Guijuan Zhao

Skyrmion-skyrmion and skyrmion-edge repulsions in skyrmion-based racetrack memory

Magnetic skyrmions are promising for building next-generation magnetic memories and spintronic devices due to their stability, small size and the extremely low currents needed to move them. In particular, skyrmion-based racetrack memory is attractive for information technology, where skyrmions are used to store information as data bits instead of traditional domain walls. Here we numerically demonstrate the impacts of skyrmion-skyrmion and skyrmion-edge repulsions on the feasibility of skyrmion-based racetrack memory. The reliable and practicable spacing between consecutive skyrmionic bits on the racetrack as well as the ability to adjust it are investigated. Clogging of skyrmionic bits is found at the end of the racetrack, leading to the reduction of skyrmion size. Further, we demonstrate an effective and simple method to avoid the clogging of skyrmionic bits, which ensures the elimination of skyrmionic bits beyond the reading element. Our results give guidance for the design and development of future skyrmion-based racetrack memory.

Magnetic skyrmion transistor: skyrmion motion in a voltage-gated nanotrack

Magnetic skyrmions are localized and topologically protected spin configurations, which are of both fundamental and applied interests for future electronics. In this work, we propose a voltage-gated skyrmion transistor within the well-established framework of micromagnetics. Its operating conditions and processes have been theoretically investigated and demonstrated, in which the gate voltage can be used to switch on/off a circuit. Our results provide the first time guidelines for practical realization of hybrid skyrmionic-electronic devices.

Competitive growth mechanisms of AlN on Si (111) by MOVPE

To improve the growth rate and crystal quality of AlN, the competitive growth mechanisms of AlN under different parameters were studied. The mass transport limited mechanism was competed with the gas-phase parasitic reaction and became dominated at low reactor pressure. The mechanism of strain relaxation at the AlN/Si interface was studied by transmission electron microscopy (TEM). Improved deposition rate in the mass-transport-limit region and increased adatom mobility were realized under extremely low reactor pressure.

Anisotropic structural and optical properties of semi-polar (11–22) GaN grown on m -plane sapphire using double AlN buffer layers

We report the anisotropic structural and optical properties of semi-polar (11–22) GaN grown on m-plane sapphire using a three-step growth method which consisted of a low temperature AlN buffer layer, followed by a high temperature AlN buffer layer and GaN growth. By introducing double AlN buffer layers, we substantially improve the crystal and optical qualities of semi-polar (11–22) GaN, and significantly reduce the density of stacking faults and dislocations. The high resolution x-ray diffraction measurement revealed that the in-plane anisotropic structural characteristics of GaN layer are azimuthal dependent. Transmission electron microscopy analysis showed that the majority of dislocations in the GaN epitaxial layer grown on m-sapphire are the mixed-type and the orientation of GaN layer was rotated 58.4° against the substrate. The room temperature photoluminescence (PL) spectra showed the PL intensity and wavelength have polarization dependence along parallel and perpendicular to the [1–100] axis (polarization degrees ~ 0.63). The realization of a high polarization semi-polar GaN would be useful to achieve III-nitride based lighting emission device for displays and backlighting.

Impact of the misfit dislocations on two-dimensional electron gas mobility in semi-polar AlGaN/GaN heterostructures

The mobility of two-dimensional electron gas as influenced by the charged misfit dislocations located at the interface of the semi-polar AlGaN/GaN heterostructure is quantitatively analyzed. The results indicate that the strength of the scattering due to the misfit dislocations in the semi-polar AlGaN/GaN heterointerface is comparable to the well-known scattering associated with the threading dislocations in the well-known polar c-plane AlGaN/GaN heterostructures.

Significant quality improvement of GaN on Si(111) upon formation of an AlN defective layer

The stress state and crystal quality of a metal–organic chemical vapor deposition (MOCVD)-grown GaN film on a Si(111) substrate with different AlN buffer layer thicknesses were investigated. The properties of the AlN layers with varying thicknesses were studied. The three-dimensional growth mode with a rough surface of AlN on Si was observed. A defective layer with mixed orientations of AlN grains formed at the AlN/GaN interface. The AlN buffer layer with the defective layer accommodated most of the lattice mismatch between the GaN film and the Si substrate. The significance of the defective layer was proved by the obtainment of a 1 μm thick nearly crack-free GaN film with high crystal quality.

Significant deterioration of energy products in exchange-coupled composite magnets

Hysteresis loops and energy products have been calculated reliably for a hard/soft/hard trilayer system with a deviation of easy axis β taken into account, which affects the coercivity significantly, and hence leads to much smaller energy products than those predicted by the previous theory. Such a deterioration is much sharper than the corresponding fall for a single-phased material and a 30° deviation of in-plane easy axis could result in a drop of the maximum energy product by more than 60%. Consequently, the advantage of the composite phase can be realized only when the material is well oriented, which offers a possible explanation to the large discrepancy between the experimental and theoretical energy products.

Growth of Well-Aligned InN Nanorods on Amorphous Glass Substrates

The growth of well-aligned nanorods on amorphous substrates can pave the way to fabricate large-scale and low-cost devices. In this work, we successfully prepared vertically well-aligned c-axis InN nanorods on amorphous glass substrate by metal-organic chemical vapor deposition. The products formed directly on bare glass are randomly oriented without preferential growth direction. By inserting a GaN/Ti interlayer, the nanowire alignment can be greatly improved as indicated by scanning electron microscopy and X-ray diffraction.

The immiscibility of InAlN ternary alloy

We have used two models based on the valence force field and the regular solution model to study the immiscibility of InAlN ternary alloy, and have got the spinodal and binodal curves of InAlN. Analyzing the spinodal decomposition curves, we obtain the appropriate concentration region for the epitaxial growth of the InN-AlN pseudobinary alloy. At a temperature most common for the epitaxial growth of InAlN (1000 K), the solubility of InN is about 10%. Then we introduce the mismatch strain item into the Gibbs free energy, and the effect of different substrates is taken into consideration. Considering Si, Al2O3, InN, GaN, AlN as a substrate respectively, it is found that all the five systems are stabilized with the upper critical solution temperature largely reduced. Finally, InN and GaN are potential substrates for In-rich InAlN, while AlN and GaN substrates are recommended in the Al-rich region. Si and Al2O3 may be ideal substrates for thin InAlN film.

Effects of V/III ratio on a-plane GaN epilayers with an InGaN interlayer

The effects of V/III growth flux ratio on a-plane GaN films grown on r-plane sapphire substrates with an InGaN interlayer are investigated. The surface morphology, crystalline quality, strain states, and density of basal stacking faults were found to depend heavily upon the V/III ratio. With decreasing V/III ratio, the surface morphology and crystal quality first improved and then deteriorated, and the density of the basal-plane stacking faults also first decreased and then increased. The optimal V/III ratio growth condition for the best surface morphology and crystalline quality and the smallest basal-plane stacking fault density of a-GaN films are found. We also found that the formation of basal-plane stacking faults is an effective way to release strain.