Low Injection Conditions Research Articles

Research on the factors influencing the width of hydraulic fractures through layers

The method of segmented hydraulic fracturing in the coal seam roof has proven to be an efficient technique for coalbed methane exploitation. However, the behavior of hydraulic fractures in multilayer formations with significant differences in mechanical properties is still unclear. This paper studied the variation in hydraulic fracture width at the coal-rock interface by employing experimental method with a true triaxial hydraulic fracturing experimental system and numerical simulation method. Results revealed that the hydraulic fracture more likely to expanded along the coal-rock interface instead of break through it with the small horizontal stress difference and low flow rate injection condition. And improving the injection flow rate lager than a critical value, the hydraulic fracture tends to break through the coal-rock interface. Hydraulic fractures in both mudstone and coal beds exhibited a trend of increasing and then decreasing of fracture width at the interface. Since the strength of the coal seam was lower compared to that of the mudstone, maintaining high pressure was no longer necessary when the hydraulic fracture crossed the interface and entered the coal seam, leading to a reduction in fracture width within the mudstone. During the later stages of fracturing, the entry of proppant into the coal seam became challenging, resulting in a phenomenon characterized by excessive fluid but insufficient sand. The time required for the fracture width to traverse the proppant was found to be inversely proportional to the difference in horizontal ground stress and the flow rate of the fracturing fluid. And it was directly proportional to the modulus of elasticity, permeability of the coal seam, and interface strength. The interface strength has the greatest influence on the width of hydraulic fractures. In conclusion, this study provides valuable insights into the behavior of hydraulic fractures in multilayer formations with varying mechanical properties. The findings contribute to a better understanding of the factors affecting hydraulic fracture width within coal seams, which can ultimately enhance the efficiency of coalbed methane exploitation.

Carrier recombination in highly Al doped 4H-SiC: dependence on the injection conditions

We investigate carrier recombination mechanisms in heavily aluminum (Al) doped p-type 4H-SiC, a material crucial for power devices. The recombination mechanisms in Al-doped p-type 4H-SiC have remained unclear, with reports suggesting various possibilities. To gain insights, we employ photoluminescence (PL) measurements, particularly time-resolved PL (TR-PL), as they are well-suited for studying carrier lifetimes in heavily Al-doped p-type 4H-SiC. We examine the temperature and excitation intensity dependencies of TR-PL and PL spectra and discuss the underlying recombination mechanisms. We observe that the dominant recombination mechanism varies with injection conditions for the samples with Al concentration less than 1019 cm−3. Under low injection conditions, recombination via the Al acceptor level appears dominant, exhibiting weak temperature dependence. However, under high injection conditions, Shockley–Read–Hall recombination takes precedence, leading to shorter carrier lifetimes with increasing temperature. This temperature dependence implies that presences of the deep recombination centers with the small capture barrier for holes.

Surface recombination velocities for 4H–SiC: Dependence of excited carrier concentration and surface passivation

Carrier lifetime plays a crucial role in optimizing the performance and reliability of SiC devices. Surface recombination is one of the factors affecting carrier lifetime. However, the reported values for surface recombination velocity (S) have been inconsistent. This study aimed to elucidate the discrepancy in S observed in various reports by conducting microwave photoconductivity decay measurements on n-type 4H–SiC freestanding epitaxial layers at different excitation carrier concentrations. Additionally, we investigated the impact of passivation by examining the surfaces subjected to oxidation and post-oxidation annealing. We observed an increase in S with increasing excited carrier concentration for the surfaces, irrespective of the crystal faces (S- and C-faces) and passivation. This suggests the presence of a potential barrier or shallow recombination centers on the surfaces. Furthermore, we observed that S remained nearly constant with increasing temperature, contrary to the behavior observed under low-injection conditions. This difference also suggests the existence of a potential barrier or shallow recombination centers on both Si- and C-faces. The comprehensive insights gained from our study have significant implications for the design of SiC devices with enhanced performance and reliability.

