How spin-orbit torques emerge from materials with weak spin-orbit coupling (e.g., light metals) is an open question in spintronics. Here, we report on a field-like spin-orbit torque (i.e., in-plane spin-orbit field transverse to the current axis) in SiO2-sandwiched permalloy (Py), with the top Py-SiO2 interface incorporating ultrathin Ti or Cu. In both SiO2/Py/Ti/SiO2 and SiO2/Py/Cu/SiO2, this spin-orbit field opposes the classical Oersted field. While the magnitude of the spin-orbit field is at least a factor of 3 greater than the Oersted field, we do not observe evidence for a significant damping-like torque in SiO2/Py/Ti/SiO2 or SiO2/Py/Cu/SiO2. Our findings point to contributions from a Rashba-Edelstein effect or spin-orbit precession at the (Ti, Cu)-inserted interface.2 An electric current in a material with spin-orbit coupling generally gives rise to a non-equilibrium spin accumulation [1-6], which can then exert torquesi.e., spin-orbit torques (SOTs)on magnetization in an adjacent magnetic medium [7][8][9]. SOTs are often classified into two symmetries: damping-like SOT that either counters or enhances magnetic relaxation, and field-like SOT (or "spin-orbit field") that acts similarly to a magnetic field. Next generations of nanomagnetic computing devices may benefit from an improved understanding of mechanisms for SOTs and the discovery of new thin-film systems enabling large SOTs.While most efforts have focused on conductors known for strong spin-orbit coupling (e.g., 5d transition metals, topological insulators, etc.) [7,8], recent reports have shown SOTs in ferromagnets interfaced with materials that are not expected to exhibit significant spin-orbit coupling [10][11][12][13][14]. For example, a large damping-like SOT has been reported in ferromagnetic Ni80Fe20 (permalloy, Py) interfaced with partially oxidized Cu [10,11]; quantum-interference transport measurements have revealed that Cu with an oxidation gradient can, in fact, exhibit enhanced spin-orbit coupling comparable to that in heavier metals (e.g., Au) [15]. As another example of SOTs that emerge by incorporating seemingly weak spin-orbit materials, Py interfaced with a Ti seed layer and Al2O3 capping layer exhibits a sizable field-like SOT [12]. The key observed features of this spin-orbit field in Ti/Py/Al2O3 [12] are: (1) it points in-plane and transverse to the current axis, irrespective of the magnetization orientation in Py; (2) its magnitude scales inversely with the Py thickness, i.e., it is interfacial in origin; (3) it is modified significantly by the addition of an insertion layer (e.g., Cu) at the Py-Al2O3 interface. Ref. [12] claims that this spin-orbit field is governed by a Rashba-Edelstein effect (REE) [1,5,16,17] at the Py/Al2O3 and Cu/Al2O3 interfaces. However, the complicated stack structures of SiO2(substrate)/Ti/Py/(Cu/)Al2O3 with multiple dissimilar interfaces in Ref.[12] obscure the mechanisms of the spin-orbit field, particularly the roles played by the Ti and Cu layers.Here, by using simpler stack structures, we gain i...

We report the temperature dependence of Er optical centers in GaN epilayers prepared by metal-organic chemical vapor deposition under the resonant excitation (4I15/2 → 4I9/2) excitation using a Ti:Sapphire laser (λexc = 809 nm). High resolution infrared spectroscopy and temperature dependence measurements of photoluminescence intensity from Er ions in GaN have been performed to identify the crystal filed splitting of the first excited state, 4I13/2. Here, we have employed a simple approach to determine activation energies which are related to the thermal population of electrons from the lowest level to the higher level of the crystal field splitting of the first excited state.

This work presents electroless deposition of palladium films onto gallium nitride from aqueous palladium dichloride using sodium L-ascorbate as the reducing agent in an acidic bath. Profilometry, four-point probe measurements, energy dispersive x-ray spectroscopy (EDS), and x-ray photoelectron spectroscopy (XPS) were used to measure film thickness, sheet resistance, resistivity, morphology, and composition. The resistivity of Pd was 1-3 x 10 -5 Ω cm. Schottky diodes were produced from plated films, and the barrier heights and ideality factors were determined from current-voltage measurements. Average barrier heights were 1.13-1.26 eV, with ideality factors from 1.02-1.05, depending on the gallium nitride epilayer and substrate. Also presented is a method for depositing palladium-gallium alloy films onto gold surfaces from palladium dichloride and gallium (III) sulfate using sodium hypophosphite as the reducing agent; however, palladium-gallium alloys were not readily deposited on gallium nitride.

