Average Carrier Research Articles

Planar microwave resonator with electrodeposited ZnO thin film for ultraviolet detection

A ZnO thin film is electrodeposited on the conducting strips of a planar microwave ring resonator to enable the formation of a novel sensor for ultraviolet irradiation. The fabrication of the sensor involves a low-cost process that basically utilizes a printed circuit board and an aqueous precursor solution. The resonator with no ZnO coating operates with a resonant frequency of 6.2 GHz and a quality factor of 170. The time-resolved microwave photoresponse of the sensor to UV illumination, under ambient conditions, is assessed through measurements of the resonance profile of the S21 parameter. The resonance frequency exhibited a highly sensitive downshift of ∼6 MHz after a UV illumination time of ∼3 min. This downshift is mostly attributed to the change in the dielectric constant of the ZnO film caused largely by the additional creation of bound charges. The usually reported long-lived and persistent post-illumination effects were not observed. The measurements of the resonance amplitude carried out at 20% and 70% relative humidity levels revealed average excess carrier relaxation lifetimes of 213 s and 185 s, respectively. Concomitantly, the measured resonance frequency downshift increased with increased humidity. These results highlight the difference in the interaction mechanisms of photogenerated carriers with water and oxygen molecules on the surface and grain boundaries of the ZnO film. To our knowledge, this UV irradiation sensor is the first ZnO-based sensor device implemented with planar microwave circuit technology. In addition, the capabilities demonstrated by this simple photosensing method to determine induced carrier lifetimes make it a valuable technique for in-depth investigation of the material properties.

Annealing Temperature Dependence of ZTO Thin Film Properties and Its Application on Thin Film Transistors by Inkjet Printing

In this study, inkjet-printed zinc tin oxide:Cl (ZTO:Cl) thin films were analyzed using x-ray diffraction, x-ray photoelectron spectroscopy (XPS) and all the films were annealed at temperatures ranging from 100°C to 600°C. XPS analysis showed that the formation of metal-oxide bonds in the films was complete with annealing temperatures at or above 500°C. Furthermore, inkjet-printed thin film transistors (TFTs) were examined using scanning electron microscopy and current–voltage characteristic measurements. The ZTO:Cl TFTs performed best when annealed at 500°C. The average carrier mobility and the on/off ratio were found to be 2.71 cm2/V s and 1.82 × 107, respectively.

Realizing High Efficiency over 20% of Low‐Bandgap Pb–Sn‐Alloyed Perovskite Solar Cells by In Situ Reduction of Sn4+

Although the theoretical power conversion efficiency (PCE) of low‐bandgap Pb–Sn‐alloyed perovskite solar cells (PSCs) is higher than that of its conventional pure Pb counterpart, its device performance currently has been severely restricted by the large open‐circuit voltage (Voc) loss. Herein, it is discovered that the Sn4+‐induced trap states of the perovskite film can be effectively suppressed by introducing excess Sn powder into the precursor solution (FASnI3) to reduce the Sn4+ content. As a result, the average charge carrier lifetime of the perovskite film increases remarkably from 115 to 701 ns due to the suppressed nonradiative recombination, and the energy levels have up‐shifted by about 0.27 eV, rendering a more favorable energy‐level alignment at the interface. Ultimately, the champion PSCs using a low‐bandgap (FASnI3)0.6(MAPbI3)0.4 perovskite film with Sn4+ reduction show a high Voc of 0.843 V corresponding to a Voc loss as low as 0.397 eV and a high fill factor of 80.34%, leading to an impressive PCE of 20.7%, which is one of the few instances of a PCE over 20% for low‐bandgap mixed Pb–Sn PSCs to date.

