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For a long time, it has been generally acknowledged that low-dimensional (lower than three-dimensions) long-range orders cannot stay stable at any finite temperature, because temperature-induced fluctuations can destroy any long-range orders in low-dimensional systems supported by isotropic short-range interactions. However, this theorem requires that the interaction must be short-range and isotropic. In fact, many low-dimensional systems do not meet these two requirements. For example, due to the strong anisotropy in two-dimensional CrI3 crystals, there is a band gap in the magnon spectrum. When the excitation energy from temperature is much lower than the band gap, the magneton cannot be excited by temperature on a large scale, and the long-range magnetic order in the two-dimensional system will not be destroyed. Various methods have been used to characterize the magnetic order in atomically thin CrI3 crystals, and a lot of attempts have been made to manipulate the magnetic structure in the system. Focusing on CrI3, in this article we review the recent studies on growth, magnetic structure measurement and manipulation of two-dimensional magnetic materials, and also discuss the prospects for the next phase of research from the perspectives of basic condensed matter physics research and electronic engineering applications.
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Keywords:
- CrI3 /
- two-dimensional magnets /
- magnetism measurement /
- magnetism manipulation
1. 引 言
近年来, 非晶氧化锌镓铟薄膜晶体管(amorphous InGaZnO thin-film transistor, InGaZnO TFT)的研究及其应用广受关注. 由于具有高迁移率、低关态电流、高透光率、均匀性良好以及低温可制备性等优点[1-5], InGaZnO TFT有望取代硅基TFT, 在平板显示、光学图像传感器、触控和指纹传感等领域拥有良好的应用前景[6-9]. 在传统的应用中, 例如薄膜晶体管液晶显示(TFT-LCD), 或者有源矩阵有机发光二极管显示(active-matrix organic light emitting display, AM-OLED), 主要利用的是InGaZnO TFT导通态的电学性能[10-12], 但在InGaZnO TFT集成的众多新兴领域中, 例如光学图像传感器, InGaZnO TFT的泄漏电流特性至关重要.
以往的研究表明, TFT中常见的泄漏电流产生机制包括陷阱辅助热发射[13,14]、陷阱辅助场发射[13]、带间隧穿[15,16]和包含Poole-Frenkel (PF)效应的陷阱辅助热电子场致发射[17]等. Lui和Migliorato [18]基于PF效应和陷阱辅助隧穿效应, 提出了载流子产生复合速率模型, 其适合于器件的数值计算. Wu等[19]在此基础上提出了多晶硅TFT泄漏电流紧凑模型, 其中包含了PF效应, 无需繁琐的数值计算, 并且可以通过实验数据提取得到模型参数. Servati和Nathan [20]研究了非晶硅(a-Si:H) TFT的泄漏电流的产生来源, 基于欧姆传导模型、反向亚阈传导模型和前沟道传导模型建立了分段式泄漏电流模型. 但是迄今为止, 几乎未见有关InGaZnO TFT的泄漏电流模型的研究报道, 这不利于InGaZnO TFT集成图像传感器等新兴领域的研究.
本文基于载流子的产生-复合机制建立了InGaZnO TFT的泄漏电流模型. 通过与TCAD模拟以及测试结果的对比来检验所提出的模型. 基于所提出的泄漏电流模型, 分析了InGaZnO TFT的沟道宽度、沟道长度和栅介质层厚度对泄漏电流的影响.
