Stacking Fault Pyramids Research Articles

Formation of Stacking Fault Pyramid in Zirconium

Formation of Stacking Fault Pyramid in Zirconium

Formation of stacking fault pyramid in zirconium

Zirconium alloys were widely used as fuel cladding in nuclear reactors. Stacking fault pyramid (SFP) is an irradiation-induced defect in zirconium. In this work, the formation process of SFP from a hexagonal vacancy plate on basal plane is studied by molecular dynamics (MD) simulations. The results show that, during the SFP formation from a basal vacancy plate, the 12[0001] (<c>/2) dislocation is firstly dissociated into two partial dislocations 118<202¯3> and 19<1¯013>. The former one resides on the basal plane, while the latter one glides on the first-order pyramidal plane. The partials on adjacent pyramidal planes react further and form a partial dislocation on the pyramidal edge, i.e. 19[101¯3¯]+19[01¯13]→19[11¯00]. A critical edge length of the initial vacancy plate is observed, below which perfect SFP is formed while above which truncated SFP is formed. The critical edge length increases with the increasing temperature. Under a compressive stress, the SFP collapses into an <c>/2 dislocation loop and then becomes a faulted loop. Under shear stress, the formation of SFP is facilitated, i.e. the critical edge length increases with the increasing shear stress. The current work is useful for understanding the irradiation effects in zirconium alloys.

Molecular dynamics studies of lattice defect effects on tritium diffusion in zirconium

Tritium diffusion in α-Zr containing point defects such as vacancies or self-interstitial atoms (SIAs) is simulated using molecular dynamics. Point defects rapidly aggregate to form extended defects, such as 3D nanoclusters and Frank loops. The geometry of extended defects is affected by the presence of tritium. At low temperature and in the absence of tritium, vacancies aggregate to form stacking fault pyramids. Addition of tritium at these temperatures promotes aggregation of vacancies to form 3D nanoclusters, within which the tritium concentration can be sufficiently high to suggest that these defects may serve as nucleation sites for hydride precipitation. Trapping of tritium in vacancy nanocluster reduces the calculated bulk diffusivity by an amount proportional to the vacancy concentration. At high temperature, vacancy clusters change shape to form planar basal dislocation loops, which bind tritium less strongly, leading to a sharp reduction in the fraction of trapped tritium and a corresponding increase in tritium diffusivity at high temperature. In contrast, SIAs increase tritium diffusion through α-Zr. Analysis of atomic trajectories shows that tritium does not interact directly with SIAs. Diffusion enhancement is instead related to expansion of the lattice.

Correlation of early-stage growth process conditions with dislocation evolution in MOCVD-based GaP/Si heteroepitaxy

To identify the complex relationships between early-stage growth processes and the resultant defect microstructure in GaP/Si heteroepitaxy, a holistic study of several key metal–organic chemical vapor deposition (MOCVD) parameters was conducted, focusing on Si surface preparation and GaP atomic layer epitaxy (ALE) based nucleation processes. Crystalline defects related to the lattice mismatch and/or interfacial heterovalency, namely misfit dislocations (MD), threading dislocations (TD), and stacking fault pyramids (SFP), were quantitatively characterized via electron channeling contrast imaging (ECCI) and correlated against the different process variations. Choice of Si surface preparation method between the two examined (dilute SiH4 annealing versus Si2H6 based homoepitaxy) had little impact on resultant GaP film morphology and defect content, whereas differing GaP ALE nucleation conditions produced much more substantial changes. In particular, the initial precursor species (tert-butylphosphine versus triethygallium) and ALE cycle purge times both yielded significant influence over threading dislocation densities (TDD) in thin (100 nm), post-critical thickness GaP/Si films, with TDD spanning two orders of magnitude, from 6.7 × 107 cm−2 to 7.1 × 105 cm−2, depending on the specific process conditions employed. SFP densities were also found to follow a similar trend, ranging from 2.0 × 107 cm−2 to 1.8 × 105 cm−2, but with no apparent causal relationship between SFP density and TDD. To help explain the dramatic differences observed, detailed, large-area MD network characterization was used to provide statistically-relevant quantitative analyses of the critical dislocation dynamics (introduction rates and glide velocities) associated with the different process variants. These extracted values are then correlated against the ALE process variants to provide insight into the potential mechanistic roles of the different growth processes.

