Kishore K. Madapu

Growth of InN quantum dots to nanorods: a competition between nucleation and growth rates

Growth evolution of InN nanostructures via a chemical vapor deposition technique is reported using In2O3 as a precursor material and NH3 as reactive gas in the temperature range of 550–700 °C. Morphology of the nanostructures solely depends on the growth temperature, evolving from quantum dot sized nanoparticles to nanorods. It is found that 630 °C is the threshold temperature for nanorod growth. At 630 °C, nucleation starts with multifaceted particles having {10–12} surface planes. Subsequently, hexagonal polyhedral NRs are grown along the [0001] direction with non-polar surfaces of m-planes {10–10}. A comprehensive study is carried out to understand the evolution of nanorods as a function of growth parameters like temperature, time and gas flow rate. The change in the morphology of nanostructures is explained based on the nucleation and the growth rates during the phase formation. Raman studies of these nanostructures show that a biaxial strain is developed because of unintentional impurity doping with the increase in growth temperature.

Nano scale investigation of particulate contribution to diamond like carbon film by pulsed laser deposition

Diamond like carbon (DLC) films were synthesized by a Pulsed Laser Deposition technique at room temperature, with the laser pulse energy varying from 100 to 400 mJ. The films synthesized at different pulse energies are labeled as DLC-100, DLC-200, DLC-300 and DLC-400. Due to laser ablation, the DLC films contained a continuous phase and particulates embedded in it. These films were characterized using X-ray Photoelectron Emission Spectroscopy (XPS), Raman spectroscopy based intensity mapping and Atomic Force Acoustic Microscopy (AFAM). Carbon co-ordination inhomogeneity in DLC films was monitored by intensity mapping of the Raman peaks. The average sp2 cluster size in the particulates was calculated from the intensity ratio of the D and G peaks of the Raman spectra (ID/IG ratio) obtained from the particulate region and was found to vary from 0.85 to 1.41 nm with various laser pulse energies. Nanometric surface spatial elasticity distribution was mapped using AFAM, which revealed the presence of nanoscale surface irregularities in all the DLC films. The number density of particulates in the present DLC films is maximum in DLC-400 and minimum in DLC-100. Relative stiffness value of all the DLC films with its particulate regions were determined and compared with respect to the relative stiffness of Si. Raman spectroscopic intensity mapping and AFAM studies revealed that carbon co-ordination and local elasticity distribution across the film and particulate of DLC films are observed to depend upon sp2 bonded carbons possessing different aromatic orders and sp3 content. The sp3 bonding content present in DLC films synthesized at different laser pulse energies was obtained from X-ray photoelectron spectra. DLC-300 was found to be the stiffest film (∼4.5 times of Si) with highest sp3 content (53%).

Effect of strain relaxation and the Burstein–Moss energy shift on the optical properties of InN films grown in the self-seeded catalytic process

For the first time, high optical quality InN films were grown on sapphire substrate using atmospheric chemical vapour deposition technique in the temperature range of 560–650 o C. Self-catalytic approach was adopted to overcome the nucleation barrier for depositing InN films. In this process, seeding of the nucleation sites and subsequent growth was performed in the presence of reactive NH3. We investigated the simultaneous effect of strain and Burstein-Moss (BM) energy shift on optical properties of InN films using Raman and photoluminescence spectroscopy. Existence of compressive strain in all films is revealed by Raman spectroscopic analysis and is found to relax with increasing growth temperature. The asymmetric broadening of the A1(LO) phonon mode is observed with the onset of plasmonphonon interaction for films grown at 620 o C. Large blue shift of the band gap of InN (1.2 eV) is observed as a collective result of compressive strain in films as well as BM shift. Carrier density is calculated using the BM shift in the photoluminescence spectra.

Interpretation of friction and wear in DLC film: role of surface chemistry and test environment

In spite of the large amount of tribological work carried out to explain the friction and wear mechanism in diamond-like carbon (DLC) films, some of the core issues relating to the evolution of reactive species across sliding interfaces and their role on the friction and wear mechanism remain unclear. The phase composition, film density and hydrogen content present in a DLC film can be tailored by substrate biasing during film deposition to achieve a nearly vanishing friction coefficient. Furthermore, nitrogen doping in DLC films significantly improves wear resistance, and sliding occurs in a nearly wearless regime. Undoped and nitrogen-doped DLC films exhibit a nearly frictionless value with ultra-low wear behavior when tests are performed in argon, nitrogen and methane atmospheres. The antifriction and antiwear properties of the DLC films were improved with the reduction of adsorbed oxygen impurities on the film surface. This behavior was understood by correlating the oxygen impurities present at the surface/subsurface region of the DLC film while using x-ray photoelectron spectroscopy and depth-resolved Auger electron spectroscopy.