Impact of Carrier Nonpinning Effect on Thermal Power Saturation in GaAs-Based High Power Diode Lasers

A root-cause analysis of thermal power saturation in broad area diode lasers is presented. Thermal power saturation limits in largest parts the optical power in continuous wave (CW) driven diode lasers, as junction temperature increases with bias. Accordingly, gain decreases and, hence, the carrier density in the quantum well increases (nonpinning) to compensate the low gain and the additional photon loss. A systematic variation of epitaxial designs was used to experimentally clarify the impact of modal gain on the temperature dependence of differential internal efficiency and optical loss. To account for this effect in simulation, a rate equation model was applied to determine temperature dependent slope efficiency. Longitudinal spatial hole burning (LSHB) causes an asymmetry of the carrier density profile, leading to high nonlinear carrier loss at the back facet, but also the average carrier density increases with temperature, enhancing the impact of LSHB. This enhancement effect leads to rapid degradation of internal differential efficiency with temperature and, hence, early power saturation. The extreme triple asymmetric vertical design is introduced, that enables devices to be realized that show minimized degradation of internal differential efficiency with temperature and therefore increased performance in CW operation.

1,1'-Biaceanthrylene and 2,2'-Biaceanthrylene: Models for Linking Larger Polycyclic Aromatic Hydrocarbons via Five-Membered Rings.

We show that polycyclic aromatic hydrocarbon (PAH) chromophores that are linked between two five-membered rings can access planarized structures with reduced optical gaps and redox potentials. Two aceanthrylene chromophores were connected into dimer model systems with the chromophores either projected outward (2,2'-biaceanthrylene) or inward (1,1'-biaceanthrylene), and the optical and electronic properties were compared. Only the planar 2,2'-biaceanthrylene system showed significant reductions of the optical gaps (1 eV) and redox potentials in relation to the aceanthrylene monomer. Organic field effect transistors based on 2,2'-biaceanthrylene were prepared and were found to operate in the hole transport regime with an average charge carrier mobilities of 1.8 × 10-4 cm2 V-1 s-1.

Robust sandwiched fluorinated graphene for highly reliable flexible electronics

The high sensitivity of graphene to the surface condition of the gate dielectric layer and its poor van der Waals adhesion with a flexible substrate result in interfacial sliding and fracturing of graphene at low strains, making the successful utilization of pristine graphene (PG) in flexible electronics challenging. Here, we report a facile method for the fabrication of flexible graphene field effect transistors (F-GFETs) using sandwiched fluorinated graphene (FG). The “FG-PG-FG” sandwich structure shows a high optical transparency (>94%) with an average carrier mobility above 340 cm2/V·s, higher than that obtained when GO and Ion gel were used as gate dielectric materials on F-GFETs and a relatively low gate leakage current of ~160 pA. Furthermore, we observed a high mechanical stability, retaining >88% of the original current output against bending deformation of up to 6 mm and >77% after 200 bending cycles by applying a tensile strain of 1.56%, compared to the control sample. This improved performance is attributed to the fact that the sandwiched FG provides a good dielectric environment by tuning the C/F ratio, which tightly fixes the PG under strain. These findings provide a new route for the future development of graphene-based flexible electronics.

Synergy effect of intraband transition and impact-Auger in optical hot-carrier solar cells

An optical hot-carrier solar cell (opt-HC-SC) is a variant of hot-carrier solar cells (HC-SCs). Hot luminescence in a specific narrow energy range is enhanced and extracted from the absorber through an optical energy-selective contact (opt-ESC) and illuminates a neighboring photovoltaic cell of a conventional type. Thus, this concept eliminates the requisites for carrier transport to achieve high conversion efficiency. In the present study, I clarified the issues for high efficiency using an improved detailed-balance model and determined the solutions. The serious issue is that the enhancement effect of the opt-ESC on the hot luminescence is considerably lower in practice than the optimal value, and this necessitates a high carrier density in the absorber for sufficient photon extraction and results in significant energy loss originating from carrier thermalization. When impact ionization occurs immediately, the average carrier energy lowers, the carrier density can decrease, and hence the thermalization energy loss significantly reduces. This, in turn, enables absorption of sub-bandgap photons arising from the intraband transition to improve the efficiency. Consequently, the synergy effect of both the aforementioned phenomena notably improves the efficiency to 53.6% under the practical upper limit of the enhancement effect, the carrier thermalization time of 1 ns, and 1000-times concentrated solar illumination, from 45.1% for no intraband transition or impact-Auger. By contrast, the individual effects are marginal. This is a unique feature of opt-HC-SCs and is not found in HC-SCs (carrier extraction). The present finding serves as a guide to material design to realize highly efficient opt-HC-SCs.

