节点文献
用导电原子力显微镜研究锗硅量子点和量子环的形貌和电学特性
【作者】 吴荣；
【作者基本信息】 复旦大学 ， 凝聚态物理， 2007， 博士
【摘要】 以半导体量子点为代表的半导体纳米结构在光电子、微电子和单电子器件领域有着重要的应用前景,在硅衬底上自组织生长的GeSi量子点由于与成熟的硅集成电路工艺兼容而更具有特殊的意义,是当前的研究热点之一。而量子点的量子物理特性是构筑许多纳米电子器件及单电子器件的基础,因而在纳米尺度上研究单个量子点的电学性质极为重要。导电原子力显微镜（CAFM）是近年来发展起来的一种扫描探针显微镜,它能同时得到样品表面纳米尺度的形貌信息和电学信息。因而CAFM可以用来表征单个量子点的微结构及其物理特性,从而为材料的控制生长和应用提供依据。本论文使用CAFM研究了自组织生长的GeSi量子点和GeSi量子环的形貌和电学性质,主要取得了以下结果:1、研究了样品表面的自然氧化物对量子点电学性质的影响。通过分析I-V特性曲线,根据针尖与量子点之间自然氧化物的存在与否将针尖与量子点间的接触分为两种类型:金属—半导体接触和金属—氧化物—半导体接触,它们对应的电流传导机制分别为热电子发射和F-N隧穿。通过对电流分布图像分析获知,电流分布并不随着施加正压力的大小而改变分布特性;同时,电流分布也不随着暴露时间延长而增加的氧化物厚度改变。这就为在大气环境下使用CAFM研究电流分布图像的可靠性提供了基本的依据。2、研究了偏压对CAFM得到的GeSi量子点的电流分布图像影响。在较小的偏压-0.5～-2.0 V内,量子点的电流分布特征保持环状,量子点中心部位的导电性弱于边缘部位。量子点区域的电流值统计表明,其符合对数正态分布函数,遵从正态分布的随机变量的均值I0和偏压的关系满足F-N隧穿关系:偏压增加至-2.5～-4.0 V范围内,量子点的电流分布特征呈圆盘状,量子点中心的导电性比边缘好。电流值的统计图中显示出多元对数正态分布特征。第一均值I0和偏压的增长关系符合热电子发射模型,在高偏压下量子点中心电流值出现了饱和现象。通过对量子点电流分布图像的电流值统计数据的对比,发现了随着偏压的增加,量子点的电流传导机制从F-N隧穿向热电子发射转变的过程。3、使用CAFM观察到了GeSi量子点的荷电现象。在量子点的I-V特性曲线中,有明显的电流峰产生,对应的载流子充放电过程速度极快,只能在快速的扫描过程中才能捕获到,因此在一般常规的稳态扫描中观察不到。电流峰的位置和强度等参数是和扫描的速度、方向、电压范围、时间等密切相关。其具体的物理机制是因为空穴被限制在了氧化物/GeSi量子点/Si衬底组成的量子阱中。这是首次观察到单个量子点的荷电现象。4、研究了GeSi量子环的电流分布特征和I-V特性曲线。使用CAFM直观地观察到了随着Si覆盖层的增加,量子点向量子环的转变过程中电流分布的变化特征。随着覆盖层的增加,电流分布从最初的量子点的环状分布特征转变为十字交叉形状,此时对应金字塔结构量子点;当覆盖层加厚,量子点形貌为mound时,电流分布特征不明显;继续增加覆盖层至量子环形成,其电流分布特征与形貌对应呈环状,且量子环环状电流分布的特征并不随着外加电压的增加而变化。量子环上的I-V特性曲线呈现出明显的共振隧穿特性,这是由于空穴在应变诱导的量子环势场的平面间的隧穿造成的。这是首次在室温条件下观察到量子环结构的共振隧穿现象。
【Abstract】 Semiconductor quantum dots(QDs) have received considerable attention because of their potential applications in photoelectron,micro-electronics and single electron devices.Self-assemble GeSi QDs have one of major advantages of its compatibility with conventional Si integrated-circuit technology.The quantum properties of quantum dots are essentially important to the application of most nano or single electron devices.Conductive atomic force microscopy(CAFM) is a promising technique capable of simultaneous topographic and conductance measurements over nano-contact area.Microstructure and physical properties of individual QD obtained by CAFM will be useful for controlling growth and application of QDs.Under the above background,this thesis is focused on topographic and conductance properties of individual QDs and quantum rings(QRs) and includes the following four parts:The first part is investigating the effects of native oxide layer on the CAFM measurements of GeSi QDs.After exposed in ambient,the current-voltage curves of the quantum dots can be well fitted by Fowler Nordheim tunneling model due to the inevitable native oxide layer,whereas Schottky emission model can be explained theⅠ-Ⅴcharacteristics well after the oxide layer was removed by HF etching.Ⅰ-Ⅴcharacteristics were also affected by the applied normal force,since it can change the effective oxide layer thickness.However,despite of the strong effect of native oxide layer on the conductive properties,the current distributions of individual QDs remain almost the same as the oxide layer thickness changed by HF etching or different normal forces applied.Thus our results provide important evidence to the application of CAFM analysis technique in ambient.The second part is the studing the effects of applied bias on the current distribution of invidual GeSi QDs.The current distribution of QDs holds ring-like characteristics under -0.5～-2.0 bias and the statistic of current magnitude can be well fitted by log-normal distribution.The relation of histogram peak position and applied bias in this field shows a good F-N tunneling fitting.The current distribution of QDs shows disk-like characteristics under -2.5～-4.0 bias and central part of QD is high conductive.The statistic of current magnitude can be well fitted by multivariant log-normal distribution.The relation of first histogram peak position and applied bias can be explained by thermal electron emission model.In addition the current magnitude is saturated at high bias.Therefore current mechanisms are affected by the applied bias.The third part is probing transient hole trapping phenomenon in individual GeSi QDs.By controlling the bias voltage sweep in certain fast sweep rate range,a novel current peak is observed in the current-voltage characteristics measured on individual GeSi quantum dots.The current peaks are detected only during the backward voltage sweep immediately after a forward sweep.The current peak position and intensity are found to depend strongly on the voltage sweep conditions.This kind of fast sweep current-voltage characteristic is very different from the ordinary steady state current behaviors of quantum dots measured previously.The origin of this phenomenon is attributed to the transient carrier trapping in the potential well formed by the quantum dot sandwiched between the top oxide layer and bottom Si substrate.The fourth part is GeSi QRs electrical property studies.The current distribution change during the transition from dome QD,pyramid QD,mound to QRs was detected by CAFM,which changes from ring-like,crisscross,disk-like to ring-like. The ring-like current characteristics of QRs are found not to depend on the applied bias.When it comes toⅠ-Ⅴcharacteristic on QRs,negative difference resistance phenomenon is observed.It is attributed to strain-induced-potential for hole resonant tunneling of planar QR structure.