O.P. Sinha

AFM study of swift gold ion irradiated silicon

Abstract AFM studies of swift (∼100 MeV) heavy ion (Au7+) irradiated crystalline p- and n-type silicon surfaces have been performed. The irradiation fluence has been varied from 1010 to 10 12 ions cm −2 . The studies have shown the formation of clear craters surrounding a hill for the irradiated p-type only whereas a hill on one side of the crater for the irradiated p- and n-type silicon surfaces has been observed. The surface roughness has been observed to be increased after the irradiation and the increase for n-type is significantly larger as compared to that for p-type. The quantitative estimate of the volume of the craters and hills has shown a larger volume (2.0–4.0 times) for the craters as compared to the hills. Moreover a larger volume for craters and hills for n-type as compared to p-type has been found. The observed features have been discussed as the radiation induced mass transfer from within the substrate to its surface and a fraction sputtered out due to the explosive evaporation as a result of heating due to electronic energy loss. The distinctly different features for n-type and p-type seem to be due to dopant dependent fault growth and shrinkage rates.

Large Area Fabrication of Semiconducting Phosphorene by Langmuir-Blodgett Assembly

Phosphorene is a recently new member of the family of two dimensional (2D) inorganic materials. Besides its synthesis it is of utmost importance to deposit this material as thin film in a way that represents a general applicability for 2D materials. Although a considerable number of solvent based methodologies have been developed for exfoliating black phosphorus, so far there are no reports on controlled organization of these exfoliated nanosheets on substrates. Here, for the first time to the best of our knowledge, a mixture of N-methyl-2-pyrrolidone and deoxygenated water is employed as a subphase in Langmuir-Blodgett trough for assembling the nanosheets followed by their deposition on substrates and studied its field-effect transistor characteristics. Electron microscopy reveals the presence of densely aligned, crystalline, ultra-thin sheets of pristine phosphorene having lateral dimensions larger than hundred of microns. Furthermore, these assembled nanosheets retain their electronic properties and show a high current modulation of 104 at room temperature in field-effect transistor devices. The proposed technique provides semiconducting phosphorene thin films that are amenable for large area applications.

Swift heavy-ion-induced epitaxial crystallization of buried Si3N4 layer

We report on swift heavy-ion-beam-induced epitaxial crystallization of a buried Si3N4 layer. We observe good epitaxial crystallization at 200 °C, which is a much lower temperature than that required for the conventional solid phase epitaxial growth. High-resolution transmission electron microscopy and selected area diffraction patterns have been used to study the 100-MeV Ag8+-ion-beam-induced epitaxial growth of the Si3N4 layer. A possible mechanism of recrystallization is discussed on the basis of synergetic effects of electronic and nuclear energy-loss processes along the trajectory of the swift heavy ions at elevated temperatures.

Recrystallization of ion-irradiated germanium due to intense electronic excitation

Germanium single crystals were irradiated at room temperature by 1.5MeV energy germanium ions and high energy silver ions of 100MeV. Based on the transmission and high-resolution electron microscopic investigations, we present the experimental evidence of complete recrystallization of the amorphized germanium layer, formed by the self-ion-implantation, due to intense electronic excitations generated by the swift Ag ions. This phenomenon is observed at room temperature—far below the solid phase epitaxial growth temperature and that at which low energy ion beam induced epitaxial crystallization takes place. The results are explained in the light of local transient melting due to a high rate of energy deposition by the silver ions and its subsequent cooling. Based on the calculations on thermal spike concept in combination with the nonequilibrium thermodynamics, we obtain a reasonably good estimate for the experimental observation.

MeV heavy ion induced recrystallization of buried silicon nitride layer: Role of energy loss processes

We report on MeV heavy ion beam induced epitaxial crystallization of a buried silicon nitride layer. Transmission electron micrographs and selected area diffraction patterns are used to study the recrystallization of an ion beam synthesized layer. We observe complete recrystallization of the silicon nitride layer having good quality interfaces with the top and substrate Si. Recrystallization is achieved at significantly lower temperatures of 100, 150, and 200°C for oxygen, silicon, and silver ions, respectively. The fact that recrystallization is achieved at the lowest temperature for the oxygen ions is discussed on the basis of the energy loss processes.

