Carrier Generation Rate Research Articles

Моделирование образования каскада смещений и переходных ионизационных процессов в кремниевых полупроводниковых структурах при нейтронном воздействии

AbstractThe formation of a disordered defect region in bulk silicon is simulated using the molecular-dynamics method for various energies of a primary recoil atom. Variations in the volume and number of radiation-induced defects in a cluster during its formation are calculated. The generation rates of nonequilibrium carriers and amplitude-temporal dependences of pulses of ionization currents in test Schottky diodes with hyperhigh frequencies are found theoretically.

Functionalizing plasmonic nanoparticles through adding a shell to improve electronic properties of c-Si thin-film solar cells

Embedding plasmonic nanoparticles (p-NPs) inside the solar cell’s active layer is capable of enhancing active layer optical absorption; however, such inclusion has some detrimental effects on the electronic properties of the solar cells. In addition, p-NPs are highly catalytic, their presence enables other non-radiative decay channels besides generation of electron–hole pairs, and the electrons usually get absorbed by these p-NPs. This results in the unfavorable fact that the potential enhancement in the carrier generation rate and the generated current is no longer in line with the enhancement in the optical absorption. In this paper, we propose to functionalize p-NPs by adding a dielectric or semiconductor shell, to passivate the p-NP without deteriorating scattering and/or plasmonic effects. Ag@SiO2 and Ag@TiO2 core–shell p-NPs have been intensely studied using extensive computational electromagnetic simulations to model the spectral response of the active layer’s optical absorption as well as electronic properties as a function of both shell composition and thickness. It is shown that a 5 nm TiO2 shell is apt to optically passivate the p-NP without any reduction in optical absorption, while improving the short circuit current density (J sc ) of the thin-film solar cell by 33.3%.

Surface Metallization Influence on Equivalence of Laser Simulation of Dose-Rate Effects

Pulsed laser simulation is a powerful method in studying dose-rate effects of semiconductor devices. However, blocking of the incident laser from the device layer by the surface metallization layers has always been an influencing factor affecting their equivalent relationship because the penetrations of laser light and gamma rays are different. For the quantitative study of the equivalent relationship, the conversion factor (CF) is defined as a comparison between the laser intensity and dose rate at the same equivalent basis [carrier generation rate (CGR), peak photocurrent]. The conventional models usually use CGR, and they suggest that the CF varies linearly with metallization coverage, but few of them considered the metallization distribution. In this paper, we propose a numerical simulation method for CF using the peak of transient response as the equivalent basis, and unambiguously taking into account the effects of metallization distribution. Backed up with the finite-element analysis, we find that the distribution of the metallization can lead to a significant change in the linear relation between CF and metallization coverage. Experimental data on discrete devices conform to these simulation results. In addition, the experimental verification in the classical diode structure further indicates the validity and the accuracy of the approach.

Contactless non-destructive imaging of doping density and electrical resistivity of semiconductor Si wafers using lock-in carrierography

A contactless non-destructive imaging method for spatially resolved dopant concentration, [2.2] Nd, and electrical resistivity, ρ, of n- and p-type silicon wafers using lock-in carrierography images at various laser irradiation intensities is presented. Amplitude and phase information from wafer sites with known resistivity was employed to derive a calibration factor for accurate determination of the absolute carrier generation rate. A frequency-domain model based on the nonlinear nature of photocarrier radiometric signals was used to extract dopant density images. Lateral variations in the resistivity of an n-type and a p-type wafer obtained by means of this methodology were found to be in excellent agreement with those obtained with conventional 4-point probe measurements. This all-optical contactless method can be used as a non-destructive tool for doping density and electrical resistivity measurements and their images over large semiconductor areas. Nd, ρ and their variances can be measured and mapped for the photovoltaic, micro- and opto-electronic industries including on wafers with oxides or surface treated layers for which contacting electrical measurements are impossible.

