Yumin Zhang

Effects of inhomogeneous oxygen content in (La,Gd)0.7Ca0.3MnO3+δ perovskites

Magnetoresistance properties of compounds with the same formula La0.7−xGdxCa0.3MnO3+δ (x=0, 0.04) but synthesized by the solid-state reaction method from different starting materials are studied. Nearly stoichiometric compounds are obtained when La(OH)3 is used for sample preparation. However, under the same condition, La2O3 causes considerable oxygen deficiency in resulting samples, characterized by a broad metal–semiconductor transition far below the magnetic transition. Inhomogeneous distribution of the oxygen content in compounds (an inhomogeneity beyond the detection of powder x-ray diffraction) can occur when both La2O3 and La(OH)3 coexist in the starting materials, leading to two metal–semiconductor transitions which are obviously developments of those in compounds prepared respectively from La2O3 and La(OH)3, and one magnetic transition corresponding to the upper resistive transition.

Defects in large single crystals Nd: YVO4

The exchange anisotropy in NiFe/NiMn bilayers was studied by using the planar Hall effect. The sputtered NiFe/NiMn films were patterned into strips of 1 mm in length and 200 mum in width and with six terminals for anisotropy magnetoresistance and planar Hall voltage measurements by a photolithographic process. It is shown that the planar Hall effect is an effective method to characterize the exchange anisotropy in ferromagnetic/antiferromagnetic (AF) systems. It can be used to accurately determine the exchange field and describe the magnetization reversal processes. The effective uniaxial anisotropy field H-K eff, the effective unidirectional anisotropy field H-ud, and AF domain wall energy H-w can be obtained by fitting the experimental results. We found that in the NiFe/NiMn bilayer system, the parameters H-K eff, H-ud, and H-w have the same values in reversible and irreversible measurements, and the domain wall energy in AF layer is larger than interfacial unidirectional anisotropy

TUNNELING ESCAPE TIME OF ELECTRONS FROM A QUANTUM-WELL WITH GAMMA-X MIXING EFFECT

By using the envelope function method we calculated the tunneling escape time of electrons from a quantum well. We adopted a simplified interface matrix to describe the Γ‐Χ mixing effect, and employed a wave packet method to determine the tunneling escape time. When the Γ state in the well was in resonance with the Χ state in the barrier, the escape time reduced remarkably. However, it was possible that the wave functions in two different channels, i.e., Γ‐Γ‐Γ and Γ‐Χ‐Γ, could interfere destructively, leading the escape time greater than that of pure Γ‐Γ‐Γ tunneling.

Surface acoustic waves in semi-insulating Fe-doped GaN films grown by hydride vapor phase epitaxy

The propagation properties of surface acoustic waves (SAWs) in semi-insulating Fe-doped GaN films grown on sapphire substrates by hydride vapor phase epitaxy are investigated. Compared with native n-type GaN, Fe-doped GaN exhibits a higher electromechanical coupling coefficient due to its high electrical resistivity. In addition, guided longitudinal leaky surface acoustic wave (LLSAW) was observed experimentally with a very high phase velocity (about 7890u2009m/s), and this mode was verified by numerical simulations. The small propagation attenuation of LLSAW along liquid/solid interfaces was demonstrated in glycerol solutions, which implies the potential applications in high-frequency chemical sensing.

Growth model of van der Waals epitaxy of films: A case of AlN films on multilayer graphene/SiC

Volmer-Weber island nucleation and step-flow growth model are the classical processes of the conventional epitaxy of films. However, a growth model of van der Waals epitaxy (vdWE) of films is still not very well-documented. Here, we present an example of vdWE of AlN films on multilayer graphene (MLG)/SiC by hydride vapor phase epitaxy at a high temperature of 1100 °C and reveal the orientation relationship of AlN, MLG, and SiC as (0001)[1-100]AlN||(0001)[1-100]MLG||(0001)[11-20]SiC, which suggests that the vdWE heterointerface is not an usual covalent bond and no excessive strain during the growth process owing to the incommensurate in-plane lattices. Remarkably, zigzag cracks are formed because of the anisotropy of strain after the films are cooled down to room temperature, indicating that the growth model of vdWE is different from that of conventional epitaxy. It is a layer-by-layer epitaxy, and a planar substrate without a miscut angle is essential for obtaining single-crystalline films. Additionally, the films can be transferred to foreign substrates by direct mechanical exfoliation without any stressor layer. An ultraviolet photosensor device illustrates an example of III-nitride heterogeneous integration application. Our work demonstrates an excellent step toward the vdWE of varieties of compound films on 2D materials for the applications of transferrable heterogeneous integration in future.

Evolution of threading dislocations in GaN epitaxial laterally overgrown on GaN templates using self-organized graphene as a nano-mask

Growth of high-quality GaN within a limited thickness is still a challenge, which is important both in improving device performance and in reducing the cost. In this work, a self-organized graphene is investigated as a nano-mask for two-step GaN epitaxial lateral overgrowth (2S-ELOG) in hydride vapor phase epitaxy. Efficient improvement of crystal quality was revealed by x-ray diffraction. The microstructural properties, especially the evolution of threading dislocations (TDs), were investigated by scanning electron microscopy and transmission electron microscopy. Stacking faults blocked the propagation of TDs, and fewer new TDs were subsequently generated by the coalescence of different orientational domains and lateral-overgrown GaN. This evolution mechanism of TDs was different from that of traditional ELOG technology or one-step ELOG (1S-ELOG) technology using a two-dimensional (2D) material as a mask.

Direct growth of GaN on sapphire with non-catalytic CVD graphene layers at high temperature

Today, heteroepitaxial GaN films on sapphire have focused on conventional two-step growth process using low temperature GaN buffer layer. Here, we show the direct growth of GaN films on sapphire by using a graphene layer at high temperature, which simplified the GaN growth process. The graphene is directly synthesized on non-catalytic sapphire substrate by chemical vapor deposition without problematic transfer processes, using C2H4 as a carbon source at the temperature of 1200 oC. The synthesized graphene has been characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM). We have compared the GaN grown on sapphire with and without graphene. The single crystal, smooth surface GaN films has been obtained on sapphire with graphene, and the nucleation of GaN films has been discussed. The GaN films illuminated high near-band-edge emission and good ultraviolet photosensor. It demonstrates that graphene is a potential, useful buffer layer for heteroepitaxy of high quality GaN films.