Bufferless Epitaxial Growth of GaAs on Step-Free Ge (001) MesaSupported by the National Natural Science Foundation of China (Grant Nos. 61975230, 61635011, 61804177 and 11804382), the National Key Research and Development Program of China (Grant No. 2018YFB2200104), Beijing Municipal Science and Technology Commission (Grant No. Z191100004819010), and the Key Research Program of Frontier Sciences, CAS (Grant No. QYZDB-SSW-JSC009).

GaAs/Ge heterostructures have been employed in various semiconductor devices such as solar cells, high-performance CMOS transistors, and III–V/IV heterogeneous optoelectronic devices. The performance of these devices is directly dependent on the material quality of the GaAs/Ge heterostructure, while the material quality of the epitaxial GaAs layer on the Ge is limited by issues such as the antiphase domain (APD), and stacking-fault pyramids (SFP). We investigate the epitaxial growth of high-quality GaAs on a Ge (001) mesa array, via molecular beam epitaxy. Following a systematic study of the Ge terrace via an in situ scanning tunneling microscope, an atomically step-free terrace on the Ge mesa measuring up to 5 × 5 μm2 is obtained, under optimized growth conditions. The step-free terrace has a single-phase c (4 × 2) surface reconstruction. The deposition of a high-quality GaAs layer with no APD and SFP is then achieved on this step-free Ge terrace. High-resolution transmission electron microscopy and electron channel contrast image characterizations reveal the defect-free growth of the GaAs layer on the step-free Ge mesa. Furthermore, InAs quantum dots on this GaAs/Ge mesa reveal photoluminescent intensity comparable to that achieved on a GaAs substrate, which further confirms the high quality of the GaAs layer on Ge.

Electron channeling contrast imaging investigation of stacking fault pyramids in GaP on Si nucleation layers

Abstract Thin gallium phosphide layers were deposited on (0 0 1) Silicon surfaces via organometallic vapor phase epitaxy and characterized by electron channeling contrast imaging (ECCI). Stacking fault pyramids at a density of up to 6 × 107 cm−2 were identified in the GaP nucleation layer by varying the diffraction conditions. We show that these defects originate at the GaP/Si interface and propagate on all four {1 1 1} planes. We observed that such stacking fault pyramids interact with the gliding of misfit dislocations during lattice-mismatched growth and enhance the threading dislocation density. The initial pulsing of TEGa and TBP during the nucleation of GaP on Silicon has been found to strongly influence the formation of those pyramidal defects. Changing the number of pulse cycles allowed us to lower their density by two orders of magnitude from 6 × 107 cm−2 to 4 × 105 cm−2.

A new scenario for ‹c› vacancy loop formation in zirconium based on atomic-scale modeling

The growth of zirconium alloys under irradiation is a phenomenon experimentally identified and associated with the development beyond a threshold dose of dislocation loops with vacancy character having a Burgers vector with a component parallel to the c axis. In this work, by combining atomic simulations (DFT and empirical potential) and continuous modeling, we show that prismatic stacking fault pyramids or bipyramids whose base rests on the basal plane of the hcp structure are likely precursors to the formation of ‹c› vacancy loops. In other words, these would not be formed by progressive accretion of vacancies but rather by collapse of the pyramids or bipyramids beyond a certain size. This mechanism could explain the fact that the ‹c› vacancy loops are never observed below a size of the order of 10 nm and their appearance at high fluence.

Shock-induced stacking fault pyramids in Ni/Al multilayers

The formations of stacking fault tetrahedra in {111}fcc/{111}fcc multilayers have been studied extensively, but the researches related to {001}fcc/{001}fcc interfaces can rarely be found in the literature. Using molecular dynamics (MD) simulations, the shock-induced dislocation structure in Ni/Al multilayers was studied in this article, the stacking fault pyramids (SFPs) initiated from Ni/Al interface was firstly observed, and the corresponding mechanism was explored. It was shown that the Shockley partial dislocations dissociated from the rectangular misfit dislocations along Ni/Al interface are first emitted into the Al interlayer and subsequently intersect with each other to form SFPs. With the propagation of the shock front in the multilayers, two adjacent SFPs interact with each other and form Lomer-Cottrell locks. The dislocation sheet composed of SFPs and Lomer-Cottrell locks can act as a barrier to dislocation transmission. The effect of shock piston velocity on the nucleation site of dislocation was also studied. It was shown that dislocations would originate from the interface at a low piston speed (<1.1km/s), and appear at the shock front if the piston speed exceeds 1.2km/s.

