Development of DLTS Microscopy in Nano-resolution

当表面下的耗尽层中存在深能级陷阱时，表面光电压（SPV）表现出延迟衰减。基于这一思想，我们发展了一种新的深能级瞬态谱（DLTS）与扫描隧道显微镜（STM）相结合的方案，在该方案中，STM针尖被用来探测SPV的瞬态截断光引起的SPV。（a）在空气中使用GaAs样品的初步实验表明，可以获得类似于光学DLTS谱的温度谱，并且可以几十nm的高分辨率成像DLTS信号的空间分布。（B）理论考虑揭示了STM-DLTS的空间分辨率由静电表面电势决定，静电表面电势受在表面下方的深能级中心处捕获的载流子的电荷的影响。在没有电荷屏蔽的情况下，分辨率约为陷阱深度的3倍，而当二维电子气在表面形成时，分辨率为1.4倍。 ...更多信息 表面屏蔽捕获的电荷。（c）等温测量试图避免由于STM针尖相对于样品位置的热位移而导致的极高分辨率的下降，这表明需要更稳定的STM单元（无热位移）和高速测量来避免由电流反馈控制引起的伪影。(d)A构建了我们原始设计的无热位移STM平台，并对其性能进行了评估。热漂移的数量级比传统的阶段和热漂移的1/200，这些阶段实现低2倍。（e）使用GaAs样品的超高真空实验表明，当时间范围减小到远短于0.1 ms时，可以避免由于STM针尖在光照射下的热膨胀而引起的明显瞬态。（f）成功组装了增益为10^8 V/A、带宽为100 kHz的电流放大器。从这些结果中，我们能够确认STM-DLTS的纳米结构样品系统的实际应用的可行性。少

Surface photovoltage (SPV) exhibits a delayed decay when a deep level trap is present in the depletion layer beneath the surface. Based on this idea, we have developed a new scheme of Deep Level Transient Spectroscopy (DLTS) combined with Scanning Tunneling Microscopy (STM) in which the STM tips are used to probe the transient of SPV on chopping the light causing the SPV.(a)Preliminary experiments using GaAs samples in air showed that temperature spectra similar to optical DLTS spectra can be obtained and the spatial distribution of the DLTS signal can be imaged in high resolution of several tens nm.(b)Theoretical considerations revealed that spatial resolution of STM-DLTS is determined by the electrostatic surface potential affected by the charge of the carrier trapped at the deep level center beneath the surface. The resolution in the absence of charge screening is approximately 3 times the depth of the trap while that is 1.4 times when the 2-dimensional electron gas formed at the su … More rface screens the trapped charge.(c)Isothermal measurements that were tried to avoid the draw-back of the extremely high resolution due to thermal shift of STM tips with respect to sample position revealed that a more stable STM unit (thermal shift-free) and high-speed measurements are necessary to circumvent artifacts arising from the current feed-back control.(d)A thermal shift-free STM stage of our original design was constructed and its performance was assessed. Thermal drifts the order of 2 times lower than the conventional stages and thermal shift 1/200 of those stages were achieved.(e)Ultra-high vacuum experiments using GaAs samples showed that apparent transient due to thermal expansion of STM tips on light illumination can be avoided when the time regime is reduced well shorter than 0.1 ms.(f)Current preamplifier with 10^8 V/A gain and a bandwidth as wide as 100 kHz was successfully assembled. From these results, we were able to confirm the feasibility of practical applications of STM-DLTS to nano-structured sample systems. Less