotonic temperature dependence, most probably caused by scattering off the transverse optical phonon mode. A quantitative explanation of charge transport in this polar metal remains a challenge to theory. For nge {n}^{* }(x), resistivity follows a T-square behavior together with slight upturns (in both Ca-free and Ca-substituted samples). The latter are reminiscent of Kondo effect and most probably due to oxygen vacancies.

High-Temperature Defect-Induced Hopping Conduction in Multilayered Germanium Sulfide for Optoelectronic Applications in Harsh Environments

Intrinsic crystal defects play a major role in tailoring the electrical and optical properties of two-dimensional (2D) materials. Here, we probe the impact of planar crystal defects on the electrical characteristics of germanium sulfide (GeS) field effect transistor (FET) at different operating temperatures varying from 300 to 575 K. Our results show that the measured mobility of the GeS field effect transistor was 0.04 × 10–3 cm2/(V s) at 300 K, and this value reached 58 × 10–3 cm2/(V s) at 575 K. It is important to note that the mobility of GeS FET at elevated temperatures in this study is greater than the mobilities in the recently reported GeS photodetector studies. Furthermore, evidence that the threshold voltage (Vth) decreases and carrier concentration increases with increasing temperature in the GeS channel is provided. We demonstrate an Arrhenius-like relation of the carrier transport as a function of temperature, a behavior that we attribute to nearest-neighbor-hopping (NNH) conduction. The existence of planar defects is revealed using transmission electron microscopy (TEM) while density functional theory (DFT) analysis supports the hypothesis that the formation of localized energy states governs hopping conduction. This study reports hopping conduction at the temperature above 300 K for the first time, whereas previous investigations on 2D materials have reported a hopping mechanism in the low-temperature (<200 K) range. These observations give insight into the fundamental charge conduction mechanisms at high temperature in other 2D materials systems which are expected to aid in the development of applications for harsh environments.

Irradiation-Induced Modification of the Superconducting Properties of Heavily-Boron-Doped Diamond

Diamond, a wide band-gap semiconductor, can be engineered to exhibit superconductivity when doped heavily with boron. The phenomena has been demonstrated in samples grown by chemical vapor deposition where the boron concentration exceeds the critical concentration for the metal-to-insulator transition of nMIT4×1020/cm3. While the threshold carrier concentration for superconductivity is generally well established in the literature, it is unclear how well correlated higher critical temperatures are with increased boron concentration. Previous studies have generally compared several samples grown under different plasma conditions, or on substrates having different crystallographic orientations, in order to vary the incorporation of boron into the lattice. Here, we present a study of a single sample with unchanging boron concentration, and instead modify the charge-carrier concentration by introducing compensating defects via high-energy ion irradiation. Superconductivity is completely suppressed when the number of defects is sufficient to compensate the hole concentration to below threshold. Furthermore, we show it is possible to recover the superconductivity by annealing the sample in vacuum to remove the compensating defects.

On the temperature delocalization of carriers in GaAs/AlGaAs/InGaAs quantum-well heterostructures

The effect of temperature delocalization in semiconductor lasers (emission wavelength λ = 1060 nm) based on symmetric and asymmetric separate-confinement heterostructures fabricated by metal-organic vapor-phase epitaxy (MOVPE) is studied. Experimental and calculated estimates show that the carrier concentration in the waveguide increases by an order of magnitude when the temperature of a semiconductor laser is raised by ∼100°C. It is found that an increase in the temperature of the active zone leads to enhancement of the temperature delocalization of both electrons and holes. It is shown that the delocalization of holes begins at higher temperatures, compared with that of electrons. It is demonstrated experimentally that the onset of temperature delocalization depends on the threshold carrier concentration in the active region of a laser at room temperature. It is found that raising the energy depth of the active region by choosing the waveguide material makes it possible to fully suppress the temperature-delocalization process up to 175°C.

Performance Enhancement of GaN-Based Laser Diodes With Prestrained Growth

Effects of the prestrained growth of a low-In InGaN/GaN quantum well (QW) before the designated light-emitting high-In InGaN/GaN multi quantum wells (MQWs) in GaN-based laser diodes (LDs) are numerically investigated. Simulation results show that the inserted low-In InGaN/GaN QW does not remarkably change the absorption loss, and the ratio of recombination current of low-In InGaN/GaN QW over injection current is very small, especially after LD is lasing. However, it weakens the polarization fields, resulting in less tilted energy bands, and the overlap of electron–hole pairs is greatly increased, which reduces threshold carrier concentrations of MQWs, leakage current, and nonradiative recombination. The performance of LD is enhanced. Threshold current is reduced by 19%, and output power is increased by 59% at the injection current of 120 mA.

