Enhanced ferromagnetism and conductivity of ultrathin freestanding La_(0.7)Sr_(0.3)MnO_(3)membranes

We report a universal method to transfer freestanding La_(0.7)Sr_(0.3)MnO_(3)membranes to target substrates.The 4-unit-cell-thick freestanding La_(0.7)Sr_(0.3)MnO_(3)membrane exhibits the enhanced ferromagnetism,conductivity and out-of-plane magnetic anisotropy,which otherwise shows nonmagnetic/antiferromagnetic and insulating behavior due to the intrinsic epitaxial strain.This work facilitates the promising applications of ultrathin freestanding correlated oxide membranes in electronics and spintronics.

Transition of photoconductive and photovoltaic operation modes in amorphous Ga_2O_3-based solar-blind detectors tuned by oxygen vacancies

We report on the transition of photovoltaic and photoconductive operation modes of the amorphous Ga_2O_3-based solar-blind photodetectors in metal–semiconductor–metal(MSM) configurations. The conversion from Ohmic to Schottky contacts at Ti/Ga_2O_3 interface is realized by tuning the conductivity of amorphous Ga_2O_3 films with delicate control of oxygen flux in the sputtering process. The abundant donor-like oxygen vacancies distributed near the Ti/Ga_2O_3 interface fascinate the tunneling process across the barrier and result in the formation of Ohmic contacts. As a consequence, the serious sub-gap absorption and persistent photoconductivity(PPC) effect degrades the performance of the photoconductive detectors. In contrast, the photovoltaic device with a Schottky contact exhibits an ultra-low dark current less than 1 pA,a high detectivity of 9.82×10^(12) cm·Hz^(1/2)·W^(-1), a fast response time of 243.9 μs, and a high ultraviolet C(UVC)-toultraviolet A(UVA) rejection ratio of 103. The promoting performance is attributed primarily to the reduction of the subgap states and the resultant suppression of PPC effect. With simple architecture, low fabrication cost, and easy fusion with modern high-speed integrated circuitry, these results provide a cost-effective way to realize high performance solar-blind photodetectors towards versatile practical applications.

The luminescence enhancement of Eu^(3+) ion and SnO_2 nanocrystal co-doped sol gel SiO_2 films

SnO_2 nanocrystal and rare-earth Eu～(3+) ion co-doped SiO_2 thin films are prepared by sol-gel and spin coating methods.The formation of tetragonal rutile structure SnO_2 nanocrystals with a uniform distribution is confirmed by X-ray diffraction and transmission electron microscopy.Fourier transform infrared spectroscopy is used to investigate the densities of the hydroxyl groups,and it is found that the emission intensity from the 5 D 0 7 F 2 transitions of the Eu～(3+) ions is enhanced by two orders of magnitude due to energy transfer from the oxygen-vacancy-related defects of the SnO_2 nanocrystals to nearby Eu～(3+) ions.The influences of the amounts of Sn and the post-annealing temperatures are systematically evaluated to further understand the mechanism of energy transfer.The luminescence intensity ratio of Eu～(3+) ions from electric dipole transition and magnetic dipole transition indicate the different probable locations of Eu～(3+) ions in the sol-gel thin film,which are further discussed based on temperature-dependent photoluminescence measurements.

Epitaxial Growth and Characteristics of Nonpolar a-Plane InGaN Films with Blue-Green-Red Emission and Entire In Content Range

Nonpolar(1120)plane In_(x)Ga_(1-x)N epilayers comprising the entire In content(x)range were successfully grown on nanoscale Ga N islands by metal-organic chemical vapor deposition.The structural and optical properties were studied intensively.It was found that the surface morphology was gradually smoothed when x increased from 0.06 to 0.33,even though the crystalline quality was gradually declined,which was accompanied by the appearance of phase separation in the In_(x)Ga_(1-x)N layer.Photoluminescence wavelengths of 478 and 674 nm for blue and red light were achieved for x varied from 0.06 to 0.33.Furthermore,the corresponding average lifetime(τ_(1/e))of carriers for the nonpolar In Ga N film was decreased from 406 ps to 267 ps,indicating that a high-speed modulation bandwidth can be expected for nonpolar In Ga N-based light-emitting diodes.Moreover,the bowing coefficient(b)of the(1120)plane In Ga N was determined to be 1.91 e V for the bandgap energy as a function of x.