Ambipolar Topological Insulator and High Carrier Mobility in Solution Grown Ultrathin Nanoplates of Sb-Doped Bi2Se3

Topological insulators (TIs) are a class of materials that can exhibit robust spin polarizations at the surfaces and have attracted much attention toward spintronic applications. Here, we optimized a solution route to synthesize ultrathin Bi2Se3 and Sb-doped Bi2Se3 nanoplates with a thickness of 6–15 nm and an average lateral size around 5 μm, up to a maximum of 10 μm. Solution chemistry provides high quality nanoplates of TIs with options to manipulate the surface states. We have synthesized Bi2Se3 and Sb-doped Bi2Se3 and characterized single nanoplates. Sb doping is used to suppress the bulk carriers, and an atomic percentage ∼6% of Sb is demonstrated by energy dispersive X-ray spectroscopy (EDS). The 2D electron carrier concentration for Sb-doped Bi2Se3 nanoplates is lowered to 5.5 × 1012 cm–2, reducing the concentration by a factor of 3 compared to the undoped Bi2Se3 nanoplate sample with an average 2D carrier concentration of 16 × 1012 cm–2. At 2 K, the pronounced ambipolar field effect is observed on the low-carrier-density Sb-doped Bi2Se3 nanoplates, further demonstrating the flexible manipulation of carrier type and concentration for these single-crystal nanoplates. Large out-of-plane magnetoresistance is measured, with a gate tunable phase coherence length.

Towards high-fidelity erosion prediction: On time-accurate particle tracking in turbomachinery

Erosion and fouling caused by ingested particles causes performance degradation and safety issues in turbomachinery components. Simulating these processes is a complex multiphysics and multiscale problem which has not reached a satisfactory level of maturity yet. The current state of the art approach is based on RANS solutions, which provide an averaged carrier phase on which the particles are advanced in an a posteriori manner. Upon wall impact, the particle quantities are then fed to the erosion and rebound models. In this work, we present as an alternative to this approach an Eulerian/Lagrangian simulation framework of high-order accuracy in space and time for the time-resolved prediction of particle motion in complex flows. We apply it to the LES of a particle-laden flow over a T106C low pressure turbine linear cascade. We then contrast time-averaged and time-accurate flow fields as carrier phases for the particles and highlight how the associated modeling assumptions influence the solution. Based on the particle Stokes number, we identify characteristic regimes and their interaction with the flow phase. By a detailed comparison of the particle statistics, we highlight the effects of turbulent small scale behavior and define the modeling challenges associated with finding accurate particle closure models for time-averaged simulations. This framework constitutes a first step towards high-fidelity erosion prediction for turbomachinery applications.

Continuous Growth of Buffer/Drift Epitaxial Stack Based on 4H-SiC by Quick Change of N<sub>2 </sub>Flow Rate under High Growth Rate Condition

N-type 4H-SiC homo-epitaxial films were grown under high growth rate condition by high speed wafer rotation vertical CVD tool, and dependence on N2 flow rate for in-wafer distribution of thickness and carrier concentration was investigated. By adjusting only the N2 flow rate from 0.33 to 130 sccm, average carrier concentration of the SiC film was controlled within a range from 3 x 1015 to 1 x 1018 cm-3, while maintaining good in-wafer uniformity less than ±7.0%. Average growth rate higher than 54 μm/h and in-wafer uniformity less than ±3.1% were maintained and no dependence for thickness distribution on N2 flow rate was observed. Buffer/drift epitaxial stack using quick change of N2 flow rate was fabricated, and the crystalline quality and steepness of N concentration at buffer/drift interface were also investigated. The epitaxial stack showed lower defect density compared with single drift layer and showed steep interface between buffer layer and drift layer.

