Uniform Distribution Of Defects Research Articles

Impact of Substrate Defects on Vertical GaN Device Leakage Behavior

In this work, the effects of the substrate defects on the reverse leakage behavior of vertical GaN Schottky and p-i-n diodes are investigated. A direct connection between the reverse leakage behavior of GaN based vertical devices and the defect characteristics of the substrate was achieved. The difference in the leakage current can be as high as 6 orders of magnitude for vertical GaN p-i-n diodes at -200V, using HVPE substrate with inhomogeneous defect distributions (with dot-cores). For comparison, using HVPE substrate with uniform defect distribution (no cores), the p-i-n diodes show much more uniform leakage behavior at -200V, varying within only an order of magnitude. The wafers with inhomogeneous defect distribution possess periodically patterned core-centers with higher defect density (up to 108 cm-2) than regions in between cores, which have very low defect concentrations (as low as 104 cm-2). The full width at 0.01 maximum of the triple axis rocking curves (GaN (0004)) shows a large variation ranging from 60 to 930 arcsec. In comparison, wafers without the periodic cores have a uniform defect density (~106 cm-2) and show a small variation in the rocking curve FW0.01M, ranging only from 70 to 280 arcsec. For Schottky diodes fabricated on substrate with cores, positioning the devices away from the core-centers results in a reduction of the reverse bias leakage by 2-4 orders of magnitude at -10 V. Similar trend are also observed in the p-i-n diodes, with device further away from the core-centers showing lower leakage. The devices in the low defect concentration region (> 200 um away from core centers) on the wafer with cores outperform the devices from the wafer without cores, showing up to 2 orders of magnitude lower leakage current at -200 V. The results from this study show that the substrate defect density and distribution play an important role on the device leakage current and avoiding highly defective regions on the substrate will improve the performance of vertical GaN devices.

Investigation of the Reverse Leakage Behavior and Substrate Defects in Vertical GaN Schottky and PIN Diodes

In this work, the effects of the substrate defect density and distribution on the reverse leakage behavior of GaN vertical Schottky diodes and p–i–n diodes are investigated. A direct connection between the reverse leakage behavior of GaN based vertical devices and the dislocation density of the underlying material was determined. The difference in the leakage current for devices on different locations of the wafer can be as high as 6 orders of magnitude (for p–i–n diodes) at −200V, for HVPE substrate with inhomogeneous but predictable defect distributions (GaN substrates with dot-core inversion domain features). For comparison, using HVPE substrates with uniform defect distribution (but with no cores), the p–i–n diodes show much more uniform leakage behavior, varying within only an order of magnitude, and that range fell within the much greater range of that for the inhomogeneous substrates. The substrates with inhomogeneous defect distribution proved to be useful to show the direct correlation. The topography measurements confirmed that the wafers with inhomogeneous defect distribution possess periodically patterned core-centers with higher defect density and larger lattice distortions surrounded by other regions, which have very low defect concentrations. Devices located away from the defective core-centers result in a reduction of the reverse bias leakage by over two orders of magnitude at −10 V for Schottky diodes. Similar trends are also observed in the p–i–n diodes; the devices close to the core centers show the highest reverse leakage (>0.01 A cm−2 at −200V). Devices further away from the core-centers (lower dislocation density) show lower reverse leakage current. Moreover, the p–i–n diodes on regions more than 300 μm away from the core-centers show the best leakage behavior (<10−7 A cm−2 at −200V) of all the devices, outperforming the devices on the substrates with uniform defect distribution (∼10−6 A cm−2 at −200V). The results from this study show that the substrate defect density and distribution play important roles in the device leakage current. X-ray topography is extremely effective for studying defect characteristics underneath individual devices. The use of the wafers with inhomogeneous, but predictable defect density clearly demonstrated the importance of low defect densities for high device performance.

RRAM Devices with Plasma Treated HfO2 with Ru as Top Electrode for In-Memory Computing Hardware