Interplay Between Strain and Thickness on the Effective Carrier Lifetime of Buffer-Mediated Epitaxial Germanium Probed by the Photoconductance Decay Technique

We report contactless effective minority carrier lifetime of epitaxially grown unstrained and in-plane <110> biaxially tensile-strained (001) germanium (ε-Ge) epilayers measured using microwave-reflectance photoconductance decay measurements. Strained Ge epilayers were grown using InxGa1–xAs linearly graded buffers on (001) GaAs substrates. Using homogeneous excitation of unstrained Ge epilayers, thickness-dependent separation of minority carrier lifetime components under low injection conditions yielded a bulk lifetime of 114 ± 2 ns and low surface recombination velocity of 21.3 ± 0.04 cm/s. More notably, an effective minority carrier lifetime of >100 ns obtained from sub-50 nm 1.6% tensile-strained Ge epilayers showed no degradation relative to the unstrained counterpart. Detailed material characterization using X-ray diffractometry revealed successful strain transfer of 0.61 and 0.89% to the Ge epilayers via InxGa1–xAs metamorphic buffers and confirms pseudomorphic growth. Lattice coherence observed at the ε-Ge epilayer and InxGa1–xAs buffer heterointerfaces via transmission electron microscopy substantiates the prime material quality achieved. The relatively high carrier lifetimes achieved are an indicator of excellent material quality and provide a path forward to realize low-threshold Ge laser sources.

Detecting striations via the lateral photovoltage scanning method without screening effect

The lateral photovoltage scanning method (LPS) detects doping inhomogeneities in semiconductors such as Si, Ge and {hbox {Si}_{hbox {x}}hbox {Ge}_{1-hbox {x}}} in a cheap, fast and nondestructive manner. LPS relies on the bulk photovoltaic effect and thus can detect any physical quantity affecting the band profiles of the sample. LPS finite volume simulation using commercial software suffer from long simulation times and convergence instabilities. We present here an open-source finite volume simulation for a 2D Si sample using the ddfermi simulator. For low injection conditions we show that the LPS voltage is proportional to the doping gradient. For higher injection conditions, we directly show how the LPS voltage and the doping gradient differ and link the physical effect of lower local resolution to the screening effect. Previously, the loss of local resolution was assumed to be only connected to the enlargement of the excess charge carrier distribution.

Physical Insights Into the ESD Behavior of Drain Extended FinFETs (DeFinFETs) and Unique Current Filament Dynamics

Single and multi-Fin behavior of drain-extended FinFET (DeFinFET) devices under low and high current injection conditions is studied using detailed 3-D TCAD simulations. For completeness, electrostatic discharge (ESD) behavior of both shallow trench isolation (STI)-type and non-STI-type DeFinFET devices is studied. Under low current injection, junction breakdown, parasitic bipolar turn-on, as well as the onset of space charge modulation and its implications on high current behavior are explored. Under high current injection, the role of space charge modulation in electrothermal instability and filament formation is discussed. Unique filament spreading behavior has been discovered in DeFinFETs. Fin-based construction was found responsible for filament spreading. The interplay among bipolar turn-on, bipolar efficiency, filament density, and nature of filament spreading is explained.

Dependence of decay time of the photocurrent induced by transient ionizing radiation on TID in NPN bipolar transistors

When an NPN bipolar junction transistor (BJT) is exposed to the pulsed gamma (or X) irradiation, photocurrent is generated, containing the primary photocurrent and the secondary photocurrent. TID experiments and transient irradiation experiments were carried out. The results show that, as accumulating total ionizing dose (TID), the secondary photocurrent decays more severely, whereas the primary photocurrent is almost impervious. Moreover, the influence of TID on the secondary photocurrent decay is related to the BJT’s collector current. If the BJT is working under the condition of high injection, the influence of TID is quite weak, but it’s evident when the BJT is working under the medium or low injection condition. The charge control model is used to analyze the decay time of the secondary photocurrent. As a result, the base minority lifetime determines the decay time, and a method how to obtain the equivalent lifetime after TID accumulation is proposed to predict the change of the decay time. Consequently, the deductive results give good agreement with the experimental results.

Assessment of AlGaN/AlN superlattices on GaN nanowires as active region of electron-pumped ultraviolet sources

In this paper, we describe the design and characterization of 400 nm long (88 periods) AlxGa1−xN/AlN (0 ≤ x ≤ 0.1) quantum dot superlattices deposited on self-assembled GaN nanowires for application in electron-pumped ultraviolet sources. The optical performance of GaN/AlN superlattices on nanowires is compared with the emission of planar GaN/AlN superlattices with the same periodicity and thickness grown on bulk GaN substrates along the N-polar and metal-polar crystallographic axes. The nanowire samples are less sensitive to nonradiative recombination than planar layers, attaining internal quantum efficiencies (IQE) in excess of 60% at room temperature even under low injection conditions. The IQE remains stable for higher excitation power densities, up to 50 kW cm−2. We demonstrate that the nanowire superlattice is long enough to collect the electron–hole pairs generated by an electron beam with an acceleration voltage VA = 5 kV. At such VA, the light emitted from the nanowire ensemble does not show any sign of quenching under constant electron beam excitation (tested for an excitation power density around 8 kW cm−2 over the scale of minutes). Varying the dot/barrier thickness ratio and the Al content in the dots, the nanowire peak emission can be tuned in the range from 340 to 258 nm.

