Effects Of Misfit Dislocations Research Articles

Predictive modeling of misfit dislocation induced strain relaxation effect on self-rolling of strain-engineered nanomembranes

Combining atomistic simulations and continuum modeling, the effects of misfit dislocations on strain relaxation and subsequently self-rolling of strain-engineered nanomembranes have been investigated. Two representative material systems including (GaN/In0.5Ga0.5N) of wurtzite lattice and II–VI materials (CdTe/CdTe0.5S0.5) of zinc-blend lattice were considered. The atomistic characteristics of dislocation and the resulting lattice distorting were first determined by generalized-stacking-fault energy profile and disregistry function obtained through Peierls-Nabarro model. Those properties were then used to calculate the accurate mismatch strain of those nanomembranes with the presence of dislocations, and as inputs into von-Karman shell theory to quantitatively evaluate the effects on self-rolling curvature and anisotropy. The theoretical results were further confirmed by atomistic simulations of different crystal geometries and dislocation configurations. Our results provide essential theoretical insights towards prediction and design of rollup configurations for strain-engineered nanomembranes containing crystalline defects.

Effect of Misfit Dislocations in Junctions and Base Thickness on Latch-up Characteristics in Two-Terminal Vertical Thyristor-Based Capacitorless Memory

Conventional dynamic random access memory (DRAM) has been facing a severe challenge to scale down to 10 nm size. The cell capacitor should be able to store sufficient charges of 25~30 fF/cell. In order to meet this requirement, high aspect ratio of cell capacitor is inevitable, resulting in capacitors leaning into each other. To overcome this issue coming from capacitor, the vertical thyristor-based two-terminal capacitorless memory was proposed as a promising candidate to replace current DRAM, which consists of p++-anode / n+-base / p+-base / n++-cathode vertical structure using conventional Si technology. The two-terminal vertical thyristor-based capacitorless memory cell having p++-anode / n+-base / p+-base / n++-cathode vertical structure is easy to construct cross-point memory array cells, as shown in Figure 1. Furthermore, Two-terminal vertical thyristor-based capacitorless memory array cells can be operated without selector since sneak current is suppressed by latch-down process. In our study, three types of two-terminal vertical thyristor-based capacitorless memory cells consist of top-p++-anode / n+-base / p+-base / bottom-n++-cathode with 80-nm-thick base region (PNPN80), top-p++-anode / n+-base / p+-base / bottom-n++-cathode with 160-nm-thick base region (PNPN160), and top-n++-cathode / p+-base / n+-base / bottom-p++-anode with 160-nm-thick base region (NPNP160) were fabricated, individually. The dopant concentration profile of each device was analyzed by secondary ion mass spectroscopy (SIMS), as shown in Figure 2. By correlating HR TEM images with SIMS analyses, misfit dislocations were observed at the depth where dopant pile-up was found, as shown in Figure 3. Figure 4 (a), (b) and (c) show I-V measurements of PNPN80, PNPN160 and NPNP160, respectively. The difference of latch-up voltages between Figure 4 (b) and (c) can be explained by carrier life-time difference. NPNP160 had less misfit dislocations compared to PNPN160, resulting in longer excess carrier life-time, since dislocations act as recombination centers. Thus, NPNP160 needed smaller anode voltage to induce sufficient excess carriers in base regions, which had smaller latch-up voltage than PNPN160. In addition, the difference of latch-up voltages between Figure 4 (a) and (b) resulting from base thickness difference can be explained by electric field. The electric field of PNPN160 is weaker than that of PNPN80 in base regions when equivalent anode voltage is applied. Therefore, PNPN160 needed larger anode voltage to lower the injection barrier of excess carriers. Besides, the effect of misfit dislocations and base dopant concentration on memory margin and off-state leakage current will be discussed. Finally, the memory cell pulse operations will be exhibited as well. Acknowledgment * This research was supported by Brain Korea 21 PLUS Program in 2018, the MOTIE (Ministry of Trade, Industry & Energy 10069063) and KSRC (Korea Semiconductor Research Consortium) support program for the development of the future semiconductor device. Figure 1

