STM Nanospectroscopic Study of Defects in Semiconductors
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STM Nanospectroscopic Study of Defects in Semiconductors Koji Maeda, Akira Hida and Yutaka Mera Dept of Appl. Phys., School of Eng., the Univ. of Tokyo, Hongo, Bunkyo-ku, Tokyo, JAPAN ABSTRACT Coupling of scanning tunneling microscopy (STM) with various schemes of optical spectroscopy was found to provide powerful tools for study of crystalline defects in bulk semiconducting solids. The simplest method was applied to a subsurface defect in a bulk GaAs crystal in which the signal was acquired by detecting the change in the tunneling current reflecting a local surface swelling that occurs when the wavelength of the chopped light used for spectroscopic measurements coincides with a photoabsorption spectral peak of the defect. Another scheme using a continuous light of variable wavelength was applied to midgap centers, assigned as arsenic antisite defects, densely populated in low-temperature-grown GaAs epifilms. Experiments at 90K revealed that light illumination causes reversible transformation of the individual defects to a metastable state with an excitation spectrum very close to one observed for the photo-quenching effect of EL2 centers in bulk GaAs. INTRODUCTION Scanning tunneling microscopy (STM) is a powerful technique not only to study regular surfaces but also to explore irregularities or defects in the crystals. The power of STM is strengthened by the use of scanning tunneling spectroscopy (STS) that allows a direct assessment of the electronic structure of the defect. However, STS can be sensitive to the variable electronic state of the STM tip and could not be applied to isolated defects which lack the path for tunneling current that is requisite for STS measurements. Recently we found that spectroscopic measurements are possible also by illuminating the sample with a light of variable photon energy. This paper reviews our invented STM-based opto-spectroscopic techniques in several schemes that have been proved to be quite useful for atomistic studies of defect in semiconductors. EXPERIMENTAL PROCEDURE Figure 1 shows the experimental setup consisting of a standard STM unit operated in an ultrahigh vacuum (UHV) chamber and an optical system that could be varied for the experimental purpose. For the simplest scheme [1], photo-induced transformation spectroscopy (PITS), the sample could be illuminated with a continuous light of variable photon energy supplied from a F6.2.1 Downloaded from https://www.cambridge.org/core. Access paid by the UCSF Library, on 05 Oct 2019 at 18:37:49, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-719-F6.2
grating monochromator equipped with a halogen lamp. The PITS spectra could be obtained by counting the events of structural changes induced by light illumination as a function of the photon energy. For the second scheme [2], photo-modulated current spectroscopy (PMCS), the spectroscopic light could be chopped at an appropriate frequency. The PMCS spectra could be measured by lock-in detecting the ampl
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