Ganesh Vanamu

Epitaxial growth of high-quality Ge films on nanostructured silicon substrates

Low (∼105cm−2) defect density, thick (∼10μm) epitaxially grown Ge films on nanostructured and planar silicon substrates using chemical vapor deposition are reported. The structural morphology of Ge films was evaluated using scanning electron microscopy, transmission electron microscopy, and high resolution x-ray diffraction measurements. The surface defect density was measured by counting etch pits. The growth quality of Ge films was significantly superior on nanostructured surfaces relative to the planar as demonstrated by (a) reduction in defect density from ∼6×108cm−2 for planar to ∼5×105cm−2, (b) reduction in Δω full width half maximum peaks of the reciprocal space maps of Ge epilayers from 373arcsec on planar (unpatterned) to 93arcsec on the nanostructured surfaces, and (c) the elimination of crosshatch pattern characteristic of planar surface growth.

Growth of high quality Ge∕Si1−xGex on nano-scale patterned Si structures

Heteroepitaxial growth of thick (∼6μm) Ge∕3μm SixGe1−x layers on nano-scale patterned Si substrates has been investigated. These nm scale structures are fabricated using interferometric lithography with reactive ion and wet-chemical etching techniques. The quality of the growth on the nanopatterned substrates was compared to growth on planar substrates. The quality of the epitaxial layers was characterized using scanning electron microscopy, transmission electron microscopy, high-resolution x-ray diffraction and etch pit density measurements. We demonstrated the nanopatterned structures produce significantly superior quality heteroepitaxial epilayers in comparison with unpatterned Si surfaces. The dislocation density of Ge epilayers on the nanopatterned Si was ∼ three orders magnitude lower than on planar Si. Growth on nanopatterned surfaces also reduced surface roughness and eliminated crosshatch patterns characterstic of planar surfaces.

Growth of high-quality GaAs on Ge∕Si1−xGex on nanostructured silicon substrates

Heteroepitaxial growth of GaAs∕Ge∕SiGe films on submicrostructured Si substrates is reported. One-dimensional, nanometer-linewidth, submicrometer period features were fabricated in Si substrates using interferometric lithography, reactive ion etching, and wet-chemical etching techniques. The quality of the GaAs layers grown on these structures was investigated using high-resolution x-ray diffraction, transmission electron microscopy, scanning electron microscopy, photoluminescence, and etch pit density measurements. The defect density of GaAs epilayers grown on submicrostructured Si at ∼6×105cm−2 was two orders of magnitude lower compared with that grown on planar silicon. The optical quality of the GaAs∕Ge∕SiGe on submicrostructured Si was comparable to that of single crystal GaAs.

Photocatalytic oxidation of a volatile organic component of acetaldehyde using titanium oxide nanotubes

Titanium oxide nanotubes are prepared and treated with Au (Au/nanotube sample) and Pt (Pt/nanotube sample), and the photoactivity of these catalysts compared to a standard Degussa P25 photocatalyst is investigated. The samples were analyzed using X-ray diffraction, field emission gun scanning transmission electron microscopy (STEM). Both high-resolution TEM images and high-angle annular dark-field (HAAD) images were recorded for the specimens. Oxidation of acetaldehyde was used to test the efficiency of the catalysts. Nanotube samples showed better photoactivity than the standard P25, because the P25 titania deactivates quickly. Enhanced reactivity of the nanotube is related to surface charge polarity developed on outer and inner surfaces due to the difference in overlap of oxygen anions that resulted from curving of octahedral sheets. A tentative and qualitative surface polarity model is proposed for enhancing electron-hole pair separation. The inner surface benefits reduction; whereas, the outer surface benefits oxidation reactions. Both the metal identity and the size of the metal particles in the nanotubes affected the photocatalytic activity. Specifically, the addition of platinum increased the activity significantly, and increased the total yield. The addition of gold had lesser impact compared to the platinum. Formation of Pt large nanoparticles on the nanotube surfaces reduces the oxidation reactivity.

