自旋记忆磁电阻与整流磁电阻的研究

发布时间：2018-06-02 13:28

本文选题：整流磁电阻 + 隧穿磁电阻　；参考：《山东大学》2017年博士论文

【摘要】：随着信息技术的快速发展,大数据、云存储等新概念不断兴起,人类对芯片提出了高速度、低功耗、高集成度的要求。在过去50年中,通过提高光刻技术精度,采用新工艺等方法,芯片的集成度遵从摩尔定律的预言快速发展。但是,随着芯片集成度的提高,量子效应的出现、单位面积功耗提升以及投入和产出失衡等因素使得摩尔定律即将失效。为了突破当前微电子技术的瓶颈,人们提出了自旋电子学的解决方案。电子拥有电荷和自旋两种属性,传统的微电子学只利用了电子的电荷属性,自旋电子学可以同时利用电子的电荷和自旋属性,可以从这两个自由度去调控器件的性能,进而实现新型多功能芯片。在自旋电子学的发展过程中,磁电阻(Magnetoresistance,MR)现象在磁传感器和磁存储领域有无可替代的作用,一直是研究的热点。1857年,Thomson在铁磁金属中发现了各向异性磁电阻(Anistropy Magnetoresistance,AMR)现象,揭开了磁电阻现象研究的序幕,其物理机制是自旋-轨道耦合导致散射截面不同。1988年,Grunberg教授和Fert教授分别独立的在反铁磁耦合的Fe/Cr多层膜中发现了巨磁电阻效应(Giant Magnetoresistance,GMR),它起源于自旋相关的散射,随后GMR效应被迅速应用于硬盘读头领域,提高了硬盘的存储密度。1994年,Jin等人在钙钛矿锰氧化物中发现了庞磁阻效应(Colossal Magnetoresistance,CMR),该效应起源于磁相关的金属-绝缘体转变。1975年发现了隧穿磁电阻(TunnelingMagnetoresistance,MR),并将其归因于自旋相关的隧穿。随后,在Al2O3和MgO作为势垒层的铁磁\绝缘体\铁磁三明治结构中获得了大的TMR效应。并且,因为MgO势垒特殊的自旋过滤(Spin Filter)效应,MgO单晶势垒磁隧道结的磁电阻比值可以高达600%(300K)。相比GMR效应,TMR磁电阻比值更高、功耗更低、更易集成,被广泛应用于磁传感器、硬盘的磁读头、磁随机存储器(MRAM)、自旋微波振荡器、自旋转移矩二极管以及自旋逻辑器件中。在硅、锗、锑化铟、硒化银、砷化镓等非磁半导体中发现了异常磁电阻效应(Extrordinary Magnetoresistance,EMR)。EMR效应随着磁场增加呈现近似线性增加,未出现饱和现象。研究表明EMR与材料内部的载流子浓度、迁移率以及电场分布的非均匀性密切相关。最近,章晓中教授课题组利用二极管的非线性输运性质和材料的磁响应,最终实现了巨大的磁电阻比值,可称之为二极管增强的磁电阻效应(Diode-enhancedMagnetoresistance,DEMR)。基于EMR效应和DEMR效应,研究者提出了可重构的磁电阻逻辑器件。可见,磁电阻效应不仅有丰富的物理内涵,而且推动了磁传感器、磁读出头、磁随机存储器以及可重构磁逻辑器件等的发展,有重要的应用前景。因此,新型磁电阻效应的发现或者将已有磁电阻效应和其它效应相结合将会为我们发展新型多功能器件,进而实现高速度、低功耗、高集成度芯片提供崭新的思路。本论文的工作包括以下五方面内容:其一,将电致阻变效应(Resistance Switching,RS)和磁电阻效应结合在Co/CoO-ZnO/Co复合势垒磁隧道结(Magnetic Tunneling Junctions,MTJs)中,进而实现了自旋记忆磁电阻效应(Spin Memory Magnetoresistance),获得四个非易失的电阻态;其二,在Al/Ge/Al肖特基结异质结中发现了整流磁电阻效应(Rectification Magnetoresistance,RMR),这是一种新型的磁电阻效应,并指出整流磁电阻效应要求器件同时具有整流效应和磁电阻效应;其三,在Co/CoO-ZnO/Co非对称势垒磁隧道结中观测到了自旋整流磁电阻效应,该效应起源于自旋极化的电子通过CoO-ZnO非对称势垒的隧穿;其四,通过交流电流和直流电流混合的方法实现了整流磁电阻的调控,获得了巨大磁电阻的比值;其五,通过整流器件和磁电阻器件并联的方式实现了整流磁电阻效应,并建立理论模型,可以很好的描述实验观测到的结果。下面我们详细地阐述论文的五部分研究内容:一、在Co/CoO-ZnO/Co复合势垒磁隧道结中实现了自旋记忆磁电阻效应。通过磁控溅射结合金属掩膜的方法在玻璃衬底上制备了结面积为100 μm ×100 μm的Co/CoO-ZnO/Co MTJs。在电场的作用下,氧离子在CoO层和ZnO层中迁移,引起CoO在绝缘态与金属态之间相互转变,发生电致阻变效应,进而实现高、低两个电阻态。在高电阻态,自旋极化的电子通过CoO-ZnO复合势垒隧穿,产生TMR效应;在低电阻态,CoO变成金属态,实现GMR效应。TMR和GMR又分别具有平行态和反平行态两个电阻态,因此总共实现了四个稳定的电阻态,在多态存储、人工神经元模拟方面有重要的应用前景。不仅如此,CoO是反铁磁材料,Co/CoO界面处存在交换偏置效应。在电致阻变的过程中,CoO的反铁磁结构被破坏和重构,从而实现了交换偏置效应的可逆电调控。在高阻态,我们观测到高达68%的隧穿磁电阻,通过Julliere模型反推出Co/CoO界面处自旋极化率高达72%。为了验证实验观测到的结果,我们对CoO和Co/CoO界面进行了第一性原理计算。计算结果表明当CoO中没有氧空位时候,为绝缘体;当CoO中氧空位浓度为25%时,CoO转变为金属态,验证了本实验中观测到的电致阻变效应。计算还表明Co/CoO界面处自旋极化率高达73.2%,同Julliere模型反推出的结果一致。二、Al/Ge/Al的肖特基异质结中的整流磁电阻效应。通过电子束蒸发结合光刻的方法在本征锗衬底上制备了 Al/Ge的肖特基结,施加一个交流电流到该结两端,测量整流电压随外磁场变化。在室温,器件的整流磁电阻高达250%,而其传统的直流磁电阻只有70%。整流磁电阻的发现不仅为磁电阻家族增添了新的一员,而且提供了一种用交流电流实现多功能器件的方法。通过一系列的对照实验,我们得出单纯具有整流或磁电阻效应的器件都不能实现整流磁电阻效应。整流磁电阻效应要求在同一个器件中同时具有整流效应和磁电阻效应。通过一系列分析发现,本征锗衬底中载流子浓度较低,电子波函数之间的交叠有限,外加磁场会引起电子波函数收缩,导致电子波函数之间的交叠减小,因此磁场通过收窄能带宽度使得能带提高,进一步改变了 Al/Ge肖特基界面处的能带弯曲,器件的整流效应发生变化,最终导致整流磁电阻效应。三、Co/CoO-ZnO/Co非对称势垒磁隧道结中的自旋整流磁电阻效应。在具有非对称势垒的Co/CoO-ZnO/Co MTJs中观测到高达116%的整流磁电阻效应,与此同时其隧穿磁电阻只有20%左右。由于Co/CoO-ZnO/Co MTJs中的整流磁电阻效应起源于自旋相关隧穿,因此称之为自旋整流磁电阻效应(Spin RMR)。不同磁场下的伏安特性曲线的测量证明该器件同时具有磁电阻效应和整流效应。进一步通过微分电导谱的测量发现,微分电导谱呈现开口向上的抛物线形状,且最小值出现在-11mV左右,符合Brinkman,Dynes and Rowell 模型(BDR模型),证明整流效应来源于非对称的CoO-ZnO势垒。通过数值拟合,我们得到了 CoO-ZnO非对称势垒的宽度和两侧的势垒高度。自旋整流磁电阻将基于电荷的整流效应和基于自旋的磁电阻效应相结合,为我们研制新型自旋电子学器件提供了新途径。四、Al/Ge/In肖特基异质结中整流磁电阻效应的电调控制备了 Al/Ge/In肖特基异质结,将直流电流和交流电流混合后施加在该异质结两端,通过调控直流分量的大小,实现了磁电阻从-530%～32500%的显著调控。