P.V. Wadekar

Above room-temperature ferromagnetism of Mn delta-doped GaN nanorods

One-dimensional nitride based diluted magnetic semiconductors were grown by plasma-assisted molecular beam epitaxy. Delta-doping technique was adopted to dope GaN nanorods with Mn. The structural and magnetic properties were investigated. The GaMnN nanorods with a single crystalline structure and with Ga sites substituted by Mn atoms were verified by high-resolution x-ray diffraction and Raman scattering, respectively. Secondary phases were not observed by high-resolution x-ray diffraction and high-resolution transmission electron microscopy. In addition, the magnetic hysteresis curves show that the Mn delta-doped GaN nanorods are ferromagnetic above room temperature. The magnetization with magnetic field perpendicular to GaN c-axis saturates easier than the one with field parallel to GaN c-axis.

Evolution of nanoripples on silicon by gas cluster-ion irradiation

Si wafers of (100), (110) and (111) orientations were bombarded by gas cluster ion beam (GCIB) of 3000 Ar-atoms/cluster on average at a series of angles. Similar surface morphology ripples developed in different nanoscales. A simple scaling functional satisfactorily describe the roughness and wavelength of the ripple patterns as a function of dosage and angle of incidence. The ripples are formed orthogonal to the incident cluster-ions at large off-normal angles. An ellipsoidal pattern was created by two consecutive irradiations incident in mutually orthogonal directions with unequal exposure times between each irradiation, from 7:1 to 10:1, beyond which the original ripple imprints would be over-written. This work was inspired by use of the ripples to seed growth of controlled nanostructures without patterning by lithography or predeposition of catalysts.

Observation of surface oxidation resistant Shubnikov-de Haas oscillations in Sb2SeTe2 topological insulator

The robustness of Sb2SeTe2 topological insulators against surface oxidation has been comparatively investigated through their magneto-transport and X-ray photoelectron spectroscopic properties with samples freshly cleaved or exposed to air over various timeframes. The magnetoresistance data exhibit Shubnikov-de Haas oscillations with the same period of oscillations for all samples regardless of surface oxidation, whereas the core-level electron binding energies of the constituent elements vary. That there is no shift in Fermi levels and no smearing-out in the amplitude of oscillations suggests that the surface states of the studied topological insulators are impervious to surface oxidation.

Effect of temperature and V/III ratio on the initial growth of indium nitride using plasma-assisted metal-organic chemical vapor deposition

The growth of Indium nitride (InN) was studied in the nucleation stage by metal-organic chemical vapor deposition technique using atomic nitrogen from an RF microwave plasma source. Deposition was carried out through a range of substrate temperatures from 375 to 550 °C and at varying V/III ratios from 950 to 3150. We found that the diffusion lifetime of In atoms on the substrate becomes maximized at the growth temperature 475 °C, in which low temperature photoluminescence exhibits the excellent optical properties of the materials with a bandgap of 0.69 eV and a width of 34 meV. In addition, we observed that nitrogen cracking efficiency is significantly improved by using plasma so that high quality InN crystallites were grown with a very low V/III ratio around 950.

Enhancement of carrier transport characteristic in the Sb 2 Se 2 Te topological insulators by N 2 adsorption

The carrier transport characteristics of Sb2Se2Te topological insulators were investigated, after exposure to different levels of nitrogen gas. The magnetoresistance (MR) slope for the Sb2Se2Te crystal increased by approximately 100% at 10 K after 2-days of exposure. The Shubnikov-de Haas (SdH) oscillation amplitude increased by 30% while oscillation frequencies remained the same. MR slopes and the mobilities had the same dependency on temperature over a wide temperature range. All measured data conformed to a linear correlation between MR slope and mobility, supporting our hypothesis that the MR increase and the SdH oscillation enhancement might be caused by mobility enhancement induced by adsorbed N2 molecular.

