A sub-bandgap optical subthreshold current spectroscopy (OSCS) is proposed for extracting the energy distribution of interface trap density (DO in nitride-based charge trap flash (CTF) memory devices. It is based on the optical response of the subthreshold slope under sub-bandgap photonic excitation. By using the OSCS technique, we comparatively investigated the dominant energy range of the program/erase (P/E) cycling-induced D-ii and observed that it is shallow in NROM-type operation and deep in NAND-type operation. Because no electrical pulse is required during extraction and the current is measured not from the substrate contact but from the drain contact, the OSCS technique is expected to be more useful for emerging nano-scale devices in comparison with the conventional charge pumping technique. (C) 2009 Elsevier Ltd. All rights reserved.