Formation of nanocrystalline SiGe in Polycrystalline-Ge/Si thin film without any metal induced crystallization

The formation of nanocrystalline SiGe without the aid of metal induced crystallization is reported. Re-crystallization of the as-deposited poly-Ge film (deposited at 450 °C) leads to development of regions with depleted Ge concentration upon annealing at 500 °C. Clusters with crystalline facet containing both nanocrystalline SiGe and crystalline Ge phase starts appearing at 600 °C. The structural phase characteristics were investigated by X-ray diffraction (XRD) and Raman spectroscopy. The stoichiometry of the SiGe phase was estimated from the positions of the Raman spectral peaks.

Aluminum induced crystallization of amorphous Ge thin films on insulating substrate

Aluminium (metal) induced crystallization of amorphous Ge in bilayer and multilayer Ge/Al thin films deposited on quartz substrate at temperature well below the crystallization temperature of bulk Ge is reported. The crystallization of poly-Ge proceeds via formations of dendritic crystalline Ge grains in the Al matrix. The observed phases were characterized by Raman spectroscopy and X-ray diffraction. The microstructure of Al thin film layer was found to have a profound influence on such crystallization process and formation of dendritic grains.

Growth of GaN nanostructures on graphene

GaN nanostructures with different morphologies are grown on few layer graphene (FLG) as template, using chemical-vapor-deposition technique in a self catalytic process using the large surface energy of graphene. Raman and photoluminescence studies reveal wurtzite GaN phase. Morphologies of these nanostructures varied depending on the number of layers in each template. Photoluminescence study reveals that growth occurs without deterioration of FLG layers and no incorporation of carbon in GaN nanostructures

Reduction in the formation temperature of Poly-SiGe alloy thin film in Si/Ge system

The role of deposition temperature in the formation of poly-SiGe alloy thin film in Si/Ge system is reported. For the set ofsamples deposited without any intentional heating, initiation of alloying starts upon post annealingat ∼ 500 °C leading to the formation of a-SiGe. Subsequently, poly-SiGe alloy phase could formonly at temperature ≥ 800 °C. Whereas, for the set of samples deposited at 500 °C, in-situ formation of poly-SiGe alloy thin film could be observed. The energetics of the incoming evaporated atoms and theirsubsequent diffusionsin the presence of the supplied thermal energy is discussed to understand possible reasons for lowering of formation temperature/energyof the poly-SiGe phase.The role of deposition temperature in the formation of poly-SiGe alloy thin film in Si/Ge system is reported. For the set ofsamples deposited without any intentional heating, initiation of alloying starts upon post annealingat ∼ 500 °C leading to the formation of a-SiGe. Subsequently, poly-SiGe alloy phase could formonly at temperature ≥ 800 °C. Whereas, for the set of samples deposited at 500 °C, in-situ formation of poly-SiGe alloy thin film could be observed. The energetics of the incoming evaporated atoms and theirsubsequent diffusionsin the presence of the supplied thermal energy is discussed to understand possible reasons for lowering of formation temperature/energyof the poly-SiGe phase.

Observation of surface plasmon polaritons in 2D electron gas of surface electron accumulation in InN nanostructures

Recently, heavily doped semiconductors have been emerging as an alternative to low-loss plasmonic materials. InN, belonging to the group III nitrides, possesses the unique property of surface electron accumulation (SEA), which provides a 2D electron gas (2DEG) system. In this report, we demonstrated the surface plasmon properties of InN nanoparticles originating from SEA using the real-space mapping of the surface plasmon fields for the first time. The SEA is confirmed by Raman studies, which are further corroborated by photoluminescence and photoemission spectroscopic studies. The frequency of 2DEG corresponding to SEA is found to be in the THz region. The periodic fringes are observed in the near-field scanning optical microscopic images of InN nanostructures. The observed fringes are attributed to the interference of propagated and back-reflected surface plasmon polaritons (SPPs). The observation of SPPs is solely attributed to the 2DEG corresponding to the SEA of InN. In addition, a resonance kind of behavior with the enhancement of the near-field intensity is observed in the near-field images of InN nanostructures. Observation of SPPs indicates that InN with SEA can be a promising THz plasmonic material for light confinement.

Near field optical and spectroscopic imaging of InN nanostructures

Conventional optical spectroscopy is limited by the diffraction limit which impose the condition of spatial resolution to be achieved ≤ λ/2. Near field optical microscopic techniques such as near field scanning optical microscopy (NSOM) and tip enhanced Raman spectroscopy (TERS) improves the spatial resolution by utilizing the evanescent field. Here, we studied the near field light matter interaction of InN nanostructures using the NSOM technique and achieved a spatial resolution of ~ 50 nm with 150 nm aperture tip and 532 nm light source. The optical contrast in the NSOM images is attributed to the local variation of dielectric constant of individual nanostructures. TERS imaging is performed with an atomic force microscopy (AFM) tip attached with a 300 nm Au particle to achieve a sub-diffraction spatial resolution of ∼200 nm using 785 nm laser excitation.