Nanoparticles (NPs) have unique electronic, optical and chemical properties due to the extreme small size. Engineered nanoparticles (ENPs) are intentionally produced for desired applications, with specific properties related to shape, size, surface properties and chemistry. Nano-sized silica (SiO2), alumina (Al2O3) and ceria (CeO2) are three important ENPs with large production and wide applications. One of the principal uses of these ENPs is in chemical and mechanical planarization (CMP), a key process applied to polish wafers when fabricating integrated circuits in semiconductor manufacturing, in which SiO2, Al2O3 and CeO2 NPs are used as abrasive particles in CMP slurries. CMP generates large amounts of waste effluents containing high levels of ENPs. Some ENPs have been proven to be able to cause toxicity to microorganisms and higher life forms, including humans. Therefore, there are concerns about the potential risks that ENPs may pose to the natural environment and human health. In addition, III-V materials like indium arsenide (InAs) and gallium arsenide (GaAs) are increasingly used in electronic and photovoltaic devices. Besides ENPs, the waste streams from III-V manufacturing also contain dissolved and particulate materials removed from III-V films during CMP. Arsenic is one of the most notorious contaminants that has been widely studied, while only very limited ecotoxicity information is available for gallium and indium. Finally, since ENPs have high surface area, it is very likely they will interact with the soluble species (such as arsenic ions) in CMP wastewater. Therefore, it is of great importance to understand whether the interactions between these materials could alter their fate and toxicity. The objective of this work is to investigate the potential environmental and health risks from the ENPs and III-V materials used in semiconductor manufacturing. To this end, the physical, chemical and toxicological characterization of ENPs used in CMP was performed (Chapter 3). Furthermore, the fate and transport of the most used ENP, SiO2, in porous media was studied (Chapter 4). In addition, acute toxicity of As(III), As(V), In(III) and Ga(III) species was evaluated using different bioassays (Chapter 5). Finally, the cytotoxicity of ENPs used in CMP slurries to human lung bronchial epithelial cells was evaluated using an impedance based real time cell analysis (RTCA) assay (Chapter 6). In Chapter 3, four model slurries containing ENPs including colloidal silica (c-SiO2), fumed silica (f-SiO2) cerium oxide (CeO2) and aluminum oxide (Al2O3) were characterized for their physical, chemical and toxicological properties. Ecotoxicity of these slurries to the marine bacterium, Aliivibrio fischeri, was evaluated by measuring its bioluminescence activity as a function of the ENP concentration dosed. The results showed that f-SiO2 and CeO2 were not toxic at concentrations up to 700 and 1000 mg/L, respectively. On the other hand, c-SiO2 and Al2O3 were inhibitory only at very high concentrations (>600 mg/L). At about 1300 mg/L, c-SiO2 and Al2O3 led to 37.6% and 28.4% decrease of cell activity after 30 min exposure, respectively. The inhibitory effect from c-SiO2 was related to additives in the slurry. In summary, the results indicate that these slurries are not likely to cause acute toxicity at environmentally relevant concentrations. The potential risks from ENPs are dependent on their fate and transport in the environment. In Chapter 4, the transport and abatement of SiO2 NPs was studied through laboratory scale column experiments. Synthetic fluorescent core-shell SiO2 NPs (83 nm) were used to facilitate NP traceability. Three widely used filtering materials, i.e., sand, anthracite and granular activated carbon (GAC), were used as porous media. Sand showed very poor capacity for the filtration of SiO2 NPs due to its limited surface area and high concentration of negative surface charge. In addition, the stability and transport of SiO2 NP was strongly dependent on the ionic strength of the solution. High ionic strength led to NP agglomeration and facilitated SiO2 NP retention, while low ionic strength resulted in release of captured NPs from the sand bed. Compared to sand, anthracite and GAC showed higher efficiency for SiO2 NP capture. The superior capacity of GAC was primarily due to its porous structure and high surface area. A process model was developed to simulate NP capture in the packed bed columns and determine fundamental attraction parameters. This model provided an excellent fit to the experimental data. Taken together the results obtained indicate that GAC is an interesting material for SiO2 NPs filtration. With the increasing usage of III-V materials, there are concerns about the ecological threats posed by III-V ions released during semiconductor manufacturing and from disposal of decommissioned electronic devices. In Chapter 5, the acute toxicity of As(III), As(V), In(III) and Ga(III) species was evaluated using different bioassays, including three microbial assays, testing for methanogenic activity, O2 uptake and bioluminescence inhibition of marine bacterium A. fischeri. Acute toxicity to the freshwater crustacean Daphnia magna was also tested. The results showed that In(III) and Ga(III) were generally not toxic or only mildly toxic in all assays, while both As(III) and As(V) showed strong inhibitory effects on different microbial activities (methanogenic and bioluminescence). The toxicity of these ions was strongly dependent on the bioassay target. For In(III) and Ga(III), D. magna was the most sensitive organism with 50% lethal concentrations (LC50) of 57.4 and 237.0 mg/L, respectively. On the other hand, As(III) and As(V) were particularly toxic to methanogens. The 50% inhibitory concentrations (IC50) of both species were about 1.5mg/L. Mixed aerobic heterotrophic culture was highly resistant to all four ions and O2 uptake by the aerobes was not affected in the tested concentrations. Overall, the results indicate that the ecotoxicity of In(III) and