Quantifying Large Lattice Relaxations in Photovoltaic Devices

Temporal variations of Cu(In,Ga)Se$_2$ photovoltaic device properties during light exposure at various temperatures and voltage biases for times up to 100 h were analyzed using the kinetic theory of large lattice relaxations. Open-circuit voltage and p-type doping increased with charge injection and decreased with temperature at low injection conditions. Lattice relaxation can account for both trends and activation energies extracted from the data were approximately 0.9 and 1.2 eV for devices with lower and higher sodium content, respectively. In these devices, increased sodium content resulted in higher initial p-type doping with greater stability. First principles calculations providing revised activation energies for the ($V_{Se}-V_{Cu}$) complex suggest that this defect does not account for the metastability observed here.

Analysis of carrier lifetimes in n-type 4H-SiC by rate equations

The authors constructed a model for carrier recombination process via the center in n-type 4H-SiC by using rate equations which adopted realistic capture cross sections. The high-injection carrier lifetime is about ten times longer than low-injection carrier lifetime, which originates from the fact that the (0) state of the center and the (-) state are dominated the under the high-injection condition and low-injection condition, respectively. The results obtained from the present model well explain the relationship between the lifetime and the carbon vacancy density experimentally obtained when the carbon vacancy density is higher than 1 − 3 × 1011 cm−3.

Analytical and numerical simulation of electron beam induced current profiles in p-n junctions

The electron beam induced current (EBIC) mode of a scanning electron microscope (SEM) is a widely used technique for the quantitative assessment of minority carrier diffusion length and surface recombination. Point source (one-dimensional) and extended source (two-dimensional) analytical models are two widely used approaches to assess this information in geometry where the electron beam (e-beam) is parallel to the p-n junction. In this article, a two-dimensional (2D) analytical model is evaluated and compared with 2D finite element numerical simulations, where the electron beam-solid interaction is modeled using a Monte Carlo simulation coupled with a drift-diffusion solver. The simulations are computed for both low and high level injection conditions. The effect of an e-beam injection level on the shape of EBIC profiles is analyzed to evaluate limitations of the analytical models.

Low frequency ranibizumab versus dexamethasone implant for macular oedema secondary to branch retinal vein occlusion

BackgroundThe aim was to make a real‐world comparison of the efficacy of ranibizumab, dexamethasone and grid laser treatments in macular oedema due to branch retinal vein occlusion (BRVO).MethodsForty‐four eyes of 44 consecutive patients with macular oedema secondary to BRVO were included. Treatment arms comprised standard care (StCARE, n = 15), intravitreal ranibizumab (RNB, n = 14) and dexamethasone implant (DEX, n = 15). No rescue laser was performed in DEX and RNB groups. Main outcome measures were mean change in visual acuity (VA) and the percentage of patients who gained 10 or more letters from baseline to six months and central retinal thickness (CRT).ResultsImprovements in mean logMAR VA (p = 0.642) and letter score from baseline to month 6 were not statistically significantly different in all three groups. Mean follow‐up was 13.9 ± 10.7 months in RNB, 11.9 ± 6.3 in DEX and 11.4 ± 6.6 in StCARE. Mean number of injections was 2.4 ± 1.4 (range: 1–6) in RNB and 1.9 ± 0.7 (range: 1–3) in DEX group over the follow‐up period. Mean letter gain was 13.5 in DEX (p = 0.067), 7.1 in RNB (p = 0.553) and 4.5 in StCARE (p = 0.362). Mean CRT at baseline was 512.8 μm in DEX, 505.1 μm in RNB and 345.5 μm in the StCARE group. At the last visit, RNB provided the maximum reduction in CRT. Mean CRT decrease was −146.5 μm (28.6 per cent) in DEX, −241.3 μm (47.8 per cent) in RNB and −45.6 μm (13.2 per cent) in StCARE (p = 0.030). A statistically significant intraocular pressure elevation occurred in the DEX group (p = 0.005).ConclusionBoth RNB and DEX provided a significant resolution in macular oedema. Low frequency injections limited the visual gain in ranibizumab therapy. Visual results could be better with higher frequency injections and early start of treatment. Dexamethasone implants may be preferable in terms of visual improvement under low frequency injection conditions. Close follow‐up is mandatory for detection of intraocular pressure elevations. Laser monotherapy is not a reasonable first‐line option in the era of injection therapies.