Water-modulated oxidation in the growth of m-ZnO epitaxial thin film by atomic layer deposition

The effects of extra H2O-modulated oxidation are reported on the structural, optical, and electrical properties of nonpolar m-plane ZnO thin films grown on m-plane Al2O3 substrates by atomic layer deposition. Films without modulation, one modulated layer, and two modulated layers are compared. Structural properties studied using x-ray reflectivity, x-ray diffraction, and transmission electron microscopy show that all the films have a largely similar thickness and epitaxial relations with their substrates, but the rocking curves grow broader as the number of modulations increases. However, the extra layer of water modulation reduces the surface roughness drastically and also improves the electrical properties as compared to the unmodulated ZnO films. Water modulation is believed to serve as a source of atomic oxygen that promotes compensation of the pre-existing oxygen vacancies. The films tend to exhibit larger mosaicity around the a-axis as compared to that around the c-axis.

Cathodoluminescence spectra of gallium nitride nanorods

Gallium nitride [GaN] nanorods grown on a Si(111) substrate at 720°C via plasma-assisted molecular beam epitaxy were studied by field-emission electron microscopy and cathodoluminescence [CL]. The surface topography and optical properties of the GaN nanorod cluster and single GaN nanorod were measured and discussed. The defect-related CL spectra of GaN nanorods and their dependence on temperature were investigated. The CL spectra along the length of the individual GaN nanorod were also studied. The results reveal that the 3.2-eV peak comes from the structural defect at the interface between the GaN nanorod and Si substrate. The surface state emission of the single GaN nanorod is stronger as the diameter of the GaN nanorod becomes smaller due to an increased surface-to-volume ratio.

Controlling Surface Morphology and Circumventing Secondary Phase Formation in Non-polar m-GaN by Tuning Nitrogen Activity

For the development of non-polar nitrides based optoelectronic devices, high-quality films with smooth surfaces, free of defects or clusters, are critical. In this work, the mechanisms governing the topography and single phase epitaxy of non-polar m-plane gallium nitride (m-GaN) thin films are studied. The samples were grown using plasma-assisted molecular beam epitaxy on m-plane sapphire substrates. Growth of pure m-GaN thin films, concomitant with smooth surfaces is possible at low radio frequency powers and high growth temperatures as judged by the high resolution x-ray diffraction, field emission scanning electron microscopy, and atomic force microscopy measurements. Defect types and densities are quantified using transmission electron microscopy, while Raman spectroscopy was used to analyze the in-plane stress in the thin films which matches the lattice mismatch analysis. Energy dispersive spectroscopy and cathodoluminescence support a congruent growth and a dominant near band edge emission. From the analysis, a narrow growth window is discovered wherein epitaxial growth of pure m-plane GaN samples free of secondary phases with narrow rocking curves and considerable smooth surfaces are successfully demonstrated.

Enhanced Ferromagnetic Interaction in Modulation-Doped GaMnN Nanorods

In this report, ferromagnetic interactions in modulation-doped GaMnN nanorods grown on Si (111) substrate by plasma-assisted molecular beam epitaxy are investigated with the prospect of achieving a room temperature ferromagnetic semiconductor. Our results indicate the thickness of GaN layer in each GaN/MnN pair, as well as Mn-doping levels, are essential for suppressing secondary phases as well as enhancing the magnetic moment. For these optimized samples, structural analysis by high-resolution X-ray diffractometry and Raman spectroscopy verifies single-crystalline modulation-doped GaMnN nanorods with Ga sites substituted by Mn atoms. Energy dispersive X-ray spectrometry shows that the average Mn concentration can be raised from 0.4 to 1.8% by increasing Mn fluxes without formation of secondary phases resulted in a notable enhancement of the saturation magnetization as well as coercive force in these nanorods.

Effects of mid-gap defects and barrier interface reactions on tunneling behaviors of ZnO-i-Si heterojunctions

Low-leakage pin diodes based on ZnO-i-Si are realized by redox reaction of aluminum with the native oxide SiOx into AlOx and by proper selection of annealing conditions. The main sources of electric leakage was found to arise from charge carrier tunneling via mid-gap states in the semiconductors or lowered tunneling barriers. Less mid-gap states in n-ZnO and high tunneling barrier of the i-layer are key to lowering the leakage. Proper post-annealing of pin diodes effectively heal the mid-gap defects, while maintaining the integrity of the tunneling layers, thus lowering the leakage currents to reach a rectification ratio of 2400, surpassing most similarly benchmarked devices reported in literature. Excessive annealing causes some part of the i-layer to transform into to ZnAl2O4 and Al:ZnO. High Al-doping and lowered potential barrier provided by ZnAl2O4 are responsible for high leakage currents in devices so fabricated.