Recently, many non-volatile memory devices such as resistive random-access memory (RRAM) have shown promise as candidates for analog in-memory computing applications related to inference and training in artificial intelligence. A RRAM cell has an MIM (metal insulator metal) structure that exhibits reversible resistive switching on application of positive or negative voltage. HfO2-based dielectrics with controlled distribution of defects or oxygen vacancies can potentially enable low power switching and multi-resistance levels. However, detailed studies of the power consumption, repeatability and retention during multi-level operation have not been undertaken previously. Transition metal oxide-based RRAM, using HfO2, executes change in resistance (switching behavior) via electrochemical migration of oxygen vacancies. Lack of enough oxygen vacancies in stoichiometric HfO2 limits the low power operation. Therefore, HfOx (with x < 2) has been studied for many RRAM devices (1). A uniform distribution of defects in HfO2 may help in switching but may not help in reducing the power. The distribution of these defects throughout the bulk of HfOx is, therefore, not well-understood. It was recently reported that high oxygen vacancy concentration close to top electrode reduces switching power (2).This work investigates the role of extra oxygen vacancies, introduced by plasma for a 6 nm thick HfO2 during deposition to reduce the power consumption. Initially TiN, which is a commonly used metal in CMOS technology, was used as the top electrode for treated HfO2 using a Si/PVD Ti/PVD TiN/HfO2/PVD TiN structure. A range of compliance currents (CC) from 1 nA to 500 nA were used to evaluate the switching characteristics. The TiN devices switch at and above a CC of 50 nA and were tested for sustainability for hundreds of cycles. In addition to oxygen vacancy rich treated-HfO2 near the top electrode, TaN and Ru electrodes were explored to enhance the switching behavior and power consumption. The role of both TaN and Ru as bottom metal was also evaluated. With Ru as top metal the Si/PVD Ti/PVD TiN/HfO2/Ru/PVD TiN structure, switched at 1 nA CC and at higher CC. Whereas when Ru was used as a bottom electrode, a Si/PVD Ti/PVD TiN/Ru/HfO2/ PVD TiN structure, devices were unable to switch below a CC of 50 mA. For TaN as top metal electrode, devices switched at and above 1 mA CC whereas with TaN as bottom metal the initial switching was at CC of 2 mA. It was concluded that use of Ru as a top metal significantly reduced the switching energy of the treated HfO2 RRAM device but was ineffective when used as a bottom metal. Potential mechanism for this observation will be discussed. References Chen, W. Lu, B. Long, Y. Li, D. Gilmer, G. Bersuker, S. Bhunia, and R. Jha, Semiconductor Science and Technology 30, 075002 (2015).Misra, P. Zhao, D. H. Triyoso, V. Kaushik, K. Tapily, R. D. Clark, S. Consiglio,2, T. Hakamata, C. S. Wajda, and G. J. Leusink, ECS J. Solid State Sci. Technol. 9, 05300, (2020).

Effect of γ-ray irradiation on thermal conductivity of tritium breeding material Li2TiO3

Thermal performance is one of the important parameters for tritium breeders in D-T fusion reactor. It has a certain significance to study the effect of irradiation damages on the thermal conductivity for enriching the database. γ-ray irradiation has been adopted due to its presence in the reactor and uniform defect distribution similar to neutron irradiation. The results of XRD and Raman indicate that the samples are damaged by irradiation. The irradiation defects have been studied by Electron Paramagnetic Resonance (EPR or ESR). E′-center and Ti3+ are determined as the main defects introduced in irradiated Li2TiO3. The spatial distribution of defects introduced in Li2TiO3 by γ-ray irradiation is uniform calculated by Fluka. The relative concentrations of E′-center, A-centers and Ti3+ have been obtained by EasySpin simulation. The thermal diffusivity at room temperature decreases as irradiation dose increases. However, there is no obvious change in trends for thermal conductivity. For isochronal annealing experiments, it can be found that the thermal conductivity changes by the effects of E′-center and Ti3+. E′-center gradually decreases with the increase of the annealing temperature. The comprehensive analysis of experimental results and EasySpin fitting results has been performed. According to analysis and comparison with other reports, it is indicated that E′-center has negative effects on thermal conductivity, while the presence of Ti3+ plays a major role in enhancing the thermal conductivity. Based on the work, the suggestion for improving thermal conductivity by adopting additional Ti3+ has been proposed.

Crystallization of self-propelled particles on a spherical substrate

In this paper, we investigate the self-propelled particles confined on a spherical substrate and explore the structural and dynamic properties of self-propelled particles by controlling the packing fraction and activity. We find that these self-propelled particles freeze into the crystal with the increase in the packing fraction. We observe the pattern evolution of inevitable topological defects due to the geometric constraints of the spherical substrate. During the process of freezing, there is a transition from twelve isolated grain boundaries to the uniform distribution of defects with the increase in the self-propelled velocity. Finally, we establish a phase diagram of the freezing process. These results may deepen our understanding of active particles in complex and crowded environments.

