Carrier Reservoir Research Articles

Determination of Surface Recombination Velocities at Contacts in Organic Semiconductor Devices Using Injected Carrier Reservoirs.

A method to determine surface recombination velocities at collecting contacts in interface-limited organic semiconductor devices, based on the extraction of injected carrier reservoirs in a single-carrier sandwich-type structure, is presented. The analytical framework is derived and verified with drift-diffusion simulations. The method is demonstrated on solution-processed organic semiconductor devices with hole-blocking TiO_{2}/organic and SiO_{2}/organic interfaces, relevant for solar cell and transistor applications, respectively.

The “electron crystal” behavior in copper chalcogenides Cu2X (X = Se, S)

In Cu2Se, the structural fluctuation of the Cu sites have minor effects on the electronic structures around valence band maximum. The electrical transport properties of Cu2Se are mainly determined by the Se sublattice; Cu atoms serve as the carrier reservoirs.

Contribution of off-resonant states to the phase noise of quantum dot lasers.

The phase noise of quantum dot lasers is investigated theoretically by coupling the Langevin noise sources into the rate equations. The off-resonant populations in the excited state and in the carrier reservoir contribute to the phase noise of ground-state emission lasers through the phase-amplitude coupling effect. This effect arises from the optical-noise induced carrier fluctuations in the off-resonant states. In addition, the phase noise has low sensitivity to the carrier scattering rates.

Strong amplitude-phase coupling in submonolayer quantum dots

Submonolayer quantum dots promise to combine the beneficial features of zero- and two-dimensional carrier confinement. To explore their potential with respect to all-optical signal processing, we investigate the amplitude-phase coupling (α-parameter) in semiconductor optical amplifiers based on InAs/GaAs submonolayer quantum dots in ultrafast pump-probe experiments. Lateral coupling provides an efficient carrier reservoir and gives rise to a large α-parameter. Combined with a high modal gain and an ultrafast gain recovery, this makes the submonolayer quantum dots an attractive gain medium for nonlinear optical signal processing.

Observation of the Transition from Lasing Driven by a Bosonic to a Fermionic Reservoir in a GaAs Quantum Well Microcavity.

We show that, by monitoring the free carrier reservoir in a GaAs-based quantum well microcavity under nonresonant pulsed optical pumping, lasing supported by a fermionic reservoir (photon lasing) can be distinguished from lasing supported by a reservoir of bosons (polariton lasing). Carrier densities are probed by measuring the photocurrent between lateral contacts deposited directly on the quantum wells of a microcavity that are partially exposed by wet chemical etching. We identify two clear thresholds in the input-output characteristic of the photoluminescence signal which can be attributed to polariton and photon lasing, respectively. The power dependence of the probed photocurrent shows a distinct kink at the threshold power for photon lasing due to an increased radiative recombination of free carriers as stimulated emission into the cavity mode sets in. At the polariton lasing threshold, on the other hand, the nonlinear increase of the luminescence is caused by stimulated scattering of exciton polaritons to the ground state which do not contribute directly to the photocurrent.

Identifying Reservoirs of Infections Caused by Kingella kingae: A Case-Control Study of Oropharyngeal Carriage of K. kingae Among Healthy Adults.

Kingella kingae is currently recognized as a significant pathogen of the pediatric population. Nevertheless, the possibility for adults to serve as a reservoir of healthy carriers has not been studied. We conducted a monocentric transversal study on 228 healthy adults to define the carriage rate. Participants were recruited among the staff of a children's hospital, a population exposed to aerosolized droplets from children. A secondary analysis using a case-control method was conducted to assess risk factors for carriage. We demonstrated an oropharyngeal carriage rate of 2.2% in this population. However, there was a striking similarity in the carriage rate among children younger than 4 years of age and adults living with children of that age group (8.8%). Use of day-care facilities for their own children was also demonstrated as a risk factor for adult carriage. We were able to demonstrate the existence of adult carriage of K. kingae but our results point to transmission from children to adults. Our results do not allow us to conclude that professional exposure in a hospital setting is a risk factor for oropharyngeal carriage.

Ultrafast gain recovery and large nonlinear optical response in submonolayer quantum dots

Submonolayer quantum dots combine the zero-dimensional charge-carrier confinement of self-assembled quantum dots with the large density of states of a quantum well. Electroluminescence and pump-probe experiments on a submonolayer-based optical amplifier show that the system exhibits a high gain of $90{\mathrm{cm}}^{\ensuremath{-}1}$ and an ultrafast gain recovery. We propose a rate equation system describing the microscopic carrier dynamics which quantitatively reproduces the observed behavior and provides deeper theoretical understanding of the material system. In contrast to Stranski-Krastanov quantum dots, the fast gain recovery is enhanced by a strong interdot coupling. Optically inactive submonolayer states form an efficient carrier reservoir and give rise to a large nonlinear optical response.

