Removal of threading dislocations from patterned heteroepitaxial semiconductors by glide to sidewalls

We have shown that threading dislocations can be removed from patterned heteroepitaxial semiconductors by glide to the sidewalls, which is driven by the presence of image forces. In principle, it should be possible to attain highly mismatched heteroepitaxial semiconductors which are completely free from threading dislocations, even though they are not pseudomorphic, by patterned heteroepitaxial processing. There are two basic approaches to patterned heteroepitaxial processing. The first involves selective area growth on a pre-patterned substrate. The second approach involves post-growth patterning followed by annealing. We have developed a quantitative model which predicts that there is a maximum lateral dimension for complete removal of threading dislocations by patterned heteroepitaxy. According to our model, this maximum lateral dimension is proportional to the layer thickness and increases monotonically with the lattice mismatch. For heteroepitaxial materials with greater than 1% lattice mismatch, our model predicts that practical device-sized threading dislocation-free regions may be realized by patterned heteroepitaxial processing.     

Deposition,post-deposition annealing,and characterization of epitaxial Ge films grown on Si(100) by pyrolysis of GeH4

As a first step towards developing heterostructures such as GaAs/Ge/Si entirely by chemical vapor deposition, Ge films have been deposited on (100) Si by the pyrolysis of GeH₄. The best films are grown at 700° C and are planar and specular, with RBS minimum channeling yields of ≈4.0% (near the theoretical value) and defect densities of 1.3 x 10⁸ cm⁻². Variations of in-situ cleaning conditions, which affect the nature of the Si substrate surface, greatly affect the ability to get good epitaxial growth at 700° C. The majority of the defects found in the Ge films are extrinsic stacking faults, formed by dissociation of misfit and thermal expansion accommodation dislocations. The stacking fault density is not significantly reduced by post-deposition annealing, as is the case for the dislocations observed in MBE Ge films. It is suggested that lowering the CVD growth temperature through the use of high vacuum deposition equipment would result in dislocation defects like those of MBE films which could then be annealed more effectively than stacking faults. Films with defect densities equivalent to MBE Ge films (~2 x 10⁷ cm⁻²) could then probably be produced.     

Compositional nonuniformities and strain relaxation at misoriented InxGa(1−x)As/GaAs interfaces

Compositional nonuniforimities and misfit dislocations are observed near misoriented In_0.2Ga_0.8As/GaAs interfaces. The compositional nonuniformities results from In interdiffusion at the interface over a distance of 3 nm and formation of InAs platelet precipitates. The misfit dislocations are of pure edge and 60° types. The core of pure edge misfit dislocations generally consists of two 60° dislocations separated by approximately 2.5 nm. One of these 60° dislocations is usually split into partials and decorated by platelets of InAs. The interface and surface morphologies are strongly influenced by the substrate tilt away from exact [001] crystallographic orientation.     

Defect Analysis of Barrier Height Inhomogeneity in Titanium 4H-SiC Schottky Barrier Diodes

Over 350 4H-SiC Schottky barrier diodes (SBDs) of varying size are characterized using current–voltage (I–V) measurements, with some also measured as a function of temperature. Devices display either a characteristic single-barrier height or atypical dual-barrier heights. Device yields are shown to decrease as device area increases. Molten KOH etching is used to highlight defects for analysis by optical microscopy and atomic force microscopy. The I–V characteristics are compared against the defect density. A positive correlation between effective barrier height and effective electrically active area of the SBDs is found. No correlation is found between threading dislocations and ideality factor or barrier height.     

Emergence of New Mechanical Functionality in Materials via Size Reduction

Julia R. Greer received her S.B. in Chemical Engineering from the Massachusetts Institute of Technology (1997) and a Ph.D. in Materials Science from Stanford University, where she worked on the nanoscale plasticity of gold with W. D. Nix (2005). She also worked at Intel Corporation in Mask Operations (2000–03) and was a post‐doctoral fellow at the Palo Alto Research Center (2005–07), where she worked on organic flexible electronics with R. A. Street. Greer is a recipient of TR‐35, Technology Review's Top Young Innovator award (2008), a NSF CAREER Award (2007), a Gold Materials Research Society Graduate Student Award (2004), and an American Association of University Women Fellowship (2003). Julia joined Caltech's Materials Science department in 2007 where she is developing innovative experimental techniques to assess mechanical properties of nanometer‐sized materials. One such approach involves the fabrication of nanopillars with different initial microstructures and diameters between 25 nm and 1 µm by using focused ion beam and electron‐beam lithography microfabrication. The mechanical response of these pillars is subsequently measured in a custom‐built in situ mechanical deformation instrument, SEMentor, comprising a scanning electron microscope and a nanoindenter. Read our interview with Prof. Greer on MaterialsViews.com

