Li He

University of Wisconsin, Madison, Mississippi, United States

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Publications (9)10.82 Total impact

  • Li He, Robert Hull
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    ABSTRACT: This work investigates the quantification of electron-phonon thermal diffuse scattering (TDS) for detection of temperature variations with nanometer spatial resolution in transmission electron microscopy (TEM). Observations of TDS intensity for (100) single crystal Si and Ge show interdependences of temperature and sample thickness which can be understood through the angular distributions of electron-phonon scattering as a function of temperature. The temperature sensitivity of the integrated TDS intensity can be of the order of 10(-3) K(-1) for Si and Ge. This shows that measurement of the TDS intensity in the TEM is a promising means for nanoscale temperature measurement; our measurements to date have demonstrated that temperature changes as small as 5 K are detectable.
    Nanotechnology 05/2012; 23(20):205705. · 3.84 Impact Factor
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    ABSTRACT: Mn ions have been reported to segregate into intermetallic precipitates or form coherent clusters in crystalline Ge(1-x)Mn(x) thin films. In this study, we investigated the microstructure of amorphous Ge(1-x)Mn(x) synthesized using low temperature molecular beam epitaxy, and observed the self-assembly of Mn rich nanostructures in Ge matrix with both cluster and column shapes by varying the Mn concentration. The magnetotransport properties were found to closely correlate with the magnetism. Negative magnetoresistance (MR) showed a dominant effect in as-grown materials, and required a very high magnetic field to saturate, whereas only positive MR was observed in post-annealed Ge(1-x)Mn(x). The anisotropic behavior in magnetism and magnetoresistance will also be discussed.
    03/2011;
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    ABSTRACT: We introduced 1.1 at. % of Mn ions into Ge thin films in order to explore the ferromagnetism in Mn implanted Ge. Rapid thermal anneal (RTA) was applied after the implantation to recrystallize the Ge and enhance the incorporation of Mn ions into the Ge lattice. A maximum saturation moment of 0.7 μB/Mn at 5 K was reached when the sample was annealed at 300 °C for 1 min, and the moment decreased with higher annealing temperatures. Two transitions temperatures Tc and Tcl were observed corresponding to the global ferromagnetism in Mn:Ge bulk and short range magnetic ordering in Mn-rich clusters. Both critical temperatures increased with RTA temperatures and Tcl even persisted close to room temperature for the 400 °C, 1 min anneal. No secondary phases were observed.
    Journal of Applied Physics 11/2010; 108(9):093919-093919-6. · 2.21 Impact Factor
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    ABSTRACT: Spin switching devices constructed using self-assembled magnetic Ge quantum dots are the ultimate goal of this effort. These devices are based on the manipulation of magnetic exchange between magnetic quantum dots and/or coherent spin states in the quantum dots. In this study, we introduced 1.1 atomic % of Mn ions into Ge thin films and 3 at.% Mn into Ge Quantum Dots by ion implantation in order to explore the ferromagnetism in Mn implanted Ge. In order to recrystallize the Ge and enhance the incorporation of Mn ions into the Ge lattice, rapid thermal annealing was applied following the implantation. A maximum saturation moment of 0.7 muB/Mn at 5 K was reached when the sample was annealed at 300° C and the moment decreased with higher annealing temperatures. The ferromagnetism in Mn:Ge thin films disappeared at ~15 K. However, superparamagnetism has been observed at 1 T up to 200 K for the 300° C annealed film and persisted to room temperature for the 400° C annealed film. Mn implanted Ge Quantum Dots were also found to be ferromagnetic at low temperatures and showed a superparamagnet transition around 200 K.
    08/2009;
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    ABSTRACT: A novel method of indirect deposition by means of a focused ion beam (FIB) is utilized to develop metal/insulator/semiconductor nanowire core-shell structures. This method is based upon depositing an annular pattern centered on a nanowire, with secondary deposition then coating the wire. Typical cross-sectional deposition area increments as a function of ion doses are 1.3 × 10(-2) µm(2) nC(-1) for Pt and 3.5 × 10(-2) µm(2) nC(-1) for SiO(2). The structures are examined with a transmission electron microscope (TEM) using a new nanowire TEM sample preparation method that allows direct examinations of individually selected core-shell nanowires fabricated under different indirect FIB deposition conditions. Elemental analyses by means of energy dispersive x-ray spectroscopy and electron energy filtered TEM imaging verify the deposition of SiO(2) and Pt layers. Relatively uniform Pt and SiO(2) coatings on individual GaP nanowires can be achieved with overall thickness deviation of about 10% for deposition up to 25-30 nm thick Pt or SiO(2) shells. It should be possible to extend this approach to any nanowire/nanotube system, and to a wide range of coatings in any desired layer sequences.