Although silicon (Si) currently dominates the semiconductor industry, its small 1.1 eV band gap limits its maximum operating temperature, which restricts its use in high-temperature, high-power devices. Gallium nitride (GaN) is an attractive semiconductor with its wide bandgap (3.4 eV), high electron mobility (1700 cm2/Vs), high electron saturation velocity (3 x 107 cm/s), large critical breakdown field (2 MV/cm), and thermal stability. The high-power capabilities of GaN allow for a reduction in device size, which can conserve physical space if used to replace conventional Si power devices. While the semiconductor itself can endure harsh operating conditions, the reliability of the metal/semiconductor contacts can be a limiting factor for its use. Schottky contacts should provide a high barrier height and low reverse leakage current, and they must be electrically stable over the lifetime of the device. In this study, three materials reported to have high work functions are compared as Schottky diodes to n-type GaN, each selected for its anticipated thermodynamic stability with GaN1 or the potential to minimize process-induced defects in the diodes or both. Rhenium (Re) diodes fabricated via electron beam deposition, molybdenum nitride (MoNx) diodes via remote plasma atomic layer deposition (ALD), and palladium (Pd) diodes via electroless deposition were investigated. Ti/Al-based ohmic contacts were employed. The Re/n-GaN Schottky diode was chosen for study because of its thermodynamic stability against metallurgical reactions1 and high work function (4.96 eV)2. The barrier heights were investigated by current–voltage (I-V) and capacitance–voltage (C-V) measurements at room temperature. Both techniques demonstrated that the barrier height increased after an anneal at 400°C for 5 min, yielding a barrier height of 0.88 eV and ideality factor of 1.02 from by I-V measurements, while the C-V measurements revealed a barrier height of 0.91 eV. These barrier heights and reverse leakage currents remained stable upon annealing in N2 at 600°C. The MoNx/n-GaN Schottky diode was chosen for study because of the reported high work function of MoNx (5.33 eV)3, its conductive and refractory nature, and its thermodynamic equilibrium with GaN4. Films were deposited with bis (tert-butylimido)-bis (dimethylamido) molybdenum and a 300 W remote N2/H2 plasma. Four-point probe measurements and x-ray photoelectron spectroscopy (XPS) were used to measure sheet resistance and composition. Barrier heights from the I-V measurements were 0.41 eV, and ideality factors were 1.4, with good stability upon annealing at 600°C. The Pd/n-GaN Schottky diode was chosen because of its high work function (5.12 eV)2 and its potential to be electrolessly deposited, offering a gentle technique to minimize process-induced defects in the GaN, although Pd is not expected to be in thermodynamic equilibrium with GaN. A new method for electroless deposition of Pd films onto GaN surfaces used palladium dichloride with sodium L-ascorbate as the reducing agent was developed. Profilometry, four-point probe measurements, energy dispersive x-ray spectroscopy, and XPS were used to measure film thickness, sheet resistance, resistivity, morphology, and composition. The process provided conductive and pure Pd films, but some challenges with nucleation of the film on GaN makes the process less robust. Barrier heights from the I-V measurements were 1.13–1.26 eV, and ideality factors were 1.02–1.05. However, Pd is not in thermodynamic equilibrium with GaN. Among the candidates tested so far, the Re diodes were overall the strongest candidate. Future work will involve stress testing followed by materials characterization to provide more information on stable metallizations for high-power GaN devices. The authors are grateful to Sandia National Laboratories (Andrew Allerman) for providing GaN epilayers. This work was funded by the Office of Naval Research under Grant N000141812360, distribution A, approved for public release, distribution is unlimited (DCN# 43-7434-20). S. E. Mohney and X. Lin, J. Electron. Mater, 25, 811–818 (1996). H. Michaelson, J. Appl. Phys. 48, 4729-4733, (1977): H. Matsuhashi and S. Nishikawa, Jpn. J. Appl. Phys., 33, 1293, (1994). H. S. Venugopalan and S. E. Mohney, Z Metallkd., 89, 184-186, (1998).