Browsing by Author "Monsegue, Niven"

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    Characterizing the Effects of Mechanical Alloying on Solid State Amorphization of Nanoscaled Multilayered Ni-Ti
    Monsegue, Niven (Virginia Tech, 2010-07-09)
    Equiatomic composition of Ni and Ti was cryomilled with varying milling times to create Ni-Ti lamella structures with average spacings of 50 nm, 470 nm, and 583 nm in powder particles to vary the interfacial surface area per volume. These surfaces form interfaces for diffusion that are essential for solid state amorphization during low temperature annealing. To compare solid state amorphization in a relatively defect free multilayer system, elemental Ni and Ti were deposited by electron beam physical vapor deposition on titanium plates with comparable spacing as above. Both milled and deposited multilayers were annealed between 225 and 350°C for up to 50 hours. X-ray diffraction characterization and in situ annealing was conducted on cryomilled and deposited multilayers of Ni-Ti. Based on this characterization, an amorphization model based on the Johnson-Mehl-Avrami nucleation and growth equation has been established to predict the amorphization of both cryomilled and deposited multilayers. Cryomilled powders experienced much larger amorphization rates during annealing than that of deposited multilayer structures, for all layer spacings. This superior amorphization is seen despite the formation of amorphous phase during the milling process; the amount of which increases with increasing milling time. The difference in amorphization rates between cryomilled and deposited multilayers is attributed to excess driving force due to the extensive preexisting defects in the powders caused by cryomilling. Serial 3D reconstruction of cryomilled Ni-Ti powders was done by scanning electron microscopy and focused ion beam. Through 3D reconstruction it was observed that a random and non-linear lamella structure has been formed in cryomilled powders. Furthermore, lamellar spacing was seen to become smaller with increased milling time while at the same time becoming more homogeneous through the material's volume. 3D reconstruction of cryomilled Ni-Ti offers a unique insight into the microstructures and surface areas of cryomilled powder particles that has never been accomplished.
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    Defect assistant band alignment transition from staggered to broken gap in mixed As/Sb tunnel field effect transistor heterostructure
    Zhu, Yizheng; Jain, Nikhil; Vijayaraghavan, S.; Mohata, Dheeraj K.; Datta, Suman; Lubyshev, Dmitri; Fastenau, Joel M.; Liu, Amy K.; Monsegue, Niven; Hudait, Mantu K. (American Institute of Physics, 2012-11-01)
    The compositional dependence of effective tunneling barrier height (E-beff) and defect assisted band alignment transition from staggered gap to broken gap in GaAsSb/InGaAs n-channel tunnel field effect transistor (TFET) structures were demonstrated by x-ray photoelectron spectroscopy (XPS). High-resolution x-ray diffraction measurements revealed that the active layers are internally lattice matched. The evolution of defect properties was evaluated using cross-sectional transmission electron microscopy. The defect density at the source/channel heterointerface was controlled by changing the interface properties during growth. By increasing indium (In) and antimony (Sb) alloy compositions from 65% to 70% in InxGa1-xAs and 60% to 65% in GaAs1-ySby layers, the E-beff was reduced from 0.30 eV to 0.21 eV, respectively, with the low defect density at the source/channel heterointerface. The transfer characteristics of the fabricated TFET device with an E-beff of 0.21eV show 2x improvement in ON-state current compared to the device with E-beff of 0.30 eV. On contrary, the value of E-beff was decreased from 0.21 eV to -0.03 eV due to the presence of high defect density at the GaAs0.35Sb0.65/In0.7Ga0.3As heterointerface. As a result, the band alignment was converted from staggered gap to broken gap, which leads to 4 orders of magnitude increase in OFF-state leakage current. Therefore, a high quality source/channel interface with a properly selected E-beff and well maintained low defect density is necessary to obtain both high ON-state current and low OFF-state leakage in a mixed As/Sb TFET structure for high-performance and lower-power logic applications. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4764880]
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    Effect of Mn substituents on the domain and local structures of Na1/2Bi1/2TiO3-BaTiO3 single crystals near a morphotropic phase boundary
    Yao, Jianjun; Yang, Yaodong; Monsegue, Niven; Li, Yanxi; Li, Jiefang; Zhang, Qinhui; Ge, Wenwei; Luo, Haosu; Viehland, Dwight D. (AIP Publishing, 2011-03-01)
    The ferroelectric domain and local structures of Na1/2Bi1/2TiO3-x% BaTiO3 (NBT-BT) and 0.14 at. % Mn substituted Na1/2Bi1/2TiO3-x% BaTiO3 (Mn:NBT-BT) single crystals with x=5.5 near a morphotropic phase boundary have been investigated by transmission electron microscopy. Increased ferroelectric ordering and enhanced in-plane octahedral tilting were observed for Mn: NBT-BT compared with NBT-BT. Bragg-filtered lattice images revealed that the size of the in-phase tilt domains of Mn:NBT-BT were on the order of 2 to 8 nm, with a tendency of alignment along {110}. (C) 2011 American Institute of Physics. [doi:10.1063/1.3573801]
