Browsing by Subject "Amorphous silicon"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item Electronic transport in mixed-phase hydrogenated amorphous/nanocrystalline silicon thin films(2013-05) Wienkes, Lee RaymondInterest in mixed-phase silicon thin film materials, composed of an amorphous semiconductor matrix in which nanocrystalline inclusions are embedded, stems in part from potential technological applications, including photovoltaic and thin film transistor technologies. Conventional mixed-phase silicon films are produced in a single plasma reactor, where the conditions of the plasma must be precisely tuned, limiting the ability to adjust the film and nanoparticle parameters independently. The films presented in this thesis are deposited using a novel dual-plasma co-deposition approach in which the nanoparticles are produced separately in an upstream reactor and then injected into a secondary reactor where an amorphous silicon film is being grown. The degree of crystallinity and grain sizes of the films are evaluated using Raman spectroscopy and X-ray diffraction respectively. I describe detailed electronic measurements which reveal three distinct conduction mechanisms in n-type doped mixed-phase amorphous/nanocrystalline silicon thin films over a range of nanocrystallite concentrations and temperatures, covering the transition from fully amorphous to ~30% nanocrystalline. As the temperature is varied from 470 to 10 K, we observe activated conduction, multiphonon hopping (MPH) and Mott variable range hopping (VRH) as the nanocrystal content is increased. The transition from MPH to Mott-VRH hopping around 100K is ascribed to the freeze out of the phonon modes. A conduction model involving the parallel contributions of these three distinct conduction mechanisms is shown to describe both the conductivity and the reduced activation energy data to a high accuracy. Additional support is provided by measurements of thermal equilibration effects and noise spectroscopy, both done above room temperature (>300 K). This thesis provides a clear link between measurement and theory in these complex materials.Item Electronic transport in nanocrystalline germanium/hydrogenated amorphous silicon composite thin films(2015-02) Bodurtha, Kent EdwardRecent interest in composite materials based on hydrogenated amorphous silicon (a-Si:H) stems in part from its potential for technical applications in thin film transistors and solar cells. Previous reports have shown promising results for films of a-Si:H with embedded silicon nanocrystals, with the goal of combining the low cost, large area benefits of hydrogenated amorphous silicon with the superior electronic characteristics of crystalline material. These materials are fabricated in a dual-chamber plasma-enhanced chemical vapor deposition system in which the nanocrystals are produced separately from the amorphous film, providing the flexibility to independently tune the growth parameters of each phase; however, electronic transport through these and other similar materials is not well understood. This thesis reports the synthesis and characterization of thin films composed of germanium nanocrystals embedded in a-Si:H. The results presented here describe detailed measurements of the conductivity, photoconductivity and thermopower which reveal a transition from conduction through the a-Si:H for samples with few germanium nanocrystals, to conduction through the nanocrystal phase as the germanium crystal fraction XGe is increased. These films display reduced photosensitivity as XGe is increased, but an unexpected increase in the dark conductivity is found in samples with XGe > 5% after long light exposures. Detailed studies of the conductivity temperature dependence in these samples exposes a subtle but consistent deviation from the standard Arrhenius expression; the same departure is found in samples of pure a-Si:H; a theoretical model is presented which accurately describes the actual conductivity temperature dependence.Item Enhanced crystallization of amorphous silicon thin films by nano-crystallite seeding(2013-12) Trask, JasonPolycrystalline silicon (poly-Si) has become popular in recent years as a candidate for low cost, high efficiency thin film solar cells. The possibility to combine the stability against light degradation and electronic properties approaching melt-grown, wafer-based crystallline silicon, with the cost advantage of Silicon thin films is highly attractive. To fully realize this goal, efforts have been focused on maximizing grain size while reducing the thermal input involved in a critical ``annealing'' step. Of the variety of processes involved in this effort, studies have shown that poly-Si films obtained from solid-phase-crystallization (SPC) of hydrogenated amorphous silicon (a-Si:H), grown from non-thermal plasma-enhanced chemical vapor deposition (PECVD), exhibit the potential to achieve the highest quality grain structures. However, reproducible control of grain size has proven difficult, with larger grains typically requiring longer annealing times. In this work, a novel technique is demonstrated for more effectively controlling the final grain structure of SPC-processed films while simultaneously reducing annealing times. The process utilized involves SPC of a-Si:H thin films containing embedded nanocrystallites, intended to serve as predetermined grain-growth sites, or grain-growth ``seeds'', during the annealing process. Films were produced by PECVD with a system in which two plasmas were operated to produce crystallites and amorphous films separately. This approach allows crystallite synthesis conditions to be tuned independently from a-Si:H film synthesis conditions, providing a large parameter space available for process optimization, including the effects of particle size, shape, quantity, and location within the film. The work contained here-in outlines the effects of select parameters on the both grain size control and thermal budget. Reproducible control of both grain size and crystallization rate were demonstrated through varying initial seed crystal concentrations. Significant reductions in annealing times were demonstrated to occur in seeded films relative to unseeded films, with both seed crystal concentration and seed crystal geometry demonstrating significant effects on crystallization rate. Furthermore, the development of this technique has resulted in potentially new insights on the material system involved, with the observation of a potentially unique phase-transformation mechanism.