Xie-Gang Zhu

Intrinsic Topological Insulator Bi2Te3 Thin Films on Si and Their Thickness Limit

High-quality Bi2Te3 films can be grown on Si by the state-of-art molecular beam epitaxy technique. In situ angle-resolved photo-emission spectroscopy measurement reveals that the as-grown films are intrinsic topological insulators and the single-Dirac-cone surface state develops at a thickness of two quintuple layers. The work opens a new avenue for engineering of topological materials based on well-developed Si technology.

Topological Insulator Thin Films of Bi2Te3 with Controlled Electronic Structure

Topological insulator thin films of Bi2Te3 with controlled electronic structure can be grown by regulating the molecular beam epitaxy (MBE) growth kinetics without any extrinsic doping. N- to p-type conversion results from the change in the concentrations of Te-Bi donors and Bi-Te acceptors. This represents a step toward controlling topological surface states, with potential applications in devices.

Intrinsic and extrinsic origins of room temperature ferromagnetism in Ni-doped ZnO films

The structural, electrical and magnetic properties of Ni-doped ZnO films with different Ni concentrations (x = 0–0.11, x: Ni concentration) and thicknesses (d = 15–330 nm, d: film thickness) prepared by radio-frequency magnetron sputtering have been systematically investigated. The structural characterizations indicate that Ni ions in the 2+ valence state, uniformly distributed in the film, almost substitute for the Zn ions when x ≤ 0.07, whereas when x increases up to 0.11, a second phase Ni is formed. Room temperature (RT) ferromagnetism (FM) has been observed for all the Ni-doped ZnO wurtzite films. The saturated magnetization varies drastically with the Ni concentration and the film thickness. A large magnetic moment of 2.80 μB/Ni is obtained in the 15 nm thick Zn0.96Ni0.04O film at RT. The results show that the FM observed is intrinsic for Ni-doped ZnO films and can be explained in terms of the bound magnetic polaron mechanism based on the presence of defects. In addition, the Ni precipitates owing to the excessive doping of Ni in ZnO, as an extrinsic origin, also contribute to the ferromagnetic properties in highly doped samples.

In situ Raman spectroscopy of topological insulator Bi 2 Te 3 films with varying thickness

AbstractTopological insulators (TIs) are a new state of quantum matter with a band gap in bulk and conducting surface states. In this work, the Raman spectra of topological insulator Bi2Te3 films prepared by molecular beam epitaxy (MBE) have been measured by an in situ ultrahigh vacuum (UHV)-MBE-Raman spectroscopy system. When the thickness of Bi2Te3 films decreases from 40 quintuple-layers (QL) to 1 QL, the spectral characteristics of some Raman modes appearing in bulk Bi2Te3 vary and a new vibrational mode appears, which has not been reported in previous studies and might be related to quantum size effects and symmetry breaking. In addition, an obvious change was observed at 3 QL when a Dirac cone formed. These results offer some new information about the novel quantum states of TIs.

Microstructure and photoluminescence study of vanadium-doped ZnO films

Zn1−xVxO films (x = 0, 1.3, 1.9, 2.6, 3.5, 4.3 and 6.2 at%) were prepared on glass substrates via the direct current reactive magnetron co-sputtering method. The microstructure, band gap, photoluminescence and Raman scattering of these films have been systematically investigated. The structural and chemical state characterizations indicate that the crystallinity of Zn1−xVxO films (x 2.6 at%) can be improved by V incorporation into the ZnO wurtzite lattice, but excess V will precipitate as V clusters when x 4.3 at%, resulting in the deterioration of crystal quality. The defects in Zn1−xVxO films (x 2.6 at%) can be suppressed by V doping, which induces the enhancement of the near band edge (NBE) emission. However, when x 4.3 at%, defects adjacent to the V dopant quench the NBE emission. On the other hand, the band gap of the films gradually increases with the increase in V concentration, which is accompanied by the blue shift of the NBE emission. (Some figures in this article are in colour only in the electronic version)

