G.-C. Wang

Physical self-assembly and the nucleation of three-dimensional nanostructures by oblique angle deposition

Growth front morphology of a thin film formed by physical vapor deposition is controlled by many factors including surface diffusion and shadowing effects. Instabilities can occur if shadowing is more dominant compared to other surface effects and can lead to many diverse physically self-assembled three-dimensional nano-size structures. In this article, we explore the fundamental nucleation and growth mechanisms of the nanostructures during oblique angle deposition. Monte Carlo simulations were carried out to predict the island density, island size distribution, and island–island correlation during the initial stages of growth. The results were compared to that obtained by the oblique angle sputter deposited tungsten films imaged by atomic force microscopy and scanning electron microscopy. Isolated islands with quasiperiodic distribution were formed as a natural consequence of the shadowing effect. Isolated columnar structures are shown to grow on these islands and the width W of the columns is predicted ...

Uniform Si nanostructures grown by oblique angle deposition with substrate swing rotation

Slanted nano-columns and square nano-springs made of amorphous silicon (a-Si) were fabricated on bare Si and patterned substrates by oblique angle deposition with a back–forth substrate swing rotation mode. Scanning electron microscopy was used to characterize the grown nanostructures. The tilt angle of slanted nano-rods is determined by the incident angle of deposition flux and the azimuthal swing rotation angle of a substrate. The controlled substrate rotation affects the uniformity and the shape of the nanostructures. On the patterned substrate, the broadening of the size of individual nano-columns is greatly reduced and the nano-columns are not connected as they grow. A simple model based on decomposing the deposition flux is used to describe the effect of substrate rotation on tilt angle, uniformity, and the top-end shape of nanostructures. The feasibility of fabricating separated and well aligned nanostructures by our swing rotation method provides an effective and controllable way to fabricate nano-devices.

Quasi-periodic nanostructures grown by oblique angle deposition

We report that tungsten nanocolumns grown by oblique angle sputter deposition develop a quasi-periodic morphology which is not observed for continuous films deposited at normal incidence. The maximum position in power spectral density of the quasi-periodic nanostructures decreases exponentially as a function of thickness. We explain the formation of the quasi-periodic nature by a “shadowing length” concept which plays a similar role to conventional surface diffusion length. Also, we show that the change of the spatial frequency of the periodicity is a result of the elimination of shorter columns due to the shadowing effect during growth.

Real-time observation of initial stages of copper film growth on silicon oxide using reflection high-energy electron diffraction

We have studied, in real time, the evolution of a thin (less than 200A) copper film deposited onto an oxidized silicon surface using reflection high-energy electron diffraction. We show that quantitative measurements of island size and shape as functions of time are possible and the results are presented. While the film texture is initially random, texture competition leads to an absence of the low-energy (111) and (200) oriented grains for later times. It is also found that the film surface is composed of facets that increase in size with time. This behavior is explained in terms of facet coalescence.

In situ reflection high energy electron diffraction surface pole figure study of biaxial texture evolution in anisotropic Mg nanoblades during shadowing growth

The reflection high energy electron diffraction (RHEED) surface pole figure technique has been applied to an in situ study of the biaxial texture evolution in the anisotropic film of Mg nanoblades. These nanoblades were grown by thermal vapor under the shadowing effect using oblique angle deposition. To compensate for the effects of the anisotropic morphology of the Mg film on the RHEED surface pole figure, a method of intensity normalization has been employed. From the normalized pole figures, we observed a development of (101¯0)[0001] biaxial texture in the film during the growth. When the film grows thicker the texture axes tilt more towards the incident vapor flux. The variation of the azimuthal angle orientation is mainly around the [0001] axis during growth and the azimuthal dispersion angle around the [0001] axis reduces. The change of texture axis tilting angle has been correlated to the change of the nanoblade tilting angle. The azimuthal angle alignment of the nanoblades is argued to be the resu...