Carrier saturation in multiple quantum well metallo-dielectric semiconductor nanolaser: Is bulk material a better choice for gain media?

Although multi quantum well (MQW) structure is frequently suggested as the appropriate medium for providing optical gain in nanolasers with low threshold current, we demonstrate that in general bulk gain medium can be a better choice. We show that the high threshold gain required for nanolasers demands high threshold carrier concentrations and therefore a highly degenerate condition in which the barriers between the quantum wells are heavily pumped. As a result, there occurs spontaneous emission from the barrier in very dissipative low Q modes or undesired confined higher Q modes with resonance wavelengths close to the barrier bandgap. This results in a competition between wells and barriers that suppresses lasing. A complete model involving the optical properties of the resonant cavity combined with the carrier injection in the multilayer structure is presented to support our argument. With this theoretical model we show that while lasing is achieved in the nanolaser with bulk gain media, the nanolaser with MQW gain structure exhibits well emission saturation due to the onset of barrier emission.

Diode lasers emitting near 3.44 µm in continuous-wave regime at 300K

A continuous-wave, room temperature operation of type-I quantum well diode lasers was extended above 3.4 μm. The laser heterostructure, optimised for minimum threshold carrier concentration, was pseudomorphically grown by solid source molecular beam epitaxy on GaSb. Multimode lasers generate 29 mW of output power at 17 °C.

A comprehensive analytical study of an undoped symmetrical double-gate MOSFET after considering quantum confinement parameter

This paper presents analytical models of threshold voltage, carrier concentration and drain current for undoped symmetrical double-gate (SDG) MOS devices. The analytical models are derived after solving the Poisson’s equation under Gradual Channel Approximation (GCA). Quantum mechanical effect in ultrathin silicon film is studied by introducing quantum confinement parameter and quantum corrected potential in the Poisson’s equation instead of solving complex Schrödinger–Poisson’s equation. The confinement parameter, which depends on film thickness, work function and gate bias decreases with film thickness. Quantum corrected potential attains its maximum value near the interface and minimum value at the centre of the silicon film. The mobile carrier density and surface potential are reduced due to quantum confinement effect. The simulation result of the threshold voltage shows excellent agreement with quantum numerical results.

Effect of Quantum Well Compressive Strain Above 1% On Differential Gain and Threshold Current Density in Type-I GaSb-Based Diode Lasers

InGaAsSb/AlGaAsSb quantum well (QW) diode laser structures with either 1% or 1.5% compressively strained QWs were grown on GaSb substrates by molecular beam epitaxy. Wide-stripe lasers fabricated from structures of both types have room-temperature operating wavelengths near 2.3 microns. The room-temperature threshold current density of 1-mm-long uncoated devices with 1.5% strained QWs was lower than threshold current density of the 1.0% strained QW devices by nearly a factor of two (120 A/cm2 versus 230 A/cm2 ). Experiment shows that the reduction in threshold current density with increasing QW strain is related to the increase in differential gain and decrease in transparency current density. Optical gain calculations prove that improvement of the QW hole confinement reduces the threshold carrier concentration in laser structures with heavily strained low arsenic content quantum wells.

Novel composite membranes comprising silver salts physically dispersed in poly(ethylene-co-propylene) for the separation of propylene/propane

Novel composite membranes, which delivered high separation performance for propylene/propane mixtures, were developed by coating inert poly(ethylene-co-propylene) rubber (EPR) onto a porous polyester substrate, followed by the physical distribution of AgBF4. Scanning electron microscopy-wavelength dispersive spectrometer (SEM-WDS) revealed that silver salts were uniformly distributed in the EPR layer. The physical dispersion of the silver salts in the inert polymer matrix, without specific interaction, was characterized by FT-IR and FT-Raman spectroscopy. The high separation performance was presumed to stem from thein-situ dissolution of crystalline silver ionic aggregates into free silver ions, which acted as an active propylene carrier within a propylene environment, leading to facilitated propylene transport through the membranes. The membranes were functional at all silver loading levels, exhibiting an unusually low threshold carrier concentration (less than 0.06 of silver weight fraction). The separation properties of these membranes, i.e. the mixed gas selectivity of propylene/propane ~55 and mixed gas permeance ~7 GPU, were stable for several days.

