Surface passivation of semiconductor quantum dots is essential to preserve their efficient and robust light emitting properties. By using a lattice matched (mismatch = 0.5%) lead halide perovskite matrix, we achieve shell-like passivation of lead sulfide QDs in crystalline films, leading to efficient infrared light emission. These structures are made from a simple one-step spin coating process of an electrostatically stabilized colloidal suspension. Photoluminescence and transient absorption spectroscopy indicate rapid energy transfer between the perovskite matrix and the QDs, suggesting an interface with few trap states. In addition to housing the efficient infrared QD emitters, lead halide perovskites themselves have good carrier mobilities and low trap densities, making these solution-processable heterostructures an attractive option for electrically pumped light emitting devices. The highest performing quantum dots for visible light applications are CdE (E=chalcogenide) core/shell heterostructures. Again, surface passivation plays a huge role in determining the brightness and robustness of visible QD emitters. Multilayer shell passivation is usually used to produce the highest quantum yield particles. Surface trap states are shown to be detrimental to luminescence output, even in thick-shelled particles. Spherical quantum wells allow for thicker shells and with good surface passivation, show promising reduction of biexciton auger recombination, as measured by a time correlated single photon counting (TCSPC) microscope. TCSPC methods were used to diagnose and identify QD architectures for LED applications and explore fundamental recombination dynamics using photon antibunching measurements, and statistical analysis of blinking traces.Introducing new surfaces onto graphitic substrates can be a useful for introducing new electronic properties, patterning device-specific geometries, or appending molecular catalysts. Metal nanoparticles were used to act as a catalyst for the gasification and etching of graphite and graphene. Several methods of controlling the initiation, propagation, and density of these trenches were explored. Patterning defects helped control where initiation occurred, while faceting existing defect sites could also enable more facile initiation and control the direction at the beginning of etching, due to the wetting mechanism of particle movement. Patterning the metal also was shown as a promising avenue to limit unwanted gasification and promote etching in specific, patterned regions. Surface functionalization using reactive gases was performed and characterized with outlook for future experiments.

Semiconductor devices are commonplace in every household. One application of semiconductors in particular, namely solid state lighting technology, is destined for a bright future. To this end, ZnO nanostructures have gained substantial interest in the research community, in part because of its requisite large direct band gap. Furthermore, the stability of the exciton (binding energy 60 meV) in this material, can lead to lasing action based on exciton recombination and possibly exciton interaction, even above room temperature. Therefore, it is very important to realize controllable growth of ZnO nanostructures and investigate their properties. The main motivation for this thesis is not only to successfully realize the controllable growth of ZnO nanorods, but also to investigate the structure, optical and electrical properties in detail by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy (steady state and time resolved) and X-ray diffraction (XRD). Furthermore, strong rectification in the ZnO/p-Si heterojunction is demonstrated. Nanorods have been successfully synthesized on silicon by a two-step process, involving the pre-coating of the substrate by a seed layer, followed by the chemical bath deposition of the nanorods. ZnO seed layers with particle sizes of about 5 nm are achieved by the thermal decomposition of zinc acetate dihydrate dissolved in ethanol. The effects of the seed layer density on the distribution, alignment and uniformity of subsequently grown nanorods were studied. The aspect ratio, orientation and distribution of nanorods are shown to be well controlled through adjusting the density of the ZnO nanoparticles pre-coated onto the substrates. It is shown that the seed layer is a prerequisite for the growth of well aligned ZnO nanorods on lattice mismatched Si substrate. The influence of various nanorod growth parameters on the morphology, optical and electrical properties of the nanorods were also systematically studied. These include the oxygen to zinc molar ratio, the pH of the growth solution, the concentration of the reactants, the growth temperature and growth time, different hydroxide precursors and the addition of surface passivating agents to the growth solution. By controlling these xii parameters different architectures of nanostructures, like spherical particles, well aligned nanorods, nanoflowers and thin films of different thicknesses are demonstrated. A possible growth mechanism for ZnO nanostructures in solution is proposed. XRD indicated that all the as-grown nanostructures produced above 45 C crystallize in the wurtzite structure and post growth annealing does not significantly enhance the crystalline quality of the material. In material grown at lower temperature, traces of zinc hydroxide were observed. The optical quality of the nanostructures was investigated using both steady-state PL and time-resolved (TR) PL from 4 K to room temperature. In the case of as-grown samples, both UV and defect related emissions have been observed for all nanostructures. The effect of post-growth annealing on the optical quality of the nanostructures was carefully examined. The effect of annealing in different atmospheres was also investigated. Regardless of the annealing environment annealing at a temperature as low as 300 C enhances the UV emission and suppresses defect related deep level emission. However, annealing above 500 C is required to out-diffuse hydrogen, the presence of which is deduced from the I4 line in the low temperature PL spectra of ZnO. TRPL was utilized to investigate lifetime decay profiles of nanorods upon different post growth treatments. The bound exciton lifetime strongly depends on the post-growth annealing temperature: the PL decay time is much faster for as grown rods, confirming the domination of surface assisted recombination. In general, the PL analysis showed that the PL of nanorods have the same characteristics as that of bulk ZnO, except for the stronger contribution from surface related bound excitons in the former case. Surface adsorbed impurities causing depletion and band bending in the near surface region is implied from both time resolved and steady state PL. Finally, although strong rectification in the ZnO/p-Si heterojunction is illustrated, no electroluminescence has been achieved. This is explained in terms of the band offset between ZnO and Si and interfacial states. Different schemes are proposed to improve the performance of ZnO/Si heterojunction light emitting devices.

