Rakesh Aluguri

Dielectric and transport properties of carbon nanotube-CdS nanostructures embedded in polyvinyl alcohol matrix

Multiwalled carbon nanotube-CdS/polyvinyl alcohol (MWCNT-CdS/PVA) composites have been grown by a simple chemical process on one-dimensional templates. The plane-view transmission electron micrographs clearly indicate the formation of nanocrystalline CdS on the nanotube surfaces. The superior dielectric behavior of the MWCNT-CdS nanostructures over MWCNT and PVA host matrices has been demonstrated. The dc and ac transport properties of CdS carbon nanotube-insulating polymer nanocomposites have been studied using impedance spectroscopy. An enhancement in optical band gap of nanocomposites over the bulk CdS has been observed due to the quantum confinement effect in CdS nanocrystals.

Optical and electrical properties of undoped and doped Ge nanocrystals

Size-dependent photoluminescence characteristics from Ge nanocrystals embedded in different oxide matrices have been studied to demonstrate the light emission in the visible wavelength from quantum-confined charge carriers. On the other hand, the energy transfer mechanism between Er ions and Ge nanocrystals has been exploited to exhibit the emission in the optical fiber communication wavelength range. A broad visible electroluminescence, attributed to electron hole recombination of injected carriers in Ge nanocrystals, has been achieved. Nonvolatile flash-memory devices using Ge nanocrystal floating gates with different tunneling oxides including SiO2, Al2O3, HfO2, and variable oxide thickness [VARIOT] tunnel barrier have been fabricated. An improved charge storage characteristic with enhanced retention time has been achieved for the devices using VARIOT oxide floating gate.

Enhancement of photoluminescence intensity of erbium doped silica containing Ge nanocrystals: distance dependent interactions

Photo-physical processes in Er-doped silica glass matrix containing Ge nanocrystals prepared by the sol-gel method are presented in this article. Strong photoluminescence at 1.54 μm, important for fiber optics telecommunication systems, is observed from the different sol-gel derived glasses at room temperature. We demonstrate that Ge nanocrystals act as strong sensitizers for Er(3+) ions emission and the effective Er excitation cross section increases by almost four orders of magnitude with respect to the one without Ge nanocrystals. Rate equations are considered to demonstrate the sensitization of erbium luminescence by Ge nanocrystals. Analyzing the erbium effective excitation cross section, extracted from the flux dependent rise and decay times, a Dexter type of short range energy transfer from a Ge nanocrystal to erbium ion is established.

Temperature dependent photoluminescence and electroluminescence characteristics of core-shell Ge–GeO2 nanowires

Ge nanowires of several micrometers long and an average diameter of  ∼70 nm have been grown by the vapour–liquid–solid technique. Different structural characteristics indicate the presence of a Ge core along with a thin (~10 nm) amorphous GeO2 shell layer. The signature of direct as well as indirect band gap emission is observed from the temperature dependent photoluminescence measurements of Ge nanowires. Room temperature near infrared electroluminescence has been achieved from the metal–insulator–semiconductor structure consisting of Ge nanowires. The phonon mediated indirect transition dominates the electroluminescence characteristics above 100 K, whereas a no phonon direct transition at low temperatures makes the device attractive for optical emitters.

Effect of growth temperature and post-growth annealing on luminescence properties of molecular beam epitaxy grown single layer Ge quantum dots

We report the detailed structural and luminescence properties of Si-embedded single layer Ge nanoislands grown via Stranski-Krastanov mechanism using molecular beam epitaxy. The luminescence energy peak positions correlated with structural properties have been studied as a function of the growth temperature and post-growth annealing. The photoluminescence peak for the annealed sample (500 °C grown) is blue shifted with increasing anneal temperature compared to the as grown sample due to intermixing of Si/Ge. Electroluminescence from the single layer Ge islands in the wavelength range 1.53 to 1.67 μm has been demonstrated using a metal-insulator-semiconductor structure, making it attractive for quantum dot light emitting devices.

