Photoabsorption spectroscopy on nanometer scale by scanning tunneling microscopy

We have developed an instrumentation that allows nanometer-scale detection of sub-surface defects in semiconductors with fingerprints of photoabsorption spectra. The experimental set-up consists of a scanning tunneling microscope (STM) and a sample whose surface is illuminated with a chopped light of variable wavelength that induces a modulation in the tunneling current when the sample absorbs photon energy. Cleaved (110) surfaces of GaAs are used to demonstrate the power of the method. In one typical case, DeltaI images, two-dimensional maps of the current modulation amplitude detected by a lock-in amplifier, exhibit bright contrasts which have no corresponding contrasts in the STM topographs, indicating that the contrast arises from a sub-surface defect. The c-ur-rent modulation amplitude measured as a function of wavelength at the bright contrast yields a photoabsorption spectrum that is characterized by a sub-gap peak originating in the defect as well as a threshold at the band-gap energy, a common feature observed everywhere on the sample. The mechanism of contrast formation is discussed in-terms of thermal expansion induced by local photon absorption and defect distortion occurring on a change in the charge state of the sub-gap level associated with the defect.