2. 基于TCAD的InGaZnO TFT泄漏电流的数值分析
图1示意了底栅顶接触型InGaZnO TFT的剖面图. 表1列出了InGaZnO TFT器件结构的几何参数, 其中 InGaZnO TFT的沟道宽长比W/L = 300
μm /50μm , a-InGaZnO半导体层厚度tIGZO = 40 nm, 栅介质层厚度tSiOx=150nm [21].表 1 InGaZnO TFT器件结构的几何参数Table 1. Geometric parameters of InGaZnO TFT device structure.参数 数值 栅介质层厚度/nm 150 a-InGaZnO半导体层厚度/nm 40 沟道宽度/μm 300 沟道长度/μm 50 图2(a)给出了在实验[21]与TCAD数值模拟中得到的InGaZnO TFT在关断区和亚阈区的IDS-VGS转移特性的曲线对比, 其中VDS取值从2 V增加到10 V. 由图可知, InGaZnO TFT具有较低的泄漏电流, 且IDS随着VDS增大而增加, IDS的变化范围为10–13—10–12 A. 泄漏电流增加的主要原因是随着TFT上所施加偏压的增加, 陷阱态中的电子获得能量跃迁至导带的几率就越大. 实验值和TCAD模拟值的变化趋势及范围大致相同, 在亚阈区内泄漏电流均呈数量级的增长.
图 2 IGZO TFT电学特性曲线实验与数值模拟比较 (a) IDS-VGS转移特性曲线; (b) IDS-VDS输出特性曲线Fig. 2. The comparison of IGZO TFT electrical characteristic curve in experiment and numerical simulation: (a) IDS-VGS transfer characteristic curve; (b) IDS-VDS output characteristic curve.图2(b)为同一InGaZnO TFT IDS-VDS转移特性曲线的实验值和TCAD模拟值的对比, 结果表明IDS和VGS, VDS均呈正相关, 在器件的饱和区(VDS ≥ VGS－VTH), IDS的值趋于稳定且不随VDS发生变化. 从图中可以看出实验值与TCAD模拟值吻合度较高, 说明TCAD模拟较为可靠. 因此, 后续将以TCAD模拟值作为参考, 验证所建立InGaZnO TFT泄漏电流解析模型的可靠性.
在InGaZnO TFT的TCAD电学仿真中, 主要通过金属功函数和UST (universal Schottky tunnling model)模型描述肖特基势垒; 有源层材料InGaZnO的特性主要由缺陷态(defects)密度模型决定. InGaZnO的缺陷态模型与泄漏电流密切相关, 具体模型参数见表2.
表 2 InGaZnO缺陷态密度模型参数Table 2. Density of states model parameters for InGaZnO.参数 描述 数值 单位 nta 导带尾类受主能态密度 1.04 × 1019 cm–3·eV–1 ntd 价带尾类施主能态密度 5.0 × 1020 cm–3·eV–1 wta 类受主态特征能量 0.04 eV wtd 类施主态特征能量 0.1 eV nga 高斯分布的受主态密度 0 cm–3·eV–1 ngd 高斯分布的施主态密度 2.0 × 1016 cm–3·eV–1 egd 高斯分布施主能态峰值能量 2.9 eV wgd 高斯分布施主能态特征能量 0.1 eV 3. 泄漏电流的紧凑模型建立与分析
根据陷阱辅助热电子场致发射机制, 在较强的栅-漏电场作用下, 大量电子将从局域态中隧穿到导带[18]. 载流子的产生率与Poole-Frenkel效应和声子辅助隧穿效应密切相关. InGaZnO层带隙内的局域态可表示为
RT=RA+RD, (1) 其中RT是陷阱态中的载流子产生复合率, RA和RD分别是受主态和施主态的载流子产生复合率. 作为n沟道器件, InGaZnO TFT中类施主态呈中性, 对自由载流子数量的影响几乎可以忽略. 但是, InGaZnO的类受主态呈正电性, 可捕获自由电子, 对泄漏电流的大小的影响较大. 在建模过程中, 为了简化计算, 忽略了施主态, 只考虑了受主态的载流子产生. 因此(1)式可简化为:
RT=RA, (2) RA=np−n2in+n1cp(χF+ΓCoulp)+p+p1cn(1+ΓDiracn)NA(Et), (3) (3)式中NA(Et)是类受主态在导(价)带附近处深能态和带尾态的密度, 受主态包括尾态和深能态级, 由于InGaZnO TFT中深能态级密度远低于带尾态, 因此, 计算过程中主要考虑带尾态密度;
