Graphoepitaxy of High-Quality GaN Layers on Graphene/6H–SiC

The implementation of graphene layers in gallium nitride (GaN) heterostructure growth can solve self-heating problems in nitride-based high-power electronic and light-emitting optoelectronic devices. In the present study, high-quality GaN layers are grown on patterned graphene layers and 6H–SiC by metalorganic chemical vapor deposition. A periodic pattern of graphene layers is fabricated on 6H–SiC by using polymethyl methacrylate deposition and electron beam lithography, followed by etching using an Ar/O 2 gas atmosphere. Prior to GaN growth, an AlN buffer layer and an Al 0.2 Ga 0.8 N transition layer are deposited. The atomic structures of the interfaces between the 6H–SiC and graphene, as well as between the graphene and AlN, are studied using scanning transmission electron microscopy. Phase separation of the Al 0.2 Ga 0.8 N transition layer into an AlN and GaN superlattice is observed. Above the continuous graphene layers, polycrystalline defective GaN is rapidly overgrown by better quality single-crys- talline GaN from the etched regions. The lateral overgrowth of GaN results in the presence of a low density of dislocations ( ≈ 10 9 cm − 2 ) and inversion domains and the formation of a smooth GaN surface

Real-time electron nanoscopy of photovoltaic absorber formation from kesterite nanoparticles

Cu2ZnSnS4 nanocrystals are annealed in a Se-rich atmosphere inside a transmission electron microscope. During the heating phase, a complete S-Se exchange reaction occurs while the cation sublattice and morphology of the nanocrystals are preserved. At the annealing temperature, growth of large Cu2ZnSnSe4 grains with increased cation ordering is observed in real-time. Thisyields an annealing protocol which is transferred to an industrially-similar solar cell fabrication process resulting in a 33% increase in the device open circuit voltage. The approach can be applied to improve the performance of any photovoltaic technology that requires annealing because of the criticality of the process step

Facet controlled anisotropic magnons in Y<inf>3</inf>Fe<inf>5</inf>O<inf>12</inf> thin films

Directional specific control on the generation and propagation of magnons is essential for designing future magnon-based logic and memory devices for low power computing. The epitaxy of the ferromagnetic thin film is expected to facilitate anisotropic linewidths, which depend on the crystal cut and the orientation of the thin film. Here, we have shown the growth-induced magneto-crystalline anisotropy in 40 nm epitaxial yttrium iron garnet (YIG) thin films, which facilitate cubic and uniaxial in-plane anisotropy in the resonance field and linewidth using ferromagnetic resonance measurements. The growth-induced cubic and non-cubic anisotropy in epitaxial YIG thin films are explained using the short-range ordering of the Fe3þ cation pairs in octahedral and tetrahedral sublattices with respect to the crystal growth directions. This site-preferred directional anisotropy enables an anisotropic magnon–magnon interaction and opens an avenue to precisely control the propagation of magnonic current for spin-transfer logics using YIG-based magnonic technology

Magnetoimpedance of Epitaxial Y3Fe5O12 (001) Thin Film in Low-Frequency Regime

The atomically flat interface of the Y3Fe5O12 (YIG) thin film and the Gd3Ga5O12 (GGG) substrate plays a vital role in obtaining the magnetization dynamics of YIG below and above the anisotropy field. Here, magnetoimpedance (MI) is used to investigate the magnetization dynamics in fully epitaxial 45 nm YIG thin films grown on the GGG (001) substrates using a copper strip coil in the MHz–GHz frequency region. The resistance (R) and reactance (X), which are components of impedance (Z), allow us to probe the absorptive and dispersive components of the dynamic permeability, whereas a conventional spectrometer only measures the field derivative of the power absorbed. The distinct excitation modes arising from the resonance in the uniform and dragged magnetization states of YIG are respectively observed above and below the anisotropy field. The magnetodynamics clearly shows the visible dichotomy between two resonant fields below and above the anisotropy field and its motion as a function of the direction of the applied magnetic field. A low value of a damping factor of ∼4.7 – 6.1 × 10–4 is estimated for uniform excitation mode with an anisotropy field of 65 ± 2 Oe. Investigation of below and above anisotropy field-dependent magnetodynamics in the low-frequency mode can be useful in designing the YIG-based resonators, oscillators, filters, and magnonic devices

