Semiconductor nanowires (NWs) are anticipated to play a crucial role in future electronic and optoelectronic devices. Their practical applications remain hindered by an urging need for feasible strategies to tailor their optical and electronic properties. Strategies based on strain and alloying are limited by issues such as defects, interface broadening and alloy scattering. In this thesis, a novel method to engineer the optoelectronic properties based on strain-free periodic structural modulations in chemically homogeneous Nanowire Twinning Superlattices (NTSLs) is experimentally demonstrated. NTSLs are an emerging new class of nanoscale material, composed of periodically arranged rotation twin-planes along the length of NWs. The main objective of this thesis is to establish the relationship between the electronic energy band gap (Eg) and the twin-plane spacing (d) in NTSLs, quantified using a periodicity parameter, based on ZnSe. ZnSe was chosen because of its excellent luminescence properties, and potential in fabrication of optoelectronic devices in the near-UV and blue region of the spectrum. A prerequisite to establishing this correspondence is a prior knowledge of the photoluminescence (PL) response and the nature of fundamental optical transitions in defect-free single crystal ZnSe NWs with zinc-blende (ZB) and wurtzite (WZ) crystal structures. There has been no systematic work done yet on understanding these fundamental optical processes, particularly on single NWs and in relation to their crystalline microstructure. Therefore, the secondary objective of this thesis is to study the influence of native point defects on the optical response of single ZnSe NWs in direct relation to their crystalline microstructure. The PL response from single ZB and WZ NWs was determined unambiguously, and excitonic emission linewidths close to 1 meV were observed, which are the narrowest reported linewidths thus far on ZnSe NWs. Based on this and extensive optical and structural characterization on individual NTSLs, a linear variation in Eg is shown through a monotonic shift in PL peak position from ZnSe NTSLs as a function of d, with Eg's that lie between those of ZB and WZ crystal structures. This linear variation in Eg was also validated by ab Initio electronic structure calculations. This establishes NTSLs as new nanoscale polytypes advantageous for applications requiring tunable band gaps.

Semiconductor nanowires (NWs) are anticipated to play a crucial role in future electronic and optoelectronic devices. Their practical applications remain hindered by an urging need for feasible strategies to tailor their optical and electronic properties. Strategies based on strain and alloying are limited by issues such as defects, interface broadening and alloy scattering. In this thesis, a novel method to engineer the optoelectronic properties based on strain-free periodic structural modulations in chemically homogeneous Nanowire Twinning Superlattices (NTSLs) is experimentally demonstrated. NTSLs are an emerging new class of nanoscale material, composed of periodically arranged rotation twin-planes along the length of NWs. The main objective of this thesis is to establish the relationship between the electronic energy band gap (Eg) and the twin-plane spacing (d) in NTSLs, quantified using a periodicity parameter, based on ZnSe. ZnSe was chosen because of its excellent luminescence properties, and potential in fabrication of optoelectronic devices in the near-UV and blue region of the spectrum. A prerequisite to establishing this correspondence is a prior knowledge of the photoluminescence (PL) response and the nature of fundamental optical transitions in defect-free single crystal ZnSe NWs with zinc-blende (ZB) and wurtzite (WZ) crystal structures. There has been no systematic work done yet on understanding these fundamental optical processes, particularly on single NWs and in relation to their crystalline microstructure. Therefore, the secondary objective of this thesis is to study the influence of native point defects on the optical response of single ZnSe NWs in direct relation to their crystalline microstructure. The PL response from single ZB and WZ NWs was determined unambiguously, and excitonic emission linewidths close to 1 meV were observed, which are the narrowest reported linewidths thus far on ZnSe NWs. Based on this and extensive optical and structural characterization on individual NTSLs, a linear variation in Eg is shown through a monotonic shift in PL peak position from ZnSe NTSLs as a function of d, with Eg's that lie between those of ZB and WZ crystal structures. This linear variation in Eg was also validated by ab Initio electronic structure calculations. This establishes NTSLs as new nanoscale polytypes advantageous for applications requiring tunable band gaps.