No description available.

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

The behaviour of nitrogen in silicon is investigated using the dislocation unlocking technique. Specimens containing well-ordered arrays of dislocations are isothermally annealed for a controlled duration, during which nitrogen segregates to and pins the dislocations. The stress required to unlock the dislocations is then measured by three-point bending at elevated temperature. By analysing the dependence of this unlocking stress on anneal duration and temperature, information about nitrogen's transport and interaction with dislocations can be deduced. Experiments are performed at anneal temperatures of 500 - 1050C using float-zone silicon with [N] = 2x10^15 cm^-3. The results are analysed to give an expression for nitrogen's effective diffusivity of D = 173,000 exp(-3.24eV/kT)cm^2 s^-1 in the 500 - 750C range, showing for the first time that nitrogen transport at low temperatures behaves in the same way as measured at higher temperatures by other groups using secondary ion mass spectrometry. If analysed in terms of monomer-dimer dissociative transport, the results give a nitrogen monomer diffusivity of D_1 = 28 exp(-(1.1 to 1.4 eV)/kT) cm^2 s^-1, which is similar to that found by another analysis in the literature. The measurements also show that nitrogen's dislocation locking strength measured at 550C is dependent on anneal temperature, peaking at 600 - 700C and falling towards zero above 1000C. The dislocation unlocking technique itself is also investigated and characterized. It is found that the measured unlocking stress is dependent on the three-point bend duration, falling with increasing duration. Analysis of these results in terms of the theory of release of dislocations from pinning points indicates that nitrogen dislocation locking is likely to be by an atomic species. This effect also has implications for the results of previous nitrogen dislocation unlocking experiments, and the technique has been modified so that a standardised set of conditions is used for every test. Other measurements show that nitrogen's dislocation locking effect is lessened by the presence of transition metal contamination, and that dislocation velocity in silicon may be affected by the nitrogen present in the material. A modified dislocation unlocking technique is developed to measure dislocation locking from near-surface ion-implanted impurities. Results from heavily N-implanted silicon show that nitrogen implantation can provide additional dislocation locking strength to that already given by the oxygen in the material. The scale of the dislocation locking effect in these experiments may provide evidence that nitrogen's effective diffusivity is reduced at high concentrations, indicating that nitrogen transport may be by a dissociative mechanism.

The trend over the last 50 years of down-scaling the silicon transistor to achieve faster computations has led to doubling of the number of transistors and computation speed over about every two years. However, this trend cannot be maintained due to the fundamental limitations of silicon as the main material for the semiconducting industry. Therefore, there is an active search for exploration of alternate materials. Among the possible candidates that can may [sic] be able to replace silicon is graphene which has recently gained the most attention. Unique properties of graphene include exceedingly high carrier mobility, tunable band gap, huge optical density of a monolayer, anomalous quantum Hall effect, and many others. To be suitable for microelectronic applications the material should be semiconductive, i.e. have a non-zero band gap. Pristine graphene is a semimetal, but by the virtue of doping the graphene surface with different molecules and radicals a band gap can be opened. Because the electronic properties of all materials are intimately related to their atomic structure, characterization of molecular and electronic structure of functionalizing groups is of high interest. The ab-inito (from the first principles) calculations provide a unique opportunity to study the influence of the dopants and thus allow exploration of the physical phenomena in functionalized graphene structures. This ability paves the road to probe the properties based on the intuitive structural information only. A great advantage of this approach lies in the opportunity for quick screening of various atomic structures. We conducted a series of ab-inito investigations of graphene functionalized with covalently and hapticly bound groups, and demonstrated possible practical usage of functionalized graphene for microelectronic and optical applications. This investigation showed that it is possible [to] produce band gaps in graphene (i.e., produce semiconducting graphene) of about 1 eV, without degrading the carrier mobility. This was archived by considering the influence of those adducts on electronic band structure and conductivity properties.

