X-ray absorption fine structure measurement using a scanning capacitance microscope: trial for selective observation of trap centers in the ∼nm region

Abstract

Recent progress in nanotechnology has been sigmficant. The combination of current fabrication techniques and theoretical proposals on advanced quantum structures has led to the various optical and electronic devices using quantum effects lJ). In these quantum structures, even one-atomic-scale electron traps, such as local distortions and point defects, mod* the quantum state, and make the device properties complex. This fact makes it difficult to understand the quantum effects in actual devices and to control their performance. Therefore, in order to achieve the well-defined quantum structures, the selective analysis of trap centers in selectable -nm regions is necessary.In recent years, the capacitance x-ray absorption fine structure (capacitance XAFS) method, in which the absorption spectrum is measured by the x-ray photon energy dependence of the capacitance involved in a diode structure, was proposed 55). Since the capacitance is sensitive to the localized electron, the photoionization of the electron trap centers owing to the x-ray inner-shell absorption induces capacitance changes, resulting in a site-selective XAFS spectrum of the trap centers. In previous papers, structural and electronic state analyses of defect in a bulk semiconductor have been performed by this method using Schottky barrier diode 3,4).In this paper, XAFS measurements using a scanning capacitance microscope (SCM-XAFS method) are performed. Though the concept of this method is the same as the capacitance XAFS method, trap centers in selectable -MY regions would be analyzed by using a scanning probe. Moreover, by controlling the bias voltage applied to the scanning probe, surface trap center selection is successfully achieved in the XAFS measurements. Figure 1 shows the schematic diagram of an experimental apparatus developed in this study. An Aucoated Si cantilever, which is normally used in scanning probe microscope techniques, is adopted for capacitance detection at a selectable local area.The point contact of the tip with a semiconductor surface locally makes a metal-oxide-semiconductor (MOS) diode structure owing to a native oxide. The MOS diode structure has the capacitance-bias voltage (C-V) characteristics schematically shown by the solid line in the inset of this figure. The x-ray irradiated into this MOS diode induces the photoionization of trap centers and the release of localized electrons, resulting in additional free carriers by AN,+ The A N d slightly shifts the C-V characteristics horizontally along the voltage axis as shown by the dashed line, resulting in capacitance changes of AC at the specific bias voltage of Vb. In our system, Cis measured by a capacitance sensor, which is a videodisc capacitance pickup circuit developed by RCA q.For the sensitive detection of AC, a lock-in amplification technique by a bias modulation is used; the capacitance XAFS signal is observed by a derivative of C at V,, (dad%, rather than the absolute C.The semiconductor sample used in this study is Sn-doped (100) oriented GaAs (GaAs:Sn). The ...

Local detection of X-ray spectroscopies with an in-situ Atomic Force Microscope

“…To be mentioned is also the work of Ishii [6] who has taken a different approach using scanning capacitance microscopy to probe localized electrons.…”