Hysteretic giant magnetoresistance curves induced by interlayer magnetostatic coupling in [Pd/Co]/Cu/Co/Cu/[Co/Pd] dual spin valves

The giant magnetoresistance (GMR) characteristics of hybrid perpendicular/Cu/in-plane magnetic anisotropy [Pd/Co]/Cu/Co single spin valves and [Pd/Co]/Cu/Co/Cu/[Co/Pd] dual spin valves with applied field perpendicular and parallel to the film plane are studied. Linear and nonhysteretic GMR behaviors are observed for the two spin valves in perpendicular-to-plane applied fields due to the coherent rotation of the Co layer driven by the in-plane shape anisotropy. The GMR value of the dual spin valve is 1.8 times as large as that of the single spin valve. The correlations between the magnetic configuration of the devices and the resistance evolvement are analyzed. In-plane GMR curves with maximum applied field of 30 kOe deviate from the linear behavior and can be well described by the Stoner–Wohlfarth model including high order anisotropy terms. Moreover, obvious hysteresis behavior is observed in the in-plane GMR curve of the dual spin valve. This phenomenon results from the hindered rotation of the Co/Pd mo...

Crystal structure and epitaxy of Bi2Te3 films grown on Si

We report comprehensive x-ray diffraction studies of the crystal structure and epitaxy of thin films of the topological insulator Bi2Te3 grown on Si (1 1 1). The films are single crystals of high crystalline quality, which strongly depend on that of their substrates, with in-plane epitaxial relationships of Bi2Te3 [2 1 −3 0] ‖ Si [1 −1 0] and Bi2Te3 [0 1 −1 0] ‖ Si [1 1 −2] along which the lattices of 1 × 3 Bi2Te3 and 2 × 2 Si supercells are well matched. As the samples age, we observe loss of crystalline Bi2Te3 film thickness accompanied with roughening of the crystalline interfaces, formation of new crystalline phases as well as compositional and structural modification of the Si substrate, consistent with the diffusion of Te into the Si substrate.

Interlayer magnetostatic coupling induced Co layer coercivity enhancement and exchange bias in [Pd/Co]/Cu/Co spin valves

Interlayer stray field coupling in hybrid perpendicular/Cu/in-plane magnetic anisotropy [Pd/Co]/Cu/Co single spin valves and [Pd/Co]/Cu/Co/Cu/[Co/Pd] dual spin valves was investigated. Enhancements of Co layer coercivity were observed after ac or dc demagnetization of the samples. This effect originates from the pinning of Co domain walls by the in-plane stray field from Co/Pd stripe domains, and the enhancements are more efficient as the Co/Pd domain size decreases. Moreover, the magnetostatic interaction between the net in-plane Co/Pd remanent moments and the Co moments leads to exchange bias in the Co layer of the dual spin valve after the dc demagnetization.

ENHANCEMENT OF SUPERCONDUCTIVITY OF Pb ULTRA-THIN FILMS BY THE INTERFACE EFFECT

We have studied the superconductivity of Pb ultra-thin films with thickness from 1 monolayer (ML) to 7 ML grown on Si(111) by molecular beam epitaxy. In situ low temperature scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) measurements were performed on the films. It is suggested that the interface effect plays a critical role in enhancing the electron-phonon coupling, which consequently increases the superconducting transition temperature when a film reaches to the two-dimensional limit.

GROWTH AND STABILITY OF ULTRA-THIN Pb FILMS ON Pb/Si(111)-α-√3 × √3

Ultra-thin Pb films with magic thicknesses of 2 monolayer (ML), 4 ML and 6 ML were prepared of atomically flat on the substrate of Si(111)-α-√3 × √3 (or SIC phase) at 145 K. Their surface morphologies and stability were studied by low temperature scanning tunneling microscopy and temperature-dependent angle resolved photoemission spectroscopy. We found that the well ordered SIC interface can lower the diffusion barrier and enhance the interface charge transfer, leading to different critical thickness compared to Pb/Si(111)-7 × 7 grown under same conditions. Enhanced thermal expansion coefficients were also observed in ultra-thin Pb films at low temperature.