Effect of misfit dislocations on strength of pearlite microstructure

Effect of misfit dislocations on strength of pearlite microstructure

Effect of Misfit Dislocation on Li Diffusion and Stress in a Phase Transforming Spherical Electrode

The Li-ion battery electrode materials generally experience significant volume change during lithium diffusion. These volume changes lead to diffusion induced stress. Diffusion induced stress(DIS) will cause fracture and nucleation in the electrode. Many electrode materials undergo formation of two or more phases during lithium insertion. By analyzing the process of lithiation, the DIS in phase transforming electrodes using a core-shell model structural is investigated. The new model considering the misfit dislocation effect is established to analyze the stress distribution in the nanoparticle electrode. We observe that the magnitude of DIS can be affected by the misfit dislocation. In addition, the concentration jumps is also affected by the misfit at phase boundaries that result in stress discontinuities, which in turn can cause cracking. The influence of the mechanical properties of the two phases on stress evolution, stress discontinuity, and misfit dislocation effect are clarified. What's more, the Tresca stress is also expounded in the Li-ion battery. The effect of misfit dislocation and the phase transforming on the Tresca stress is clarified. The trends obtained with the model may be used to help tune electrode materials with appropriate interfacial and the misfit dislocation so as to increase the durability of battery electrodes.

Residual stresses in thin film systems: Effects of lattice mismatch, thermal mismatch and interface dislocations

This paper explores the mechanisms of the residual stress generation in thin film systems with large lattice mismatch strain, aiming to underpin the key mechanism for the observed variation of residual stress with the film thickness. Thermal mismatch, lattice mismatch and interface misfit dislocations caused by the disparity of the material layers were investigated in detail. The study revealed that the thickness-dependence of the residual stresses found in experiments cannot be elucidated by thermal mismatch, lattice mismatch, or their coupled effect. Instead, the interface misfit dislocations play the key role, leading to the variation of residual stresses in the films of thickness ranging from 100nm to 500nm. The agreement between the theoretical analysis and experimental results indicates that the effect of misfit dislocation is far-reaching and that the elastic analysis of dislocation, resolved by the finite element method, is sensible in predicting the residual stress distribution. It was quantitatively confirmed that dislocation density has a significant effect on the overall film stresses, but dislocation distribution has a negligible influence. Since the lattice mismatch strain varies with temperature, it was finally confirmed that the critical dislocation density that leads to the measured residual stress variation with film thickness should be determined from the lattice mismatch strain at the deposition temperature.

1538 異種材界面に沿った応力場におけるミスフィット転位の影響

異種材界面においては,格子定数の差が原因でミスフィット転位が生じる ミスフィット転位か異種材界面周辺の応力場に与える影響は非常に大きく,その影響を弾性論の見地から評価することは材料開発の観点からも重要となる 本研究では,異種材界面の応力場の弾性解を,整合界面を仮定した場合の弾性ひずみ場と,ミスフィット転位の影響による弾性ひずみ場とを重ね合わせることによって求めた また,異種材界面の応力場を,分子静力学法を用いた原子シミュレーションによって求め,弾性解との比較を行うことにより,弾性解の妥当性を検証した 検証にはレナードジョーンズポテンシャルで表すことができる仮想材料を用い,異種材間の格子定数の差が33%の場合と10%の場合の二通りで解析を行うことによって,弾性解の精度に対して格子定数の差がどのように影響するのかを検証した ミスフィット転位の弾性解は, Stroh Formalism を基礎式とする異方性弾性論を用いて求め,材料を等方性として近似した場合と異方性を考慮した場合の比較も行った.

