Subsurface Second Harmonic Speckle Defect Microscopy*

Growing crystals epitaxialy over substrates is a critically important process for opto-electronic applications. Due to generally occurred atomic-level mismatches between the crystal and substrate, these semiconductor thin films are vulnerable to varieties of defects (like threading dislocation in GaAs-Si), which act as scattering sites for electrons and photons. Multi-scattering from these defects can result in the localization of the light. We demonstrate that a signature of this localization (in the case of III-V films) can be picked by a fiber scanning probe microscope in nonlinear regime. The size of these light localizations, which optically appear like speckles, have strong correlation to the density of the film dislocations. We introduce this noninvasive optical technique capable of identifying the presence of threading dislocations, their density, as well as orientation and structural arrangements of these defects.