Small-Signal Analysis of Ultra-High-Speed Multi-Mode VCSELs

In this paper, we show that the dynamic performance of multi-mode vertical-cavity surface-emitting lasers (VCSELs) can be modeled by single-mode rate equations developed for edge emitters as long as the lasing modes share a common carrier reservoir. However, this assumption does not hold for ultra-high performing VCSEL devices. Due to the high photon densities inside these optimized VCSELs, the common carrier reservoir splits up as a result of the spatial hole burning effect. This is caused by the high intensity of the multiple transverse modes. In this case, a small-signal modulation response with a different shape is expected. We derive an easy-to-apply fitting function, which allows the extraction of consistently expanded figures of merit. This novel function works for all VCSELs, particularly, including devices with carrier reservoir splitting. Furthermore, we use this new model to perform a detailed analysis of our latest VCSEL generation with a modulation bandwidth of up to 32.7 GHz.

Mechanism of hole injection enhancement in light-emitting diodes by inserting multiple hole-reservoir layers in electron blocking layer

In this study, gallium nitride (GaN) based light-emitting diodes (LEDs) with single and multiple hole-reservoir layers (HRLs) inserted in the electron-blocking layer (EBL) have been investigated numerically and experimentally. According to simulation results, a better electron confinement and a higher hole injection level can be achieved by the multiple HRLs inserted in the EBL region. To further reveal the underlying mechanism of hole injection enhancement experimentally, the active regions were intentionally designed to emit photons with three different wavelengths of 440 nm, 460 nm, and 480 nm, respectively. Based on the experimental results of photoluminescence (PL) and time-resolved PL (TRPL) measurements conducted at 298 K, the remarkable enhancement (148%) of PL intensities and significant increase in the decay times of the quantum wells close to p-GaN can be obtained. Therefore, the mechanism is proposed that carriers are able to reserve in the EBL region with multiple HRLs for a much longer time. Meanwhile, carriers could diffuse into the active region by tunnelling and/or thermo-electronic effect and then recombine efficiently, leading to the better carrier reservoir effect and higher hole injection in LEDs. As a result, by inserting multiple HRLs in the EBL region instead of single HRL, the experimental external quantum efficiency is enhanced by 19.8%, while the serious droop ratio is markedly suppressed from 37.0% to 27.6% at the high current injection of 100 A/cm2.

Exhumation charge: The last gasp of a petroleum source rock and implications for unconventional shale resources

Petroleum (oil and gas) forms from the bacterial or thermal breakdown of kerogen during progressive burial in sedimentary basins. During times of petroleum generation, kerogens in organic-rich source rocks expel petroleum to form a fluid phase in the pore system, capable of migrating under hydrodynamic and buoyancy forces to ultimately escape to the surface or accumulate within petroleum traps in the subsurface. The relative timing of petroleum charge and trap formation is a vital component in the accumulation of petroleum deposits. Exhumed basins have been historically viewed as higher-risk targets for conventional petroleum exploration because of, inter alia, the switch-off of petroleum generation in the source rock at the commencement of cooling during exhumation. However, even at the switch-off point, the source rock may retain a significant volume of petroleum sorbed in kerogen and within its pore system. Herein we demonstrate that if the source rock is exhumed to shallower depths after peak burial, pore pressure reduction and the associated volumetric expansion of the petroleum—particularly of the gaseous—phase in the pore system will result in the discharge of additional petroleum into the adjacent carrier bed or reservoir formations. Because most onshore sedimentary basins are characterized by major exhumation events at some point in their history, this represents an additional and underappreciated mechanism for a late-stage petroleum charge in exhumed sedimentary basins. The modeling also indicates that both the initial, pre-exhumation, total gas storage capacity and the exhumation gas charge are likely to be volumetrically more significant for gas-bearing source rocks that have been exposed to higher initial pressures and lower thermal gradients. The concepts presented here also have implications for petroleum resources retained within unconventional shale reservoirs because high-graded shale plays may be associated with systems where the magnitude or rate of relative overpressure dissipation has limited exhumation charge from the unconventional to conventional reservoirs within the basin.