Influence of oriented β‐lamellae on deformation and pore formation in β‐nucleated polypropylene

ABSTRACTFour β‐nucleated polypropylene samples with increasing die draw ratio (DDR) were prepared to modify lamellae arrangement. The DSC, SEM, and 2D‐XRD results show that all four cast films had similar crystallinity, high contents of β‐crystal but lowering stability of β‐lamellae with ascending DDR. Meanwhile, the anisotropy of β‐lamellae distribution strengthens gently and the stacked lamellae structure perpendicular to the machine direction (MD) predominates dramatically. Tensile testing at 25 °C and 90 °C were conducted along MD and transverse direction (TD), respectively. The markedly expanding difference of deformation indicates the anisotropy highlighted significantly. Additionally, when the samples stretched along MD, a more homogeneous deformation occurs with ascending anisotropy, which is completely opposite to the β‐lamellae stability. But samples deformed more heterogeneous when stretched along TD. The characterization of morphological evolutions during stretching shows that the stacked lamellae debonds uniformly and abundant microvoids formed when the sample stretched along MD with higher anisotropy, resulting in evenly dispersion of stress, consequently making a more uniform distribution of defects and a better isotropic deformation. Moreover, the microfibrils and defects distributed uniformly within higher orientation sample after longitudinal stretching stretched along MD, leading to the dramatic improvement of pore size distribution of the membrane after biaxial stretching. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1745–1759

Effects of precursor topology on polymer networks simulated with molecular dynamics

Molecular modeling of crosslinked polymers often follows arbitrary pathways for network generation, with different precursor topology from experimental systems. We use coarse-grained molecular simulation to study the effects of precursor choice on the predicted network structure and properties. Three sets of precursors with different molecular architectures are designed such that they would form identical networks at the limit of perfect conversion. Little difference is observed between the resulting networks in typical properties including the radial distribution function, macroscopic statistics of network connectivity, and glass transition behaviors. However, the stress-strain relationship in tensile deformation clearly depends on the formation pathway when compared at the same crosslinking density. The elastic modulus of the network is found to correlate strongly with the number of elastic strands in the network, except at the highly-crosslinked limit where substantial discrepancy is observed between networks from different precursors. Although these final networks contain a similar average density of structural defects, the choice of precursor has significant impact on their spatial distribution, leading to the precursor dependence of their mechanical properties. Uniform defect distribution and fast defect elimination can be achieved by designing precursor units with a proper stoichiometric ratio of different monomers.

First principle study of Nb defects in anatase (101) TiO2 surface

In this work, effects of Niobium (Nb) defects on TiO2 surface using density functional theory (DFT) are investigated. Based on formation energy of the defects, their occurrences in two different extreme conditions, O-rich and O-poor conditions, are evaluated. Effects of Nb defects on surface and its electronic structure are studied and it is demonstrated that Nb doping widens valence band in deep energy level leaving the band gap without any change and it also lowers oxygen vacancy defect concentration due to the stronger bonding of Nbsub defect with oxygen atoms specially bridging oxygen (most probable defect site for Oxygen vacancy). Higher density of Nb substitutional defects (Nbsub) are examined and it is shown that higher density doping of TiO2 surface leads to uniform distribution of defects over the anatase structure as a result of interaction of Nb defects when they are close and this fact prevents segregation of Nb atoms in Nb-doped TiO2.

A Classification of the Symmetries of Uniform Discrete Defective Crystals

Crystals which have a uniform distribution of defects are endowed with a Lie group description which allows one to construct an associated discrete structure. These structures are in fact the discrete subgroups of the ambient Lie group. The geometrical symmetries of these structures can be computed in terms of the changes of generators of the discrete subgroup which preserve the discrete set of points. Here a classification of the symmetries for the discrete subgroups of a particular class of three-dimensional solvable Lie group is presented. It is a fact that there are only three mathematically distinct types of Lie groups which model uniform defective crystals, and the calculations given here complete the discussion of the symmetries of the corresponding discrete structures. We show that those symmetries corresponding to automorphisms of the discrete subgroups extend uniquely to symmetries of the ambient Lie group and we regard these symmetries as (restrictions of) elastic deformations of the continuous defective crystal. Other symmetries of the discrete structures are classified as ‘inelastic’ symmetries.

The non-Euclidean model of failure of the deep rock masses under the deformation incompatibilty condition

The deep rock masses are considered as granular materials containing uniform distribution of defects. Defects in the deep rock masses are characterized by the damage variable. The damage variable of the deep rock masses is determined using Mori-Tanaka’s method and Sidoroff’s method, which consider the interaction among microcracks. Effect of the pre-existing microcracks and secondary microcracks on scalar curvature and the free energy density is investigated. A new non-Euclidean mode is established, considering effect of the pre-existing microcracks and secondary microcracks. Contributions of pre-existing microcracks and secondary microcracks to the self-equilibrated stresses are derived from the free energy and the deformation incompatibilty condition. The stress fields of the surrounding rock masses around a deep circular tunnel are determined from a new non-Euclidean model.

