D.P. Datta

Transition from ripples to faceted structures under low-energy argon ion bombardment of silicon: understanding the role of shadowing and sputtering

In this study, we have investigated temporal evolution of silicon surface topography under 500-eV argon ion bombardment for two angles of incidence, namely 70° and 72.5°. For both angles, parallel-mode ripples are observed at low fluences (up to 2 × 1017 ions cm-2) which undergo a transition to faceted structures at a higher fluence of 5 × 1017 ions cm-2. Facet coarsening takes place at further higher fluences. This transition from ripples to faceted structures is attributed to the shadowing effect due to a height difference between peaks and valleys of the ripples. The observed facet coarsening is attributed to a mechanism based on reflection of primary ions from the facets. In addition, the role of sputtering is investigated (for both the angles) by computing the fractional change in sputtering yield and the evolution of surface roughness.PACS81.05.Cy, 81.16.Rf, 61.80.Jh, 87.64.Dz

Temporal evolution of nanoporous layer in off-normally ion irradiated GaSb

Room temperature irradiation of GaSb by 60 keV Ar+-ions at an oblique incidence of 60° leads to simultaneous formation of a nanoporous layer and undulations at the interface with the underlying substrate. Interestingly, with increasing ion fluence, a gradual embedding of the dense nanoporous layer takes place below ridge-like structures (up to the fluence of 1 × 1017 ions cm−2), which get extended to form a continuous layer (at fluences ≥4 × 1017 ions cm−2). Systematic compositional analyses reveal the co-existence of Ga2O3 and Sb2O3 in the surface layer. The results are discussed in terms of a competition between ion-induced defect accumulation and re-deposition of sputtered atoms on the surface.

60 keV Ar+-ion induced modification of microstructural, compositional, and vibrational properties of InSb

Room temperature irradiation of InSb(111) by 60 keV Ar+-ions at normal (0°) and oblique (60°) angles of incidence led to the formation of nanoporous structure in the high fluence regime of 1 × 1017 to 3 × 1018 ions cm−2. While a porous layer comprising of a network of interconnected nanofibers was generated by normal ion incidence, evolution of plate-like structures was observed for obliquely incident ions. Systematic studies of composition and structure using energy dispersive x-ray spectroscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, Raman mapping, grazing incidence x-ray diffraction, and cross-sectional transmission electron microscopy revealed a high degree of oxidation of the ion-induced microstructures with the presence of In2O3 and Sb2O3 phases and presence of nanocrystallites within the nanoporous structures. The observed structural evolution was understood in terms of processes driven by ion-induced defect accumulation within InSb.

Facile synthesis of a superhydrophobic and colossal broadband antireflective nanoporous GaSb surface

This paper reports the facile synthesis of tunable hydrophobic and colossal antireflective nanoporous GaSb by alteration of its porosity. In particular, it is observed that the contact angle of a water droplet on the GaSb surface increases as the nanoporous structures undergo different stages of growth and finally exceeds 150°, indicating the transition to a superhydrophobic surface. The observed correlation between the contact angle and the surface morphology is qualitatively understood in light of the Cassie–Baxter model. It is found that with the temporal evolution of nanostructures, a decrease in the fraction of the solid surface wetted by the water droplet and a corresponding increase in the air–water interface fraction lead to the enhancement in hydrophobicity, where the chemistry of the porous surface also plays a role. The temporal evolution of the contact angle is also studied to understand the interaction of the sessile drop with the hydrophobic surface and the ambient. In addition to an increase in the contact angle, we also observe a colossal broadband antireflection (in the range of 200–800 nm), which is correlated to a large reduction in the refractive index due to increasing porosity. Such a surface, with the combination of superhydrophobicity and colossal antireflection, can be very useful in the applications of GaSb nanostructures in thermophotovoltaic cells or photodiodes.

Tunable wettability of Si through surface energy engineering by nanopatterning

We synthesize nanoscale ripple patterns on Si surfaces by ion sputtering and show a systematic variation in the wettability of the surface depending upon its structure and chemical composition. As a matter of fact, our experiments reveal a hydrophilic to hydrophobic transition of the Si surface as a function of morphology and composition. Observed variation in the contact angle is found to be consistent with Wenzels law up to the onset of a sinusoidal ripple pattern formation on the Si surface, although a clear deviation from Wenzel-like behavior is detected after ripple formation. This deviation is attributed to a reduction in the surface free energy of ion implanted Si surfaces due to the formation of ripples. Further, we detect the existence of a top amorphous layer and the presence of Ar atoms near the top surface for all ion implanted Si samples. Thus, to understand our results, we take into account the compositional modification of the implanted surface due to incorporation of Ar atoms and attempt to correlate the observed evolution of contact angle with this compositional heterogeneity following the ripple formation. We have shown that the pinning behavior of a water droplet on the rippled-Si surface, due to the presence of ripple height, can be altered through a change in the surface composition.