Fast gain and phase recovery of semiconductor optical amplifiers based on submonolayer quantum dots

Submonolayer quantum dots as active medium in opto-electronic devices promise to combine the high density of states of quantum wells with the fast recovery dynamics of self-assembled quantum dots. We investigate the gain and phase recovery dynamics of a semiconductor optical amplifier based on InAs submonolayer quantum dots in the regime of linear operation by one- and two-color heterodyne pump-probe spectroscopy. We find an as fast recovery dynamics as for quantum dot-in-a-well structures, reaching 2 ps at moderate injection currents. The effective quantum well embedding the submonolayer quantum dots acts as a fast and efficient carrier reservoir.

A New Capacitance-Voltage Model for Hydrogen-Terminated Diamond Mosfet

Hydrogen-terminated diamond (HTD) metal-oxide-semiconductor field-effect transistor (MOSFET) is a relatively new electronic device made on diamond substrate. Due to the unique properties of diamond, the device shows comparable performance to those made on III-V compound semiconductors and good potential in radio frequency (RF) and high-power applications [1]. The most distinguishing property of HDT device is that it shows p-type conductivity at the diamond surface even though there is no intentional doping process involved during the fabrication process, and the hole carrier density is high (~1013cm-2) [2]. However, the origin of this p-type conductivity is still not fully understood, and the capacitance-voltage (C-V) characteristics of the HTD MOSFET is quite different from the regular MOSFET devices made on other semiconductor materials such as silicon [3]. In our present work, we derive a device model that takes into account of a surface adsorbate layer (ADL) to study the C-V characteristics of the HTD MOSFET. In this model, the ADL acts like a reservoir of hole carriers and forms a hetero-structure with the hydrogen-terminated diamond surface. Due to the Fermi level difference and band offset between the ADL and diamond, the hole carriers will flow from the ADL into diamond to form the conductive channel. As the gate bias is varied from positive to negative, the C-V curve will go through three different regions viz., depletion, accumulation, and injection regions. In the depletion region, both the ADL and the hole channel at diamond surface are depleted, therefore the gate capacitance is very small. In the accumulation region, the hole carriers are accumulated at diamond surface by the electric field but the ADL is still fully depleted, so the total gate capacitance equals the capacitance of gate oxide and ADL in series. In the injection region, the hole carriers overcome the band offset at the ADL/HTD interface and move into the ADL, which will cause the gate capacitance to increase again. Finally, the gate capacitance will reach its maximum value which equals the capacitance of the gate oxide. Based on our device model, a 2D device simulation is performed by using Sentaurus Technology Computer-Aided Design (TCAD) software and an analytical model is derived. The results from both the device simulation and analytical model show good match with the reported data for a practical device, and thus our model is effective in describing the charge transport inside the device. In addition, the influence of device parameters such as the thickness of gate oxide and hole carrier density on the C-V characteristics is studied, which is helpful in computer-aided design and optimization of future HDT devices.

A New Capacitance-Voltage Model for Hydrogen-Terminated Diamond Mosfet

A new device model that takes into account of a surface adsorbate layer (ADL) is generated to study the C-V characteristics of the hydrogen-terminated diamond (HTD) MOSFET. In this model, the ADL acts like a reservoir of hole carriers and forms a hetero-structure with the hydrogen-terminated diamond surface. Our device model is applied to the C-V characteristics of a practical device and the results match well. This work is helpful in computer-aided design and optimization of future HDT devices.

Vacancy associates-rich ultrathin nanosheets for high performance and flexible nonvolatile memory device.

On the road of innovation in modern information technology, resistive switching random access memory (RRAM) has been considered to be the best potential candidate to replace the conventional Si-based technologies. In fact, the key prerequisite of high storage density and low power consumption as well as flexibility for the tangible next generation of nonvolatile memories has stimulated extensive research into RRAM. Herein, we highlight an inorganic graphene analogue, ultrathin WO3·H2O nanosheets with only 2-3 nm thickness, as a promising material to construct a high performance and flexible RRAM device. The abundant vacancy associates in the ultrathin nanosheets, revealed by the positron annihilation spectra, act not only carrier reservoir to provide carriers but also capture center to trap the actived Cu(2+) for the formation of conductive filaments, which synergistically realize the resistive switching memory with low operating voltage (+1.0 V/-1.14 V) and large resistance ON/OFF ratio (>10(5)). This ultrathin-nanosheets-based RRAM device also shows long retention time (>10(5) s), good endurance (>5000 cycles), and excellent flexibility. The finding of the existence of distinct defects in ultrathin nanosheets undoubtedly leads to an atomic level deep understanding of the underlying nature of the resistive switching behavior, which may serve as a guide to improve the performances and promote the rapid development of RRAM.

