Carrier Transport in Silicon Nanocrystallite-Based Multilayer Electroluminescent Devices
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photoluminescence behavior of a variety of forms of visible light emitting silicon, possible optoelectronic applications require the study of the electroluminescence behavior. To this end, we have constructed electroluminescent devices based on Si nanocrystallite thin films. The devices use Al and indium tin oxide (ITO) electrodes and transparent polymer capping layers to provide the necessary carrier injection and chemical and electronic passivation for the silicon nanocrystallite active layer. The devices have been evaluated for their I-V characteristics, stability, and luminescence behavior. Contributing factors to device performance include: the availability of radiative/nonradiative recombination pathways, carrier injection across heterojunctions, and the motion of carriers through the films. Several types of carrier injection and transport mechanisms are present in the multilayer polymer-nanocrystallite devices, making it difficult to distinguish the role each layer plays in overall device performance. Taking advantage of the flexibility of pulsed laser ablation, a series of less-intricate electrode/Si nanocrystallite/electrode heterostructures were fabricated and used to elucidate the carrier injection and/or transport mechanisms present in the devices and the regimes in which they are dominant. EXPERIMENTAL A pulsed laser ablation supersonic expansion source was employed to deposit thin films of silicon nanocrystallites [9]. For the carrier transport studies, films were grown on p-type silicon wafers and aluminum sheet, while patterned ITO on glass substrates were used for the electroluminescent devices. Our three-layer electroluminescent devices consist of Si nanocrystallites sandwiched directly between a hole-injecting contact and an electron-injecting contact. The 271 Mat. Res. Soc. Symp. Proc. Vol. 405 01996 Materials Research Society
charge carriers are injected into the light emitting Si nanocrystallite region, where they either recombine sina2fmm or pass through to the electrode of opposite TOP active area polarity. A schematic of a three-layer device is VIEW shown in Figure 1. Device fabrication begins with a glass substrate coated with strips of ITO 2 mm wide and -200 nm thick. ITO is a high work function material serves as the hole-injecting electrode. Since it is optically transparent, it offers SIDE enhanced external quantum efficiency over VIEW
semitransparent metal contacts. A thin film of Si
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4- Al 4,Si nanocrystallites
nanocrystallites (50-100 nm thick) is then glass deposited onto the patterned ITO substrate. Vacuum evaporation of a top electrode of aluminum lines (2 mm wide, 90-200 nm thick) placed perpendicular to the ITO strips results in a Figure 1. Schematic top- and side-view of the matrix configuration in which each of the 2 mm x 2 thiree-layer electroluainescent device. mm active devices ("pixels") can be addressed individually. The emission efficiency and stability of our three-layer device was improved by incorporating additional conducting polymer l
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