Fabrication and characterization of Si nanocrystal based light emitting devices

University of Minnesota Ph.D. dissertation. December 2012. Major: Electrical Engineering. Advisor: Stephen A. Campbell. 1 computer file (PDF); vii, 100 pages.As the demands on the high efficiency light emitting devices continue to grow, alternative materials such as Si nanocrystals are attracting more and more attention from both academic and industry. To date, quite a few Si nanocrystal fabrication techniques have been developed targeting for high photo generated quantum efficiency. However, the devices made of Si nanocrystals do not exhibit the expected quantum efficiency mostly due to the lack of effective surface passivating of nanocrystals. Furthermore, the optical properties of Si nanocrystals cannot be preserved during the device fabrication process. Therefore, the surface treatment of bare Si nanocrystals is a prerequisite for solution-based device fabrication and eventually high efficiency LEDs. In this thesis, the non-thermal plasma synthesized Si nanocrystals were hydrosilylated with reactive alkenes and the resulting functionalized Si nanocrystals maintained relatively high quantum efficiency up to 20%; the surface functional groups such as hexene, octene and dodecene not only prevented attacking from oxygen and water, but also facilitated the solution process with polymers and organic solvents. These formed Si nanocrystals in toluene can be readily dispersed into PMMA toluene solution without any precipitation. Thin films of Si nanocrystals embedded into PMMA matrix can be obtained on ITOs by simple spin-coating. The device structure studied was composed of Al/Si nanocrystals PMMA/ITO. The film of Si nanocrystal/PMMA was formed by spin-coating onto the pre-cleaned ITO and Al was deposited by thermal evaporation. Three devices made of hexene, octene and dodecene Si nanocrystals were fabricated. Among the different aliphatic molecules, octene was chosen for further study as it was found to be the shortest ligand which fully passivated the Si nanocrystals but still prevented liquid phase agglomeration. The conduction mechanism of the fabricated LEDs was then studied. Two conduction regimes were clearly seen on the fabricated devices. At low field, the behavior follows space charge limited current with a weak temperature dependence. Evidence suggests that the small energy barrier we measure may be due to the size distribution of the nanocrystals. The low field behavior was also used to extract the Si nanocrystal density. The trap density is in reasonable agreement with the nanocrystal density suggesting that each trap is one nanocrystal. At high field, trap assisted tunneling was observed. An estimate of the electron tunneling effective mass was found to be 0.23mo. Although tunneling has been observed in epitaxially grown nanocrystals and in 2D arrays of metal nanoparticles, this is the first time that tunneling between nanocrystals has been seen in an electroluminescent composite film. This could be an important step toward making low cost nanocrystal based light emitting devices since the conduction mechanism appears to be on the nanocrystals, eliminating the need to transfer charge or excitons from the matrix to the nanocrystals. The efficiency of the LED of simple device structure was determined to be relatively low at the operational voltages. One reason for the relatively low device efficiency is believed to be the direct contact of the Al electrode to the Si nanocrystal/PMMA film. The fact that the majority of the electron/hole recombination events are non-radiative may also be due to the imbalance of electrons and holes injected into the composite film. To improve the LED efficiency, it's necessary to incorporate metal oxides as charge transporting layers. Therefore, the ZnO and MoO3 were chosen for this purpose as electron and hole transporting layers, respectively. A thorough material characterization of ALD ZnO was discussed by utilizing techniques such as XRD, Auger, XPS, and RBS as a function of deposition temperature. A literature review on MoO3 was also included. The fabrication process used to synthesize the devices which incorporated ZnO and MoO3 was discussed in detail. The band alignment of the device indicated that electron and hole recombination was thermodynamically favored. The emission intensity as a function of device current was also discussed between the device of simple structure and the device with metal oxides. The device incorporated with metal-oxide transport layers exhibited improved emission intensity at lower currents than that of the device with simple structure. Improved device efficiency was also observed on metal-oxides Si nanocrystal/PMMA LEDs. However, both devices still showed low device efficiency. Further improvement is needed to balance the electron and hole densities at Si nanocrystals