Rajani K. Vijayaraghavan

Highly transparent and reproducible nanocrystalline ZnO and AZO thin films grown by room temperature pulsed-laser deposition on flexible Zeonor plastic substrates

Zeonor plastics are highly versatile due to exceptional optical and mechanical properties which make them the choice material in many novel applications. For potential use in flexible transparent optoelectronic applications, we have investigated Zeonor plastics as flexible substrates for the deposition of highly transparent ZnO and AZO thin films. Films were prepared by pulsed laser deposition at room temperature in oxygen ambient pressures of 75, 150 and 300 mTorr. The growth rate, surface morphology, hydrophobicity and the structural, optical and electrical properties of as grown films with thicknesses∼65–420 nm were recorded for the three oxygen pressures. The growth rates were found to be highly linear both as a function of film thickness and oxygen pressure, indicating high reproducibility. All the films were optically smooth, hydrophobic and nanostructured with lateral grain shapes of∼150 nm wide. This was found compatible with the deposition of condensed nanoclusters, formed in the ablation plume, on a cold and amorphous substrate. Films were nanocrystalline (wurtzite structure), c-axis oriented, with average crystallite size∼22 nm for ZnO and∼16 nm for AZO. In-plane compressive stress values of 2–3 GPa for ZnO films and 0.5 GPa forAZO films were found. Films also displayed high transmission greater than 95% in some cases, in the 400–800 nmwavelength range. The low temperature photoluminescence spectra of all the ZnO and AZO films showed intense near band edge emission. A considerable spread from semi-insulating to n-type conductive was observed for the films, with resistivity∼103 Ω cm and Hall mobility in 4–14 cm2 V−1 s−1 range, showing marked dependences on film thickness and oxygen pressure. Applications in the fields of microfluidic devices and flexible electronics for these ZnO and AZO films are suggested.

Fabrication and characterisation of GaAs nanopillars using nanosphere lithography and metal assisted chemical etching

We present a low-cost fabrication procedure for the production of nanoscale periodic GaAs nanopillar arrays, using the nanosphere lithography technique as a templating mechanism and the electrochemical metal assisted etch process (MacEtch). The room-temperature photoluminescence (PL) and Raman spectroscopic properties of the fabricated pillars are detailed, as are the structural properties (scanning electron microscopy) and fabrication process. From our PL measurements, we observe a singular GaAs emission at 1.43 eV with no indications of any blue or green emissions, but with a slight redshift due to porosity induced by the MacEtch process and characteristic of porous GaAs (π-GaAs). This is further confirmed via Raman spectroscopy, where additionally we observe the formation of an external cladding of elemental As around our nanopillar features. The optical emission is enhanced by an order magnitude (∼300%) for our nanopillar sample relative to the planar unprocessed GaAs reference.

Nondestructive Monitoring of Die Warpage in Encapsulated Chip Packages

We describe an X-ray diffraction imaging technique for nondestructive, in situ measurement of die warpage in encapsulated chip packages at acquisition speeds approaching real time. The results were validated on a series of samples with known inbuilt convex die warpage, and the measurement of wafer bow was compared with the results obtained by optical profilometry. We use the technique to demonstrate the impact of elevated temperature on a commercially sourced micro quad flat nonlead chip package and show that the strain becomes locked in at a temperature between 94 °C and 120 °C. Using synchrotron radiation at the Diamond Light Source, warpage maps for the entire 2.2 mm × 2.4 mm × 150-μm Si die were acquired in 50 s, and individual line scans in times as short as 500 ms.

Pulsed-Plasma Physical Vapor Deposition Approach Toward the Facile Synthesis of Multilayer and Monolayer Graphene for Anticoagulation Applications.

We demonstrate the growth of multilayer and single-layer graphene on copper foil using bipolar pulsed direct current (DC) magnetron sputtering of a graphite target in pure argon atmosphere. Single-layer graphene (SG) and few-layer graphene (FLG) films are deposited at temperatures ranging from 700 °C to 920 °C within <30 min. We find that the deposition and post-deposition annealing temperatures influence the layer thickness and quality of the graphene films formed. The films were characterized using atomic force microscopy (AFM), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and optical transmission spectroscopy techniques. Based on the above studies, a diffusion-controlled mechanism was proposed for the graphene growth. A single-step whole blood assay was used to investigate the anticoagulant activity of graphene surfaces. Platelet adhesion, activation, and morphological changes on the graphene/glass surfaces, compared to bare glass, were analyzed using fluorescence microscopy and SEM techniques. We have found significant suppression of the platelet adhesion, activation, and aggregation on the graphene-covered surfaces, compared to the bare glass, indicating the anticoagulant activity of the deposited graphene films. Our production technique represents an industrially relevant method for the growth of SG and FLG for various applications including the biomedical field.