这是一种新型磁电调控手段,该技术可推广到所有具有整流磁电阻效应的器件,将推动多功能器件的发展。该效应的产生是由于当将直流电流和交流电流混合后施加在Al/Ge/In肖特基异质结器件两端时,直流电流产生的电压降和交流电流的整流电压叠加在一起被探测到。因此,通过改变直流分量和交流分量的大小可以实现对探测电压的调控,从而实现整流磁电阻效应的调控。五、用分立器件实现整流磁电阻效应,并建立理论模型。交替溅射制备的CoZnO磁性半导体薄膜作为磁电阻器件,1N5817肖特基二极管作为整流器件。通过将两者并联,耦合整流效应和磁电阻效应,构成整流磁电阻器件,实现了整流磁电阻效应。将直流电流和交流电流混合后施加到该器件两端,通过调节直流分量的大小,该器件的磁电阻可以在-11300%～13500%范围内被显著调控。该方法拓宽了整流磁电阻的应用范围。根据整流磁电阻的定义以及分立的整流器件和磁电阻器件的电输运性质,我们建立了理论模型,该模型能够通过分立器件的参数仿真器件的整流磁电阻效应及其电调控特性,与实验观测到的结果相吻合。这将有助于我们通过独立调整分立器件的参数,优化整流磁电阻器件的性能。
[Abstract]:With the rapid development of information technology, large data, cloud storage and other new concepts, human chips have raised the requirements of high speed, low power consumption and high integration. In the past 50 years, the integration degree of the chip has developed rapidly from the prophecy of Moore's law by improving the precision of lithography technology and using the new technology. In order to break through the current microelectronic technology bottlenecks, people have proposed a solution to spintronics, which have two properties of charge and spin, and traditional microelectronics only use electrons. In the development process of spintronics, the Magnetoresistance (MR) phenomenon has an irreplaceable role in the field of magnetic sensors and magnetic storage, which can be used to regulate the performance of the device from the two degrees of freedom. In.1857 years of research, Thomson found the anisotropic magnetoresistance (Anistropy Magnetoresistance, AMR) in ferromagnetic metals, which uncovered the prelude to the study of magnetoresistance. The physical mechanism is that the spin orbit coupling causes the scattering cross section to be different for.1988 years, and Professor Grunberg and Professor Fert are independent of the antiferromagnetic coupling. The giant magnetoresistance effect (Giant Magnetoresistance, GMR) was found in the Fe/Cr multilayer film. It originated from the spin dependent scattering, and then the GMR effect was quickly applied to the field of hard disk reading head, which increased the storage density of the hard disk for.1994 years, and Jin et al. In the perovskite manganese oxide (Colossal Magnetoresistance, CMR). The effect originated from the magnetic related metal insulator transition in.1975 and found the tunneling magnetoresistance (TunnelingMagnetoresistance, MR) and attributed it to spin related tunneling. Subsequently, the large TMR effect was obtained in the ferromagnetic sandwich structure of the barrier layer of Al2O3 and MgO, and the special spin filtration of the MgO barrier. (Spin Filter) effect, the magnetoresistance ratio of the MgO single crystal barrier magnetic tunnel junction can be as high as 600% (300K). Compared to the GMR effect, the TMR magnetoresistance ratio is higher, the power consumption is lower, and it is easier to integrate. It is widely used in magnetic sensors, magnetic read head of hard disk, magnetic random memory (MRAM), spin microwave oscillator, spin transfer moment diode and spin logic device. In non magnetic semiconductors such as silicon, germanium, indium selenide, silver selenide, gallium arsenide, and other non magnetic semiconductors, the effect of abnormal magnetoresistance (Extrordinary Magnetoresistance, EMR).EMR showed an approximate