Ga(III) is much lower than that of the As species. This finding is important in filling the knowledge gap regarding the ecotoxicology of In and Ga. Besides ecotoxicity, ENPs and III-V materials in CMP effluents could also pose a threat to human health. In Chapter 6, the cytotoxicity of CMP slurries to human bronchial epithelial cells (16HBE14o-) was assessed using a novel impedance based real time cell analyzer (RTCA). Cell death and detachment was observed in assays supplied with high concentrations of c-SiO2 and f-SiO2 NPs (≥250 mg/L). On the other hand, CeO2 and Al2O3 slurries were not inhibitory at concentrations up to 1250 mg/L. In addition, since CMP wastewater generated during the planarization of III-V films contains a mixture of ENPs and soluble III-V species, it is important to understand whether the interactions between these materials could alter their fate and toxicity. As(III) toxicity to human lung cells in the presence and absence of CeO2 NPs was evaluated using the RTCA assay. Exposure to As(III) (0.5 mg/L) for 48 h resulted in 81.3% inhibition of cell viability and proliferation, while cell inhibition decreased to only 13.0% when As(III) was dosed together with sub-toxic levels of CeO2 NPs (250 mg/L). This detoxification effect was mainly due to As(III) adsorption onto CeO2 NPs. When the NPs were added, the soluble arsenic concentration was reduced significantly from 0.5 mg/L to 0.03 mg/L. This work demonstrates that adsorption of As(III) on CeO2 NPs can lower As(III) concentration in the solution and reduce its bioavailability and subsequently result in As(III) detoxification. In conclusion, this dissertation indicates that the ENPs (SiO2, CeO2 and Al2O3) used in semiconductor industry are not expected to cause acute toxicity to the natural environment and human health under environmentally relevant concentration (<1 mg/L). Among the soluble III-V species, In(III) and Ga(III) showed no or mild acute inhibitory effects in different bioassays even at comparatively high concentration. However arsenic species are highly toxic to various important microbial populations in the environment and human cells. The results showed that arsenic could induce toxic effects under current discharge limit set for semiconductor industry. Finally, we demonstrated that the adsorption of As(III) on CeO2 NPs can lower the concentration of soluble As(III) and subsequently resulted in As(III) detoxification.

abstract: This dissertation addresses challenges pertaining to multi-junction (MJ) solar cells from material development to device design and characterization. Firstly, among the various methods to improve the energy conversion efficiency of MJ solar cells using, a novel approach proposed recently is to use II-VI (MgZnCd)(SeTe) and III-V (AlGaIn)(AsSb) semiconductors lattice-matched on GaSb or InAs substrates for current-matched subcells with minimal defect densities. CdSe/CdTe superlattices are proposed as a potential candidate for a subcell in the MJ solar cell designs using this material system, and therefore the material properties of the superlattices are studied. The high structural qualities of the superlattices are obtained from high resolution X-ray diffraction measurements and cross-sectional transmission electron microscopy images. The effective bandgap energies of the superlattices obtained from the photoluminescence (PL) measurements vary with the layer thicknesses, and are smaller than the bandgap energies of either the constituent material. Furthermore, The PL peak position measured at the steady state exhibits a blue shift that increases with the excess carrier concentration. These results confirm a strong type-II band edge alignment between CdSe and CdTe. The valence band offset between unstrained CdSe and CdTe is determined as 0.63 eV±0.06 eV by fitting the measured PL peak positions using the Kronig-Penney model. The blue shift in PL peak position is found to be primarily caused by the band bending effect based on self-consistent solutions of the Schrödinger and Poisson equations. Secondly, the design of the contact grid layout is studied to maximize the power output and energy conversion efficiency for concentrator solar cells. Because the conventional minimum power loss method used for the contact design is not accurate in determining the series resistance loss, a method of using a distributed series resistance model to maximize the power output is proposed for the contact design. It is found that the junction recombination loss in addition to the series resistance loss and shadowing loss can significantly affect the contact layout. The optimal finger spacing and maximum efficiency calculated by the two methods are close, and the differences are dependent on the series resistance and saturation currents of solar cells. Lastly, the accurate measurements of external quantum efficiency (EQE) are important for the design and development of MJ solar cells. However, the electrical and optical couplings between the subcells have caused EQE measurement artifacts. In order to interpret the measurement artifacts, DC and small signal models are built for the bias condition and the scan of chopped monochromatic light in the EQE measurements. Characterization methods are developed for the device parameters used in the models. The EQE measurement artifacts are found to be caused by the shunt and luminescence coupling effects, and can be minimized using proper voltage and light biases. Novel measurement methods using a pulse voltage bias or a pulse light bias are invented to eliminate the EQE measurement artifacts. These measurement methods are nondestructive and easy to implement. The pulse voltage bias or pulse light bias is superimposed on the conventional DC voltage and light biases, in order to control the operating points of the subcells and counterbalance the effects of shunt and luminescence coupling. The methods are demonstrated for the first time to effectively eliminate the measurement artifacts. / Dissertation/Thesis / Ph.D. Electrical Engineering 2012