Effect of Induced Strain on Band Lineups in CdxZn1-xS/ZnS/BeyZn1-yS Separate-Confinement Heterostructures

We have performed a theoretical study of conduction and valence band offsets and effective band gap energy in CdxZn1-xS/ZnS/BeyZn1-yS separate-confinement heterostructures (SCHs). Valence band offset increases slightly with increasing Be content (y) in CdxZn1-xS/ZnS/BeyZn1-yS SCHs. The variation in effective band gap energy on threshold carrier concentration for production of a positive optical gain is very slight. We indicate that the BeyZn1-yS cladding layer is useful for the fabrication of optoelectric devices based on CdxZn1-xS/ZnS quantum wells (QWs).

Transport mechanism of chromium(III) through the unmixed bulk liquid membrane containing dinonylnaphthalenesulfonic acid as a carrier

Cr(III) transport through the unmixed bulk liquid membrane containing di-nonylnaphthalenesulfonic acid (DNNSA) as a carrier has been investigated. The concentration of chromium in the strip solution and the viscosity of the membrane versus DNNSA concentration in it was measured. The results indicated that in the present of threshold carrier content in the membrane (near 0.32 mol dm −3) the common facilitated transport is changing to the another kind of transport which is more effective than a facilitated diffusion mechanism. The maximum in the curve of Cr(III) flux versus DNNSA concentration was found and increase of membrane viscosity versus carrier concentration was observed. It seems that in the threshold carrier concentration in the membrane the distance between neighboring molecules decreases and there is a possibility to a chromium ions jump from one complexing center to another. We suppose that might be a “jumping” transport theoretically predicted by Cussler et al. [27]. It was shown that to obtain “jumping” effect a system needs a certain time-lag (about 0.5 h). During this time the new transport structure is self-organized inside the membrane.

HIGH POWER TYPE-I GASB-BASED LASERS

Challenges and limitations in development of the mid-infrared room temperature CW operated diode lasers with wavelength longer than 2 μm are discussed. The major breakthrough in terms of high power performance in this spectral range was achieved with type-I true quaternary InGaAsSb heavily compressively strained QW GaSb -based lasers. We have demonstrated 1 W CW at 2.5 μm, 500 mW and 160 mW CW at 2.7 and 2.8 μm, respectively. High power 2.3 μm linear laser arrays output 10W CW at room temperature. Experimental results show that there is no fundamental limitation to extend CW RT operating wavelength of these devices to over 3 μm spectral region. It is carrier leakage and material quality issues that limit device performance at wavelength longer than 2.5 μm. The role of Auger recombination is not decisive in type-I MQW GaSb -based lasers. We speculate that it is high differential gain and, as a result, low threshold carrier concentration that can account for muted effect of Auger on type-I GaSb -based laser performance. Dilute-nitride GaSb -based type-I QWs are proposed for development of CW room temperature operated lasers in spectral range 3 - 4 μm.

Effect of Induced Strain Due to Lattice Mismatch between MgyZn1-yS Cladding Layers and CdxZn1-xS/ZnS Quantum Wells on Effective Band Gap Energy in CdxZn1-xS/ZnS/MgyZn1-yS Separate-Confinement Heterostructures

We calculate the effect of the induced strain due to lattice mismatch between CdxZn1-xS/ZnS quantum wells (QWs) and MgyZn1-yS cladding layers on the effective band gap energy in CdxZn1-xS/ZnS/MgyZn1-yS separate-confinement heterostructures (SCHs). The effective band gap energy in the CdxZn1-xS/ZnS/MgyZn1-yS SCHs increases with increasing Mg content (y) for wider wells. The variation in effective band gap energy reflects the peak energy of optical gain spectra. The effect of the induced strain due to MgyZn1-yS cladding layers on the threshold carrier concentration for the production of a positive optical gain is very weak. The lattice-matching condition for the MgyZn1-yS cladding layers with respect to the CdxZn1-xS/ZnS MQWs is estimated. The induced strain due to the lattice mismatch between MgyZn1-yS cladding layers and CdxZn1-xS/ZnS MQWs is suppressed by adjusting y.

