Characterization of III-V compound semiconductor heterostructures grown by metalorganic chemical vapor deposition

III-V compound semiconductor materials have had much attention because of their application to high speed electronic and optoelectronic devices. For achieving these purposes, it is required to produce high quality samples with uniform layer thickness, no defects, and abrupt interfaces. For this metalorganic chemical vapor deposition (MOCVD) is one of the most important growth methods. In this study, transmission electron microscopy (TEM) was used for the characterization of epilayer structures grown by the MOCVD technique. High resolution electron microscopy (HREM), the two beam technique and the convergent beam technique (CBED) were used. Cross sectional, plan view and cleavage samples using the ion milling or chemical etching method were used for TEM sample preparation. Tetragonal distortion occurs in the strained layer superlattice (SLS). Misfit dislocations are found above a certain layer thickness (critical thickness) and the critical thickness is related to the total strain state in SLS. Composition measurements of In\sb{\rm 1-x}Ga\sb{\rm x}As in SLS using TEM has restrictions because of the misfit strain and the similarity of atomic scattering factors of Ga and In. But a low In concentration layer can be determined from the (002) dark field intensity ratio. The interface quality of heterostructures can be distinguished by 5 beam, 9 beam or more conditions at a (100) zone axis. Digital vector pattern recognition was found to be a powerful tool for quantization of interface quality.U of I OnlyETDs are only available to UIUC Users without author permissio

Initial Deformation Behaviors in Lean Duplex Stainless Steel

The deformation behaviors of the austenite phase in lean duplex stainless steels were investigated through uniaxial tension tests with different amounts of deformation. Microstructural analysis showed that in the initial deformation stage the deformation in austenite grains had a predominant effect on the strain hardening behavior of the LDX-2101 steel. The initial deformation in the austenite grains was found to be mainly accommodated by the formation of stacking faults. As the deformation increased further, mechanical twins were generated by the initial stacking faults and sequentially interacted with dislocations to accommodate the strain. The analysis of dislocation behavior revealed that the deformation twinning process followed the three-layer twin formation mechanism

Temperature-dependent universal dislocation structures and transition of plasticity enhancing mechanisms of the Fe40Mn40Co10Cr10 high entropy alloy

The FCC Fe40Mn40Co10Cr10 (at.%) high entropy alloy exhibits deformation twinning for room temperature deformation and deformation-induced HCP transformation for subzero deformation. Since the systematic investigation of temperature-dependent dislocation structures is not available, we present an in-depth characterization of the defects involved in deformation at room temperature (298 K) and subzero temperature (223 K) of the cold-rolled and annealed Fe40Mn40Co10Cr10 alloy. The material deformed at 223 K shows a higher strain hardening rate than the material deformed at room temperature while both materials show large ductility, 48% for the 223 K deformation and 55% for the 298 K deformation. The main deformation mechanisms of the investigated HEA include the development of inhomogeneous dislocation structures and interaction between dislocations and deformation twin/mechanically induced HCP martensite. The stacking fault energy measured using TEM weak-beam dark-field imaging of dissociated dislocations is 20 +/- 9 mJ/m(2) at 298 K. The Fe40Mn40Co10Cr10 alloy exhibiting a positive temperature dependence of SFE leads to a decrease of SFE as deformation temperature decreases from 298 K to 223 K. The decrease of SFE results in the transition from deformation twinning to deformation-induced HCP transformation. Further, at higher strains at 223 K, kink banding of HCP and reverse transformation from HCP to FCC were observed, which could account for strain accommodation and stress relaxation, and the large ductility. The 298 K deformation leads to various types of dislocation structures: Extended dislocations, Taylor lattice of perfect dislocations, dislocation loops, highly dense dislocation walls, cell blocks, and cell structures. The observed dislocation structures at 298 K and 223 K are similar suggesting the minor effect of SFE on dislocation structures.11Nsciescopu