Atomic structure of thin films and heterostructure of Bi2Te3 and Bi2Se3 topological insulators

The atomic structure of transition metals doped three dimensional (3D) topological insulators (TIs) and the bonding nature of Bi2Te3 with FeSe layers and Ge(111) substrate were studied. Motivation behind transition metal doping of 3D TIs is driven by achieving long range ferromagnetism of Bi2Se3 and Bi2Te3, which is expected to give rise to different spintronic effects that can be utilise in device applications. The nature of this magnetisation depends on the location of the dopants in the Bi chalcogenide matrix. Dopants in Bi based TIs can substitute for Bi, Te, or incorporate between the quintuple layers in the van der Waals gap. Long range ferromagnetism is observed in both Cr doped Bi2Se3 and Mn doped Bi2Te3; however, the main goal of achieving room-temperature ferromagnetism in homogeneously doped TIs has proven to be difficult. In this thesis it is shown that 4.6 at-% of Cr is incorporated substitutionally on Bi sites with no phase segregation. The presences of grain boundaries can cause Cr segregation; hence by controlling the defect density a homogeneous Cr distribution could in principle be achieved even at higher concentrations. In case of Mn as a dopant, we show that the local environment of Mn in Bi2Te3 is heterogeneous. The first principal calculations revealed that the Mn dopants ferromagnetically couple in Bi2Te3 lattice. In addition, we have shown that doping of Bi2Te3 with Mn should be limited to low concentrations (< 6 at-%), higher dopants concentrations results in the formation of secondary phases. Next we have demonstrated that epitaxial growth of FexCu1-xSe on Bi2Te3 is possible regardless of their different lattice symmetries and large lattice mismatch of 19%. First-principles energy calculations revealed that this is realised through van der Waals-like bonding between the Se and Te atomic planes at the interface. Finally, we have shown that the weak van der Waals bonding between the Bi2Te3 and Ge(111) substrate can be strengthen by formation of a Te monolayer at the interface. The electronic band structure calculations revealed that this is due to the stronger atomic p-type orbital hybridization at the interface

The antiphase boundary in half-metallic Heusler alloy Co2Fe(Al,Si) : atomic structure, spin polarization reversal, and domain wall effects

Atomic resolution scanning transmission electron microscopy reveals the presence of an antiphase boundary in the half-metallic Co2Fe(Al,Si) full Heusler alloy. By employing the density functional theory calculations, we show that this defect leads to reversal of the sign of the spin-polarization in the vicinity of the defect. In addition, we show that this defect reduces the strength of the exchange interactions, without changing the ferromagnetic ordering across the boundary. Atomistic spin calculations predict that this effect reduces the width of the magnetic domain wall compared to that in the bulk

Controlling the half-metallicity of Heusler/Si(1 1 1) interfaces by a monolayer of Si–Co–Si

By using first-principles calculations we show that the spin-polarization reverses its sign at atomically abrupt interfaces between the half-metallic Co₂ (Fe,Mn)(Al,Si) and Si(1 1 1). This unfavourable spin-electronic configuration at the Fermi-level can be completely removed by introducing a Si–Co–Si monolayer at the interface. In addition, this interfacial monolayer shifts the Fermi-level from the valence band edge close to the conduction band edge of Si. We show that such a layer is energetically favourable to exist at the interface. This was further confirmed by direct observations of CoSi₂ nano-islands at the interface, by employing atomic resolution scanning transmission electron microscopy

Magnetic and structural depth profiles of Heusler alloy Co2FeAl0.5Si0.5 epitaxial films on Si(1 1 1)

The depth-resolved chemical structure and magnetic moment of Co2FeAl0.5Si0.5, thin films grown on Si(1 1 1) have been determined using x-ray and polarized neutron reflectometry. Bulk-like magnetization is retained across the majority of the film, but reduced moments are observed within 45˚A of the surface and in a 25˚A substrate-interface region. The reduced moment is related to compositional changes due to oxidation and diffusion, which are further quantified by elemental profiling using electron microscopy with electron energy loss spectroscopy. The accuracy of structural and magnetic depth-profiles obtained from simultaneous modeling is discussed using different approaches with different degree of constraints on the parameters. Our approach illustrates the challenges in fitting reflectometry data from these multi-component quaternary Heusler alloy thin films

The role of chemical structure on the magnetic and electronic properties of Co2FeAl0.5Si0.5/Si(111) interface