χF 是Poole-Frenkel增强效应因子; ni是本征载流子浓度; n1和p1分别代表陷阱能级上的电子浓度和空穴浓度, cn,p代表俘获系数, 且n1=niexp(Et−EikT), (4) p1=niexp(Ei−EtkT), (5) χF=exp(ΔECkT), (6) 式中k为玻尔兹曼常数, T为温度; 其中
ΔEC 可表征为ΔEC=q√qFπεIGZO, (7) 其中q代表电荷量, F为垂直方向的电场强度,
εIGZO 为InGaZnO材料的介电常数. (7)式来源于描述Poole-Frenkel效应下泄漏电流与电场强度关系的公式[22], 在Poole-Frenkel效应作用下, 泄漏电流与电场强度的指数正相关,ΔEC 表示在电场强度F作用下势垒的降低量. (3)式中ΓpCoul与ΓDiracn 分别是Dirac阱与Coulombic阱中载流子的隧穿比率, 可表征为:ΓDiracn=ΔEnkT∫10exp(ΔEnkT−knu32)du, (8) ΓCoulp=ΔEpkT∫1ΔECΔEpexp{ΔEpkTu−kpu32[1−(ΔECuΔEp)53]}du, (9) ΔEn=EC−Et, (10) ΔEp=Et−EV, (11) kn,p=4√2m∗e,h(ΔEn,p)33qℏF. (12) (12)式中
m∗e,h 表示隧穿效应中电子(空穴)的有效质量,ℏ 表示普朗克常数.NA(Et)=NTailAexp[Et−ECETailA], (13) U=∫ECEVRT(Et)dEt=∫ECEVRA(Et)dEt. (14) 将(1)—(13)式代入(14)式中, 可以求解得到InGaZnO TFT的载流子产生率. 在上述模型中, 包含了InGaZnO TFT可能的泄漏电流产生机制: 场致电子发射、带间隧穿以及Poole-Frenkel效应[23-25]. 由于InGaZnO TFT器件一般工作于常温, 热电子发射产生的泄漏电流极小, 这里忽略了热电子发射效应. 同时, (14)式也是一双积分表达式, 计算过程繁琐且需要消耗大量的内存资源. 因此, 本文采用分区段计算的方法来简化建模过程.
分析式(14)可知, 第一重积分计算的是场效应积分即
ΓCouln,p和ΓDiracn,p 的值, 积分对象是u, 积分过程与Poole-Frenkel增强因子χF 无关; 第二重积分的对象是Et, 积分过程同样与Poole-Frenkel增强因子χF 无关; 在整个积分过程中Poole-Frenkel增强因子χF 始终保持指数形式, 在最终积分结果中也保持原状.在低负电场区域(F ≤ FC), Poole-Frenkel效应增强下的热发射是泄漏电流产生的主要机制, 在载流子产生复合率U的表达式里Poole-Frenkel增强因子
χF 的值占主导地位, 因此低负电场区载流子产生复合率U可简化成U=a1exp(b1√F),F⩽FC. (15) 在高负电场区域(F ≥ FC), 陷阱中的载流子首先热激发到一个中间能级Et, 然后再从该中间能级隧穿到导带或者价带; 根据文献[26]可知在高负电场区域热离子场助发射是产生泄漏电流的主要原因. 文献[26]中计算得出, 在高负电场区域lnU与电场强度F呈线性关系, 故在高负电场区载流子产生复合率U可表示为
U=a2exp(b2⋅F),F>FC. (16) U={a1exp(b1√F),F⩽FC,a2exp(b2F),F>FC, (17) 其中a1, a2, b1, b2均为拟合参数. 泄漏电流ILeak可由(18)式描述[27]:
ILeak=qVolU, (18) 其中
Vol=W⋅Lov⋅Xd, (19) 式中q是电荷量, Vol是源漏极交叠耗尽区的体积, W是InGaZnO TFT沟道宽度, Lov是栅漏交叠区长度, Xd为耗尽区厚度. 将(19)式代入(18)式中得到关断区泄漏电流模型:
ILeak-off={qVol⋅a1exp(b1√F),F⩽FC,qVol⋅a2exp(b2F),F>FC, (20) 其中
F=VDS−VGStSiOx, (21) tSiOx 为栅氧化层厚度.由图2(a)可知在亚阈区后半段lnIDS与VGS呈线性关系:
ILeak-sub=a3exp(b3VGS). (22) 使用平滑函数tanh(x)[28]得到InGaZnO TFT泄漏电流的统一模型为
ILeak=qWLovXd⋅[a1exp[b1(VDS−VGStSiOx)]×{1−tanh[β(VGS−VC)]2}+a2exp(b2√VDS−VGStSiOx)×(1+tanh[β(VGS−VC)]2)]×(1−tanh(βVGS)2)+a3exp(b3VGS)⋅[1+tanh(βVGS)2], (23) 其中a1, a2, a3, b1, b2, b3均为拟合参数.