Modelling of the evolution of crack of nanoscale in iron

Metal owns the ability of self-healing to some extent, and the ability of the internal crack healing is most desirable for improving the reliability of metal. A molecular dynamics simulation has been further developed to investigate the evolution of a nanoscale crack in body centred cubic Fe crystal under the conditions of heating or compressive pressure. When system temperature drops, the evolution of the crack that was at elevated temperature has been studied for the first time. N-body potential according to the embedded atom method has been adopted. The original nanoscale crack is expressed by removing some atoms in the centre of the cell, and the minimum vertical distance between the atoms on the top and bottom crack surfaces has been defined as Dm for assessing the process of crack evolution. The results show that a crack healing process can be accelerated significantly with an increase of temperature. When the system temperature decreases, Dm of the crack that was in healing process does not change significantly but fluctuates in a narrow range. This means that the crack healing is the result of Fe atoms diffusing into the crack area but not the thermal stress incurred in the simulation cell at elevated temperature. The pre-compressive pressure under the condition of both biaxial and uniaxial loadings can help promote the crack healing significantly and results in more uniform distribution of defects after healing.

Very low 1/f barrier noise in sputtered MgO magnetic tunnel junctions with high tunneling magnetoresistance

Low frequency 1/f barrier noise has been investigated in sputtered MgO magnetic tunnel junctions (MTJs) with a tunneling magnetoresistance ratio of up to 330% at room temperature. The lowest normalized noise parameter α of the tunnel barrier reaches 2.5 × 10−12–2.1 × 10−11 μm2, which is comparable to that found in MTJs with the MgO barrier grown by MBE or electron–beam evaporation. This normalized barrier noise is almost bias independent in the voltage range of up to ±1.2 V. The low noise level and high voltage stability may reflect the high quality of the sputtered MgO with a uniform distribution of defects in the MgO layer.

Thermal ideality factor of hydrogenated amorphous silicon p-i-n solar cells

The performance of hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells is limited, as they contain a relatively high concentration of defects. The dark current voltage (JV) characteristics at low forward voltages of these devices are dominated by recombination processes. The recombination rate depends on the concentration of active recombination centers and the recombination efficacy of each of these centers. The first factor causes the ideality factor of the devices to be non-integer and to vary with voltage. The temperature dependence of the dark current can be expressed by its activation energy. For microcrystalline silicon solar cells the activation energy varies with voltage with a so-called thermal ideality factor of 2. This value was derived for devices with a spatially uniform defect distribution and reflects the recombination efficacy. Here we present results of a thickness series of a-Si:H p-i-n solar cells. We have matched the experimental curves with computer simulations, and show that the voltage-dependent ideality factor curve can be used to extract information on the cross sections for electron and hole capture. Also, the activation energy is used as a measure for the mobility gap, resulting in a mobility gap for a-Si:H of 1.69 eV. We find a thermal ideality factor close to 2 for all samples. This is explained with a theoretical derivation, followed by a comparison between the internal electric field strength and the spatial variation of the defect density in the intrinsic layer. The thermal ideality factor is shown to be insensitive to the defect distribution and the recombination profile in the device. It is, therefore, an appropriate parameter to characterize a-Si:H p-i-n devices, providing direct insight on the recombination efficacy.

Size effect of quantum conductance in carbon nanotube Y-Junctions

This paper studies the quantum conductance properties of three-terminated carbon nanotube Y-junctions, which are built by connecting three (5,5) single-walled carbon nanotubes. The results show that the quantum conductance at the Fermi energy oscillates periodically with the junction's size, and the number of oscillating periodic layers is 3 which is the same as that in the two terminated (10,0)/m(5, 5)/(10,0) junctions. Moreover, this Y-junction with different size exhibits an obviously different distribution of electron current in the two drain branches, called shunt valve effect of electronic current. Thus the degree of this effect can be controlled and modulated directly by constructing the three branches' sizes or the distribution of defect. The results show in detail that the difference between the two drain currents can be up to two times for some constructions with special sizes. In addition, the uniform distribution of defects in the Y-junction leads to lower quantum conductance than that of other defect configurations.

Analysis of Coupling Capacitance among Interconnections Based on Defect's Uniform Distribution

The parasitical effect is one of the key factors that restrict the VLSI development forward high speed and high density. In this paper,the parasitical capacitance affected by the defects in the IC manufacturing among interconnections is analyzed and the model of the parasitical capacitance is given. The simulation results show that the extra electric defect makes the parasitical capacitance increase under given a space between interconnections in this paper.

