Carrier Deactivation Research Articles

Ultrafast Charge Transfer and Recombination Dynamics in Monolayer-Multilayer WSe2 Junctions Revealed by Time-Resolved Photoemission Electron Microscopy.

The ultrafast carrier dynamics of junctions between two chemically identical, but electronically distinct, transition metal dichalcogenides (TMDs) remains largely unknown. Here, we employ time-resolved photoemission electron microscopy (TR-PEEM) to probe the ultrafast carrier dynamics of a monolayer-to-multilayer (1L-ML) WSe2 junction. The TR-PEEM signals recorded for the individual components of the junction reveal the sub-ps carrier cooling dynamics of 1L- and 7L-WSe2, as well as few-ps exciton-exciton annihilation occurring on 1L-WSe2. We observe ultrafast interfacial hole (h) transfer from 1L- to 7L-WSe2 on an ∼0.2 ps time scale. The resultant excess h density in 7L-WSe2 decays by carrier recombination across the junction interface on an ∼100 ps time scale. Reminiscent of the behavior at a depletion region, the TR-PEEM image reveals the h density accumulation on the 7L-WSe2 interface, with a decay length ∼0.60 ± 0.17 μm. These charge transfer and recombination dynamics are in agreement with ab initio quantum dynamics. The computed orbital densities reveal that charge transfer occurs from the basal plane, which extends over both 1L and ML regions, to the upper plane localized on the ML region. This mode of charge transfer is distinctive to chemically homogeneous junctions of layered materials and constitutes an additional carrier deactivation pathway that should be considered in studies of 1L-TMDs found alongside their ML, a common occurrence in exfoliated samples.

Chemical-looping combustion in a 100 kW unit using a mixture of synthetic and natural oxygen carriers – Operational results and fate of biomass fuel alkali

Biomass fuel use in chemical looping combustion enables negative CO2 emissions through BECCS (Bio-Energy Carbon Capture and Storage). Effective biomass utilization in CLC requires an economical and effective oxygen carrier to achieve high fuel conversion, effective CO2 capture, and management of the harmful effects of biomass alkali release (bed agglomeration, oxygen carrier deactivation, fouling and corrosion). These issues were addressed in 100 kW CLC pilot experiments.Building on previous work, a mixture of a synthetic calcium manganite perovskite and natural ilmenite was used as the oxygen carrier. Four biomass fuels of varied alkali content were tested: black pellets of steam-exploded stem wood (BP), BP impregnated with K2CO3, a mixture of 50% BP with 50% straw pellets, and wood char. Experiments showed high fuel conversion and very high CO2 capture, with overall performance exceeding that of ilmenite and manganese ore. More than 95% gas conversion was achieved with black pellets at around 950 °C.The fate of biomass alkali, previously virtually unknown in CLC research, was explored by implementing online surface-ionization-based measurement of alkali released in the flue gases of the fuel reactor (FR) and air reactor (AR). Release levels were found to correlate with the fuel alkali content. The flue gas measurements and bed material elemental analyses suggest that most of the fuel alkali are accumulated in the oxygen carrier. Unexpectedly, it was found that flue gas alkali release occurs in both the FR and AR, with AR exhibiting an equal or higher rate of release vs. the FR.

Insights into the relationship between microstructural evolution and deactivation of Al2O3 supported Fe2O3 oxygen carrier in chemical looping combustion

The Fe2O3-Al2O3 oxygen carrier is regarded to a promising candidate for chemical looping combustion (CLC). However, Fe2O3-Al2O3 oxygen carrier was often subjected to a decrease of oxygen carrying capacity. The deactivation mechanism of Fe2O3-Al2O3 oxygen carrier in chemical looping combustion was investigated over consecutive 20 cycles. The Fe2O3-Al2O3 oxygen carrier exhibited relatively poor redox stability in a fixed bed reactor. The physico-chemical properties of cycled Fe2O3-Al2O3 oxygen carrier were characterized by different characterization methods. No carbon deposit was detected in the reduction process, indicating that carbon deposition was not the cause for Fe2O3-Al2O3 oxygen carrier deactivation under this experimental condition. The characterization results showed that the deactivation of Fe2O3-Al2O3 oxygen carrier in redox cycles was mainly attributed to thermal sintering. The cycled Fe2O3-Al2O3 oxygen carrier showed lower surface area, lower reducibility and oxygen mobility than that of fresh sample. After a certain number of redox cycles, Fe diffusion and enrichment at the surface of cycled oxygen carrier was observed. The surface Fe is unstable under high reaction temperature, leading to accelerate thermal sintering. A deactivation mechanism of Fe2O3-Al2O3 oxygen carrier in CLC was proposed based on the Fe diffusion and surface sintering.