linear increase with the increase of the magnetic field, without saturation. The study showed that the carrier concentration, the mobility and the distribution of the electric field within the EMR and the material were inhomogeneous. There is a close relationship. Recently, Professor Zhang Xiaozhong's team, using the nonlinear transport properties of the diode and the magnetic response of the material, finally achieved a huge magnetoresistance ratio, which is called the Diode-enhancedMagnetoresistance (DEMR). Based on the EMR effect and the DEMR effect, the researchers have proposed a reconfigurable magnetoelectricity. It can be seen that the magnetoresistance effect not only has rich physical connotation, but also promotes the development of magnetic sensors, magnetic reading heads, magnetic random memory and reconfigurable magnetic logic devices. Therefore, the discovery of the new magnetoresistance effect or the combination of the existing magnetoresistance effects and other effects will be for me We develop new multi-functional devices to achieve high speed, low power, and high integration chips. The work of this paper includes the following five aspects: first, it combines the Resistance Switching, RS and magnetoresistance effects in the Co/CoO-ZnO /Co composite barrier magnetic tunnel junction (Magnetic Tunneling Junctions, MTJ) In s), the spin memory magnetoresistance effect (Spin Memory Magnetoresistance) is realized and four nonvolatile resistance states are obtained. Secondly, the rectifying magnetoresistance effect (Rectification Magnetoresistance, RMR) is found in the Al/Ge/Al Schottky junction heterojunction, which is a new type of magnetoresistance effect and points out the requirement of the rectifying magnetoresistance effect. The device has both rectification and magnetoresistance effects. Thirdly, the spin rectifying magnetoresistance effect is observed in the Co/CoO-ZnO/Co asymmetrical barrier magnetic tunnel junction, which originates from the tunneling of the spin polarized electrons through the CoO-ZnO asymmetric barrier; fourthly, the rectifying magnetoresistance is realized by the method of mixing the AC current and the DC current. The ratio of the giant magnetoresistance is obtained. Fifthly, the rectifying magnetoresistance effect is realized through the parallel connection of the rectifier device and the magnetoresistance device, and the theoretical model is set up. The results can be well described by the experimental observation. In the following five parts of the paper are described in detail: first, the Co/CoO-ZnO/Co composite barrier magnetic tunnel. The spin memory magnetoresistance effect is realized in the path junction. By the method of magnetron sputtering and metal mask, the Co/CoO-ZnO/Co MTJs. with a junction area of 100 mu x 100 mu is prepared on the glass substrate, and the oxygen ions migrate in the CoO layer and the ZnO layer under the action of the electric field, causing the CoO to change between the insulating state and the metal state, and the electrodrag change occurs. In the high resistance state, in the high resistance state, the spin polarized electrons are tunneling through the CoO-ZnO composite barrier, producing the TMR effect. In the low resistance state, the CoO becomes the metal