Ion induced dewetting of Au–Si on a SiO2 surface: composite nanodot evolution and wettability transition

A nanodot array morphology gradually develops on SiO2 surface when a thin bi-layer of Au and Si undergoes ion irradiation. An increasing amount of gold silicide is detected as islands on the insulator surface evolve into nanodots as a function of increasing ion fluence. Different stages of evolution from islands to nanodots are found to be driven by the localized melting of Au along the ion-track and dewetting of the metal film. Dewetting is accompanied by sputter-erosion and mixing of Au and Si at the bi-layer interface due to ion energy deposition. Interestingly, a gradual transition in wettability of the surface from the hydrophilic to the hydrophobic one is observed with the growth of nanodots, which is correlated with the compositional variation. The experimental results indicate a route towards the controlled growth of composite nanodots on an insulator surface having hydrophobic properties using ion irradiation.

Nanoporosity-induced superhydrophobicity and large antireflection in InSb

A porous nanostructure evolves in InSb due to keV ion implantation which leads to superhydrophobic and large antireflective property, indicating a single-step facile fabrication to introduce both functionalities. In particular, it is observed that the contact angle of a water droplet on the nanoporous InSb surface exceeds 150°, revealing the transition to a superhydrophobic surface. Correlation between the contact angle and the porous nanostructures is qualitatively understood in light of the Cassie-Baxter model. It is found that a decrease in the fraction of solid surface wetted by the water droplet and a corresponding increase in the air-water interface fraction lead to the enhancement in the hydrophobicity. We further observe that the large broadband antireflection (in the range of 200–800 nm) is also correlated to the nanoporous structure, arising out of a large reduction in the refractive index due to its increasing porosity. Such a surface with the combination of superhydrophobicity and large antire...

Statistical analysis of ripple morphology on Si surfaces due to 60 keV Ar + -ions

We report on analysis of ion-beam patterned surface morphology in terms of regularity of pattern shape and orientation, homogeneity over irradiated surface, and the effective increment in its surface area, which are critical in deciding the applications for the corresponding surface. As a case study, we have chosen Si surface, which is exposed to 60 keV Ar+-ions at different angles of incidence and ion fluence and have performed detail statistical analysis of topographic images of the patterned surfaces. By using the Scanning Probe Image Processor (SPIP) software, morphological parameters, viz. surface area ratio, texture direction index, texture aspect ratio, ratio of system correlation length to ripple wavelength, directional roughness exponents, and anisotropy ratio are calculated as functions of ion incidence angle and fluence. From angle-dependent studies, we observe that ripple patterns become more regular with increasing angle of incidence. On the other hand, fluence-dependent study of these parameters shows that ripple shapes are most regular for the fluence of 3 × 1018 ions cm−2, while ripples are most unidirectional for the fluence of 2 × 1018 ions cm−2. Our analysis method shows a route towards optimization of ion-patterned surfaces in terms of nanostructure quality or effective surface area, which is vital for applications. Further, using scaling analysis, we associate Si surfaces generated within particular angular or fluence range to different universality classes, which can help towards understanding of their formation mechanism.

Argon-ion-induced formation of nanoporous GaSb layer: Microstructure, infrared luminescence, and vibrational properties

Room temperature implantation of 60 keV Ar+-ions in GaSb to the fluences of 7 × 1016 to 3 × 1018 ions cm−2 is carried out at two incidence angles, viz 0° and 60°, leading to formation of a nanoporous layer. As the ion fluence increases, patches grow on the porous layer under normal ion implantation, whereas the porous layer gradually becomes embedded under a rough top surface for oblique incidence of ions. Grazing incidence x-ray diffraction and cross-sectional transmission electron microscopy studies reveal the existence of nanocrystallites embedded in the ion-beam amorphized GaSb matrix up to the highest fluence used in our experiment. Oxidation of the nanoporous layers becomes obvious from x-ray photoelectron spectroscopy and Raman mapping. The correlation of ion-beam induced structural modification with photoluminescence signals in the infrared region has further been studied, showing defect induced emission of additional peaks near the band edge of GaSb.

Solid state reaction induced phase evolution of Ni/Bi thin films

Effect of thermal annealing on the structural and electrical properties of Ni/Bi layer deposited on Si substrates are studied as a function of temperature. Agglomeration of the thin films into island-like structures is observed after annealing whereas grazing angle x-ray diffraction studies shows formation of both NiBi and NiBi3 phases. We discuss the phase evolution in terms of diffusion of Bi and NiBi3/NiBi phase formation at the interfaces. Semi-metallic nature of the as-deposited film is revealed by resistivity versus temperature study, which transforms to metallic behavior after annealing.