Transmission electron microscope imaging of plasma grown electroformed silicon nitride-based light emitting diode for direct examination of nanocrystallization

Abstract

We report for the first time a direct transmission electron microscope (TEM) imaging of a cross-section of a silicon nitride-based light emitting diode (LED), produced via a method patented by our research group. Grown by plasma enhanced chemical vapor deposition (PECVD) technique the LED structure (glass/Cr/p+-nc-Si:H/i-SiNx:H/n+-nc-Si:H/ITO) was then subjected to a high forward voltage stress for one time only, i.e. electroforming process. After electroforming the LED exhibited a boosted visible light emission and memory effect. To study the structural effect of the electroforming on the as-deposited LED the cross-section was extracted by focused ion beam (FIB) technique directly from the electroformed diode and thus prepared for TEM imaging. Since the electroforming process caused crystallization of ITO and its breakup in some parts of the diode surface, the FIB was conducted for the cross-section containing some regions with ITO layer and some without ITO. TEM examination revealed the nanocrystalline phase formation within the intrinsic layer (i-SiNx:H) caused by the electroforming process. The average size and distribution of Si nanocrystallites formed inside i-SiNx:H was determined. The Si nanocrystallization within i-SiNx:H was compared for the regions with and without ITO layer. The previously proposed model describing the changes taken place in the diode during electroforming process was reconsidered in the light of this TEM analysis.