We study the heavy metal layer thickness dependence of the current-induced spin-orbit torque (SOT) in perpendicularly magnetized Hf|CoFeB|MgO multilayer structures. The damping-like (DL) current-induced SOT is determined by vector anomalous Hall effect measurements. A non-monotonic behavior in the DL-SOT is found as a function of the thickness of the heavy-metal layer. The sign of the DL-SOT changes with increasing the thickness of the Hf layer in the trilayer structure. As a result, in the current-driven magnetization switching, the preferred direction of switching for a given current direction changes when the Hf thickness is increased above ∼7 nm. Although there might be a couple of reasons for this unexpected behavior in DL-SOT, such as the roughness in the interfaces and/or impurity based electric potential in the heavy metal, one can deduce a roughness dependence sign reversal in DL-SOT in our trilayer structure.
Temperature and nitrogen dependence of 2D carrier mobility in as-grown and annealed Ga 1−x In x N y As 1−y /GaAs quantum well (QW) structures (x = 0.32; y = 0, 0.009, and 0.012) are investigated. An analytical model that accounts for the most prominent scattering mechanisms is used to explain the characteristic of temperature dependence of the carrier mobility. An expression for alloy scattering-limited mobility in N-related alloys is developed to explain the behavior of hole mobility for N-containing p-type samples. Analytical modeling of temperature dependence of the electron mobility indicates that N-related alloy scattering and interface roughness scattering are the dominant mechanism at the entire temperature range of interest. The temperature insensitivity of the electron mobility is explained in terms of the overriding effect of N-related alloy scattering and high 2D electron density. A deviation between theoretical and experimental electron mobility at low temperatures is observed not to have any dependency on N concentration. We, therefore, suggest that C NM interaction parameter of the band anti-crossing (BAC) model must be defined as temperature dependent in order to explain the observed low temperature characteristics of electron mobility. The hole mobility is mainly restricted by interface roughness and alloy scatterings at temperatures lower than 100 K, whilst high temperature hole mobility is drastically affected from optical phonon scattering. Moreover, the hole mobility at high temperatures exhibits an N-independent characteristic and hole density starts to increase at temperatures above 70 K, which is explained using the concept of parallel conduction. Extraction of the hole density in each transport channel (QW and barrier) by using a simple parallel conduction extraction method (SPCEM) shows that, in p-type samples, low temperature hole mobility takes place in quantum well, while as temperature increases barrier channel also contribute to the hole mobility and becomes dominant at high temperatures. The experimental and calculated Hall mobility results reveal that thermal annealing has decreased interface roughness and alloy scatterings.
In this study, we investigate the effect of annealing and nitrogen amount on electronic transport properties in n- and p-type-doped Ga0.68In0.32NyAs1 − y/GaAs quantum well (QW) structures with y = 0%, 0.9%, 1.2%, 1.7%. The samples are thermal annealed at 700°C for 60 and 600 s, and Hall effect measurements have been performed between 10 and 300 K. Drastic decrease is observed in the electron mobility of n-type N-containing samples due to the possible N-induced scattering mechanisms and increasing effect mass of the alloy. The temperature dependence of electron mobility has an almost temperature insensitive characteristic, whereas for p-type samples hole mobility is decreased drastically at T > 120 K. As N concentration is increased, the hole mobility also increased as a reason of decreasing lattice mismatch. Screening effect of N-related alloy scattering over phonon scattering in n-type samples may be the reason of the temperature-insensitive electron mobility. At low temperature regime, hole mobility is higher than electron mobility by a factor of 3 to 4. However, at high temperatures (T > 120 K), the mobility of p-type samples is restricted by the scattering of the optical phonons. Because the valance band discontinuity is smaller compared to the conduction band, thermionic transport of holes from QW to the barrier material, GaAs, also contributes to the mobility at high temperatures that results in a decrease in mobility. The hole mobility results of as-grown samples do not show a systematic behavior, while annealed samples do, depending on N concentration. Thermal annealing does not show a significant improvement of electron mobility.
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