Morphologies of epitaxial islands on a lattice-misfitted substrate

Under certain growth conditions for systems with a film/substrate lattice misfit, the deposited material is known to aggregate into island-like shapes. We have obtained an analytical expression of the total free energy, which consists of strain energy, surface energy and interfacial energy of a coherent island/substrate system, and the change of equilibrium aspect ratio versus the volume of the island and the misfit of lattices in the system, which provides a broad perspective on island behaviour. These then were used to study the equilibrium shapes of the system. The results show that in order to minimize the total free energy, a coherent island will have a particular height-to-width aspect ratio, called equilibrium aspect ratio, that is a function of the island volume and misfit. The aspect ratio is increased with increasing island volume at a fixed misfit, and with increasing misfit strain between the island and substrate at a fixed island volume. Moreover, the effect of misfit dislocation on the equilibrium shape of the island is also examined. The results obtained are in good agreement with experiment of observations and thus can serve as a basis for interpreting the experiments.

Misfit dislocations in nanocomposites with quantum dots, nanowires and their ensembles

We review theoretical concepts and experimental results on the physics of misfit dislocations in nanocomposite solids with quantum dots (QDs) and nanowires (quantum wires). Special attention is paid to thermodynamic theoretical models of formation of misfit dislocations in QDs and nanowires, including composite core–shell nanowires. The effects of misfit dislocations on the film growth mode during heteroepitaxy and phase transitions in QD systems are analysed. Experimental results and theoretical models of the ordered spatial arrangement of QDs growing on composite substrates with misfit dislocation networks are discussed. The influence of subsurface dislocations in composite substrates on the nucleation of QDs and nanowires on the substrate surface is considered. Models of misfit strain relaxation and dislocation formation in nanofilms on compliant substrates are also reviewed.

Lattice location of implanted tellurium in GaN heteroepitaxial films

The lattice site of implanted tellurium in gallium nitrideheteroepitaxial films grown on sapphire substrates has been studied. GaNsamples were implanted with 400 keV 130Te ions at 300 °C. Theion dose was 1.5×1015 cm-2. The samples were analysedwith Rutherford backscattering spectrometry channelling using 2.0 MeV4He ions. The maximum Te concentration was 0.2 at.%. Based onmeasured channelling angular scans for Ga and Te along the ⟨0001⟩ and ⟨101̅1⟩ axial directions,analysed by comparison with Monte Carlo simulations, it was concluded thatin the as-implanted sample about 70% of Te atoms were on the Ga atomlattice sites, slightly displaced. The measured ⟨101̅1⟩ scan showed no evidence of substitutional Te on Nlattice site. The post-implantation annealing at 900 °C was foundto decrease the substitutional fraction of Te while little recovery of thehost lattice was observed. This indicates an interaction process betweenTe atoms and implantation-induced lattice defects. No migration of Teduring annealing at 900 °C was observed. In addition, the effect ofmisfit dislocations in the host lattice on the channelling yield wasvisible in the ⟨101̅1⟩ angular scan measured at a47° tilt angle.}\\fnm{3}{Present address: Vaisala Oyj, FIN-00421 Helsinki, Finland.

Relaxation enhancing interlayers (REIs) in threading dislocation reduction

An approach is proposed to understand the role of relaxation enhancing interlayers in the evolution of threading and misfit dislocations in growing lattice mismatched films. The approach based on the ability of misfit dislocations to reduce their local stresses due to the influence of relaxation enhancing interlayers. The diminishing of these local stresses eliminates blocking effect of misfit dislocations and allows efficient motion of threading dislocations with consequent overall strain relaxation in the heteroepitaxial film. An analytical model is developed which quantitatively describes the dislocation density changes during growth with relaxation enhancing interlayers. The applications of the approach to lattice engineered substrates (exploring the techniques of internal oxidation) and weakly bonded compliant layers are discussed.

Passivation of misfit dislocations by atomic hydrogen irradiation in lattice-mismatched heteroepitaxy

We investigated the passivation effect of misfit dislocations in an (InAs)1(GaAs)4 strained short-period superlattice grown on GaAs with atomic hydrogen (H) by transmission electron microscopy and electron-beam-induced current. The misfit dislocations in the 〈1 1 0〉 directions are generated at the heterointerface in order to accommodate the misfit strain. They were effectively passivated by atomic H irradiation during the growth and cooling processes. Furthermore, it was found that the misfit dislocation in the [1 1 0] direction was effectively passivated rather than that in the [1 1 0] direction. The desorption temperature of atomic H from the [1 1 0] misfit dislocation was higher than that from the [1 1 0] misfit dislocation. These phenomena could be attributed to the fact that the bonding strength of As-H is larger than that of GaH.

