Ajay Tiwari

Effect of Growth Mode on Eu-Incorporation and Luminescence of Eu-Doped GaN Epitaxial Film Grown by Plasma-Assisted Molecular Beam Epitaxy

The growth mode of europium (Eu)-doped GaN epitaxial films grown on a GaN template by rf plasma-assisted molecular beam epitaxy (PAMBE) was investigated with different III/V ratios under a constant Eu beam equivalent pressure ratio [PEu/(PEu+PGa)]. The reflection high-energy electron diffraction (RHEED) patterns and atomic force microscopy (AFM) images revealed the transition of the growth mode from three-dimensional (3D) to step-flow/two-dimensional (2D) by increasing the III/V ratio. When the films were grown in the 3D growth mode, Eu concentrations estimated by Rutherford backscattering spectrometry/channeling (RBS/channeling) were almost constant, although the III/V ratios varied. However, when the growth mode was transferred from 3D to step-flow/2D, precipitates on the surface abruptly increased while the Eu concentration abruptly decreased, indicating the abrupt degradation of Eu-incorporation in the film. Luminescence sites of Eu3+ were sensitive to the III/V ratio, and Eu atoms have different luminescence sites in both growth modes. Furthermore, luminescence efficiency abruptly increased when the growth mode was transferred from 3D to step-flow/2D.

Improved read disturb and write error rates in voltage-control spintronics memory (VoCSM) by controlling energy barrier height

A hybrid writing scheme that combines the spin Hall effect and voltage-controlled magnetic-anisotropy effect is investigated in Ta/CoFeB/MgO/CoFeB/Ru/CoFe/IrMn junctions. The write current and control voltage are applied to Ta and CoFeB/MgO/CoFeB junctions, respectively. The critical current density required for switching the magnetization in CoFeB was modulated 3.6-fold by changing the control voltage from −1.0 V to +1.0 V. This modulation of the write current density is explained by the change in the surface anisotropy of the free layer from 1.7 mJ/m2 to 1.6 mJ/m2, which is caused by the electric field applied to the junction. The read disturb rate and write error rate, which are important performance parameters for memory applications, are drastically improved, and no error was detected in 5 × 108 cycles by controlling read and write sequences.

Effect of post annealing on spin accumulation and transport signals in Co2FeSi/MgO/n+-Si on insulator devices

The post annealing temperature dependence of spin accumulation and transport signals in Co2FeSi/MgO/n+-Si on insulator were investigated. The spin signals were detected using 3- and 4-terminal Hanle, 2-terminal local and 4-terminal nonlocal magnetoresistance measurements. The post annealing temperature (TA) dependence of the magnitude in 3-terminal narrow Hanle signals is nearly constant up to TA < 400°C, however a slight decrease above TA ≥ 400°C is observed. This behavior is consistent with the TA dependence of the magnitude of 4-terminal nonlocal magnetoresistance (MR) signals. The spin polarization estimated from the 3-terminal narrow Hanle signals and the magnitude of 2-terminal local MR signals show a slight improvement with increasing post annealing temperature with a peak at around 325°C and then start reducing slowly. The slight increase in the spin signal would be due to high spin polarization of Co2FeSi as a result of structural ordering. The 2-terminal local MR signals do not vary significantl...

Room temperature observation of high spin polarization in post annealed Co2FeSi/MgO/n+-Si on insulator devices

The post annealing temperature dependence of room temperature spin signals in Co2FeSi/MgO/n+-Si on insulator fabricated on Si(2×1) surface was investigated. For the devices fabricated on the Si(2×1) surface, the large and reliable three- and four-terminals spin signals were obtained even at room temperature. The magnitude of three-terminal narrow Hanle signals has a peak around 325 °C with respect to post annealing temperature. The trend of increasing spin accumulation signal with decreasing bias voltage was observed for both as deposited and sample annealed at 325 °C in the bias voltage range 600–800 mV. The enhancement of three- and four-terminals non-local magnetoresistance (MR) for post annealed sample at 325 °C, indicates that the spin polarization increases due to the structural ordering of Heusler alloy Co2FeSi. As a result, we observed large spin injection efficiency into Si (P ~ 41.7%) even at room temperature. These results will pave a way to the future Si spintronics devices such a spin-MOSFET.