Initiation strategies for simultaneous control of antiphase domains and stacking faults in GaAs solar cells on Ge

Incorporating a Ge junction into a lattice-matched GaInP/GaAs/GaInNAsSb triple-junction cell grown by molecular beam epitaxy (MBE) could enable concentrated efficiencies of ∼50%. Epitaxial integration allows lift-off and wafer bonding steps to be avoided, but growth of III–Vs on Ge by MBE can lead to antiphase domains (APD) and stacking fault pyramids (SFP), both of which diminish solar cell performance. Initiating growth by migration-enhanced epitaxy (MEE) is typically cited as necessary to obtain high-quality III–Vs on Ge. In this work, the authors show that typical MEE growth conditions force a compromise between APD height (hAPD) and SFP density (ρSFP). As APDs can readily self-terminate while SFPs cannot, a two-step initiation strategy was employed, where MEE is performed under conditions that minimize ρSFP followed by low-temperature MBE conditions that encourage APD termination. By doing so, the authors obtained ρSFP &amp;lt; 104 cm−2 with hAPD ≤ 57 nm. The authors also demonstrated that high-quality GaAs on Ge can be grown without MEE initiation using conventional MBE conditions, though with taller APDs. Both the two-step initiation and conventional MBE initiation yield GaAs cells with high open-circuit voltage and internal quantum efficiency, demonstrating promising paths toward epitaxial integration of high-efficiency solar cells on Ge.

Diffusion of point defects, nucleation of dislocation loops, and effect of hydrogen in hcp-Zr: Ab initio and classical simulations

Diffusion of point defects, nucleation of dislocation loops, and the associated dimensional changes of pure and H-loaded hcp-Zr have been investigated by a combination of ab initio calculations and classical simulations. Vacancy diffusion is computed to be anisotropic with Dvac,basal=8.6×10−6e−Q/(RT) (m2/s) and Dvac,axial=9.9×10−6e−Q/(RT) (m2/s), Q=69 and 72kJ/mol for basal and axial diffusion, respectively. At 550K vacancy diffusion is about twice as fast in the basal plane as in a direction parallel to the c-axis. Diffusion of self-interstitials is found to be considerably faster and anisotropic involving collective atomic motions. At 550K diffusion occurs predominantly in the a-directions, but this anisotropy diminishes with increasing temperature. Furthermore, the diffusion anisotropy is very dependent on the local strain (c/a ratio). Interstitial H atoms are found to diffuse isotropically with DH=1.1×10−7e−42/(RT) (m2/s). These results are consistent with experimental data and other theoretical studies. Molecular dynamics simulations at 550K with periodic injection of vacancies and self-interstitial atoms reveal the formation of small nanoclusters, which are sufficient to cause a net expansion of the lattice in the a-directions driven by clusters of self-interstitials and a smaller contraction in the c-direction involving nanoclusters of vacancies. This is consistent with and can explain experimental data of irradiation growth. Energy minimizations show that vacancy c-loops can collapse into stacking-fault pyramids and, somewhat unexpectedly, this is associated with a contraction in the a-directions. This collapse can be impeded by hydrogen atoms. Interstitial hydrogen atoms have no marked influence on self-interstitial diffusion and aggregation. These simulations use a new Zr–H embedded atom potential, which is based on ab initio energies.

Transmission Electron Microscopy Study of Stacking Fault Pyramids Formed in Multiple Oxygen Implanted Silicon-on-Insulator Material

The microstructure of various shapes of stacking fault pyramids (SFPs) formed in multiple implant/anneal Separation by Implanted Oxygen (SIMOX) material were investigated by plan-view and cross-sectional transmission electron microscopy. In the multiple implant/anneal SIMOX, the defects in the top silicon layer are confined at the interface of the buried oxide layer at a density of <TEX>${\sim}10^6\;cm^{-2}$</TEX>. The dominant defects are perfect and imperfect SFPs. The perfect SFPs were formed by the expansion and interaction of four dissociated dislocations on the {111} pyramidal planes. The imperfect SFPs show various shapes of SFPs, including I-, L-, and Y-shapes. The shape of imperfect SFPs may depend on the number of dissociated dislocations bounded to the top of the pyramid and the interaction of Shockley partial dislocations at each edge of {111} pyramidal planes.

