Capture of the energy in sunlight relies mainly on the use of light-absorbing semiconductors, in solar cells and in water-splitting devices. While solar cell efficiency has increased dramatically since the first practical device was made in 1954, production costs for the most-efficient solar absorbers, III-V semiconductors, remain high. This is largely a result of use of expensive, slow growth methods which rely on hazardous gas-phase precursors. Alternative growth methods are necessary to lower the cost for III-V materials for use in solar cells and improve the practicality of water-splitting devices. The research goal of this dissertation is two-fold: to expand the capabilities of close-spaced vapor transport, an alternative growth method for III-Vs to demonstrate its compatibility with current technologies; and to explore the fundamental chemistry of close-spaced vapor transport as a growth method for these materials. This dissertation surveys plausibly lower-cost growth methods for III-V semiconductors (Chapter II) and presents in-depth studies on the growth chemistry of two ternary III-Vs: GaAs1-xPx (Chapter III) and Ga1-xInxP (Chapter IV). Finally, the growth of GaAs microstructures which could be utilized in a water-splitting device is studied (Chapter V). This dissertation includes previously published and unpublished co-authored material. / 2019-01-09

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.

In this dissertation, the extraordinary magnetoresistance (EMR) effect in semiconductor/metal hybrid structures is studied to improve the performance in sensing applications. Using two-dimensional finite element simulations, the geometric dependence of the output sensitivity, which is a more relevant parameter for EMR sensors than the magnetoresistance (MR), is studied. The results show that the optimal geometry in this case is different from the geometry reported before, where the MR ratio was optimized. A device consisting of a semiconductor bar with length/width ratio of 5~10 and having only 2 contacts is found to exhibit the highest sensitivity. A newly developed three-dimensional finite element model is employed to investigate parameters that have been neglected with the two dimensional simulations utilized so far, i.e., thickness of metal shunt and arbitrary semiconductor/metal interface. The simulations show the influence of those parameters on the sensitivity is up to 10 %. The model also enables exploring the EMR effect in planar magnetic fields. In case of a bar device, the sensitivity to planar fields is about 15 % to 20 % of the one to perpendicular fields. 5 A “top-contacted” structure is proposed to reduce the complexity of fabrication, where neither patterning of the semiconductor nor precise alignment is required. A comparison of the new structure with a conventionally fabricated device shows that a similar magnetic field resolution of 24 nT/√Hz is obtained. A new 3-contact device is developed improving the poor low-field sensitivity observed in conventional EMR devices, resulting from its parabolic magnetoresistance response. The 3-contact device provides a considerable boost of the low field response by combining the Hall effect with the EMR effect, resulting in an increase of the output sensitivity by 5 times at 0.01 T compared to a 2-contact device. The results of this dissertation provide new insights into the optimization of EMR devices for sensor applications. Two novel concepts are presented, which are promising for realizing EMR devices with high spatial resolution and for opening new applications for EMR sensors in the low-field regime.

The electrical properties of nanowire-based n-InSb-n-InAs heterojunctions grown by chemical beam epitaxy were investigated both theoretically and experimentally. This heterostructure presented a type-III band alignment with the band bendings at 0.12 eV for InAs side and 0.16 − 0.21 eV in InSb. Analysis of the temperature dependent current voltage characteristics showed that the current through the heterojunction is caused mostly by generation-recombination processes in the InSb and at the heterointerface. Due to the partially overlapping valence band of InSb and the conduction band of InAs, the second process was fast and activationless. Theoretical analysis showed that, depending on the heterojunction parameters, the flux of non-equilibrium minority carriers may have a different direction, explaining the experimentally observed non-monotonic coordinate dependence of the electron beam induced current at the vicinity of heterointerface.