state, and the GMR effect.TMR and GMR have parallel and anti parallel states respectively. Therefore, four stable resistance states are realized in total, and polymorphic in a total of four states. Storage, artificial neuron simulation has an important application prospect. Not only that, CoO is antiferromagnetic material, and there is an exchange bias effect at the Co/CoO interface. In the process of electrical impedance change, the antiferromagnetic structure of CoO is destroyed and reconstructed, thus the reversible electrical regulation of the exchange bias effect is realized. In the high resistivity state, we observed up to 68% of the tunnel. Through the magnetoresistance, the Julliere model was used to reverse the spin polarization of the Co/CoO interface up to 72%. in order to verify the results of the experimental observation. We carried out the first principle calculation of the CoO and Co/CoO interfaces. The calculation shows that when there is no oxygen vacancy in the CoO, it is an insulator; when the oxygen vacancy concentration in CoO is 25%, the CoO is transformed into a metal state. The electroresistance effect observed in this experiment is also proved. The calculation also shows that the spin polarization of the Co/CoO interface is up to 73.2%, which is in accordance with the reverse results of the Julliere model. The rectifying magnetoresistance effect in the Schottky heterojunction of two, Al/Ge/Al is prepared by electron beam evaporation and photolithography to prepare the Schottky on the intrinsic germanium substrate. An AC current is applied to the junction at both ends of the junction to measure the change of the rectifying voltage with the external magnetic field. At room temperature, the rectifier's rectifying magnetoresistance is up to 250%, and the discovery of its traditional DC magnetoresistance only with the 70%. rectifying magnetoresistance not only adds a new member of the magnetoresistance family, but also provides a formula for realizing multifunction devices with AC current. Method. Through a series of controlled experiments, we conclude that a single rectifier or magnetoresistance effect can not achieve the rectifying magnetoresistance effect. The rectifying magnetoresistance effect requires simultaneous rectification and magnetoresistance effects in the same device. Through a series of analyses, the carrier concentration in the intrinsic germanium substrate is low, and the electron wave is found. The overlap between the functions is limited, and the external magnetic field will cause the contraction of the electronic wave function, which leads to the reduction of the overlap between the electronic wave functions, so the magnetic field increases the energy band by narrowing the energy band width, and further changes the band bending of the Al/Ge Schottky interface. The rectifier effect should be changed, and the rectifying magnetoresistance effect is finally resulted. Three, The spin rectifying magnetoresistance effect in Co/CoO-ZnO/Co asymmetrical barrier magnetic tunnel junction. The rectifying magnetoresistance effect of up to 116% is observed in the Co/CoO-ZnO/Co MTJs with asymmetric barrier, while its tunneling magnetoresistance is only about 20%. Because the rectifying magnetoresistance effect in Co/CoO-ZnO/Co