We show that Co2FeAl0.5Si0.5 film deposited on Si(111) has a single crystal structure and twin related epitaxial relationship with the substrate. Sub-nanometer electron energy loss spectroscopy shows that in a narrow interface region there is a mutual inter-diffusion dominated by Si and Co. Atomic resolution aberration-corrected scanning transmission electron microscopy reveals that the film has B2 ordering. The film lattice structure is unaltered even at the interface due to the substitu- tional nature of the intermixing. First-principles calculations performed using structural models based on the aberration corrected electron microscopy show that the increased Si incorporation in the film leads to a gradual decrease of the magnetic moment as well as significant spin-polarization reduction. These effects can have significant detrimental role on the spin injection from the Co2FeAl0.5Si0.5 film into the Si substrate, besides the structural integrity of this junction

Effect of annealing on the structure and magnetic properties of Co2FeAl0.5Si0.5 thin films on Ge(111)

Abstract We present a magnetic and structural properties study of epitaxially grown B2-ordered full Heusler Co2FeSi0.5Al0.5 single crystal films on Ge(111) substrates, as a function of annealing temperature. Hysteresis loop measurements reveal that the magnetic properties of Co2FeSi0.5Al0.5 are stable up to 450 °C while ferromagnetic resonance linewidth measurements show a reduction of Gilbert damping from 5.6 × 10−3 to 2.9 × 10−3 for as-grown and annealed film, respectively. Above 500 °C, the films have increased coercivity, decreased saturation magnetisation, and show characteristic two-magnon scattering resonance line-shapes. Magnetic inhomogeneities developed within the film when annealed above 500 °C were correlated to significant interdiffusion at the film-substrate interface, as confirmed by scanning transmission electron microscopy and electron energy loss spectroscopy. By performing first-principles calculations based on atomistic models developed from atomically-resolved microscopy images, we show the magnetic moment of the Co2FeSi0.5Al0.5 film reduces upon Co substitution by Ge atoms

Correlation between spin transport signal and Heusler/semiconductor interface quality in lateral spin-valve devices

We show direct evidence for the impact of Heusler/semiconductor interfaces atomic structure on the spin transport signals in semiconductor-based lateral spin-valve (LSV) devices. Based on atomic scale Z-contrast scanning transmission electron microscopy and energy dispersive x-ray spectroscopy we show that atomic order/disorder of Co2FeAlSi (CFAS)/-Ge LSV devices is critical for the spin injection in Ge. By conducting a postannealing of the LSV devices, we find 90% decrease in the spin signal while there is no difference in the electrical properties of the CFAS /n-Ge contacts and in the spin diffusion length of the n-Ge layer. We show that the reduction in the spin signals after annealing is attributed to the presence of intermixing phases at the Heusler/semiconductor interface. First-principles calculations show how that intermixed interface region has drastically reduced spin polarization at the Fermi level, which is the main cause for the significant decrease of the spin signal in the annealed devices above 300 C

Modification of the van der Waals interaction at the Bi2Te3{\mathrm{Bi}}_{2}{\mathrm{Te}}_{3} and Ge(111) interface

We present a structural and density-functional theory study of the interface of the quasi-twin-free grown three-dimensional topological insulator Bi2Te3 on Ge(111). Aberration-corrected scanning transmission electron microscopy and electron energy-loss spectroscopy in combination with first-principles calculations show that the weak van der Waals adhesion between the Bi2Te3 quintuple layer and Ge can be overcome by forming an additional Te layer at their interface. The first-principles calculations of the formation energy of the additional Te layer show it to be energetically favorable as a result of the strong hybridization between the Te and Ge

Correlating point defects with mechanical properties in nanocrystalline TiN thin films

Defects significantly affect the mechanical properties of materials. However, quantitatively correlating the point defects with mechanical property could be a challenge. In this study, we explore the point defect effects on the structure and property of magnetron sputtered TiN nanocrystalline films (synthesized using different negative bias potential) via a combination of analytical techniques and density functional theory (DFT) calculations. We gain insights into the structural evolution and properties of nanocrystalline films at different length scales. It is found that nanocrystal microstructure and local electronic structure triggered by various point defects remarkably change. Along with the structural evolution and point defect changes, the electrical conductivity and the fracture toughness of TiN are improved. Furthermore, the fracture toughness, Young’s modulus, and cleavage energy and stresses for TiN films with different point defect structures are calculated. The experimental data is in excellent agreement with first-principle calculations. Our results suggest a direct correlation of the point defect structure with TiN films' mechanical properties