Runaway lattice-mismatched interface in an atomistic simulation of femtosecond laser irradiation of Ag film–Cu substrate system

The atomic mixing and structural transformations in a Ag film–Cu substrate system irradiated by a femtosecond laser pulse are investigated in a simulation performed with a model that couples the classical molecular dynamics method with a continuum-level description of the laser excitation and subsequent relaxation of the conduction-band electrons. The higher strength of the electron–phonon coupling in Cu compared to Ag results in preferential sub-surface heating and melting of the Cu substrate. The melting is followed by fast cooling and rapid resolidification occurring under conditions of strong undercooling below the equilibrium melting temperatures of Cu and Ag. The rapid resolidification results in a complex structure of the interfacial region, where the lattice-mismatched interface is separated from the Ag–Cu mixing region by an intermediate pseudomorphic bcc Cu layer that grows epitaxially on the (001) face of the fcc Ag film during the final stage of the resolidification process. The new lattice-mismatched interface has a three-dimensional structure consisting of a periodic array of stacking fault pyramids outlined by stair-rod partial dislocations. The intermediate bcc layer and the stacking fault pyramid structure of the mismatched interface are likely to present a strong barrier for dislocation propagation, resulting in the effective hardening of the layered structure treated by the laser irradiation. The concentration profiles in the atomic mixing region are substantially wider compared to the width of the equilibrium Cu–Ag interface and have a pronounced asymmetric shape that reflects the preferential melting of the Cu substrate.

Structural analysis of life tested 1.3 μm quantum dot lasers

We present the results of an accelerated life test study of quantum dot lasers operating at 1310 nm. The devices were run at 1 and 2 kA/cm2 (∼10 and ∼70 times Ith, depending on facet coatings), at temperatures of 80 and 100 °C for 1350 h. Some devices, particularly those with higher current densities, showed significant drops in output power and increase in threshold current over this time. The devices were examined using electroluminescence, which shows nonradiative recombination centers in the active region of the device as dark spots. A clear correlation between the density of dark spots and degradation is observed. The defect structure responsible for the dark spots has been identified using conventional and high-resolution cross-section transmission electron microscopy of selected structures. The defects consist of an inverted stacking fault pyramid or microtwin enclosing the dot. The more extensive defects observed after the life test are consistent with their growth by climb, i.e., addition and/or removal of point defects. It is proposed that quantum dot devices show enhanced resistance to the growth of these defects in comparison with quantum well lasers.

A transmission electron microscopy study of defects formed through the capping layer of self-assembled InAs∕GaAs quantum dot samples

Plan-view and cross-sectional transmission electron microscopy have been used for a detailed study of the defects formed in capped InAs∕GaAs quantum dot (QD) samples. Three main types of defects, V-shaped defects, single stacking faults, and stacking fault pyramids, were found to form under growth conditions that led to either very large, indium enriched, or coalesced islands. All three types of defects originate at the buried quantum dot layer and then travel through the GaAs cap to the surface on the {111} planes. The V-shaped defects were the most common and typically consisted of two pairs of closely spaced 60° Shockley partials with a ⟨211⟩ line direction. The two pairs originate together at the buried QD layer and then travel in “opposite” directions on different {111} planes. The second type of defect is the single stacking fault which consists of a single pair of partial dislocations separated by an ≈50nm wide stacking fault. Finally, both complete and incomplete stacking fault pyramids were observed. In the case of the complete stacking fault pyramid the bounding dislocations along the [110], [11¯0], [101¯], and [101] directions were identified as stair rods. A possible mechanism for the stacking fault pyramid formation, which can also account for the creation of incomplete stacking fault pyramids, is presented.