Zn Doped Nanocrystalline CuCl Thin Films for Optoelectronic Applications

We report on the use of Zn as an n-type dopant in CuCl thin films for optoelectronic applications, wherein maximum n-type doping of the order of 1018 cm -3 has been achieved. Zn doped nanocrystalline CuCl thin films are successfully deposited on glass and Si substrates by pulsed dc magnetron sputtering. Structural and morphological properties are investigated using X-ray diffraction (XRD) studies and Scanning Electron Microscopy (SEM), respectively. The conductivity of the CuCl:Zn films is examined using the four point probe technique. An order of magnitude increase in the conductivity of CuCl, by the doping with Zn is reported herein. The doped CuCl films display strong room temperature cathodoluminescence (CL) at ~ 385nm, which is similar to that of the undoped films. Hall Effect measurements show an n-type conductivity of the doped films.

Highly enhanced UV responsive conductivity and blue emission in transparent CuBr films: implication for emitter and dosimeter applications

Highly efficient transparent blue emitters have been pursued for many years, driven in large part by solid-state lighting technology, and the need for blue/UV spectroscopic sources. CuBr is a strong candidate material, chiefly due to its relatively large excitonic binding energies. However, the semiconductor copper halides (CuCl, CuBr, CuI) have been plagued by their relatively inefficient light emission properties, often attributed to intrinsic defect structures. Here we report a novel UV-treatment based approach to achieve massive and persistent enhancements in the room temperature electrical conductivity and blue photoluminescence (PL) in CuBr films in ambient air, an important finding for future light emitting application. After this treatment (typically UV exposure for ∼20 minutes), we have observed an approximately 5 orders of magnitude enhancement in electrical conductivity, and 2 orders of magnitude enhancement in PL, respectively. These enhancements are correlated with the cumulative UV exposure. We also found that the emission energies of the films can be tuned by varying the excitation wavelength. A mechanism based on the formation of luminescent excimer/exciplexes with cuprophilic interactions is proposed to explain the observed unusual photophysical characteristics. The potential of these films for UV dosimeter application is demonstrated by fabricating a device and monitoring the current readout as a function of UV dose, which is substantially sensitive to a dose range of 1–1000 mJ cm−2. This work provides valuable insights into the interesting photophysical properties of CuBr films, and opens a pathway to their deployment across a wide range of applications.

Nondestructive X-ray diffraction measurement of warpage in silicon dies embedded in integrated circuit packages

X-ray diffraction imaging in both monochromatic and white beam section mode has been used to measure quantitatively the displacement and warpage stress in encapsulated silicon devices.

Nondestructive, In Situ Mapping of Die Surface Displacements in Encapsulated IC Chip Packages Using X-Ray Diffraction Imaging Techniques

We have recently shown that x-ray synchrotron-based B-Spline X-Ray Diffraction Imaging (B-XRDI), and laboratory-based X-Ray Diffraction (XRD) techniques, can be used to non-destructively and in situ evaluate die warpage and strain inside fully encapsulated chip packages. In this paper, we show that both synchrotron-based X-Ray Diffraction Imaging (XRDI) and laboratory-based XRD can be used to usefully evaluate die surface shape f(x, y) and vertical displacements (Δz) induced in commercially packaged chips at room temperature and at elevated temperatures consistent with typical package processing temperatures, e.g. 25 - 200 oC. We describe a suite of algorithms, which have been developed to generate full die Δz and warpage maps for the encapsulated die using the rocking curve (RC) data collected by laboratory-based XRD and these are correlated with section topography (ST) data acquired via synchrotron-based white beam XRDI. Confidence in the efficacy of the techniques was established through the correlation of the laboratory-based Δz vertical displacement maps with optical interferometry analysis for a series of golden samples of known and predetermined warpage.

Measurement of deposition rate and ion energy distribution in a pulsed dc magnetron sputtering system using a retarding field analyzer with embedded quartz crystal microbalance

A compact retarding field analyzer with embedded quartz crystal microbalance has been developed to measure deposition rate, ionized flux fraction, and ion energy distribution arriving at the substrate location. The sensor can be placed on grounded, electrically floating, or radio frequency (rf) biased electrodes. A calibration method is presented to compensate for temperature effects in the quartz crystal. The metal deposition rate, metal ionization fraction, and energy distribution of the ions arriving at the substrate location are investigated in an asymmetric bipolar pulsed dc magnetron sputtering reactor under grounded, floating, and rf biased conditions. The diagnostic presented in this research work does not suffer from complications caused by water cooling arrangements to maintain constant temperature and is an attractive technique for characterizing a thin film deposition system.