Olefin-induced dissolution of silver salts physically dispersed in inert polymers and their application to olefin/paraffin separation

Remarkably high-performance membranes are developed by physically dispersing silver ionic aggregates in inert polymer such as poly(dimethyl siloxane) (PDMS). The mixed gas selectivity for propylene/propane reaches nearly 200, to our knowledge the highest value ever reported for the solid state, whereas other membranes reported up to now show ∼60 of propylene/propane selectivity. Physical dispersion of silver salt in the inert polymers without specific interaction is observed by FT-IR, FT-Raman and wide angle X-ray scattering (WAXS). It is also found that the crystalline silver ionic aggregates are completely dissolved under the propylene environment, representing that propylene is a good ligand for coordination to silver ions. The inert membranes are functional at any loading level and hence a threshold carrier concentration does not exist.

促進輸送膜による高選択的ガス分離の最近の展開

Recent developments in the facilitated transport membranes (FTMs) for gas separations with high selectivity are described focusing on CO2/N2, 02/N2 and olefin/paraffin separations. There are two types of FTM, i.e., mobile carrier membrane and fixed carrier membrane. For both types of membrane, the rate and equilibrium of the reaction between the carrier and the penetrant gas have decisive effects on the membrane performance. With respect to fixed carrier membranes for O2/N2 separation, a composite membrane with a very thin active layer of Co (II) -porphyrin complex of about 80nm thickness on a porous support exhibits promising O2 permeance and selectivity. Very efficient solid polymer electrolyte membranes containing silver salts have been developed for olefin/paraffin separations, and the threshold carrier concentration behavior has been elucidated from the coordination behavior of silver ion in the membrane. Finally, a novel type of “facilitated transport membrane” accompanied by permeation of a carrier solution, which overcomes the drawback of liquid membranes such as instability and low permeability, is shown to be very effective for CO2/N2 and C2H4/C2H6 separations.

Internal quantum efficiency of stimulated emission of (λ=1.55 µm) InGaAsP/InP laser diodes

The stimulated emission (ηist) of InGaAsP/InP separate-confinement double heterostructure lasers operating at λ=1.5–1.6 µm has been studied experimentally and theoretically. Laser heterostructures with a varied design of the waveguide layer were grown by MOCVD. The maximum internal quantum efficiency ηist≈97% was obtained in a structure with a double-step waveguide characterized by minimum leakage into the p-emitter above the generation threshold. The high value of ηist is provided by low threshold and nonequilibrium carrier concentrations at the interface between the waveguide and p-emitter. The calculation yields ηist values correlating well with the experimental data.

Design of a 364 nm Electrically Pumped Multi-Quantum Well Continuous Wave Nitride Vertical Cavity Surface Emitting Laser

ABSTRACTShort-wavelength vertical cavity surface emitting lasers (VCSELS) are typically important for high-density optical memory and optical imaging systems. An AlGaN/GaN multiquantum well (MQW) VCSEL was designed for operation at wavelength of 364 nm. The design process consisted of careful selection of materials and optimization of the parameters for distributed Bragg reflector (DBR) stacks, active region, and calculation of the threshold current density, external quantum efficiency, and threshold carrier concentration. InGaN/AlGaN material system was selected for constructing DBR stacks in order to minimize power dissipation and to achieve high reflectivity. A reflectivity of 99% was calculated for 33 pairs in the bottom DBR stack and 97% was calculated for 30 pairs in the top DBR stack. An external quantum efficiency of 85% has been achieved through parametric optimization. Results were comparable to that of the similar structures based on the review of recent literatures.

Auger coefficients in type-II InAs/Ga1−xInxSb quantum wells

Two different approaches, a photoconductive response technique and a correlation of lasing thresholds with theoretical threshold carrier concentrations have been used to determine Auger lifetimes in InAs/GaInSb quantum wells. For energy gaps corresponding to 3.1–4.8 μm, the room-temperature Auger coefficients for seven different samples are found to be nearly an order-of-magnitude lower than typical type-I results for the same wavelength. The data imply that at this temperature, the Auger rate is relatively insensitive to details of the band structure.