3D free carrier localization in light emitting diodes via two-photon optical beam-induced current microscopy

Abstract

We demonstrate three-dimensional free carrier localization in light emitting diodes (LEDs) using the inherent axial resolution of two-photon optical beaminduced current (2P-OBIC) microscopy. The carrier distribution cannot be mapped out by single photon OBIC (1P-OBIC) in two-dimensions more so in three-dimensions because of the very weak axial discrimination. We utilized a Ti:Sapphire femtosecond laser source operating at 800 nm to derive the 2P-OBIC signal from a 605-nm bandgap LED. The spatial confinement of free carrier generation only at the focus and quadratic dependence of the 2P-OBIC signal with excitation power are the key principles in two-photon excitation. As a consequence, superior image quality evident in the 2P-OBIC images of LEDs are obtained. These features decrease the linear absorption and wide-angle scattering effects plaguing single-photon optical beam-induced current (1P-OBIC) technique thereby increasing the resolution of the imaging system in the axial and lateral directions. Thus, attainment of good axial discrimination in the LED samples is obtained even without a confocal pinhole.