Comprehensive study on PVDF-HFP/BMImBF4/AgBF4 membranes for propylene purification

In this work, a comprehensive analysis of PVDF-HFP/BMImBF4/AgBF4 facilitated transport membranes for olefin/paraffin separation is presented. Previous works of our research group have reported high flux and propylene selectivity under dry conditions and using synthetic gas mixtures, highlighting the promising potential of these membranes for industrial applications. This work advances in the understanding of the phenomena involved in membrane performance and moves one step forward in the knowledge of the industrial viability of this membrane system. First, the internal interactions between the silver cations and the polymer backbone, the silver salt dissociation and the silver degradation have been studied using FTIR, Raman and XPS spectroscopic techniques. Secondly, the experimental membrane performance during 110 days and working at changing relative humidity conditions in the feed gas has been assessed. Thermogravimetric techniques helped determining the water uptake capability of the facilitated transport membrane. Thirdly, real gas mixtures from a fluid catalytic cracking unit were provided by the industry and used in permeation experiments to check the membrane behavior under industrial-like conditions. The results provide experimental evidence for the previously theorized facilitated transport mechanisms and reveal a major influence of feed gas humidity on membrane performance. On the other hand, the industrial gas mixture produces no deviation from synthetic feed conditions due to trace contaminants. Finally, the carrier deactivation in long-term permeation has been quantified through a mathematical expression.

Energy Saving Scheme for Multicarrier HSPA + Under Realistic Traffic Fluctuation

In the near future, an increase in cellular network density is expected to be one of the main enablers to boost the system capacity. This development will lead to an increase in the network energy consumption. In this context, we propose an energy efficient dynamic scheme for HSDPA + (High Speed Downlink Packet Access-Advanced) systems aggregating several carriers and which adapts dynamically to the network traffic. The scheme evaluates whether node-B deactivation is feasible without compromising the user flow throughput. Furthermore, instead of progressive de-activation of carriers and/or node-B switch-off, we evaluate the approach where feasible combination of inter-site distance and number of carriers is searched to obtain best savings. This is done by also considering the effect of transition delays between network configuration changes. The solution exploits the fact that re-activation of carriers might permit turning off other BSs earlier at relatively higher load than existing policies. Remote electrical downtilt is also considered as a means to maximize the utilization of higher modulation and coding schemes in the extended cells. This approach promises significant energy savings when compared with existing policies - not only for low traffic hours but also for medium load scenarios.

Deactivation Improvement of Advanced CMOS Devices Using a Flood BC PLAD Process

Flood buried-contact (FBC) plasma doping (PLAD) is a promising scheme to improve both pMOS and nMOS device performance with a significant cost reduction. Deactivation characteristics of CMOS devices doped by an FBC implant using an ultra-low energy (ULE) BF3 PLAD process is extensively evaluated from inline [front end of line (FEOL)] to final [back end of line (BEOL)] to investigate the impact of the backend thermal processing. nMOS devices show more serious deactivation than pMOS devices from FEOL to BEOL. However, both pMOS and nMOS devices obtained further improvements at BEOL processing. The mechanism of nMOS device performance improvement can be attributed to the Schottky barrier height lowering effect and carrier deactivation improvement, as well as other effects including silicide stability improvement and mechanical stress. It is observed that the deactivation of nMOS device can be improved by an ULE BF3 PLAD process which can be interpreted by less dopant loss and $F$ passivation on the interface between the source/drain and the dielectric spacer. Another important observation is that the deactivation improvement is more profound for the small-size devices. Deactivation characteristics of CMOS devices are consistent with our recent deactivation mechanism studies of blanket wafers (electrical-assisted diffusion mechanism), and provide further evidence for this mechanism.

