Abstract
Electrical, forming and switching characteristics of metal-semiconductor-metal (MSM) memory switches of ion bombarded hydrogenated amorphous silicon (a-Si:H) and its alloys are presented. MSM devices for memory switching applications are known to be characterised by instabilities as well as non-uniformity and irreproducibility of the forming and switching characteristics. It is believed that the presence of defect states in the semiconductor layer plays a significant role in the observation of memory switching in these MSM devices. Gas-phase doping and current stressing of the semiconductor are some of the techniques that have been used in the past to introduce mid-gap defect states. In this work, we use for the first time, ion bombardment as a novel tool for defect introduction into the semiconductor material of the MSM devices and we compare the electrical, forming and switching characteristics of these devices to those fabricated using the previous techniques mentioned above. A significant observation is that as the density of defects is increased in the semiconductor film with increasing implantation dose, conduction in the devices changes from barrier-controlled thermionic emission to bulk controlled where carriers hop through the defect states in a Poole-Frenkel manner. This transformation eliminates problems associated with Schottky barriers such as quality of contacts, oxidation, etc. In the forming characteristics, not only do we report enhanced uniformity of the forming voltages (VF) but also the magnitude of VF is observed to vary systematically with the implantation dose used. The ON states and switching characteristics in the bombarded devices are also observed to be much more stable presumably as a result of the uniformity of the defects introduced by ion bombardment. We report also an enhanced switching ratio in the ion bombarded devices, especially after partial annealing of the bombardment induced Si dangling bond defects.