Molecular beam epitaxy growth of resonant-cavity separate-absorption-and-multiplication avalanche photodiodes

We discuss the design and growth considerations of a resonant-cavity separate-absorption- and-multiplication avalanche photodiode and the effects of misfit dislocations that originate in the lattice-mismatched absorption layer. This device is grown on a GaAs substrate using molecular beam epitaxy and is designed to detect light near wavelengths of 900 nm. Such photodetectors have exhibited the following characteristics: an external quantum efficiency of nearly 80%, a spectral linewidth of less than 8 nm, an avalanche gain in excess of 40, and low dark current. In addition, a low excess noise factor corresponding to 0.2≤k≤0.3 has been achieved.

Effect of misfit dislocations on leakage currents in strained multiquantum well structures

The reverse leakage current in strained multiple quantum well (MQW) p-i-n structures has been measured for a range of different dimensions and strain. The magnitude of the leakage current is found to be dependent on the average strain of the MQW, the total MQW thickness and the thickness of the capping layer. Plan view transmission electron microscopy shows that misfit dislocation arrays form primarily at the upper and lower MQW interfaces and the total density of these determine the leakage current.

MESFET performance and limitations of optimized GaAs strained buffer layer grown on InP by molecular beam epitaxy

The effects of the thickness of the GaAs strained buffer layer on the electrical performance of GaAs field effect transistors are demonstrated. The GaAs MESFET has been fabricated on GaAs/InP epitaxial material grown by molecular beam epitaxy. From experimental results a minimum thickness of the strained buffer layer appears to be needed to compensate the effect of misfit dislocation due to lattice mismatch between the GaAs and InP substrate. The minimum required thickness amounts to 2 μm. The electron mobility and transconductance behavior demonstrate the effects of thickness of the strained buffer layer.

Electronic structure and total energy of diamond/BeO interfaces

Electronic structure calculations are used to study the bonding at diamond/BeO interfaces. The {110} interface between zinc blende BeO and diamond is used as a representative model for general reconstructed interfaces characterized by an equal amount of Be–C and O–C bonds. The interface energy is calculated to be 2 J/m2 and combined with the estimated free surface energies to obtain an estimate of the adhesion energy. It is found to be close to the adhesion of BeO to itself, but somewhat lower than that of diamond to itself. The effects of the 7% lattice mismatch on the total energy and the band structure for a biaxially strained pseudomorphic diamond film are investigated. The effect of misfit dislocations, expected to occur for thicker films, on the adhesion energy is estimated to be lower than 10%. The bulk properties, such as equilibrium lattice constant, bulk modulus, cohesive energy, and band gap of BeO are shown to be in good agreement with experimental values and previous calculations. The valence-band offset is calculated to be 3.9 eV and found to take up most of the large band gap discontinuity. The nature of the bonding is discussed in terms of the local densities of states near the interface. The interface localized features are identified in terms of Be–C and O–C bonding and antibonding states.

High-spatial-resolution photoluminescence studies on misfit dislocations in lattice-mismatched III-V heterostructures

The effect of misfit dislocations on the electro-optical behaviour of strained III-V heterostructures has been studied by high-resolution spatially resolved photoluminescence (PL) imaging. Both epitaxial layers under tensile stress (in this study GaAs on InGaAs) as well as layers under compressive stress (InGaAs on GaAs) have been examined. Dislocation lines in both (011) directions are observed in both systems. In contrast to the compressive system, where no as-grown morphological asymmetry is observed, the dislocations in the tensile system have been preferably formed in the (011) direction. This is explained by assuming (i) that the relaxation of the strained layer is initiated by the most mobile dislocation in GaAs, which is the arsenic glide As(g) dislocation, and (ii) that cross-slip of these As(g) dislocations is only possible in the compressive system. PL imaging reveals the presence of many non-radiative deep levels around the dislocation lines and also shows an opto-electronic asymmetry of the two (011) directions for both systems, in contrast to the as-grown morphology. For the compressive system this is explained by the difference in depletion between the dislocation lines along the (011) direction, of which the core is believed to consist of Ga atoms, and those along the (011) direction, formed with As atoms. For the tensile system just the opposite holds. DSL (defect-revealing) etching confirms the results obtained with PL imaging.