The electrical properties of nanowire-based n-InSb-n-InAs heterojunctions grown by chemical beam epitaxy were investigated both theoretically and experimentally. This heterostructure presented a type-III band alignment with the band bendings at 0.12 eV for InAs side and 0.16 − 0.21 eV in InSb. Analysis of the temperature dependent current voltage characteristics showed that the current through the heterojunction is caused mostly by generation-recombination processes in the InSb and at the heterointerface. Due to the partially overlapping valence band of InSb and the conduction band of InAs, the second process was fast and activationless. Theoretical analysis showed that, depending on the heterojunction parameters, the flux of non-equilibrium minority carriers may have a different direction, explaining the experimentally observed non-monotonic coordinate dependence of the electron beam induced current at the vicinity of heterointerface.

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abstract: Recently a new materials platform consisting of semiconductors grown on GaSb and InAs substrates with lattice constants close to 6.1 A was proposed by our group for various electronic and optoelectronic applications. This materials platform consists of both II-VI (MgZnCdHg)(SeTe) and III-V (InGaAl)(AsSb) compound semiconductors, which have direct bandgaps spanning the entire energy spectrum from far-IR (~0 eV) up to UV (~3.4 eV). The broad range of bandgaps and material properties make it very attractive for a wide range of applications in optoelectronics, such as solar cells, laser diodes, light emitting diodes, and photodetectors. Moreover, this novel materials system potentially offers unlimited degrees of freedom for integration of electronic and optoelectronic devices onto a single substrate while keeping the best possible materials quality with very low densities of misfit dislocations. This capability is not achievable with any other known lattice-matched semiconductors on any available substrate. In the 6.1-A materials system, the semiconductors ZnTe and GaSb are almost perfectly lattice-matched with a lattice mismatch of only 0.13%. Correspondingly, it is expected that high quality ZnTe/GaSb and GaSb/ZnTe heterostructures can be achieved with very few dislocations generated during growth. To fulfill the task, their MBE growth and material properties are carefully investigated. High quality ZnTe layers grown on various III-V substrates and GaSb grown on ZnTe are successfully achieved using MBE. It is also noticed that ZnTe and GaSb have a type-I band-edge alignment with large band offsets (delta_Ec=0.934 eV, delta_Ev=0.6 eV), which provides strong confinement for both electrons and holes. Furthermore, a large difference in refractive index is found between ZnTe and GaSb (2.7 and 3.9, respectively, at 0.7 eV), leading to excellent optical confinement of the guided optical modes in planar semiconductor lasers or distributed Bragg reflectors (DBR) for vertical-cavity surface-emitting lasers. Therefore, GaSb/ZnTe double-heterostructure and ZnTe/GaSb DBR structure are suitable for use in light emitting devices. In this thesis work, experimental demonstration of these structures with excellent structural and optical properties is reported. During the exploration on the properties of various ZnTe heterostructures, it is found that residual tensile strains exist in the thick ZnTe epilayers when they are grown on GaAs, InP, InAs and GaSb substrates. The presence of tensile strains is due to the difference in thermal expansion coefficients between the epilayers and the substrates. The defect densities in these ZnTe layers become lower as the ZnTe layer thickness increases. Growth of high quality GaSb on ZnTe can be achieved using a temperature ramp during growth. The influence of temperature ramps with different ramping rates in the optical properties of GaSb layer is studied, and the samples grown with a temperature ramp from 360 to 470 C at a rate of 33 C/min show the narrowest bound exciton emission peak with a full width at half maximum of 15 meV. ZnTe/GaSb DBR structures show excellent reflectivity properties in the mid-infrared range. A peak reflectance of 99% with a wide stopband of 480 nm centered at 2.5 um is measured from a ZnTe/GaSb DBR sample of only 7 quarter-wavelength pairs. / Dissertation/Thesis / Ph.D. Physics 2012

