Dipak K. Goswami

Nanoscale self-affine surface smoothing by ion bombardment

The topography of silicon surfaces irradiated by a 2-MeV Si + ion beam at normal incidence and ion fluences in the range 10 1 5 -10 1 6 ions/cm 2 has been investigated using scanning tunneling microscopy. At length scales below ∼50 nm, surface smoothing is observed; the smoothing is more prominent at smaller length scales. The smoothed surface is self-affine with a scaling exponent α=0.53′0.03.

Ion-irradiation-induced mixing, interface broadening and period dilation in Pt/C multilayers

Pt/C multilayers of nanometric dimension have been irradiated with 2-MeV-Au ions to a fluence of 1×1014 ions/cm2 and analyzed by x-ray reflectivity and x-ray standing wave measurements. The multilayer period has expanded by ∼9%, the expansion of the Pt layers being larger than that of the C layers. Ion-induced displacement of atoms across the interfaces led to an increased interface roughness and an increase of 2 at. % Pt in C layers. Monte Carlo simulations for ion-induced atomic displacement have been used to explain the observed effects.

Enhanced Environmental Stability Induced by Effective Polarization of a Polar Dielectric Layer in a Trilayer Dielectric System of Organic Field-Effect Transistors: A Quantitative Study

We report a concept fabrication method that helps to improve the performance and stability of copper phthalocyanine (CuPc) based organic field-effect transistors (OFETs) in ambient. The devices were fabricated using a trilayer dielectric system that contains a bilayer polymer dielectrics consisting of a hydrophobic thin layer of poly(methyl methacrylate) (PMMA) on poly(vinyl alcohol) (PVA) or poly(4-vinylphenol) (PVP) or polystyrene (PS) with Al2O3 as a third layer. We have explored the peculiarities in the device performance (i.e., superior performance under ambient humidity), which are caused due to the polarization of dipoles residing in the polar dielectric material. The anomalous behavior of the bias-stress measured under vacuum has been explained successfully by a stretched exponential function modified by adding a time dependent dipole polarization term. The OFET with a dielectric layer of PVA or PVP containing hydroxyl groups has shown enhanced characteristics and remains highly stable without any degradation even after 300 days in ambient with three times enhancement in carrier mobility (0.015 cm(2)·V(-1)·s(-1)) compared to vacuum. This has been attributed to the enhanced polarization of hydroxyl groups in the presence of absorbed water molecules at the CuPc/PMMA interface. In addition, a model has been proposed based on the polarization of hydroxyl groups to explain the enhanced stability in these devices. We believe that this general method using a trilayer dielectric system can be extended to fabricate other OFETs with materials that are known to show high performances under vacuum but degrade under ambient conditions.

Study of sputtered particles from gold nano-islands due to MeV self-ion irradiation

Abstract Very thin films of gold deposited on silicon substrates form isolated nano-island structures due to the non-wetting nature of gold. Thick films are more homogeneous and do not have the isolated island structures. Thin gold films of various thicknesses (≈0.4–21.4 nm) are deposited under high vacuum condition and irradiated with 1.5 MeV Au2+ ions. The sputtered particles are collected on catcher grids (carbon coated) during the irradiation and are analyzed with transmission electron microscopy (TEM) and Rutherford backscattering spectrometry (RBS). The average sputtered particle size is determined from TEM measurements, whereas the amount of gold on the catcher grid is found by RBS. The average sputtered particle size from thin (up to a thickness of ≈2 nm) discontinuous films is larger compared to the average particle size from thick continuous films. The coverage of the sputtered particles on the catcher grids is also discussed. Energy spike and its distribution in the nano-islands is proposed to be the main reason for the variation in the particle size and the coverage of the sputtered particles on the catcher grid.

Crater formation in gold nanoislands due to MeV self-ion irradiation

The modification of gold nanoislands, grown on silicon substrates under high-vacuum conditions, by MeV self-ion irradiation has been studied by using scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and x-ray reflectivity. Upon irradiation with 1.5 MeV Au2+, two types of craters are observed on the Au islands: Empty craters and craters with a central hillock. The contribution of plastic flow, pressure spike, and sputtering to the crater formation during the ion impacts on the gold islands is analyzed. Thermal spike confinement within the gold islands is also proposed to be one of the possible reasons for crater formation in nanoislands.

