J. K. Dash

Temperature-dependent electron microscopy study of Au thin films on Si (1 0 0) with and without a native oxide layer as barrier at the interface

Real-time electron microscopy observation on morphological changes in gold nanostructures deposited on Si (1?0?0) surfaces as a function of annealing temperatures has been reported. Two types of interfaces with silicon substrates were used prior to gold thin film deposition: (i) without native oxide and on ultra-clean reconstructed Si surfaces and (ii) with native oxide covered Si surfaces. For ?2.0?nm thick Au films deposited on reconstructed Si (1?0?0) surfaces using the molecular beam epitaxy method under ultra-high vacuum conditions, aligned four-fold symmetric nanogold silicide structures formed at relatively lower temperatures (compared with the one with native oxide at the interface). For this system, 82% of the nanostructures were found to be nanorectangle-like structures with an average length of ?27?nm and aspect ratio of 1.13 at ?700??C. For ?5.0?nm thick Au films deposited on Si (1?0?0) surface with native oxide at the interface, the formation of a rectangular structure was observed at higher temperatures (?850??C). At these high temperatures, desorption of gold silicide followed the symmetry of the substrate. Native oxide at the interface was found to act like a barrier for the inter-diffusion phenomena. Structural characterization was carried out using advanced electron microscopy methods.

Oxide mediated liquid–solid growth of high aspect ratio aligned gold silicide nanowires on Si(110) substrates

Silicon nanowires grown using the vapor-liquid-solid method are promising candidates for nanoelectronics applications. The nanowires grow from an Au-Si catalyst during silicon chemical vapor deposition. In this paper, the effect of temperature, oxide at the interface and substrate orientation on the nucleation and growth kinetics during formation of nanogold silicide structures is explained using an oxide mediated liquid-solid growth mechanism. Using real time in situ high temperature transmission electron microscopy (with 40 ms time resolution), we show the formation of high aspect ratio ( approximately 15.0) aligned gold silicide nanorods in the presence of native oxide at the interface during in situ annealing of gold thin films on Si(110) substrates. Steps observed in the growth rate and real time electron diffraction show the existence of liquid Au-Si nano-alloy structures on the surface besides the un-reacted gold nanostructures. These results might enable us to engineer the growth of nanowires and similar structures with an Au-Si alloy as a catalyst.

Growth of oriented Au nanostructures: Role of oxide at the interface

We report on the formation of oriented gold nanostructures on Si(100) substrate by annealing procedures in low vacuum (≈10−2 mbar) and at high temperature (≈975 °C). Various thicknesses of gold films have been deposited with SiOx (using high vacuum thermal evaporation) and without SiOx (using molecular beam epitaxy) at the interface on Si(100). Electron microscopy measurements were performed to determine the morphology, orientation of the structures and the nature of oxide layer. Interfacial oxide layer, low vacuum and high temperature annealing conditions are found to be necessary to grow oriented gold structures. These gold structures can be transferred by simple scratching method.

A study of the initial stages of the growth of Au-assisted epitaxial Ge nanowires on a clean Ge(100) surface

We report on the interfacial phenomena that occur at the initial stages of Ge nanowire growth using gold as the catalyst on Ge(100) substrates under ultra high vacuum (UHV) conditions using molecular beam epitaxy (MBE). Room temperature deposition of a thin Au layer using MBE showed a wetting nature while de-wetting has been observed at 500 °C and higher temperatures. The deposition of a thin layer of Ge at this condition resulted in the formation of Ge nanostructures and Au islands on Ge pedestals, corresponding to the initial growth of the Ge nanowires. Ge deposition at 600 °C yielded larger Ge nanowires below the Au/AuGe catalyst interface due to the enhancement of the lateral material transport.

Nano scale phase separation in Au-Ge system on ultra clean Si(100) surfaces

We report on the phase separation in Au–Ge system leading to the formation of lobe-lobe (bi-lobed) Au-Ge nanostructures under ultra high vacuum (UHV) conditions (≈3 × 10−10 mbar) on clean Si(100) surfaces. For this study, ≈2.0 nm thick Au samples were grown on the substrate surface by molecular beam epitaxy. Thermal annealing was carried out inside the UHV chamber at temperature ≈500 °C and following this, nearly square shaped AuxSi1-x nano structures of average length ≈48 nm were formed. A ≈2 nm Ge film was further deposited on the above surface while the substrate was kept at a temperature of ≈500 °C. Well ordered Au-Ge nanostructures where Au and Ge residing side by side (lobe-lobe structures) were formed. In our systematic studies, we show that, gold-silicide nanoalloy formation at the substrate (Si) surface is necessary for forming phase separated Au-Ge bilobed nanostructures. These results show that the Au–Ge bonding is unstable in nature. Electron microscopy (TEM, STEM–EDS, SEM) studies were carrie...

A simple growth method for Nb2O5 films and their optical properties

A simple method for the synthesis of Nb2O5 films of thicknesses ranging from tens to several hundreds of nanometers on amorphous silicon dioxide or quartz substrates is presented. Nb2O5 films were formed by annealing the sputter deposited Nb films under an Ar flow and without oxygen plasma in a quartz tube within a furnace at 850 °C. The structural, compositional, optical, and vibrational properties were characterized by grazing incidence X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet visible spectroscopy, and Raman scattering. Each of the Nb2O5 films is polycrystalline with an orthorhombic crystal structure. We observed vibrational modes including longitudinal optical, transverse optical, and triply degenerate modes, and measured the indirect optical band gap to be ∼3.65 eV. The transmittance spectrum of the ∼20 nm thick Nb2O5 film shows over 90% transmittance below the band gap energy in the visible wavelength range and decreases to less than 20% in the ultraviolet regime. The optical properties of the films in the UV-vis range show potential applications as UV detectors.