Filmes finos de óxido de nióbio têm sido usados em muitas aplicações tecnológicas. Existem pelo menos três óxidos estáveis de nióbio: NbO, NbO2 e Nb2O5 e cada um deles tem propriedades específicas. O Nb2O5 é a forma termodinamicamente mais estável e apresenta propriedades físicas e químicas únicas, como alto índice de refração, band gap largo, excelente estabilidade química e resistência à corrosão, baixa absorção óptica no campo da luz visível até regiões próximas ao infra-vermelho, sendo amplamente utilizados como filtros de interferência óptica de alta qualidade. Neste trabalho foram depositados filmes de óxido de nióbio por meio da técnica de sputtering reativo sobre substratos de silício e borossilicato. Os filmes finos foram obtidos com vazão de oxigênio variando entre 15 e zero sccm. O objetivo deste trabalho foi a caracterização das propriedades ópticas dos filmes. Foram avaliados o índice de refração e a espessura pela técnica de elipsometria, o band gap pelo método de Tauc e a razão atômica e a densidade superficial por meio de Espectrometria de Retroespalhamento Rutherford (RBS). Foram obtidos espectros de transmitância e refletância por Espectrofotometria UV/Vis/NIR e o coeficiente de extinção foi calculado pelo método de Hong. Todos estes parâmetros são importantes para aplicação em dispositivos ópticos. Nos filmes depositados com vazão de oxigênio de 15 sccm foi observado que o índice de refração aumenta com o aumento da espessura dos filmes e que o composto formado foi Nb2O5. Para uma vazão de 2,0 sccm foi encontrado o composto NbO2 e o filme apresentou alta absorção ótica. Os resultados sugerem que outros óxidos de nióbio ou nióbio metálico foram incorporados nos filmes conforme o fluxo de oxigênio foi diminuído. Foi observada uma relação direta entre a diminuição da vazão de oxigênio durante a deposição e um aumento da quantidade de nióbio nos filmes, acompanhados de um aumento do índice de refração e da densidade superficial. / Niobium oxide thin films have been used in many technological applications. There are at least three stable niobium oxides: NbO, NbO2 and Nb2O5 and each one has specific properties. The Nb2O5 is the most stable thermodynamically form having unique physical and chemical properties such as high refractive index, wide band gap, excellent chemical stability and corrosion resistance, low optical absorption in the field of visible light to near the infrared regions, being widely used as high quality optical interference filters. In this work niobium oxide thin films were deposited by reactive sputtering technique on silicon and borosilicate substrates. The films were obtained by using oxygen flow rates varying from 15 up to zero sccm. The objective of this work was the optical characterization of niobium oxide films. The refractive index and the thickness were evaluated by ellipsometry technique, the band gap by TAUC method and the atomic ratio and the surface density by means of Rutherford backscattering spectrometry (RBS). Spectral transmittance and reflectance spectrophotometry by UV / Vis / NIR curves were obtained and the extinction coefficient was calculated by the method of Hong. All of these parameters are important for application in optical devices. In the films deposited with oxygen flow of 15 sccm it was observed that the refractive index increases with increasing thickness of the films and analyzes indicated that Nb2O5 forms for when a flow rate of 15 sccm is used. For a flow rate of 2.0 sccm it was found NbO2 and a high optical absorption. As the flow of oxygen used in the deposition of the films was decreased, there were signs that were incorporated in the films other oxides of niobium or niobium metal itself. A direct relationship between decreased of oxygen flow during deposition and a increase in niobium quantity in the films, accompanied by an increase in the refractive index and surface density was observed.

