Development of DLTS Microscopy in Nano-resolution

当表面下的耗尽层中存在深能级陷阱时，表面光伏(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变光谱(DLTS)和扫描隧道显微镜(STM)相结合的方案，其中STM针尖被用来探测引起SPV的光斩波时的SPV的瞬变。(A)利用空气中的GaAs样品进行的初步实验表明，可以获得与光学DLTS谱相似的温度谱，并且DLTS信号的空间分布可以以几十nm的高分辨率成像。(B)理论分析表明，STM-DLTS的空间分辨率取决于表面深能级中心处捕获的载流子电荷对表面静电势的影响。在没有电荷屏蔽的情况下，分辨率大约是陷阱深度的3倍，而当在su…上形成二维电子气时，分辨率是1.4%更多的表面屏蔽了捕获的电荷。(C)为了避免由于STM针尖相对于样品位置的热移位而导致极高分辨率的回落，等温测量表明，为了避免当前反馈控制带来的伪影，需要更稳定的STM单元(无热移位)和高速测量。(D)构建了我们原始设计的无热移位STM工作台，并对其性能进行了评估。获得了比常规级低2个数量级的热漂移和1/200的热漂移。(E)利用GaAs样品的超高真空实验表明，当时间范围大大缩短到0.1ms时，可以避免STM针尖在光照射下由于热膨胀而产生的明显瞬变。(F)成功地组装了增益为10^8V/A、带宽高达100 kHz的电流前置放大器。从这些结果中，我们能够证实STM-DLTS在纳米结构样品系统中实际应用的可行性。较少

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