Heavy and light hole minority carrier transport properties in low-doped n-InGaAs lattice matched to InP

Minority carrier diffusion lengths in low-doped n-InGaAs using InP/InGaAs double-heterostructures are reported using a simple electrical technique. The contributions from heavy and light holes are also extracted using this methodology, including minority carrier mobilities and lifetimes. Heavy holes are shown to initially dominate the transport due to their higher valence band density of states, but at large diffusion distances, the light holes begin to dominate due to their larger diffusion length. It is found that heavy holes have a diffusion length of 54.5 ± 0.6 μm for an n-InGaAs doping of 8.4 × 1015 cm–3 at room temperature, whereas light holes have a diffusion length in excess of 140 μm. Heavy holes demonstrate a mobility of 692 ± 63 cm2/Vs and a lifetime of 1.7 ± 0.2 μs, whereas light holes demonstrate a mobility of 6200 ± 960 cm–2/Vs and a slightly longer lifetime of 2.6 ± 1.0 μs. The presented method, which is limited to low injection conditions, is capable of accurately resolving minority carrier transport properties.

Minority carrier diffusion length, lifetime and mobility in p-type GaAs and GaInAs

Minority carrier diffusion lengths and lifetimes were determined for p-type Ga(1-x)InxAs with an In-content of 0 ≤ x ≤ 0.53 by cathodoluminescence and time-resolved photoluminescence measurements respectively under low injection conditions; the resulting minority carrier mobilities are also reported. Highly p-doped samples (3 × 1018 cm−3) demonstrate a constant minority carrier diffusion length of (5.0 ± 0.7) μm and a constant lifetime of (3.7 ± 0.7) ns for an In-content up to 21%. Lower doped samples (3 × 1017 cm−3), on the other hand, show an increase in minority carrier diffusion length and lifetime with In-content from (6.3 ± 0.2) μm and (6.2 ± 0.5) ns respectively for GaAs to (14 ± 2) μm and (24.4 ± 0.5) ns respectively for Ga0.79In0.21As. Increasing the In-content to 53% results in a drop in the minority carrier diffusion length independently of the p-doping concentration .This is interpreted as a change in the energy of the Shockley-Read-Hall trap levels within the bandgap as a function of indium concentration.

Absolute measurement of effective radiative-efficiency in GaAs grown with molecular-beam-epitaxy

In this work, we have decoupled the radiative and nonradiative contributions to recombination by developing analytical models to fit observed non-monoexponential photoluminescence decay and bulk-lifetime temperature-dependence, which has enabled absolute measurement of the effective radiative efficiency and characterization of the dominant defect in lightly doped GaAs samples grown using molecular-beam-epitaxy. The measured effective radiative efficiency values under low level injection conditions range from (88.9 ± 0.1)% and (81.2 ± 0.1)% at 77 K to (0.028 ± 0.001)% and (0.034 ± 0.001)% at 700 K, with 300 K values of (17.3 ± 0.1)% and (10.5 ± 0.1)% in the p-type and n-type samples, respectively. A common defect-signature is observed in the temperature-dependence of the nonradiative lifetime in samples of both doping types, and our measured parameters are consistent with characteristics previously measured for the EL2 defect using deep-level transient spectroscopy.

