Co与ZnO半导体等复合膜磁电阻效应及自旋注入的研究
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摘要
自从在Fe/Cr金属多层膜中发现巨磁电阻效应以来，人们在磁性金属/非磁性材料的多层膜、颗粒膜和磁性隧道结中都相继发现了磁电阻效应，而且其在磁传感器、计算机读头及磁随机存储器等自旋电子器件上得以广泛应用。相对于金属/绝缘体薄膜而言，以半导体为势垒来研究磁电阻效应的报道却很少，半导体材料具有较低的势垒高度，能显著降低材料的电阻率，同时通过增加半导体的厚度，可以有效减少针孔效应的发生。尽管已有文献报道了磁性金属/半导体薄膜的室温磁电阻效应，但如何进一步提高磁电阻的数值和磁电阻效应的产生机制等有待进一步探究。
本论文采用磁控溅射超薄分层交替沉积的方法，室温下制备了Co/ZnO等薄膜，对薄膜的结构、磁性、磁电阻效应及金属/半导体界面的自旋电子注入进行了研究。①比较了Co/ZnO与Co/Al_2O_3、Co/C和Co/Cu薄膜的结构和磁电阻效应，探明Co/ZnO薄膜磁电阻效应的来源；②通过在ZnO中加入少量Al来提高薄膜的磁性和磁电阻效应；③在Co/ZnO薄膜中发现了磁电阻效应对其电阻的依赖关系。主要研究内容如下：
(1)采用固定Co层厚度(0.6 nm)，改变ZnO层厚度(0.4-3.0 nm)的方法制备了Co/ZnO薄膜，发现薄膜形成了Co纳米颗粒包裹在ZnO半导体中的颗粒膜结构，薄膜中有少量Co~(2+)取代了ZnO中的Zn~(2+)离子；同时薄膜的室温和低温负磁电阻值分别达到11.9%和26%，薄膜电阻与温度间的lnρ与T~(-1/2)线性关系说明薄膜的磁电阻效应来源于磁性纳米颗粒间电子自旋相关的隧穿输运机制；而高温时lnρ与T~(-1/2)发生微小的非线性偏离说明温度的升高使自旋无关的高阶跳跃输运逐渐增多；薄膜低温时磁电阻效应的加强源于自旋相关的高阶隧穿。
(2)通过对Co/ZnO、Co/Al_2O_3、Co/C和Co/Cu薄膜结构和磁电阻效应的对比研究，发现四种不同基质材料的薄膜均形成了Co颗粒包裹在非磁性基质中的颗粒膜，随基质材料与金属Co表面能差的增加，薄膜中Co颗粒尺寸逐渐减小；金属Co和半金属C之间较大的电导失配使Co/C薄膜没有室温磁电阻效应；在Co/ZnO薄膜中金属Co颗粒使部分ZnO中的电子极化，较大的室温磁电阻效应可能与Co颗粒与ZnO基质界面处部分极化的电子有关。
(3)采用磁控溅射方法在ZnO中掺入金属Al制备了Co/ZnAlO (Al: 2at.%)薄膜，发现Co/ZnO与Co/ZnAlO薄膜均形成了磁性纳米颗粒包裹在半导体中的颗粒膜，薄膜的磁性来源于金属Co颗粒以及Co颗粒与半导体基质界面处的梯度磁性半导体；在Co/ZnAlO薄膜中获得12.3%的室温负磁电阻值，这是目前为止在磁性金属/半导体薄膜中得到的最大室温负磁电阻值；Al的加入使Co/ZnAlO薄膜的磁性和室温磁电阻效应同时提高，磁性的增加是由于Al的加入增加了薄膜中载流子浓度，从而增强了磁性半导体的磁性；磁性半导体磁性的增加使其对传导电子的自旋过滤效应增强，提高了Co/ZnAlO薄膜的室温磁电阻效应和自旋电子注入效率。
(4)通过改变溅射气压和ZnO厚度等一系列实验条件制备了Co/ZnO薄膜，通过大量实验数据系统分析发现Co/ZnO薄膜室温磁电阻效应对其电阻具有明显的依赖性，当1300＜R＜6000时，薄膜具有8%以上的较大室温负磁电阻值，电阻值太小或过大时磁电阻效应都会减弱；薄膜的结构、磁性及电输运性质等结果表明薄膜磁电阻效应与电阻的依赖关系源于薄膜中电子的输运机制，当薄膜中电子输运以隧穿电导为主时才会表现出较大的室温磁电阻效应，但电阻过大又会使电子隧穿几率减小，降低了磁电阻值；电阻过小时薄膜中磁性颗粒间较大的耦合作用使其磁电阻效应减弱。
总之，我们在Co/ZnO和Co/ZnAlO薄膜中获得了较大的室温磁电阻效应和较高的室温自旋电子注入效率，阐明了这种磁电阻来源于磁性颗粒间的隧穿输运机制，发现较大室温磁电阻效应与磁性金属颗粒和半导体界面处磁性半导体的自旋过滤效应有关，这为研究金属/半导体界面室温自旋电子注入提供了新的途径，同时其有望在自旋电子学器件中得以广泛应用。
Since the discovery of giant magnetoresistance effect in Fe/Crmultilayers, the magnetoresistance (MR) in magnetic multilayers,granular films and magnetic tunnel junctions composed of magneticmetals and non-magnetic materials have been studied extensively forpotential application in spintronic devices, such as magnetic sensors,hard disk reading heads and magnetic random access memories.There is little research about the MR of semiconductor barrier.Semiconductor can provide a much lower barrier height compared toinsulator with a decrease of resistivity. Moreover, the increasedthickness of semiconductor reduces the probability of pinholes.According to the reports in the literature, room temperature MR hasbeen observed in magnetic metal/semiconductor films. However,there is ample room for further investigation of exploring the origin ofthe MR in magnetic metal/semiconductor films and improving the MRratio.