     

Controlled surface crystallization of fused silica by Li+-Ion implantation

A new method of producing a glass-ceramic surface layer on fused silica has been demonstrated using Li⁺-ion implantation and relatively low-temperature annealing. Infrared reflection spectroscopy (IRS) was used to study the effects of ion implantation on structural changes. Isochronal annealing of samples implanted with 250 keV Li⁺/cm² brings about a dramatic change in the IRS spectra at 800°C in that it becomes identical with that of α-quartz. The dependence of the degree of crystallization on temperature, Li⁺-ion fluence, and silica type was studied.     

Revisiting the defect structure of MAX phases: the case of Ti4AlN3

Microstructural study of as-grown Ti₄AlN₃ MAX phase has been performed by transmission electron microscopy. Dislocation walls, dislocation nucleation sites and stacking faults are described. In particular, diffraction contrast analysis combined with high-resolution images give a new insight into the nature of the stacking faults: contrarily to what is usually postulated, it is shown that the stacking faults possess a shear component in the basal plane. The stacking faults are created by the insertion of MX layers in the lattice via diffusion mechanisms. Their possible role on the deformation mechanism of MAX phases is discussed.     

Determination of elastic strain fields and geometrically necessary dislocation distributions near nanoindents using electron back scatter diffraction

The deformation around a 500-nm deep Berkovich indent in a large grained Fe sample has been studied using high resolution electron back scatter diffraction (EBSD). EBSD patterns were obtained in a two-dimensional map around the indent on the free surface. A cross-correlation-based analysis of small shifts in many sub-regions of the EBSD patterns was used to determine the variation of elastic strain and lattice rotations across the map at a sensitivity of ～±10^?4. Elastic strains were smaller than lattice rotations, with radial strains found to be compressive and hoop strains tensile as expected. Several analyses based on Nye's dislocation tensor were used to estimate the distribution of geometrically necessary dislocations (GNDs) around the indent. The results obtained using different assumed dislocation geometries, optimisation routines and different contributions from the measured lattice rotation and strain fields are compared. Our favoured approach is to seek a combination of GND types which support the six measurable (of a possible nine) gradients of the lattice rotations after correction for the 10 measurable elastic strain gradients, and minimise the total GND line energy using an L¹ optimisation method. A lower bound estimate for the noise on the GND density determination is ～±10¹² m^?2 for a 200-nm step size, and near the indent densities as high as 10¹⁵ m^?2 were measured. For comparison, a Hough-based analysis of the EBSD patterns has a much higher noise level of ～±10¹⁴m^?2 for the GND density.     

Precipitate assemblies formed on dislocation loops in aluminium-silver-copper alloys

The precipitation microstructure of the γ′ (AlAg₂) intermetallic phase has been examined in aluminium-silver-copper alloys. The microstructure developed in an Al-0.90at.%Ag-90at.Cu alloy was significantly different from that reported for binary Al-Ag alloys. The orientation relationship between the matrix and precipitate was unchanged; however, the γ′ phase formed assemblies with a two-dimensional, open arrangement of precipitates. Each such assembly contained two variants of the γ′ phase alternately arranged to form a faceted elliptical unit. The θ′ (Al₂Cu) phase formed on these assemblies after further ageing. Each assembly was formed via repeated precipitation of the γ′ phase on dissociated segments of a single dislocation loop. This faceted elliptical assembly has not been previously reported for the γ′ precipitate. The difference between the precipitation behaviour of the γ′ phase in Al-Ag and Al-Ag-Cu alloys was attributed to copper modifying the as-quenched defect structure of the matrix. The formation of faceted elliptical γ′ phase assemblies clarifies earlier observations on the precipitate number density and mechanical properties of aluminium-silver-copper alloys.