First-principles quantum corrections for carrier correlations in double-layer two-dimensional heterostructures

We present systematic ab initio calculations of the charge carrier correlations between adjacent layers of two-dimensional materials in the presence of both charged impurity and strain disorder potentials using the examples of monolayer and bilayer graphene. For the first time, our analysis yields unambiguous first-principles quantum corrections to the Thomas--Fermi densities for interacting two-dimensional systems described by orbital-free density functional theory. Specifically, using density-potential functional theory, we find that quantum corrections to the quasi-classical Thomas-Fermi approximation have to be taken into account even for heterostructures of mesoscopic size. In order for the disorder-induced puddles of electrons and holes to be anti-correlated at zero average carrier density for both layers, the strength of the strain potential has to exceed that of the impurity potential by at least a factor of ten, with this number increasing for smaller impurity densities. Furthermore, our results show that quantum corrections have a larger impact on puddle correlations than exchange does, and they are necessary for properly predicting the experimentally observed Gaussian energy distribution at charge neutrality.

Unraveling the Radiative Pathways of Hot Carriers upon Intense Photoexcitation of Lead Halide Perovskite Nanocrystals.

The slowdown of carrier cooling in lead halide perovskites (LHP) may allow the realization of efficient hot carrier solar cells. Much of the current effort focuses on the understanding of the mechanisms that retard the carrier relaxation, while proof-of-principle demonstrations of hot carrier harvesting have started to emerge. Less attention has been placed on the impact that the energy and momentum relaxation slowdown imparts on the spontaneous and stimulated light-emission process. LHP nanocrystals (NCs) provide an ideal testing ground for such studies as they exhibit bright emission and high optical gain, while the carrier cooling bottleneck is further pronounced compared to their bulk analogues due to confinement. Herein, the luminescent properties of CsPbBr3, FAPbBr3, and FAPbI3 NCs in the strong photoexcitation regime are investigated. In the former two NC systems, amplified spontaneous emission is found to dominate over the radiative recombination at average carrier occupancy per nanocrystal larger than 5-10. On the other hand, under the same photoexcitation conditions in the FAPbI3 NCs, a longer lived population of hot carriers results in a competition between hot luminescence, stimulated emission, and defect recombination. The dynamic interplay between the aforementioned three emissive channels appears to be influenced by various experimental and material parameters that include temperature, material purity, film morphology, and excitation pulse width and wavelength.

Effects of grain boundaries on conversion efficiencies of single-crystal-like GaAs thin-film solar cells on flexible metal tapes

We present a numerical simulation study of the effects of low-angle grain boundaries (GBs) in a single-crystal-like GaAs thin-film material on its photovoltaic performance. Here, 1D and 2D modeling are employed to simulate a solar-cell (SC) device based on the properties of single-crystal-like GaAs thin films grown on metal tape. Properties include minority carriers’ mobility, carrier lifetimes, and diffusion length. The 1D model, by incorporating a uniform biaxially textured GaAs compared to single crystal GaAs, predicts an efficiency ∼21% for average carrier lifetimes of 1 ns. This result is not consistent with an experimental results showing an efficiency of 4.3%. 1D simulation generally works well for the prediction of conventional crystalline SC I-V characteristics, but it fails for newly-developed single-crystal-like GaAs SC devices with different material properties and non-vertical device geometry. Hence, we develop a 2D model to study the effect of localized recombination centers in the material by defining different regions of defective low-angle grain boundaries and single crystalline intra-grains. The 2D model is comparable to the experimental results mainly due to its capability to consider localized material inhomogeneity and lateral carriers dynamic. The effect of grain boundary density on SC performance is studied by 2D modeling. Increasing the grain size of GaAs from 2 μm to 50 μm can improve efficiency from 4.8% to 12.3%. The open-circuit voltage (Voc) shows more sensitivity to varying grain boundary densities than other SC characteristic factors. The 2D model is also employed to study bulk passivation of GBs and suggests that an efficiency of ∼19.5% is achievable in the single-crystal-like GaAs SCs even with a small grain size of 2 μm, if effective grain boundary passivation is applied.