根据上述模型可得到InGaZnO TFT泄漏电流的计算值, 此外还可利用TCAD模拟的方式得到InGaZnO TFT泄漏电流的模拟值. 图3是模型计算结果和TCAD模拟结果的对比图. 从模拟结果来看, VDS从2 V增大到10 V的过程中, InGaZnO TFT泄漏电流也随之增大. 这与(20)式和(21)式描述的情况相符, VDS增大导致垂直方向电场F增大, 在强电场的作用力下, Poole-Frenkel效应愈加明显, 在陷阱态的电子只需要获得较小的热能便能脱离陷阱态变成自由载流子, 沟道内自由载流子增多使得泄漏电流增大.
图 3 模型计算值与TCAD模拟值的对比(VDS = 2, 4, 6, 8, 10 V)Fig. 3. Comparison between model calculation and numerical simulation results (VDS = 2, 4, 6, 8, 10 V).图4给出了不同沟道宽度情况下的InGaZnO TFT的TCAD模拟结果和模型计算结果的对比. TFT的沟道宽度W从200
μm 增大至500μm , 泄漏电流随TFT的沟道宽度线性增大. 这与(19)式描述的情况较为符合. 这是因为TFT的沟道宽度W增大导致耗尽区的面积和体积增大, 从而沟道内的感应载流子增多, 致使泄漏电流增大. 由于(19)式中的Lov是指栅漏交叠区域的长度, 因此, TFT的沟道长度与泄漏电流的大小并无关系[29]. 由图5可知, TFT的沟道长度L从50μm 增大至90μm , 在关断区域内漏电流大小增长幅度仅为0.6%. 然而, 在导通区域, 漏电流随着TFT沟道长度L的增大而减小. 该模拟结果表明TFT的沟道长度L对关断区泄漏电流几乎无影响.
图 4 InGaZnO TFT在不同宽度(W = 200, 300, 400, 500μm Fig. 4. Relationship between leakage current and gate-source voltage under different widths of InGaZnO TFT (W = 200, 300, 400, 500μm 
图 5 InGaZnO TFT在不同沟道长度 (L = 50, 60, 70, 80, 90μm Fig. 5. Relationship between leakage current and gate-source voltage for different lengths of InGaZnO TFT (L = 50, 60, 70, 80, 90μm 图6为InGaZnO TFT不同栅氧化层厚度
tSiOx 下模型计算和TCAD模拟的泄漏电流对比图. 可见, 泄漏电流随着栅氧化层厚度tSiOx 的增大而减小, 栅氧化层厚度tSiOx 从150 nm增大至250 nm过程中, 泄漏电流的值减小了20%. 这与(21)式和(23)式描述的情况一致, 栅氧化层厚度tSiOx 增加导致电场强度F减弱, 陷阱态内电子无法获得足够的动力挣脱束缚, 沟道内自由载流子减少, 载流子复合产生率减小. 且在高电场作用下, 栅氧化层厚度tSiOx 的增加也会导致器件内隧穿电流减小, 最终亦会导致泄漏电流减小.