The relationship between the coherent size, binding energy and dissociation dynamics of charge transfer excitons at organic interfaces

At organic semiconductor interfaces, an electron and a hole can be bound Coulombically to form an interfacial charge transfer (CT) exciton. It is still under debate how a CT exciton can overcome its strong binding and dissociate into free carriers. Experimentally, capturing the evolution of the CT exciton on time (fs-ps) and length scales (nm) in which the dissociation process occurs is challenging. To overcome this challenge, time-resolved two photon photoemission spectroscopy is used to measure the binding energies and electronic coherent sizes of a series of CT states at organic interfaces, and capture the temporal dynamics of these CT excitons after their excitation. Using zinc phthalocyanine (ZnPc)/fullerene (C60) interface as a model system, it is shown that the interfacial CT process first populates a hot CT state with a coherent size of ~4 nm. Hot and delocalized CT excitons subsequently relax into CT excitons with lower energies and smaller coherent sizes. To correlate the CT exciton properties with the dissociation efficiency, we develop a method that exploits graphene field effect transistors to probe the rate and yield of free carrier generation at the interface. Our results show that exciton dissociation can be more efficient if one can extract electrons from the hot and delocalized CT state. We propose a cascade structure that would serve this purpose.

Dynamic Photoelectrochemical Device with Open-Circuit Potential Insensitive to Thermodynamic Voltage Loss.

The open-circuit potential ( Voc) represents the maximum thermodynamic potential in a device, and achieving a high Voc is crucial for self-biased photoelectrochemical (PEC) devices that use only solar energy to produce chemical energy. In general, Voc is limited by the photovoltage ( Vph), which is a potential difference generated by light-induced thermodynamic processes at semiconductor photoelectrodes, such as the generation and recombination of charge carriers. Therefore, low light intensity and nanostructured semiconductor materials degrade Vph (and Voc) by inefficient carrier generation and by enhancing recombination loss, respectively. Here, we report that Voc in dynamic PEC devices employing a porous NiO x/Si photocathode is insensitive to thermodynamic losses, which was clarified by varying the carrier generation and recombination rates. The Voc values were observed to be unchanged even under a low light intensity of 0.1 sun, as well as for different morphologies such as nanostructured and polycrystalline Si. These findings shed light on the potential merit of dynamically operated PEC systems.

Efficiency enhancement through flat intermediate band in Quantum dot solar cell

Abstract Quantum dots (QD) are playing a vital role due to their multifunctionality such as small size and tuning the range of band and have achieved a considerable attention in the recent decade. In recent research, the efficiency of the solar cell (SC) has greatly been increased by application of QD. If these dots are grown systematically, may lead to the formation of the intermediate band (IB) which leads to the absorption of photons with energy less than the actual band through intermediate absorption process and hence increasing the carrier generation rate which ultimately results in high current density keeping the output voltage unaffected. In this paper, we have proposed a method to control the band flatness of the IB in InAs/GaAs QDSC. The flatness is achieved by optimizing the doping concentration and Fermi level (FL) of the surrounding layers. Results show that the maximum 44.12% efficiency can be achieved under maximum optical intensity.

Rigorous mathematical model of through‐silicon via capacitance

Through-silicon vias (TSVs) are a key technology for three-dimensional integrated circuits. As the integration of circuits increases, high temperature has a greater effect on the performance of the TSV interconnections. The metal–oxide semiconductor (MOS) effect is one of the most important temperature-dependent characteristics of a TSV. This study introduces the mathematical model of a TSV to predict the MOS effect more accurately. The thermal effect that varies due to the change in the TSV capacitance and depletion region can be modelled by the non-linear the Poisson equation including mobile charge carriers. In procedures to solve this equation, the proposed method considers not only the thermal effect of intrinsic carrier concentration and silicon bandgap energy but also the shift effect of the flat band voltage due to the Si–SiO 2 interface charges. In addition, since it considers the minority carrier generation rate, which is dependent on the change of gate voltage, the MOS effect in a TSV can be explained more accurately using equations derived from these procedures. To verify the proposed mathematical model, comparison with the numerical method is carried out, and these results show that the proposed method is very accurate in explaining the MOS effect in a TSV.

