Development of DLTS Microscopy in Nano-resolution

当表面下的损耗层中存在深能级阱时，表面光电压（SPV）表现出延迟衰减。基于这一思想，我们开发了一种新的深层瞬态光谱（DLTS）与扫描隧道显微镜（STM）相结合的方案，其中STM尖端用于探测引起SPV的光的瞬态。(a)空气中GaAs样品的初步实验表明，可以获得与光学DLTS光谱相似的温度光谱，并且可以在几十nm的高分辨率下成像DLTS信号的空间分布。(b)理论考虑表明，STM-DLTS的空间分辨率是由表面下深层中心被捕获的载流子电荷影响的静电表面电位决定的。无电荷屏蔽时的分辨率约为陷阱深度的3倍，而在表面形成二维电子气体时的分辨率约为1.4倍。(c)等温测量试图避免由于STM尖端相对于样品位置的热移位而导致的极高分辨率的损失，结果表明，为了避免当前反馈控制产生的伪影，需要更稳定的STM单元（无热移位）和高速测量。(d)构建了原始设计的无热移STM阶段，并对其性能进行了评估。热漂移比常规阶段低2倍，热位移是常规阶段的1/200。(e)利用砷化镓样品进行的超高真空实验表明，当时间周期缩短到0.1 ms以下时，可以避免STM尖端在光照下热膨胀引起的瞬态现象。(f)成功组装了增益为10^8 V/A、带宽为100 kHz的电流前置放大器。从这些结果中，我们能够确认STM-DLTS在纳米结构样品系统中实际应用的可行性。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