Highly stretchable fiber transistors with all-stretchable electronic components and graphene hybrid electrodes

Abstract Textile-based electronic devices should be not only bendable but also highly stretchable for human-friendly wearable electronic applications. Herein, a highly stretchable and mechanically durable fiber transistor was designed and prepared by combining all-stretchable electronic components. First, electrically conductive and stretchable electrodes were fabricated by simple prestraining-then-buckling of graphene/silver hybrid fibers on a highly elastic thermoplastic polyurethane (TPU) monofilament, which maintained a low resistivity (∼30 Ω cm) at a high stretching strain (∼70%) and under many stretch/release cycles (∼1,000). A highly stretchable active channel was prepared by directly blending semiconducting poly(3-hexylthiophene) (P3HT) and viscoelastic TPU without using pre-grown P3HT nanofibrils. The P3HT/TPU blend (5/5 w/w blend ratio) active layer possessed reasonably high mobility (1.46 × 10−3 cm2 V−1 s−1) and adequate mechanical strength (21 MPa) with a high elongation strain (∼120%). In addition, a stretchable dielectric layer and gate electrode were prepared using an elastic ion-gel film and a liquid metal composite, respectively. The assembled fiber transistor exhibited excellent stretchability (∼50%) while maintaining good electrical properties (average charge carrier mobility of 1.74 cm2 V−1 s−1, on/off current ratio of 104) and showed outstanding electrical stability up to 1,000 cycles of stretch/release testing. To the best of our knowledge, these superior stretchability and stability have not been reported elsewhere in the area of fiber-type transistors. We believe that our work can serve as an important step toward the development of core components in wearable devices, such as wearable displays, computers, and biomedical sensors.

Extreme radiation resistance in InN

Carrier dynamics were studied in InN, which is a promising material for radiation-resistant optoelectronic devices. InN epilayers with different background electron densities were prepared by metal-organic chemical vapor deposition and irradiated with protons. Application of the light-induced transient grating technique in the sub-picosecond domain allowed to study the carrier lifetime and diffusion coefficient as a function of the total carrier density (intrinsic, photoexcited, and irradiation-induced). The dominance of the direct Auger process in carrier recombination was revealed. The irradiation of InN by protons, such as those encountered in space applications, decreases the carrier lifetime while simultaneously increasing the diffusion coefficient (up to 190 cm2/s at the highest total carrier density studied). As a result, the average carrier diffusion length does not decrease below ∼140 nm, and exceeds the light absorption depth of InN. The lifetime damage factor was determined, and the lifetime dependence on the displacement damage dose was measured. The radiation resistance of InN surpasses that of GaAs, InGaAs, and InGaN. InN-based photovoltaic devices may work for long space missions.

Double GaAs/Si Heterojunction Layers in Si Solar Cells Fabricated by Electron Beam Evaporation

We investigated silicon solar cells with double gallium arsenide heterojunctions. Both p-type and n-type GaAs were deposited on the back and front side of the Si pn junction using an electron beam evaporator under a high vacuum condition to fabricate ppnn cells, respectively. The ppnn cell with a micro-textured Si surface showed enhanced quantum efficiency by about 10% in the shorter wavelength region and by about 1% in the longer wavelength region compared to ppnn cell with nanotextured Si surface. Moreover, the average minority carrier lifetime was increased by 1 μs and conversion efficiency was increased by 0.74% for the micro-ppnn cell despite its high series resistance. Morphological analysis showed that GaAs thin-film on Si surface was As-rich in the as-deposition case. It became distorted at higher annealing temperatures, leading to formation of cracks. Moreover, annealing of the deposited GaAs on a nano-textured Si surface at 100 °C for 30 min resulted in formation of GaAs nanodots in an amorphous GaAs matrix.