abstract: Nanowires are one-dimensional (1D) structures with diameter on the nanometer scales with a high length-to-diameter aspect ratio. Nanowires of various materials including semiconductors, dielectrics and metals have been intensively researched in the past two decades for applications to electrical and optical devices. Typically, nanowires are synthesized using the vapor-liquid-solid (VLS) approach, which allows defect-free 1D growth despite the lattice mismatch between nanowires and substrates. Lattice mismatch issue is a serious problem in high-quality thin film growth of many semiconductors and non-semiconductors. Therefore, nanowires provide promising platforms for the applications requiring high crystal quality materials. With the 1D geometry, nanowires are natural optical waveguides for light guiding and propagation. By introducing feedback mechanisms to nanowire waveguides, such as the cleaved end facets, the nanowires can work as ultra-small size lasers. Since the first demonstration of the room-temperature ultraviolet nanowire lasers in 2001, the nanowire lasers covering from ultraviolet to mid infrared wavelength ranges have been intensively studied. This dissertation focuses on the optical characterization and laser fabrication of two nanowire materials: erbium chloride silicate nanowires and composition-graded CdSSe semiconductor alloy nanowires. Chapter 1 – 5 of this dissertation presents a comprehensive characterization of a newly developed erbium compound material, erbium chloride silicate (ECS) in a nanowire form. Extensive experiments demonstrated the high crystal quality and excellent optical properties of ECS nanowires. Optical gain higher than 30 dB/cm at 1.53 μm wavelength is demonstrated on single ECS nanowires, which is higher than the gain of any reported erbium materials. An ultra-high Q photonic crystal micro-cavity is designed on a single ECS nanowire towards the ultra-compact lasers at communication wavelengths. Such ECS nanowire lasers show the potential applications of on-chip photonics integration. Chapter 6 – 7 presents the design and demonstration of dynamical color-controllable lasers on a single CdSSe alloy nanowire. Through the defect-free VLS growth, engineering of the alloy composition in a single nanowire is achieved. The alloy composition of CdSxSe1-x uniformly varies along the nanowire axis from x=1 to x=0, giving the opportunity of multi-color lasing in a monolithic structure. By looping the wide-bandgap end of the alloy nanowire through nanoscale manipulation, the simultaneous two-color lasing at green and red colors are demonstrated. The 107 nm wavelength separation of the two lasing colors is much larger than the gain bandwidth of typical semiconductors. Since the two-color lasing shares the output port, the color of the total lasing output can be controlled dynamically between the two fundamental colors by changing the relative output power of two lasing colors. Such multi-color lasing and continuous color tuning in a wide spectral range would eventually enable color-by-design lasers to be used for lighting, display and many other applications. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2015

Filmes finos de óxido de nióbio têm sido usados em muitas aplicações tecnológicas. Existem pelo menos três óxidos estáveis de nióbio: NbO, NbO2 e Nb2O5 e cada um deles tem propriedades específicas. O Nb2O5 é a forma termodinamicamente mais estável e apresenta propriedades físicas e químicas únicas, como alto índice de refração, band gap largo, excelente estabilidade química e resistência à corrosão, baixa absorção óptica no campo da luz visível até regiões próximas ao infra-vermelho, sendo amplamente utilizados como filtros de interferência óptica de alta qualidade. Neste trabalho foram depositados filmes de óxido de nióbio por meio da técnica de sputtering reativo sobre substratos de silício e borossilicato. Os filmes finos foram obtidos com vazão de oxigênio variando entre 15 e zero sccm. O objetivo deste trabalho foi a caracterização das propriedades ópticas dos filmes. Foram avaliados o índice de refração e a espessura pela técnica de elipsometria, o band gap pelo método de Tauc e a razão atômica e a densidade superficial por meio de Espectrometria de Retroespalhamento Rutherford (RBS). Foram obtidos espectros de transmitância e refletância por Espectrofotometria UV/Vis/NIR e o coeficiente de extinção foi calculado pelo método de Hong. Todos estes parâmetros são importantes para aplicação em dispositivos ópticos. Nos filmes depositados com vazão de oxigênio de 15 sccm foi observado que o índice de refração aumenta com o aumento da espessura dos filmes e que o composto formado foi Nb2O5. Para uma vazão de 2,0 sccm foi encontrado o composto NbO2 e o filme apresentou alta absorção ótica. Os resultados sugerem que outros óxidos de nióbio ou nióbio metálico foram incorporados nos filmes conforme o fluxo de oxigênio foi diminuído. Foi observada uma relação direta entre a diminuição da vazão de oxigênio durante a deposição e um aumento da quantidade de nióbio nos filmes, acompanhados de um aumento do índice de refração e da densidade superficial. / Niobium oxide thin films have been used in many technological applications. There are at least three stable niobium oxides: NbO, NbO2 and Nb2O5 and each one has specific properties. The Nb2O5 is the most stable thermodynamically form having unique physical and chemical properties such as high refractive index, wide band gap, excellent chemical stability and corrosion resistance, low optical absorption in the field of visible light to near the infrared regions, being widely used as high quality optical interference filters. In this work niobium oxide thin films were deposited by reactive sputtering technique on silicon and borosilicate substrates. The films were obtained by using oxygen flow rates varying from 15 up to zero sccm. The objective of this work was the optical characterization of niobium oxide films. The refractive index and the thickness were evaluated by ellipsometry technique, the band gap by TAUC method and the atomic ratio and the surface density by means of Rutherford backscattering spectrometry (RBS). Spectral transmittance and reflectance spectrophotometry by UV / Vis / NIR curves were obtained and the extinction coefficient was calculated by the method of Hong. All of these parameters are important for application in optical devices. In the films deposited with oxygen flow of 15 sccm it was observed that the refractive index increases with increasing thickness of the films and analyzes indicated that Nb2O5 forms for when a flow rate of 15 sccm is used. For a flow rate of 2.0 sccm it was found NbO2 and a high optical absorption. As the flow of oxygen used in the deposition of the films was decreased, there were signs that were incorporated in the films other oxides of niobium or niobium metal itself. A direct relationship between decreased of oxygen flow during deposition and a increase in niobium quantity in the films, accompanied by an increase in the refractive index and surface density was observed.