[Truncated abstract] Infrared (IR) detectors have many applications across a wide range of industries. HgCdTe is the leading semiconductor material for fabrication of high-performance IR detectors due to a number of superior fundamental material properties. However, significant technological challenges are involved in working with this narrow bandgap material, primarily due to its low damage threshold. Exposure of HgCdTe to H2/CH4/Ar plasma in IR detector fabrication processes is known to generally cause modifications to the electrical properties of the material, specifically including p-to-n type conductivity conversion. This is an undesirable side-effect when aiming to perform physical etching for device delineation. However, it has previously been exploited as a novel means of planar n-on-p junction formation for high-performance HgCdTe photodiode fabrication. This technique offers significant advantages over established junction formation techniques such as ion implantation and ion beam milling. These include not requiring a postimplant anneal to activate dopants and repair ion-induced damage, and not necessitating reapplication of the passivation layer after junction formation. Previous work has demonstrated high-performance photodiodes based on H2/CH4 plasmainduced junction formation using a parallel-plate reactive ion etching (RIE) tool. The newer hybrid inductively coupled plasma (ICP) RIE technology is capable of greater control of the plasma condition, and therefore potentially greater control of the plasma-induced type conversion process. ... Differential profiling has been performed using wet chemical etch-backs between measurements to investigate the depth profiles of the carrier species. This investigation has revealed that the ICPRIE-induced type conversion depth is most sensitive to the sample temperature during exposure. The other ICPRIE process parameters, including the process pressure, RIE power, and ICP power, have also been shown to affect the type conversion depth and the electron concentration and mobility in the type converted layer. Based on this carrier transport characterisation work, a set of ICPRIE process conditions was identified as being suitable for formation of n-on-p junctions for high-performance photodiode fabrication. Three sets of photodiodes have been fabricated and characterised. The ICPRIE process parameters for junction formation were refined based on the parametric study of the carrier transport properties. The performance of photodiodes from each sample was measured to improve with each set of variations to the conditions for ICPRIE-induced junction formation, based on performance characterisation by current-voltage and noise measurements. Dynamic resistance area products up to 2.5 × 106 O.cm2 at 77K were measured for these mid-wave (MW) IR photodiodes (cutoff wavelength 5.3 µm), which is equivalent to the best reported results in the literature for devices based on established fabrication techniques. Gated photodiode structures were used to demonstrate that surface passivation is the performance-limiting factor for these photodiodes. This indicates not only that the set of ICPRIE conditions developed in this work to date is suitable for producing high-performance photodiodes, but that there is also potential for further improvement.

No description available.

Thesis (Ph.D)--Electrical and Computer Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Yoder, Douglas; Committee Member: Graham, Samuel; Committee Member: Allen, Janet; Committee Member: Klein, Benjamin; Committee Member: Voss, Paul. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Wide band gap based nanostructures have being attracting much research interest because of their promise for application in optoelectronic devices. Among those wide band gap semiconductors, gallium nitride (GaN) and zinc oxide (ZnO) are the most commonly studied and optoelectronic devices based on GaN and ZnO have been widely investigated. This thesis concentrates on the growth, optical and electrical properties of GaN and ZnO nanostructures, plus their application in solar cells and light emitting diodes (LEDs). GaN-nanowire based dye sensitized solar cells were studied. Different post-growth treatments such as annealing and coating with a TiOx shell were applied to enhance dye absorption. It was found that TiOx increased the dye absorption and the performance of the dye sensitized solar cell. ZnO nanorods were synthesized by vapor deposition and electrodeposition. Post-growth treatments such as annealing and hydrothermal processing were used to modify the defect chemistry and optical properties. LEDs based on GaN/ZnO heterojunctions were studied. The influence of ZnO seed layers on GaN/ZnO LEDs was investigated. GaN/ZnO LEDs based on ZnO nanorods with MgO and TiOx shells were also prepared in order to modify the LED performance. The coating condition of the shell was found to influence the current-voltage (I-V) characteristics and device performance. Moreover, high brightness LEDs based on GaN with InGaN multiple quantum wells were also fabricated. The origin of the emission from GaN/ZnO LEDs was studied using different kinds of GaN substrates. Direct metal contacts on bare GaN substrates were also employed to investigate the optical emission and electrical properties. It is found that the emission from the GaN/ZnO LEDs probably originated from the GaN substrate. GaN/ZnO LEDs with MgO as an interlayer were also fabricated. The MgO layer was expected to modify the band alignment between the GaN and the ZnO. It was shown that GaN/MgO/ZnO heterojunctions (using both ZnO nanorods and ZnO films) have quite different emission performance under forward bias compared to those that have no MgO interlayer. An emission peak was around 400 nm could originate from ZnO. Nitrogen doped ZnO nanorods on n-type GaN have been prepared by electrodeposition. Zinc nitrate and zinc acetate were used as ZnO precursors and NH4NO3 was used as a nitrogen precursor. Only the ZnO nanorods made using zinc nitrate showed obvious evidence of doping and coherent I-V characteristics. Cerium doped ZnO based LEDs were fabricated and showed an emission that depended on the cerium precursor that was employed. This indicates that the choice of precursor influences the growth, the materials properties and the optical properties of ZnO. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy