Giant injection magnetoresistance in the heterostructure gallium arsenide/granular film with cobalt nanoparticles

We have studied the electron transport and have observed new phenomenon—the positive injection magnetoresistance on heterostructures gallium arsenide/granular film SiO₂ with Co nanoparticles and gallium arsenide/granular film TiO₂ with Co island layers.     

Study of the Au/SiO<Subscript>2</Subscript> + Co(65 at %)/GaAs heterostructure interfaces by the polarized neutron reflectometry method

Using the polarized neutron reflectometry method, we studied Au/SiO₂ + Co(60 at %)/GaAs granular films, which display a giant injection magnetoresistance effect in a narrow temperature range close to T = 300 K. We revealed the existence of a layer having particular magnetic properties at the boundary of a film with a semiconductor GaAs substrate. Experiments carried out at temperatures T = 300 K and T = 100 K showed an insignificant difference in the magnetic profile of the heterostructure.     

Polarized neutron reflectometry from the interface of the heterostructures SiO2(Co)/Si and SiO2(Co)/GaAs

Polarized neutron reflectometry is used to investigate SiO₂(Co) granular films (70 at% of Co nanoparticles in SiO₂ matrix) deposited on Si and GaAs substrates. The aim of the study is to compare magnetization depth profiles in two systems: in SiO₂(Co)/GaAs heterostructure which shows at room temperature giant injection magnetoresistance (IMR) with the system SiO₂(Co)/Si which reveals almost no IMR effect. We found that at room temperature and at the same value of external magnetic field mean magnetization in the SiO₂(Co)/GaAs sample is much higher than in the case of SiO₂(Co)/Si. We also demonstrate that magnetic scattering length density, and hence, magnetization profile strongly depends on the substrate. We show that SiO₂(Co)/Si heterostructure is ferromagnetically ordered within the temperature range between 120 and 460 K what could explain a weak IMR.     

Investigation of the SiO<Subscript>2</Subscript>(Co)/GaAs heterostructures using the surface scattering of synchrotron radiation

The giant injection magnetoresistive effect has been observed in a granulated Co/SiO₂ film on a semiconductor GaAs substrate in a narrow temperature range near T = 300 K. According to the existing theory, the nature of the effect is due to the structure and physical problems of the interface layer. The spatial distribution of cobalt nanoparticles in the bulk of the Co/SiO₂ granular film and at the granular film/semiconductor substrate (GF/SS) interface has been investigated by the reflectometry and small-angle scattering of synchrotron radiation in the grazing geometry. It has been shown that the characteristic average distance between the cobalt granules in the bulk of the film is 7.3 nm. At the same time, the average distance between the granules with a vertical size of about 7.5 nm at the GF/SS interface is 32 nm. The experimental data indicate the low concentration of cobalt at the interface and the point character of the contact of the main bulk of the Co/SiO₂ film with the GaAs substrate through a relatively diluted layer of ferromagnetic cobalt granules.     

Positive and negative magnetoresistance in Fe3O4-based heterostructures

Fe₃O₄-based heterostructures, including Fe₃O₄/MgO/Fe₃O₄, Fe₃O₄/MgO/Si and Fe₃O₄/SiO₂/Si, were fabricated by magnetron sputtering to investigate the perpendicular-to-plane magneto-transport properties. In the Fe₃O₄/MgO/Fe₃O₄ and Fe₃O₄/MgO/Si heterostructures, the typical magneto-transport properties of single Fe₃O₄ films, such as negative magnetoresistance (MR) and extreme values of MR−T curves at 120 K, were observed, suggesting that the spin polarization of conducting electrons conserves through MgO barrier. MR in the Fe₃O₄/MgO/Fe₃O₄ heterostructure is larger than that in the Fe₃O₄/MgO/Si heterostructure, because the spin of electrons is disturbed in the depletion layer of Si and the SiO₂ layer introduced by Fe₃O₄/MgO growth. The Fe₃O₄/SiO₂/Si heterostructure has a positive MR of 2% at 120 K, which may originate from the scattering of conducting electrons in amorphous SiO₂ and the spin polarization reversal at the Fe₃O₄/SiO₂ interface.     

退火对Co35(SiO2)65颗粒膜巨磁电阻效应的影响

采用多靶离子束溅射镀膜机制备了一系列不同退火温度的Co₃₅(SiO₂)₆₅（体积百分数）颗粒膜样品,发现样品的巨磁电阻效应随着退火温度T_a的升高而单调下降.应用磁力显微镜对样品的磁结构进行了观测,发现随着退火温度的升高,近邻的Co颗粒的磁矩倾向于平行排列,形成磁畴结构,从而导致Co₃₅(SiO₂)₆₅颗粒膜巨磁电阻效应单调下降. 关键词：     

Properties of thin manganite films grown on semiconducting substrates for spintronics applications

La_0.7Sr_0.3MnO₃ (LSMO) manganite thin films were grown by pulsed plasma deposition on silicon (Si) and gallium arsenide (GaAs) substrates covered by an amorphous oxide. Manganite films are characterized by polycrystalline structure. Ferromagnetic transition is above room temperature and for 50 nm thick film the Curie temperature was as high as 325 K and 305 K for LSMO/SiO_x/Si and LSMO/AlO_x/GaAs, respectively.     

同步辐射光激励的二氧化硅薄膜刻蚀研究

利用热氧化法在硅晶片上生长SiO₂薄膜，结合光刻和磁控溅射技术在SiO₂薄膜表面制备接触型钴掩模，通过掩模方法在硅表面开展了同步辐射光激励的表面刻蚀研究，在室温下制备了SiO₂薄膜的刻蚀图样.实验结果表明：在同步辐射光照射下，通入SF₆气体可以有效地对SiO₂薄膜进行各向异性刻蚀，并在一定的气压范围内，刻蚀率随SF₆气体浓度的增加而增加，随样品温度的下降而升高；如果在同步辐射光照射下，用SF₆和O₂的混合气体作为反应气体，刻蚀过程将停止在SiO₂/Si界面，即不对硅刻蚀，实现了同步辐射对硅和二氧化硅两种材料的选择性刻蚀；另外，钴表现出强的抗刻蚀能力，是一种理想的同步辐射光掩模材料. 关键词： 同步辐射刻蚀 接触型钴掩模 二氧化硅薄膜     

Fabrication of quantum wires by selective intermixing induced in GaAs/AlGaAs quantum well heterostructures by SiO2 capping and subsequent annealing

Results are presented demonstrating that selective intermixing of GaAs/AlGaAs quantum well heterostructures by SiO₂ capping and subsequent annealing can be spatially localized with a length scale compatible with the observation of lateral quantum confinement effects. Patterning of a 400 nm-thick SiO₂ encapsulation layer deposited by rapid thermal chemical vapor deposition into arrays of wires was performed using high resolution electron beam lithography and subsequent reactive ion etching. After high temperature (850°C) annealing, photoluminescence experiments indicate the creation of double barrier quantum wires when small trenches (< 100 nm) are etched in the SiO₂ film at a period greater than 800 nm. Signatures of the formation of one-dimensional subbands are observed both in photoluminescence excitation spectroscopy and linear polarization anisotropy analysis. A mechanism involving the ability of the stress field generated during annealing at the SiO₂ film edges to pilot the diffusion of the excess gallium vacancies which are responsible for the enhanced interdiffusion under SiO₂ is suggested to account for the high lateral selectivity achievable with this novel process.     

Optical properties of magnetic quasi-2D nanocomposites in the IR spectral region

The optical absorption spectra of magnetic quasi-2D Co island films and of hybrid Co/SiO₂ multilayers in which island films are separated by SiO₂ layers were studied. It was found that, when the Co film was masked by a thin SiO₂ layer, the absorption considerably increased and exceeded 0.8. This was observed when the Co fraction was substantially lower than its critical value for the percolation transition. Moreover, the maximal absorption in this case weakly depends on both the effective film thickness and the number of Co layers. Numerical analysis shows that the observed peculiarities of the absorption spectra of the structures based on quasi-2D granulated Co films are connected to a great extent with the peculiar features of the interference effects occurring in such systems.     

Salient features of the electrical characteristics of oxygen-implanted gallium arsenide Schottky-barrier structures

We have studied the deep levels in gallium arsenide implanted with oxygen ions at a low dose of 2·10¹⁰ cm⁻² and the influence of those levels on the current-voltage characteristics of Ni/Pd/Ga Schottky-barrier structures. The deep levels were checked by the thermally-stimulated-current (TSC) method. The number of peaks on the TSC curves and their amplitudes are shown to depend on the oxygen-ion energy and the temperature of the post-implantation anneal. An important role in the formation of centers associated with oxygen is played by gallium diffusion into the protective SiO₂/Si₃N₄ insulator layer as well as by elastic and plastic strain phenomena that occur during deposition of the coating and post-implantation annealing of the specimens. The deep levels due to oxygen are responsible for the appearance of excess currents resulting from recombination as well as for ohmic leakages. V. D. Kuznetsov Siberian Physicotechnical Institute at Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 30–36, May, 1996.     

Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript>/Nd<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript> MnO<Subscript>3</Subscript> heteroepitaxial structure: Optical and magnetotransport properties

Sm_0.55Sr_0.45MnO₃/Nd_0.55Sr_0.45MnO₃ heterostructure consisting of layers with different Curie temperatures is studied. By comparing data for IR transmission, resistivity, magnetotransmission, magnetoresistance, and Kerr effect measured on the side of the film and substrate, the Curie temperatures of the layers are determined and the contributions of the layers to the magnetoresistance and magnetotransmission are estimated. A weak temperature dependence of the magnetotransmission and magnetoresistance makes manganites with a colossal magnetoresistance and magnetotransmission candidate materials for devices without temperature stabilization.     

Hole injection and surface state effects at gallium arsenide electrodes

We report capacitance and luminescence measurements at gallium arsenide electrodes in 1 M H₂SO₄ electrolyte that indicate the injection of holes on cathodic polarization. The steady-state potential-current relations and photoeffects are reviewed critically. We show that the assumption of hole injection is equally plausible to that of electron extraction provided the surface recombination velocity is fast compared to the bulk diffusion and recombination process. In addition to the space charge there is a substantial surface charge of the type commonly observed at III–V semiconductor surfaces.     

Magneto-optic effects in ferromagnetic films: Implications for spin devices

We present results on the magneto-optic properties of ferromagnetic films deposited on GaAs and SiO₂ substrates. Using left- and right-circularly polarized light, we have measured the polarization-dependent photoresponse and reflectivity of Co/GaAs, Fe/GaAs and NiFe/GaAs Schottky diodes and the polarization-dependent reflection and transmission of NiFe/SiO₂ and Co/SiO₂ structures as a function of ferromagnetic film thickness, reported here in the range of 7.5-15 nm. Films were prepared by sputtering and molecular-beam epitaxy. Measurements were made in the presence of magnetic fields ranging from −1.2 to +1.2 T both parallel and perpendicular to the sample surface. We find maximum polarization-dependent transmission and photoresponse effects (with respect to left- versus right-circularly polarized light) of 2-4% in magnitude. Taken together the work suggests that magneto-optic effects intrinsic to the films, rather than spin injection across the ferromagnetic/semiconductor interface, are responsible for the observed phenomenology. The work has direct implications for the interpretation of results in ferromagnetic/semiconductor spintronic systems.     

Large positive magnetoresistance effect in metal-insulator nanocomposites in weak magnetic fields

Studies of the magnetoresistance of granular Fe/SiO₂ films within the compositional range corresponding to insulators revealed a large positive magnetoresistance. The positive magnetoresistance reaches 10% in magnetic fields ～100 Oe at room temperature and exhibits slow response times (～2 min) to stepped magnetic-field variation. The nature of the effect is apparently associated with the influence of ferromagnetic cluster aggregates of iron nanoparticles on the magnetoresistance.     

Tunneling magnetoresistance and electroresistance properties of composite‐barrier ferroelectric tunnel junctions

The multiferroic properties of ferroelectric tunnel junctions with a composite barrier comprising a fully epitaxial La_0.7Sr_0.3MnO₃/BaTiO₃/SrTiO₃/La_0.7Sr_0.3MnO₃ heterostructure are reported in this study. The patterned junctions having extended top electrodes show tunnel magnetoresistance ratios ranging from 20% to 110% at 77 K. Furthermore, tunneling electroresistance – induced by ferroelectric polarization switching and showing two‐state memory effect in the dynamic resistance – has also been observed in these junctions. Thus, with the concurrence of tunneling electroresistance and magnetoresistance, these tunnel junctions serve as memory devices with four resistance states. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoelectric properties of ZnO: In nanorods/SiO2/Si heterostructure assembled in aqueous solution

In-doped zinc oxide (ZnO:In) nanorods were grown onto SiO₂/n-Si substrate without catalyst in aqueous solution. The ZnO:In nanorods/SiO₂/n-Si heterostructure photovoltaic device was prepared. The structural and photoelectric properties of the as-grown ZnO:In nanorods were analyzed. ZnO:In nanorods had a strong and broad UV surface photovoltage response in the range of 300–400 nm, and the bands were identified. The photoelectric conversion properties of ZnO:In nanorods/SiO₂/n-Si heterostructure were investigated. ZnO:In/SiO₂/n-Si heterostructure showed a wide range photocurrent spectral response with high intensity in the UV and visible region. The rectifying behavior of this heterostructure was observed. Moreover, the device had a low turn-on voltage and a high breakdown voltage. Current–voltage characteristic was studied for the heterostructure, and the open-circuit voltage and short-circuit current were obtained. PACS 73.40.Lq; 85.35.Be; 81.16.Dn     

Structure and properties of interphase boundaries of gallium arsenide-metal (dielectric)

To study the nature and properties of potential barriers in gallium arsenide devices, we have investigated structural phase transitions in GaAs contacts with multilayer films containing refractory transition-metal borides (TiB₂, LaB₆). We verified the important role in degrading Schottky barrier device performance played by local mechanical stresses introduced at the interface by lateral nonuniformities in interphase interactions. We examine the electrical properties of MIS gallium arsenide devices, taking into account the high density of electronic surface states (ESS). We show it is possible to control the density of ESS by selecting the dielectric, and we discuss its deposition and annealing with a pulsed laser. We discuss the nature of potential barriers in gallium arsenide devices, drawing upon our data and previously published data and modern theoretical models.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, No. 10, pp. 52–62, October, 1993.     

Formation of the buffer layer of silicon suboxides SiOx in the Si/SiO2 low-dimensional heterosystem after Si+ ion implantation: Si L2, 3 X-ray emission spectra

Nanosized heterostructures n-Si/SiO₂ with different thicknesses of the oxide film (20, 500 nm) after implantation by Si⁺ ions with energies of 12 and 150 keV have been investigated using Si L _{2, 3} X-ray emission spectroscopy (the Si 3d3s → Si 2p _{1/2, 3/2} electronic transition). The ion-beam modification of the interface has been revealed and studied for the heterostructure with a silicon dioxide thickness of 20 nm. An analysis of the Si L _{2, 3} X-ray emission spectra has demonstrated that the Si⁺ ion implantation leads to the self-ordering of the structure of the initially amorphous SiO₂ film 20 nm thick due to the effect of high doses. A mechanism of ion-beam modification of the insulator-semiconductor interface has been proposed. No substantial transformation of the atomic and electronic structures of the heterostructure with a silicon dioxide thickness of 500 nm has been revealed after the ion implantation.     

Characteristics of a type-II n-MoS2/p-Ge van der Waals heterojunction

A few-atomic-layer molybdenum disulfide (MoS₂) film on Si/SiO₂ substrates grown by metal-organic chemical vapor deposition was investigated. The few-atomic-layer MoS₂ film was subsequently transferred onto a (100) p-Ge substrate to build a van der Waals n-p heterojunction. The as-grown few-atomic-layer MoS₂ film and the MoS₂/Ge heterostructure were characterized atomic force microscopy, spectroscopic ellipsometry, high-resolution scanning transmission electron microscopy, Raman spectroscopy analyses, photoluminescence (PL) measurements at room temperature (RT, 300 K), and type-II band alignment of the heterostructure determined by ultraviolet photoelectron spectroscopy. The RT-PL measurements showed dominant peaks at 1.96 and 1.8 eV for the as-grown MoS₂ and red-shifted PL peaks for that transferred onto Ge. We examined the electrical characteristics of the few-atomic-layer MoS₂ by forming a type-II band alignment van der Waals heterojunction with a highly doped p-Ge. The heterojunction solar cell exhibited an open-circuit voltage of 0.15 V and a short-circuit current density of 45.26 μA/cm². The external quantum efficiency measurements showed a spectral response up to approximately 500 nm owing to the absorption by the few-atomic-layer MoS₂ film.