Thesis advisor: John T. Fourkas / One can extract a tremendous amount of information about the organizational and dynamic states of molecules, in situ and in real-time, through highly sensitive and noninvasive single particle optical probing. The highly efficient, multi-photon excited luminescence from stabilized metal nanoclusters renders these species useful as optical probes that can be used in detecting single particle and molecular dynamics. We generate stable, and monodisperse samples of Ag nanoclusters as small as 1 nm in diameter, and find that through substitution of various stabilizer molecules we can precisely tune the size of the clusters over a 1-6 nm range of diameters, ensuring monodispersity and stability at every stage. These clusters also exhibit highly efficient, polarized luminescence upon two photon excitation at 800 nm and remain highly photostable, not exhibiting the deleterious blinking that occurs with many single-molecule fluorophores. In order to demonstrate the utility of these clusters as single-molecule probes, we track their emission polarization over long periods in deeply supercooled liquids such as 4'(octahydro-4,7-methano-5H-inden-5-yliden) bisphenol dimethyl ether (ODE). Our results suggest that these clusters can detect nanoscale dynamics with high sensitivity. / Thesis (BS) — Boston College, 2004. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Chemistry. / Discipline: College Honors Program.

The development of next generation devices for high speed switching, high efficiency energy conversion, spintronic devices require the development of advanced material systems. While conventional group IV, group II-VI and group III-V based materials systems have served as a base material in many modern device structures, they posses fundamental materials properties that limit their suitability in next generation device structures. The group III-N material system is very promising for the development of advanced device structures. GaN is currently widely used in high efficiency lighting applications. However, the development of this material system has been limited to material systems with limited indium. The growth of high indium concentration materials such as InN and GaxIn1-xN has proven difficulty due to the high thermal decomposition pressure of InN. In response to this difficulty, a high pressure chemical vapor deposition reactor system has been developed for the growth of InN which enables elevated processing temperatures as compared to conventional low-pressure growth techniques. The design criteria and implementation of this unique design is presented here. In addition, the results of in-situ real time optical characterization capabilities of this reactor system are presented as applied to thermal characterization, flow dynamics, gas phase kinetics and surface reactions. Ex-situ InN thin films grown on sapphire substrates and GaN epilayers have been analyzed by x-ray diffraction, transmission spectroscopy and raman spectroscopy. These results indicated single crystal indium nitride films with an optical absorption edge which varies between 0.7 and 1.9 eV as a function of precursor flow stoichiometry.

In this study, InSe single crystals were grown from the melt using conventional Bridgman-Stockbarger system. The grown crystals were implanted by N-ions to investigate the doping effect. the stoichiometry and the structural features were examined by scanning electron microscope and X-ray diffraction method, respectively. We have observed that the ingot was stoichiometric and the structure was hexagonal. Temperature dependent conductivity and Hall effect measurements were carried out to investigate the electrical properties of as-grown, as-implanted and annealed samples within the temperature range of 80-400 K. To investigate the annealing effect on both the absorption and photoluminescence (PL) spectra, absorption and PL measurements were performed at room temperature. N-implantation reduced the resistivity order from 103 to 101 (&amp / #937 / -cm). We have used temperature dependent conductivity and Hall effect measurements to analyze the dominant scattering mechanisms. Hall measurements showed that all the samples had n-type conduction. Absorption measurements showed that InSe had direct band gap. It was observed that annealing had almost no effect an both room temperature absorption and PL spectra of the samples.

