摘要
通过实验测量,研究了HgCdTe反型层中自旋轨道耦合、塞曼效应及界面粗糙涨落效应。采用理论模型对不同温度及不同平行磁场下的反弱局域效应进行分析,结果表明,在平行磁场中,界面粗糙涨落效应与塞曼效应均会对HgCdTe反型层的反弱局域效应产生抑制作用。其中,界面粗糙涨落效应表现为产生一个二维电子气法向的弱局域效应,对样品施加平行磁场会首先抑制界面粗糙涨落效应导致的法向弱局域效应,然后才以塞曼效应继续抑制反弱局域效应。通过对参数与的分析表明,塞曼效应对反弱局域效应的抑制与温度无关。
窄禁带直接带隙半导体材料碲镉汞(Hg1-xCdxTe)是一种由CdTe与HgTe形成的赝二元半导体材料,其具有禁带宽度随组分可调的性质,红外探测范围覆盖了从近红外到甚长波红外波段,加之光吸收率高等优点,使得其被广泛应用于各红外探测领
二维电子气(Two-Dimensional Electron Gas,2DEG)体系中的自旋轨道耦合已经在多种材料体系中得到大量研究,多种理论模型也被提出以对实验数据进行分析,比较著名的有ILP模
在InGaAs等异质结体系中的研究表明,除了塞曼效应外,异质结样品的界面粗糙度也会对体系的自旋动力学产生影
目前关于HgCdTe体系中的界面粗糙涨落效应的影响下的自轨道耦合与塞曼效应的竞争还未被研究过。本文通过对具有界面微粗糙的HgCdTe反型层样品在不同倾斜磁场以及不同温度下的磁输运性质进行测量,研究了在有较强粗糙涨落效应的情况下,自旋轨道耦合与塞曼效应之间的竞争。
实验中所使用的p型HgCdTe样品是在CdTeZn衬底上由液相外延的方法制备而成。随后将样品在240℃的环境下退火24 h以形成氧空位受主。通过将样品置于碱溶液中进行阳极氧化,可以在表面形成氧化层,氧化层与HgCdTe的界面处会发生能带弯曲进而形成二维电子
实验测量使用范德堡测试法,样品被为切为5 mm×5 mm大小,随后通过在样品的四个顶角上使用金属铟电极以实现欧姆接触。磁输运性质测量所使用的设备为牛津仪器公司的TesltronPT低温超导磁体系统。为获取所需的平行磁场,使用了可转动的样品杆,可实现二维电子气平面法向与磁场夹角的连续变化。首先保持磁场方向固定,而后转动样品台,使其从略小于90°的初始位置,逐渐旋转到略大于90°的最终位置。在此过程中,磁场的二维电子气面内分量为,垂直分量为,二者关系满足。通过这一方法,可以得到一个近似不变的平行磁场和一个小的连续变化的垂直磁场。角度的具体大小可以根据霍尔电阻的大小仅与垂直磁场有关的原理,通过测量一个与样品平行放置的参考样品的霍尔电阻获得。

图1 1.32 K温度下,样品在垂直磁场中纵向电阻SdH振荡(红线)及霍尔电阻(黑线)
Fig.1 The SdH oscillation of longitudinal resistivity (red line) and the Hall resistivity (black line) as a function of perpendicular magnetic field at 1.32 K

图2 垂直磁场中在变温条件下的反弱局域效应,点线为实验数据,实线为ILP模型的拟合数据。插图为拟合得到的和随温度的变化,红线为的拟合曲线。
Fig.2 Conductance correction and fitting results of WAL effect using ILP model in perpendicular magnetic field at different temperatures. The dotted lines are experimental data and the solid lines are the fitting results of ILP model. The inset gives the temperature dependence of the parameters and with red solid line is the fitting curve of
1.32 K温度时,样品在大小不同平行磁场下的磁电导测量结果如

图3 1.32 K时在不同大小平行磁场中的反弱局域效应,平行磁场大小在相应曲线旁标出,点线为实验数据,实线为ILP模型的拟合结果
Fig.3 Magnetoconductance and fitting results of WAL effect using modified ILP model in various in-plane magnetic fields at 1.32 K. The in-plane magnetic field intensity is given besides corresponding curve. The dotted lines are experimental results while the solids are fitting curve of the ILP model.
没有施加平行磁场时,体系的反弱局域效应被弱局域效应部分抵消;存在平面内磁场时,将先抑制弱局域效应,使体系表现为反弱局域效应先增强后减弱。弱局域效应随着温度升高而减弱的规律也可解释界面微粗糙度效应随着温度升高而逐渐减弱的现象。因此,界面粗糙涨落效应可以等效为一定大小的平行磁场,可以忽略系统中的电子在法向的运动来探索自旋轨道耦合和塞曼效应直接的竞争关系,系统近似于无界面粗糙涨落效应的二维电子气系统。
使用代替来研究塞曼效
, | (1) |
其中,, ,为表征排除界面粗糙涨落效应影响后的塞曼效应强度的参数,类似于无界面粗糙涨落效应体系中的有效g因子,应随温度保持不变。
利用修正ILP模型对不同温度下的平行磁场中的反弱局域效应进行拟合,得到了随平行磁场变化关系,并用
, | (2) |
得到的值与温度的关系如

图4 不同温度下通过用ILP模型拟合反弱局域效应得到的参数随着平行磁场的变化规律。不同颜色的点和线分别代表不同温度下的实验结果和拟合曲线,拟合式为式(1)。插图圆点为不同温度下拟合得到的的值,方形点为由式(4)推导得出的时的值
Fig.4 The experimental and fitting results of the dependence of ratio on parallel magnetic field extracted by fitting the ILP model at various temperatures. The points are experimental data while the solid lines are fitting curves of Eq. 1. The inset gives the temperature dependence of the zero-field values of . The circles are obtained with Eq.2 and the square is inferred from fitting result of Eq.4
接下来将继续研究同一平行磁场下系统反弱局域效应的随温度变化关系。类似于无界面粗糙涨落效应体系与温度的线性关
, | (3) |
其中与均为拟合参量,可以看到,不同平行磁场下,

图5 不同平行磁场下的反弱局域效应用ILP模型拟合得到的值随温度的变化,不同颜色点和线表示不同的平行磁场,拟合公式为式(3)。插图四角星为拟合得到的时的的值,实线为式(4)拟合结果
Fig.5 The ratio as a function of temperature extracted by the ILP model. The dots are experimental data and the solid lines are fitting results of Eq. 3. The inset gives the dependence of on when . The points are value of obtained by Eq. 3 while the solid line is the fitting curve of Eq.4
由于时
. | (4) |
拟合曲线如
通过对存在粗糙涨落效应HgCdTe样品在不同温度和不同大小平行磁场下进行系统性的磁输运测试,并采用理论模型对数据进行分析发现,存在界面粗糙涨落效应的样品在平行磁场中的反弱局域效应会受到两种抑制作用,其一是界面粗糙涨落效应导致二维电子气法向上的弱局域效应,这一作用可以用一定大小的平行磁场抵消;另一种是塞曼效应,其对反弱局域效应的抑制作用与温度无关。对样品施加平行磁场会首先抑制界面粗糙涨落效应导致的法向弱局域效应,然后才以塞曼效应逐渐抑制反弱局域效应。
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