图 6 InGaZnO TFT在不同栅氧化层厚度(tSiOx = 150, 200, 250 nm)下泄漏电流与栅源电压的关系Fig. 6. Relationship between leakage current and gate-source voltage of InGaZnO TFT with different gate oxide thickness (tSiOx = 150, 200, 250 nm).4. 结 论
本文基于载流子产生-复合机制提出了一种InGaZnO TFT的泄漏电流模型. 基于低负电场区的Poole-Frenkel增强热发射、高负电场的场辅助热发射的泄漏电流产生机制, 在不同电场条件下提出了双积分载流子产生-复合率的近似模型, 并利用平滑函数将其统一成适用于关断区和亚阈区的泄漏电流模型. 将模型计算结果与TCAD模拟结果进行了对比, 在器件的亚阈区和关断区, 泄漏电流IDS的模型计算值和TCAD模拟值吻合程度较高. 基于上述模型, 本文讨论了InGaZnO TFT的沟道宽度、沟道长度和栅介质层厚度等关键参数对泄漏电流的影响. 模型计算值与TCAD模拟值的对比结果证明该模型在预测InGaZnO TFT沟道宽度、沟道长度和栅介质层厚度等结构参数对泄漏电流的影响方面较为可靠.
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图 1 化学气相传输法制备块体晶体 (a)化学气相传输法生长块状单晶材料原理图; (b) CrI3单晶照片; (c) CrCl3单晶照片; (d) CrBr3单晶照片; (e) FePS3单晶照片; (f) NiPS3单晶照片. 数据来源于参考文献[48—51]
Figure 1. Sythesizing bulk crystal via chemical vapor transportation technology: (a) The scheme of chemical vapor transportation; (b) the optical photo of CrI3; (c) the optical photo of CrCl3; (d) the optical photo of CrBr3; (e) the optical photo of FePS3; (f) the optical photo of NiPS3. Data from Ref. [48–51].
图 2 在Au (111)及HOPG基底上生长单层CrI3 (a) Au (111)表面单层CrI3的STM图像(插图为单层高度图); (b) 原子分辨的STM图像; (c)傅里叶变换图; (d)在HOPG上生长20 min的单层CrI3的大范围STM图像; (e)在HOPG上生长40 min的单层CrI3的大范围STM图像; (f)原子分辨图像; 数据来源于参考文献[53]
Figure 2. Growing monolayer CrI3 on Au (111) and HOPG substrates: (a) STM image of CrI3 monolayer on Au (111) substrate (the inset is a height profile along the red dashed line); (b) atomic-resolution STM image; (c) Fourier transform map of monolayer CrI3 on Au (111) substrate; STM images after (d) 20 min, (e) 40 min, andatomic-resolution image (f) of CrI3 on HOPG substrate. Data from Ref. [53].
图 3 MOKE显微镜装置搭建图. 633 nm的激光由He-Ne激光器发出, 采用机械斩波器和光弹性调制器分别进行光强调制和偏振调制. 调制的光束通过偏振分束器定向到样品中, 样品被放置在15 K的封闭循环低温恒温器中, 使用7 T螺线管超导磁体对样品施加磁场. 反射光束通过一个分析器到一个光电二极管上进行测量反射强度和克尔旋转角. 数据来源于参考文献[19].
Figure 3. Schematic of MOKE microscopy setup. A power-stabilized He-Ne laser (633 nm) is used for the optical excitation. A mechanical chopper and photoelastic modulator modulate the intensity and polarization of the exciting beam. The modulated beam is then conducted to the sample through a polarizing beam splitter. The sample is subjected to cryostat at T = 15 K with magnetic field upto 7 T. A lock in amplifier is used to detect the intensity and Kerr rotation of the reflected beam. Schematic of the setup from Ref. [19].