Tilting of lattice planes in InP epilayers grown on miscut GaAs substrates: the effect of initial growth conditions

The relative misorientation (tilt) between the epilayer and substrate (400) lattice planes of InP epilayers grown on (100) GaAs substrates misoriented towards the (100) plane was studied by high resolution X-ray diffraction. The epilayers were grown by gas-source molecular beam epitaxy. For the growth temperature of 490–500°C, the direction of the relative tilt was nearly coincident with the direction of the lattice plane tilting of the substrate, according to previous models. In contrary, when a buffer layer was deposited at a lower temperature of 400–450°C prior to growth, an azimuthal rotation of about 45° was found between the directions of the relative tilt and the substrate lattice plane tilting. In order to explain the results, a temperature-dependent anisotropic nucleation model is proposed. The effect of misfit dislocations is also considered.

Structural properties of GaAs/InGaAs/GaAs (001) and InGaAs/GaAs (001) heterostructures and correlation with electronic properties

In this study we have examined a series of strained heteropeitaxial GaAs/InGaAs/GaAs and InGaAs/GaAs structures, both on (001) GaAs substrates. These heterostructures are potentially very interesting from a device standpoint because of improved band gap properties (InAs has a much smaller band gap than GaAs so there is a large band offset at the InGaAs/GaAs interface), and because of the much higher mobility of InAs. However, there is a 7.2% lattice mismatch between InAs and GaAs, so an InxGa1-xAs layer in a GaAs structure with even relatively low x will have a large amount of strain, and misfit dislocations are expected to form above some critical thickness. We attempt here to correlate the effect of misfit dislocations on the electronic properties of this material.The samples we examined consisted of 200Å InxGa1-xAs layered in a hetero-junction bipolar transistor (HBT) structure (InxGa1-xAs on top of a (001) GaAs buffer, followed by more GaAs, then a layer of AlGaAs and a GaAs cap), and a series consisting of a 200Å layer of InxGa1-xAs on a (001) GaAs substrate.

Effects of misfit dislocations and thermally induced strain on the film properties of heteroepitaxial GaAs on Si

GaAs hereroepitaxial films on Si contain a high number of misfit dislocations and large internal stresses due to the lattice and thermal mismatch of the two materials. These greatly affect the structural, optical, and electrical properties of the films. We report a study of these effects in films grown by metalorganic chemical vapor deposition using x-ray diffraction, sample curvature, photoluminescence, and carrier concentration measurements. The x-ray data indicate that the lattice misfit strain is almost entirely relieved by the generation of dislocations, but that the difference in thermal expansion between the film and substrate causes significant tetragonal distortion of the GaAs lattice which results in wafer bowing and, for thicker GaAs layers, film cracking. Wafer bowing was successfully eliminated by growth of GaAs films on both sides of the Si substrate. Photoluminescence spectra of crack-free GaAs layers indicated that the thermally induced strain was distributed in a nonuniform but continuous manner throughout the film. The magnitude of the strain, as determined from x-ray diffraction, wafer curvature, and photoluminescence spectroscopy, was consistently 10% lower than the value calculated from simple thermal relaxation. Finally, for large numbers of misfit defects (>108 cm−2) the electrical properties of the sample were found to be correlated to the mean dislocation density of the GaAs film.

Direct scattering of channelling He ions at dislocations

The effects of misfit dislocations in epitaxial PbS/PbSe bi-crystals on the channeling of energetic He ions were studied. The direct scattering of channelling ions at dislocations was detected and its dependence on the energy of the incident ions was measured.