Phase-amplitude coupling characteristics in directly modulated quantum dot lasers

We present a semi-analytical model for studying the phase-amplitude coupling (α-factor) in quantum dot (QD) semiconductor lasers, which takes into account the influence of carrier populations in the excited state and in the two-dimensional carrier reservoir on the refractive index change. Calculations of the α-factor based on the amplified spontaneous emission method and on the “FM/AM” technique are both investigated. It is shown that the α-factor of a QD laser strongly depends on the energy separation between the ground state and the off-resonant states. Through band structure engineering, the α-factor can be reduced by enlarging this energy separation.

Ultrafast nonequilibrium carrier dynamics in semiconductor laser mode locking

Semiconductor disk lasers have been shown to be ideal as wavelength-agile, high-brightness sources for producing high average power under various pulsed mode-locking scenarios. Systematic microscopic modeling reveals that ultrafast nonequilibrium kinetic hole burning in electron/hole carrier distributions dictates the outcome of femtosecond duration mode-locked pulse formation. The existence of a large reservoir of unsaturated carriers within the inverted distributions leads to the emergence of multiple pulse waveforms (not necessarily harmonically mode-locked pulse trains) that inefficiently draw on these carrier reservoirs. The concept of gain is no longer meaningful in this limit, and the dynamical inversion of electrons and holes primarily in the active medium establishes the final dynamical state of the system. The simulation results explain much of the generic behavior observed in key recent experiments and point to the difficulty of pushing semiconductor mode-locked lasers to pulse durations below 100 fs.

Comparison of gain and phase recovery dynamics among optical pumping schemes in quantum-dot semiconductor optical amplifiers

We numerically compare the gain and phase recovery acceleration of quantum-dot (QD) semiconductor optical amplifiers (SOAs) caused by four optical pumping schemes: the single-color optical pumping tune to the quantum-well (QW) carrier reservoir; the QD excited state (ES) ensembles; and the QD ground state (GS) ensembles together with multicolor optical pumping tuned to the GS and ES ensembles of QDs. The temporal responses of the gain and phase recoveries caused by the four optical pumping schemes are calculated and compared by solving 1088 coupled rate equations at the same pumping conditions. Among the four optical pumping schemes, the single-color optical pumping to the ES ensemble shows the fastest response of the gain and phase recovery. This results from the large carrier occupation ratio at the GS and ES ensembles and the high modal gain at the pumping wavelength, which facilitates the reduction of the carrier recovery time within the GS and ES ensembles due to the increased stimulated recombination rate caused by the optical pumping beam.

Dynamical behavior of quantum-dot laser

In this work we have investigated the dynamical properties of InAs/InGaAs laser emitting 1.3 m pumped by injection current density of fast rise and falling times with a duration of 5ns . In (capture) and out (escape) scattering rates of electrons and holes are studied against temperature, injection current density, Auger coefficient .Scattering times are studied against carrier reservoir loss coefficient , injection current density and temperature. Temporal behaviorof photons density against injection current density is given too.

The determination of charge carrier mobility from the current transients in organic field effect transistor

In this paper, we present features of current transients in organic field effect transistors obtained by the computer simulation and experimentally. We analyse the influence of the charge carrier reservoir at the organic semiconductor-insulator interface and the load resistance on the current transients. We demonstrate how to determine the amount of charge carriers in the reservoir and how to take it into account when calculating the mobility values estimated from current transients. Such correction reduces the estimated mobility values and changes mobility dependence on the electric field.

Theoretical simulation of carrier capture and relaxation rates in quantum-dot semiconductor optical amplifiers

Based on Auger scattering mechanism, carrier-carrier scattering dynamics between the two-dimensional carrier reservoir (also called wetting layer, i.e., WL) and the confined quantum dot ground and first excited state in quantum-dot semiconductor optical amplifiers (QD-SOAs) are investigated theoretically in this paper. The scattering rates for independent electron and hole densities are calculated. The results show an ultra-fast carrier capture (relaxation) rate up to 1 ps−1, and there is a complex dependence of the Coulomb scattering rates on the WL electron and hole densities. In addition, due to the different effective mass and the level distribution, the scattering rates for electron and hole are very different. Finally, in order to provide a direction to control (increase or decrease) the input current in realistic QD-SOA systems, a simple method is proposed to determine the trends of the carrier recovery rates with the WL carrier densities in the vicinity of the steady-state.