Enhanced Spin Hall Effect in S-Implanted Pt

High efficiency of charge–spin interconversion in spin Hall materials is a prime necessity to apprehend intriguing functionalities of spin–orbit torque for magnetization switching, auto-oscillations, and domain wall motion in energy-efficient and high-speed spintronic devices. To this end, innovations in fabricating advanced materials that possess not only large charge–spin conversion efficiency but also viable electrical and spin Hall conductivity are of importance. Here, a new spin Hall material designed by implanting low energy 12 keV sulfur ions in heavy metal Pt, named as Pt(S), is reported that demonstrates eight times higher conversion efficiency as compared to pristine Pt. The figure of merit, spin Hall angle (θSH), up to θPt(S)SH of 0.502 together with considerable electrical conductivity σPt(S)xx of 1.65 × 10 6 Ω–1 m–1 is achieved. The spin Hall conductivity σPt(S)SH increases with increasing σPt(S)xx, as σPt(S)SH∝σPt(S)1.7xx, implying an intrinsic mechanism in a dirty metal conduction regime. A comparatively large σPt(S)SH of 8.32 × 10 5 (ℏ/2e) Ω–1 m–1 among the reported heavy-metals-based alloys can be useful for developing next-generation spintronic devices using spin–orbit torque

Growth and optical, electrical and mechanical characterization of zinc oxide nanowires

The topic of this dissertation is embedded into the new-born field of inorganic nanowires. The research was focused on zinc oxide (ZnO) nanowires, in particular, which besides posing fundamental questions in physics, promise broad range of applications. ZnO nanowires were grown by chemical-vapour-deposition (CVD) using the vapour-liquid-solid (VLS) mechanism with gold particles as catalysts. Oxygen was introduced in the argon carrier gas and zinc was carboreduced from the ZnO/C mixture. We produced micrometer long ZnO nanowires and their diameter was controlled by the size of the gold catalyst particles. The diameter range of our ZnO nanowires is between 30 to 100nm. After growth, the alloyed particles were found at the top of the nanowires. ZnO nanowires are composed of single crystals of ZnO in the wurtzite phase. Growth direction is defined by their epitaxial relation with the substrate. From photoluminescence (PL) and cathodoluminescence (CL) characterizations, a deep impurity energy level at 2.3eV was found in a band gap of 3.3 eV, which could be related to oxygen off-stoichiometry. PL measurements showed that this energy level does not depend on the oxygen concentration used during the growth. Scanning-Tunnelling-Microscopy-based CL (STM-CL) measurements were made on single ZnO nanowires and showed that this energy level is likely to come from curved nanowires. Electrical measurements were performed on single nanowires, by using dielectrophoresis to place individual ZnO nanowires at a predefined location. E-beam lithography was used to deposit four metallic contacts on top of individual micrometer long ZnO nanowires. In order to avoid oxidation and the Shockley effect, we used chromium nitride (CrN) as electrical contacts. By measuring the temperature dependence on the resistance we extracted the activation energy of 2eV for charge transport, possibly arising from the same impurity level, as measured in PL. In the field-effect-transistor (FET) configuration, by using PMMA as dielectric layer and Cr as gate electrode, we extracted the charge carrier mobility of an individual ZnO nanowire as being 9.5 cm2/(Vs). In order to discover the origin of the energy level in the band gap, we measured the relaxation time of the current, after UV-illumination was switched off, and we found out that the surface states have significant effects. The elastic properties of individual ZnO nanowires were studied as a function of diameter and point defect/dislocation concentration. Using dielectrophoresis, nanowires were placed over a hole of a microfabricated Si3N4 membrane and, from bending measurements using an atomic force microscope (AFM) the Young's modulus was extracted. The diameter dependence of the Young's modulus showed a maximal value for a diameter of 50nm. Annealing under argon atmosphere showed an improvement of the Young's modulus which is explained by the removal of the defects/dislocations left during the growth. For comparison, the elastic properties of ZnS nanotubes produced by atomic layer deposition (ALD) were also measured. A clear dependence of the Young's modulus on the diameter and the wall thickness were found and attributed to the variation of the crystal structure of the nanotubes