Application of defect engineering to silicon Raman lasers and amplifiers

Silicon Raman lasers and amplifiers are the only silicon-based, monolithic device structures in which optical gain has been unambiguously observed. The main limitation on this gain is optical absorption due to free carriers, generated by two photon absorption. Here we explore a means to mitigate carrier effects via defect engineering. The optical and electrical properties of the defected silicon waveguides are modeled for both a uniform defect distribution and a remote, localized defect distribution. Simulation results indicate that a uniform defect distribution provides no improvement with any increase in net gain provided solely by surface modification. In contrast, for devices with remote defect volumes the reduction of carrier lifetime and limited optical absorption results in a significant improvement to net gain.

Crystallization of a Polypropylene Terpolymer Made by a Ziegler−Natta Catalyst: Formation of γ-phase

The appearance of the gamma phase of a polypropylene-containing terpolymer prepared with a Ziegler-Natta catalyst was analyzed, and a novel strategy to make a polypropylene terpolymer which contains a high content of the gamma phase was experimentally sought. Wide-angle X-ray scattering and differential scanning calorimetry measurements have been conducted to investigate gamma-phase formation. Using a mixed flow of both elongational and shear components (a foaming process), we could introduce a lot of nuclei in the melt by flow-induced crystallization (FIC). The large number of nuclei accounting for the structural disorder during crystallite formation was ascribed to be the reason for gamma-phase formation in the terpolymer crystallization. A uniform distribution of defects in the melt crystallization was the key to obtaining a high content of the gamma phase.

Deformation-induced nanocrystallization: A comparison of two amorphous Al-based alloys

Using conventional and high-resolution transmission electron microscopy (HRTEM), the effects of rolling at room temperature on the microstructures of amorphous Al90Fe5Gd5and Al86.8Ni3.7Y9.5were compared. In rolled Al90Fe5Gd5, nanocrystallites were observed at shear bands, whereas none were observed in rolled Al86.8Ni3.7Y9.5. When HRTEM was combined with with Fourier transform filtering, nanoscale, low-density defects were imaged. In Al90Fe5Gd5, the shear bands contain few defects, which are concentrated at the boundary zone between the shear bands and undeformed region, whereas in Al86.8Ni3.7Y9.5, the shear bands contain a uniform distribution of defects with a density higher than the undeformed region. The preferential precipitation of nanocrystallites in rolled Al90Fe5Gd5is attributed to a kinetic effect due to uniformly-distributed excess free volume in the shear bands.

Characterization of the charge transport uniformity of CdZnTe crystals for large-volume nuclear detector applications

Mounting evidence indicates that charge transport nonuniformity is one of the most important obstacles limiting the energy resolution of large-volume (/spl ges/1 cm/sup 3/) CdZnTe nuclear detectors. Better understanding of the defects responsible for charge transport nonuniformity is needed to develop advanced crystal growth processes producing CdZnTe crystals with more uniform defect distribution and, hence, more uniform charge transport. Better material screening techniques are also required to determine the charge transport uniformity of the CdZnTe crystals at the ingot or slice level in order to maximize the detector fabrication yields in manufacturing large-volume CdZnTe nuclear detectors. Here, we present results from a program aimed at implementing a defect and charge transport uniformity characterization process to provide feedback to crystal growth process development efforts and material selection for large-volume CdZnTe detector manufacturing. We first focused on recent results suggesting that structural nonuniformity of CdZnTe crystals around Te inclusions is responsible for electron transport nonuniformity and diminished energy resolution of large-volume CdZnTe coplanar-grid detectors. The spatial size and density distribution of Te inclusions was measured with infra-red microscopy. A charge transport mapping station employing a collimated alpha source was used to study the uniformity of carrier transport in CdZnTe crystals.

The effect of compression and tension on shear-band structure and nanocrystallization in amorphous Al 90Fe 5Gd 5: a high-resolution transmission electron microscopy study

Using both conventional and high-resolution transmission electron microscopy (HRTEM), the effect of bending at room temperature on the microstructure of amorphous Al 90Fe 5Gd 5 was investigated. In the compressive region, nanocrystallites formed at shear bands, along small cracks and at the fracture surface; in the tensile region, nanocrystallites were observed only at the fracture surface. Combining HRTEM with frequency filtering, low-density, nanoscale defects at shear bands were imaged. In the compressive region, both the shear bands and the undeformed matrix contain few defects. In the tensile region, there is a uniform distribution of defects within the shear bands. The preferential precipitation of nanocrystallites in the compressive region is attributed to a kinetic effect due to the uniformly distributed free volume in the shear bands. In contrast, the formation of the nanocrystallites at the fracture surfaces is likely due to adiabatic heating induced by fracture.