Templated Control of Au nanospheres in Silica Nanowires

The formation of regularly spaced metal nanostructures in selectively placed insulating nanowires is an important step toward realization of a wide range of nanoscale electronic and optoelectronic devices. Here we report templated synthesis of Au nanospheres embedded in silica nanowires, with nanospheres consistently spaced with a period equal to three times their diameter. Under appropriate conditions, nanowires form exclusively on Si nanostructures because of enhanced local oxidation and reduced melting temperatures relative to templates with larger dimensions. We explain the spacing of the nanospheres with a general model based on a vapor-liquid-solid mechanism, in which an Au/Si alloy dendrite remains liquid in the nanotube until a critical Si concentration is achieved locally by silicon oxide-generated nanowire growth. Additional Si oxidation then locally reduces the surface energy of the Au-rich alloy by creating a new surface with smaller area inside of the nanotube. The isolated liquid domain subs...

Process challenges in advanced photomask etch processes

Plasma etch challenges such as resolution enhancement, etch error reduction, and process reliability improvement are investigated in next generation phase shift photomask processes for ≤14nm technology node. Etch resolution predominantly depends on etch bias and linearity while overall process resolution is also determined by resist thickness. Several resolution enhancement techniques including thin hardmasks and new absorber materials are tested in terms of etch profile, linearity, and minimum feature printability. New approaches provide improvement on etch bias as well as good pattern fidelity for sub-resolution patterns. Reduction of etch profile errors is also critical to maintain high pattern resolution. It is found that some of etch profile distortion can be minimized by changing plasma conditions. To meet tighter process reliability requirement, we investigated a couple of advanced process control techniques in alternating phase shift mask manufacturing. Integration of real-time monitor is essential to obtain good process reliability with no degradation on defects or throughput.

GaAs Growth on Micro and Nano Patterned Ge/Si1-XGeX and Si Surfaces

We show heteroepitaxial growth of GaAs on Ge/SiGe grown on nanometer-scale grating structures. Conventional lithography techniques were combined with reactive ion and wet-chemical etching to fabricate 1-D patterns of silicon posts. The quality of the GaAs layers was investigated using high-resolution x-ray diffraction (HRXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), photoluminescence (PL) and etch pit density (EPD) measurements. Our results show significant improvement in the quality of heteroepitaxial layers grown on nano patterned structures compared to those on the unpatterned silicon. The optical quality of the GaAs/Ge/SiGe on nano-scale patterned silicon was comparable to that of single crystal GaAs.

Heteroepitaxial growth and characterization of Ge and Si X Ge 1−X films on patterned silicon structures

We describe novel 2-D structures that facilitate strain relief and allow us to obtain Ge epilayers that are free of defects. These structures can potentially absorb thermal expansion and lattice expansion mismatch as well as enable liftoff of heteroepitaxial layers for subsequent wafer reuse. Conventional lithography techniques were combined with reactive ion and wet-chemical etching to fabricate 2-D patterns of silicon posts. The dimensions of the posts were varied keeping the pitch (center to center distance) constant. Heteroepitaxial growth of Ge/Si x Ge 1−x on these micrometer-scale structures was investigated. While, keeping the growth parameters constant, the geometry of the structures was varied to determine the optimum configuration for the highest quality heteroepitaxial growth. The quality of the Si 1−x Gex buffer system was investigated using high-resolution x-ray diffraction. Transmission electron microscopy (TEM) was used to analyze the epilayer cross-sections. Surface morphology was analyzed using scanning electron microscopy (SEM), atomic force microscopy (AFM) and optical microscopy. Our results show that the quality of the heteroepitaxial layers improves as the width of the posts in the 2-D pattern was decreased.

Modeling the Elastic Fields in Epitaxially Grown Multilayers

A model was developed to calculate the elastic fields, including strain energy density, in multilayers grown epitaxially on a planar substrate. This model works well for compliant and non-compliant substrates. In particular we illustrate the model for four layer heterostructure and apply it for graded Ge (Si x Ge 1−x ) grown on a planar silicon substrate. Using the equations for static equilibrium and Hookes law for isotropic materials under a plane stress condition, the elastic fields associated with each layer were calculated. The strain partitioning in this model reduces to the limiting case of a two- layer structure available in the literature. As it turns out here, strain partitioning is a function of the bulk unstrained lattice parameters, elastic constants and thicknesses of the layers. The model was qualitatively verified by comparing the strain energy density with the dislocation density away from a relatively thick substrate. This model helps shed some light on the factors important in achieving defect free multilayers for optoelectronic devices.