    Nanotechnology 11/2008; 19(44):445610. · 3.84 Impact Factor
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    ABSTRACT: We have been studying the magnetic and transport properties of Mn doped Group IV semiconductors. Mn ions have been implanted into both boron doped P type and phosphorus doped N type Ge thin films respectively. A ferromagnetic hysteresis loop has been observed in P type samples at low temperatures and supermagnetism remains strong at 300 K. The P type samples show much stronger ferromagnetism than N type samples. At 5K, the ferromagnetic saturation moment (Ms) of 5% Mn doped p-Ge sample is ˜0.65 Bohr magneton per Mn, which is almost twice as much as that of the 5% Mn doped n-Ge. Rapid thermal annealing has been used to reduce the ion implantation damage as well as to help Mn ions to incorporate into Ge lattice. In this talk we will present magnetic, transport and electron microscopy characterization of these samples. We are in the process of trying to understand the nature of the ferromagnetism in these films and its correlation to carrier type.
    03/2008;
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    ABSTRACT: Exploring ferromagnetism in Group IV semiconductors is of great interest due to their potential application to spintronics. In this presentation, we discuss the ferromagnetism induced in thin Ge films by Mn^+ ion implantation as well as the correlation between their magnetism and their transport properties. The as-received Germanium on insulator (GOI) wafer consists of 200nm of (100) oriented Ge on 400nm of oxide both on a Si wafer. Mn ions were implanted at 300 ^oC into the Ge layer at 200 KeV. The ferromagnetism has been observed in Ge with a range of Mn concentration from 0.5 to 2 atom %. The sample with 2 % Mn doping has a Curie temperature near 300K and has a moment of ˜ 0.7 muB/Mn at 10 K. Transmission electron microscopy (TEM) reveals the formation of second phase clusters of which are probably responsible for the majority of the magnetism in this sample. In contrast, the 0.5 % and 1 % Mn as implanted Ge thin films behave like diluted ferromagnetic semiconductors, both have Curie temperatures is around 100 ˜150 K and the 0.5% sample doesn't show TEM evidence of a second phase. Our data indicates that the transport properties of Mn doped Ge correlates with the magnetism.
    03/2007;
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    ABSTRACT: We have used reactive-biased target-ion beam-sputter deposition to prepare CoxTi1-xO2 thin films on LaAlO3 (100) and SrTiO3 (100) substrates for 0.005
    03/2007;
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    ABSTRACT: First generation spintronics has entered the mainstream of information technology through its utilization of the magnetic tunnel junction in applicable devices such as read head sensors for hard disk drives and magnetic random access memory. The future of spintronic devices requires next generation spintronic materials (Wolf etal. in IBM J. Res. Dev. 9:101, [2006]). Here we report on the structural, transport, and magnetic characteristics of V1−x Cr x O2 (0.1≤x≤0.2) thin films deposited on (001) Al2O3 substrates. We show that the metal-insulator transition of VO2 is suppressed and the rutile structure is stable down to 100K. The films are remarkably smooth having a root-mean squared surface roughness of 0.3nm. Films are conductive at room temperature and appear to follow a variable-range-hopping conduction mechanism below that. Ferromagnetism is observed at room temperature and is dependent on Cr concentration. The combination of these characteristics makes V1−x Cr x O2 a viable candidate material for next generation spintronic multilayer devices.
    Journal of Superconductivity and Novel Magnetism 21(2):87-92. · 0.93 Impact Factor

Publication Stats

3 Citations
10.82 Total Impact Points

Institutions

  • 2012
    • University of Wisconsin, Madison
      • Department of Materials Science and Engineering
      Mississippi, United States
  • 2010–2012
    • Rensselaer Polytechnic Institute
      • Department of Materials Science and Engineering
      Troy, NY, United States
  • 2008
    • University of Virginia
      • Department of Materials Science and Engineering
      Charlottesville, VA, United States