General Expression for Plasma Extraction Transit-Time Oscillations From Silicon-Bipolar Power Semiconductor Devices

We propose a general expression for plasma extraction transit-time oscillations (PETT-Oscs) from silicon-bipolar power semiconductor devices. PETT-Oscs are generated when the transition time for the hole to fly into the depletion region and be accelerated by the electric field harmonizes with the resonant frequency of the loops through stray inductances and p-n junction capacitances. A physical model of PETT leads to an expression that takes into account its dependence on voltage and dopant density. The average carrier speed in the depletion region and the junction temperature dependence are fitted to the experimental results. The validity of the proposed general expression for PETT-Osc is verified successfully through state-of-the-art insulated-gate bipolar transistor module design. PETT-Osc from the package is estimated to generate at the typical operating conditions. The expression guiding the prevention method for PETT-Osc and the completed module does not show PETT-Osc in the operational range. The expression indicates the existence of PETT-Osc outside of the operational range, although it does not appear in the voltage and current for the collector or the gate. However, precise measurement using an antenna revealed the presence of PETT-Osc at the predicted conditions.

Integration of highly crystalline C8-BTBT thin-films into simple logic gates and circuits

Abstract Highly crystalline organic thin films possess the charge carrier mobilities needed for high-performance, low-cost flexible electronics. However, only few reports exist that show the integration of these films into short-channel organic circuits. This work describes the integration of highly crystalline layers of the thermally and chemically fragile small molecule C8-BTBT. Thin films of this material are processed by a combination of zone-casting and homoepitaxial vacuum evaporation and display an average charge carrier mobility of 7.5 cm2/V in long channel transistors. The integration of these films into a circuit technology based on a 5 μm channel-length bottom-gate bottom-contact transistor topology results in inverters with gains up to 40 as well as a robust 19-stage ring oscillator. This circuit requires the simultaneous operation of 80 TFTs and displays a stage delay of 40 μs, resulting in an operating frequency of 630 Hz at an operating voltage of 10 V. With the help of circuit modelling, we quantify the relationship between the speed of ring oscillators and the contact resistance of individual transistors. Indeed, the successful integration of highly-crystalline layers with high intrinsic mobility stresses the need for advances in contact engineering.

Compact Model of Carrier Transport in Monolayer Transition Metal Dichalcogenide Transistors

Monolayer 2-D transition metal dichalcogenide (TMDC) field-effect transistors (FETs) have excellent scaling properties due to an extremely thin body and reduced quantum-mechanical tunneling. High-field and ballistic carrier transport can play an important role in aggressively scaled TMDC FETs. A compact model is developed to describe carrier transport in 2-D monolayer FETs based on a TMDC material. The model has the following features: 1) it treats high-field transport and velocity saturation; 2) it can be applied from ballistic to quasi-ballistic to diffusive transport regimes; and 3) it incorporates analytical expressions for carrier mobility values based on examining physical carrier scattering mechanisms in TMDC materials. Monte Carlo simulation, which solves the Boltzmann transport equation stochastically, is used to validate the model. The carrier transport model is further integrated with a virtual-source model to calculate the ${I}$ – ${V}$ characteristics of 2-D monolayer FETs. Based on the model, the effects of phonon scattering and ionized impurity scattering on the average carrier injection velocity and transistor ballisticity are discussed.

Demographic assessment and adjustment of census age distribution in selected West African Countries

Context/background: Despite the public acknowledgment of improvement in sub-Saharan African censuses and surveys in the recent time, the quality of age-sex data still remains relatively poor. This study assessed the quality of reported age-sex data in selected West African countries. Data source and methods: We utilized the available recent West African census data to address the study objective. Data adjustments were made using the UN-moving average technique, Carrier Far-rag, Karup-king newton and Arriaga methods. Results: First, the age- distribution in five years was examined for errors. The UN-Age-sex accuracy index showed that all the selected countries had inaccurate age-distribution, with the exception of Cameroon (2005) and Senegal (2002) censuses which revealed a lesser distorted age- distribution. Utilizing the strong smoothing techniques, the results showed that virtually all the censuses were plaqued with the overestimation error at older ages, particularly the female population. The error margin ranged between 10-25%. Conclusion: The study concludes that inaccuracies in West African census data persist and recommends more quality age-sex data in region.