Effects of the unintentional background concentration, indium composition and defect density on the performance of InGaN p-i-n homojunction solar cells

We theoretically investigate the effects of the unintentional background concentration, indium composition and defect density of intrinsic layer (i-layer) on the photovoltaic performance of InGaN p-i-n homojunction solar cells by solving the Poisson and steady-state continuity equations. The built-in electric field and carrier generation rate depend on the position within the i-layer. The collection efficiency, short circuit current density, open circuit voltage, fill factor, and conversion efficiency are found to depend strongly on the background concentration, thickness, indium composition, and defect density of the i-layer. With increasing the background concentration, the maximum thickness of field-bearing i-layer decreases, and the width of depletion region may become even too small to cover the whole i-layer, resulting in a serious decrease of the carrier collection. Some oscillations as a function of indium composition are found in the short circuit current density and conversion efficiency at high indium composition and low defect density due to the interference between the absorbance and the generation rate of carriers. The defect density degrades seriously the overall photovoltaic performance, and its effect on the photovoltaic performance is roughly seven orders of magnitude higher than the previously reported values [Feng et al., J. Appl. Phys. 108 (2010) 093118]. As a result, the high crystalline quality InGaN with high indium composition is a key factor in the device performance of III-nitride based solar cells.

Improvement in electrostatic characteristics of doped TFETs by hole layer formation

We present a distinct approach to enhance the performance of physically doped tunnel field-effect transistors (TFETs) based on creation of a layer of positive charge at the semiconductor–insulator interface in the source region. Formation of such a hole layer resolves the issue related to material solubility and improves direct-current (DC) as well as high-frequency figures of merit. To implement this approach, a typical $$P^{+}$$ -I- $$N^{+}$$ -type physically doped TFET structure is considered. Furthermore, a metal electrode with workfunction of 4.53 eV is placed over the heavily doped $$P^{+}$$ source region with a negative supply voltage. The negative voltage at the source electrode attracts holes from the source region and creates a hole layer just below the semiconductor–insulator interface. This phenomenon makes the source–channel junction abrupt and reduces the tunneling barrier width, resulting in higher tunneling generation rate of charge carriers at the source–channel junction. Thus, the proposed device shows 100-fold increased ON-state current and a threshold voltage reduction of 300 mV. Analog/radiofrequency (RF) parameters are also greatly improved compared with the conventional device. Furthermore, optimization of the spacer length ( $$L_\mathrm{SG}$$ ), i.e., the gap between the source and gate electrode, and application of a negative voltage ( $$-V_\mathrm{SE}$$ ) at the source electrode (SE), were applied to achieve the optimum performance. Moreover, device linearity was also analyzed in a comparative manner.

Material, Size, and Environment Dependence of Plasmon-Induced Hot Carriers in Metallic Nanoparticles

Harnessing hot electrons and holes resulting from the decay of localized surface plasmons in nanomaterials has recently led to new devices for photovoltaics, photocatalysis, and optoelectronics. Properties of hot carriers are highly tunable, and in this work, we investigate their dependence on the material, size, and environment of spherical metallic nanoparticles. In particular, we carry out theoretical calculations of hot carrier generation rates and energy distributions for six different plasmonic materials (Na, K, Al, Cu, Ag, and Au). The plasmon decay into hot electron–hole pairs is described via Fermi’s golden rule using the quasistatic approximation for optical properties and a spherical well potential for the electronic structure. We present results for nanoparticles with diameters up to 40 nm, which are embedded in different dielectric media. We find that small nanoparticles with diameters of 16 nm or less in media with large dielectric constants produce most hot carriers. Among the different mat...

Performance prediction of p-i-n HgCdTe long-wavelength infrared HOT photodiodes.

Recently, an enhanced computer program was applied to explain in detail the influence of different recombination mechanisms (Auger, radiative, and Shockley-Read-Hall) on the performance of high-operation-temperature, long-wavelength, infrared p-i-n HgCdTe heterojunction photodiodes. It is shown that the photon recycling effect drastically limits the influence of radiative recombination on the performance of small pixel HgCdTe photodiodes. The computer program is based on a solution of the carrier transport equations, as well as the photon transport equations for semiconductor heterostructures. Both the distribution of thermal carrier generation and recombination rates, and spatial photon density distribution in photodiode structures have been obtained. In comparison with two previously published papers in the Journal of Electronics Materials [J. Electron. Mater.45, 4587 (2016)JECMA50361-523510.1007/s11664-016-4566-6 and J. Electron. Mater.46, 6295 (2017)JECMA50361-523510.1007/s11664-017-5736-x], our paper provides an additional insight on the ultimate performance of long-wavelength infrared, high-operation-temperature HgCdTe arrays with pixel densities that are fully consistent with background- and diffraction-limited performance due to system optics.