Ion-beam induced transformations in nanoscale multilayers: Evolution of clusters with preferred length scales

Ion-irradiation-induced modifications of a periodic Pt∕C multilayer system containing a small amount of Fe have been analyzed by transmission electron microscopy and grazing incidence x-ray diffraction (GIXRD) studies. The multilayer stack with 16 Pt∕C layer pairs (period of 4.23nm) was fabricated on a glass substrate. A 2MeV Au2+ ion beam was rastered on the sample to obtain uniformly irradiated strips with fluences from 1×1014to1×1015ions∕cm2. Ion irradiation has been found to cause preferential migration of Fe towards Pt layers [Bera et al., Nucl. Instrum. Methods Phys. Res. B 212, 530 (2003)]. Cross-sectional transmission electron microscopy (XTEM) shows considerable atomic redistribution for irradiation at the highest ion fluence (1×1015ions∕cm2). This structure is composed of small clusters. Phase separation and cluster formation processes are discussed. Periodic multilayers have periodicity only in the direction normal to the multilayer surface. However, Fourier transform (FT) of the XTEM images of...

Low bias stress and reduced operating voltage in SnCl2Pc based n-type organic field-effect transistors

Vacuum deposited tin (IV) phthalocyanine dichloride (SnCl2Pc) field-effect transistors were fabricated on polymethylmethacrylate/aluminum oxide (PMMA/Al2O3) bilayer gate dielectric, with reduced operating voltage and low contact resistance. The devices with top contact Ag electrodes exhibit excellent n-channel behavior with electron mobility values of 0.01 cm2/Vs, low threshold voltages ∼4 V, current on/off ratio ∼104 with an operating voltage of 10 V. Bias stress instability effects are investigated during long term operation using thin film devices under vacuum. We find that the amount of bias stress of SnCl2Pc based thin film transistor is extremely small with characteristic relaxation time >105 s obtained using stretched exponential model. Stressing the SnCl2Pc devices by applying 10 V to the gate for half an hour results in a decrease of the source drain current, IDS of only ∼10% under low vacuum. These devices show highly stable electrical behavior under multiple scans and low threshold voltage inst...

High carrier mobility of CoPc wires based field-effect transistors using bi-layer gate dielectric

Polyvinyl alcohol (PVA) and anodized Al2O3 layers were used as bi-layer gate for the fabrication of cobalt phthalocyanine (CoPc) wire base field-effect transistors (OFETs). CoPc wires were grown on SiO2 surfaces by organic vapor phase deposition method. These devices exhibit a field-effect carrier mobility (μEF) value of 1.11 cm2/Vs. The high carrier mobility for CoPc molecules is attributed to the better capacitive coupling between the channel of CoPc wires and the gate through organic-inorganic dielectric layer. Our measurements also demonstrated the way to determine the thicknesses of the dielectric layers for a better process condition of OFETs.

Local diffusion induced roughening in cobalt phthalocyanine thin film growth.

We have studied the kinetic roughening in the growth of cobalt phthalocyanine (CoPc) thin films grown on SiO2/Si(001) surfaces as a function of the deposition time and the growth temperature using atomic force microscopy (AFM). We have observed that the growth exhibits the formation of irregular islands, which grow laterally as well as vertically with coverage of CoPc molecules, resulting rough film formation. Our analysis further disclosed that such formation is due to an instability in the growth induced by local diffusion of the molecules following an anomalous scaling behavior. The instability relates the (ln(t))(1/2), with t as deposition time, dependence of the local surface slope as described in nonequilibrium film growth. The roughening has been characterized by calculating different scaling exponents α, β, and 1/z determined from the height fluctuations obtained from AFM images. We obtained an average roughness exponent α = 0.78 ± 0.04. The interface width (W) increases following a power law as W ∼ t(β), with growth exponent β = 0.37 ± 0.05 and lateral correlation length (ξ) grows as ξ ∼ t(1/z) with dynamic exponent 1/z = 0.23 ± 0.06. The exponents revealed that the growth belongs to a different class of universality.

Ion beam studies in strained layer superlattices

Abstract The potential device application of semiconductor heterostructures and strained layer superlattices has been highlighted. Metal organic chemical vapour deposition grown In 0.53 Ga 0.47 As/InP lattice-matched structure has been irradiated by 130 MeV Ag 13+ and studied by RBS/Channelling using 3.5 MeV He 2+ ions. Ion irradiation seems to have induced a finite tensile strain in the InGaAs layer, indicating thereby that ion beam mixing occurs at this energy. Other complementary techniques like high resolution XRD and STM are needed to conclude the structural modifications in the sample.