Investigating the Reasons for the Difficult Erase Operation of a Charge‐Trap Flash Memory Device with Amorphous Oxide Semiconductor Thin‐Film Channel Layers

A charge‐trap flash (CTF) device is fabricated using atomic‐layer‐deposited zinc tin oxide (ZTO) as an n‐type amorphous oxide semiconductor (AOS) channel layer and its program/erase characteristics are examined. When a positive voltage of 20 V is applied to the gate electrode, electrons are fluently injected into the adopted SiNx charge trap layer, resulting in a program operation showing a threshold voltage (Vth) shift of 3.7 V. However, even when −20 V is applied for up to 10 s, the trapped electrons are not detrapped. Instead, it is possible to recover Vth to its initial value only by the irradiation of white light. To understand this phenomenon, a ZTO CTF device is modeled using the technology computer‐aided design (TCAD) simulation package. The high hole injection barrier between the source/drain and the ZTO channel prohibits hole injection, which is the cause of the inefficient erasing. It is also confirmed that an AOS CTF with sufficient program and erase speed would be limited by the inherent energy band structure of AOS with its wide bandgap of over 3 eV. An alternative method of using the fringing field effect according to channel length scaling is proposed.