Nanometer-scale measurements of photoabsorption spectra of individual defects in semiconductors

Photoabsorption measurements using scanning tunneling microscopy, in which a modulated component of the tunneling current induced by a wavelength-variable chopped light is detected, were conducted for a cleaved surface of GaAs to demonstrate that a simple scheme enables nanometer-scale imaging of individual subsurface defects isolated in the crystal with fingerprints of photoabsorption spectra associated with them. The origin of the signal modulation and the image contrast formation mechanism are discussed in terms of photothermal expansion caused by nonradiative recombinations at the defect, photoinduced defect transformation, and an electrostatic surface potential change due to a charge alteration of the defect. (C) 2001 American Institute of Physics.