Effect of gas composition in Chemical-Looping Combustion with copper-based oxygen carriers: Fate of sulphur

Chemical-Looping Combustion (CLC) is an emerging technology for CO 2 capture because separation of this gas from the other flue gas components is inherent to the process and thus no energy is expended for the separation. Natural or refinery gas can be used as gaseous fuels and they may contain different amounts of sulphur compounds, such as H 2S and COS. This paper presents the combustion results obtained with a Cu-based oxygen carrier using mixtures of CH 4 and H 2S as fuel. The influence of H 2S concentration on the gas product distribution and combustion efficiency, sulphur splitting between the fuel reactor (FR) and the air reactor (AR), oxygen carrier deactivation and material agglomeration was investigated in a continuous CLC plant (500 W th). The oxygen carrier to fuel ratio, ϕ, was the main operating parameter affecting the CLC system. Complete fuel combustion were reached at 1073 K working at ϕ values ≥1.5. The presence of H 2S did not produce a decrease in the combustion efficiency even when working with a fuel containing 1300 vppm H 2S. At these conditions, the great majority of the sulphur fed into the system was released in the gas outlet of the FR as SO 2, affecting to the quality of the CO 2 produced. Formation of copper sulphide, Cu 2S, and the subsequent reactivity loss was only detected working at low values of ϕ ≤ 1.5, although this fact did not produce any agglomeration problem in the fluidized beds. In addition, the oxygen carrier was fully regenerated in a H 2S-free environment. It can be concluded that Cu-based oxygen carriers are adequate materials to be used in a CLC process using fuels containing H 2S although quality of the CO 2 produced is affected.

Effect of Fuel Gas Composition in Chemical-Looping Combustion with Ni-Based Oxygen Carriers. 1. Fate of Sulfur

Chemical-looping combustion (CLC) has been suggested among the best alternatives to reduce the economic cost of CO2 capture using fuel gas because CO2 is inherently separated in the process. For gaseous fuels, natural gas, refinery gas, or syngas from coal gasification can be used. These fuels may contain different amounts of sulfur compounds, such as H2S and COS. An experimental investigation of the fate of sulfur during CH4 combustion in a 500 Wth CLC prototype using a Ni-based oxygen carrier has been carried out. The effect on the oxygen carrier behavior and combustion efficiency of several operating conditions such as temperature and H2S concentration has been analyzed. Nickel sulfide, Ni3S2, was formed at all operating conditions in the fuel reactor, which produced an oxygen carrier deactivation and lower combustion efficiencies. However, the oxygen carrier recovered their initial reactivity after certain time without sulfur addition. The sulfides were transported to the air reactor where SO2 was produced as final gas product. Agglomeration problems derived from the sulfides formation were never detected during continuous operation. Considering both operational and environmental aspects, fuels with sulfur contents below 100 vppm H2S seem to be adequate to be used in an industrial CLC plant.

Damage-free microwave-excited plasma etching without carrier deactivation of heavily doped Si under thin silicide layer

Microwave-excited high-density plasma etching equipment with a dual shower-plate structure has been developed to overcome various disadvantageous of current reactive-ion etching equipment. Disadvantages include severe charge-up damage, heavy dependence of etching speed on pattern size, and very limited etching conditions, where the plasma uniformity over the entire wafer surface is only maintained for predetermined gas species, gas working pressure, self-bias voltage, and wafer surface patterns and materials. The authors’ new system has been confirmed to be free from charge-up damage, have pattern-size-dependent etching speed, and maintain the plasma uniformity over the entire wafer surface even if gas species, gas working pressure, self-bias voltage, and wafer surface patterns and materials are changed. To establish damage-free contact-hole etching without degradation of contact resistance of source and drain electrodes of metal-oxide-semiconductor transistors, damage of heavily doped Si induced by ion bombardment through the silicide (TaSi2) is investigated by using this new system. Carrier deactivation and Si lattice damage are found at the heavily doped Si layer even by ion-bombardment through the silicide when the energy of the bombarding ions to the wafer surface is larger than the critical value, which depends on gas species and frequency of rf power applied to the wafer electrode. Highly productive damage-free etching has been established for source and drain contact-hole etching by combining high-speed etching and damage-free etching using this new system, where the self-bias voltage is set at high voltages for high-speed etching and decreased to −300to−500V for damage-free etching at the final stage of contact-hole etching. In this new system, the plasma uniformity is widely maintained even if the self-bias voltage is widely varied during processes.

Strain compensation in boron-indium coimplanted laser thermal processed silicon

Strain in B-implanted laser thermal processed (LTP) silicon is reduced by coimplantation of In. Strain in the codoped layer is calculated using lattice constants measured by high-resolution x-ray diffraction. Compensation of the strain with increasing In dose corresponds to suppression of the carrier deactivation during post-LTP annealing.