Multilayer Ge nanocrystals embedded within Al2O3 matrix for high performance floating gate memory devices

Metal-insulator-silicon devices with Ge nanocrystals dispersed in Al2O3 have been studied with a view to exploit them for floating gate memory applications. Multilayer devices comprising of five layers Ge nanocrystals have exhibited superior memory characteristics over the single layer Ge and multilayer Si nanocrystals reported in literature. The effect of interface traps on the memory behavior using frequency dependent capacitance- and conductance-voltage measurements has been investigated. This study has demonstrated an enhanced memory window with superior retention characteristics, owing to the Coulomb blockade effect, due to the introduction of multi-layer nanocrystals in the floating gate.

MBE-grown Si and Si1−xGex quantum dots embedded within epitaxial Gd2O3 on Si(111) substrate for floating gate memory device

Si and Si(1-x)Ge(x) quantum dots embedded within epitaxial Gd2O3 grown by molecular beam epitaxy have been studied for application in floating gate memory devices. The effect of interface traps and the role of quantum dots on the memory properties have been studied using frequency-dependent capacitance-voltage and conductance-voltage measurements. Multilayer quantum dot memory comprising four and five layers of Si quantum dots exhibits a superior memory window to that of single-layer quantum dot memory devices. It has also been observed that single-layer Si(1-x)Ge(x) quantum dots show better memory characteristics than single-layer Si quantum dots.

Direct band gap optical emission from Ge islands grown on relaxed Si0.5Ge0.5/Si (100) substrate

Strained Ge islands have been grown on fully relaxed Si0.5Ge0.5 substrate by pulsed laser ablation technique. The formation of strained Ge islands has been found for film with higher thickness following Stranski–Krastanov growth mechanism. The variation of strain with changing Ge layer thickness has been analyzed using Raman spectroscopy and high-resolution X-ray diffraction techniques. X-ray photoelectron spectra have shown the absence of any Si-Ge intermixing and oxidation of Ge films. A strong no-phonon photoluminescence emission from Ge islands has been observed, showing the superior optical characteristics of the islands grown on relaxed substrate.

Enhanced charge storage characteristics of nickel nanocrystals embedded flash memory structures

Nickel nanocrystals (NCs) embedded flash memory structure with SiO2 tunnelling and pulsed laser deposition grown Al2O3 blocking oxide is reported here. The post-deposition thermal annealing of an ultrathin (∼5 nm) nickel film evaporated on thermally grown SiO2 tunnelling barrier has been performed to form the nickel NCs. The formation of tiny nickel NCs is confirmed from the high-resolution transmission electron microscope micrographs. The electrical capacitance–voltage (C–V) characteristics of the optimised 1000°C, 5 min annealed sample shows a large memory window of 20 V at ±20 V sweeping voltage. The charge storage properties are found to be significantly improved as compared to control samples. The frequency dependent C–V measurements indicate the dominant charge trapping in Ni NCs, making the memory structure attractive for use in future nanoscale high-performance applications.

Conductive bridge random access memory characteristics of SiCN based transparent device due to indium diffusion

In this work, the transparent bipolar resistive switching characteristics of a SiCN-based ITO/SiCN/AZO structure due to In diffusion from ITO is studied. The SiCN based device is found to be 80% transparent in the visible wavelength region. This device, with AZO as both top and bottom electrodes, does not show any RRAM property due to deposition of the high quality O2-free SiCN film. Replacing the AZO top electrode with ITO in this device results in good resistive switching (RS) characteristics with a high on/off ratio and long retention. Replacing the SiCN film with ZrO2 also results in excellent RS characteristics due to the formation of an oxygen vacancies filament inside the ZrO2 film. A resistance ratio of on/off is found to be higher in the SiCN based device compared to that of the ZrO2 device. Diffusion of In from ITO into the SiCN film on application of high positive voltage during forming can be attributed to the occurrence of RS in the device, which is confirmed by the analyses of energy dispersive spectroscopy and secondary-ion mass spectrometry. This study shows a pathway for the fabrication of CBRAM based transparent devices for non-volatile memory application.