S. E. Harrison

Two-step growth of high quality Bi2Te3 thin films on Al2O3 (0001) by molecular beam epitaxy

Large-area topological insulator Bi2Te3 thin films were grown on Al2O3 (0001) using a two-temperature step molecular beam epitaxy growth process. By depositing a low temperature nucleation layer to serve as a template for high temperature epitaxial film growth, a high quality terrace-step surface morphology with a significant reduction in three-dimensional defect structures was achieved. X-ray diffraction measurements indicate that high crystalline quality Bi2Te3 layers were grown incoherently by van der Waals epitaxy using this technique. Angle resolved photoemission spectroscopy measurements verified the integrity of this growth method by confirming the presence of metallic surface states on cleaved two-step Bi2Te3 samples.

Magnetic ordering in Cr-doped Bi2Se3 thin films

We report the structural and magnetic study of Cr-doped Bi2Se3 thin films using x-ray diffraction (XRD), magnetometry and polarized neutron reflectometry (PNR). Epitaxial layers were grown on c-plane sapphire by molecular beam epitaxy in a two-step process. High-resolution XRD shows the exceptionally high crystalline quality of the doped films with no parasitic phases up to a Cr concentration of 12% (in % of the Bi sites occupied by substitutional Cr). The magnetic moment, measured by SQUID magnetometry, was found to be per Cr ion. The magnetic hysteresis curve shows an open loop with a coercive field of . The ferromagnetic transition temperature was determined to be analyzing the magnetization-temperature gradient. PNR shows the film to be homogeneously ferromagnetic with no enhanced magnetism near the surface or interface.

Study of Gd-doped Bi2Te3 thin films: Molecular beam epitaxy growth and magnetic properties

Incorporation of magnetic dopants into topological insulators to break time-reversal symmetry is a prerequisite for observing the quantum anomalous Hall (QAHE) effect and other novel magnetoelectric phenomena. GdBiTe3 with a Gd:Bi ratio of 1:1 is a proposed QAHE system, however, the reported solubility limit for Gd doping into Bi2Te3 bulk crystals is between ∼0.01 and 0.05. We present a magnetic study of molecular beam epitaxy grown (GdxBi1–x)2Te3 thin films with a high Gd concentration, up to x ≈ 0.3. Magnetometry reveals that the films are paramagnetic down to 1.5 K. X-ray magnetic circular dichroism at the Gd M4,5 edge at 1.5 K reveals a saturation field of ∼6 T, and a slow decay of the magnetic moment with temperature up to 200 K. The Gd3+ ions, which are substitutional on Bi sites in the Bi2Te3 lattice, exhibit a large atomic moment of ∼7 μB, as determined by bulk-sensitive superconducting quantum interference device magnetometry. Surface oxidation and the formation of Gd2O3 lead to a reduced moment ...

X-ray magnetic spectroscopy of MBE-grown Mn-doped Bi2Se3 thin films

We report the growth of Mn-doped Bi2Se3 thin films by molecular beam epitaxy (MBE), investigated by x-ray diffraction (XRD), atomic force microscopy (AFM), SQUID magnetometry and x-ray magnetic circular dichroism (XMCD). Epitaxial films were deposited on c-plane sapphire substrates by co-evaporation. The films exhibit a spiral growth mechanism typical of this material class, as revealed by AFM. The XRD measurements demonstrate a good crystalline structure which is retained upon doping up to ∼7.5 atomic-% Mn, determined by Rutherford backscattering spectrometry (RBS), and show no evidence of the formation of parasitic phases. However an increasing interstitial incorporation of Mn is observed with increasing doping concentration. A magnetic moment of 5.1 μB/Mn is obtained from bulk-sensitive SQUID measurements, and a much lower moment of 1.6 μB/Mn from surface-sensitive XMCD. At ∼2.5 K, XMCD at the Mn L2,3 edge, reveals short-range magnetic order in the films and indicates ferromagnetic order below 1.5 K.

Magnetic properties of gadolinium substituted Bi2Te3 thin films

Thin film GdBiTe3 has been proposed as a candidate material in which to observe the quantum anomalous Hall effect. As a thermal non-equilibrium deposition method, molecular beam epitaxy (MBE) has the ability to incorporate large amounts of Gd into Bi2Te3 crystal structures. High-quality rhombohedral (GdxBi1−x)2Te3 films with substitutional Gd concentrations of x ≤ 0.4 were grown by MBE. Angle-resolved photoemission spectroscopy shows that the topological surface state remains intact up to the highest Gd concentration. Magnetoresistance measurements show weak antilocalization, indicating strong spin orbit interaction. Magnetometry reveals that the films are paramagnetic with a magnetic moment of 6.93 μB per Gd3+ ion.