AFM studies of swift heavy ion-irradiated surface modification in Si and GaAs

Abstract Atomic force microscopy (AFM) studies of Swift Heavy Ions (SHI ∼100 MeV Si 7+ and Au7+) irradiated Si and GaAs surfaces have been performed for a variable fluence in the range of 1010– 10 13 ions cm −2 . The craters with piled up material, which is called hill, are clearly seen in the micrographs. A significant direct observation of amorphization (or melting due to SHI irradiation damage), plastic flow and subsequent recrystallization in the form of platelets has been made. The quantitative estimation of the features revealed that the volume of the craters for silicon ion irradiation is smaller than the gold ion irradiation. However, surface roughness has been found to be enhanced after the irradiation. Moreover, the GaAs surfaces were found to be less rough than the Si surface. The features are related to the difference in electronic energy loss of incident ions, thermal diffusivity, thermal conductivity and density of the target materials.

HIGH ENERGY HEAVY ION IRRADIATION IN SEMICONDUCTORS

Abstract Pd/n-Si and Pd/n-GaAs devices have been irradiated from high energy (∼100 MeV) heavy ions of Au7+ (gold) and Si7+ (silicon) to study the irradiation effects in these junction devices on semiconductor substrates. The devices have been characterized from I–V and C–V studies for electronic flow characterization. It has been found that the devices become high resistive on the irradiation and the substrates change the conductivity type from n- to p- on the irradiation of fluence of ∼1012–1013 ions/cm2. The change in conductivity type has been understood as a result of creation of deep acceptors on the irradiation.

In situ I–V study of swift (~100 MeV) O6+ ion-irradiated Pd/n-Si devices

An in situ I–V study of Pd/n-Si devices irradiated to swift (~100 MeV) O6+ ions for a fluence of 1011–1013 cm−2 has been carried out. The devices have been irradiated at room and LN2 temperatures. The irradiated devices have been hydrogenated in ex situ condition by molecular hydrogen. It has been observed that resistivity increases after the irradiation and there is a progressive increase with the increase of irradiation fluence. On hydrogenation, the devices irradiated at LN2 temperature show that the irradiation-induced increased resistivity decreases back to the pre-irradiated condition, whereas the devices irradiated at room temperature do not show any change. The results have been understood in the realm of irradiation-induced defects.

Application of 2D-MoO3 nano-flakes in organic light emitting diodes: effect of semiconductor to metal transition with irradiation

The current work demonstrates efficient utilization of 2D-MoO3 nano-flakes as a hole injection layer (HIL) in organic light emitting diodes (OLEDs). Nano-flakes are synthesized using an organic solvent-assisted grinding and sonication method of liquid exfoliation for MoO3, and 8–16 nm thick flakes are obtained. The effect of solar illumination on the hole injection properties of these nano-flakes is then studied by exposing the nano-flakes for 0, 15, 30, 45, 60 and 120 min and using them as HIL in green OLED. The device results are then compared with the OLED having bulk MoO3 as HIL. OLEDs with nano-flakes as the HIL have shown better performance than the OLED with bulk MoO3 as the HIL due to the better semiconducting properties in the nano-flake phase. The luminous intensity is increased by increasing the duration of irradiation and was found to be optimum in case of nano-flakes irradiated for 30 or 45 min and then started to decrease with the increase of duration of irradiation. The current density in the OLEDs with nano-flakes as the HIL shows a switching from high resistance to low resistance; however, the sequential pattern of switching voltage was missing with the duration of irradiation. The current density also decreased for nano-flakes with 60 and 120 min of irradiation. Transition from the semiconducting to metal nature of nano-flakes by solar irradiation is suggested to be the reason behind this decrease in current density and luminous intensity with a longer duration of illumination.

Surface modified ZnO nanoparticles: structure, photophysics, and its optoelectronic application

Nanostructured ZnO has been synthesized by wet chemical route. Poly(vinylpyrrolidone) is used for stabilization and surface passivation of synthesized nanoparticles, thus tailoring the growth of ZnO at nanoscale. Structural characterization using X-ray diffraction, scanning electron microscopy, high-resolution transmission electron micoroscopy and Fourier transformed infrared spectroscopy of the synthesized nanoparticles confirm the evolution of nanocrystalline ZnO prevailing in hexagonal wurtzite phase. UV–Vis and photoluminescence spectroscopy studies show blue shift phenomenon in the synthesized nanoparticle in contrast to the bulk ZnO furnishing evidence in support of quantum size effect. The nanocomposites of ZnO and poly [9,9-dioctylfluorenyl-2,7-diyl] are prepared and characterized to investigate its luminescent and spectral emission effects. The nanocomposites are then incorporated in light-emitting diodes, and influence of ZnO on the device performance has been explored via electroluminescence, current density evaluation, and corresponding CIE coordinates calculation.