Gamma non-ionizing energy loss: Comparison with the damage factor in silicon devices

The concept of non-ionizing energy loss (NIEL) has been demonstrated to be a successful approach to describe the displacement damage effects in silicon materials and devices. However, some discrepancies exist in the literature between experimental damage factors and theoretical NIELs. 60Co gamma rays having a low NIEL are an interesting particle source that can be used to validate the NIEL scaling approach. This paper presents different 60Co gamma ray NIEL values for silicon targets. They are compared with the radiation-induced increase in the thermal generation rate of carriers per unit fluence. The differences between the different models, including one using molecular dynamics, are discussed.

Design of double gate vertical tunnel field effect transistor using HDB and its performance estimation

In this paper, we demonstrate the double gate vertical tunnel field-effect transistor using homo/hetero dielectric buried oxide (HDB) to obtain the optimized device characteristics. In this concern, the existence of double gate, HDB and electrode work-function engineering enhances DC performance and Analog/RF performance. The use of electrostatic doping helps to achieve higher on-current owing to occurrence of higher tunneling generation rate of charge carriers at the source/epitaxial interface. Further, lightly doped drain region and high-k dielectric below channel and drain region are responsible to suppress the ambipolar current. Simulated results clarifies that proposed device have achieved the tremendous performance in terms of driving current capability, steeper subthreshold slope (SS), drain induced barrier lowering (DIBL), hot carrier effects (HCEs) and high frequency parameters for better device reliability.

4H-SiC Detector in High Photons and Ions Irradiation Regime

A Silicon carbide detector, based on a Schottky diode technology with an interdigitated front electrode design, was employed to monitor photons and low-energy ions. Photons were generated by a He-Cd laser with maximum optical power density of 64 mW/cm2, while a plasma generated by a pulsed laser at low intensity (1010 W/cm2), was used to produce low-energy Al and C ions. The measured detector yield in the case of the photon detection increases linearly with the optical power density, and shows saturation at high power densities, being the saturation value depending on the applied detector bias. A similar behavior is observed for ion detection, where the detector response saturates with increasing ion energy. Carrier generation rate was extracted from the experimental data obtaining almost equal values for photon and ion irradiations, allowing also to extract a maximum generation rate of about $8 \times 10^{19}$ carriers/cm $^{3}\cdot \text {s}$ as upper limit for the linear operation of the detector.

Iodide-modified Bi4O5Br2 photocatalyst with tunable conduction band position for efficient visible-light decontamination of pollutants

Band-gap regulation of photocatalysts has been turned out to be a very important approach to achieve high effective utilization of solar spectrum and obtain an optimal photocatalytic property. Herein, a novel iodide modified Bi4O5Br2 photocatalyst with controlled band-gap position was successfully synthesized for the first time via a simple alcoholysis-hydrolysis method at room temperature. The band structure, optical absorption properties, photo-induced charges transfer/separation rate of the synthesized samples were characterized by UV-vis DRS, Mott-Schottky method, valence band (VB) XPS, electrochemical impedance spectra and photoelectric conversion spectra. The results demonstrated that the conduction band position of Bi4O5Br2 continuously decreased from 2.46 eV to 2.16 eV, and then the visible-light response and photo-generated carrier separation changes took place. Owing to the higher visible-light absorption capacity, lower interfacial charge-transfer resistance, and more efficient separation of the photoinduced charges, the iodide modified Bi4O5Br2 exhibited a higher visible-light activity than pure Bi4O5Br2 for the photocatalytic degradation of Bisphenol A (BPA) and NO removal under visible light irradiation. Ultimately, the repeated photodegradation experiments of BPA displayed that the obtained sample is stable and resistant to chemical or photochemical corrosion. We believe such a simple and effective strategy could pave a new way for narrowing continuously the band gap of an existed photocatalyst to control the generation and recombination rate of photo-induced carriers.