<p>An ammonium polysulfide (NH₄)₂S_x solution was optimized through a series of experiments to be used for surface passivation of III-V nanowires . The effectiveness of sulfur passivation was investigated by measuring the photoluminescence from p-InP nanowires before and after passivation. The optimized parameters included solvent type, molarity and passivation time. According to the experiments, passivation of nanowires in 0.5 M solution diluted in isopropyl alcohol for 5 min produced the maximum photoluminescence improvement. It was also demonstrated that the whole surface passivation of vertical nanowires in ensemble samples caused a 40 times increase in the photoluminescence intensity while top surface passivation of individual nanowires resulted in a 20 times increase of photoluminescence intensity. A model was developed to calculate the photoluminescence from single nanowires under different surface recombination and surface potential. The model showed that the 40 times increase in the photoluminescence is mainly due to the reduction of surface state density from 10¹² cm^-2before passivation to 5×10¹⁰ cm^-2after passivation.</p> <p>The effect of sulfur passivation on core-shell p-n junction GaAs nanowire solar cells has been investigated. The relative cell efficiency increased by 19% after passivation.</p> / Doctor of Philosophy (PhD)

Organic molecules adsorbed on inorganic substrates are the topics of interest in this thesis. Interfaces of this kind are found in dye sensitized solar cells that convert solar energy to electricity, a promising environmentally friendly energy source which might provide a route to replace fossil fuels. Another field where these interfaces play a role is in molecular electronics, an approach to solve the down scaling in the ever increasing hunt for miniaturized electronic devices. The motivation for this work lies among other in these applications and surface science is a suitable approach to investigate the electronic and morphologic properties of the interfaces as it provides detailed knowledge on an atomic level. Phthalocyanines are the organic molecules investigated and the inorganic substrates range from wide band gap via narrow band gap semiconductors to metals. Photoelectron and X-ray spectroscopy experiments are performed to shed light on the electronic properties of the adsorbed molecules and the substrate, as well as the chemical interaction between adsorbate and substrate at the interface. The ordering of the adsorbate at the interface is important as ordered molecular thin films may have other properties than amorphous films due to the anisotropic electronic properties of the organic molecules; this is investigated using scanning tunneling microscopy. We find that the phthalocyanines are affected by adsorption when the substrate is TiO2 or Ag, where charge transfer from the molecule occurs or an interface state is formed respectively. The molecules are adsorbed flat on these surfaces giving a large contact area and a relatively strong bond. On Ag, ordered structures appear with different symmetry depending on initial coverage. The reactivity of the TiO2 surface is not ideal in the solar cell application and by modifying the surface with a thin organic layer, the negative influence on the adsorbed phthalocyanine is reduced. ZnO is not as reactive as TiO2, thanks maybe to the upright adsorption mode of the phthalocyanines. The semiconductor InSb is less reactive leading to self-assembled molecular structures on the (001) surface, either homogenously distributed in a one monolayer thick film or in strands along the reconstruction rows. InAs on the other hand has a larger influence on the adsorbed molecules resulting in a metallic film upon thermal treatment. / QC 20100812

The global increase in internet usage requires an upgrade of the existing infrastruc- ture. Lasers are a key proponent to improving existing systems, and engineering better gain materials aids in this effort. (InGa)As is the leading material in this field for 1.55 μm communication wavelengths, but can be improved on by changing the substrate from InP to GaAs. Another improvement would be reducing the losses due to Auger recombination. (InGa)(BiAs) is suggested to alleviate many of these issues, as it can be grown on a GaAs substrate and is capable of decreased Auger recombination. By analyzing prospective alloys (and existing ones) using spatial electron localization, a superior candidate for industrial use can be suggested. The localization captures the disorder introduced by alloying and can be associated with material properties such as the gain characteristics and photoluminescence linewidths. These properties are important factors in determining a successor. The subject of two-dimensional materials is another topic which has shown promise in various applica- tions. Examples include flexible, transparent, and miniaturized electronics. Recent research done by Al Balushi et al. suggests that GaN may be stabilized in a two-dimensional sys- tem. By extending the material modelling approach from the telecommunication application to this system, we were able to show which III-V isoelectronic elements can be substituted into GaN. This two-dimensional system may be the only candidate capable of spanning the visible spectrum. We found Phosphorus to be the strongest candidate for decreasing the band gap. / Thesis / Master of Applied Science (MASc)