Shallow p-type SiGeC layers synthesized by ion implantation of Ge, C, and B in Si

We investigated the characteristics of p-type SiGeC layers for possible application to source/drain regions of sub-100 nm metal–oxide–semiconductor field-effect transistors. We synthesized the SiGeC layer by the high-dose ion implantation of Ge and C into Si substrate and subsequent annealing. We succeeded in fabricating shallow p+/n junctions by the incorporation of C, of which the peak concentration is more than 1%. B diffusion in this layer was significantly suppressed, and the depth profile of B was reproduced with a process simulation in which a diffusion coefficient much lower than the reported value was assumed. This should be attributed to decreased Si interstitials. We examined the sheet resistance and contact resistance of the SiGeC layer, and found that the increase in resistance is tolerable as compared with a SiGe layer. This is because the carrier deactivation by the incorporation of C was not serious, which was confirmed by the spreading resistance measurement. The junction leakage in the p+/n diode was reduced with the increase in the concentration of C. Hence, SiGeC is a promising material for use in shallow and low resistance p+/n junctions.

Athermal effects in ion implanted layers

Abstract Defect structure and electrical characterization of boron and arsenic implanted layers has been investigated for implantation under athermal (light) excitation. This Photon Assisted (PA) implantation owes its specific properties to an additional electric field acting on charged particles including carriers and charged defects. It was shown that in case of n-type silicon this extra field draws charged vacancies and self-interstitials towards each other and, thus, diminishes transient diffusion of boron. This effect resulted in junctions which are about 20% shallower compared to conventionally processed reference wafers. Experiments using light of an Ar-ion laser and white light of a high pressure Xe arc lamp were compared. Some deactivation of carriers in the deeper laying parts of the p-region was always a by-product.

Anomalous electrical activation in Si-implanted GaAs/AlGaAs superlattices

Electrical activation in Si-implanted and annealed GaAs/AlGaAs superlattices (SLs) has been studied as a function of implantation temperature and dose. Carrier concentration, measured by the electrochemical capacitance-voltage profiling, is shown to be strongly dependent on implantation temperature. When SLs are implanted at room temperature or lower, electrical activation is severely suppressed in the near-surface region, where the carrier concentration is one to two orders of magnitude lower than the maximum (1–3) × 10 18/cm 3 observed near the implant peak position. Similar deactivation behavior was observed in Si-implanted GaAs crystals, but the disparity between the minimum and maximum of the carrier concentration was significantly less than one order of magnitude. The minimum in carrier concentration profiles is explained in terms of carrier deactivation by the near-spherical voids of 25–200 Å in diameter which are formed only in the near-surface regions, as revealed by transmission electron microscopy analysis. The larger carrier deactivation in the SL materials is attributed to the combined effects of electron confinement by the AlGaAs into the GaAs regions together with the preferential formation of voids in the GaAs layers.

Formation of low reverse current ion-implanted n+p junctions by low-temperature annealing

By employing stress compensation technology in conjunction with an ultrahigh vacuum ion implanter, we have obtained two orders of magnitude smaller reverse-bias current levels in n+p junctions annealed at 550 °C as compared to the previous data. When samples are made with an optimum stress compensation condition, the magnitude of reverse-bias currents in n+p junctions as well as the carrier deactivation during post-implantation anneal becomes smallest. In order to further reduce the reverse-bias current level, it is shown that the elimination of contamination involvement during the implantation process is most essential.

Severe loss of dopant activity due to CHF3+CO2 reactive ion etch damage

We report direct evidence from spreading resistance carrier profiling that an effective reduction of up to 5×1019 carriers cm−3 is caused by etching 10–150 nm of doped silicon with a CHF3+CO2 plasma in a commercial hexode reactor. During an 80-nm etch of bulk p+- and n+-Si, over 99 and 90% respectively of the carriers are trapped or complexed to a depth extending 200 nm from the etched surface. Thermal annealing at 450 °C restores all of the electrical activity, a behavior which is similar to the recovery of phydrogen-passivated acceptor impurities. More complicated behavior was observed for carrier deactivation of both p+ and n+ implanted junctions, where only partial recovery occurs after a 450 °C anneal. Further annealing at 850 °C does not reactivate any further p-type dopant, but a 950 °C anneal does increase the n-type dopant activity.