Ashutosh Rath

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.

Atomic Layer Epitaxy of h-BN(0001) Multilayers on Co(0001) and Molecular Beam Epitaxy Growth of Graphene on h-BN(0001)/Co(0001)

The direct growth of hexagonal boron nitride (h-BN) by industrially scalable methods is of broad interest for spintronic and nanoelectronic device applications. Such applications often require atomically precise control of film thickness and azimuthal registry between layers and substrate. We report the formation, by atomic layer epitaxy (ALE), of multilayer h-BN(0001) films (up to 7 monolayers) on Co(0001). The ALE process employs BCl3/NH3 cycles at 600 K substrate temperature. X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) data show that this process yields an increase in h-BN average film thickness linearly proportional to the number of BCl3/NH3 cycles, with BN layers in azimuthal registry with each other and with the Co(0001) substrate. LEED diffraction spot profile data indicate an average BN domain size of at least 1900 Å. Optical microscopy data indicate the presence of some domains as large as ∼20 μm. Transmission electron microscopy (TEM) and ambient exposure studies demonstrate macroscopic and microscopic continuity of the h-BN film, with the h-BN film highly conformal to the Co substrate. Photoemission data show that the h-BN(0001) film is p-type, with band bending near the Co/h-BN interface. Growth of graphene by molecular beam epitaxy (MBE) is observed on the surface of multilayer h-BN(0001) at temperatures of 800 K. LEED data indicate azimuthal graphene alignment with the h-BN and Co(0001) lattices, with domain size similar to BN. The evidence of multilayer BN and graphene azimuthal alignment with the lattice of the Co(0001) substrate demonstrates that this procedure is suitable for scalable production of heterojunctions for spintronic applications.

Coherently Embedded Ag Nanostructures in Si: 3D Imaging and their application to SERS

Surface enhanced Raman spectroscopy (SERS) has been established as a powerful tool to detect very low-concentration bio-molecules. One of the challenging problems is to have reliable and robust SERS substrate. Here, we report on a simple method to grow coherently embedded (endotaxial) silver nanostructures in silicon substrates, analyze their three-dimensional shape by scanning transmission electron microscopy tomography and demonstrate their use as a highly reproducible and stable substrate for SERS measurements. Bi-layers consisting of Ag and GeOx thin films were grown on native oxide covered silicon substrate using a physical vapor deposition method. Followed by annealing at 800°C under ambient conditions, this resulted in the formation of endotaxial Ag nanostructures of specific shape depending upon the substrate orientation. These structures are utilized for detection of Crystal Violet molecules of 5 × 10−10 M concentrations. These are expected to be one of the highly robust, reusable and novel substrates for single molecule detection.

Thermal Resistance of Transferred-Silicon-Nanomembrane Interfaces.

We report measurements of the interfacial thermal resistance between mechanically joined single crystals of silicon, the results of which are up to a factor of 5 times lower than any previously reported thermal resistances of mechanically created interfaces. Detailed characterization of the interfaces is presented, as well as a theoretical model incorporating the critical properties determining the interfacial thermal resistance in the experiments. The results demonstrate that van der Waals interfaces can have very low thermal resistance, with important implications for membrane-based micro- and nanoelectronics.

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...

Real time nanoscale structural evaluation of gold structures on Si (100) surface using in-situ transmission electron microscopy

Transmission electron microscopy (TEM) study on morphological changes in gold nanostructures deposited on Si (100) upon annealing under different vacuum conditions has been reported. Au thin films of thickness ~2.0 nm were deposited under high vacuum condition (with the native oxide at the interface of Au and Si) using thermal evaporation. In-situ, high temperature (from room temperature (RT) to 850\degreeC) real time TEM measurements showed the evaluation of gold nanoparticles into rectangular/square shaped gold silicide structures. This has been attributed to selective thermal decomposition of native oxide layer. Ex-situ annealing in low vacuum (10-2 mbar) at 850\degreeC showed no growth of nano-gold silicide structures. Under low vacuum annealing conditions, the creation of oxide could be dominating compared to the decomposition of oxide layers resulting in the formation of barrier layer between Au and Si.Transport behavior of gold nanostructures on Si(100) substrate during annealing under high vacuum has been investigated using in-situ real time transmission electron microscopy (TEM). A comparative study has been done on the morphological changes due to annealing under different vacuum environments. Au thin films of thickness ∼2.0 nm were deposited on native oxide covered silicon substrate by using thermal evaporation system. In-situ real time TEM measurements at 850 °C showed the isotropic growth of rectangular/square shaped gold-silicon alloy structures. During the growth, it is observed that the alloying occurs in liquid phase followed by transformation into the rectangular shapes. For similar system, ex-situ annealing in low vacuum (10−2 millibars) at 850 °C showed the spherical gold nanostructures with no Au-Si alloy formation. Under low vacuum annealing conditions, the rate of formation of the oxide layer dominates the oxide desorption rate, resulting in the creation of a barrier layer between Au an...

DC heating induced shape transformation of Ge structures on ultraclean Si(5 5 12) surfaces

We report the growth of Ge nanostructures and microstructures on ultraclean, high vicinal angle silicon surfaces and show that self-assembled growth at optimum thickness of the overlayer leads to interesting shape transformations, namely from nanoparticle to trapezoidal structures, at higher thickness values. Thin films of Ge of varying thickness from 3 to 12 ML were grown under ultrahigh vacuum conditions on a Si(5 5 12) substrate while keeping the substrate at a temperature of 600 °C. The substrate heating was achieved by two methods: (i) by heating a filament under the substrate (radiative heating, RH) and (ii) by passing direct current through the samples in three directions (perpendicular, parallel and at 45° to the (110) direction of the substrate). We find irregular, more spherical-like island structures under RH conditions. The shape transformations have been found under DC heating conditions and for Ge deposition more than 8 ML thick. The longer sides of the trapezoid structures are found to be along (110) irrespective of the DC current direction. We also show the absence of such a shape transformation in the case of Ge deposition on Si(111) substrates. Scanning transmission electron microscopy measurements suggested the mixing of Ge and Si. This has been confirmed with a quantitative estimation of the intermixing using Rutherford backscattering spectrometry (RBS) measurements. The role of DC heating in the formation of aligned structures is discussed. Although the RBS simulations show the presence of a possible SiO(x) layer, under the experimental conditions of the present study, the oxide layer would not play a role in determining the formation of the various structures that were reported here.

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.