Development of DLTS Microscopy in Nano-resolution

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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