[论文解读] Impact of interface traps on charge noise, mobility and percolation density in Ge/SiGe heterostructures

Leonardo Massai, Bence Hetényi+9 more|arXiv (Cornell University)|Oct 9, 2023

Quantum and electron transport phenomena被引用 5

一句话总结

本研究确定，SiGe-氧化物异质界面处的界面陷阱是Ge/SiGe异质结构中电荷噪声、迟滞效应及输运性能退化的主要原因。通过施加逐渐负向的栅压，作者证明了陷阱填充导致持久的静电紊乱，降低低密度载流子迁移率与渗流密度，并使低频电荷噪声增加，其谱特性为1/f²，且在约1天内逐渐衰减，凸显了界面质量对稳定自旋量子比特运行的关键作用。

ABSTRACT

Hole spins in Ge/SiGe heterostructure quantum dots have emerged as promising qubits for quantum computation. The strong spin-orbit coupling (SOC), characteristic of heavy-hole states in Ge, enables fast and all-electrical qubit control. However, SOC also increases the susceptibility of spin qubits to charge noise. While qubit coherence can be significantly improved by operating at sweet spots with reduced hyperfine or charge noise sensitivity, the latter ultimately limits coherence, underlining the importance of understanding and reducing charge noise at its source. In this work, we study the voltage-induced hysteresis commonly observed in SiGe-based quantum devices and show that the dominant charge fluctuators are localized at the semiconductor-oxide interface. By applying increasingly negative gate voltages to Hall bar and quantum dot devices, we investigate how the hysteretic filling of interface traps impacts transport metrics and charge noise. We find that the gate-induced accumulation and trapping of charge at the SiGe-oxide interface leads to an increased electrostatic disorder, as probed by transport measurements, as well as the activation of low-frequency relaxation dynamics, resulting in slow drifts and increased charge noise levels. Our results highlight the importance of a conservative device tuning strategy and reveal the critical role of the semiconductor-oxide interface in SiGe heterostructures for spin qubit applications.

研究动机与目标

• 确定用于自旋量子比特的Ge/SiGe异质结构中电压诱导迟滞与电荷噪声的根源。
• 确定SiGe-氧化物界面处的界面陷阱是否导致电荷噪声增加与输运性能退化。
• 评估陷阱填充对量子点与霍尔条带中低密度迁移率、渗流密度及电荷噪声动力学的影响。
• 评估栅压循环后陷阱诱导效应的可逆性与恢复 timescales。
• 建立保守的调控策略，以实现Ge基量子器件的稳定、可重复运行。

提出的方法

• 对霍尔条带与量子点器件进行电压扫描，施加逐渐负向的栅压，以探测陷阱填充动力学。
• 通过测量磁阻与微分电导，提取作为最小栅压函数的低密度迁移率与渗流密度。
• 采用库仑峰追踪（CPT）与功率谱密度（PSD）分析量化电荷噪声，并拟合至幂律模型（S₀/f^α）。
• 在栅压循环后监测超过30小时的电荷噪声时间演化，以评估被捕获电荷的恢复动力学。
• 对阻挡栅极进行循环电压扫描，确认栅电极附近局部陷阱填充，证实其界面起源。
• 通过热循环重置被捕获电荷，将输运指标与噪声变化与界面陷阱填充程度相关联。

Figure 1: Device layouts and Ge/SiGe heterostructure : a , Schematic illustration of the measurement setup and Hall bars used for magnetoresistance measurements. The Hall bar gate is defined either in GL1 (green) or GL2 (blue) for HB 1 and HB 2 , respectively. Nominally, the channel width is $W=20$

实验结果

研究问题

• RQ1 为何在基于SiGe的量子器件中普遍存在电压诱导迟滞？
• RQ2 SiGe-氧化物异质界面处的界面陷阱如何影响Ge/SiGe异质结构中低密度迁移率与渗流密度？
• RQ3 在负向栅压下观察到的低频电荷噪声的起源及其时间尺度是什么？
• RQ4 为何峰值迁移率保持不变，尽管低密度输运性能显著退化？
• RQ5 通过栅压复位能否逆转陷阱诱导的紊乱与噪声，还是必须依赖热循环？

主要发现

• 电压诱导迟滞源于SiGe-氧化物界面处界面陷阱的逐步填充，而非体缺陷所致。
• 随着负向栅压增大，低密度迁移率与渗流密度以强烈相关、反相关的趋势下降，表明存在持久的静电紊乱。
• 峰值迁移率基本保持不变，表明其不适合作为低密度、少载流子区域器件的基准指标。
• 电荷噪声在低频段（f < 10⁻² Hz）表现出1/f²的功率谱密度分量，且随更负的V_min而增强，表明被捕获电荷具有缓慢的弛豫动力学。
• 增加的低频噪声在约1天内单调衰减，证实了界面捕获电荷的时间依赖性弛豫过程。
• 热循环可完全重置界面陷阱分布，但将栅压恢复至零电压则不能，表明电压反转无法完全清除陷阱态。

Figure 2: Hall bar measurement data and analysis for HB 2 : a , Schematic diagram illustrating the measurement protocol. b , Channel turn-on curves for $V_{\text{min}}$ decreasing from $-0.15$ V (red) to $-3$ V (blue). The grey dashed line marks 90 $\%$ of $I_{\rm{xx,max}}$ , used to extract the tur