In this paper, the films composed of Co and ZnO semiconductoretc. were deposited on glass substrates by sequential magnetronsputtering at room temperature. The structure, magnetic properties,magnetoresistance and spin injection at the interface between metaland semiconductor were studied.①the magnetization and roomtemperature MR in Co/ZnO films was enhanced by Al doping;②theorigin of the MR in Co/ZnO films was studied compared to that inCo/Al_2O_3, Co/C, and Co/Cu films;③the room temperature MR ofCo/ZnO films depends on the resistance of Co/ZnO films. The resultsare summarized as follows:
(1) Co/ZnO films were deposited by sequential deposition ofultrathin Co layer fixed 0.6 nm and ZnO layer varied from 0.4 to 3.0 nm.The granular films consist of Co particles dispersed in ZnO matrix anda few of +2 valence cobalt ions substitute for Zn2+ions in ZnO lattices.The large negative MR of 11.9% and 26% at room temperature and lowtemperature, respectively, are observed in the Co/ZnO film. Theobserved linear relation between lnρand T~(.1/2)indicates aninter-particle tunneling conduction mechanism in the films. At highertemperatures, lnρstarts to slightly deviate from linear relation,suggesting the presence of spin-independent high-order hopping athigher temperatures due to defects within the ZnO matrix. Theenhanced MR at low temperature is ascribed to spin-dependent high-order tunneling.
(2) The structure and MR effect for Co/ZnO, Co/Al_2O_3, Co/C andCo/Cu granular films were studied comparatively. Four types of filmsexhibit similarities in structure, comprising Co particles embedded innonmagnetic matrix. The average size of Co particles decreases withan increase of the difference of surface energy between metallic Coand the nonmagnetic matrix, which are smaller than that of cobalt.The absence of MR effect in Co/C films at room temperature isprobably due to the conductivity mismatch between the metallic Coand the semimetal carbon. The large room temperature MR in Co/ZnOgranular films is related to the partially polarized electrons at theinterface between Co particles and ZnO matrix.
(3) The Co/ZnAlO films were prepared using sequential magnetronsputtering at room temperature by Al doped ZnO (Al: 2at.%). The filmshave a similar structure consisting of Co nanoparticles dispersed insemiconductor matrix. The magnetization of the films comes frommetallic Co particles and the graded magnetic semiconductor at theinterface between Co particles and semiconductor matrix. The MR ofCo/ZnAlO granular film reaches -12.3% at room temperature, which isthe largest value in magnetic metal/semiconductor films by far. Boththe magnetization and the MR effect are higher for films containingCo/ZnAlO than for Co/ZnO. The enhanced magnetization of the films with Al doping is probably due to the enhanced carrier concentrationenhancing the magnetization of the graded magnetic semiconductor.The larger MR in the film with ZnAlO occurs because of the larger spinpolarization of the semiconductor which acts as the spin filter, whichis very promising to improve the spin injection efficiency intosemiconductors.