Synthesis of [100] Wurtzite InN Nanowires and [011] Zinc-Blende InN Nanorods

一个维的 wurtzite 客栈 nanowires 和锌闪锌矿客栈 nanorods 被化学蒸汽免职(CVD ) 准备 GaAs 的自然劈开飞机(110 ) 上的方法。客栈 nanowires 的生长方向是[100 ] ，与 wurtzite 结构。客栈的稳定的水晶结构是 wurtzite (w 客栈) ，锌闪锌矿结构(z 客栈) 以前仅仅为 2D 客栈晶体被报导。然而在这个工作，锌闪锌矿客栈 nanorods [011 ] 被综合并且描绘。SEM 和 TEM 图象证明每 nanorod 塑造一个圆锥形的尖端，它能被生长过程和 Ehrlich-Schwoebel 障碍的理论的 anisotropy 解释。

Plasma assisted molecular beam epitaxial growth of GaN with low growth rates and their properties

A systematic investigation on PA-MBE grown GaN with low growth rates(less than 0.2μm/h)has been conducted in a wide growth temperature range,in order to guide future growth of sophisticated fine structures for quantum device applications.Similar to usual growths with higher growth rates,three growth regions have been revealed,namely,Ga droplets,slightly Ga-rich and N-rich 3D growth regions.The slightly Ga-rich region is preferred,in which GaN epilayers demonstrate optimal crystalline quality,which has been demonstrated by streaky RHEED patterns,atomic smooth surface morphology,and very low defect related yellow and blue luminescence bands.The growth temperature is a critical parameter to obtain high quality materials and the optimal growth temperature window(~700-760℃)has been identified.The growth rate shows a strong dependence on growth temperatures in the optimal temperature window,and attention must be paid when growing fine structures at a low growth rate.Mg and Si doped GaN were also studied,and both p-and n-type materials were obtained.

Crystallographic and magnetic properties of van der Waals layered FePS_3 crystal

The crystallographic and magnetic properties are presented for van der Waals antiferromagnetic FePS_3. High-quality single crystals of millimeter size have been successfully synthesized through the chemical vapor transport method. The layered structure and cleavability of the compound are apparent, which are beneficial for a potential exploration of the interesting low dimensional magnetism, as well as for incorporation of FePS_3 into van der Waals heterostructures. For the sake of completeness, we have measured both direct current(dc) and alternating current(ac) magnetic susceptibility.The paramagnetic to antiferromagnetic transition occurs at approximately T_N 115 K. The effective moment is larger than the spin-only effective moment, suggesting that an orbital contribution to the total angular momentum of the Fe^(2+) could be present. The ac susceptibility is independent of frequency, which means that the spin freezing effect is excluded.Strong anisotropy of out-of-plane and in-plane susceptibility has been shown, demonstrating the Ising-type magnetic order in FePS_3 system.

Fabrication and Characterization of a GaN-Based 320×256 Micro-LED Array

Design, fabrication and characterizations of GaN-based blue micro light emitting diode(LED) arrays are reported.The GaN micro-LED array consists of 320×256 pixels with a pitch size of 30 fjym. Each pixel is 25×25 μm^2 in size, which is designed for backside emission and high density flip-chip packaging. The selected LED pixels being tested exhibit good uniformity in terms of turn-on voltage and reverse leakage current. The eficiency droop behavior and reliabilit.y behavior under high forward current stress are also studied. The micro-LED pixel shows improved reliability, which is likely caused by enhanced heat dissipation.

Experimental observation of interlayer perpendicular standing spin wave mode with low damping in skyrmion-hosting[Pt/Co/Ta]_(10)multilayer

An interlayer perpendicular standing spin wave mode is observed in the skyrmion-hosting[Pt/Co/Ta]_(10) multilayer by measuring the time-resolved magneto-optical Kerr effect.The observed interlayer mode depends on the interlayer spin-pumping and spin transfer torque among the neighboring Co layers.This mode shows monotonically increasing frequency-field dependence which is similar to the ferromagnetic resonance mode,but within higher frequency range.Besides,the damping of the interlayer mode is found to be a relatively low constant value of 0.027 which is independent of the external field.This work expounds the potential application of the[heavy-metal/ferromagnetic-metal]_(n) multilayers to skyrmion-based magnonic devices which can provide multiple magnon modes,relatively low damping,and skyrmion states,simultaneously.

Influence of Fluorine on the Conductivity and Oxidation of Silicon Nanomembranes after Hydrofluoric Acid Treatment

After immersion in hydrofluoric acid,the sheet resistance of a 220-nm-thick silicon nanomembrane,measured in dry air by van der Pauw method,drops around two orders of magnitude initially,then increases and reaches the level of a sample with a native oxide surface in about one month.The surface component and oxidation rate are also characterized by x-ray photo electronic spectroscopy measurement.Fluorine is found to play a significant role in improving conductivity and has no apparent influence on the oxidation rate after hydrofluoric acid treatment.

Growth and Properties of Blue and Amber Complex Light Emitting InGaN/GaN Multi-Quantum Wells

Blue-red complex light emitting InGaN/GaN multi-quantum well(MQW)structures are fabricated by metal organic chemical vapor deposition(MOCVD).The structures are grown on a 2-inch diameter(0001)oriented(c−face)sapphire substrate,which consists of an approximately 2-µm−thick GaN template and a five-period layer consisting of a 4.9-nm-thick In0.18Ga0.82N well layer and a GaN barrier layer.The surface morphology of the MQW structures is observed by an atomic force microscope(AFM),which indicates the presence of islands of several tens of nanometers in height on the surface.The high resolution x−ray diffraction(XRD)θ/2θscan is carried out on the symmetric(0002)of the InGaN/GaN MQW structures.At least four order satellite peaks presented in the XRD spectrum indicate that the thickness and alloy compositions of the individual quantum wells are repeatable throughout the active region.Besides the 364 nm GaN band edge emission,two main emissions of blue and amber light from these MQWs are found,which possibly originate from the carrier recombinations in the InGaN/GaN QWs and InGaN quasi-quantum dots embedded in the QWs.

A Band-Gap Energy Model of the Quaternary Alloy In_(x)Ga_(y)Al_(1−x−y)N using Modified Simplified Coherent Potential Approximation

Based on modification of the simplified coherent potential approximation,a model for the band-gap energy of In_(x)Ga_(y)Al_(1−x−y)N is developed.The parameters of the model are obtained by fitting the experimental band-gap energy of their ternary alloys.It is found that the results agree with the experimental values better than those reported by others,and that the band-gap reduction of In_(x)Ga_(y)Al_(1−x−y)N with increasing In or Ga content is mainly due to enhanced intraband coupling within the conduction band,and separately within the valence band.

Site Preference of Se and Te in Bi2Se3-xTex Thin Films

The ternary topological insulators Bi2Se3-xTex have attracted a great deal of attention due to their exotic physical and chemical properties.While most of the studies focus on the properties of these ternary TIs,limited research was performed to investigate the dynamic atomic stack of its crystal structure.We prepared highquality Bi2Se3-xTex thin films on Ga As(111)B substrates using molecular beam epitaxy,characterized with Raman spectroscopy,x-ray diffraction and photoelectron spectroscopy.It is found that when Se is replaced by Te,the preferred substituting sites are the middle layer at 0<x<1,and this is also valid for Se substituting Te at 2<x<3.In the middle region,the substituting atoms prefer to go to the first and the fifth layer.

Direct observation of the distribution of impurity in phosphorous/boron co-doped Si nanocrystals

Doping in Si nanocrystals is an interesting topic and directly studying the distribution of dopants in phosphorous/boron co-doping is an important issue facing the scientific community.In this study,atom probe tomography is performed to study the structures and distribution of impurity in phosphorous/boron co-doped Si nanocrystals/SiO_(2) multilayers.Compared with phosphorous singly doped Si nanocrystals,it is interesting to find that the concentration of phosphorous in co-doped samples can be significantly improved.Theoretical simulation suggests that phosphorous-boron pairs are formed in co-doped Si nanocrystals with the lowest formation energy,which also reduces the formation energy of phosphorous in Si nanocrystals.The results indicate that co-doping can promote the entry of phosphorous impurities into the near-surface and inner sites of Si nanocrystals,which provides an interesting way to regulate the electronic and optical properties of Si nanocrystals such as the observed enhancement of conductivity and sub-band light emission.

Tuning Surface Spin Polarization of CoFeB by Boron Diffusion Detected by Spin Resolved Photoemission

Research of spin polarization of magnetic CoFeB thin films is of practical importance in spintronic applications.Here,using a direct characterization technique of spin-resolved photoemission spectroscopy,we obtain the surface spin polarization of amorphous Co_(40)Fe_(40)B_(20)thin films with different annealing temperatures from 100℃to 500℃prepared by magnetron sputtering.After high annealing temperature,a quasi-semiconductor state is gradually formed at the CoFeB surface due to the boron diffusion.While the global magnetization remains almost constant,the secondary electrons’spin polarization,average valence band spin polarization and the spin polarization at Fermi level from spin-resolved photoemission spectroscopy show a general trend of decreasing with the increasing annealing temperature above 100℃.These distinct surface properties are attributed to the enhanced Fe-B bonding due to the boron segregation upon surface after annealing as confirmed by x-ray photoelectron spectroscopy and scanning transmission electron microscopy with energy dispersive spectroscopy.Our findings provide insight into the surface spin-resolved electronic structure of the CoFeB thin films,which should be important for development of high-performance magnetic random-access memories.