Successful integration of daylighting systems requires the ability to predict their performance for given climates. In this dissertation, a bottom-up approach is applied to evaluate the optical performance of a selection of daylighting systems. The evaluations are based on the optical properties of the included materials, and part of the dissertation focuses on developing new optical characterization methods. The work on characterization techniques uses an integrating sphere method to characterize the transmittance of light scattering samples more accurately. The method's principle is to reduce the discrepancy in light distribution between the reference and the sample scans by using an entry port beam diffuser. For samples exhibiting distinct light scattering patterns, the benefits of improved uniformity outweigh the errors introduced by the diffusing material. The method is applicable to any integrating sphere instrument, and its simplicity makes it suitable for standard measurements. In addition to normal-hemispherical properties, many daylighting applications require knowledge of the system's spatial light distribution. This dissertation presents a method combining experimental techniques and ray tracing simulations to assess the light distribution from a Venetian blind system. The method indicates that ray tracing based on simplified optical data is inadequate to predict the light distribution for slat materials exhibiting both specular and diffuse properties. Ray tracing is a promising complement to experimental methods used to characterize light guiding or light redirecting systems. Here, spectrophotometric measurements of a scaled mirror light pipe validate a ray tracing model. The model shows excellent agreement with experimental results for both direct and diffuse incident light. The spectral evaluation shows no dramatic color changes for the transmitted light. The ray tracing model is used to evaluate four daylighting systems for a selection of Swedish locations. The percentage of occupied time when the studied systems achieve full design illuminance is relatively low, but the systems provide a valuable contribution to the required illuminance. Additionally, this dissertation provides an overview of available energy efficient windows and illustrates the importance of including the solar energy transmittance when evaluating window energy performance. Overall, this dissertation presents optical characterization techniques for improved performance evaluations of daylighting systems.

III-nitride semiconductors suffer significant efficiency limitations; ‘efficiency’ being an umbrella term that covers an extensive list of challenges that must be overcome if they are to fulfil their vast potential. To this end, it is imperative to understand the underlying phenomena behind such limitations. In this dissertation, I combine powerful optical and structural characterization techniques to investigate the effect of different defects on the carrier dynamics in III-nitride materials for light emitting devices. The results presented herein will enhance the current understanding of the carrier mechanisms in such devices, which will lead to device efficiency improvements. In the first part of this dissertation, the effects of some important types of crystal defects present in III-nitride structures are investigated. Here, two types of defects are studied in two different III-nitride-based light emitting structures. The first defects of interest are V-pit defects in InGaN/GaN multiple quantum well (MQW) blue LEDs, where their contribution to the high-efficiency of such LEDs is discussed. In addition, the effect of these defects on the efficiency droop phenomenon in these LEDs is elucidated. Secondly, the optical effects of grain boundary defects in AlN-rich AlGaN/AlGaN MQWs is studied. In this study, it is shown that grain boundary defects may result in abnormal carrier localization behavior in these deep ultraviolet (UV) structures. While both defects are treated individually, it is evident from these studies that threading dislocation (TD) defects are an underlying contributor to the more undesirable outcomes of the said defects. In the second part, the dissertation reports on the carrier dynamics of III-nitride LED structures grown on emerging substrates—as possible efficiency enhancing techniques—aimed at mitigating the effects of TD defects. Thus, the carrier dynamics of GaN/AlGaN UV MQWs grown, for the first time, on (2̅01) – oriented β-Ga2O3 is studied. It is shown to be a candidate substrate for highly efficient vertical UV devices. Finally, results from the carrier dynamics investigation of an AlGaN/AlGaN MQW LED structure homoepitaxially grown on AlN substrate are discussed, where it is shown that its high-efficiency is sustained at high temperatures through the thermal redistribution of carriers to highly efficient recombination sites.