In-flight gas phase passivation of silicon nanocrystals for novel inorganic-silicon nanocrystal based electroluminescent devices.

Abstract

Silicon nanocrystals (SiNCs) have become a heavily researched material over the past several years. Researchers envision that this material can be used in many diverse applications such as electronic devices, non-toxic biological tags, optical devices such as LEDs, lasers or displays, thermoelectrics, and photovoltaic (PV) applications. For many of these proposed applications one needs to properly control the NC size and the surface chemistry via passivation. Current passivation techniques allow for the creation of highly efficient SiNC optical emitters, however the emission of these NCs are fixed in the red- NIR range. To resolve this issue several novel in-flight passivation techniques were investigated. A novel dual-plasma setup which allows for the in-flight passivation of SiNCs through a thermal or LPCVD based nitridation process was developed first. FTIR and XPS analysis were used to study the surface chemistry on of the nitride passivated NCs while TEM was used to investigate whether or not a “shell” was grown on the surface. PL measurements and thermal stability tests were performed on the nitride passivated NCs to gain a further understanding of the stability (in both air as well as other ambients) of the NCs and their surface chemistry. Tunable full color emission from SiNCs was developed for the dual-plasma reactor utilizing CF4 as both an etching and passivating source. F radicals generated in the etching plasma remove Si from the surface of the NC, while at the same time CF2 radicals lead to the formation of a fluorocarbon passivation layer on the NC surface. By controlling the parameters of the reactor (CF4 flow rate, power), the NC size and thus its color can be controlled. Red to green luminescence was observed from SiNCs and is believed to be due to the quantum confinement effect. The blue emission observed from the NCs is appears to be related to oxide related surface states. Despite the defects, high QY was observed from these CF4-etched NCs. The fluorocarbon passivation layer, although stable, prevents further functionalization of the NCs. To counteract this problem another silicon-based dry etch chemistry, SF6 was investigated. Full-color emission was observed from SF6 etched NCs, with QY 2X higher than that of CF4-etched NCs. A maximum QY of nearly 55% at 700 nm was observed after several weeks in air, comparable to that observed with alkyl passivation. The native oxidation of the bare oxidized and SF6-etched NCs were also studied. Results show that the NC oxidation follows the Cabrera-Mott mechanism for low temperature oxidation. Inorganic-NC based LED structures were then investigated. Fabrication processes for the inorganic hole and electron transport layers were developed by RF sputtering and atomic layer deposition (ALD). Thorough characterization was performed on the metal-oxide films (ZnO, TiO2, NiO) to verify their stoichiometry as well as study their optical and electrical properties. Novel inorganic-NC device structures were fabricated. Inorganic NC devices which use a metal-oxide HTL but no ETL, emit light, however their emission is so weak. The addition of an ETL increases the light output by a factor of 4, but the device reproducibility is poor. To improve efficiency two insulating matrix layers were investigated. In both cases, the film deposited on the top of the NC is rough, porous, discontinuous, and potentially full of traps – certainly not the ideal film for a device. Therefore, more work is needed, specifically on the NC layer to improve the structure of the as-deposited NC film, but efficient device structures appear to be possible.