(4) Many influential factors of room temperature MR in Co/ZnOfilms was studied by a lot of experimental data by changing theconditions of the sputtering pressure and ZnO normal thickness. Thestudy indicates that the room temperature MR is related to theresistance of Co/ZnO films instead of the sputtering pressure and ZnOthickness. The large room temperature MR (more than 8%) of thefilms is observed on the scale of 1300Ω In conclusion, large room temperature negative MR and spin injection efficiency were observed in Co/ZnO and Co/ZnAlO films,which is caused by spin-dependent tunneling throughsemiconducting barriers. We found that the enhanced MR comesfrom the spin filtering of the magnetic semiconductor at theinterface between the metallic particles and semiconductor matrix.This provides a new way for the effective electron spin injection atroom temperature between the metal and semiconductor, which isexpected for potential application in spintronics devices.
本论文采用磁控溅射超薄分层交替沉积的方法，室温下制备了Co/ZnO等薄膜，对薄膜的结构、磁性、磁电阻效应及金属/半导体界面的自旋电子注入进行了研究。①比较了Co/ZnO与Co/Al_2O_3、Co/C和Co/Cu薄膜的结构和磁电阻效应，探明Co/ZnO薄膜磁电阻效应的来源；②通过在ZnO中加入少量Al来提高薄膜的磁性和磁电阻效应；③在Co/ZnO薄膜中发现了磁电阻效应对其电阻的依赖关系。主要研究内容如下：
(1)采用固定Co层厚度(0.6 nm)，改变ZnO层厚度(0.4-3.0 nm)的方法制备了Co/ZnO薄膜，发现薄膜形成了Co纳米颗粒包裹在ZnO半导体中的颗粒膜结构，薄膜中有少量Co~(2+)取代了ZnO中的Zn~(2+)离子；同时薄膜的室温和低温负磁电阻值分别达到11.9%和26%，薄膜电阻与温度间的lnρ与T~(-1/2)线性关系说明薄膜的磁电阻效应来源于磁性纳米颗粒间电子自旋相关的隧穿输运机制；而高温时lnρ与T~(-1/2)发生微小的非线性偏离说明温度的升高使自旋无关的高阶跳跃输运逐渐增多；薄膜低温时磁电阻效应的加强源于自旋相关的高阶隧穿。
(2)通过对Co/ZnO、Co/Al_2O_3、Co/C和Co/Cu薄膜结构和磁电阻效应的对比研究，发现四种不同基质材料的薄膜均形成了Co颗粒包裹在非磁性基质中的颗粒膜，随基质材料与金属Co表面能差的增加，薄膜中Co颗粒尺寸逐渐减小；金属Co和半金属C之间较大的电导失配使Co/C薄膜没有室温磁电阻效应；在Co/ZnO薄膜中金属Co颗粒使部分ZnO中的电子极化，较大的室温磁电阻效应可能与Co颗粒与ZnO基质界面处部分极化的电子有关。
(3)采用磁控溅射方法在ZnO中掺入金属Al制备了Co/ZnAlO (Al: 2at.%)薄膜，发现Co/ZnO与Co/ZnAlO薄膜均形成了磁性纳米颗粒包裹在半导体中的颗粒膜，薄膜的磁性来源于金属Co颗粒以及Co颗粒与半导体基质界面处的梯度磁性半导体；在Co/ZnAlO薄膜中获得12.3%的室温负磁电阻值，这是目前为止在磁性金属/半导体薄膜中得到的最大室温负磁电阻值；Al的加入使Co/ZnAlO薄膜的磁性和室温磁电阻效应同时提高，磁性的增加是由于Al的加入增加了薄膜中载流子浓度，从而增强了磁性半导体的磁性；磁性半导体磁性的增加使其对传导电子的自旋过滤效应增强，提高了Co/ZnAlO薄膜的室温磁电阻效应和自旋电子注入效率。
(4)通过改变溅射气压和ZnO厚度等一系列实验条件制备了Co/ZnO薄膜，通过大量实验数据系统分析发现Co/ZnO薄膜室温磁电阻效应对其电阻具有明显的依赖性，当1300＜R＜6000时，薄膜具有8%以上的较大室温负磁电阻值，电阻值太小或过大时磁电阻效应都会减弱；薄膜的结构、磁性及电输运性质等结果表明薄膜磁电阻效应与电阻的依赖关系源于薄膜中电子的输运机制，当薄膜中电子输运以隧穿电导为主时才会表现出较大的室温磁电阻效应，但电阻过大又会使电子隧穿几率减小，降低了磁电阻值；电阻过小时薄膜中磁性颗粒间较大的耦合作用使其磁电阻效应减弱。
总之，我们在Co/ZnO和Co/ZnAlO薄膜中获得了较大的室温磁电阻效应和较高的室温自旋电子注入效率，阐明了这种磁电阻来源于磁性颗粒间的隧穿输运机制，发现较大室温磁电阻效应与磁性金属颗粒和半导体界面处磁性半导体的自旋过滤效应有关，这为研究金属/半导体界面室温自旋电子注入提供了新的途径，同时其有望在自旋电子学器件中得以广泛应用。
Since the discovery of giant magnetoresistance effect in Fe/Crmultilayers, the magnetoresistance (MR) in magnetic multilayers,granular films and magnetic tunnel junctions composed of magneticmetals and non-magnetic materials have been studied extensively forpotential application in spintronic devices, such as magnetic sensors,hard disk reading heads and magnetic random access memories.There is little research about the MR of semiconductor barrier.Semiconductor can provide a much lower barrier height compared toinsulator with a decrease of resistivity. Moreover, the increasedthickness of semiconductor reduces the probability of pinholes.According to the reports in the literature, room temperature MR hasbeen observed in magnetic metal/semiconductor films. However,there is ample room for further investigation of exploring the origin ofthe MR in magnetic metal/semiconductor films and improving the MRratio.