Study of Dy-doped Bi2Te3: thin film growth and magnetic properties

Breaking the time-reversal symmetry (TRS) in topological insulators (TIs) through ferromagnetic doping is an essential prerequisite for unlocking novel physical phenomena and exploring potential device applications. Here, we report the successful growth of high-quality (Dy(x)Bi(1-x))2Te3 thin films with Dy concentrations up to x = 0.355 by molecular beam epitaxy. Bulk-sensitive magnetisation studies using superconducting quantum interference device magnetometry find paramagnetic behaviour down to 2 K for the entire doping series. The effective magnetic moment, μeff, is strongly doping concentration-dependent and reduces from ∼12.6 μ(B) Dy(-1) for x = 0.023 to ∼4.3 μ(B) Dy(-1) for x = 0.355. X-ray absorption spectra and x-ray magnetic circular dichroism (XMCD) at the Dy M4,5 edge are employed to provide a deeper insight into the magnetic nature of the Dy(3+)-doped films. XMCD, measured in surface-sensitive total-electron-yield detection, gives μ(eff )= 4.2 μ(B) Dy(-1). The large measured moments make Dy-doped films interesting TI systems in which the TRS may be broken via the proximity effect due to an adjacent ferromagnetic insulator.

Preparation of layered thin film samples for angle-resolved photoemission spectroscopy

Materials with layered van der Waals crystal structures are exciting research topics in condensed matter physics and materials science due to outstanding physical properties associated with their strong two dimensional nature. Prominent examples include bismuth tritelluride and triselenide topological insulators (TIs), which are characterized by a bulk bandgap and pairwise counter-propagating spin-polarized electronic surface states. Angle-resolved photoemission spectroscopy (ARPES) of ex-situ grown thin film samples has been limited by the lack of suitable surface preparation techniques. We demonstrate the shortcomings of previously successful conventional surface preparation techniques when applied to ternary TI systems which are susceptible to severe oxidation. We show that in-situ cleaving is a simple and effective technique for preparation of clean surfaces on ex-situ grown thin films for high quality ARPES measurements. The method presented here is universally applicable to other layered van der Waals systems as well.

Controlled removal of amorphous Se capping layer from a topological insulator

We report on the controlled removal of an amorphous Se capping layer from Bi2Te3 and Bi2Se3 topological insulators. We show that the Se coalesces into micron-sized islands before desorbing from the surface at a temperature of ∼150 °C. In situ Auger Electron Spectroscopy reveals that Se replaces a significant fraction of the Te near the top surface of the Bi2Te3. Rutherford Backscattering Spectrometry and Transmission Electron Microscopy show that after heating, Se has been incorporated in the Bi2Te3 lattice down to ∼7 nm from its top surface while remaining iso-structural.

Study of Ho-doped Bi2Te3 topological insulator thin films

Breaking time-reversal symmetry through magnetic doping of topological insulators has been identified as a key strategy for unlocking exotic physical states. Here, we report the growth of Bi2Te3 thin films doped with the highest magnetic moment element Ho. Diffraction studies demonstrate high quality films for up to 21% Ho incorporation. Superconducting quantum interference device magnetometry reveals paramagnetism down to 2 K with an effective magnetic moment of ∼5 μB/Ho. Angle-resolved photoemission spectroscopy shows that the topological surface state remains intact with Ho doping, consistent with the materials paramagnetic state. The large saturation moment achieved makes these films useful for incorporation into heterostructures, whereby magnetic order can be introduced via interfacial coupling.

Nanoscale Probing of Local Electrical Characteristics on MBE-Grown Bi2Te3 Surfaces under Ambient Conditions

The local electrical characteristics on the surface of MBE-grown Bi2Te3 are probed under ambient conditions by conductive atomic force microscopy. Nanoscale mapping reveals a 10-100× enhancement in current at step-edges compared to that on terraces. Analysis of the local current-voltage characteristics indicates that the transport mechanism is similar for step-edges and terraces. Comparison of the results with those for control samples shows that the current enhancement is not a measurement artifact but instead is due to local differences in electronic properties. The likelihood of various possible mechanisms is discussed. The absence of enhancement at the step-edges for graphite terraces is consistent with the intriguing possibility that spin-orbit coupling and topological effects play a significant role in the step-edge current enhancement in Bi2Te3.