Molecular dynamics simulation of single asperity contact

We present a series of molecular dynamics simulations of single asperity contact and deformation that occurs as two rough surfaces are brought together, loaded and then separated. We monitor the force between the two surfaces, the total potential energy, the Q6 order parameter, the effective minimum contact radius and the conductance (using the modified Sharvin equation). The deformation is observed using the local bond order parameter. During loading, the force–displacement relation exhibits a sawtooth form, associated with repetitive generation and motion of partial dislocations and the concomitant creation and annihilation of stacking fault pyramids. Unloading is characterized by an extended elastic deformation regime followed by plastic deformation of a type that is distinct from that in loading. The qualitative features of the force, contact area and conductance versus displacement plots are in excellent qualitative agreement with experimental observations. The simulations and experiments both show that Johnson–Kendall–Roberts theory should not be applied to describe asperity loading, but is applicable to the case of unloading, where the deformation is nearly elastic.

In-Situ Transmission Electron Microscopic Study of Perovskite-type Niobate Nanosheets under Electron-Irradiation and Heating

In-situ transmission electron microscopic (TEM) observation has been performed in the study of effects of electron irradiation and heating on the perovskite-type (TBA, H)Ca2Nb3O10 nanosheet material. Radiation-induced migration of vacancies was found to play an important role in the generation of various structural defects under electron irradiation at room temperature. Irradiation-induced defects include loops and stacking fault pyramids (SFP). SFPs are the new structural features since they neither have been discovered in bulk perovskite materials nor have been generated as extended defects under irradiation. Extraction of interlayer molecules via thermal dynamic effects of electron illumination has produced a variety of domain boundaries including antiphase boundaries, δ-boundaries, and amorphous interfaces due to irradiation damage. The present nanosheet material has also been heated to ∼500 °C during the in-situ TEM observation. Periodic antiphase domain boundary structures have been observed after h...

Stacking-fault pyramids formed in perovskite-type niobate nanosheet aggregates under electron irradiation

We have observed stacking-fault pyramids (SFPs) formed as extended defects in (TBA, H)Ca2Nb3O10 perovskite-type nanosheet aggregates (TBA = tetrabutyl-ammonium) under electron irradiation in a transmission electron microscope. Using the classical SFP as reference, characterization of the structure is discussed. However, irradiation-induced SFPs are not stable and quickly transform to loops. The formation under irradiation of SFPs rather than stacking-fault tetrahedra indicates a strong dependence of extended defect structures on the host crystal structure.

Transmission electron microscopy study of stacking-fault trapezoids and stacking-fault tubes in ZnSe/GaAs(001) pseudomorphic epitaxial layers

Using the classical stacking-fault pyramid as an internal reference, the fault planes and stair-rod dislocations in stacking-fault trapezoids and stacking-fault tubes in pseudomorphic ZnSe/GaAs(001) epitaxial layers have been characterized. The acute-obtuse stair-rod dislocation pair in a stacking-fault trapezoid form an extended C dislocation dipole. Stacking faults with a lozenge-shaped cross-section in a stacking-fault tube intersect to give rise to dislocation dipoles of alternating acute-obtuse stair-rod dislocations. The extended dislocation dipoles of the stacking-fault trapezoids and stacking-fault tubes can act as diffusion channels for pipe diffusion of point defects during the degradation process.

Determination of the orientation of a stacking fault by large-angle convergent-beam electron diffraction (LACBED)

The asymmetrical bending of the stacking fault fringes in the dark-field large angle convergent beam electron diffraction patterns is reported and utilized to determine the sense of the stacking fault. A stacking fault pyramid in GaAs is studied by analysing the enteraction between the stacking fault and the diffraction lines in the LACBED patterns so as to determine the intrinsic/extrinsic stacking faults and the displacement vector R.

Transmission electron microscopy study of stacking faults and the associated partial dislocations in pseudomorphic epilayers of ZnSe/GaAs(001)

V-shaped double triangular faults in ZnSe/GaAs epilayers have been identified to be incomplete stacking fault pyramids and single stacking faults identified as stacking fault trapezoids. It is confirmed by transmission electron microscopy that 〈101〉 open boundaries of incomplete stacking fault pyramids are 30° Shockley partial dislocations. The closely spaced parallel acute and obtuse stair-rod dislocation pair in stacking fault trapezoids form a dislocation dipole.