In this paper, the films composed of Co and ZnO semiconductoretc. were deposited on glass substrates by sequential magnetronsputtering at room temperature. The structure, magnetic properties,magnetoresistance and spin injection at the interface between metaland semiconductor were studied.①the magnetization and roomtemperature MR in Co/ZnO films was enhanced by Al doping;②theorigin of the MR in Co/ZnO films was studied compared to that inCo/Al_2O_3, Co/C, and Co/Cu films;③the room temperature MR ofCo/ZnO films depends on the resistance of Co/ZnO films. The resultsare summarized as follows:
(1) Co/ZnO films were deposited by sequential deposition ofultrathin Co layer fixed 0.6 nm and ZnO layer varied from 0.4 to 3.0 nm.The granular films consist of Co particles dispersed in ZnO matrix anda few of +2 valence cobalt ions substitute for Zn2+ions in ZnO lattices.The large negative MR of 11.9% and 26% at room temperature and lowtemperature, respectively, are observed in the Co/ZnO film. Theobserved linear relation between lnρand T~(.1/2)indicates aninter-particle tunneling conduction mechanism in the films. At highertemperatures, lnρstarts to slightly deviate from linear relation,suggesting the presence of spin-independent high-order hopping athigher temperatures due to defects within the ZnO matrix. Theenhanced MR at low temperature is ascribed to spin-dependent high-order tunneling.
(2) The structure and MR effect for Co/ZnO, Co/Al_2O_3, Co/C andCo/Cu granular films were studied comparatively. Four types of filmsexhibit similarities in structure, comprising Co particles embedded innonmagnetic matrix. The average size of Co particles decreases withan increase of the difference of surface energy between metallic Coand the nonmagnetic matrix, which are smaller than that of cobalt.The absence of MR effect in Co/C films at room temperature isprobably due to the conductivity mismatch between the metallic Coand the semimetal carbon. The large room temperature MR in Co/ZnOgranular films is related to the partially polarized electrons at theinterface between Co particles and ZnO matrix.
(3) The Co/ZnAlO films were prepared using sequential magnetronsputtering at room temperature by Al doped ZnO (Al: 2at.%). The filmshave a similar structure consisting of Co nanoparticles dispersed insemiconductor matrix. The magnetization of the films comes frommetallic Co particles and the graded magnetic semiconductor at theinterface between Co particles and semiconductor matrix. The MR ofCo/ZnAlO granular film reaches -12.3% at room temperature, which isthe largest value in magnetic metal/semiconductor films by far. Boththe magnetization and the MR effect are higher for films containingCo/ZnAlO than for Co/ZnO. The enhanced magnetization of the films with Al doping is probably due to the enhanced carrier concentrationenhancing the magnetization of the graded magnetic semiconductor.The larger MR in the film with ZnAlO occurs because of the larger spinpolarization of the semiconductor which acts as the spin filter, whichis very promising to improve the spin injection efficiency intosemiconductors.
(4) Many influential factors of room temperature MR in Co/ZnOfilms was studied by a lot of experimental data by changing theconditions of the sputtering pressure and ZnO normal thickness. Thestudy indicates that the room temperature MR is related to theresistance of Co/ZnO films instead of the sputtering pressure and ZnOthickness. The large room temperature MR (more than 8%) of thefilms is observed on the scale of 1300Ω
引文
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