图 4 MOKE测量揭示单层CrI3晶体的铁磁性 (a) 单层CrI3的MOKE信号随磁场的变化; (b) 不同激光强度的MOKE信号, 其中蓝色、粉色和红色曲线对应的激光强度分别为3, 10 和30 μW; (c) 不同磁场下MOKE信号对温度的依赖关系, 其中红色曲线为磁场为0时的信号, 蓝色曲线为磁场为0.15 T时的信号; 数据来源于参考文献[19]
Figure 4. MOKE measurements of monolayer CrI3: (a) MOKE signal for a CrI3 monolayer varies with magnetic field; (b) power dependence of the MOKE signal taken at incident powers of 3 μW (blue), 10 μW (pink), and 30 μW (red); (c) temperature dependence of MOKE signal with the sample initially cooled at μ0H = 0 (blue) and 0.15 T (red). Data from Ref. [19].
图 5 利用磁圆二色谱探测原子级厚度CrI3晶体中的磁序随外加磁场的变化 (a) 多层CrI3隧穿器件示意图; 温度为2 K 下, 双层(b)、三层(c)和四层(d) CrI3的RMCD信号对磁场的变化关系. 数据来源于参考文献[63]
Figure 5. Probing magnetism of atomic CrI3 via RMCD: (a) Schematic of two-dimensional spin-filter magnetic tunnel junction; magnetic field dependent RMCD signal of (b) bilayer, (c) trilayer and (d) four-layer CrI3 devices at T = 2 K. Data from Ref. [63].
图 6 自旋过滤效应探测原子级厚度CrI3晶体中磁序随磁场的变化 (a) 金属/铁磁绝缘体/金属隧穿结的能量示意图, 其中势垒区域的红线和蓝线分别表示自旋向上和自旋向下的势垒; (b) 石墨/CrI3/石墨隧穿结器件照片和示意图; (c) 双层CrI3的隧穿电导对磁场的依赖关系; 自旋向上电子和自旋向下电子隧穿经过双层反铁磁(d)和铁磁(e) CrI3时的势垒; (f) 四层CrI3的隧穿电导随磁场的变化; 自旋向上电子和自旋向下电子隧穿经过四层反铁磁(g)和铁磁(h) CrI3时的势垒; 数据来源于参考文献[29]
Figure 6. Probing the magnetism in atomically-thin CrI3 via spin filter effect. (a) Energy digram of metal/ferromagnetic insulator/ metal junction. The blue and red horizontal lines stand for the energy barriers of spin up and spin down as indicated by the red and blue arrows. (b) Optical picture and structure diagram of a graphite/tetralayer CrI3/graphite tunnel junction device. (c) Tunneling conductance of bilayer CrI3 device. The insets indicate the corresponding magnetic configurations. (d) and (e) The diagram of layer resolved, spin dependent tunneling barrier in antiferromagnetic phase (d) and in ferromagnetic phase (e). (f)–(h) Analogous data and schematics for tetralayer CrI3 device. Data from Ref. [29].
图 7 静电掺杂调控CrI3晶体磁结构 (a) Huang等[64]构筑的双栅极双层CrI3器件示意图; (b)在温度为15 K时, 同一器件的RMCD信号与磁场的关系, 掺杂水平约从0(黑色)到4.4 × 1012 cm–2 (红色); (c) 同掺杂水平, 在从0 V·nm–1 (红色)到0.6 V·nm–1(黑色)不等的几个外加场的作用下RMCD信号与磁场的关系; (d) Jiang等[20]构筑的双栅极双层CrI3场效应器件示意图; (e) 4 K时单层CrI3中饱和磁化强度、矫顽磁场和居里温度随电子掺杂水平和栅极电压的变化; (f) 4 K时双层CrI3掺杂密度-磁场强度相图, 左轴为栅极电压, 右轴为掺杂密度, FM和AFM相分别对应高、低RMCD信号的区域. 数据来源于参考文献[20,64].
