On metastable properties of plasma treated amorphous Si:H thin films

Amorphous hydrogenated silicon (a-Si:H) is well known as a semiconductor with metastable properties. This paper deals with structural and electrical properties of a-Si:H surfaces in virgin state as well as on low-energy ion exposition. Two ion sources were used, namely a monoenergetic ion beam produced by Kaufmann source and ions extracted by the plasma immersion ion implantation technique (PIII). The structural and electronic changes induced by ion impacts, as investigated by the X-ray diffraction at grazing incidence, capacitance-voltage measurements and charge version of deep level transient spectroscopy (Q-DLTS) are reported. The changes induced in the gap-state distribution of a-Si:H due to an interaction with low energy Ar+ ions followed in situ by the short exposure to both hydrogen/oxygen ion beam or to molecular high-purity oxygen are presented. The X-ray measurements confirmed that the most important reflection, which enables us to trace the evolution of the structural changes of a-Si:H layers caused by ion impacts, has the position at 2q ~ 28°. It is related to the existence of Si80H20 complexes inside the layer. The existence of only two types of deep metastable distributions Dz and De was observed in MIS structures prepared for the first time by the plasma immersion ion implantation technique. The distribution corresponding to positively charged defects Dh is missing. The use of the standard monoenergetic ion beam technique for the preparation of MIS structure confirmed the existence of three types of deep metastable distributions in a-Si:H (Dh, Dz and De). The differences in the results are explained by the application of a relatively high negative potential (1000 V) on the sample during the PIII experiment