Field ionization of argon using β-phase W nanorods

We report testing of β-phase W nanorods as a gas ionizer for neutral argon atoms. These W nanorods having square-base pyramidal apexes were grown on oxidized Si(100) substrate using glancing angle sputter deposition technique with substrate rotation. Only a few volts of positive anode voltage (3–4V) applied to the W nanorods generates a high electric field, which ionizes gas-phase argon atoms and generates ion currents up to several tenths of microamperes. The low anode voltage and high ion current observed in this study indicates that β-phase W nanorods can be used as battery-operated gas ionizers.

In situ reflection high energy electron diffraction study of dehydrogenation process of Pd coated Mg nanoblades

The near surface structural evolution in dehydrogenation process of air exposed Pd coated Mg nanoblades was characterized in situ from room temperature to ∼573 K using reflection high energy electron diffraction (RHEED). The evolved normalized diffraction intensity and the full width at half maximum of diffraction peaks have been correlated with the growth of crystal and the change in crystal size, respectively. With RHEED, we are able to detect crystal sizes smaller than ∼2 nm. At room temperature the dominant structures near surface were Pd and MgH2. With the substrate heating MgH2 started to gradually decompose at ∼380 K. When the temperature increased to ∼480 K MgH2 was nearly depleted and Mg6Pd alloys started to form. In addition, at high temperatures pure Mg reappeared and MgO was enhanced significantly even in high vacuum condition. We have discussed the effect of Mg oxide on the dehydrogenation process and the alloy formation between Pd and Mg as well as the accompanying migration of Mg to the sur...

Deformation of amorphous silicon nanostructures subjected to monotonic and cyclic loading

An atomic force microscope (AFM) was used to characterize the deformation behavior of amorphous Si (a-Si) nanostructures subjected to monotonic and cyclic loading. The sample geometry was specially designed (in the form of elbow) using finite element modeling for the purpose of these tests, and the samples were grown by glancing angle deposition. When deformed monotonically at room temperature, the a-Si specimens exhibited a nonlinear force–displacement response at forces larger than a critical force, a phenomenon not observed in bulk silicon. A fatigue testing methodology based on the use of the AFM was established. The fatigue life of the a-Si specimens was observed to increase by five orders of magnitude with a 50% reduction in the applied force amplitude. It was verified that this delayed failure is caused by progressive damage accumulation during cyclic loading. These results are compared with literature data obtained from micron-size specimens.

van der Waals epitaxy of Ge films on mica

To date, many materials have been successfully grown on substrates through van der Waals epitaxy without adhering to the constraint of lattice matching as is required for traditional chemical epitaxy. However, for elemental semiconductors such as Ge, this has been challenging and therefore it has not been achieved thus far. In this paper, we report the observation of Ge epitaxially grown on mica at a narrow substrate temperature range around 425 °C. Despite the large lattice mismatch (23%) and the lack of high in-plane symmetry in the mica surface, an epitaxial Ge film with [111] out-of-plane orientation is observed. Crystallinity and electrical properties degrade upon deviation from the ideal growth temperature, as shown by Raman spectroscopy, X-ray diffraction, and Hall effect measurements. X-ray pole figure analysis reveals that there exist multiple rotational domains in the epitaxial Ge film with dominant in-plane orientations between Ge 1¯10 and mica[100] of (20n)°, where n = 0, 1, 2, 3, 4, 5. A supe...

Surface etch-front morphologies using in-plane light scattering

By using a diode array detector and an in-plane scattering geometry, we have investigated the diffraction from various etch-front morphologies. We can obtain an angular distribution of light intensity profile within 30 milliseconds. A series of experimental work, including the detailed characterization of Si backside surfaces and the morphology of Al films on Si during chemical corrosion, will be presented. The corresponding roughness parameters for different surfaces were extracted from light scattering profiles, and compared with those from real-space images. Real time measurements have been performed to study the evolution of Si surface morphology during wet chemical etching. RMS roughness, pits density, correlation length, and pits formation rate can be determined in real time.