Figure 7. Manipulating the magnetism of two-dimensional CrI3 crystals via electrostatic doping: (a) Schematic of a dual-gate bilayer CrI3 device fabricated by Huang et al.[64]; (b) RMCD signal of the same device varies with magnetic field at varying doping levels from 0 (black) to 4.4
× 1012 cm–2 (red) with temperature fixed at T = 15 K; (c) the dependences of RMCD signal on magnetic field for displacement field varying from 0 V·nm–1 (red) to 0.6 V·nm–1 (black) with doping level fixed at zero; (d) schematic of a dual-gate bilayer CrI3 device fabricated by Jiang et al.[20]; (e) normalized Coercive field (magenta), saturation magnetization (purple) measured at T = 4 K and curie temperature (orange) to their values at zero gate voltage as functions of gate voltage (bottom axis) and doping density (top axis); (f) magnetic field-doping density phase diagram at T = 4 K. Data from Ref. [20,64].
图 8 电场通过磁电效应调节CrI3晶体的磁性 (a) 单层和双层CrI3原子结构图; (b) 双层AFM相CrI3电场诱导产生净磁示意图; (c) 4 K时MCD信号在不同外加电场下与磁场的关系; (d) 4 K时不同磁场下电场产生的磁化强度ΔM, 以及相对磁化强度ΔM/M0关于外加电场的关系; (e) 4 K温度下磁化强度对外加电场变化率随磁场的变化; (f) 双层CrI3的磁化强度M和归一化磁化强度M/M0在两个固定磁场(± 0.44 T下)与外加电场的关系图. 数据来源于参考文献 [67]
Figure 8. Control the magnetism of CrI3 via magnetoelectric effect: (a) Top view of monolayer (left panel) and side view of bilayer (right panel) CrI3 crystal; (b) antiferromagnetic bilayer CrI3 consists of two individual ferromagnetic monolayers with antiferromagnetic interlayer couplings. Schematic of nonzero net magnetization induced by electric field in bilayer CrI3 crystals; (c) MCD signal varies with magnetic field at different displacement electric field as indicated in the legends. The black and red curves stand for forward and backward sweeps of magnetic field; (d) electric field induced relative (left axis) and absolute (right axis) of magnetization as function of electric field at T = 4 K; (e) change rate of magnetization with displacement electric field as a function of magnetic field for bilayer and monolayer CrI3 flakes at 4 K; (f) absolute and relative magnetization of bilayer CrI3 as a function of electric field at fixed vertical magnetic field (filled: 0.44 T; empty: －0.44 T). Data from Ref. [67].
图 9 拉曼光谱揭示静压力引起的隧穿器件中CrI3 堆叠方式的变化 (a) 单斜相和三方相CrI3的层间堆叠顺序, 蓝色、红色和黑色球分别代表不同层中的Cr原子; (b) 单斜相CrI3的偏振拉曼光谱; (c) 三方相CrI3的偏振拉曼光谱. 数据来源于参考文献[30]
Figure 9. Raman spectrum reveal the change of CrI3 layer stacking order in tunneling devices: (a) Monoclinic (left panel) and rhombohedral (right panel) crystals order of CrI3; (b), (c) polarization dependences of Raman spectrum of CrI3 in monoclinic and rhombohedral phases respectively. Data from Ref. [30].
图 10 施加压力前后磁圆二色谱、偏振拉曼和隧穿电导的变化 (a) 施加1.8 GPa压力前两层和五层CrI3的MCD图像; (b)施加1.8 GPa压力后两层和五层CrI3的MCD图像; (c), (d) 在温度为300 K时, 施加1.8 GPa 静压力之前(c)与之后(d)五层CrI3拉曼光谱随偏转角度的变化; (e)温度为1.7 K时, 施加压力为0, 1, 1.8, 0 GPa (从上到下)时双层CrI3隧穿结器件的隧穿电导随磁场强度的变化图像; 数据来源于参考文献[30].
Figure 10. Comparing the MCD signal, polarized Raman spectrum and tunneling conductance before and after applying a high pressure: MCD signals of bilayer and five layer CrI3 crystals before (a) and after (b) applying the pressure; polarized Raman spectrum of five-layer CrI3 before (c) and after (d) applying a pressure of 1.8 GPa; (e) tunneling conductance of bilayer CrI3 device at pressure of 0, 1, 1.8 and 0 GPa (from top panel to bottom panel) at T = 1.7 K. Data from Ref. [30].
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