MTJs originates from spin related tunneling, It is called the spin rectifying magnetoresistance effect (Spin RMR). The measurement of the volt ampere characteristic curves under different magnetic fields shows that the device has both magnetoresistance and rectifying effects. Further through the measurement of differential conductance spectrum, the differential conductance spectrum presents an upwards parabolic shape, and the minimum value appears at about -11mV, which is in line with Brinkman, Dyn The ES and Rowell model (BDR model) proves that the rectifying effect derives from the asymmetric CoO-ZnO barrier. By numerical fitting, we get the width of the asymmetric barrier of CoO-ZnO and the barrier height on both sides. The spin rectifying magnetoresistance combines the rectification effect of the charge and the spin based magnetoresistance effect to develop a new spin for us. The electronic device provides a new way. Four, the Al/Ge/In Schottky heterojunction is prepared by the modulation of the rectifying magnetoresistance effect in the Al/Ge/In Schottky heterojunction. The DC current and AC current are mixed and applied to the two ends of the heterojunction. By regulating the size of the DC component, the magnetoresistance from the -530% to 32500% is realized. This technique can be extended to all the devices with the rectifying magnetoresistance effect, which will promote the development of the multifunction device. The effect is due to the voltage drop produced by direct current current and the integral of AC current when the DC current and AC current are mixed at both ends of the Al/Ge/In Schottky heterojunction device. The current voltage superposition is detected together. Therefore, by changing the DC component and the size of the AC component, the detection voltage can be regulated and the rectifying magnetoresistance effect is realized. Five, the rectifying magnetoresistance effect is realized by the discrete device, and the theoretical model is established. The magnetic semiconductor thin film prepared by alternate sputtering is used as the magnetic field. Five Resistance device, 1N5817 Schottky diode is used as a rectifier. By parallel, coupled rectifier effect and magnetoresistance effect, a rectifier magnetoresistance device is formed to realize the rectifying magnetoresistance effect. The DC current and AC current are mixed to the two ends of the device. The magnetoresistance of the device can be adjusted by adjusting the size of the DC component. This method broadens the application range of the rectifying magnetoresistance in the range of -11300% to 13500%. Based on the definition of the rectifying magnetoresistance and the electrical transport properties of the discrete rectifier devices and magnetoresistance devices, a theoretical model is established. The model can simulate the rectifying magnetoresistance effect of the device through the parameters of the discrete device and its effect. The electrical regulation characteristics coincide with the experimental results. This will help us to optimize the performance of the rectifier by adjusting the parameters of the discrete device independently.
【学位授予单位】：山东大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TN303

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