Silicon nanocrystals based light emitting diodes integrated using all inorganic metal oxides as the charge transport layers

Air-stable luminescence silicon nanocrystals (Si-NCs) were synthesized using a novel in-flight system composed of a Si-NC synthesis SiH4/Ar plasma and an SF6 plasma which etches and passivates the NCs. The etch plasma can efficiently tailor the Si-NC size and the surface functionalities by tuning the gas flow rate, applied power, and pressure of the plasma. Si-NCs based light emitting diodes (LEDs) were fabricated by using the Si-NCs as the recombination center for injected electron-hole pairs. Si-NCs were deposited in between two inorganic metal oxide layers, nickel oxide (NiO) and zinc oxide (ZnO), which served as the hole transport layer (HTL) and electron transport layer (ETL), respectively. NiO and ZnO have been chosen by considering their energy band offsets with respect to Si-NCs, and their band offsets to the electrodes which should produce roughly comparable carrier concentrations once the contacts are forward biased, to get charge balance at the Si-NCs. The as-prepared metal oxides were confirmed to be stoichiometric using Auger Electron Spectroscopy (AES). Four-point probes measurements show the oxide sheet resistances in the range of 2-5×10⁶ Ω/□. The as-prepared etched Si-NCs generate orange photoluminescence at a peak intensity of 650nm with a quantum efficiency of 23%. I-V characteristics and light intensities of the Si-NCs LED without depositing the ZnO ETL have been studied with respected to the Si-NCs thickness. LEDs made using a two minute deposition of Si-NCs (approximately 250nm thick) showed an easily visible air-stable light emission; however, the light intensity decreased by 50% for thicker (1.5μm) Si-NC films. The LED performance was improved by using an ITO/ZnO/SiNCs/NiO/Al device structure. The turned on voltage increased to 7V but the current saturated to 0.1A very rapidly. The Si-NCs LED EL spectrum was collected at a bias voltage of 8.5V. The emission peaked at 653 nm for the Si-NCs LED in good agreement with the PL results. At the highest current densities some degradation of the device was observed, otherwise device operation was consistent and yield was good. The I-V characteristics of the Si-NC LED made using all inorganic metal oxides showed Schottky behavior as well as good light intensity.