Excitation and temperature dependent exciton-carrier transport in CVD diamond: Diffusion coefficient, recombination lifetime and diffusion length

Time-resolved induced absorption (IA) and light induced transient grating (LITG) techniques were applied for the investigation of nonequilibrium exciton-carrier diffusion and recombination processes in a high-purity CVD diamond. Injection range from 1015 to 1020cm−3 carrier density was achieved by combining one photon and two photon excitations. The measurements were performed in the 10–750K temperature range. The LITG diffusion coefficient peaked at 44cm2/s value at room temperature under low injection conditions. At lower temperatures it transferred to much lower exciton diffusion coefficient. A strong decrease of diffusion coefficient under higher injection conditions was explained by exciton formation with a low diffusion coefficient and many body effects, as polyexciton and electron-hole droplet formation. High temperature phonon-limited diffusion coefficient was weakly injection dependent. Low excitation carrier lifetime was about 700ns above 200K. At lower temperatures, the decay time reduced by two orders of magnitude, which was explained by the formation of biexcitons. At lowest temperatures, an increase of the carrier recombination rate at high injection was attributed to Auger recombinations of polyexcitons and electron-hole droplets. While at high temperatures, the increase of the recombination rate with 490meV activation energy was observed. The combination of IA and LITG measurements provided effective diffusion lengths in a 0.3–50µm range, being strongly dependent on the excess carrier density and temperature.

Analysis of the reverse recovery oscillation of superjunction MOSFET body diode

The voltage and current oscillations occasionally occur during the reverse recovery transient of the superjunction MOSFET body diode. This paper identifies the unique reverse recovery oscillation characteristics of the superjunction MOSFET body diode. Base on the experimental investigation and theoretical analysis, the various reverse recovery oscillation mechanisms are clarified. At first, with a discrete 650V/47A superjunction MOSFET utilized, the high-frequency and low-frequency oscillation characteristics during the reverse recovery transient are obtained by the double-pulse test. After the theoretical analysis, it is found that the superjunction MOSFET body diode has various oscillation mechanisms depending on its drift region injection level. Under high-level injection condition, the high-frequency oscillation occurs due to the plasma extraction transient-time (PETT) effect. Under low-level injection condition, the body diode’s snappy reverse recovery results in the low-frequency LC oscillation. In the end of the paper, the oscillation behaviors under both high and low level injection conditions are reproduced by the mixed-mode numerical simulation, the simulation results validate the proposed oscillation mechanisms.

Factors influencing photoluminescence and photocarrier lifetime in CdSeTe/CdMgTe double heterostructures

CdSeTe/CdMgTe double heterostructures were produced with both n-type and unintentionally doped absorber layers. Measurements of the dependence of photoluminescence intensity on excitation intensity were carried out, as well as measurements of time-resolved photoluminescence decay after an excitation pulse. It was found that decay times under very low photon injection conditions are dominated by a non-radiative Shockley-Read-Hall process described using a recombination center with an asymmetric capture cross section, where the cross section for holes is larger than that for electrons. As a result of the asymmetry, the center effectively extends photoluminescence decay by a hole trapping phenomenon. A reduction in electron capture cross section appeared at doping densities over 1016cm−3. An analysis of the excitation intensity dependence of room temperature photoluminescence revealed a strong relationship with doping concentration. This allows estimates of the carrier concentration to be made through a non-destructive optical method. Iodine was found to be an effective n-type dopant for CdTe, allowing controllable carrier concentrations without an increased rate of non-radiative recombination.

저압 분사조건에 따른 직접분사 LPG의 분무 및 연소특성 연구

저압 분사조건에 따른 직접분사 LPG의 분무 및 연소특성 연구

Current-voltage Curve Analysis for Non-linear Silicon Solar Cells and Operation Conditions Beyond Low Injection

This paper addresses the limitation of the double-diode model to low injection conditions. Assuming a constant carrier lifetime, we re-formulate the current of the first diode for general injection. The new parameter Y is introduced to quantify the share of the diode saturation current J01 that is due to lightly doped regions that go into high injection at higher voltages. The remaining J01 (1-Y) accounts for the highly doped regions that remain in low injection. When Y equals 0, the extended model is identical to the conventional model. A least square fit of the Isc-Voc characteristics to the general-injection model makes the determination of J01 more reliable and indicates whether the recombination is dominated by the highly doped regions. We also propose to use a double-diode model with a variable J01(V) to evaluate non-linear silicon solar cells. A J01 range between ∼400 mV and the maximum power point can be obtained from the bias-light dependent internal quantum efficiency (IQE). Based on the electrical and optical model that describes the IQE, the excess carrier concentration in the base when the solar cell is in short circuit is calculated for every bias light and used to attribute a voltage. The J01(V) for larger voltages is extracted from the ISC-VOC characteristics after an appropriate extrapolation of the parasitic currents through the second diode and the shunt.