表面等离子激元与F-P共振耦合平衡钙钛矿太阳能电池有源层内载流子产生速率

表面等离子激元与F-P共振耦合平衡钙钛矿太阳能电池有源层内载流子产生速率

Near-Field Enhanced Photochemistry of Single Molecules in a Scanning Tunneling Microscope Junction.

Optical near-field excitation of metallic nanostructures can be used to enhance photochemical reactions. The enhancement under visible light illumination is of particular interest because it can facilitate the use of sunlight to promote photocatalytic chemical and energy conversion. However, few studies have yet addressed optical near-field induced chemistry, in particular at the single-molecule level. In this Letter, we report the near-field enhanced tautomerization of porphycene on a Cu(111) surface in a scanning tunneling microscope (STM) junction. The light-induced tautomerization is mediated by photogenerated carriers in the Cu substrate. It is revealed that the reaction cross section is significantly enhanced in the presence of a Au tip compared to the far-field induced process. The strong enhancement occurs in the red and near-infrared spectral range for Au tips, whereas a W tip shows a much weaker enhancement, suggesting that excitation of the localized plasmon resonance contributes to the process. Additionally, using the precise tip-surface distance control of the STM, the near-field enhanced tautomerization is examined in and out of the tunneling regime. Our results suggest that the enhancement is attributed to the increased carrier generation rate via decay of the excited near-field in the STM junction. Additionally, optically excited tunneling electrons also contribute to the process in the tunneling regime.

Performance improvement of doped TFET by using plasma formation concept

Formation of abrupt doping profile at tunneling junction for the nanoscale tunnel field effect transistor (TFET) is a critical issue for attaining improved electrical behaviour. The realization of abrupt doping profile is more difficult in the case of physically doped TFETs due to material solubility limit. In this concern, we propose a novel design of TFET. For this, P+ (source)-I (channel)-N (drain) type structure has been considered, wherein a metal electrode is deposited over the source region. In addition to this, a negative voltage is applied to the source electrode (SE). It induces the surface plasma layer of holes in the source region, which is responsible for steepness in the bands at source/channel junction and provides the advantage of higher doping in source region without any addition of the physical impurity. The proposed modification is helpful for achieving steeper band bending at the source/channel interface, which enables higher tunneling generation rate of charge carriers at this interface and overcomes the issue of low ON-state current. Thus, the proposed device shows the increment of 2 decades in drain current and 252 mV reduction in threshold voltage compared with conventional device. The optimization of spacer length (LSG) between source/gate (LSG) and applied negative voltage (Vpg) over source electrode have been performed to obtain optimum drain current and threshold voltage (Vth). Further, for the suppression of ambipolar current, drain region is kept lightly doped, which reduces the ambipolar current up to level of Off state current. Moreover, in the proposed device gate electrode is underlapped for improving RF performance. It also reduces gate to drain capacitances (Cgd) and increases cut-off-frequency (fT), fmax, GBP, TFP. In addition to these, linearity analysis has been performed to validate the applicability of the device.

A comparative study of photoluminescence internal quantum efficiency determination method in InGaN/GaN multi-quantum-wells

The photoluminescence (PL) measurement is considered to be a powerful and convenient method to determine the internal quantum efficiency (IQE) of InGaN/GaN multi-quantum-wells. However, as different calculation models are developed for different PL experiments, the calculated IQE values can sometimes differ a lot, mainly due to different experimental conditions. In this paper, a comparative study is presented among the temperature-dependent PL (TDPL) measurement and power-dependent PL (PDPL) measurement and time-resolved PL (TRPL) measurement, and the relationship between different IQE calculation models is analyzed both theoretically and experimentally. It is revealed that despite distinct IQE values, different PL methods are actually compatible with each other. The discrepancy in TDPL and TRPL results comes from the temperature dependent relationship between the carrier concentration and carrier generation rate. Through a careful conversion of the excitation condition, both TDPL and TRPL results are correspondent to an IQE value under a specific excitation power in the PDPL measurement. Finally, through the combination of different PL measurements, an accurate measurement of IQE and carrier recombination coefficients can be achieved at different temperatures and excitation conditions.