G. Devaraju

Energy-transfer from ultra-small Au nanoclusters to Er3+ ions: a short-range mechanism

Sub-nanometric Au nanoclusters are known to act as very efficient sensitizers for the luminescent emission of Er(3+) ions in silica through a non-resonant broad-band energy-transfer mechanism. In the present work the energy-transfer process is investigated in detail by room temperature photoluminescence characterization of Er and Au co-implanted silica systems in which a different degree of coupling between Er(3+) ions and Au nanoclusters is obtained. The results allow us to definitely demonstrate the short-range nature of the interaction in agreement with non-radiative energy-transfer mechanisms. Moreover, an upper limit to the interaction length is also set by the Au-Au intercluster semi-distance which is smaller than 2.4 nm in the present case.

Synthesis and tailoring of GaN nanocrystals at room temperature by RF magnetron sputtering

We report here the synthesis of hexagonal GaN nanocrystals on p-silicon substrates by Radio-Frequency (RF) magnetron sputtering without substrate heating or by post-deposition annealing treatment. GaN nanocrystals are synthesized and tailored as a function of RF power at constant Ar and N2 flow rates (working pressure). The observed reduction in grain sizes as a function of RF power has been correlated with an increase in compressive strain. The effect of RF power on crystallite orientation has been determined by diffraction intensities using the degree of c-axis orientation. Atomic force microscopy shows uniform lateral nanocrystal sizes with spherical in shape by insignificant difference in Root-Mean-Square (RMS) roughness values for all the deposited thin films. Transmission electron microscope and field emission-scanning electron microscope studies have been performed to understand the surface morphologies and grain sizes. Thus, tailoring the size of the nanocrystals has been discussed by correlating RF powers, working pressures and cathode voltages on lattice vibrations.

SHI irradiation induced effects in functionalized MWCNTs

Multi-walled carbon nanotubes (MWCNTs) have attracted extensive attention globally due to their applications in modern nanotechnology. It is very important to study the effects on these MWCNTs under swift heavy ion (SHI) irradiation since ion irradiation has been recognized as one of the best tools for nanostructuring of materials, in general. Here, we present the effects of 80 MeV Ni ions with a fluence ranging from 3×1012 to 3×1013 ions/cm2 on functionalized MWCNT mats. The properties of pristine and irradiated samples were studied using X-ray diffraction, Raman spectroscopy and transmission electron microscopy. The effects of SHI irradiation on these samples are discussed in detail.

Ion beam-mixing effects in nearly lattice-matched AlInN/GaN heterostructures by swift heavy ion irradiation

Ion beam-induced intermixing is of great interest and has been observed in various systems such as metal–semiconductor interfaces. However, similar effects in III-nitrides have not been studied in any detail, yet. In the present study, swift heavy ion-induced intermixing of nearly lattice-matched Al(1−x) In x N/GaN heterostructures have been investigated using Rutherford backscattering spectrometry and high-resolution X-ray diffraction techniques. Inter-diffusion of Ga and In elements across the Al(1−x) In x N/GaN interface and the consequent formation of a new mixed quaternary alloy (AlGaInN) layer have been observed. The influence of electronic energy loss of SHI on intermixing effects has been studied.

Characterization Of GeO2 Nanocrystals Prepared By Microwave Annealing

GeOx films have been deposited on silicon substrate using RF magnetron sputtering. The as‐deposited samples were annealed at 900 °C using microwave annealing. All the samples were subsequently characterized by X‐ray diffraction (XRD) to observe the GeO2 nanocrystal formation. Raman spectroscopy and Transmission electron microscopy (TEM) measurements were also carried out to confirm the presence of the nanocrystals. The film topography was studied by atomic force microscopy (AFM).

Effects of concentration and thermal annealing on the optical activation of Er implanted into GaN layers

The wide band gap semiconductor, GaN, has emerged as an important host for rare earth-electroluminescence. The annealing behaviour and lattice site location of Er implanted into GaN were studied with the Rutherford Backscattering Spectrometry (RBS)/channelling and photoluminescence (PL) techniques. Also Er site dependence on the annealing temperature and implantation dose has been studied in detail. The optical properties of the Er-doped GaN system, evidencing their dependence on the parameters adopted during the synthesis procedure (Er implantation dose, annealing temperature) have been discussed. RBS/channelling measurements suggested that mostly Er occupy substitutional site and depends on the Er concentration. The main result is the activation of a typical Er giving rise to PL emission in the 1450–1650 nm range, related to radiative 4 I 13/2→4 I 15/2 transitions. Depending on the Er dose, we observe a specific behaviour linked to variation of the annealing temperature that strongly determines PL emission band. We observed a PL spectral shape with the main peak located at 1542 nm and shoulder peak at 1558 nm (and full width at half maximum (FWHM) of 33 nm) with a series of weaker PL structures at 1519, 1572 and 1591 nm, due to the Stark sub-level splitting.

Investigation of Strain in AlGaN/GaN Multi Quantum Wells by Complementary Techniques

Al0.49Ga0.51N (12 nm)/GaN (13 nm) Multi Quantum Wells of 15 periods are grown on sapphire by MOCVD technique. GaN/AlN, each of thickness 200 nm and 20 nm respectively, are used as buffer layers between substrate and epilayer to incorporate the strain in epilayers. It is a well established technique to engineer the band gap in AlxGa1−xN by adjusting alloy composition. These samples are used in visible & UV light emitters. In the present study, we employ a photoluminescence technique to estimate the composition and luminescence peak positions of AlGaN and GaN. Crystallinity and quality of interfaces have been studied by Rocking curve scan. The Threading Dislocations formed at the GaN buffer layer travel across the entire layers to the surface to form good quality films. Photo‐luminescence results show a very sharp GaN peak at 3.4 eV, as observed and reported by others, which shows that samples are free from point defects.

Ion-beam-treated strained AlGaN/GaN multi-quantum wells: HAADF-STEM, HRTEM, Raman and HRXRD characterizations

Ion beams are extensively used for the modification and analysis of a wide range of materials. However, basic mechanisms of defect propagation with the effect of different electronic energy losses are not understood yet and are essential to improve device performance. In the present study, AlGaN/GaN multi-quantum wells (MQWs), which were grown on sapphire by metal organic chemical vapor deposition, have been subjected to swift heavy ion irradiation at a fixed fluence as a function of type of projectile and its energy to understand the defect formation. Irradiated materials have been compared with the as-deposited ones to see the effects on composition, periodicity and strain. The composition gradient as a function of S e has been investigated with analytical and high-angle annular dark field scanning transmission electron microscopy (TEM) and the results have been further confirmed by high-resolution X-ray diffraction. The interface quality has been analyzed with aberration-corrected high-resolution TEM images. Effects of electronic energy loss on Raman modes have been investigated. Finally, the effects of ion beams on MQWs interfaces are discussed.

SHI Effects On Ge+SiO2 Composite Films Prepared By RF Sputtering

Ge+SiO2 composite films were deposited on Silicon substrate using RF magnetron sputtering. The as‐deposited samples were irradiated with 150 MeV Ag+12 ions at a fixed fluence of 3×1013 ions/cm2. These samples were subsequently characterized by X‐ray diffraction (XRD) and Raman spectroscopy to understand the crystallization behavior. Formation of Ge nanocrystal in amorphous silicon dioxide film was studied using transmission electron microscopy (TEM). We also studied the surface morphology of these high energy irradiated samples by Atomic Force Microscopy (AFM). The basic mechanism for ion beam induced crystallization in these films has been discussed.