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    Role of coexisting tetragonal regions in the rhombohedral phase of Na0.5Bi0.5TiO3-xat.%BaTiO3 crystals on enhanced piezoelectric properties on approaching the morphotropic phase boundary
    Yao, Jianjun; Monsegue, Niven; Murayama, Mitsuhiro; Leng, W. N.; Reynolds, William T. Jr.; Zhang, Qinhui; Luo, Haosu; Li, Jiefang; Ge, Wenwei; Viehland, Dwight D. (AIP Publishing, 2012-01-01)
    The ferroelectric domain and local structures of Na0.5Bi0.5TiO3-xat.%BaTiO3 (NBT-x%BT) crystals for x = 0, 4.5, and 5.5 have been investigated by transmission electron microscopy. The results show that the size of polar nano-regions was refined with increasing xat. %BT. The tetragonal phase volume fraction, as identified by in-phase octahedral tilting, was found to be increased with BT. The findings indicate that the large electric field induced strains in morphotropic phase boundary compositions of NBT-x%BT originate not only from polarization rotation but also polarization extension. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3673832]
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    Role of InAs and GaAs terminated heterointerfaces at source/channel on the mixed As-Sb staggered gap tunnel field effect transistor structures grown by molecular beam epitaxy
    Zhu, Yizheng; Jain, Nikhil; Vijayaraghavan, S.; Mohata, Dheeraj K.; Datta, Suman; Lubyshev, Dmitri; Fastenau, Joel M.; Liu, W. K.; Monsegue, Niven; Hudait, Mantu K. (American Institute of Physics, 2012-07-15)
    The structural, morphological, defect properties, and OFF state leakage current mechanism of mixed As-Sb type-II staggered gap GaAs-like and InAs-like interface heterostructure tunnel field effect transistors (TFETs) grown on InP substrates using linearly graded InxAl1-xAs buffer by molecular beam epitaxy are investigated and compared. Symmetric relaxation of >90% and >75% in the two orthogonal < 110 > directions with minimal lattice tilt was observed for the terminal GaAs0.35Sb0.65 and In0.7Ga0.3As active layers of GaAs-like and InAs-like interface TFET structures, respectively, indicating that nearly equal numbers of alpha and beta dislocations were formed during the relaxation process. Atomic force microscopy reveals extremely ordered crosshatch morphology and low root mean square roughness of similar to 3.17 nm for the InAs-like interface TFET structure compared to the GaAs-like interface TFET structure of similar to 4.46 nm at the same degree of lattice mismatch with respect to the InP substrates. The GaAs-like interface exhibited higher dislocation density, as observed by cross-sectional transmission electron microscopy, resulting in the elongation of reciprocal lattice point of In0.7Ga0.3As channel and drain layers in the reciprocal space maps, while the InAs-like interface creates a defect-free interface for the pseudomorphic growth of the In0.7Ga0.3As channel and drain layers with minimal elongation along the Delta omega direction. The impact of the structural differences between the two interface types on metamorphic TFET devices was demonstrated by comparing p(+)-i-n(+) leakage current of identical TFET devices that were fabricated using GaAs-like and InAs-like interface TFET structures. Higher OFF state leakage current dominated by band-to-band tunneling process due to higher degree of defects and dislocations was observed in GaAs-like interface compared to InAs-like interface where type-II staggered band alignment was well maintained. Significantly lower OFF state leakage current dominated by the field enhanced Shockley-Read-Hall generation-recombination process at different temperatures was observed in InAs-like TFET structure. The fixed positive charge at the source/channel heterointerface influences the band lineup substantially with charge density greater than 1 x 10(12)/cm(2) and the band alignment is converted from staggered gap to broken gap at similar to 6 x 10(12)/cm(2). Clearly, InAs-like interface TFET structure exhibited 4x lower OFF state leakage current, which is attributed primarily to the impact of the layer roughness, defect properties on the carrier recombination rate, suggesting great promise for metamorphic TFET devices for high-performance, and ultra-low power applications. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737462]
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    Solid State Synthesis of Bulk Amorphous Ni – 50AT% Ti Alloy
    Monsegue, Niven (Virginia Tech, 2007-11-21)
    The mechanical alloying (MA) process and hot isostatic pressing (HIP) were used to synthesize bulk amorphous Ni-Ti alloy as an alternative to the traditional methods of casting multi-component metallic alloys. Samples milled cryogenically for 10 hours provided a homogeneous lamella structure with spacing of 30-110 nm. X-ray diffraction and transmission electron microscopy studies indicated that there were alloying and amorphous phase within the layers of the MA powder prior to annealing or HIPing. The amount of amorphous phase increased with time when the milled powder was annealed at a constant temperature and with temperature when annealing time was held constant. The microhardness of the powder correspondingly increased with the amount of amorphous formed in the powders. The HIPing of the MA powder produced a close to 100% amorphous compact with some dispersion of nanocrystals in the amorphous matrix. However, densification was not achieved.