Metal-enhanced Ge1−xSnx alloy film growth on glass substrates using a biaxial CaF2 buffer layer

Ge1−xSnx alloyed films were grown on glass substrates by sequential physical vapor deposition of a biaxial CaF2 buffer layer and a Sn heteroepitaxial layer at room temperature, followed by a Ge layer grown at low temperatures (200–350 °C). The predeposited Sn on the CaF2 layer enhances Ge diffusion and crystallization. Sn is substituted into the Ge lattice to form a biaxial Ge1−xSnx alloyed film. The epitaxial relationships were obtained from X-ray pole figures of the samples with Ge1−xSnx 〈01〉∥CaF2 〈01〉 and Ge1−xSnx 〈10〉∥CaF2 〈10〉. Crystallization and biaxial texture formation start at about 200 °C with the best biaxial Ge1−xSnx film grown at about 300 °C, which is 100 °C lower than the growth temperature of biaxial pure Ge film without Sn on the CaF2/glass substrate. The microstructure, texture and Sn concentration of the Ge1−xSnx films were characterized by X-ray diffraction, X-ray pole figure analysis, and transmission electron microscopy. The spatial chemical composition of Sn in Ge1−xSnx was measured by energy-dispersive X-ray spectroscopy and was found to be nearly uniform throughout the thickness of the alloyed film. Raman spectra show shifts of Ge–Ge, Ge–Sn, and Sn–Sn vibration modes due to the percentage change of substitutional Sn in Ge as a function of growth temperature. This growth method is an alternative cost-effective way to grow biaxial semiconductor films on amorphous substrates.

Universality in shape evolution of Si1?xGex structures on high-index silicon surfaces

The shape evolution of MBE grown Si1?xGex islands on ultraclean reconstructed high-index Si(5 5 12), Si(5 5 7) and Si(5 5 3) surfaces has been studied experimentally and explained using a phenomenological kinetic Monte Carlo (kMC) simulation. We show that a self-assembled growth at optimum thickness leads to interesting shape transformations, namely spherical islands to rectangular rods up to a critical size beyond which the symmetry of the structures is broken, resulting in a shape transition to elongated trapezoidal structures. We observe a universality in the growth dynamics in terms of aspect ratio and size exponent, for all three high-index surfaces, irrespective of the actual dimensions of Ge-Si structures. The shape evolution has been simulated using kMC by introducing a deviation parameter (?) in the surface barrier term (ED) to take the effect of anisotropic diffusion as one of the plausible mechanisms.

Morphological variations in Au x Si y nanostructures under variable pressure and annealing conditions

To understand surface structural modifications for Au/Si (100) system, a thin gold film of ~2.0 nm was deposited under ultra high vacuum (UHV) condition on reconstructed Si surfaces using molecular beam epitaxy (MBE). Post annealing was done at 500{\deg}C in three different vacuum conditions: (1) low vacuum (LV) furnace (10-2 mbar), (2) UHV (10-10 mbar) (MBE chamber), (3) high vacuum (HV) chamber. The variation in the overall shape of the gold nanostructures and finer changes at the edges, like rounding of corners has been reported in this work. Although well aligned nano rectangles were formed in both HV and LV cases, corner rounding is more prominent in LV case. Furthermore in UHV case, random structures were formed having sharp corners. In all the above three cases, samples were exposed to air (for half an hour) before annealing. To study the effect of surface oxide, in-situ annealing inside UHV-MBE chamber was done without exposing to air. Well aligned rectangles with sharp corners (no corner rounding) were formed. The details about the role of surface oxides in the corner rounding process are discussed.

Shape transformation of Si1−xGex structures on ultra clean Si(5 5 7) and Si(5 5 12) surfaces

We report growth of Ge nano/micro structures on ultra clean, high vicinal silicon surfaces of Si(5 5 7) and Si(5 5 12) under two substrate heating conditions: direct current (DC) and radiative heating (RH). These were grown under ultra high vacuum conditions while keeping the substrate at a temperature of 600°C. The results for 10 monolayer (ML) and 12 ML thick Ge deposited on the above surfaces show spherical island structures for RH conditions while aligned trapezoidal structures were observed under DC conditions of heating. We find that the longer side of trapezoid structures are along irrespective of DC current direction. In the case of 10 ML Ge deposited on Si (5 5 7), elongated SixGe1−x nanostructures with an average length of ≈300 nm and a length/width ratio of ≈3.3 have been formed along the step edges. Under similar conditions for 10 ML Ge growth on Si(5 5 12), we found aligned SixGe1−x trapezoidal microstructures of length ≈6.25 μm and an aspect ratio of ≈3.0. Scanning transmission electron microscopy (STEM) measurements showed the mixing of Ge and Si at the interface and throughout the over-layer. Detailed electron microscopy studies (scanning electron microscopy (SEM) and STEM) reveal the structural aspects of these microstructures.